LES and DNS

Wind turbines operate in the surface layer of the atmospheric boundary layer, where they are subjected to strong wind shear and relatively high turbulence levels. These incoming boundary layer flow characteristics are expected to affect the structure of wind turbine wakes. The near-wake region is characterized by a complex coupled vortex system (including helicoidal tip vortices), unsteadiness and strong turbulence heterogeneity. Limited information about the spatial distribution of turbulence in the near wake, the vortex behavior and their influence on the downwind development of the far wake hinders our capability to predict wind turbine power production and fatigue loads in wind farms. This calls for a better understanding of the spatial distribution of the 3D flow and coherent turbulence structures in the near wake. Systematic wind-tunnel experiments were designed and carried out to characterize the structure of the near-wake flow downwind of a model wind turbine placed in a neutral boundary layer flow. A horizontal-axis, three-blade wind turbine model, with a rotor diameter of 13 cm and the hub height at 10.5 cm, occupied the lowest one-third of the boundary layer. High-resolution particle image velocimetry (PIV) was used to measure velocities in multiple vertical stream-wise planes (x–z) and vertical span-wise planes (y–z). In particular, we identified localized regions of strong vorticity and swirling strength, which are the signature of helicoidal tip vortices. These vortices are most pronounced at the top-tip level and persist up to a distance of two to three rotor diameters downwind. The measurements also reveal strong flow rotation and a highly non-axisymmetric distribution of the mean flow and turbulence structure in the near wake. The results provide new insight into the physical mechanisms that govern the development of the near wake of a wind turbine immersed in a neutral boundary layer. They also serve as important data for the development and validation of numerical models.

A numerical methodology to simulate two-phase electrohydrodynamic flows under the volume-of-fluid paradigm is proposed. The electric force in such systems acts only at the interface and is zero elsewhere in the two fluids. Continuum surface force representations are derived for the electric field force in a system of dielectric-dielectric and conducting-conducting fluids. On the basis of analytical calculations for simple flow problems we propose a weighted harmonic mean interpolation scheme to smoothen the electric properties in the diffused transition region (interface). It is shown that a wrong choice of interpolation scheme (weighted arithmetic mean) may lead to a transition region thickness dependent electric field in the bulk. We simulate a set of problems with exact or approximate analytical solutions to validate the numerical model proposed. A coupled level set and volume-of-fluid (CLSVOF) algorithm has been used for simulations presented here.

This paper presents a comparative study for the weakly compressible (WCSPH) and incompressible (ISPH) smoothed particle hydrodynamics methods by providing numerical solutions for fluid flows over an airfoil and a square obstacle. Improved WCSPH and ISPH techniques are used to solve these two bluff body flow problems. It is shown that both approaches can handle complex geometries using the multiple boundary tangents (MBT) method, and eliminate particle clustering-induced instabilities with the implementation of a particle fracture repair procedure as well as the corrected SPH discretization scheme. WCSPH and ISPH simulation results are compared and validated with those of a finite element method (FEM). The quantitative comparisons of WCSPH, ISPH and FEM results in terms of Strouhal number for the square obstacle test case, and the pressure envelope, surface traction forces, and velocity gradients on the airfoil boundaries as well as the lift and drag values for the airfoil geometry indicate that the WCSPH method with the suggested implementation produces numerical results as accurate and reliable as those of the ISPH and FEM methods.

We derive and analyze a model for implicit Large Eddy Simulation (LES) of compressible flows that is applicable to a broad range of Mach numbers and particularly efficient for LES of shock-turbulence interaction. Following a holistic modeling philosophy, physically sound turbulence modeling and numerical modeling of unresolved subgrid scales (SGS) are fully merged, in a manner quite different from that of traditional implicit LES approaches. The implicit subgrid model is designed in such a way that asymptotic consistency with incompressible turbulence theory is maintained in the low Mach number limit. Compressibility effects are properly accounted for by a novel numerical flux function, which can capture strong shock waves in supersonic flows and also ensures an accurate representation of smooth waves and turbulence without excessive numerical dissipation. Simulations of shock-tube problems, Noh's three-dimensional implosion problem, large-scale forced and decaying three-dimensional homogeneous isotropic turbulence, supersonic turbulent boundary layer flows, and a Mach = 2.88 compression-expansion ramp flow demonstrate the good performance of the SGS model; across this range of flows, predictions are in excellent agreement with theory, direct numerical simulations, and experimental reference data. Results for implicit LES of canonical shock-turbulence interaction are compared with results of explicit LES using the dynamic Smagorinsky model. The analysis shows that details of the numerical method used for shock capturing clearly outweigh the effect of different turbulence modeling strategies in explicit and implicit LES. The implicit LES model recovers the ideal 2nd-order grid convergence of shock-capturing errors that has been predicted using Rapid Distortion Theory. The dynamic Smagorinsky model in conjunction with a hybrid method that combines sixth-order central differences with a seventh-order weighted essentially non-oscillatory scheme yields turbulence statistics that are very similar to the implicit LES results. However, while the explicit LES requires a tailored high-order low-dissipative numerical method that applies numerical dissipation only in shock normal direction, no such ad hoc adjustments are necessary with the proposed implicit LES method.

Objective: While the correlation of atherosclerotic plaque locations with local wall shear stress magnitude has been evaluated previously by other investigators in both right (RCA) and left coronary arteries (LCA), the relative performance of average wall shear stress (AWSS), average wall shear stress gradient (AWSSG), oscillatory shear index (OSI) and relative residence time (RRT) as indicators of potential atherosclerotic plaque locations has not been studied for the LCA. Here we determine the performance of said wall shear parameters in the LCA for the prediction of plaque development locations and compare these results to those previously found in the RCA. Methods: We obtained 30 patient-specific geometries (mean age 67.1 (±9.2) years, all with stable angina) of the LCA using dual-source computed tomography and virtually removed any plaque present. We then performed computational fluid dynamics simulations to calculate the wall shear parameters. Results: For the 96 total plaques, AWSS had a higher sensitivity for the prediction of plaque locations (86 ± 25%) than AWSSG (65 ± 37%, p < 0.05), OSI (67 ± 32%, p < 0.01) or RRT (48 ± 38%, p < 0.001). RRT had a higher PPV (49 ± 36%) than AWSS (31 ± 20%, p < 0.05) or AWSSG (16 ± 12%, p < 0.001). Segment 5 of the LCA presented with overall low values for sensitivity and PPV. Parameter performance in the remainder of the LCA was comparable to that in the RCA. Conclusions: AWSS features remarkably high sensitivity, but does not reach the PPV of RRT. This may indicate that while low wall shear stress is necessary for plaque formation, its presence alone is not sufficient to predict future plaque locations. Time dependent factors have to be taken into account as well.

