Bulletin of the American Physical Society
61st Annual Meeting of the APS Division of Fluid Dynamics
Volume 53, Number 15
Sunday–Tuesday, November 23–25, 2008; San Antonio, Texas
Session EB: Turbulence Simulations III |
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Chair: Andrew Duggleby, Texas A&M University Room: 001B |
Sunday, November 23, 2008 4:10PM - 4:23PM |
EB.00001: A narrow-stencil formulation of subgrid-scale models in large-eddy simulation: application to the stretched vortex model for compressible flows Ramji Kamakoti, Carlos Pantano Finite-difference approximations of second-order derivatives involving variable coefficients, as those discretizing subgrid-scale models of large-eddy simulation, are investigated. It is observed that use of the first-order derivative operators successively result in poor resolution and negligible contribution of the subgrid-scale models at the highest wavenumber supported by the mesh. This affects the stability of these simulations negatively. Low and high-order, narrow stencil, numerical discretizations are developed and applied to large-eddy simulation of compressible turbulent flows using the stretched-vortex subgrid scale model. These discretizations are based on a narrow-stencil formulation, which is constructed by requiring that the ellipticity property of the operators, when the variable coefficients are positive, is preserved. Such an approach is found to be discretely conservative, stable, resolves the subgrid model contribution at the smallest wavenumber supported by the mesh better and enables energy transfer from the resolved to the subgrid scales at all discrete modes of the mesh. We investigate the new discretizations in homogeneous decaying turbulence and temporally evolving shear layers. [Preview Abstract] |
Sunday, November 23, 2008 4:23PM - 4:36PM |
EB.00002: Hybrid RANS/LES simulation of a flow with mild separation Ugo Piomelli, Senthil Radhakrishnan The flow past a ramp with mild separation zone has been computed using LES and hybrid approaches. In the hybrid calculations, the RANS equations are solved throughout the boundary layer in the equilibrium region of the flow, while a wall-resolved LES is used to compute the separation and recovery regions. The LES results are used to assess the accuracy of hybrid RANS/LES approach. The accuracy of the hybrid approach depends on the generation of resolved fluctuations at the RANS/LES interface and often needs a long transition region before realistic turbulent fluctuations are generated. When the RANS/LES interface is near the separation point, forcing provided by the addition of synthetic turbulence at the interface results in faster generation of turbulent fluctuations, which is aided by the inflectional instability of the mean velocity profile. When the RANS/LES interface is in the equilibrium boundary layer region, on the other hand, even the addition of synthetic turbulence does not cause fast generation of turbulence fluctuations, resulting in reduction of skin-friction and early separation of the flow. Strategies that promote the fast generation of realistic eddies in the equilibrium region, resulting in a short transition zone, will be discussed. [Preview Abstract] |
Sunday, November 23, 2008 4:36PM - 4:49PM |
EB.00003: Large Eddy Simulation of Ducted Propulsors in Crashbac Hyunchul Jang, Krishnan Mahesh Flow around a ducted marine propulsor is computed using the large eddy simulation methodology under crashback conditions. Crashback is an operating condition where a propulsor rotates in the reverse direction while the vessel moves in the forward direction. It is characterized by massive flow separation and highly unsteady propeller loads, which affect both blade life and maneuverability. The simulations are performed on unstructured grids using the algorithm developed by Mahesh at al. (2004, J. Comput. Phys 197). The flow is computed at the advance ratio J=-0.7 and Reynolds number Re=480,000 based on the propeller diameter. Average and RMS values of the unsteady loads such as thrust, torque, and side force on the blades and duct are compared to experiment. It is seen that even though effects of the duct on thrust and torque are not large enough, those on the side force are significant. The rms of side forces is much higher in the presence of the duct. Pressure distributions on blade surfaces and duct surface are examined and used to explain this effect. This work was supported by the United States Office of Naval Research under ONR Grant N00014-05-1-0003. [Preview Abstract] |
Sunday, November 23, 2008 4:49PM - 5:02PM |
