Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session L34: CFD: Turbulence Modeling I |
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Chair: Dale Pullin, California Institute of Technology Room: 2024 |
Monday, November 24, 2014 3:35PM - 3:48PM |
L34.00001: RANS study of flow Characteristics Over flight deck of Simplified frigate Ship Shrish Shukla, Sidh Nath Singh, Balaji Srinivasan The combined operation of a ship and helicopter is ubiquitous in every naval organization. The operation of ship with the landing and takeoff of a helicopter over sea results in very complex flow phenomena due to presence of ship air wakes, strong velocity gradients and widely varying turbulence length scales. This complexity of flow is increased with the addition of helicopter downwash during landing and takeoff. The resultant flow is therefore very complicated and accurate prediction represents a computational challenge. We present Reynolds-averaged-Navier-Stokes (RANS) of turbulent flow over a simple frigate ship to gain insight into the flow phenomena over a flight deck. Flow conditions analysis is carried out numerically over the generic simplified frigate ship. Profiles of mean velocity across longitudinal and transverse plane have been analyzed along the ship. Further, we propose some design modifications in order to reduce pilot load and increase the ship helicopter operation limit (SHOL). Computational results for these modified designs are also presented and their efficacy in reducing the turbulence levels and recirculation zone in the ship air wakes is discussed. [Preview Abstract] |
Monday, November 24, 2014 3:48PM - 4:01PM |
L34.00002: DNS of Supersonic Turbulent Flows in a DLR Scramjet Intake Xinliang Li, Changping Yu Direct numerical simulation (DNS) of supersonic/hypersonic flow through a DLR scramjet intake GK01 is performed. The free stream Mach numbers are 3, 5 and 7, and the angle of attack is zero degree. The DNS cases are performed by using an optimized MP scheme with adaptive dissipation (OMP-AD) developed by the authors, and the blow-and-suction perturbations near the leading edge are used to trigger the transition. To stabilize the simulation, locally non-linear flittering is used in high-Mach-number case. The transition, separation, and shock-turbulent boundary layer interaction are studied by using both flow visualization and statistical analysis. A new method, OMP-AD, is also addressed in this paper. The OMP-AD scheme is developed by using joint MP method and optimized technique, and the coefficients in the scheme are flexible to show low dissipation in the smoothing region, and to show high robust (but high dissipation) in the large gradient region. Numerical tests show that the OMP-AD is more robust than the original MP schemes, and the numerical dissipation of OMP-AD is very low. [Preview Abstract] |
Monday, November 24, 2014 4:01PM - 4:14PM |
L34.00003: Assessing the numerical dissipation rate and viscosity in CFD simulations of fluid flows F.S. Schranner, J.A. Domaradzki, S. Hickel, N.A. Adams We describe a method for quantifying the effective numerical dissipation rate and the effective numerical viscosity in Computational Fluid Dynamics simulations. Differently from the previous approach that was formulated in spectral space, the proposed method is developed in a physical-space representation and allows for determining numerical dissipation rates and viscosities locally, i.e., at the individual cell level or for arbitrary subdomains of the computational domain. The method is self-contained using only results produced by the Navier-Stokes solver being investigated. Since no extraneous information is required, the method is suitable for a straightforward quantification of the numerical dissipation as a post-processing step. We demonstrate the method's capabilities on the example of implicit large-eddy simulations of three-dimensional Taylor-Green vortex flows that exhibit laminar, transitional, and turbulent flow behavior at different stages of time evolution. For validation, we compare the numerical dissipation rate obtained using this method with exact reference data obtained with an accurate, spectral-space approach. [Preview Abstract] |
Monday, November 24, 2014 4:14PM - 4:27PM |
L34.00004: Wall boundary condition for LES using particle method Akihiko Nakayama, Nobuyuki Hisasue In large eddy simulation that does not resolve the near-wall viscous sub-layer, the no-slip boundary condition cannot be applied and a wall model is needed. A method is proposed in which a special wall model is applied to the motion of particles that lie within a thin layer next to solid wall to accomplish the wall modeling in a SPH formulation. It is verified in fully developed open channel flows and is applied to dam break flow with reasonable results. [Preview Abstract] |
Monday, November 24, 2014 4:27PM - 4:40PM |
