### Session OM: Turbulence Simulations V

Chair: Bamin Khomami, Washington University at St. Louis
Room: Tampa Marriott Waterside Hotel and Marina Meeting Room 10

 Tuesday, November 21, 2006 12:15PM - 12:28PM OM.00001: ABSTRACT WITHDRAWN Tuesday, November 21, 2006 12:28PM - 12:41PM OM.00002: Simulation of the Rayleigh-Taylor instability using atomistic methods John Barber , Kai Kadau , Tim Germann , Peter Lomdahl , Brad Holian , Berni Alder Recent increases in computational capacity and speed have allowed the application of particle-based algorithms, such as molecular dynamics (MD) and direct simulation Monte Carlo (DSMC), to the simulation of turbulent fluid behavior. The use of such techniques has a number of advantages. In particular, they capture several physical effects not resolved by more traditional continuum methods, for example the discontinuous breakup of flow features and the influence of micro-scale fluctuations. In addition, they can be used for length and time scales at which the continuum approximation does not hold. In this work, we present the results of various MD and DSMC simulations of the Rayleigh-Taylor instability, in which a heavy fluid lies on top of a light fluid in the presence of gravity. Our results for the initial growth spectrum of the interface and the development in time of the mixing zone width are compared with the analogous results from both continuum simulations and experiment. Tuesday, November 21, 2006 12:41PM - 12:54PM OM.00003: On the physics of the bimodal coherent dynamics of the turbulent horseshoe vortex at Re=\boldmath{$1.16\times10^5$} Joongcheol Paik , Cristian Escauriaza , Fotis Sotiropoulos Due to the strong adverse pressure gradients, the boundary layer approaching a wall-mounted obstacle undergoes a three-dimensional separation with horseshoe vortices which wrap around the obstacle like necklace. Devenport and Simpson (J Fluid Mech., Vol. 210, P. 23 1990) reported the large scale unsteady bimodal nature of the horseshoe vortex system in the leading edge region of a wing at Re=$1.16\times10^5$ which switches aperiodically from one mode to another at time intervals and accounts for turbulent energy production and turbulent stresses an order of magnitude higher than those from conventional shear mechanism in the upstream boundary layer. We carried out detached eddy simulations of the flow past a wing experimentally investigated by Devenport and Simpson and for the first time numerically confirmed the experimental finding of the bimodal velocity probability phenomenon in the horseshoe vortex region. Detailed quantitative comparisons with the measurements and analysis of the 3D nature of large scale coherent vortical structures in the wing-body junction flow will be present in the presentation. Tuesday, November 21, 2006 12:54PM - 1:07PM OM.00004: Vortex structures in turbulent channel flow behind an orifice Soichiro Makino , Kaoru Iwamoto , Hiroshi Kawamura Direct numerical simulation of a channel flow with an orifice has been performed for $Re_{\tau0}=10 - 600$, where $u_{\tau0}$ is the friction velocity calculated from the mean pressure gradient, $\delta$ the channel half width and $\nu$ the kinematic viscosity. In the wake region, the mean flow becomes asymmetric by the Coanda effect. The degree of asymmetry increases with increasing the Reynolds number for the laminar flow at $Re_{\tau0} < 50$. The degree decreases abruptly at $Re_{\tau0} =50$, where the transition from the laminar to the turbulent flow take places. Large-scale spanwise vortices generated at the orifice edges. They become deformed and break up into disordered small-scale structures in shear layer. The small-scale vortices are convected towards the channel center. The large-scale vortices have an important effect upon the reattachment locations and streamwise vortices near the wall in the wake region. Tuesday, November 21, 2006 1:07PM - 1:20PM OM.00005: Reynolds number effects on the coherent dynamics of the turbulent horseshoe vortex Fotis Sotiropoulos , Joongcheol Paik , Cristian Escauriaza The streamwise adverse pressure gradient set up by a wall-mounted obstacle causes the approaching boundary layer to separate and form horseshoe vortices (HSV) in the leading edge region. Reynolds number (Re) is one of most dominant factors on the locations of the boundary layer separation line and