On Three-Dimensionality of Turbulent Buoyant Channel Flow

Journal of Fluid Mechanics, 2010

The behaviour of the velocity and pressure fluctuations in the outer layers of wall-bounded turbulent flows is analysed by comparing a new simulation of the zero-pressure-gradient boundary layer with older simulations of channels. The 99 % boundary-layer thickness is used as a reasonable analogue of the channel half-width, but the two flows are found to be too different for the analogy to be complete. In agreement with previous results, it is found that the fluctuations of the transverse velocities and of the pressure are stronger in the boundary layer, and this is traced to the pressure fluctuations induced in the outer intermittent layer by the differences between the potential and rotational flow regions. The same effect is also shown to be responsible for the stronger wake component of the mean velocity profile in external flows, whose increased energy production is the ultimate reason for the stronger fluctuations. Contrary to some previous results by our group, and by others, ...

Physics of Fluids, 2015

An experimental investigation of the flow over two-and three-dimensional large-scale wavy walls was performed using high-resolution planar particle image velocimetry in a refractive-index-matching (RIM) channel. The 2D wall is described by a sinusoidal wave in the streamwise direction with amplitude to wavelength ratio a/λ x = 0.05. The 3D wall is defined with an additional wave superimposed on the 2D wall in the spanwise direction with a/λ y = 0.1. The flow over these walls was characterized at Reynolds numbers of 4000 and 40 000, based on the bulk velocity and the channel half height. Flow measurements were performed in a wall-normal plane for the two cases and in wall-parallel planes at three heights for the 3D wavy wall. Instantaneous velocity fields and time-averaged turbulence quantities reveal strong coupling between large-scale topography and the turbulence dynamics near the wall. Turbulence statistics show the presence of a well-structured shear layer that enhances the turbulence for the 2D wavy wall, whereas the 3D wall exhibits different flow dynamics and significantly lower turbulence levels. It is shown that the 3D surface limits the dynamics of the spanwise turbulent vortical structures, leading to reduced turbulence production and turbulent stresses and, consequently, lower average drag (wall shear stress). The likelihood of recirculation bubbles, levels and spatial distribution of turbulence, and rate of the turbulent kinetic energy production are shown to be severely affected when a single spanwise mode is superimposed on the 2D sinusoidal wall. Differences of one and two order of magnitudes are found in the turbulence levels and Reynolds shear stress at the low Reynolds number for the 2D and 3D cases. These results highlight the sensitivity of the flow to large-scale topographic modulations; in particular the levels and production of turbulent kinetic energy as well as the wall shear stress.

Experiments in Fluids, 2014

The experimental conditions required for a turbulent channel flow to be considered fully-developed and nominally two-dimensional remain a challenging objective. In this study we show that the flow obtained in a high-aspect-ratio channel facility cannot be reproduced by DNSs of spanwise-periodic channel flows; therefore, we reserve the term "channel" for spanwiseperiodic DNSs, and denote the experimental flow by the term "duct". Oil film interferometry (OFI) and static pressure measurements were carried out over the range 200 < Re τ < 900 in an adjustable-geometry duct flow facility. Three-dimensional effects were studied by considering different aspect ratio (AR) configurations, and also by fixing the AR and modifying the hydraulic diameter D H of the section. The conditions at the centerplane of the duct were characterized through the local skin friction from the OFI and the centerline velocity 2 Ricardo Vinuesa et al. at four different streamwise locations, and through the wall shear based on the streamwise global pressure gradient. The skin friction obtained from pressure gradient overestimated the local shear measurements obtained from the OFI, and did not reproduce the same ARdependence observed with OFI. Differences between the local and global techniques were also reflected in the flow development. For the range of Reynolds numbers tested, the development length of high-aspect-ratio ducts scales with the duct full-height H and is around 200H, much larger than the values of around 100 − 150H previously reported in the literature.

