Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session A17: Flow Control: Separation |
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Chair: Ari Glezer, Georgia Tech Room: 205 |
Sunday, November 22, 2015 8:00AM - 8:13AM |
A17.00001: Characterization and Control of Separated Entrance Flow in a Branched Channel C.J. Peterson, B. Vukasinovic, A. Glezer The evolution of the flow downstream of the inlet of a rectangular channel that is branched along the entire span of the side wall of a primary channel of the same height is investigated experimentally in an air facility. Of particular interest is the formation and scaling of a separated flow domain downstream of the entrance plane into the secondary channel and its interaction with the flow surfaces at speeds up to $M = 0.4$. The separation is actively controlled using a spanwise array of fluidic actuators on the primary channel's surface upstream of the inlet plane of the secondary duct. The effects of the actuation on the evolution of the separation and attachment of the vorticity layer between upstream surface of the primary duct and the surface of the secondary duct downstream of the branched inlet in the presence of a strong confined adverse pressure gradient are investigated using particle image velocimetry coupled with detailed static surface pressure distributions. The effects of the controlled separation within the secondary channel on the global flow within the primary duct and on flow split between primary and secondary channels are assessed, and it is demonstrated that actuation can effect significant changes in the flow fractions between the channels. [Preview Abstract] |
Sunday, November 22, 2015 8:13AM - 8:26AM |
A17.00002: Aerodynamic Control using Distributed Active Bleed John Kearney, Ari Glezer The global aerodynamic loads on a stationary and pitching airfoil at angles of attack beyond the static and dynamic stall margins, respectively are controlled in wind tunnel experiments using regulated distributed bleed driven by surface pressure differences. High-speed PIV and proper orthogonal decomposition of the vorticity flux on the static airfoil show that the bleed engenders trains of discrete vortices that advect along the surface and are associated with a local instability that is manifested by a time-averaged bifurcation of the vorticity layer near the bleed outlets and alters the vorticity flux over the airfoil and thereby the aerodynamic loads. Active bleed is used on a dynamically pitching airfoil (at reduced frequencies up to k $=$ 0.42) to modulate the evolution of vorticity concentrations during dynamic stall. Time-periodic bleed improved the pitch stability by reducing adverse pitching moment (``negative damping'') that can precipitate structural instabilities. At the same time, the maintains the cycle-average loads to within 5{\%} of the base flow levels by segmenting the vorticity layer during upstroke and promoting early flow attachment during downstroke segments of the pitch cycle. [Preview Abstract] |
Sunday, November 22, 2015 8:26AM - 8:39AM |
A17.00003: Understanding the Response of Separated Flow over an Airfoil under a Single-pulse Actuation Andre Fernando de Castro da Silva, Tim Colonius Experiments have shown that short-duration pulses of actuation near the leading edge of an airfoil at high angle of attack produce a lift response that consists of an initial lift reversal followed by a larger lift increment that decays over about 10 convective time units. To investigate the physical mechanisms that lead to the observed forces, we consider a simple model of two-dimensional flow over a NACA 0009 airfoil at moderate Reynolds number. We model actuation as a momentum source that imposes a specified velocity in a small region near the leading edge. The actuation parameters are varied to determine how the instantaneous and phase-averaged lift scale with the strength and duration of actuation. The results are compared with instantaneous and phase-averaged PIV data from the experiments, and the flow structures responsible for the lift response are identified. [Preview Abstract] |
Sunday, November 22, 2015 8:39AM - 8:52AM |
A17.00004: Control of Trapped Vorticity in an Offset Diffuser Travis J. Burrows, Bojan Vukasinovic, Ari Glezer Vorticity concentrations trapped within in a recessed section in the moldline of an offset diffuser are manipulated using fluidic actuation to alter the flow evolution within the diffuser. Trapped vorticity is engendered by deliberate local flow separation owing to the aggressive moldline curvature. The strength and scale of the trapped vortex and its interaction with the cross flow are controlled by a spanwise array of streamwise, surface-integrated fluidic actuators that are placed just upstream of the recessed moldline. The local and global characteristics of the diffuser flow in the absence and presence of the actuation are investigated at Mach numbers up to $M \quad =$ 0.7, using static pressure distributions, hot-wire anemometry, and particle image velocimetry. It is shown that flow distortion as measured by cross sectional variations of the total pressure distribution within the diffuser can be significantly modified by manipulation of the trapped vorticity, and is reduced (by over 50{\%}) with increasing momentum of the actuation jets. The mitigation of flow distortion by trapped vorticity actuation is associated with manipulation of the evolution of streamwise secondary vortices within the diffuser. [Preview Abstract] |
Sunday, November 22, 2015 8:52AM - 9:05AM |