EB.00004: Visualization of Vortex Shedding in the Turbulent Flow Over A Surface Mounted Obstacle Nikolaos Malamataris The three dimensional turbulent flow over a surface mounted obstacle is studied as a numerical experiment that takes place in a wind tunnel. The transient Navier Stokes equations are solved directly with Galerkin finite elements. The Reynolds number defined with respect to the height of the wind tunnel is 12518. Instantaneous streamline patterns are shown, that give a complete picture of the flow phenomena which include the vortex shedding phenomenon and the flapping of the recirculation bubble downstream the obstacle. Both phenomena are considered as inherent unsteady features of separated flows and have not been visualized before apart from one attempt in a two-dimensional simulation of the same flow by the same author. A movie is going to be shown where the motion of the vortical structures is demonstrated. The energy spectrum yields the -5/3 law dependence with respect to the frequency. Mean values of velocities and root mean square fluctuations are compared with available experimental results. Other statistical characteristics of turbulence such as Eulerian autocorrelation coefficients, longitudinal and lateral coefficients are also computed. Finally, oscillation diagrams of computed velocity fluctuations yield the chaotic behavior of turbulence. The computer code developed for this work is a parallel program written in Fortran 90 that uses the MPI-paradigm and runs in distributed memory systems. [Preview Abstract] |
Sunday, November 23, 2008 5:02PM - 5:15PM |
EB.00005: A dynamic multi-scale approach for turbulent inflow generation in spatially-developing boundary layers with streamwise pressure gradients Guillermo Araya, Luciano Castillo, Kenneth Jansen, Charles Meneveau A novel method for generating realistic turbulent velocity and thermal inlet boundary conditions is presented for simulations of evolving turbulent boundary layers. The approach is based on the rescaling-recycling method proposed by Lund et al. (1998). The standard rescaling process requires prior knowledge about how the appropriate velocity and length scales are related between the inlet and recycle stations (e.g. classic scaling laws). Here a dynamic approach is proposed in which such information is deduced dynamically by involving an additional plane, the ``test plane'', which is located between the inlet and recycle stations. This improvement, as well as the use of multiple velocity scales, permits the simulations of turbulent boundary layers subjected to arbitrary pressure gradients. DNS for zero (ZPG), adverse (APG) and favorable (FPG) pressure gradient flows are discussed. The agreement obtained by comparing present results with experimental and numerical data demonstrates the suitability of the present method as a turbulent inflow generator. [Preview Abstract] |
Sunday, November 23, 2008 5:15PM - 5:28PM |
EB.00006: Turbulence production and shear stress partitioning in rough-walled channel flow DNS Richard Leighton The effects of roughness in an incompressible turbulent boundary layer include the increased production of turbulence kinetic energy (TKE) and altered the nature of the skin drag. By formulating the exact Reynolds-averaged Navier-Stokes turbulence kinetic energy equations in a manner that includes an arbitrary roughness, the averaged terms representing the roughness production of TKE and the roughness drag can be written explicitly. Similar transport equations for TKE can be formulated wherein the roughness geometry is represented using the immersed boundary methodology. These terms are calculated from a collection of direct numerical simulations (DNS). The roughness geometry is limited to a simple array of vertically oriented cylinders with roughness heights range from $2 l^+$ to $50 l^+$. The primary results include an examination of the partitioning of the production of TKE into canonical shear production and into production by roughness, and the partitioning of drag into form drag and viscous shear drag. The relevance of the partitioning will be discussed in the context of turbulence modeling. [Preview Abstract] |
Sunday, November 23, 2008 5:28PM - 5:41PM |
