Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 55, Number 16
Sunday–Tuesday, November 21–23, 2010; Long Beach, California
Session HJ: Flow Control V |
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Chair: Tim Colonius, California Institute of Technology Room: Long Beach Convention Center 201A |
Monday, November 22, 2010 10:30AM - 10:43AM |
HJ.00001: High-Speed Jet Noise Source Identification by Wavelet Filtering Jacques Lewalle, Zachary Berger, Kerwin Low, Mark Glauser Pressure sensors in a Mach 0.6 jet provide near-field data at 2 sections (x/D = 3 and 6), and simultaneous far-field data at 5 angular locations from 15$^{\circ}$ to 90$^{o\circ}$ degrees relative to the jet axis. Continuous wavelets allow some feature recognition at the various stations. In the absence of sustained oscillations, the Mexican hat wavelet is used on the Fourier mode 0 in the near-field. At each scale, some local extrema (presumed signature of a nearby vortex) are recognized, with delay, between the 2 stations, and a scale-dependent convection speed is calculated. Non-linear filtering isolates the recognized, ``matched'' features, at the 2 near-field stations, and a `residue' presumed to include the result of vortex pairing or breakdown. The physical relevance of this decomposition is established by the cross-correlation of the filtered near-field data with far-field noise. A scale-dependent cross-correlation was calculated, showing distinct scales and convection propagation delays for the various pairs of traces, for which different causes will be discussed. At the time of writing, the distinctive characteristics are used for pattern recognition in the raw data. [Preview Abstract] |
Monday, November 22, 2010 10:43AM - 10:56AM |
HJ.00002: Shear Layer Excitation of a High Speed Turbulent Jet Kerwin Low, Basman El Hadidi, Mark Glauser, Zachary Berger Simultaneous pressure and acoustic measurements are acquired in the hydrodynamic and acoustic fields of a Mach 0.6 cold jet (Re = 680,000). The two axisymmetric sensor arrays in the near-field (x/D = 3 and 6) are positioned 10cm from the developing shear layer. The far-field microphones (x/D = 75) are positioned at five angular locations from 15$^{\circ}$ to 90$^{\circ}$ degrees relative to the jet axis is. Presented here are a set of experiments geared towards characterizing the system response of the near field jet shear layer to different modes of forcing. Several open and closed loop control tests were conducted. The open loop control cases included simple sinusoidal forcing (with varying coefficient of momentum and frequency), phased forcing and amplitude modulated forcing. The closed loop cases included feeding back the Fourier filtered signals from 3 diameters and 6 diameters downstream. The correlations between the near field Fourier filtered pressure modes and the far field noise are changed significantly for all control cases; demonstrating the ability to effect control authority in the near field region. An examination of the far field noise spectra, however, demonstrates only minor changes from the control. [Preview Abstract] |
Monday, November 22, 2010 10:56AM - 11:09AM |
HJ.00003: Three Dimensional Vortex Formation of a Finite-Span Synthetic Jet Tyler Van Buren, Michael Amitay Synthetic jets have been a topic for multiple investigations in the field of flow control. Understanding the flow physics associated with these jets is crucial in the development and application of this technology. Synthetic jets are commonly used in boundary layer control, and a large part of that comes from the flow interaction with the vortices created at the synthetic jet orifice. Three dimensional particle image velocimetry experiments have been conducted on the flow field near the orifice of a synthetic jet issued into a quiescent flow with interests in exploring the effects of multiple geometric features (such as throat length, aspect ratio, exit angle, etc.) of a rectangular orifice. [Preview Abstract] |
Monday, November 22, 2010 11:09AM - 11:22AM |
HJ.00004: Eliminating turbulence in spatially intermittent flows Bjoern Hof, Alberto de Lozar, Marc Avila, Tobias Schneider When transferring large quantities of fluid it is energetically far more efficient if the fluid motion is laminar since here friction losses are low. Flows through pipes and channels however are sensitive to minute disturbances even at moderate velocities and in practice most flows are turbulent. We here isolate an amplification mechanism which constantly feeds energy from the mean shear into turbulent eddies. In pipe and channel experiments a simple control strategy is applied to intercept this energy transport at intermediate flow-rates . When activated an immediate collapse of turbulence is observed and the flow re-laminarises. While in experiments this simple method is limited to moderate Reynolds numbers, numerical simulations show that the same principle works at much larger Re. Possible extensions to experiments at higher flow rates are discussed. [Preview Abstract] |
Monday, November 22, 2010 11:22AM - 11:35AM |