An efficient numerical scheme to compute flows past rigid solid bodies moving through viscous incompressible fluid is presented. Solid obstacles of arbitrary shape are taken into account using the volume penalization method to impose no-slip boundary condition. The 2D Navier–Stokes equations, written in the vorticity-streamfunction formulation, are discretized using a Fourier pseudo-spectral scheme. Four different time discretization schemes of the penalization term are proposed and compared. The originality of the present work lies in the implementation of time-dependent penalization, which makes the above method capable of solving problems where the obstacle follows an arbitrary motion. Fluid–solid coupling for freely falling bodies is also implemented. The numerical method is validated for different test cases: the flow past a cylinder, Couette flow between rotating cylinders, sedimentation of a cylinder and a falling leaf with elliptical shape.

In this paper, we investigate the high-speed dynamics of symmetric and asymmetric cavitation bubble-collapse. For this purpose, a sharp-interface numerical model is employed, that includes a numerically effi-cient evaporation/condensation model. The underlying assumption is that phase change occurs in thermal non-equilibrium and that the associated timescale is much larger than that of the wave-dynamics described by the interfacial Riemann problem. The sharp-interface model allows for an accurate tracking of the interface evolution throughout collapse and rebound. With a first set of simulations, we investigate the influence of the non-equilibrium on the relaxation behaviour of an oscillating vapour bubble. We observe that a good prediction of the phase-change rate is essential. Of high practical interest is the col-lapse of cavitation bubbles near walls under high ambient-pressure conditions. We investigate the differ-ences in collapse evolution for detached and attached bubbles. It is shown that the maximum wall pressure strongly depends on the symmetry of the collapse mechanisms, and regions with a high proba-bility of bubble rebound are identified. Asymmetric attached bubbles lead to significantly different topol-ogy changes during collapse than symmetric bubbles but exhibit roughly the same range of maximum pressures.

The aim of this paper is to present an experimental and numerical investigation of dam-break flow over initially dry bed with a bottom obstacle. This test case highlights not only the bottom slope effects but also those of abrupt change in channel topography. Dam-break flow was applied in a smooth prismatic channel of rectangular cross-section over a trapezoidal bottom sill on the downstream bed. The present study scrutinized the formation and propagation of negative bore behind the sill. The flow was numerically simulated by the VOF-based commercially available CFD program, Flow-3D, solving the Reynolds Averaged Navier Stokes equations with the k-epsilon turbulence model (RANS) and the Shallow Water Equations (SWE). To validate CFD models an experiment was carried out. Using an advanced measuring technique, digital image processing, the flow was recorded simply through the glass walls of channel; thus, continuous free surface profiles were acquired synchronously with three cameras along the channel. The adopted measuring technique is non-intrusive and yields accurate and valuable results without flow disturbances. Comparison of the computed results with experimental data shows that RANS model reproduces the flow under investigation with reasonable accuracy while simple SWE model indicates some discrepancies particularly in predicting the negative wave propagation.

Keywords: dam-break; digital image processing; obstacle; CFD; VOF; k-epsilon turbulence model

The importance of three-dimensional effects for flapping wings is addressed by means of numerical simulation. In particular, the clap–fling–sweep mechanism is examined. The flow at the beginning of the downstroke is shown to be in reason ableagreement with the two-dimensional approximation. After the wings move farther than one chord length apart, three-dimensional effects become essential. Two values of the Reynolds number are considered. At Re=128, the spanwise flow from the wing roots to the wing tips is driven by the centrifugal forces acting on the mass of the fluid trapped in the recirculation bubble behind the wings. It removes the excess of vorticity and delays the periodic vortex shedding. At Re=1400, vortex breakdown occurs past the outer portion of the wings, and multiple vortex filaments are shed into the wake.

Journal Homepage: http://pof.aip.org/ Journal Information: http://pof.aip.org/about/about_the_journal Top downloads: http://pof.aip.org/features/most_downloaded Information for Authors: http://pof.aip.org/authors Downloaded 22 Jul 2013 to 143.215.126.126. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://pof.aip.org/about/rights_and_permissions A wide range of relative two-particle dispersion statistics from the Lagrangian Kinematic Simulation KS model, which contains turbulent-like flow structures, compares well with Yeung's Phys. Fluids 6, 3416 1994 DNS results. In particular, the Lagrangian flatness factor 4 (t) compares excellently better than Heppe's J. Fluid Mech. 357, 167 1998 nonlinear stochastic model. For higher Reynolds numbers the results from KS show that 4 (t) is significantly greater than 3 over a wide range of times within the inertial range of time scales.

A numerical algorithm for the simulation of flow past immersed objects with heat transfer is proposed and validated which conforms with the ideas of the fictitious domain method. A momentum source term is added to account for the presence of the object and a heat source term is proposed to impose the Dirichlet boundary condition on the surface of the objects. The algorithm is an implicit fictitious domain based method where the entire fluid-immersed object domain assumed to be an incompressible fluid. The flow domain is constrained to be divergence free, whereas a rigidity constraint is imposed on the body domain. Heat transfer is similarly considered by assuming that the object domain is filled with a fluid with different thermal properties. The SIMPLE algorithm with a collocated grid arrangement is used for pressure–velocity coupling which is unconditionally stable. The algorithm is validated by considering stationary, forced motion and freely moving objects with both isothermal and freely variable temperature inside the object. Good agreement with previous numerical and experimental studies for all the test cases is observed.

The interaction between an incident shock wave and a Mach-6 undisturbed hypersonic laminar boundary layer over a cold wall is addressed using direct numerical simulations (DNS) and wall-modeled large-eddy simulations (WMLES) at different angles of incidence. At sufficiently high shock-incidence angles, the boundary layer transitions to turbulence via breakdown of near-wall streaks shortly downstream of the shock impinge-ment, without the need of any inflow free-stream disturbances. The transition causes a localized significant increase in the Stanton number and skin-friction coefficient, with high incidence angles augmenting the peak thermomechanical loads in an approximately linear way. Statistical analyses of the boundary layer downstream of the interaction for each case are provided that quantify streamwise spatial variations of the Reynolds analogy factors and indicate a breakdown of the Morkovin's hypothesis near the wall, where velocity and temperature become correlated. A modified strong Reynolds analogy with a fixed turbulent Prandtl number is observed to perform best. Conventional transformations fail at collapsing the mean velocity profiles on the incompressible log law. The WMLES prompts transition and peak heating, delays separation, and advances reattachment, thereby shortening the separation bubble. When the shock leads to transition, WMLES provides predictions of DNS peak thermomechanical loads within ±10% at a computational cost lower than DNS by two orders of magnitude. Downstream of the interaction, in the turbulent boundary layer, WMLES agrees well with DNS results for the Reynolds analogy factor, the mean profiles of velocity and temperature, including the temperature peak, and the temperature/velocity correlation.

