Numerical Simulations of Rough-wall Turbulent Boundary Layers PDF Download
Are you looking for read ebook online? Search for your book and save it on your Kindle device, PC, phones or tablets. Download Numerical Simulations of Rough-wall Turbulent Boundary Layers PDF full book. Access full book title Numerical Simulations of Rough-wall Turbulent Boundary Layers by . Download full books in PDF and EPUB format.
Author:
Publisher:
ISBN:
Category :
Languages : en
Pages : 302
Get Book Here
Book Description
At sufficiently high Reynolds number, all surfaces are rough, and roughness affects most flows in engineering and the natural sciences. Examples range from atmospheric boundary layers over buildings and canopies, to engineering surfaces with erosion, deposits, etc. To study the roughness effects, we take a high-resolution approach to capture the flow around individual roughness elements using direct and large-eddy simulations (DNS and LES); the goal is to elucidate phenomena which have been difficult to access using physical experiments, and to help develop engineering correlations and models. First, most experiments and turbulence models are based on a standardized type of roughness, sand-grain roughness, which can be described using a single length scale. The relationship between the geometry of an arbitrary surface and the canonical one must be known, to predict critical flow parameters such as the drag, using either experimental correlations or turbulence models. Using numerical experiments, we relate this length-scale to the roughness geometry, and propose a guideline for its prediction in the industrial setting. Next, to explain the dependence of drag on the topographical details, we examine the role of the wake of the roughness elements in the drag generation of a rough surface. The wake field is found to promote vertical momentum transfer and near-wall instability; it might provide a link between geometry details and the engineering modeling of roughness effects. Lastly, we focus on a more realistic flow scenario -- the one with freestream accelerations -- and study the combined effects of roughness and acceleration, a phenomenon widely present in engineering flows over airfoils or complex landscapes. It is first shown, by comparing equilibrium accelerating flows obtained in the present study with the non-equilibrium flows in the literature, that the roughness and acceleration effects are interdependent and depend on the flow equilibrity. Then, using DNS data of a spatially developing flat-plate boundary layer, it is found that the effect coupling develops as the roughness affects the turbulence time scale and thus the flow susceptibility of the acceleration stabilization, while acceleration changes the wake velocity and ultimately the roughness destabilization level.
Author:
Publisher:
ISBN:
Category :
Languages : en
Pages : 302
Get Book Here
Book Description
At sufficiently high Reynolds number, all surfaces are rough, and roughness affects most flows in engineering and the natural sciences. Examples range from atmospheric boundary layers over buildings and canopies, to engineering surfaces with erosion, deposits, etc. To study the roughness effects, we take a high-resolution approach to capture the flow around individual roughness elements using direct and large-eddy simulations (DNS and LES); the goal is to elucidate phenomena which have been difficult to access using physical experiments, and to help develop engineering correlations and models. First, most experiments and turbulence models are based on a standardized type of roughness, sand-grain roughness, which can be described using a single length scale. The relationship between the geometry of an arbitrary surface and the canonical one must be known, to predict critical flow parameters such as the drag, using either experimental correlations or turbulence models. Using numerical experiments, we relate this length-scale to the roughness geometry, and propose a guideline for its prediction in the industrial setting. Next, to explain the dependence of drag on the topographical details, we examine the role of the wake of the roughness elements in the drag generation of a rough surface. The wake field is found to promote vertical momentum transfer and near-wall instability; it might provide a link between geometry details and the engineering modeling of roughness effects. Lastly, we focus on a more realistic flow scenario -- the one with freestream accelerations -- and study the combined effects of roughness and acceleration, a phenomenon widely present in engineering flows over airfoils or complex landscapes. It is first shown, by comparing equilibrium accelerating flows obtained in the present study with the non-equilibrium flows in the literature, that the roughness and acceleration effects are interdependent and depend on the flow equilibrity. Then, using DNS data of a spatially developing flat-plate boundary layer, it is found that the effect coupling develops as the roughness affects the turbulence time scale and thus the flow susceptibility of the acceleration stabilization, while acceleration changes the wake velocity and ultimately the roughness destabilization level.