L34.00005: Quantifying numerical dissipation rate in a commercial CFD code Giacomo Castiglioni, J. Andrzej Domaradzki Recently it has become increasingly clear that the role of a numerical dissipation, originating from the discretization of Navier-Stokes equations, rarely can be ignored regardless of the formal order of accuracy of a numerical scheme used in explicit or implicit Large Eddy Simulations (LES). The numerical dissipation inhibits the predictive capabilities of LES whenever it is of the same order of magnitude or larger than the subgrid-scale dissipation. The need to estimate the numerical dissipation is most pressing for lower order methods employed by commercial CFD codes. Following the recent work of Schranner et al. the equations and procedure for estimating the numerical dissipation rate and the numerical viscosity in a commercial code will be presented. The method allows to compute the numerical dissipation rate and numerical viscosity in the physical space for arbitrary sub-domains in a self-consistent way, using only information provided the code in question. It is the first time this analysis has been applied to low order solvers. Two equivalent ways to compute the numerical dissipation rate are described and compared. The procedure is tested for a 3D Taylor-Green vortex flow and compared with benchmark results obtained using an accurate, incompressible spectral solver. [Preview Abstract] |
Monday, November 24, 2014 4:40PM - 4:53PM |
L34.00006: Large-eddy simulations of impinging jet with embedded vortices on rough surface Wen Wu, Rayhaneh Banyassady, Ugo Piomelli Large-eddy simulations (LES) are used to study round jets impinging on rough surface at nozzle-to-plate distance $H/D=1$ ($D$ is the nozzle exit diameter) and Reynolds numbers $Re=U_oD/\nu= 6.6\times10^4$ ($U_o$ is the mean jet velocity). Our aim is to explore the effect of roughness on the evolution of vortices in the analysis of impinging jet. Two cases, one with turbulent, the other with laminar inflow, are performed. Roughness is represented by uniformly distributed but randomly oriented ellipsoids of equivalent sand-grain height $k_s/D = 0.02$, modelled by immersed boundary method. Results are compared to our previous LES simulations of jets impinging on a smooth surface. A wider and weaker wall jet is observed in the rough surface turbulent jet, compared to the reference turbulent one with smooth surface. The vortices and the peak of wall jet velocity shift away from the surface. Secondary vorticity is formed and lifted up, as in the smooth-surface case.The wall shear stress increases significantly; the separated vorticity, however, has the same strength as the one in the smooth case. The roughness causes higher turbulent fluctuations, and leads to the transition to turbulent wall jet even when the inflow is laminar, changing the vortex dynamics during vortex interaction. [Preview Abstract] |
Monday, November 24, 2014 4:53PM - 5:06PM |
L34.00007: High-Order Velocity and Pressure Statistics from Direct Numerical Simulations of a Zero-Pressure-Gradient Turbulent Boundary Layer Bryan Kaiser, Svetlana Poroseva High-order turbulence statistics in a zero-pressure-gradient turbulent boundary layer are important for developing turbulence models. They also provide an insight into the physics of turbulent flows. A complete database of one-point statistics extracted from the dataset of direct numerical simulations (DNS) of a zero-pressure-gradient turbulent boundary layer by the Universidad Politecnica de Madrid Fluid Dynamics Group was collected. Third-, fourth-, and fifth-order velocity central moments and second-order pressure-velocity correlations at two Reynolds numbers of 4101 and 5200 based on the momentum thickness will be presented. DNS data are in a good agreement with experimental data. Results of the validation of Millionshtchikov's hypothesis of quasi-normality and the Gram-Charlier series expansions for representing higher-order velocity moments in terms of lower-order moments will be reported. The two approaches are used as closing procedures in third- and fourth-order statistical closures, respectively. [Preview Abstract] |
Monday, November 24, 2014 5:06PM - 5:19PM |
L34.00008: Compressible DNS study of separation bubbles for flow past a low pressure turbine blade Rajesh Ranjan, Suresh Deshpande, Roddam Narasimha A representative low pressure turbine blade T106A is subjected to a direct numerical simulation (DNS) study for low Reynolds Number (Re $=$ 51831 based on inflow velocity and axial chord) and angle of incidence (45.5 deg from the axial chord). The DNS code used here solves the compressible Navier-Stokes equations and uses a semi-kinetic energy preserving scheme. A hybrid grid is used for the computational domain, with a very fine wall-bounded boundary layer grid near the surface of the blade and an unstructured grid for rest of the domain. Total grid size for the current simulation is around 160 million. In the mean flow, a long but shallow separation bubble is found near the trailing edge. However, the instantaneous flow reveals a train of bubbles at this location. These instantaneous bubbles continually break and merge in time. The presence of these separation bubbles make the flow very complicated, as the bubbles are responsible for tripping the otherwise laminar flow to a transitional state. Skin friction and heat transfer co-efficient are also computed over the blade to understand the effect of these bubbles on parameters of engineering importance. [Preview Abstract] |
