Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session G27: Separated Flows II |
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Chair: Manoochehr Koochesfahani, Michigan State University Room: 31C |
Monday, November 19, 2012 8:00AM - 8:13AM |
G27.00001: Shock wave unsteadiness in an over-expanded nozzle Britton Olson, Sanjiva Lele A suite of Large-Eddy Simulations has recently elucidated the unsteadiness of a shock wave in an over-expanded planar nozzle. The simulations model the nozzle used by Johnson and Papamoschou ({\emph Phys. Fluids} {\bf 22}, 2010), who found that the exhaust jet was destabilized by the shock wave oscillations. Shock wave unsteadiness has been observed in several experiments with similar nozzle geometries. The mechanism which drives the instability is a feedback loop between the nozzle exit and the shock wave. The shock boundary layer interaction causes flow separation and reversal, which then causes an obstruction at the exit of the nozzle. The obstruction is seen as a change in the effective exit area, which in turn causes the shock to readjust its position. When the shock moves, the nature of the shock induced separation changes and the cycle repeats, never becoming stationary. Parametric variation of the nozzle geometry and pressure ratio demonstrate that the instability has a dependence on the Mach number and Reynolds number. A reduced order model (ROM) which is based on the proposed mechanism and the LES data is developed. Preliminary results indicate that the ROM predicts the frequency of the instability to within 10\% when compared to the LES data. [Preview Abstract] |
Monday, November 19, 2012 8:13AM - 8:26AM |
G27.00002: Sensitivity of an asymmetric three-dimensional diffuser to inlet condition perturbations Emily Sayles, Sven Grundmann, Christopher Elkins, John Eaton Experiments were performed to investigate the flow in an asymmetric 3D diffuser that is highly sensitive to inlet condition perturbations. Magnetic Resonance Velocimetry (MRV) revealed the development of a three-dimensional separation bubble in the baseline case. The shape and size of this separation bubble could be manipulated through the introduction of longitudinal counter-rotating vortices produced by small delta-wing vortex generators placed in the inlet duct. The changes to the separation bubble were reflected in significant changes in the diffuser's pressure recovery. Similar pressure recovery effects were observed by perturbing the inlet flow with dielectric barrier discharge plasma actuators oriented to generate forcing in the spanwise direction. The plasma actuators can both improve and degrade the diffuser's performance. Two cases, a continuous forcing case which decreases the pressure recovery and a pulsed forcing case which increases the pressure recovery, were selected for further study. Particle image velocimetry was used to better understand how the secondary flows introduced by the plasma actuators interact with the separation bubble, and why they have such a marked effect on the diffuser's performance. [Preview Abstract] |
Monday, November 19, 2012 8:26AM - 8:39AM |
G27.00003: Triple deck solutions for supersonic flow past obstacles Ramesh Yapalparvi, Pierre-Yves Lagree This study is based on the numerical investigation of the boundary-layer separation of supersonic flow at high Reynolds number past obstacles (flat plate with a hump) based on the concept of viscous-inviscid interaction. The ``triple-deck'' model is used to describe the interaction process. We observe a separation region both ahead and behind the hump whereas a separation region is centered for the indents. In case of humps, at higher values of hump height, the pressure has both positive and negative value of ``plateau'' (observed up to now only upstream wedges). This novel feature has been compared with the ``interacting boundary-layer'' model which shows an excellent agreement. [Preview Abstract] |
Monday, November 19, 2012 8:39AM - 8:52AM |
G27.00004: An experimental study of flow past a rotationally oscillating cylinder. Sanjay Kumar, Carlos Lopez, Oliver Probst, Davood Askari, Yingchen Yang Flow past a circular cylinder executing sinusoidal rotary oscillations about its own axis is studied experimentally. The experiments are carried out at Re = 185, oscillation amplitudes varying from $\pi $/8 to $\pi $, and forcing frequency ratios varying from 0 to 5. It is found that the phenomenon of lock-on occurs in a forcing frequency range which depends not only on the oscillation amplitude but also the downstream location from the cylinder. The experimentally measured lock-on diagram in the forcing amplitude and frequency plane is presented at various downstream locations ranging from 2 to 23 diameters. The upper limit of the lock-on forcing frequency band depends strongly on the downstream location whereas the lower limit is fairly insensitive. The far field wake decouples, after the lock-on at higher forcing frequencies and behaves more like a regular Karman vortex street from a stationary cylinder with a vortex shedding frequency mostly lower than the one from a stationary cylinder. The dependence of circulation values of shed vortices on the forcing frequency revealed a universal decay curve independent of forcing amplitude beyond forcing frequency of $\sim $ 1.0. [Preview Abstract] |
Monday, November 19, 2012 8:52AM - 9:05AM |
G27.00005: Shear-Layer Interactions Between Surface-Mounted Obstacles at Varying Streamwise Spacings T. Kim, J.L. Best, K.T. Christensen Surface obstacles occur in a variety of flows, such as roughness elements in engineering flows and barchan dunes in natural eolian environments on both the Earth and Mars. Depending upon the arrangement and spacing between such obstacles, the flow over one obstacle can significantly alter the flow over those positioned downstream. Such flow interactions occur in fields of barchan dunes that are closely spaced and aligned in the flow direction, and where flow sheltering may play a significant role. To better understand these flow interactions, experiments were conducted for a pair of identical, upright cylinders extending into the log layer and aligned at various spacings in the streamwise direction of a turbulent channel flow at $\mathrm{Re}_\tau\sim 1200$. Particle-image velocimetry measurements of the flow around the cylinders reveal strong interactions between the shear layers generated downstream of the cylinders, and particularly a weakening of the downstream-most shear layer for small cylinder spacings ($<4-6D$). Modifications of the vortex-shedding processes at the downstream cylinder are under investigation, as these interactions are thought to play a critical role in the formation and evolution of surface obstacles when the surface is cohesionless and mobile. [Preview Abstract] |