This paper describes a scenario of transition from laminar to turbulent flow in a spatially developing boundary layer over a flat plate. The base flow is the Blasius non-parallel flow solution; it
is perturbed by optimal disturbances yielding the largest energy growth over a short time interval. Such perturbations are computed by a non-linear global optimization approach based on a Lagrange multiplier technique. The results show that non-linear optimal perturbations are characterized by a localized basic building block, called the minimal seed, defined as the smallest flow structure  which maximizes the energy growth over short times. It is formed by vortices inclined in the streamwise direction surrounding a region of intense streamwise disturbance velocity. Such a basic structure appears to be a robust feature of the base flow since it is practically invariant with respect to the initial energy of the perturbation, the target time, the Reynolds number and the dimensions of the computational domain. The minimal seed grows very rapidly in time while spreading, and it triggers non-linear effects which bring the flow to turbulence in a very efficient manner, through the formation of a turbulence spot. This evolution of the initial optimal disturbance has been studied in detail by direct numerical simulations. Using a perturbative formulation of the Navier--Stokes equations, each linear and non-linear convective term of the equations has been analyzed.
The results show the fundamental role of the streamwise inclination of the vortices in the process. The non-linear coupling of the finite amplitude disturbances is crucial to sustain such streamwise inclination, as well as to generate dislocations within the flow structures, and local inflectional velocity distributions. The analysis provides a picture of the transition process characterized by a sequence of structures appearing successively in the flow, namely, Lambda vortices, hairpin vortices, streamwise streaks. Finally,  a disturbance regeneration cycle is conceived, initiated by the fast non-linear amplification of the minimal seed, providing a possible scenario for the continuous regeneration of the same fundamental
flow structures at smaller space and time scales.

"We present the development of a sliding mesh capability for an unsteady high order (order>3) h/p Discontinuous Galerkin solver for the three-dimensional incompressible Navier-Stokes equations. A high order sliding mesh method is developed and implemented for flow simulation with relative rotational motion of an inner mesh with respect to an outer static mesh, through the use of curved boundary elements and mixed triangular-quadrilateral meshes.

A second order stiffly stable method is used to discretise in time the Arbitrary Lagrangian-Eulerian form of the incompressible Navier-Stokes equations. Spatial discretisation is provided by the Symmetric Interior Penalty Galerkin formulation with modal basis functions in the x-y plane, allowing hanging nodes and sliding meshes without the requirement to use mortar type techniques. Spatial discretisation in the z-direction is provided by a purely spectral method that uses Fourier series and allows computation of spanwise periodic three-dimensional flows. The developed solver is shown to provide high order solutions, second order in time convergence rates and spectral convergence when solving the incompressible Navier-Stokes equations on meshes where fixed and rotating elements coexist.

In addition, an exact implementation of the no-slip boundary condition is included for curved edges; circular arcs and NACA 4-digit airfoils, where analytic expressions for the geometry are used to compute the required metrics.

The solver capabilities are tested for a number of two dimensional problems governed by the incompressible Navier-Stokes equations on static and rotating meshes: the Taylor vortex problem, a static and rotating symmetric NACA0015 airfoil and flows through three bladed cross-flow turbines. In addition, three dimensional flow solutions are demonstrated for a three bladed cross-flow turbine and a circular cylinder shadowed by a pitching NACA0012 airfoil."

The purpose of this paper is to evaluate the accuracy of the mesoscopic approach proposed by Février et al. [P. Février, O. Simonin, K.D. Squires, Partitioning of particle velocities in gas–solid turbulent flows into a continuous field and a spatially uncorrelated random distribution: theoretical formalism and numerical study, J. Fluid Mech. 533 (2005) 1–46] by comparison against the Lagrangian approach for the simulation of an ensemble of non-colliding particles suspended in a decaying homogeneous isotropic turbulence given by DNS. The mesoscopic Eulerian approach involves to solve equations for a few particle PDF moments: number density, mesoscopic velocity, and random uncorrelated kinetic energy (RUE), derived from particle flow ensemble averaging conditioned by the turbulent fluid flow realization. In addition, viscosity and diffusivity closure assumptions are used to compute the unknown higher order moments which represent the mesoscopic velocity and RUE transport by the uncorrelated velocity component. A detailed comparison between the two approaches is carried out for two different values of the Stokes number based on the initial fluid Kolmogorov time scale, StK=0.17and2.2. In order to perform reliable comparisons for the RUE local instantaneous distribution and for the mesoscopic kinetic energy spectrum, the error due to the computation method of mesoscopic quantities from Lagrangian simulation results is evaluated and minimized. A very good agreement is found between the mesoscopic Eulerian and Lagrangian predictions for the small particle Stokes number case corresponding to the smallest particle inertia. For larger particle inertia, a bulk viscous term is included in the mesoscopic velocity governing equation to avoid spurious spatial oscillation that may arise due to the inability of the numerical scheme to resolve sharp number density gradients. As a consequence, for StK=2.2StK=2.2, particle number density and RUE spatial distribution predicted by the mesoscopic Eulerian approach are more smooth with respect to the ones measured from the Lagrangian simulations results. Similarly, the Eulerian approach underestimates the mesoscopic kinetic energy for the high wavenumber modes while the agreement remains very good for the low wavenumber modes. For both cases, the mesoscopic Eulerian approach provides a good prediction of the time dependent particle and fluid-particle velocity correlations measured by spatial averaging in the whole computational domain.

In the framework of the IAVCEI (International Association of Volcanology and Chemistry of the Earth Interior) in-tercomparison study on volcanic plume models, we present three-dimensional (3D) numerical simulations carried out with the ASHEE (ASH Equilibrium Eulerian) model. The ASHEE model solves the compressible balance equations of mass, momentum, and enthalpy of a gas-particle mixture and is able to describe the kinematic decoupling for particles characterized by Stokes number (i.e., the ratio between the particle equilibrium time and the flow characteristic time) lower than 0.2 (or particles smaller than about 1 mm). The computational fluid dynamic model is designed to accurately simulate a turbulent flow field using a Large Eddy Simulation approach , and is thus suited to analyze the role of particle non-equilibrium in the dynamics of turbulent volcanic plumes. The two reference scenarios analyzed correspond to a weak (mass eruption rate = 1.5* 10 6 kg/s) and a strong volcanic plume (mass eruption rate = 1.5*10 9 kg/s) in absence of wind. For each scenario, we compare the 3D results, averaged in space and time, with theoretical results obtained from integral plume models. Such an approach enables quantitative evaluation of the effects of grid resolution and the subgrid-scale turbulence model, and the influence of gas-particle non-equilibrium processes on the large-scale plume dynamics. We thus demonstrate that the uncertainty on the numerical solution associated with such effects can be significant (of the order of 20%), but still lower than that typically associated with input data and integral model approximations. In the Weak Plume case, 3D results are consistent with the predictions of integral models in the jet and plume regions, with an entrainment coefficient around 0.10 in the plume region. In the Strong Plume case, the self-similarity assumption is less appropriate and the entrainment coefficient in the plume region is more unstable, with an average value of 0.24. For both cases, integral model predictions diverge from the 3D plume behavior in the umbrella region. The presented analysis of 3D numerical simulations thus enables identification of the critical hypotheses that underlie integral models used in operational studies. In addition, high-resolution 3D runs allow reproduction of observable quantities (such as infrasound signals) which can be useful for constraining eruption dynamics during real events.