Author: Morad Alvarez
Publisher:
ISBN:
Category :
Languages : en
Pages : 130
Get Book Here
Book Description
The effects of surface roughness on compressible turbulent flow have not been studied as closely as the effects of surface roughness on incompressible flow. To date, our knowledge of fully-rough high-speed turbulent flows comes from experiments, large eddy simulations, or direct numerical simulations of rough-wall channel flows. This dissertation seeks to extend our understanding of rough-wall boundary layers by examining the effect of the freestream Mach number. A previously-verified fifth-order hybrid weighted essentially non-oscillatory scheme with geometries imposed by a third-order cut-stencil method was modified to handle turbulent inflow boundary conditions and spanwise periodicity. The turbulence capabilities of the code were then validated against published results of a Mach 2.5 smooth-wall turbulent boundary layer. Two direct numerical simulations of different freestream Mach numbers, 2.5 and 5.0, were conducted. The results show that scaling the root mean square (RMS) of velocity and vorticity fluctuations with the local density accounts for the difference in magnitude. Scaling the RMS of non-dimensionalized temperature fluctuations by the ratio of wall temperature and freestream temperature, provides reasonable collapse between both simulations. Similarly, scaling by the ratio of wall density and freestream density offers reasonable collapse for the RMS of density fluctuations. Both of these scalings offer good collapse regardless of the surface topology. The Favre-averaged Reynolds shear stresses exhibit increased magnitude in regions with local compression. Conversely the Favre-averaged Reynolds shear stresses decreased in regions with increased expansion. A similar trend was observed for the wall-normal Favre-averaged Reynolds stress, but is not as pronounced. The location of the expansion and compression waves from the edges of the roughness is directly affected by the local Mach angle. For the \mf case, the Mach angles varied much more resulting in regions of decreased dilatation. The freestream Mach number plays an indirect role in setting the shift in the log-layer. The compressible results from both Mach numbers do not compare well to incompressible results. This could be due to the different topologies.
Author: Haosen Xu
Publisher:
ISBN:
Category :
Languages : en
Pages :
Get Book Here
Book Description
This dissertation presents numerical and theoretical studies of wall-bounded turbulence. The dissertation is divided into three major parts. The first part focuses on flow physics and modeling of statistical quantities in turbulent boundary layers. We model pressure statistics. The modeling strategy is based on a combination of Townsend's attached eddy hypothesis with Komolgorov's 1941 theory on small-scale turbulence. Specifically, we account for small-scale motions related to pressure inside larger scale wall-attached eddies. With this strategy, we are able to model the even order moments and the scaling of pressure spectrum. In the second part of the dissertation, we study rough wall turbulent boundary layer with densely packed roughness elements. The roughness elements are cubes. Direct numerical simulations are carried out. We report mean flow statistics, Reynolds and dispersive stresses, as well as terms in the turbulent/dispersive kinetic energy budget equations. We show that roughness with high packing densities do not necessarily have similar behaviors and therefore they cannot simply be categorized as d-type roughness. The third part of this dissertation aims to apply computational fluid dynamics to realistic engineering problems. A few problems are considered. We start by studying turbulent boundary layers with heat transfer. We focus on low-speed flows(Mach number within 0.2) with heat transfer, and the performance of wall-modeled large eddy simulation with equilibrium wall model is assessed. The study shows that the Mach number limit for incompressible assumption for thermal fields is lower than the often-quoted value 0.2 due to the associated viscous heating. In addition, we show that the first grid point implementation of the equilibrium wall model outperforms the third grid point implementation for heat transfer problems. Next, we apply wall-modeled large eddy simulation for flows in turbo-machinery. The study focuses specifically on the return channel of a multistage centrifugal compressor. We compare the results from wall-modeled large eddy simulation with those from Reynolds-averaged Navier-Stokes equations and experimental measurements. We show the potential advantages of wall-modeled large eddy simulations for practical engineering applications.
Author: Debra J. Olejniczak
Publisher:
ISBN:
Category :
Languages : en
Pages : 278
Get Book Here
Book Description
Author: Stephen Jay Kline
Publisher:
ISBN:
Category : Boundary layer
Languages : en
Pages : 60
Get Book Here
Book Description
Author:
Publisher:
ISBN:
Category :
Languages : en
Pages : 34
Get Book Here
Book Description
Author: K. Tsujimoto
Publisher:
ISBN:
Category :
Languages : en
Pages : 8
Get Book Here
Book Description
Rough-wall turbulent flows are more common in engineering application than smooth-wall turbulent flows. Modification of mean flow and turbulence property have been established by enormous experiments. However, details of mechanism of rough-wall turbulence has been understood little because spatial resolution is limited in experiments. Meanwhile, Direct Numerical Simulation (DNS) of a rough-wall turbulent flows which is capable of giving high resolution data requires prohibitively heavy computer power and accordingly, no other DNS data are available than those published by the present authors (Miyake et al., 1999). Our first DNS considered sandgrain roughness whose effect was implemented by profile drag based on Stokes drag and could successfully reproduce experimentally established rough-wall turbulent flow such as downward shift of straight line of logarithmic mean velocity distribution and vanishing of viscous sublayer. It was confirmed that the layer adjacent to the wall up to several tens in wall unit where smooth-wall turbulence exhibits autonomous property independent on the layer above it, is taken over by the layer having property of logarithmic layer, in rough-wall turbulence. While quasi-streamwise vortices play major role to generate high turbulent shear stress in this near-wall layer in smooth-wall turbulent flow, roughness destroy this vortical system and consequently, different mixing system which replaces the role of quasi-streamwise vortices should be found in rough-wall layer. Present work intends to investigate the turbulent mixing in the layer close to the wall of rough-wall turbulence by a DNS of more sound numerical conditions, i.e., without using any model for roughness element.
