Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Fluid Dynamics
Volume 56, Number 18
Sunday–Tuesday, November 20–22, 2011; Baltimore, Maryland
Session L16: Separated Flows II |
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Chair: Ralph Volino, United States Naval Academy Room: 319 |
Monday, November 21, 2011 3:35PM - 3:48PM |
L16.00001: Effect of wakes from moving upstream rods on boundary layer separation from a high lift airfoil Ralph J. Volino Highly loaded airfoils in turbines allow power generation using fewer airfoils. High loading, however, can cause boundary layer separation, resulting in reduced lift and increased aerodynamic loss. Separation is affected by the interaction between rotating blades and stationary vanes. Wakes from upstream vanes periodically impinge on downstream blades, and can reduce separation. The wakes include elevated turbulence, which can induce transition, and a velocity deficit, which results in an impinging flow on the blade surface known as a ``negative jet.'' In the present study, flow through a linear cascade of very high lift airfoils is studied experimentally. Wakes are produced with moving rods which cut through the flow upstream of the airfoils, simulating the effect of upstream vanes. Pressure and velocity fields are documented. Wake spacing and velocity are varied. At low Reynolds numbers without wakes, the boundary layer separates and does not reattach. At high wake passing frequencies separation is largely suppressed. At lower frequencies, ensemble averaged velocity results show intermittent separation and reattachment during the wake passing cycle. [Preview Abstract] |
Monday, November 21, 2011 3:48PM - 4:01PM |
L16.00002: Experimental investigation of frequency lock-on in separated flow John Griffin, Lou Cattafesta, Jonathan Tu, Clancy Rowley, Ehsan Aram, Rajat Mittal Separated flow is a complex phenomenon comprised of several flow dynamics. Recent experimental and computational investigations propose that, depending on flow conditions, the nature of flow separation is governed by three dominant mechanisms: the shear layer, separation bubble, and wake instabilities. The nonlinear interactions of these instabilities provide potential for separation characterized by various lock-on states. This study provides an experimental investigation into the lock-on type dynamics for separated flows from various two-dimensional airfoil shapes, Reynolds numbers, and angles of attack. Two simultaneously sampled hot-wire probes are used to acquire velocity in two of the three regions of interest. The data quantify nonlinear coupling between the instabilities observed in the shear layer, the wake, and, in the case of mean flow reattachment, the separation bubble. The locations of these phenomena are determined with simple two-component particle image velocimetry. The coupling is assessed via higher-order spectral and moment analysis. Several of the cases that demonstrate resonant behavior are included for discussion. [Preview Abstract] |
Monday, November 21, 2011 4:01PM - 4:14PM |
L16.00003: Wake Modes of Rotationally Oscillating Cylinders at low Re Prabu Sellappan, Tait Pottebaum Vortex shedding from bluff bodies is important in various engineering applications because the wake can have many effects, including exciting vibrations in structures and altering convective heat transfer. While vortex shedding from cylinders in cross-flow and cylinders undergoing transverse and in-line oscillations has been studied extensively, only limited data is available for rotational oscillations and is mainly limited to spectral analysis of the wake. Water tunnel experiments were carried out at Re = 150 to investigate the wake of a rotationally oscillating cylinder 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 study both the near and far wake within this parameter space. Well-defined patterns of wake vortices were observed in distinct regions of the parameter space, similar to the wake modes of transversely oscillating cylinders in cross-flow. In portions of the parameter space for which information exists in the literature the wake modes are well-related to spectral data. Variants of modes in previously unexplored regions are explained in terms of harmonics. The initial application of these results to understanding heat transfer enhancement from rotationally oscillating cylinders will also be addressed. [Preview Abstract] |
Monday, November 21, 2011 4:14PM - 4:27PM |
L16.00004: Separation Control in a Centrifugal Bend Using Plasma Actuators Michael Arthur, Thomas Corke An experiment and CFD simulation are presented to examine the use of plasma actuators to control flow separation in a 2-D channel with a 135$^{\circ}$ inside-bend that is intended to represent a centrifugal bend in a gas turbine engine. The design inlet conditions are $P=330$\,psia., $T=1100^{\circ}$F, and $M=0.24$. For these conditions, the flow separates on the inside radius of the bend. A CFD simulation was used to determine the location of the flow separation, and the conditions (location and voltage) of a plasma actuator that was needed to keep the flow attached. The plasma actuator body force model used in the simulation was updated to include the effect of high-pressure operation. An experiment was used to validate the simulation and to further investigate the effect of inlet pressure and Mach number on the flow separation control. This involved a transient high-pressure blow-down facility. The flow field is documented using an array of static pressure taps in the channel outside-radius side wall, and a rake of total pressure probes at the exit of the bend. The results as well as the pressure effect on the plasma actuators are presented. [Preview Abstract] |
Monday, November 21, 2011 4:27PM - 4:40PM |