The present work is dedicated to the numerical study of statistical characteristics of spatially-evolving supersonic turbulent boundary layers (STBL) with cooled walls. Large-Eddy Simulations (LESs) are performed to gain further insight into the role of wall temperature on the mean and fluctuating-flow properties of STBL. The velocity fluctuations, which are scaled according to the Morkovin's hypothesis, have shown acceptable agreement with available experimental and DNS results of literature. However, the van Driest transformed skin friction C f inc lies below the incompressible theoretical curves as a function of Re h inc for cold STBL, whereas compressible skin friction is found to be relatively higher for cold wall boundary layers than adiabatic boundary layers. The variation of total shear stress remains unaffected throughout the boundary layer for 0:5 6 T w =T r 6 1. The total temperature fluctuations are non-negligible for cold cases and surface cooling changes the near-wall turbulent structures. Additionally, the streamwise velocity and temperature fluctuations for the coldest isothermal STBL case are strongly correlated compared to the anti-correlation behavior of the adiabatic STBL in the near-wall region (y þ % 9). Furthermore, the pressure fluctuations are found to be non-negligible for cooled boundary layers and a positive correlation between pressure and density fluctuations are observed in the log-layer. These tendencies have also been verified through a detailed statistical analysis of the unsteady flow-field.

An approach for Eulerian–Lagrangian large-eddy simulation of bubble plume dynamics is presented and its performance evaluated. The main numerical novelties consist on defining the gas-liquid coupling based on the bubble size to mesh resolution ratio (Dp/Δx) while the interpolation between Eulerian and Lagrangian frameworks is achieved through the use of delta functions. The model’s performance is thoroughly validated for a bubble plume in a cubic tank in initially quiescent water using experimental data obtained from high-resolution ADV and PIV measurements. The predicted time-averaged averaged velocities and second-order statistics show a good agreement with the measurements, including the reproduction of the anisotropic nature of the plume’s turbulence. Further, the predicted Eulerian and Lagrangian velocity fields, second-order turbulence statistics and interfacial gas-liquid forces are quantified and discussed as well as the visualization of the time-averaged primary and secondary flow structure in the tank.

In this paper we establish a benchmark data set of a generic high-pressure turbine vane generated by direct numerical simulation (DNS) to resolve fully the flow. The test conditions for this case are a Reynolds number of 0.57 million and an exit Mach number of 0.9, which is representative of a modern transonic high-pressure turbine vane. In this study we first compare the simulation results with previously published experimental data. We then investigate how turbulence affects the surface flow physics and heat transfer. An analysis of the development of loss through the vane passage is also performed. The results indicate that free-stream turbulence tends to induce streaks within the near wall flow, which augment the surface heat transfer. Turbulent breakdown is observed over the late suction surface, and this occurs via the growth of two-dimensional Kelvin-Helmholtz spanwise roll-ups, which then develop into lambda vortices creating large local peaks in the surface heat transfer. Turbulent dissipation is found to significantly increase losses within the trailing-edge region of the vane.

Momentum and heat transfer characteristics of a semi-circular cylinder immersed in unconfined flowing Newtonian fluids have been investigated numerically. The governing equations, namely, continuity, Navier-Stokes and energy, have been solved in the steady flow regime over wide ranges of the Reynolds number (0.01 6 Re 6 39.5) and Prandtl number (Pr 6 100). Prior to the investigation of drag and heat transfer phenomena, the critical values of the Reynolds number for wake formation (0.55 < Re c < 0.6) and for the onset of vortex shedding (39.5 < Re c < 40) have been identified. The corresponding values of the lift coefficient, drag coefficient, and Strouhal number are also presented. After establishing the limit of the steady flow regime, the influence of the Reynolds number (0.01 6 Re 6 39.5) and Prandtl number (Pr = 0.72, 1, 10, 50 and 100) on the global flow and heat transfer characteristics have been elucidated. Detailed kinematics of the flow is investigated in terms of the streamline and vorticity profiles and the variation of pressure coefficient in the vicinity of the cylinder. The functional dependence of the individual and total drag coefficients on the Reynolds number is explored. The Nusselt number shows an additional dependence on the Prandtl number. In addition, the isotherm profiles, local Nusselt number (Nu L ) and average Nusselt number (Nu) are also presented to analyze the heat transfer characteristic of a semicircular cylinder in Newtonian media.

In this research, we numerically investigate the physics of pulsatile flows confined within a 3-dimensional channel with a modelled stenosis formed eccentrically on the upper wall using the method of large-eddy simulation (LES). An advanced dynamic
nonlinear subgrid-scale stress model was utilized to conduct numerical simulations and its predictive performance was examined in comparison with that of the conventional dynamic model. The Womersley number tested in the simulation was fixed at 10.5 and the Reynolds numbers tested were set to 750 and 2000, which are characteristics of human blood flows in large arteries. An in-house LES code, based on curvilinear Cartesian coordinates, has been developed to conduct the unsteady numerical simulations using three different grid systems. The physical characteristics of the flow field have been studied in terms of the resolved mean velocity, turbulence kinetic energy, viscous wall shear stress, resolved and subgrid-scale turbulent
shear stresses, local kinetic energy fluxes between the filtered and subgrid scales, and turbulence energy spectra along the central streamline of the domain. Triggered by  the stenosis, the flow field driven by the pulsatile inlet condition undergoes laminarturbulent-laminar patterns in the streamwise direction. Correspondingly, the slope of the energy spectra deviates significantly from the well-known −5/3 law for the inertial
subrange to reflect the transition in the flow patterns.

In this study, the melting process of ice as a phase-change material (PCM) saturated with a nickel– steel porous matrix inside a horizontal elliptical tube is investigated. Due to the low thermal conductivity of the PCM, it is motivated to augment the heat transfer performance of the system simultaneously by finding an optimum value of the aspect ratio and impregnating a metallic porous matrix into the base PCM. The lattice Boltzmann method with a double distribution function formulated based on the enthalpy method, is applied at the representative elementary volume scale under the local thermal equilibrium assumption between the PCM and porous matrix in the composite. While reducing or increasing the aspect ratio of the circular tubes leads to the expedited melting, the 90 • inclination of each elliptical tube in the case of the pure PCM melting does not affect the melting rate. With the reduction in the porosity, the effective thermal conductivity and melting rate in all tubes promoted. Although the natural convection is fully suppressed due to the significant flow blockage in the porous structure, the melting rates are generally increased in all cases.

problem of an incompressible two-phase immiscible fluid in a stratified inviscid shear flow with interfacial tension. The time-dependent evolution of the two-fluid interface over a wide range of Richardson number (Ri) and for three different density ratios is numerically investigated. The simulation results are compared with analytical solutions in the linear regime. Having captured the physics behind KHI, the effects of gravity and surface tension on a two-dimensional shear layer are examined independently and together. It is shown that the growth rate of the KHI is mainly controlled by the value of the Ri number, not by the nature of the stabilizing forces. It was observed that the SPH method requires a Richardson number lower than unity (i.e. Ri ∼ = 0.8) for the onset of KHI, and that the artificial viscosity plays a significant role in obtaining physically correct simulation results that are in agreement with analytical solutions. The numerical algorithm presented in this work can easily handle two-phase fluid flow with various density ratios. NUMERICAL MODELING OF KELVIN-HELMHOLTZ INSTABILITY USING SPH Figure 7. Time evolution of the interface in the two-dimensional KHI problem for the density ratio of 2 / 1 = 10 at various Ri numbers: (a) t * = 0.5; (b) t * = 1.0; (c) t * = 1.5; and (d) t * = 2.0 ( = 0.001).

