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Turbulent Boundary Layer (turbulent + boundary_layer)
Selected AbstractsSimulation of coherent structures in turbulent boundary layer using Gao,Yong equations of turbulenceHEAT TRANSFER - ASIAN RESEARCH (FORMERLY HEAT TRANSFER-JAPANESE RESEARCH), Issue 5 2004Bo Liu Abstract The equations of incompressible turbulent flow developed by the Gao,Yong turbulence model have two important features. First, they do not contain any empirical coefficients or wall functions. Second, the series representation of turbulence energy equation reflects multi-scale structures of the nonlinearity of turbulence, and, therefore, is capable of describing both statistical mean flows and the coherent structures. This paper presents some simulation results of a two-dimensional turbulent boundary layer with zero pressure gradient based on Gao,Yong equations of turbulence. With a staggered grid arrangement, an incompressible SIMPLE code was used in the simulations. The simulated coherent structures have verified the adaptability of the newly derived equations to real intermittent turbulent flows. The effect of the orders of the energy equation and computational grid scales on the detection of coherent structures is also investigated. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(5): 287,298, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.20019 [source] Numerical simulation of turbulent free surface flow with two-equation k,, eddy-viscosity modelsINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 4 2004V. G. Ferreira Abstract This paper presents a finite difference technique for solving incompressible turbulent free surface fluid flow problems. The closure of the time-averaged Navier,Stokes equations is achieved by using the two-equation eddy-viscosity model: the high-Reynolds k,, (standard) model, with a time scale proposed by Durbin; and a low-Reynolds number form of the standard k,, model, similar to that proposed by Yang and Shih. In order to achieve an accurate discretization of the non-linear terms, a second/third-order upwinding technique is adopted. The computational method is validated by applying it to the flat plate boundary layer problem and to impinging jet flows. The method is then applied to a turbulent planar jet flow beneath and parallel to a free surface. Computations show that the high-Reynolds k,, model yields favourable predictions both of the zero-pressure-gradient turbulent boundary layer on a flat plate and jet impingement flows. However, the results using the low-Reynolds number form of the k,, model are somewhat unsatisfactory. Copyright © 2004 John Wiley & Sons, Ltd. [source] Evaluation of one- and two-equation low- Re turbulence models.INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 12 2003Axisymmetric separating, Part I, swirling flows Abstract This first segment of the two-part paper systematically examines several turbulence models in the context of three flows, namely a simple flat-plate turbulent boundary layer, an axisymmetric separating flow, and a swirling flow. The test cases are chosen on the basis of availability of high-quality and detailed experimental data. The tested turbulence models are integrated to solid surfaces and consist of: Rodi's two-layer k,, model, Chien's low-Reynolds number k,, model, Wilcox's k,, model, Menter's two-equation shear-stress-transport model, and the one-equation model of Spalart and Allmaras. The objective of the study is to establish the prediction accuracy of these turbulence models with respect to axisymmetric separating flows, and flows of high streamline curvature. At the same time, the study establishes the minimum spatial resolution requirements for each of these turbulence closures, and identifies the proper low-Mach-number preconditioning and artificial diffusion settings of a Reynolds-averaged Navier,Stokes algorithm for optimum rate of convergence and minimum adverse impact on prediction accuracy. Copyright © 2003 John Wiley & Sons, Ltd. [source] Turbulence in a three-dimensional wall-bounded shear flowINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 8 2010A. Holstad Abstract A new turbulent flow with distinct three-dimensional characteristics has been designed in order to study the impact of mean-flow skewing on the turbulent coherent vortices and Reynolds-averaged statistics. The skewing of a unidirectional plane Couette flow was achieved by means of a spanwise pressure gradient. Direct numerical simulations of the statistically steady Couette,Poiseuille flow enabled in-depth explorations of the turbulence field in the skewed flow. The imposition of a modest spanwise gradient turned the mean flow about 8° away from the original Couette flow direction and this turning angle remained nearly the same over the entire cross section. Nevertheless, a substantial non-alignment between the turbulent shear stress angle and the mean velocity