L16.00005: An experimental study of flow past an oscillating cylinder Sanjay Kumar, Carlos Lopez, German Francisco, Davood Askari, Cesar Alejandro Toledo Suarez, Oliver Probst We present preliminary experimental results on flow past an oscillating cylinder at frequency ratios varying from 0 to 5 and oscillation amplitudes varying from $\pi /8$ to $\pi $. The experiments are conducted at Reynolds number of 185. The frequency ratio, f$_{R}$, is defined as the ratio of cylinder oscillation frequency to vortex shedding frequency from a non-oscillating cylinder. The diagnostic is done using hydrogen bubble technique for flow visualization in a plane. It is found that at one diameter downstream from the cylinder, vortex shedding frequency matches the forcing frequency (lock-on) at all f$_{R} \quad >$1 and all amplitudes; however, for f$_{R} \quad <$1 there is a window adjacent to f$_{R}$ = 1 where lock-on occurs and this depends on oscillation amplitude. In the far wake at nine diameters from the cylinder, the lock-on region is centered around f$_{R}$ = 1 and depends strongly on amplitude. The visualization of the vortical structures showed that near f$_{R}$ =1.0 the vortices became very compact and well formed in the visualization plane. Their size decreases considerably at much higher f$_{R}$'s in the near-wake. The motion pictures reveal interesting phenomenon of merger of vortices at certain f$_{R}$'s. [Preview Abstract] |
Monday, November 21, 2011 4:40PM - 4:53PM |
L16.00006: Experimental Study of Wake Instabilities of a Blunt Trailing Edge Profiled Body at Intermediate Reynolds Numbers Arash Naghib-Lahouti, Philippe Lavoie, Horia Hangan The periodic shedding of von K\'arm\'an vortices is the primary instability in the wake of nominally 2D bluff bodies, beyond a critical Reynolds number around 45-49. When Reynolds number passes a second threshold, which can be as high as 700 depending on profile geometry, secondary instabilities emerge and accompany the von K\'arm\'an vortices. For most bluff bodies, these instabilities appear as pairs of counter-rotating streamwise vortices, and spanwise undulations of the von K\'arm\'an vortices. The mechanism and scale of these instabilities depend on the bluff body geometry and Reynolds number. The focus of the present study is to identify and characterize the dominant secondary instability in the wake of a blunt trailing edge profiled body at intermediate Reynolds numbers between 8,000 and 20,000 based on the body thickness. The experiments, which include PIV and hot-wire measurements in the wake, complement previous studies involving the same bluff body at higher and lower Reynolds numbers, and make it possible to determine the scale and mechanism of the secondary instability at intermediate Reynolds numbers. [Preview Abstract] |
Monday, November 21, 2011 4:53PM - 5:06PM |
L16.00007: Comparison of turbulent separation over a smooth surface and mako shark skin on a NACA 4412 hydrofoil Drew Smith, Amy Lang, Redha Wahidi Shark skin is being investigated as a means of passive flow separation control due to the flexibility and preferential flow direction of the scales covering the skin. In this study, the effect of the scales is observed in a tripped turbulent boundary layer by comparing the flow over a NACA 4412 hydrofoil with a smooth surface to that over the same hydrofoil with samples of mako shark skin affixed to its upper surface. These samples were taken from the flank area of the shark because the scales at that location have been shown to have the greatest angle of erection, and thus the best potential for separation control. All flow data in this study was obtained using Time-Resolved Digital Particle Image Velocimetry and recorded at multiple angles of attack (between 8 and 16 degrees) and two Reynolds numbers. The flow was primarily analyzed by means of the backflow coefficient (a value based on the percentage of time that flow in a region over the hydrofoil is reversed) and the time history of instantaneous flow velocity values at specific points in the boundary layer over the hydrofoil models. [Preview Abstract] |
Monday, November 21, 2011 5:06PM - 5:19PM |
L16.00008: An Experimental Study of Flow Separation over 2D Transverse Grooves Emily Jones, Amy Lang, Farhana Afroz, Jennifer Wheelus, Drew Smith A shark's scales help to reduce drag over its body by controlling boundary layer separation over its skin. It is theorized that the scales bristle when encountering a reversing flow, thereby trapping vortices between the scales, creating a partial slip condition over the surface and inducing turbulence augmentation in the boundary layer. In an attempt to replicate and study these effects, a spinning cylinder was used in a water tunnel to induce separation over a flat plate with 2 millimeter square 2D transverse grooves. The results were compared to separation occurring over a flat plate without grooves using DPIV. The angular speed of the cylinder was varied. The observed delays in separation, changes in separation bubble shedding frequency and other effects upon the boundary layer are discussed. [Preview Abstract] |
Monday, November 21, 2011 5:19PM - 5:32PM |
L16.00009: Flow around the tip of a circular cylinder in proximity with a channel bottom wall Christopher Hocut, Ralph Budwig Measurement probes create a region of disturbed flow in the vicinity of the probe tip. The present investigation examined the disturbance due to an idealized probe (a circular cylinder) that penetrated the free surface of an open channel flow as well as potential interactions of the disturbance with the channel bottom. Stereoscopic particle image velocimetry (PIV) was used to measure the three components of velocity in the vertical plane of symmetry downstream the cylinder with the tip of the cylinder located zero to four diameters from the channel bottom. The disturbance characteristics (change in velocity and turbulence quantities), vertical distance to the extent of disturbance as measured from cylinder tip, and downstream distance to the point of maximum disturbance have been determined. The cylinder tip depth that caused interaction with the channel bottom leading to onset of scour was also investigated. [Preview Abstract] |
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