A computational investigation of unsteady separated flows

Iranian Journal of Science and Technology, Transactions of Mechanical Engineering, 2019

In this paper, we investigate the unsteady Reynolds-averaged Navier-Stokes simulations of the turbulent cavitating flow around a hydrofoil CLE. The primary objective of this study was to highlight the effect of subgrid-scale turbulence method in the prediction of unsteady vortical flow. In this work, the Smagorinsky and wall-adapting local eddy viscosity (WALE) models are selected to close the large eddy simulation equations. To investigate the limitations of these two models, the interactions between cavitation and vortices, the predicted streamlines, the velocities components, and the pressure oscillations have been discussed. The numerical results were compared with experimental results. These results showed that regardless of the selected turbulence model the development cycle of cavitation pocket is characterized by three levels: (first stage) the growth of attached pocket, (second stage) the separation of the attached pocket, and (last stage) the development and collapse of detached structures. The comparison between these two models shows that the WALE model takes into account both the rotation and strain rates, whereas the Smagorinsky model only takes into account the deformation rate of the turbulent structure. Besides, the WALE model is highly adaptive for wall-bounded flows. This advantage is explained by this ability to recover the near-wall scaling for the eddy viscosity. The WALE model has adopted a treatment without damping function or wall functions. Due to its algebraic nature, it offers a high-speed and efficient scheme compared to the Smagorinsky model. The study of the flow structures by iso-surface of the Q-criterion showed that the WALE model can predict the transition from laminar to the turbulent regime.

Wind Energy, 2009

Direct numerical simulations of the Navier-Stokes equations are performed to achieve a better understanding of the behaviour of wakes generated by wind turbines. The simulations are performed by combining the in-house developed computer code EllipSys3D with the actuator-line methodology. In the actuator-line method, the blades are represented by lines along which body forces representing the loading are introduced. The body forces are determined by computing local angles of attack and using tabulated aerofoil coefficients. The advantage of using the actuator-line technique is that it is not needed to resolve blade boundary layers and instead the computational resources are devoted to simulating the dynamics of the flow structures. In the present study, approximately 5 million mesh points are used to resolve the wake structure in a 120-degree domain behind the turbine. The results from the computational fluid dynamics (CFD) simulations are evaluated and the downstream evolution of the velocity field is depicted. Special interest is given to the structure and position of the tip vortices. Further, the circulation from the wake flow field is computed and compared to the distribution of circulation on the blades.

Cornell University - arXiv, 2022

We investigate the spatial distributions and production mechanisms of vorticity and turbulent kinetic energy around a finite NACA 0018 wing with square wingtip profile at Re c = 10 4 and 10 • angle of attack with the aid of Direct Numerical Simulation (DNS). The analysis focuses on the highly inhomogeneous region around the tip and the near wake; this region is highly convoluted, strongly three-dimensional, and far from being self-similar. The flow separates close to the leading edge creating a large, open recirculation zone around the central part of the wing. In the proximity of the tip, the flow remains attached but another smaller recirculation zone forms closer to the trailing edge; this zone strongly affects the development of main wing tip vortex. The early formation mechanisms of three vortices close to the leading edge are elucidated and discussed. More specifically, we analyse the role of vortex stretching/compression and tilting, and how it affects the strength of each vortex as it approaches the trailing edge. We find that the three-dimensional flow separation at the sharp tip close to the leading edge plays an important role on the subsequent vortical flow development on the suction side. The production of turbulent kinetic energy and Reynolds stresses is also investigated and discussed in conjunction with the identified vortex patterns. The detailed analysis of the mechanisms that sustain vorticity and turbulent kinetic energy improves our understanding of these highly three dimensional, non-equilibrium flows and can lead to better actuation methods to manipulate these flows.

Physical Review Fluids

We investigate the spatial distributions and production mechanisms of vorticity and turbulent kinetic energy around a finite NACA 0018 wing with square wingtip profile at Re c = 10 4 and 10 • angle of attack with the aid of Direct Numerical Simulation (DNS). The analysis focuses on the highly inhomogeneous region around the tip and the near wake; this region is highly convoluted, strongly three-dimensional, and far from being self-similar. The flow separates close to the leading edge creating a large, open recirculation zone around the central part of the wing. In the proximity of the tip, the flow remains attached but another smaller recirculation zone forms closer to the trailing edge; this zone strongly affects the development of main wing tip vortex. The early formation mechanisms of three vortices close to the leading edge are elucidated and discussed. More specifically, we analyse the role of vortex stretching/compression and tilting, and how it affects the strength of each vortex as it approaches the trailing edge. We find that the three-dimensional flow separation at the sharp tip close to the leading edge plays an important role on the subsequent vortical flow development on the suction side. The production of turbulent kinetic energy and Reynolds stresses is also investigated and discussed in conjunction with the identified vortex patterns. The detailed analysis of the mechanisms that sustain vorticity and turbulent kinetic energy improves our understanding of these highly three dimensional, non-equilibrium flows and can lead to better actuation methods to manipulate these flows.