— This work reports investigations on Diesel spray transients, accounting for internal nozzle flow and needle motion, and demonstrates how seamless calculations of internal flow and external jet can be accomplished in a Large-Eddy Simulation (LES) framework using an Eulerian mixture model. Sub-grid stresses are modeled with the Dynamic Structure (DS) model, a non-viscosity based one-equation LES model. Two problems are studied with high level of spatial and temporal resolution. The first one concerns an End-Of-Injection (EOI) case where gas ingestion, cavitation, and dribble formation are resolved. The second case is a Start-Of-Injection (SOI) simulation that aims at analyzing the effect of residual gas trapped inside the injector sac on spray penetration and rate of fuel injection. Simulation results are compared against experiments carried out at Argonne National Laboratory (ANL) using synchrotron X-ray. A mesh sensitivity analysis is conducted to assess the quality of the LES approach by evaluating the resolved turbulent kinetic energy budget and comparing the outcomes with a length-scale resolution index. LES of both EOI and SOI processes have been carried out on a single hole Diesel injector, providing insights in to the physics of the processes, with internal and external flow details, and linking the phenomena at the end of an injection event to those at the start of a new injection. Concerning the EOI, the model predicts ligament formation and gas ingestion, as observed experimentally, and the amount of residual gas in the nozzle sac matches with the available data. The fast dynamics of the process is described in detail. The simulation provides unique insights into the physics at the EOI. Similarly, the SOI simulation shows how gas is ejected first, and liquid fuel starts being injected with a delay. The simulation starts from a very low needle lift and is able to predict the actual Rate-Of-Injection (ROI) and jet penetration, based only on the prescribed needle motion. Finally, guidelines and future improvements of the model are discussed concerning the simulation of the transient injection phases.

To study the mixed convection flows in a square double lid-driven cavity with various inclination angles, a numerical method based on the finite volume method is used. In this investigation Al2O3/water nanofluid with particle diameter of 47 nanometers in different solid volume fractions have been utilized. Flow is obtained by the top and bottom walls, which slide in opposite direction at constant speed. The study has been conducted for the Richardson number of 0.1 to 10, Reynolds number (Re) of 1 to 100, while solid volume fraction (φ) of nanoparticles altered from 0 to 0.06. The heat transfer increases with increasing solid volume fraction for a constant Re. It also increases with increasing Richardson number and Reynolds number for a particular volume fraction. Also at low Reynolds number, the solid volume fraction does not have much effect on flow and heat transfer.

Keywords: nanofluid, heat transfer, mix convection, double lid-driven cavity, numerical study

A reliable and efficient aerodynamic design optimization tool using evolutionary algorithm has been developed for transonic compressor blades. A real-coded adaptive-range genetic algorithm is used to improve efficiency and robustness in design optimization. To represent flow fields accurately and produce reliable designs, three-dimensional Navier-Stokes computation is used for aerodynamic analysis.

Direct numerical simulations (DNS) of supersonic turbulent boundary layers (STBL) over adiabatic and isothermal walls are performed to investigate the effects of wall heat transfer on turbulent statistics and near wall behaviors. Four different cases of adiabatic, quasi-adiabatic, and uniform hot and cold wall temperatures are considered. Based on the analysis of the current database, it is observed that even though the turbulent Mach number is below 0.3, the wall heat transfer modifies the behavior of near-wall turbulence. These modifications are investigated and identified using both instantaneous fields (i.e. scatter plots) and mean quantities. Morkovin's hypothesis for compressible turbulent flows is found to be valid for neither heated nor cooled case. It is further uncovered that although some near-wall asymptotic behaviors change upon using weak or strong adiabatic walls, respectively denote the isothermal and isoflux walls, basic turbulent statistics are not affected by the thermal boundary condition itself. We also show that among different definition of Reynolds number used in STBL, the Reynolds number based on the friction velocity has some advantages data comparison regarding the first and second order statistical moments. More in depth analyses are also performed using the balance equation for turbulent kinetic energy (TKE) budget, as well as the dissipation rate. It is found that the dilatational to solenoidal dissipation ratio increases/decreases when heating/cooling the walls. The DNS of the current STBLs are available online for the community.

The effect of wind turbine blade tip geometry is numerically analysed using Computational Fluid Dynamics (CFD). Three different rotating blade tips are compared for attached flow conditions and the flow physics around the geometries are analysed. To this end, the pressure coefficient (Cp) is defined based on the stagnation pressure rather than on the inflow dynamic pressure. The tip geometry locally modifies the angles of attack (AOA) and the inflow dynamic pressure at each of the studied sections. However not all 3D effects could be reduced to a change of these two variables. An increase in loadings (particularly the normal force) towards the tip seem to be associated to a spanwise flow component present for the swept-back analysed tip. Integrated loads are ranked to asses wind turbine tip overall performance.It results from the comparison that a better tip shape that produced better torque to thrust ratios in both forces and moments is a geometry that has the end tip at the pitch axis. The work here presented shows that CFD may prove to be useful to complement 2D based methods on the design of new wind turbine blade tips.

Atmospheric motions are governed by turbulent motions associated to nontrivial energy transfers at small scales (direct cascade) and/or at large scales (inverse cascade). Although it is known that the two cascades coexist, energy fluxes have been previously investigated from the spectral point of view but not on their instantaneous spatial and local structure. Here, we compute local and instantaneous subfilter-scale energy transfers in two sets of reanalyses (NCEP–NCAR and ERA-Interim) in the troposphere and the lower stratosphere for the year 2005. The fluxes are mostly positive (toward subgrid scales) in the troposphere and negative in the stratosphere, reflecting the baroclinic and barotropic nature of the motions, respectively. The most intense positive energy fluxes are found in the troposphere and are associated with baroclinic eddies or tropical cyclones. The computation of such fluxes can be used to characterize the amount of energy lost or missing at the smallest scales in climate and weather models.

Over the last decade the adjoint method has been consolidated as one of the most versatile and successful tools for aerodynamic design. It has become a research area on its own, spawning a large variety of applications and a prolific literature. Yet, some relevant aspects of the method remain relatively less explored in the literature. Such is the case with the adjoint boundary conditions and, more specifically, with regard to permeable boundaries. The present work discusses at length a novel approach to the continuous adjoint boundary problem, with emphasis on the full characteristic formulation of the farfield boundary conditions. The main goal of this approach is to ensure the well-posedness of the adjoint equations and consistency with the primal problem. * Graduate Student, Department of Mechanical Engineering.