Monday, November 24, 2014 5:19PM - 5:32PM |
L34.00009: Improving large-eddy simulation on adaptive mesh refinement grids using the turbulence closure Lauren Goodfriend, Fotini Chow, Marcos Vanella, Elias Balaras Large-eddy simulation (LES) and adaptive mesh refinement (AMR) reduce the computational cost of turbulence modeling by restricting resolved length scales, but combining these techniques generates additional errors. The grid refinement interfaces in AMR grids can create interpolation errors and reflect resolved energy. This talk will explore using the turbulence closure to mitigate grid interface errors in LES. Specifically, explicit filtering of the advection term and the mixed model are compared to implicit filtering and the eddy viscosity model. I will present a half-channel case study in which the domain is split into two structured static grids, one fine and one coarse. This simple test case allows observation of the effects of the grid interfaces. It is found that explicitly filtering the advection term allows both mass and momentum to be conserved across grid refinement interfaces by reducing interpolation errors. The mixed model decreases unphysical energy accumulation generated by wave reflection. These results inform the use of LES on block-structured non-uniform grids, including dynamic AMR grids. [Preview Abstract] |
Monday, November 24, 2014 5:32PM - 5:45PM |
L34.00010: Mean wall shear stress boundary condition for large eddy simulation using near-wall streamwise momentum equation Minjeong Cho, Haecheon Choi, Jungil Lee The mean wall shear stress boundary condition based on the log law has been proven as an appropriate boundary condition for large eddy simulations (LES) of turbulent channel and boundary layer flows without resolving near-wall region (Lee, Cho \& Choi, PoF, 2013). In the present study, we use near-wall streamwise momentum equation following Chung \& Pullin (JFM, 2009), to determine the mean shear stress at the wall. In this procedure, the near-wall streamwise momentum equation is averaged over a few off-wall grid points, in which the velocity at the first grid point is approximated with the Taylor series expansion. We test this wall boundary condition for turbulent channel and boundary layer flows, showing good prediction capability at high Reynolds numbers. The result of applying this boundary condition to a separating flow will be also shown at the presentation. [Preview Abstract] |
Monday, November 24, 2014 5:45PM - 5:58PM |
L34.00011: Tensor Diffusivity and Lagrangian Particle Methods A. Leonard We consider the tensor diffusivity model for large-eddy simulation and its connection with lagrangian particle methods for the vorticity transport equation and for the scalar advection equation. If $\phi$ is a scalar field being advected by an incompressible velocity field $\mathbf{u}$ then application of the tensor diffusivity model results in the term $ - \frac{\sigma^2}{2} \bar{S_{ij}} \partial^2 \bar{\phi}/\partial x_i \partial x_j$ on the RHS of the filtered advection equation, where $\bar{S_{ij}}$ is the filtered strain rate tensor and $\sigma$ is the width of the gaussian filter. Previous investigators have shown, in a priori tests, that this model yields results that are relatively well correlated with the actual subgrid scale terms. Also previously noted is the fact that one has negative diffusion along the the principal axes of $\bar{S_{ij}}$ that correspond to positive eigenvalues. Examples are given where the negative diffusivity produces meaningful backscatter of energy into the resolved scales. Also shown is that lagrangian particle methods have a truncation error that produces exactly the tensor diffusivity model. For 3D vorticity transport and scalar advection, the particle velocity must be slightly modified from the local velocity. [Preview Abstract] |
Monday, November 24, 2014 5:58PM - 6:11PM |
L34.00012: A 3D Computational fluid dynamics model validation for candidate molybdenum-99 target geometry Lin Zheng, Greg Dale, Peter Vorobieff Molybdenum-99 ($^{99}$Mo) is the parent product of technetium-99m ($^{99m}$Tc), a radioisotope used in approximately 50,000 medical diagnostic tests per day in the U.S. The primary uses of this product include detection of heart disease, cancer, study of organ structure and function, and other applications. The US Department of Energy seeks new methods for generating $^{99}$Mo without the use of highly enriched uranium, to eliminate proliferation issues and provide a domestic supply of $^{99}$mTc for medical imaging. For this project, electron accelerating technology is used by sending an electron beam through a series of $^{100}$Mo targets. During this process a large amount of heat is created, which directly affects the operating temperature dictated by the tensile stress limit of the wall material. To maintain the required temperature range, helium gas is used as a cooling agent that flows through narrow channels between the target disks. In our numerical study, we investigate the cooling performance on a series of new geometry designs of the cooling channel. [Preview Abstract] |
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