the stagnation point, the instability of the primary HSV causing the bursting into other vortices, the number of vortices and their interplays in the junction region, and the unsteady bimodal behavior of the HSV. We carried out detached eddy simulations of flows past a circular cylinder normally wall-mounted on a flat bed at Res ranging from 2.0 x 10$^4$ to 1.0 x 10$^5$ and analyzed the effect of Re on the large scale unsteady behavior of the turbulent HSV system. The computed results confirmed Dargahi's flow visualizations and mean flow measurements of the junction flow at Res of 2.0 x 10$^4$ and 3.9 x 10$^4$ (Exp. Fluids, Vol. 8. p. 1, 1989) which consists of a complex system of multiple vortices shedding, merging and interacting quasi-periodically in the junction region. Numerical solutions further confirmed distinct bimodal velocity histogram of the turbulent HSV system produced by its natural instability at high Re. Tuesday, November 21, 2006 1:20PM - 1:33PM OM.00006: Polymer Induced Drag Reduction: The Interplay Between Vortex Dynamics and Drag Reduction C.-F. Li , R. Sureshkumar , B. Khomami Hi-fidelity DNS of polymer induced DR in turbulent channel flows up to the maximum drag reduction (MDR) limit have been performed using a fully spectral method in conjunction with kinetic theory based elastic dumbbell models for the description of polymer chain dynamics. The simulation results over the friction Re number range of 125 to 395 have been extensively analyzed to decipher the effect of polymer additives on vortex dynamics and the extent of DR. Specifically, we have observed that from the onset of DR to MDR, the Deborah number defined as the product of an effective Weissenberg number and the rms streamwise vorticity fluctuation remains O(1) in the near wall region. This observation further underlines the intricate balance between elastic forces and average rotation speed of the near-wall axial vortices that mediate upwash and downwash events that give rise to Reynolds stress production. Moreover, it is shown that the average lifetime of axial vortices increases with increasing DR while their rotation speed decreases. However, the rate of decrease in the rotation speed exceeds the enhancement in lifetime of axial vortices as a function of DR. Hence. MDR is achieved when these time scales become nearly equal. This simple framework is capable and can adequately describe the influence of polymer additives on extent of DR from onset to MDR as well as the universality of the MDR in flow systems with polymer additives. Tuesday, November 21, 2006 1:33PM - 1:46PM OM.00007: Large Eddy Simulations of a Stationary Smooth-Wall Isothermal Serpentine Passage Frederic Felten , Laskowski Gregory Gas turbine blade cooling strategies consist of serpentine passages with streamline curvature, rib-roughened walls and are subjected to strong rotational and thermal effects. The ability to predict the heat transfer is a major problem and is dependant on the ability to predict the turbulent flowfield. LES have been conducted for an internal cooling passage model in order to determine the ability of LES to capture strong curvature effects. Simulations of fully developed turbulent flow in an isothermal, smooth-wall, stationary serpentine passage have been performed and compared to the DNS data of Laskowski(2004). The geometry is 12$\pi \delta$x2$\delta$x3$\pi \delta$, in the streamwise, wall-normal and spanwise directions, respectively, where $\delta$ is $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$}$ the passage height. The inner radius of the bends is $\delta$. The Reynolds number based on the bulk velocity and $\delta$ is Re$_{b}$=2800. A kinetic-energy conserving, finite-volume scheme, using a collocated-mesh arrangement for simulation of turbulence in complex geometries, as described by Felten {\&} Lund(2006), is applied to treat the streamwise and wall-normal directions, while Fourier collocation is used in the spanwise direction. A 3$^{rd}$ order Runge-Kutta explicit marching scheme is used to advance the solution in time while the pressure Poisson equation is solved using a multigrid technique. The LES results are presented and close agreement with DNS is reported. Simulations focusing on the rotating case of Laskowski(2004) are ongoing. Tuesday, November 21, 2006 1:46PM - 1:59PM OM.00008: ABSTRACT WITHDRAWN