A fundamental study has been performed to understand the effect of unsteady forcing on turbulence statistics in channel flow with rough walls using direct numerical simulation. Unsteady flows have been generated by applying an unsteady nonzero mean forcing in the form of time varying pressure gradient such that the amplitude of oscillations is between 19% and 26% of mean centerline velocity and covering a range of forcing frequencies. The analysis has revealed unsteady forcing, depending on the forcing frequency, results in enhanced roughness compared to steady channel flow. The rough-wall flow dynamics have been categorized into high-, intermediate-, and low-frequency regimes. In the regime of high-frequency forcing, unsteadiness alters the mean velocity and turbulence intensities only in the inner layer of the turbulent boundary layer. Further, the turbulence intensities are out of phase with each other and also with the external forcing. In the regime of intermediate-frequency forcing, mean velocity and turbulence intensities are altered beyond the inner layer. In the inner layer, the turbulence intensities are out of phase with each other. The Reynolds stress is in phase with the external forcing in the inner layer, but it is out of phase in the outer layer. In the regime of low-frequency forcing, the mean velocity and turbulence intensities are significantly altered throughout the turbulent boundary layer.

Extensive numerical results on the flow and thermal fields are presented for free convection from a semicircular cylinder (flat base upward) immersed in quiescent power-law fluids for the following ranges of conditions: Grashof number, 10 6 Gr 6 10 5 , Prandtl number, 0.72 6 Pr 6 100, and power-law index, 0.2 6 n 6 1.8. The heat transfer characteristics are analyzed in terms of the isotherm patterns, local and average Nusselt number as functions of the pertinent dimensionless parameters. The flow field is visualized in terms of the streamline patterns adjacent to the surface of the cylinder for a range of values of the Grashof number, Prandtl number and power-law index. A separated flow region forms at as low values of the Prandtl number as Pr = 0.72 for n P 1 (Newtonian and shear-thickening fluids); whereas for shear-thinning fluids (n < 1), the flow remains attached to the cylinder surface over the range of conditions encompassed here. The bubble size grows with Grashof number and it shrinks with Prandtl number. In order to quantify the deviation from the Newtonian behaviour, the normalized values of average Nusselt number are analyzed as a function of the power-law index. In addition, a correlation is proposed for average Nusselt number as a function of the Grashof number, Prandtl number and power-law index. In general terms, shear-thinning fluid behaviour enhances heat transfer whereas shear-thickening has adverse influence on it.

The continuity, momentum and energy equations describing the flow and heat transfer of power-law fluids over a semi-circular cylinder have been solved numerically in the two-dimensional steady flow regime. The influence of the Reynolds number (Re), Prandtl number (Pr) and power-law index (n) on the local and global flow and heat characteristics have been studied over wide ranges of conditions as follows: 0.01 6 Re 6 30, 1 6 Pr 6 100 and 0.2 6 n 6 1.8. The variation of drag coefficient and Nusselt number with the Reynolds number, Prandtl number and power-law index is shown over the aforementioned ranges of conditions. In addition, streamline and isotherm profiles along with the recirculation length and distribution of pressure coefficient and Nusselt number over the surface of the semi-circular cylinder are also presented to gain further insights into the nature of the underlying kinematics. The wake size (recirculation length) shows almost linear dependence on the Reynolds number (Re P 1) for all values of power-law index studied herein. The drag values show the classical inverse variation with the Reynolds number, especially for shear-thinning fluids at low Reynolds numbers. The point of maximum pressure coefficient is found slightly displaced from the front stagnation point for highly shearthinning fluids, whereas for shear-thickening and Newtonian fluids, it coincides with the front stagnation point. For fixed values of the Prandtl number and Reynolds number, the rate of heat transfer decreases with the gradual increase in power-law index; this effect is particularly striking at high Prandtl numbers due to the thinning of the thermal boundary layer. Conversely, as expected, shear-thinning behavior facilitates heat transfer and shear-thickening impedes it. The effect of power-law index on both momentum and heat-transfer characteristics is seen to be appreciable at low Reynolds numbers and it gradually diminishes with the increasing Reynolds number.

Turbulent flows over rough surfaces are encountered in many engineering and geo-
physical applications. Flows of this nature, due to their increasing technological
interests, have been a subject of rigorous investigation in recent years. Of the
particular interest to the oceanographic community is the study of turbulent oscil-
latory flow over rough surfaces, representative of sediment-bed in a coastal environ-
ment. In particular, formulation of predictive criteria for onset of sediment motion
requires detailed knowledge of the dynamics of the near-bed turbulence structure
and resultant variations in the magnitudes and time-scales of the destabilizing
drag and lift forces on sediment grains. The primary objectives of this work are (i)
to quantify, using high-fidelity numerical experiments, sediment grain–turbulence
interactions and (ii) provide data on the temporal variations in the magnitude of
drag and lift forces on sediment grains, the time-scales associated with these vari-
ations, and their correlations with the near-bed turbulence in an oscillatory flow
environment. To this end, particle-resolved direct numerical simulations (DNS) are performed
to investigate the behavior of an oscillatory flow field over a bed of closely packed
fixed spherical particles for a range of Reynolds numbers in transitional and rough
turbulent flow regime. Presence of roughness leads to a substantial modification of
the underlying boundary layer mechanism resulting in increased bed shear stress,
reduction in the near-bed anisotropy, modification of the near-bed sweep and ejec-
tion motions along with marked changes in turbulent energy transport mechanisms.
Characterization of such resulting flow field is performed by studying statistical
descriptions of the near-bed turbulence for different roughness parameters. A
double-averaging technique is employed to reveal spatial inhomogeneities at the
roughness scale that provide alternate paths of energy transport in the turbulent
kinetic energy (TKE) budget. Spatio–temporal characteristics of unsteady particle
forces by studying their spatial distribution, temporal auto-correlations, frequency
spectra as well as cross-correlations with near-bed turbulent flow variables are
reported. Intermittency in the forces by means of impulse is also investigated.

We investigate a passive flow-control technique for the interaction of an oblique shock generated by an 8.8° wedge with a turbulent boundary-layer at a free-stream Mach number of Ma∞=2.3 and a Reynolds number based on the incoming boundary-layer thickness of Reδ0=60 500 by means of large-eddy simulation (LES). The compressible Navier–Stokes equations in conservative form are solved using the adaptive local deconvolution method (ALDM) for physically consistent subgrid scale modeling. Emphasis is placed on the correct description of turbulent inflow boundary conditions, which do not artificially force low-frequency periodic motion of the reflected shock. The control configuration combines suction inside the separation zone and blowing upstream of the interaction region by a pressure feedback through a duct embedded in the wall. We vary the suction location within the recirculation zone while the injection position is kept constant. Suction reduces the size of the separation zone with strongest effect when applied in the rear part of the separation bubble. The analysis of wall-pressure spectra reveals that all control configurations shift the high-energy low-frequency range to higher frequencies, while the energy level is significantly reduced only if suction acts in the rear part of the separated zone. In that case also turbulence production within the interaction region is significantly reduced as a consequence of mitigated reflected shock dynamics and near-wall flow acceleration.