gradient angle was observed. The structure parameter turned out to slightly exceed that in the pure Couette flow, contrary to the observations made in some other three-dimensional shear flows. Coherent flow structures, which are known to be associated with the Reynolds shear stress in near-wall regions, were identified by the ,2 -criterion. Instantaneous and ensemble-averaged vortices resembled those found in the unidirectional Couette flow. In the skewed flow, however, the vortex structures were turned to align with the local mean-flow direction. The conventional symmetry between Case 1 and Case 2 vortices was broken due to the mean-flow three-dimensionality. The turning of the coherent vortices and the accompanying symmetry-breaking gave rise to secondary and tertiary turbulent shear stress components. By averaging the already ensemble-averaged shear stresses associated with Case 1 and Case 2 vortices in the homogeneous directions, a direct link between the educed near-wall structures and the Reynolds-averaged turbulent stresses was established. These observations provide evidence in support of the hypothesis that the structural model proposed for two-dimensional turbulent boundary layers remains valid also in flows with moderate mean three-dimensionality. Copyright © 2009 John Wiley & Sons, Ltd. [source] Turbulence energetics in stably stratified geophysical flows: Strong and weak mixing regimesTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 633 2008S. S. Zilitinkevich Abstract Traditionally, turbulence energetics is characterised by turbulent kinetic energy (TKE) and modelled using solely the TKE budget equation. In stable stratification, TKE is generated by the velocity shear and expended through viscous dissipation and work against buoyancy forces. The effect of stratification is characterised by the ratio of the buoyancy gradient to squared shear, called the Richardson number, Ri. It is widely believed that at Ri exceeding a critical value, Ric, local shear cannot maintain turbulence, and the flow becomes laminar. We revise this concept by extending the energy analysis to turbulent potential and total energies (TPE, and TTE = TKE + TPE), consider their budget equations, and conclude that TTE is a conservative parameter maintained by shear in any stratification. Hence there is no ,energetics Ric', in contrast to the hydrodynamic-instability threshold, Ric,instability, whose typical values vary from 0.25 to 1. We demonstrate that this interval, 0.25 < Ri < 1, separates two different turbulent regimes: strong mixing and weak mixing rather than the turbulent and the laminar regimes, as the classical concept states. This explains persistent occurrence of turbulence in the free atmosphere and deep ocean at Ri , 1, clarifies the principal difference between turbulent boundary layers and free flows, and provides the basis for improving operational turbulence closure models. Copyright © 2008 Royal Meteorological Society [source] Flow over a hill covered with a plant canopyTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 596 2004J. J. Finnigan Abstract We develop an analytical model for atmospheric boundary-layer flow over a hill that is covered with a vegetation canopy. The slope of the hill is assumed to be small enough that the flow above the canopy can be treated within the linear framework of Hunt. Perturbations to the flow within the canopy are driven by the pressure gradient associated with the flow over the hill. In the upper canopy this pressure gradient is balanced by downwards turbulent transport of momentum and the canopy drag. The flow there can be calculated from linearized dynamics, which show that the maximum streamwise winds are where the perturbation pressure is at a minimum, i.e. near the crest of the hill. Deep within the canopy the pressure gradient associated with the flow over the hill is balanced by the canopy drag, here the nonlinear canopy drag. This nonlinear balance shows how the streamwise winds are largest where the perturbation pressure gradient is largest, i.e. on the upwind slope of the hill. In the lee of the hill this nonlinear solution shows how the pressure gradient decelerates the wind deep within the canopy, leading to separation with a region of reversed flow when the canopy is sufficiently deep. Coupling between the out-of-phase flows within and above the canopy means that the maximum velocity is further upwind of the hill crest than in flow over a rough hill, while the extra turbulent mixing caused by the canopy significantly reduces the magnitude of the velocity speed-up over the hill. Finally, we find that there is no formal limit process where the solutions with a canopy yield the well-known solutions for flow over a rough hill. This finding calls into question the very use of a roughness length in accelerating or decelerating turbulent boundary layers. Copyright © 2004 Royal Meteorological Society [source] | ||||||||||