A17.00005: Aerodynamic Control of a Dynamically Pitching Airfoil using Transitory Pulsed Actuation Yuehan Tan, Thomas Crittenden, Ari Glezer Transitory control and regulation of trapped vorticity concentrations are exploited for control of the aerodynamic loads on an airfoil that is dynamically pitching beyond the dynamic stall margin in wind tunnel experiments. Actuation is effected using a spanwise array of integrated miniature chemical (combustion based) high impulse actuators that are triggered intermittently relative to the airfoil's motion on characteristic time scales that are an order of magnitude shorter than the airfoil's convective time scale. The effects of the actuation on the aerodynamic characteristics of the airfoil are assessed using time-dependent measurements of the lift force and pitching moment coupled with time-resolved particle image velocimetry that is acquired phased-locked to the motion of the airfoil. The aerodynamic loading can be significantly altered by a number of actuation programs using multiple actuation bursts during the pitch cycle. While actuation during the upstroke primarily affects the formation, evolution, and advection of the dynamic stall vortex, actuation during the downstroke accelerates flow attachment. Superposition of such actuation programs leads to enhancement of cycle lift and pitch stability, and reduced cycle hysteresis. [Preview Abstract] |
Sunday, November 22, 2015 9:05AM - 9:18AM |
A17.00006: Fast-flap Actuation for Attenuating Gust Response Michael OL, Kenneth Granlund`, Albert Medina Airfoil flow control actuators can respond at perhaps O(10E-3) convective-times, but the flowfield response requires typically 2-4 convective times, and initial force-transients can be negative. A conventional trailing-edge mechanical flap is ``slow'' to deflect in flight applications, of questionable efficacy in separated flows, and is plagued by a response nonlinear with deflection angle. We consider a half-chord airfoil flap actuated O(10) times faster than one convective time, taking advantage of scaling-effects in a water tunnel. The motivation is recent work on accelerating flat plates at high incidence, where despite zero bound-circulation, the lift transient follows Wagner's solution. Force-measurements for high-rate large-incidence flap deflection show similar trends, and offer promise in cancelling lift-transients from gusts (modeled by plunging or surging the airfoil). Parameter-studies of rate, amplitude and initial incidence suggest first-order-system relaxation to steady-state, with a time constant commensurate with 1-3 convective times, no negative transients and no discernible lag. Rapid flap actuation induces entrainment that augments the flow's propensity to attach/reattach, perhaps paradoxically comporting with theory better, the faster the actuation. [Preview Abstract] |
Sunday, November 22, 2015 9:18AM - 9:31AM |
A17.00007: Control of flow separation in a turbulent boundary layer Minjeong Cho, Sangho Choi, Haecheon Choi Towards the development of successful control methods for separation delay in a turbulent boundary layer, we adopt a model flow field, in which a turbulent separation occurs above a flat plate (Na and Moin 1998 JFM), and apply controls to this flow for reducing the size of the separation bubble and investigating the interaction between the forcing and flow near the separation bubble. We provide a single-frequency forcing with zero net mass flow rate at the upstream of the separation bubble. At low forcing frequencies, spanwise vortices are generated and travel downstream, bringing high momentum toward the wall and reducing the size of the separation bubble. Also, these vortices cause the separation and reattachment points to travel downstream. On the other hand, at high forcing frequencies, the size of the separation bubble becomes smaller and larger in time, respectively, due to the pressure gradient alternating favorably and adversely in time. [Preview Abstract] |
Sunday, November 22, 2015 9:31AM - 9:44AM |
A17.00008: Field Test Results from a 10 kW Wind Turbine with Active Flow Control Thomas Rice, Veronika Bychkova, Keith Taylor, Dan Clingman, Michael Amitay Active flow control devices including synthetic jets and dynamic vortex generators were tested on a 10 kW wind turbine at RPI. Previous work has shown that load oscillations caused by dynamic stall could be modified through the use of active flow control by injecting momentum into the flow field near the leading edge of a dynamically pitching model. In this study, this work has been extended to its logical conclusion, field-testing active flow control on a real wind turbine. The blades in the current study have a 0.28m chord and 3.05m span, no twist or taper, and were retrofitted with six synthetic jets on one blade and ten dynamic vortex generators on a second blade. The third blade of this turbine was not modified, in order to serve as a control. Strain gauges were installed on each blade to measure blades' deflection. A simple closed loop control was demonstrated and preliminary results indicate reduced vibrational amplitude. Future testing will be conducted on a larger scale, 600kW machine at NREL, incorporating information collected during this study. [Preview Abstract] |
Sunday, November 22, 2015 9:44AM - 9:57AM |
A17.00009: Performance Comparison of Sweeping/Steady Jet Actuators Damian Hirsch, Justin Mercier, Flavio Noca, Morteza Gharib Flow control through the use of steady jet actuators has been used on various aircraft models since the late 1950's. However, the focus of recent studies has shifted towards the use of sweeping jets (fluidic oscillators) rather than steady jet actuators. In this work, experiments using various jet actuator designs were conducted at GALCIT's Lucas Wind Tunnel on a NACA 0012 vertical tail model similar to that of the Boeing 767 vertical stabilizer at Reynolds numbers ranging from 0.5 to 1.2 million. The rudder angle was fixed at 20 degrees. A total of 32 jet actuators were installed along the wingspan perpendicular to the trailing edge and the rudder shoulder of the vertical stabilizer. It is known that these types of flow control prevent separation. However, the goal of this work is to compare different jet designs and evaluate their performance. Parameters such as the number of actuators, their volumetric flow, and the wind tunnel speed were varied. The lift generation capabilities of steady and sweeping jet actuators were then compared. Another set of experiments was conducted to compare a new sweeping jet actuator design with one of the standard versions. [Preview Abstract] |
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