Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session H4: Separated Flows II - Wakes and Flows past Special Surfaces |
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Chair: Gary Coleman, NASA Langley Room: 326 |
Monday, November 25, 2013 10:30AM - 10:43AM |
H4.00001: Low-Reynolds number compressible flow around a triangular airfoil Phillip Munday, Kunihiko Taira, Tetsuya Suwa, Daiju Numata, Keisuke Asai We report on the combined numerical and experimental effort to analyze the nonlinear aerodynamics of a triangular airfoil in low-Reynolds number compressible flow that is representative of wings on future Martian air vehicles. The flow field around this airfoil is examined for a wide range of angles of attack and Mach numbers with three-dimensional direct numerical simulations at $Re = 3000$. Companion experiments are conducted in a unique Martian wind tunnel that is placed in a vacuum chamber to simulate the Martian atmosphere. Computational findings are compared with pressure sensitive paint and direct force measurements and are found to be in agreement. The separated flow from the leading edge is found to form a large leading-edge vortex that sits directly above the apex of the airfoil and provides enhanced lift at post stall angles of attack. For higher subsonic flows, the vortical structures elongate in the streamwise direction resulting in reduced lift enhancement. We also observe that the onset of spanwise instability for higher angles of attack is delayed at lower Mach numbers. [Preview Abstract] |
Monday, November 25, 2013 10:43AM - 10:56AM |
H4.00002: Viscous-Inviscid Interaction Analysis in High-Reynolds Number Flows Using Complex Singularities K.W. Cassel, F. Gargano, M. Sammartino, V. Sciacca Interaction between the viscous boundary layer and inviscid outer flow occurs during unsteady separation on two distinct spatial scales depending upon the Reynolds number regime. Using the impulsively-started flow about a circular cylinder, it is illustrated how these regimes can be identified by tracking singularities in the complex plane based on numerical solutions of the unsteady Navier-Stokes equations. Such an analysis also allows for clear identification in Navier-Stokes solutions of the precursor to the van Dommelen singularity, which occurs in the classical non-interactive boundary-layer equations. The first interaction developing in the separation process is large-scale interaction that is visible for all the Reynolds numbers considered, and it signals the first relevant differences between the boundary-layer and Navier-Stokes solutions. For $Re\geq O(10^4)$, a small-scale interaction corresponding to the van Dommelen singularity follows the large-scale interaction. [Preview Abstract] |
Monday, November 25, 2013 10:56AM - 11:09AM |
H4.00003: Investigation of Modal Excitation of a Flexible Cylinder on Vortex Induced Vibrations Ersegun D. Gedikli, Harrison Zimmer, Jason M. Dahl The Vortex-Induced Vibration (VIV) of low mode number flexible cylinders is investigated to observe the effect of modal excitation on synchronization of the wake in a uniform flow. Previous studies have focused on the analysis of two degree of freedom vibrations of a rigid cylinder by controlling the ratio between in-line and cross-flow natural frequencies. The present study holds this natural frequency relation constant while varying the anticipated mode shape associated with structural natural frequencies in air. It is found that a combination of an odd mode shape excited in the cross-flow direction with an even mode shape in the in-line direction results in an incompatible synchronization condition, where the dominant forcing frequency in-line may experience a frequency equal to the cross-flow forcing frequency, a condition not typically observed in rigid cylinder experiments. Excitations of odd mode shapes in both in-line and cross-flow directions result in typical VIV excitation of the flexible body, which compares well with rigid cylinder experiments. Observed motions of the flexible body are forced using a rigid cylinder to visualization the wake. [Preview Abstract] |
Monday, November 25, 2013 11:09AM - 11:22AM |
H4.00004: Dynamic mode decomposition of supersonic and transonic wakes of generic space launcher configurations Vladimir Statnikov, Taraneh Sayadi, Matthias Meinke, Wolfgang Schroeder, Peter Schmid Dynamic mode decomposition (DMD) is applied to supersonic and transonic wakes of generic space launcher configurations of Mach numbers 0.7 and 6 computed using a zonal RANS/LES approach. The axisymmetric geometry includes a backward facing step that causes the flow to separate. In addition to the separation bubble, acoustic waves are also radiated from the downstream region of the flow. Experimental and numerical observations clearly demonstrate the existence of peaks in the pressure spectra which can be attributed to both the flow inside the wake and the acoustic waves in the freestream. The objective of this work is to apply DMD to the set of numerical data in order to firstly, extract the spatial shape of the modes and secondly, identify their respective frequencies. This allows the dynamics associated to the separation bubble and those of the acoustic waves to be differentiated properly. In addition, with the help of DMD the modes responsible for pressure-loading on the backward face of the step are extracted and analyzed. [Preview Abstract] |
Monday, November 25, 2013 11:22AM - 11:35AM |
