### Session G23: Separated Flows I

Chair: Steven Brunton, Princeton University
Room: 326

 Monday, November 21, 2011 8:00AM - 8:13AM G23.00001: Unsteady aerodynamic models for separated flow past a flat plate at Re=100 Steven Brunton , Clarence Rowley This work develops reduced-order models for the unsteady aerodynamic forces on a small wing in response to agile maneuvers and gusts. Models are based on direct numerical simulations performed using the immersed boundary projection method for a flat plate at Reynolds number 100. In particular, we investigate models linearized at various angles of attack, up to the Hopf bifurcation, which occurs around $\alpha_{\rm{crit}}=28^{\circ}$. A main result is that we present a flexible, low-order representation that includes viscous separated effects as well as added-mass effects, and may be seen as a linearization of a physically motivated nonlinear dynamical system. The performance of models below the Hopf bifurcation are investigated in the frequency domain as well as on large amplitude, rapid pitch maneuvers in the time domain. Additionally, we investigate models and flow field structures in the fully nonlinear vortex shedding regime using force, vorticity, and finite-time Lyapunov exponent field measurements. Monday, November 21, 2011 8:13AM - 8:26AM G23.00002: Anisotropy-resolving models for predicting separation in 3--D asymmetric diffusers Elbert Jeyapaul , Paul Durbin All linear eddy-viscosity models are qualitatively incorrect in predicting separation in 3--D asymmetric diffusers. The failure to predict normal stress and shear stress anisotropy at high production-dissipation ratios is the cause. The Explicit algebraic Reynolds stress model (Wallin and Johansson, 2000) predicts the mean flow field in the diffuser accurately, but not the wall pressure and Reynolds stresses. Recalibrating the coefficients of the rapid part of pressure-strain model improves the wall pressure prediction. Including the convective, diffusive, streamline curvature effects on anisotropy has not been beneficial. The model has been tested using a family of diffusers having the same nominal streamwise pressure gradient, LES data is used as a reference. Monday, November 21, 2011 8:26AM - 8:39AM G23.00003: A numerical investigation of the laminar, three-dimensional backward-facing step Nikolaos Malamataris , Rainald Lohner The three-dimensional, backward-facing step is a benchmark problem in CFD. However, only recently, Malamataris and L\"{o}hner (2011) were able to compute the eddy along the upper wall of the step for this flow in laboratory feasible conditions, although the existence of this eddy is known from the experiments of Armaly et al (1983) and Lee and Mateescu (1998). There are some misinterpretations regarding three-dimensional effects for this flow due to the fact that at the time of the work of Armaly et al (1983) only two-dimensional computations were available that perpetuated until the work of Malamataris and L\"{o}hner. The issues with three-dimensional effects are shown to leave the flow phenomena unaffected in the plane of symmetry for the conditions of Armaly et al's experiment by showing streamlines in planes parallel/perpendicular to the plane of symmetry. The Reynolds number ranges from 100 to 1050. The steady state, three-dimensional Navier Stokes equations are solved with finite elements using two different codes FEFLO and FEM3D. Monday, November 21, 2011 8:39AM - 8:52AM G23.00004: Flow features from the wake of a rotating sphere Andrew Ooi , Eric Poon , Matteo Giacobello , Raymond Cohen The flow features in the wake of a rotating sphere is investigated numerically. Simulations are carried out at Reynolds numbers up to Re = 1000, with non-dimensional rotational rates \textbf{$\Omega$}= 0 to 1. The axis of rotation is orientated from \textit{$\alpha$} = 0 (streamwise rotation) to \textit{$\alpha$} = \textit{$\pi$}/2 (transverse rotation). At Re=250 and 300, the flow field could be steady or unsteady, depending on the value of \textbf{$\Omega$} and \textit{$\alpha$}. Phase diagrams of \textbf{(}CLx\textbf{,}CLy\textbf{) } and (CD\textbf{,}CL ) are used to illustrate the dynamics of the unsteady forces acting on a rotating sphere. By employing the phase diagram \textbf{(}CLx\textbf{,}CLy\textbf{)}, the direction of the resultant lift force can be shown to be dependent on the non-dimensional rotation rate. The shape of the curves which appear on the phase diagrams \textbf{(}CLx\textbf{,}CLy\textbf{) } are highly correlated to the behaviour of the vortical structures in the wake. This study will give a more complete picture of the physics associated with the incompressible flow past a rotating sphere. Monday, November 21, 2011 8:52AM - 9:05AM G23.00005: A Nonlinear Model for Wind-Induced