Author: Ronald Lee Panton
Publisher: Computational Mechanics
ISBN:
Category : Science
Languages : en
Pages : 448
Get Book Here
Book Description
Why is wall turbulence self-sustaining? In this book well-regarded researchers not only discuss what they know and believe, but also speculate on ideas that still require numerical or experimental testing and verification. An initial brief history of boundary layer structure research is followed by chapters on experimental information and specific topics within the subject. There are then sections on computational aspects.
Author: Stanford University. Thermosciences Division. Thermosciences Division
Publisher:
ISBN:
Category :
Languages : en
Pages : 46
Get Book Here
Book Description
Author: Hyun Ji Bae
Publisher:
ISBN:
Category :
Languages : en
Pages :
Get Book Here
Book Description
Most turbulent flows cannot be calculated by direct numerical simulation (DNS) of the Navier-Stokes equations because the range of scales of motions is so large that the computational cost becomes prohibitive. In large-eddy simulation (LES), only the large eddies are resolved and the effect of the small scales on the larger ones is modeled through a subgrid-scale (SGS) model. Given that accurate representation and prediction of turbulence is needed in many engineering and scientific applications, development of accurate yet computationally efficient SGS models is an important task. Additionally, wall models are necessary to overcome the prohibitive near-wall resolution requirements for the large scales in high-Reynolds-number turbulent flows. This study investigates a new SGS model, the anisotropic minimum-dissipation (AMD) model, which is constructed to provide the minimum eddy viscosity required to avoid energy pile-up in the smallest resolved scales. The AMD model is successfully applied in simulations of decaying grid turbulence for isotropic grids, and temporal mixing layer and turbulent channel flow for anisotropic grids. This model is more cost-effective than the dynamic Smagorinsky model (DSM) and appropriately switches off in laminar and transitional flows. The formulation of the AMD model is extended to the transport equation for scalar concentration to model the subfilter scalar flux. The performance of the model is tested in the simulation of high-Reynolds-number rough-wall boundary-layer flow with a constant and uniform surface scalar flux. The simulation results obtained from the scalar model show good agreement with well-established empirical correlations and theoretical predictions of the resolved flow statistics. The accuracy of the SGS models is tested by studying the convergence properties in the outer region of a channel flow at moderate to high Reynolds numbers. As LES requires scale separation of the resolved and subgrid scales, the convergence study must be conducted in high-Reynolds-number flows. However, the analysis shows that the errors from the near-wall region are dominant for SGS models in usual LES grid resolutions, where the grid is not refined in the wall-parallel directions. For evaluation of SGS models, in order to overcome the grid requirements imposed by the near-wall turbulent eddies as well as the errors accumulated near the wall, a possible solution is to isolate the outer region of wall-bounded flows. This is made possible by one of two ways: suppressing the near-wall dynamics through a modified wall, or supplying the correct mean stress at the wall with a wall model. Theoretical analysis of the error scaling of SGS models for the mean velocity profile, turbulence intensities, and energy spectra is performed. The numerical convergence studies of the DSM and AMD models show that both models are first-order accurate in terms of the mean velocity profile, which is consistent with the theoretical assessments. Lastly, a new dynamic wall model based on the slip boundary condition is proposed. The use of the slip boundary condition for wall-modeled LES is motivated through theoretical analysis and a priori study of DNS data. The effect of the slip boundary condition on the one-point statistics of the flow is investigated in LES of turbulent channel and flat-plate turbulent boundary layer. The slip boundary condition provides a framework to compensate for the deficit or excess of mean momentum at the wall. The requirements for the slip lengths to be used in conjunction with wall models are discussed, and the equation that connects the slip boundary condition with the stress at the wall is derived. A dynamic procedure based on the invariance of wall stress under test filtering is formulated for the slip condition, providing a dynamic slip wall model free of any a priori specified coefficients. The performance of the proposed dynamic wall model is tested in a series of LES of turbulent channel flow at varying Reynolds numbers, non-equilibrium three-dimensional transient channel flow, and zero-pressure-gradient flat-plate turbulent boundary layer. The results show that the dynamic wall model is able to accurately predict mean and turbulence intensities for various flow configurations, Reynolds numbers, and grid resolutions.