Journal of Fluid Mechanics, 1982

Fully developed turbulent channel flow has been simulated americally at . Reynolds number 19800, based on centerline velocity and channel half width. The large-scale flow field has been obtained by directly integrating the filtered, three-dimensional, tine-dependent, Davies-Stokes equations. The smallscale field motions were simulated through an eddy viscosity model. The calculations were carried out on the ILLIAC IV computer with up to 516,096 grid points. The computed flow field was used to study the statistical properties of the flow as well as its time-dependent features. The agreement of the computed mean velocity profile, turbulence statistics, and detailed flow structures with experimental data is good. The resolvable portion of the statistical correlations appearing in the Reynolds stress equations are calculated. Particular attention is given to the examination of the flow structure in the vicinity of the wall. I. Introduction Large-eddy simulation (LBS) is a relatively new approach to the calculation of turbulent flows. The basic idea stems from two experimental observations. First, the large-scale structure of turbulent flows varies greatly from flow to flow (e.g., jets vs. boundary layers) and consequently is difficult, if not impossible, to model in a general way. Second, the small-scale turbulence structures are nearly isotropic, very universal in character (Chapman, 1979) and hence such sore amenable to general modeling. In LBS, one actually calculates the large-scale notions in a time-dependent, three-*Portions of this work were carried out while the authors held NRC Research Associateships at Ames Research Center. a locally isotropic: part and an iahomc.-Qeneous part, he employed a separate partial differential equation for SGS turbulent kinetic energy. However, the added differential equation did not improve the results over the calculations in which only an eddy viscosity model was used ( Schumann, 1975). Gr8tzbach and Schumann ( 1977) extended their channel flow calculations to account for temperature fluctuations and heat transfer. Later extensions by GrBtzbach include calculations of secondary flows in partly roughened chanr nels, inclusion of buoyancy effects, and liquid metal flows in plane channels and annuli. A recent -4view of this group's work in LES was given by Schumann et al. (1979). In all of the above computations, the dynamics of the inner region of the boundary layer (viscous sublayer and butter layer) was essentially ignored. It is in this region that virtually all of the production of turbulence kinetic energy takes place (Townsend, 1956;. Artificial boundary conditions in the logarithmic region were used to simulate the inner layers. Aside from the fact that these boundary conditions are designed to be consistent iu the mean with the law of thx wall, there is little justification or experimental evidence to warrant their use for the detailed flow field. however, the computations of and especially those of the Karlsruhe group have produced successful comparisons with experimental data in the regions away from the walls. With a relatively modest number of grid points, they have been able to extract considerable information of practical interest from their computations. The first numerical simulation of turbulent channel flow that computed ^rather than modeled the flow in the immediate neighborhood of the wall was that of . In this calculation only 16 uniformly spaced grid points were used in each of the streamwise, x, and spanwise, z, directions

Journal of Fluid Mechanics, 2006

The interaction between the wall and the core region of turbulent channels is studied using direct numerical simulations at friction Reynolds number Re τ ≈ 630. In these simulations the near-wall energy cycle is effectively removed, replacing the smooth-walled boundary conditions by prescribed velocity disturbances with non-zero Reynolds stress at the walls. The profiles of the first-and second-order moments of the velocity are similar to those over rough surfaces, and the effect of the boundary condition on the mean velocity profile is described using the equivalent sand roughness. Other effects of the disturbances on the flow are essentially limited to a layer near the wall whose height is proportional to a length scale defined in terms of the additional Reynolds stress. The spectra in this roughness sublayer are dominated by the wavenumber of the velocity disturbances and by its harmonics. The wall forcing extracts energy from the flow, while the normal equilibrium between turbulent energy production and dissipation is restored in the overlap region. It is shown that the structure and the dynamics of the turbulence outside the roughness sublayer remain virtually unchanged, regardless of the nature of the wall. The detached eddies of the core region only depend on the mean shear, which is not modified beyond the roughness sublayer by the wall disturbances. On the other hand, the large scales that are correlated across the whole channel scale with U LOG = u τ κ −1 log(Re τ ), both in smooth-and in rough-walled flows. This velocity scale can be interpreted as a measure of the velocity difference across the log layer, and it is used to modify the scaling proposed and validated by delÁlamo et al. (J.