Bulletin of the American Physical Society
70th Annual Meeting of the APS Division of Fluid Dynamics
Volume 62, Number 14
Sunday–Tuesday, November 19–21, 2017; Denver, Colorado
Session F31: Computational Fluid Dynamics: WallBounded, WallModeled and Implicit LESBoundary Layers CFD

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Chair: Catherine Gorle, Stanford University Room: 108 
Monday, November 20, 2017 8:00AM  8:13AM 
F31.00001: Turbulent intensities in largeeddy simulation of wallbounded flows Hyunji Jane Bae, Adrian LozanoDuran, Sanjeeb Bose, Parviz Moin A persistent problem in wall bounded largeeddy simulations (LES) with Dirichlet noslip boundary conditions is that the nearwall streamwise velocity fluctuations are overpredicted, while those in the wallnormal and spanwise directions are underpredicted. The problem is particularly pronounced when the nearwall region is underresolved. The prediction of the fluctuations is known to improve for wallmodeled LES, where the noslip boundary condition at the wall is typically replaced by Neumann and notranspiration conditions for the wallparallel and wallnormal velocities, respectively. However, the turbulent intensity peaks are sensitive to the grid resolution and the prediction may degrade when the grid is refined. In the present study, a physical explanation of this phenomena is offered in terms of the behavior of the nearwall streaks. We also show that further improvements are achieved by introducing a slip boundary condition with transpiration. By using a slip condition, the inner energy production peak is damped, and the blocking effect of the wall is relaxed such that the splatting of eddies at the wall is reduced. As a consequence, the slip condition provides an accurate and consistent prediction of the turbulent intensities regardless of the nearwall resolution. [Preview Abstract] 
Monday, November 20, 2017 8:13AM  8:26AM 
F31.00002: Optimizing inflow boundary conditions for LES of wind loading. Giacomo Lamberti, Catherine Gorle Largeeddy simulations (LES) have promising capabilities to complement wind tunnel testing for assessing wind hazards, but modeling the mean flow and turbulent statistics representative of a specific atmospheric boundary layer (ABL) is not straightforward. In the present work, we focus on applying a divergencefree digital filter method to generate an ABL inflow condition. The method requires the specification of the mean velocity profile, the turbulence length scales, and the Reynolds stresses at the inlet, to produce a turbulent inflow condition with exponential correlation functions in space and time. However, when imposing the profiles for the target ABL at the inlet, they develop downstream towards an equilibrium solution that depends on the numerics, the subgrid model, and the wall functions used. The resulting ABL statistics at the location of interest in the computational domain can vary considerably from those of the target ABL. To overcome this problem, we propose adjusting the inflow parameters using a gradientbased optimization, such that the required statistics are obtained at the building location. The implementation of the method is tested for an ABL simulated in an empty wind tunnel, and the results are compared to the experimental data. The expected monotonic behavior is found, where higher Reynolds stresses at the inflow result in higher Reynolds stresses at the location of interest. Hence, this approach is a promising tool for defining inflow boundary conditions for LES of wind loading [Preview Abstract] 
Monday, November 20, 2017 8:26AM  8:39AM 
F31.00003: ReInnovating Recycling for Turbulent Boundary Layer Simulations Joseph Ruan, Guillaume Blanquart Historically, turbulent boundary layers along a flat plate have been expensive to simulate numerically, in part due to the difficulty of initializing the inflow with ``realistic'' turbulence, but also due to boundary layer growth. The former has been resolved in several ways, primarily dedicating a region of at least 10 boundary layer thicknesses in width to rescale and recycle flow or by extending the region far enough downstream to allow a laminar flow to develop into turbulence. Both of these methods are relatively costly. We propose a new method to remove the need for an inflow region, thus reducing computational costs significantly. Leveraging the scale similarity of the mean flow profiles, we introduce a coordinate transformation so that the boundary layer problem can be solved as a parallel flow problem with additional source terms. The solutions in the new coordinate system are statistically homogeneous in the downstream direction and so the problem can be solved with periodic boundary conditions. The present study shows the stability of this method, its implementation and its validation for a few laminar and turbulent boundary layer cases. [Preview Abstract] 
Monday, November 20, 2017 8:39AM  8:52AM 
F31.00004: How do rigidlid assumption affect LES simulation results at high Reynolds flows? Ali Khosronejad, Ali Farhadzadeh This research is motivated by the work of Kara et al., \textit{JHE}, 2015. They employed LES to model flow around a model of abutment at a \textit{Re} number of 27,000. They showed that firstorder turbulence characteristics obtained by rigidlid (RL) assumption compares fairly well with those of levelset (LS) method. Concerning the secondorder statistics, however, their simulation results showed a significant dependence on the method used to describe the free surface. This finding can have important implications for open channel flow modeling. The Reynolds number for typical open channel flows, however, could be much larger than that of Kara et al.'s test case. Herein, we replicate the reported study by augmenting the geometric and hydraulic scales to reach a \textit{Re} number of one order of magnitude larger (\textasciitilde 200,000). The Virtual Flow Simulator (VFSGeophysics) model in its LES mode is used to simulate the test case using both RL and LS methods. The computational results are validated using measured flow and freesurface data from our laboratory experiments. Our goal is to investigate the effects of RL assumption on both firstorder and second order statistics at high Reynolds numbers that occur in natural waterways. \textbf{Acknowledgment:} Computational resources are provided by the Center of Excellence in Wireless {\&} Information Technology (CEWIT) of Stony Brook University. [Preview Abstract] 
Monday, November 20, 2017 8:52AM  9:05AM 