Rotor tip-clearance induced noise, both in the form of rotor self noise and rotor-stator interaction noise, constitutes a significant component of total fan noise. Innovative yet cost effective techniques to suppress rotor-generated noise are, therefore, of foremost importance for improving the noise signature of turbofan engines. To that end, the feasibility of a passive porous treatment strategy to positively modify the tip-clearance flow field is addressed. The present study is focused on accurate viscous flow calculations of the baseline and the treated rotor flow fields. Detailed comparison between the computed baseline solution and experimental measurements shows excellent agreement. Tip-vortex structure, trajectory, strength, and other relevant aerodynamic quantities are extracted from the computed database. Extensive comparison between the untreated and treated tipclearance flow fields is performed. The effectiveness of the porous treatment for altering the rotor-tip vortex flow field in general and reducing the intensity of the tip vortex, in particular, is demonstrated. In addition, the simulated flow field for the treated tip clearly shows that substantial reduction in the intensity of both the shear layer roll-up and boundary layer separation on the wall is achieved.

In this work, the governing partial differential equations (continuity and Cauchy's momentum equations) describing the flow of power-law type non-Newtonian fluids across a semicircular cylinder (oriented with its curved surface in the upstream direction) have been solved numerically. In particular, consideration has been given to the delineation of the critical Reynolds numbers denoting the onset of flow separation from the surface of the cylinder and the onset of the laminar vortex shedding regime. This information is germane to establish the scaling of the macroscopic characteristics like drag coefficient and Strouhal number on the governing parameters, namely, Reynolds number and power-law index. The present results clearly suggest that the transitional Reynolds numbers show a strong dependence on the type (shear-thinning and shear-thickening) of fluid behavior as well as on the severity of the shear-dependence of the viscosity. With reference to the behavior seen in Newtonian fluids, the flow remains not only attached to the surface up to higher Reynolds numbers, but shear-thinning behavior also delays the onset of the laminar vortex shedding regime. As expected, shear-thickening fluids, of course, display the opposite characteristics.

We present a new numerical scheme for the simulation of deformable objects immersed in a viscous incompressible fluid. The two-dimensional Navier-Stokes equations are discretized with an efficient Fourier pseudo-spectral scheme. Using the volume penalization method arbitrary inflow conditions can be enforced, together with the no-slip conditions at the boundary of the immersed flexible object. With respect to Kolomenskiy and Schneider (2009) [1], where rigid moving obstacles have been considered, the present work extends the volume penalization method to account for moving deformable objects while avoiding numerical oscillations in the hydrodynamic forces. For the solid part, a simple and accurate one-dimensional model, the non-linear beam equation, is employed. The coupling between the fluid and solid parts is realized with a fast explicit staggered scheme. The method is applied to the fluttering instability of a slender structure immersed in a free stream. This coupled non-linear system can enter three distinct states: stability of the initial condition or maintenance of an either periodic or chaotic fluttering motion. We present a detailed parameter study for different Reynolds numbers and reduced free-stream velocities. The dynamics of the transition from a periodic to a chaotic state is investigated. The results are compared with those obtained by an inviscid vortex shedding method [2] and by a viscous linear stability analysis [3], yielding for both satisfactory agreement. New results concerning the transition to chaos are presented.

A fundamental study was conducted to shed light on entrainment and mixing in buoyancy-driven Boussinesq density currents. Large-eddy simulation was performed on lock-exchange (LE) release density currents—an idealized test bed to generate density currents. As dense fluid was released over a sloping surface into an ambient lighter fluid, the dense fluid slumps to the bottom and forms a characteristic head of the current. The dynamics of the head dictated the mixing processes in LE currents. The key contribution of this study is to resolve an ongoing debate on mixing: We demonstrate that substantial mixing occurs in the early stages of evolution in an LE experiment and that entrainment is highly inhomogeneous and unsteady during the slumping regime. Guided by the flow physics, entrainment is calculated using two different but related perspectives. In the first approach, the entrainment parameter (E) is defined as the fraction of ambient fluid displaced by the head that entrains into the current. It is an indicator of the efficiency in which ambient fluid is displaced into the current and it serves as an important metric to compare the entrainment of dense currents over different types of surfaces, e.g., roughness configuration. In the second approach, E measures the net entrainment in the current at an instantaneous time t over the length of the current. Net entrainment coefficient is a metric to compare the effects of flow dynamical conditions, i.e., lock-aspect ratio that dictates the fraction of buoyancy entering the head, and also the effect of the sloping angle. Together, the entrainment coefficient and the net entrainment coefficient provide an insight into the entrainment process. The " active " head of the current acts as an engine that mixes the ambient fluid with the existing dense fluid, the 3-D lobes and clefts on the frontal end of the current causes recirculation of the ambient fluid into the current, and Kelvin-Helmholtz rolls are the mixers that entrain the ambience into the current. Buoyancy and shear production occur at the interface in the head region of the current, and transport of turbulence kinetic energy (TKE) by Reynolds stresses results in high TKE. Published by AIP Publishing. [http://dx.doi.org/10.1063/1.4974353]

This work provides a global optimization analysis, looking for perturbations inducing the largest energy growth at a finite time in a boundary-layer flow in the presence of smooth three-dimensional roughness elements. Amplification mechanisms are described which can by-pass the asymptotical growth of Tollmien-Schlichting waves. Smooth axisymmetric roughness elements of different height have been studied, at different Reynolds numbers. The results show that even very small roughness elements, inducing only a weak deformation of the base flow, can localize the optimal disturbance characterizing the Blasius boundary-layer flow. Moreover, for large enough bump heights and Reynolds numbers, a strong amplification mechanism has been recovered, inducing an increase of several order of magnitude of the energy gain with respect to the Blasius case. In particular, the highest value of the energy gain is obtained for an initial varicose perturbation, differently to what found for a streaky parallel flow. Optimal varicose perturbations grow very rapidly by transporting the strong wall-normal shear of the base flow, which is localized in the wake of the bump. Such optimal disturbances are found to lead to transition for initial energies and amplitudes considerably smaller than sinuous optimal ones, inducing hairpin vortices downstream of the roughness element.

Large eddy simulation (LES) runs are performed to calculate flows over heterogeneous surfaces imitating small forest lakes. Regularities in the turbulent exchange of heat and momentum over such objects are examined. A weak sensitivity of turbulence characteristics over a "lake" to thermal stratification is noted. Problems of the representativeness of field eddy covariance measurements of turbulent fluxes over such objects are discussed.

