Large eddy simulation of backward-facing step flow (original) (raw)
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Journal of Applied Mechanical Engineering, 2015
Large-eddy simulation (LES) of turbulent flow over a backward facing step is studied to test the influence of grid resolution, inlet turbulence, wall boundary treatments, and eddy viscosity models. The computational results are validated with experimental data. A grid resolution with 10 cells spanning the step height adequately models the flow, although a doubling of resolution results in the realization of smaller scale kinematic features. The inlet turbulence conditions are determined to be the most significant contributor to downstream flow evolution. Reattachment length is found to be strongly dependent on the magnitude of the inlet root-mean-square velocity. The choice of eddy viscosity model is found to have negligible influence, while the no-slip condition and an LES log-law-based wall function performed similarly for the given flow. Journal of Applied Mechanical Engineering J ou rnal of A p p li e d Me cha n ic a l E n gi neerin g
Journal of Turbulence, 2007
The unsteady backward-facing step flow is investigated using Large Eddy Simulation. Two different inlet conditions are tested to study the sensitivity of the separated flow to a modification of the upstream boundary layer. The first inlet condition consists of a mean turbulent profile perturbed by a white noise. The second relies upon a more realistic condition, in which fully turbulent inflow data are derived from an auxiliary simulation of a quasi-temporal boundary layer. The temporal pressure spectra in different locations downstream of the step exhibit four different characteristic frequencies in both cases. Pressure and velocity statistics, supplemented by visualizations, demonstrate how the flow in the shear layer is strongly influenced by the upstream conditions. The turbulent boundary layer triggers a rapid destabilization of the mixing layer, resulting in a shortening of the mean recirculation length. The temporal spectra reveal that the precursor simulation leads to an increase of the characteristic frequencies associated to the Kelvin-Helmholtz vortices and to the flapping of the shear layer. Streaks and quasi-longitudinal vortices created in the turbulent boundary layer induce high and low longitudinal velocity modulations upstream of the step edge. The spanwise modulation of the velocity seems to be responsible for the wavy destabilization of the Kelvin-Helmholtz vortices. The dependency of the mean flow and of the characteristic frequencies of pressure fluctuations to the incoming boundary layer clearly demonstrates the importance of defining as realistic boundary conditions as possible for the simulation of 3D separated flows.
Comptes Rendus Mécanique, 2006
We use Large Eddy Simulation to investigate the influence of upstream boundary conditions on the development of a backward facing step flow. The first inlet condition consists of a mean turbulent boundary layer velocity profile perturbed by a white noise. The second relies upon a precursor calculation where the development of a quasi-temporal turbulent boundary layer is simulated. In this case, the quasi-longitudinal vortices in the upstream turbulent boundary-layer trigger the destabilization of the shear layer just behind the step, resulting in a shortening of the recirculation length and an increase of the characteristic frequency associated to the Kelvin-Helmholtz vortices. The mean flow and the characteristic frequencies of pressure fluctuations are strongly dependent of the upstream flow. It demonstrates the importance of realistic boundary conditions for the simulation of complex 3D flows or for flow control simulations. To cite this article: J.-L. Aider, A. Danet, C. R. Mecanique 334 (2006).
Direct numerical simulation of turbulent flow over a backward-facing step
Journal of Fluid Mechanics, 1997
Turbulent flow in a channel with a sudden expansion is simulated using the incompressible Navier-Stokes equations. The objective is to provide statistical data on the dynamical properties of flow over a backward-facing step that could be used to improve turbulence modeling. The expansion ratio is E R = 2.0 and the Reynolds number, based on the step height and mean inlet velocity, is Re h = 9000. The discretisation is performed using a spanwise periodic spectral/hp element method. The inlet flow has turbulent velocity and pressure fields that are formed by a regenerating channel segment upstream of the inlet. Time and spanwise averages show secondary and tertiary corner eddies in addition to the primary recirculation bubble, while streamlines show a small eddy forming at the downstream tip of the secondary corner eddy. This eddy has the same circulation direction as the secondary vortex. Analysis of three-dimensional time-averages shows a wavy spanwise structure that leads to spanwise variations of the mean reattachment location. The visualisation of spanwise averaged pressure fluctuations and streamwise velocity shows that the interaction of vortices with the recirculation bubble is responsible for the flapping of the reattachment position, which has a characteristic frequency of St = 0.078.
Parallel Simulation of Turbulent Flow in a Backward-Facing Step
2005
The adiabatic three-dimensional turbulent flow over a backward-facing step has been studied using the Finite Volume Method and parallel processing techniques applied to the incompressible Navier-Stokes equations. The aim of the work is to explore the use of a less expensive solution, such as parallel processing in Beowulf Clusters, to solve weighted CFD cases . The classical flow over backward-facing step is a benchmark for new fluid dynamics codes due to the fact that, despite its simple geometry, it presents a very complex generation of three-dimensional structures, influencing the transition phenomenon and properties such as characteristics frequencies of vortex emission and reattachment length. Based on the step height (h) and the free stream velocity (U0) the flow was simulated at Reynolds 5,100 for an expansion ratio of 1.20. According to the literature, backward-facing step flow having such characteristics presents a critical Reynolds number around 748. It was employed a Larg...