F31.00005: Towards WallModeled LES using HighOrder Discontinuous Galerkin Methods Yu Lv, Xiang Yang, George Park, Matthias Ihme This talk presents a DGbased wall model technique for largeeddy simulations.~Wall modeling capability plays a key role for enabling highfidelity simulations of external aerodynamics and wallbounded flows~at~very high Reynoldsnumbers. Previous development of wall models was primarily based on finitedifference or finitevolume schemes. The performance of wall models in the DG framework has~not yet been well understood.~~In this study, we investigate the equilibrium wall model in a configuration of turbulent channel flow. The detailed implementation for integrating the wall model into a DG solver will be presented. The accuracy of the DGbased WMLES technique will be compared against the conventional WMLES that were developed based~on~finitevolume methods. The importance~of utilizing subgridscale models~will be highlighted. Practical implication~of this new WMLES technique onto more complex flow configurations will be discussed. [Preview Abstract] 
Monday, November 20, 2017 9:05AM  9:18AM 
F31.00006: Implicit and explicit subgridscale modeling in discontinuous Galerkin methods for largeeddy simulation Pablo Fernandez, NgocCuong Nguyen, Jaime Peraire Over the past few years, highorder discontinuous Galerkin (DG) methods for LargeEddy Simulation (LES) have emerged as a promising approach to solve complex turbulent flows. Despite the significant research investment, the relation between the discretization scheme, the Riemann flux, the subgridscale (SGS) model and the accuracy of the resulting LES solver remains unclear. In this talk, we investigate the role of the Riemann solver and the SGS model in the ability to predict a variety of flow regimes, including transition to turbulence, wallfree turbulence, wallbounded turbulence, and turbulence decay. The TaylorGreen vortex problem and the turbulent channel flow at various Reynolds numbers are considered. Numerical results show that DG methods implicitly introduce numerical dissipation in underresolved turbulence simulations and, even in the high Reynolds number limit, this implicit dissipation provides a more accurate representation of the actual subgridscale dissipation than that by explicit models. [Preview Abstract] 
Monday, November 20, 2017 9:18AM  9:31AM 
F31.00007: Diffusion and dispersion characteristics of hybridized discontinuous Galerkin methods for underresolved turbulence simulations Rodrigo Moura, Pablo Fernandez, Gianmarco Mengaldo We investigate the dispersion and diffusion characteristics of hybridized discontinuous Galerkin (DG) methods. This provides us with insights to develop robust and accurate highorder DG discretizations for underresolved flow simulations. Using the eigenanalysis technique introduced in (Moura et al., JCP, 2015 and Mengaldo et al., Computers \& Fluids, 2017), we present a dispersiondiffusion analysis for the linear advectiondiffusion equation. The effect of the accuracy order, the Riemann flux and the viscous stabilization are investigated. Next, we examine the diffusion characteristics of hybridized DG methods for underresolved turbulent flows. The implicit largeeddy simulation (iLES) of the inviscid and viscous TaylorGreen vortex (TGV) problems are considered to this end. The inviscid case is relevant in the limit of high Reynolds numbers $Re$, i.e. negligible molecular viscosity, while the viscous case explores the effect of $Re$ on the accuracy and robustness of the simulations. The TGV cases considered here are particularly crucial to underresolved turbulent free flows away from walls. We conclude the talk with a discussion on the connections between hybridized and standard DG methods for underresolved flow simulations. [Preview Abstract] 
Monday, November 20, 2017 9:31AM  9:44AM 
F31.00008: Towards gridconverged wallmodeled LES of atmospheric boundary layer flows Shashank Yellapantula, Ganesh Vijayakumar, Marc Henry de Frahan, Matthew Churchfield, Michael Sprague Accurate characterization of incoming atmospheric boundary layer (ABL) turbulence is a critical factor in improving accuracy and predictive nature of simulation of wind farm flows. Modern commercial wind turbines operate in the log layer of the ABL that are typically simulated using wallmodeled largeeddy simulation (WMLES). One of the longstanding issues associated with wall modeling for LES and hybrid RANSLES for atmospheric boundary layers is the overprediction of the meanvelocity gradient, commonly referred to as loglayer mismatch. Kawai and Larsson in 2012, identified underresolution of the nearwall region and the incorrect information received by the wall model as potential causes for the loglayer mismatch in WMLES of smoothwall boundarylayer flows. To solve the log layer mismatch issue, they proposed linking the wall model to the LES solution at a physical of height of $y_m$, instead of the first grid point. In this study, we extend their wall modeling approach to LES of the roughwall ABL to investigate issues of loglayer mismatch and grid convergence. [Preview Abstract] 
Monday, November 20, 2017 9:44AM  9:57AM 
F31.00009: Multidimensional upwindingbased implicit LES for the vorticity transport equations Daniel Foti, Karthik Duraisamy Complex turbulent flows such as rotorcraft and wind turbine wakes are characterized by the presence of strong coherent structures that can be compactly described by vorticity variables. The vorticityvelocity formulation of the incompressible NavierStokes equations is employed to increase numerical efficiency. Compared to the traditional velocitypressure formulation, high order numerical methods and subgrid scale models for the vorticity transport equation (VTE) have not been fully investigated. Consistent treatment of the convection and stretching terms also needs to be addressed. Our belief is that, by carefully designing sharp gradientcapturing numerical schemes, coherent structures can be more efficiently captured using the vorticityvelocity formulation. In this work, a multidimensional upwind approach for the VTE is developed using the generalized Riemann problembased scheme devised by Parish et al. (Computers \& Fluids, 2016). The algorithm obtains high resolution by augmenting the upwind fluxes with transverse and normal direction corrections. The approach is investigated with several canonical vortexdominated flows including isolated and interacting vortices and turbulent flows. The capability of the technique to represent subgrid scale effects is also assessed. [Preview Abstract] 
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