Oscillations of Open-Grown Trees Lakshmi Narayanan Ramanujam , Mark Reiland , Yahya Modarres-Sadeghi , Brian Kane Wind-induced oscillations of trees constitute a traditional fluid-structure interaction problem. The highly nonlinear interaction between fluid and structure makes the prediction of such oscillations very challenging. We use a nonlinear analytical model to predict the wind-induced oscillations of the open-grown trees, by taking into account the geometrical as well as fluid-related nonlinearities. We validate this model using a series of experimental measurements of the amplitudes and frequencies of oscillations for the red pine trees at various wind speeds. Then we use this model to study the influence of several parameters such as tree stiffness, drag coefficient, canopy density, etc. on the resulting wind-induced oscillations. Monday, November 21, 2011 9:05AM - 9:18AM G23.00006: DNS study of large-scale structures in a separated turbulent boundary layer Hiroyuki Abe , Yasuhiro Mizobuchi , Yuichi Matsuo Direct numerical simulations (DNSs) of a separated flat-plate turbulent boundary layer have been carried out. The inlet data are prescribed by DNSs of a zero-pressure-gradient turbulent boundary layer with the rescaling-recycling method; blowing and suction are imposed at the upper boundary for producing a separation bubble. The Reynolds numbers at the inlet are set to be $Re_\theta$=300, 600 and 900, where $Re_\theta$ is the Reynolds number based on the freestream velocity and the momentum thickness. Particular attention is given to large- scale structures existing in a separated region. Results indicate that large-scale organized structures of the streamwise velocity fluctuation appear in a detached shear layer when a large separated region is formed. The latter structures consist of positive and negative regions alternating in the spanwise direction with a spacing of about $2\sim3 \delta_{99}$ ($\delta_{99}$ denotes the 99\% boundary layer thickness at the inlet), which become more apparent with increasing Reynolds number. They are most likely associated with large-scale spanwise meandering of the separation line. There is also close relationship between the large-scale structures and vortical structures, the latter tending to form vortex clusters where hairpin-like vortices are also observed. Monday, November 21, 2011 9:18AM - 9:31AM G23.00007: Three--dimensional Instability in the Wake of Two Side--by--side Square Cylinders Choon-Bum Choi , Yong-Jun Jang , Kyung-Soo Yang Three-dimensional instability in the flow past two square cylinders in side-by-side arrangements has been numerically studied via a Floquet stability analysis. The distance between the neighboring faces of the two cylinders (G) is the key parameter which affects the secondary instability under consideration. In this talk, we present the critical Reynolds number for the 3D instability and the spanwise wave number corresponding to the most unstable (or least stable) wave for each G. When the two cylinders are close to each other, the Floquet mode resembles mode A of flow past a single cylinder. For large G, the Floquet mode for each cylinder is independent of the neighboring cylinder as expected. In the range of intermediate G, interference of the two wakes is clearly reflected in the Floquet mode. We also discuss about the effect of gap flow, and visualization of the dominant 3D vortical structures is provided by using Q contours. Our results shed light on a complete understanding of the onset of 3D instability in the presence of two side-by-side square cylinders. Monday, November 21, 2011 9:31AM - 9:44AM G23.00008: Separation Bubble Dynamics in Single and Multiple STBLIs Brandon Morgan , Nghia Nguyen , Sanjiva Lele We first analyze the low-frequency motion of incipient shock-induced separation using the results of large-eddy simulation (LES) of an oblique shock impinging on a supersonic turbulent boundary layer ($M_{\infty }$ = 2.28, \textit{$\varphi$} = 8\r{ }, Re$_{\theta }$ = 2300). Through analysis of low-pass filtered data fields, evidence is found of a correlation between the low-frequency motion and a periodic exchange of mass between the separation bubble and shear layer. Integral mass and momentum budgets in filtered bubbles are computed from the LES data to gain insight into the mechanism of low-frequency unsteadiness, and time signals of several indicators of bubble dynamics are analyzed. Additionally, a preview of results is presented from LES of a normal shock train in a constant area duct. Significant three-dimensionality is observed, and its effects on separation dynamics are described, when viscous side-wall boundary conditions are imposed in the spanwise direction. Monday, November 21, 2011 9:44AM - 9:57AM G23.00009: ABSTRACT WITHDRAWN