Re´sume´-Conditions aux limites et mode`les SGS pour les simulations LES d'e´coulements se´pare´s de´limite´s par des parois : une application aux ge´ome´tries de type moteur -L'imple´mentation et la combinaison de conditions aux limites avance´es et de mode`les des e´chelles infe´rieures a`la maille dans des simulations des grandes e´chelles (Large Eddy Simulations, LES) sont pre´sente´es. Le but est d'effectuer des simulations LES fiables des e´coulements froids dans des ge´ome´tries complexes, comme dans les cylindres des moteurs a`combustion interne. L'imple´mentation de conditions aux limites a`l'entre´e pour la ge´ne´ration de turbulence synthe´tique et de deux mode`les des e´chelles infe´rieures a`la maille, le mode`le local dynamique de Smagorinsky et le mode`le SGS (Sub-Grid Scales) de viscosite´WALE (Wall-Adapting Local Eddy), est de´crite. Le mode`le WALE est baseś ur le carre´du tenseur du gradient de vitesse. Il prend en compte a`la fois les effets de la pression et du taux de rotation des plus petites fluctuations turbulentes de´tectables et il de´crit de manie`re ade´quate l'e´chelle y 3 a`proximite´des parois de la viscosite´turbulente sans ne´cessiter de pression dynamique. Il est ainsi cense´repre´senter un mode`le tre`s fiable pour les simulations ICE. La validation du mode`le a e´te´effectue´e sur deux bancs laminaires stationnaires : une ge´ome´trie a`gradin inverse´(backward-facing) et une ge´ome´trie simple de moteur a`combustion interne avec une unique vanne centrale. L'exhaustivite´de la simulation LES (i.e. la qualite´de la simulation LES) est aussi discute´e.

This paper presents a mesh adaptation scheme for direct minimization
of output error using a selection process for choosing the optimal
refinement option from a discrete set of choices.  The scheme is
geared for viscous aerodynamic flows, in which solution anisotropy
makes certain refinement options more efficient compared to others.
No attempt is made, however, to measure the solution anisotropy
directly or to incorporate it into the scheme.  Rather, mesh
anisotropy arises naturally from the minimization of a cost function
that incorporates both an output error estimate and a count of the
additional degrees of freedom for each refinement option.  The method
is applied to output-based adaptive simulations of the laminar and
Reynolds-averaged compressible Navier-Stokes equations on body-fitted
meshes in two and three dimensions. Two-dimensional results for
laminar flows show a factor of 2-3 reduction in the degrees of freedom
on the final adapted meshes when the discrete choice optimization is
used compared to pure isotropic adaptation.  Preliminary results on a
wing-body configuration show that these savings improve in three
dimensions and for higher Reynolds-number flows.

Direct numerical simulations are used to study the postformation evolution of a laminar vortex ring. The vortex structure is described by calculating the embedded boundaries of the vortex inner core, vortex core, and vortex bubble. The topology of the vortex ring is found to be self-similar during the entire postformation phase. We also show that extracting the vortex inner core provides an objective method in setting the upper value for the cutoff vorticity level separating the vortex from its tail. The computed power laws describing the decay of the translation velocity and integrals of motion ͑circulation, impulse, and energy͒ are shown to be consistent with both studies of Dabiri and Gharib ͓J. Fluid Mech. 511, 311 ͑2004͔͒ and Maxworthy ͓J. Fluid Mech. 51, 15 ͑1972͔͒. We prove that the apparently different scaling laws reported in these two studies collapse if a virtual time origin is properly defined. Finally, the computationally generated vortex rings are matched to the classical Norbury-Fraenkel model and the recent model proposed by Kaplanski and Rudi ͓Phys. Fluids 17, 087101 ͑2005͔͒. The former model provides not only a good prediction of normalized energy and circulation but also a good estimation of individual integrals of motion. The latter model offers, in addition to a good prediction of integral quantities, a more accurate description of the vortex ring topology when comparing the contours of the inner cores or vortex signatures. Both models underestimate the volume of fluid carried inside the vortex bubble.

The evaluation of the dispersion in vegetated beds may allow indentifying mechanisms that affect the transport and reaction of solutes, namely organic and nitrogen compounds. A set of non-reactive tracer experiments (slag injection) was performed in a vegetated bed (a mesocosm with a LECA-based substratum and colonized with Phragmites australis) used for the removal of organic and nitrogen pollutant loads. Loads of approximately 300 mg COD/L and 30 mg NH4-N/L and a hydraulic loading rate of 3.5 cm/d were used. The results showed a delay in all the residence time distribution (RTD) curves and a variation in the dimensionless residence time (μ(m,θ)) of the E(θ) curves, which means that the mass centre of the impulse was late relatively to the expected one. A strong dispersion and tracer retention (due to the presence of stagnated areas and internal recirculation) was observed, especially in the first 33 cm of the bed, which seems to have been related to the presence of complex clusters of roots, solid material, biofilm and LECA particles. An analytical solution of the Multiple-Tanks-in-Series (MTS) model well represents the RTD curves obtained in the tracer experiments. The detected dispersion and dead volume ratios (7% to 12%) did not affect the performance of the bed, which presented mean removal efficiencies of 85% and 60.4% for COD and NH4-N, respectively.

Whilst electrohydrodynamic (EHD) flow actuation of dielectric fluids has been widely demonstrated, the fundamental mechanisms responsible for their behaviour is not well understood. By highlighting key distinguishing features of the various EHD mechanisms discussed in the literature, and proposing a more general mechanism based on Maxwell (electric) pressure gradients that arise due to induced polarization, we suggest that it is possible to identify the dominant EHD mechanisms that are responsible for an observed flow. We demonstrate this for a class of low conductivity dielectric fluids -Electro-Conjugate Fluids (ECFs) -that have recently been shown to exhibit EHD flow phenomena when subjected to nonuniform fields of low intensities. Careful inspection of the salient attributes of the flow, at least at low field strengths (<1 kV/cm) -for example, the absence of a threshold voltage for the onset of flow, the quadratic scaling of the flow velocity with the applied voltage, and flow from the high to the low field region -eliminate the possibility of mechanisms based on space charge. Instead, we suggest that flow can be attributed to the existence of a Maxwell pressure gradient. This is further corroborated by good agreement between our experimental results and theoretical analysis.

This study focuses on the dispersion and diffusion characteristics of high-order energy-stable flux reconstruction (ESFR) schemes via the spatial eigensolution analysis framework proposed in [1]. The analysis is performed for five ESFR schemes, where the parameter 'c' dictating the properties of the specific scheme recovered is chosen such that it spans the entire class of ESFR methods, also referred to as VCJH schemes, proposed in [2]. In particular, we used five values of 'c', two that correspond to its lower and upper bounds and the others that identify three schemes that are linked to common high-order methods, namely the ESFR recovering two versions of discontinuous Galerkin methods and one recovering the spectral difference scheme. The performance of each scheme is assessed when using different numerical intercell fluxes (e.g. different levels of upwinding), ranging from " under-" to " over-upwinding ". In contrast to the more common temporal analysis, the spatial eigensolution analysis framework adopted here allows one to grasp crucial insights into the diffusion and dispersion properties of FR schemes for problems involving non-periodic boundary conditions, typically found in open-flow problems, including turbulence, unsteady aerodynamics and aeroacoustics.

Phone Number: 0 (+33) 232 959 794, Fax Number: 0 (+33) 232 959 780 ABSTRACT In a recent paper by Zhang et al. (2012), a Mach number-invariant scaling was proposed to account for the effect of variation of free-stream Mach number in supersonic turbulent boundary layers. The present work focuses on the effect of variation of wall temperature with strong heating and cooling at the wall. Direct numerical simulation is used to study scaling and turbulence structure of a spatially evolving Mach 2 supersonic boundary layer at a friction Reynolds number of 500. A new scaling law is proposed to account for temperature dependent fluid-property variations.