EB.00007: A dynamic surface rougness model for large-eddy simulation of atmospheric boundary layer flow over fractal-like evolved fluvial landscapes William Anderson, Paola Passalacqua, Fernando Port\'e-Agel, Charles Meneveau Evolved fluvial landscapes are characterized by a multi-scale composition of channels which display scale-invariance properties. A high-resolution evolved landscape is obtained through solution of a modified version of the Kardar-Parisi-Zhang equation. This landscape is spatially filtered to various resolutions for large-eddy simulations (LES) of the atmospheric boundary layer flowing over such surfaces. In the LES the subgrid-scale motions are parameterized using the Lagrangian scale-dependent dynamic model (Bou-Zeid et al. 2005). The boundary condition at the lower boundary is prescribed using a roughness length that is modeled as the product of local standard deviation of the unresolved height field and an unknown dimensionless coefficient. This coefficient is evaluated dynamically by comparing the plane-average force due to wall-stress at two resolutions. The results illustrate that the challenges posed by the multi-scale interactions of fluvial fractal-like lower surface boundary condition with the atmospheric turbulence may be addressed using a dynamic model for unresolved surface roughness. [Preview Abstract] |
Sunday, November 23, 2008 5:41PM - 5:54PM |
EB.00008: On the modification of particle dispersion in isotropic turbulence by free rotation of particle Yongnam Park, Changhoon Lee Effect of a particle's spin is investigated numerically by considering the effect of lift occurring due to difference of rotations of a particle and of fluid such as the Saffman lift and Magnus force. These lift forces have been neglected in many previous works on particle-laden turbulence. The trajectory of particles can be changed by the lift forces, resulting in significant modification of the stochastic characteristics of heavy particles. Probability density functions and autocorrelations are examined of velocity, acceleration of solid particle and acceleration of fluid at the position of solid particle. Changes in velocity statistics are negligible but statistics related with acceleration are a little bit changed by particle's rotation. When a laden particle encounters with coherent structures during the motion, the particle's rotation might significantly affects the motion due to intermittently large fluid acceleration near coherent structures. The result can be used for development of stochastic model for particle dispersion. Detailed physical interpretation will be presented in the meeting. [Preview Abstract] |
Sunday, November 23, 2008 5:54PM - 6:07PM |
EB.00009: Behavior of small particles in isotropic turbulence in the presence of gravity Seonggee Cho, Changhoon Lee The motion of small heavy particles in homogeneous isotropic turbulence in the presence of gravity is investigated using Direct Numerical Simulations (DNS) at moderate Reynolds number. The Lagrangian velocity and acceleration statistics of particles and of flow for a wide range of Stokes number, defined as the ratio of the particle response time to the Kolmogorov time scale of turbulence, were obtained for the direction of the gravity and normal direction, respectively. It is found that particles lose their correlation faster than the case without gravity. Then, a significant increase in the average settling velocity was observed for a certain range of Stokes number. Our focus is placed on gravitational effect on very small particles. Our simulations show that as the Stokes number reduces to zero, their mean settling velocity approaches the terminal velocity in still fluid, which is rather odd considering that the trajectory of a small particle approaches the trajectory of a fluid particle which does not settle. Detailed physical mechanism will be presented in the meeting. [Preview Abstract] |
Sunday, November 23, 2008 6:07PM - 6:20PM |
EB.00010: Comparing experimental and Direct Numerical Simulation results from a turbulent channel flow Jason Monty, Min Chong Remarkable progress has been made in the direct numerical simulation (DNS) of wall-bounded turbulence; particularly of turbulent channel flow, with numerical data now available above $Re_{\tau} \approx 2000$. Yet there are only very limited comparisons with experimental data in the literature. As such, this investigation compares a well-documented, high Reynolds number ($Re_{\tau} = 934$), large box size DNS from Del Alamo, Jimenez, Zandonade \& Moser (JFM, 2004) and laboratory channel flow data measured by the authors. Results show excellent agreement of streamwise velocity statistics. The spectra are also very similar, however, throughout the logarithmic region the secondary peak in energy is significantly reduced in the DNS results. Since the wavelengths associated with the energy difference are close to the DNS box length, it is recommended that longer box lengths should be investigated. Another spectral discrepancy results from an incorrect convection velocity when using Taylor's hypothesis for the temporal laboratory data. A convection velocity modification function is tentatively proposed giving good agreement between the data sets. [Preview Abstract] |
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