Monday, November 19, 2012 9:05AM - 9:18AM |
G27.00006: Generalised phase average with applications to sensor-based flow estimation of the wall-mounted square cylinder wake Robert Martinuzzi, Jason Bourgeois, Bernd R. Noack We experimentally investigate the three-dimensional wake behind a finite wall-mounted square cylinder at $Re=12,000$ and aspect ratio of $4$. Focus is placed on the base flow and oscillatory fluctuation. Time-resolved 3D velocity fields are constructed from high-frame-rate particle image velocimetry (PIV) and simulatenously recorded surface pressure measurements. All three velocity components are resolved in a rectangular near-wake region by two orthogonal dense arrays of parallel PIV planes. A key enabler is a generalized phase-average incorporating slowly varying base flow, a variable oscillation amplitude and higher harmonics. These generalizations reduce the residual 30\% below those of a traditional phase average. Moreover, the resolved variations reveal analytical constraints of the mean flow and oscillation levels, like the mean-field paraboloid. The proposed methodology for generalized phase averaging and for construction of 3D velocity fields from 2D PIV data is applicable to a large class of turbulent flows with oscillatory dynamics. [Preview Abstract] |
Monday, November 19, 2012 9:18AM - 9:31AM |
G27.00007: Low-Reynolds-number vortex dynamics around moving wings Ryan Jantzen, Kunihiko Taira, Michael Ol, Kenneth Granlund The focus for the present research is to investigate the fundamental flow physics around low-aspect-ratio flat-plate wings undergoing pitching and surging motions. Numerical simulations performed with the immersed boundary projection method are used to investigate the three-dimensionality of the low-Reynolds-number flow around these wings. Of particular interest is the influence of wing motion on the formation of the leading-edge, trailing-edge, and tip vortices. To determine the relationship that these pure motions have on the formation of these vortices, we vary the aspect ratio, pitching rate, and pivot-point location. The spanwise variation in the roll-up of the leading and trailing-edge vortices under the influence of tip effects is analyzed. The aerodynamic forces generated during these unsteady wing motions will be compared to force measurements obtained from moderate-Reynolds-number towing tank experiments. A further understanding of the underlying flow physics for these idealized motions is necessary in order to understand more complex wing maneuvers. [Preview Abstract] |
Monday, November 19, 2012 9:31AM - 9:44AM |
G27.00008: Wake Modes and Heat Transfer from Rotationally Oscillating Cylinder Prabu Sellappan, Tait Pottebaum Wake formation is an important problem in engineering due to its effect on phenomena such as vortex induced vibrations and heat transfer. While prior work has focused on the wake formation due to vortex shedding from stationary and oscillating cylinders, limited information is available on the relationship between wake modes and heat transfer from rotationally oscillating cylinders. Experiments were carried out at Re=150 and 750, using an electrically heated cylinder, in a water tunnel for oscillation frequencies from 0.67 to 3.5 times the natural shedding frequency and peak-to-peak oscillation amplitudes up to 320\r{ }. DPIV was used to identify and map wake modes to various regions of the parameter space. Temperature data from a thermocouple embedded in the cylinder was used to calculate heat transfer rates. Correlation between heat transfer enhancement and certain wake mode regions were observed in the parameter space. The relationship between wake formation and heat transfer enhancement will be described. [Preview Abstract] |
Monday, November 19, 2012 9:44AM - 9:57AM |
G27.00009: Effect of approach flow on the bluff body wake behind a ship superstructure Cody Brownell, Luksa Luznik, Hyung Suk Kang, Murray Snyder Air velocity measurements are obtained in situ aboard a 108 ft naval training vessel operating in the Chesapeake Bay. Three-component sonic anemometers are placed on a vertical mast at the bow of the ship, for approach flow measurement, and at numerous locations above a flight deck at the stern of the ship. The mean flow structure resembles that of a 3D backward-facing step, with a recirculation region covering much of the flight deck. The flow at the bow, mostly undisturbed by the presence of the ship, is characterized in the region up to 11-m above the sea surface. The effects of approach angle and atmospheric stability on the wake turbulence is discussed. [Preview Abstract] |
Monday, November 19, 2012 9:57AM - 10:10AM |
G27.00010: Reducing the Drag and Damage of a High-Speed Train by Analyzing and Optimizing its Boundary Layer Separation and Roll-up into Wake Vortices Chung-Hsiang Jiang, Philip Marcus We present numerical calculations of the boundary layers and shed wake vortices behind several aerodynamic bodies and generic models of high-speed trains. Our calculations illustrate new visual diagnostics that we developed that clearly show where the separation of a boundary layer occurs and where, how, and with what angles (with respect to the stream-wise direction) the wake vortices form. The calculations also illustrate novel 3D morphing and mesh ``pushing and pulling'' techniques that allow us to change the shapes of aerodynamic bodies and models in a controlled and automated manner without spurious features appearing. Using these tools we have examined the patterns of the shed vortices behind generic bodies and trains and correlated them with the changes in the drag as well as with the effects of the shed vortices on the environment. In particular, we have applied these techniques to the end car of a next-generation, high-speed train in order to minimize the drag and to minimize the adverse effects of the shed vortices on the track ballast. [Preview Abstract] |
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