H4.00005: On the relationship between boundary-layer thickness, base drag and near-wake flow of an axisymmetric bluff body Maria Vittoria Salvetti, Alessandro Mariotti, Guido Buresti A large contribution to the aerodynamic drag of a bluff body is given by the low pressures on its base, i.e. the surface lying within the separated wake. In order to develop strategies to decrease drag, a critical issue is the relationship between the base pressure and the flow parameters. The main application is the development of methods for the reduction of the drag of road vehicles. We consider the flow around an axisymmetric body which can be viewed as a simplified model of a road vehicle, for which experiments, LES and DNS were carried out. Both experiments and simulations showed that an increase of the boundary layer thickness before separation reduces the pressure drag of the body. This is connected with an increase of the length of the mean recirculation region behind the body. A detailed analysis of the near wake dynamics is carried out to ascertain whether the variations of the recirculation length, and hence of the base pressure, caused by the modifications of the boundary layer thickness, may be connected with changes in the dynamics of the vorticity structures originating from the instability of the separated shear layers. More generally, this analysis may be also a useful step towards devising further strategies for pressure drag reduction. [Preview Abstract] |
Monday, November 25, 2013 11:35AM - 11:48AM |
H4.00006: Turbulent flow past an obstacle embedded in a hydraulically rough and porous bed Nikolaos Apsilidis, Panayiotis Diplas, Clinton Dancey, Pavlos Vlachos, Polydefkis Bouratsis The pressure gradients imposed by wall-mounted obstacles give rise to large-scale, coherent flow structures. Past studies have linked the unsteadiness of these organized flow motions to phenomena such as increased turbulence intensities, momentum and heat transfer over the junction region [Simpson, \textit{Annu. Rev. Fluid Mech.} 2001]. The typical configuration under study refers to a bluff body mounted vertically on an impermeable and hydraulically smooth wall. These characteristics of the bottom boundary, however, do not model accurately a number of flows of environmental (river flow around a boulder) or engineering interest (flow past a bridge pier). Motivated by this inconsistency, we carried out experiments for the turbulent flow at the upstream junction of a cylinder placed within a permeable, hydraulically rough bed. Using 2D2C Particle Image Velocimetry, we captured the rich dynamics of the coherent flow structures developing over the region of interest. We compared results with those from a benchmark test run at a similar Reynolds number, but over a smooth and impermeable bed. Comparisons show that the unsteadiness of coherent flow structures at a wall-body junction increases significantly in the presence of a permeable and rough wall. [Preview Abstract] |
Monday, November 25, 2013 11:48AM - 12:01PM |
H4.00007: Flow around a superhydrophobic cylinder Jessica Shang, Brian Rosenberg, Peter Dewey, Howard Stone, Alexander Smits The boundary condition on a circular cylinder is varied through the use of superhydrophobic surfaces, which introduce a partial-slip boundary condition. We examine the effect of these surfaces on the separation behind a cylinder in the Reynolds number range $15 < Re_D < 1600$. Two different superhydrophobic surfaces are compared with a smooth untreated surface: a conventional superhydrophobic surface consisting of a aluminum hydroxide networked nanostructure with an air-water interface, and a slippery liquid-infused surface (SLIPS) that is similarly nanostructured. We find no change in the critical $Re_D$ or the vortex shedding frequency. An increase in the vortex formation length, generally associated with a decrease in base suction, occurs for the air-water interface for $300 < Re_D < 900$, but not for SLIPS. Superhydrophobic surfaces appear to have no similar effect at higher $Re_D$ in the shear layer transition regime. [Preview Abstract] |
Monday, November 25, 2013 12:01PM - 12:14PM |
H4.00008: Flow over Barnacles--Characterization of Barnacle Geometry and Some Initial Flow Characteristics Jasim Sadique, Xiang Yang, Charles Meneveau, Michael Schultz, Rajat Mittal Macrobiofouling is a serious concern for the marine industry, costing billions in preventive and control measures. Accurate modelling of flows over surfaces with such complex geometry and wide range of length scales is still a huge challenge. Such simulations are required in predicting the effects of fouling, like surface drag and also forces experienced by individual barnacles. DNS or wall resolved LES are impractical due to constraints imposed by the nature of the geometry. We aim to develop and test a computational tool for accurate simulation of such flows. The method being proposed incorporates generalized dynamic wall models along with sharp-interface Immersed Boundary Methods. The results from these simulations will help us understand the effects on surface drag caused by variations in parameters like roughness density, roughness heights, spatial heterogeneity etc. Along with this, detailed studies on a single barnacle will help us in understanding flow structures in the presence of boundary layers. In this talk we will give a brief overview of the problem and some results from our investigation on the characterization of Barnacle geometries and on the characteristics of flow over a single barnacle. [Preview Abstract] |
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