Validation of a novel very large eddy simulation method for simulation of turbulent separated flow
International Journal for Numerical Methods in Fluids, 2013
The paper describes the validation of a newly developed very LES (VLES) method for the simulation of turbulent separated flow. The new VLES method is a unified simulation approach that can change seamlessly from Reynolds-averaged Navier-Stokes to DNS depending on the numerical resolution. Four complex test cases are selected to validate the performance of the new method, that is, the flow past a square cylinder at Re D 3000 confined in a channel (with a blockage ratio of 20%), the turbulent flow over a circular cylinder at Re D 3900 as well as Re D 140, 000, and a turbulent backward-facing step flow with a thick incoming boundary layer at Re D 40, 000. The simulation results are compared with available experimental, LES, and detached eddy simulation-type results. The new VLES model performs well overall, and the predictions are satisfactory compared with previous experimental and numerical results. It is observed that the new VLES method is quite efficient for the turbulent flow simulations; that is, good predictions can be obtained using a quite coarse mesh compared with the previous LES method. Discussions of the implementation of the present VLES modeling are also conducted on the basis of the simulations of turbulent channel flow up to high Reynolds number of Re D 4000. The efficiency of the present VLES modeling is also observed in the channel flow simulation. From a practical point of view, this new method has considerable potential for more complex turbulent flow simulations at relative high Reynolds numbers.
Comparison of Turbulence Models in the Flow over a Backward-Facing Step
—This work presents the numerical simulation and analysis of the turbulent flow over a two-dimensional channel with a backward-facing step. The computational simulation performed in this study is based on the Reynolds equations using a technique denominated Reynolds Average Navier-Stokes (RANS). The main objective of the present work is the comparison of different models of turbulence applied to the turbulent flow over a backward-facing step. The performance of each RANS model used will be discussed and compared with the results obtained through a direct numerical simulation present in the literature. The RANS turbulence models used are k-ω, k-ε, Shear Stress Transport k-ω (SST k-ω) and the second-order closure model called Reynolds Stress Model (RSM). The Reynolds number used in all the numerical simulations constructed in this study is equal to 9000, based on the height of the step h and the inlet velocity U b. The results are the reattachment length, the mean velocity profiles and the turbulence intensities profiles. The k-ε model obtained poor results in most of the analyzed variables in this study. Among the RANS turbulence models, the SST k-ω model presented the best results of reattachment length, mean velocity profile and contour when compared to results obtained in the literature. The RSM model found the best results of turbulence intensity profile, when compared to the models of two partial differential equations that use the Boussines hypothesis.
EPJ Web of Conferences, 2012
The impact of the choice of inlet boundary condition treatment on the fluid flow is studied in this work. The correct representation of the turbulence at the inlet to the domain is essential for the accuracy of Large Eddy Simulation. The inappropriate specification of the inlet velocity has significant effect on the downstream flow pattern. The case of backward-facing step was used as a test case. Three different approaches of the inlet boundary conditions were studied: uniform velocity profile, mean velocity profile of the fully developed channel flow and velocity obtained from mapping velocity from plane positioned behind the inlet back to the inlet. The results of the simulations were compared with experimental results. It has shown, that using uniform velocity profile on inlet and even prescribing turbulent mean velocity profile without proper representation of turbulence fluctuations leads to unrealistic results.
2017
We present the parametric studies of near-wall turbulence modeling for Large Eddy Simulation (LES) of incompressible flows. Flow across a step channel is considered for the numerical studies as the complex wall bounded flows can be highly sensitive to the friction enforced and results are analyzed according to the physics of the flow. We apply the wall shear stress model to model the boundary layers in LES which we refer to as near-wall turbulence modeling. This paper is mainly concerned with the investigation of sensitivity of friction parameter of wall shear stress model and the reliability of near-wall turbulence modeling for LES of incompressible flows. The accuracy and efficiency of LES with near-wall turbulence modeling for complex turbulent flows are investigated by the parametric studies of model parameters. It is observed that wall shear stress model computes the reattachment length for flows in a step channel for both constant and parabolic inflow profile efficiently and w...
Large Eddy Simulation of Flow Over a Backward Facing Step using Fire Dynamics Simulator (FDS)
2013
Flow over a backward-facing step is a widely used benchmark case in the field of Computational Fluid Dynamics (CFD). This paper presents the numerical simulation of backward-facing step using Fire Dynamics Simulator (FDS), open source software CFD developed by National Institute of Standards and Technology (NIST), US. Large Eddy Simulation (LES) is the default mode of its operation. In this paper, the latest version, FDS 6, is used for the numerical simulation of turbulent flow over a backward-facing step. This recent version of FDS incorporates four different eddy viscosity models namely a Constant Coefficient Smagorinsky model, a Dynamic Smagorinsky model, Deardroff’s and Vreman’s Models. The principal objective of this paper is to compare these turbulence models with a proposed benchmark case. Moreover, these simulated results are compared with standard experimental results of Jovic and Driver to assess the accuracies of the various models.