HAL (Le Centre pour la Communication Scientifique Directe), 2018

HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

2007

Direct Numerical Simulation of channel flow was utilized to study the evolution of various vortex configurations presented as flow initial conditions. Simulations of longitudinally, laterally and cross-flow oriented vortices suggested that the predominant form of turbulent structure was the half hairpin vortex. This vortical structure was dominant in the simulations seen in this as well as other investigations. In all cases hairpin vortices quickly degenerated to half hairpin or inclined vortical structures. It is hypothesized that these structures function as the predominant momentum transfer mechanism within the boundary layer, entraining fluid into the vortex cores like miniature tornados and transporting this fluid to the top of the boundary layer while simultaneously dragging fluid viscously around the inclined core of the vortex causing mixing of low-speed and high-speed flows.

European Journal of Mechanics - B/Fluids

Dynamical behavior of the turbulent channel flow separation induced by a wall-mounted two-dimensional bump is studied, with an emphasis on unsteadiness characteristics of vortical motions evolving in the separated flow. The present investigations are based on an experimental approach and Direct Numerical Simulation (Dns). The main interests are devoted to give further insight on mean flow properties, characteristic scales and physical mechanisms of low-frequencies unsteadiness. The study also aims to clarify the Reynolds number effects. Results are presented for turbulent flows at moderate Reynolds-number Re τ ranging from 125 to 730 where Re τ is based on friction velocity and channel half-height. A large database of time-resolved two-dimensional Piv measurements is used to obtain the velocity distributions in a region covering the entire shear layer and the flow surrounding the bump. An examination of both high resolved velocity and wallshear stress measurements showed that for moderate Reynolds numbers, a separated region exists until a critical value. Under this conditions, a thin region of reverse flow is formed above the bump and a large-scale vortical activity is clearly observed and analyzed. Three distinct self-sustained oscillations are identified in the separated zone. The investigation showed that the flow exhibits the shear-layer instability and vortexshedding type instability of the bubble. A low-frequency self-sustained oscillations associated with a flapping phenomenon is also identified. The experimental results are further emphasized using post-processed data from Direct Numerical Simulations, such as flow statistics and Dynamic Mode Decomposition. Physical mechanisms associated with observed self-sustained oscillations are then suggested and results are discussed in the light of instabilities observed in a laminar regime for the same flow configuration.

2015

The effects on the wakes of unsteady flows past a cylinder with sharp separation edges are investigated. Time evolution of the incompressible Navier–Stokes equations is achieved by means of a high-order spectral-element method in conjunction with a third-order-accurate time integration scheme. For this study, the Reynolds number was varied up to Reh = 200, where h is the projected frontal height of the cylinder having an equilateral triangular cross-section. The critical Reynolds numbers for vortex shedding for the cylinder at various inclinations are determined using the Stuart–Landau equation, and its dependence on the cylinder inclination described. As the Reynolds number is increased, the Kármán vortex street is observed to first develop spatially into a bi-layered wake profile, which then, depending on the cylinder inclination, observes a transition to either a secondary meandering profile or wakes reminiscent of the 2P and P+S modes found for oscillating circular cylinders. Th...

2017

Industry standard aerodynamic design tool RFOIL's performance can be improved at high angles of attack by incorporating the double wake inviscid model in the separated flow region. As a precursor, single wake inviscid model is developed using 2-D panel method to replicate the outcome of the standard tools. Then the double wake inviscid model is established on top with changes in the Kutta condition and local vorticity at the separation point. The solution is calculated by iterative procedure by establishing two wake sheets one from the trailing edge and the other from the separation point. The wake shapes are established from induced velocities of the airfoil vorticity distribution. The double wake inviscid model could establish better results than XFOIL and results closer to the experiment in the separated flow region over airfoils. This model can be combined with the viscous effects to mitigate the convergence problem at very high angles of attack with separated flows. Further...

2005

A computational method is described that has been designed to capture thin vortical regions in high Reynolds number incompressible flows. The principal objective of the method-Vorticity Confinement (VC)-is to capture the essential features of these small-scale vortical structures and model them with a very efficient difference method directly on an Eulerian computational grid. Essentially, the small scales are modeled as nonlinear solitary waves that "live" on the lattice indefinitely. The method allows convecting structures to be modeled over as few as 2 grid cells with no numerical spreading as they convect indefinitely over long distances, with no special logic required for merging or reconnection. It also serves as a very efficient substitute for RANS models of attached and separating boundary layers and vortex sheets and filaments. Further, the method easily allows boundaries with no-slip conditions to be treated as "immersed" surfaces in uniform, non-conforming grids, with no requirements for complex logic involving "cut" cells. In this paper a description of the basic VC method is given. This is more comprehensive than has been previously available. There are close analogies between VC and well-known shock and contact discontinuity capturing methodologies. These are discussed to explain the basic ideas behind VC, since it is somewhat different than conventional CFD methods. Some of the possibilities that VC offers towards very efficient computation of turbulent flows in the LES approximations are explored. These stem from the ability of VC to act as a negative dissipation at scales just above a grid cell, but that saturates and does not lead to divergence. This feature allows 1. approximate cancellation of numerical diffusion, so that more complex, high order-low dissipation schemes can be avoided. Small-scale vortical structures at the grid cell level can then be captured, resulting in very efficient use of the available degrees of freedom on the grid. 2. approximate treatment of backscatter. This involves the addition of (modeled) subgrid kinetic energy to the flow in a natural way, without requiring stochastic forcing, and which restores some of the instabilities that are removed by the (implicit) filtering. Although used for a number of years for complex, attached and separating flows, and trailing vortices, its use as an LES method is relatively recent. In Ref. [0], some initial LES results are presented.