HJ.00005: Separation control in a conical diffuser with an annular inlet Kin Pong Lo, Christopher Elkins, John Eaton Conical diffusers are commonly used in turbomachines to slow down the flow and recover pressure. In typical applications such as the diffuser behind a power turbine, the inlet to the diffuser is an annulus. A large central separation bubble forms if the central hub ends abruptly. A long streamlined tail cone can eliminate the separation, but it is often unfeasible for structural reasons. Experiments were performed to investigate various means to manage both the central separation bubble and any separation on the outer diffuser walls. The Reynolds number is 66000 based on the annulus bulk velocity and hydraulic diameter. Full-field, three-component velocity data were measured using magnetic resonance velocimetry. The central separation bubble behind the hub extends the full length of the diffuser in the absence of any control. A Coanda jet at the end of the hub can strongly reduce or completely eliminate the central separation bubble. However this can cause separation from the conical diffuser walls in some cases. A step in the outer diffuser wall acts to fix the location of separation making it more amenable to control. Several control mechanisms for this outer separation bubble are under investigation. [Preview Abstract] |
Monday, November 22, 2010 11:35AM - 11:48AM |
HJ.00006: Experimental Investigation of Actuators for Flow Control in Inlet Ducts John Vaccaro, Yossef Elimelech, Michael Amitay Attractive to aircraft designers are compact inlets, which implement curved flow paths to the compressor face. These curved flow paths could be employed for multiple reasons. One of which is to connect the air intake to the engine embedded in the aircraft body. A compromise must be made between the compactness of the inlet and its aerodynamic performance. The aerodynamic purpose of inlets is to decelerate the oncoming flow before reaching the engine while minimizing total pressure loss, unsteadiness and distortion. Low length-to-diameter ratio inlets have a high degree of curvature, which inevitably causes flow separation and secondary flows. Currently, the length of the propulsion system is constraining the overall size of Unmanned Air Vehicles (UAVs), thus, smaller more efficient aircrafts could be realized if the propulsion system could be shortened. Therefore, active flow control is studied in a compact (L/D=1.5) inlet to improve performance metrics. Actuation from a spanwise varying coanda type ejector actuator and a hybrid coanda type ejector / vortex generator jet actuator is investigated. Special attention will be given to the pressure recovery at the AIP along with unsteady pressure signatures along the inlet surface and at the AIP. [Preview Abstract] |
Monday, November 22, 2010 11:48AM - 12:01PM |
HJ.00007: On the lower bound of net driving power in controlled duct flows Koji Fukagata, Kazuyasu Sugiyama, Nobuhide Kasagi We examine mathematically the lower bound of the net driving power of a controlled flow under a constant flow rate. The net power in a duct with arbitrary cross-section in the presence of the inertial term, blowing/suction from the wall, and arbitrary body forces can be decomposed into four terms: (1) dissipation due to the velocity profile of Stokes flow; (2) dissipation due to deviation of mean velocity from the Stokes flow profile; (3) dissipation due to velocity fluctuations; and (4) correlation between the wall-pressure of Stokes flow and the time-averaged blowing/suction velocity. Among these, the first three terms are shown to be non-negative, while the sign of the fourth term is indefinite. The fourth term vanishes in the cases where the duct has a constant-shape cross-section, such as cylindrical pipes and plane channels. Namely, in such cases, the lower bound of net power is exactly given by the dissipation rate of the Stokes flow at the same flow rate. [Preview Abstract] |
Monday, November 22, 2010 12:01PM - 12:14PM |
HJ.00008: Optimal localized control of the onset of turbulence in a channel flow Rashad Moarref, Binh K. Lieu, Mihailo R. Jovanovic For the problem of controlling the onset of turbulence in a channel flow, we study the design of optimal localized state-feedback controllers. The actuation is generated by blowing and suction at the walls and we assume that (i) the actuators are placed along a two-dimensional lattice of equally spaced points; and that (ii) each actuator uses information from only a limited number of nearby sensors. We utilize recently developed tools for designing structured optimal feedback gains to reduce receptivity of velocity fluctuations to flow disturbances in the presence of control. Our preliminary DNS result, conducted at low Reynolds numbers, show that this approach can indeed maintain the laminar flow. This is in contrast to the localized strategies obtained by spatial truncation of optimal centralized controllers, which may introduce instability and promote transition even in the situations when the uncontrolled flow stays laminar. [Preview Abstract] |
Monday, November 22, 2010 12:14PM - 12:27PM |
HJ.00009: Pumping or drag reduction? J\'er\^ome Hoepffner, Koji Fukagata Two types of wall actuation in channel flow are considered: traveling waves of wall deformation (peristalsis) and traveling waves of blowing and suction. The flow response and its mechanisms are analyzed using nonlinear and weakly nonlinear computations. We show that both actuations induce a flux in the channel in absence of imposed pressure gradient and can thus be characterized as pumping. In the context of flow control, pumping and drag reduction are strongly connected, and we seek to define them properly based on these two actuation examples. [Preview Abstract] |
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