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 D25: Flow Control II: Jets |
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Chair: Onkar Sahni, Rensselaer Polytechnic Institute Room: 320 |
Sunday, November 24, 2013 2:15PM - 2:28PM |
D25.00001: Performance Enhancement of a Vertical Tail Using Synthetic Jet Actuators: Flow Physics Nicholas Rathay, Edward Whalen, Michael Amitay Using aerodynamic flow control techniques, it is possible to reduce the severity or even eliminate the flow separation that occurs over the rudder of a vertical tail on a commercial airplane when it is deflected to high angles. Decreasing the extent of separated flow would result in a larger side force generated by the tail. This could allow for the size of the tail to be reduced, decreasing the overall weight and drag, and potentially creating considerable savings in fuel costs. In this work, wind tunnel experiments were conducted at Rensselaer Polytechnic Institute on a 1/19$^{\mathrm{th}}$ scale model of a vertical tail. It was shown that synthetic jet (zero-net-mass-flux) actuators were capable of decreasing the separated flow over the rudder and increasing the side force. Furthermore, Stereo Particle Image Velocimetry was used to understand the interaction of the synthetic jets with the flow over the rudder. The measurements showed regions of reduced and enhanced velocity (relative to the baseline) in the vicinity of the jet trajectory. These regions were believed to in part be the result of an interaction between the synthetic jets' edge vortices and the crossflow. The data suggested that in this application the synthetic jet flow control system could more effective at augmenting side force if the jet orifices were designed or aligned in such a way to modify the weighted contribution of these vortices. [Preview Abstract] |
Sunday, November 24, 2013 2:28PM - 2:41PM |
D25.00002: Interactions of a finite span synthetic jet with a cross flow Chia Min Leong, Tyler Van Buren, Edward Whalen, Michael Amitay A synthetic jet is a zero-net-mass-flux flow control actuator that produces alternating ejection and suction of fluid momentum across an orifice. It has been used in numerous applications as an active flow control device to improve aerodynamic performance. Though their aerodynamic performance effects are well known, this present study seeks to understand the fluid dynamic effects of synthetic jets. Specifically, the work investigates the interactions of a finite span synthetic jet with a zero-pressure-gradient laminar boundary layer. This study was performed in a small-scale subsonic wind tunnel with an adjustable test section upper wall that was used to generate a zero-pressure-gradient boundary layer. Several finite span rectangular orifices were chosen for this study. Time and phase-averaged Stereoscopic Particle Image Velocimetry (SPIV) measurements were acquired at multiple planes upstream and downstream of the synthetic jet orifice to explore the interaction of the synthetic jet with the cross flow. The effects of the orifice aspect ratio (12, 18, and 24) and blowing ratio (0.5, 1, and 1.5) were investigated. The unsteady vortical structures observed in the near field and the steady structures in the far field are discussed. [Preview Abstract] |
Sunday, November 24, 2013 2:41PM - 2:54PM |
D25.00003: Numerical Investigation of Synthetic-jet based Flow Control on Vertical-axis Wind Turbine Blades Ashwin Menon, Steven Tran, Onkar Sahni Vertical-axis wind turbines encounter large unsteady aerodynamic loads in a sustained fashion due to the continuously varying angle of attack that is experienced by turbine blades during each revolution. Moreover, the detachment of the leading edge vortex at high angles of attack leads to sudden change in aerodynamic loads that result in structural vibrations and fatigue, and possibly failure. This numerical study focuses on using synthetic-jet based fluidic actuation to reduce the unsteady loading on VAWT blades. In the simulations, the jets are placed at the dominant separation location that is observed in the baseline case. We consider different tip-speed ratios, O(2-5), and we also study the effect of blowing ratio (to be in O(0.5-1.5)) and reduced frequency, i.e., ratio of jet frequency to flow frequency (to be in O(5-15)). For all cases, unsteady Reynolds-averaged Navier-Stokes simulations are carried out by using the Spallart-Allamaras turbulence model, where stabilized finite element method is employed for spatial discretization along with an implicit time-integration scheme. [Preview Abstract] |
Sunday, November 24, 2013 2:54PM - 3:07PM |
D25.00004: Active Flow Control Integrated Diffuser for increased Energy Efficiency in Variable Air Volume Systems Hermanus Van Der Schijff, David Menicovich, Jason Vollen, Michael Amitay An experimental investigation was performed to study the application of flow control on an HVAC diffuser using synthetic jets distributed evenly along the diffuser edges. The study was conducted on 1:3 scale typical office space (150 ft$^{2})$, which included a simulated scale HVAC system supplied by compressed air. Two different jet momentum coefficients were investigated for two inlet flow rates of 40 and 60 CFM. The flow field was measured using hot wire anemometry and Particle Image Velocimetry. Current Variable Air Volume HVAC systems vary the incoming airflow to adjust to changing temperature conditions in the conditioned space. However, when the air flow rate drops below ideal, air distribution becomes inefficient. This study demonstrates the effectiveness of synthetic jets at controlling the incoming airflow and the distribution in the room, showing ability to affect throw coefficient parameters for different flow rates within the test chamber. The use of such devices has the potential to improve air quality and air distribution in building while simultaneously lowering energy demands of HVAC systems. [Preview Abstract] |
Sunday, November 24, 2013 3:07PM - 3:20PM |
D25.00005: Open loop control of an axisymmetric turbulent wake using pulsed jet blowing Jonathan Morrison, Anthony Oxlade We investigate the effects of pulsed jet blowing on the turbulent wake of an axisymmetric bullet-shaped body with a sharp trailing edge. The jet is formed from an annular orifice situated immediately below the trailing edge and oriented in the direction of the freestream. By varying the frequency and amplitude of the perturbation, we achieve a mean pressure increase on the base of the body of up to 33\%. Modal decomposition of the base-pressure fluctuations reveals a nonlinear coupling between the symmetric ($m=0$) perturbation and higher order azimuthal modes ($m\pm1,\pm2$) that results in an asymmetric mean pressure distribution. The pressure recovery is shown to be proportional to the strength of the jet vortices and is accompanied by a broadband suppression of energy across all modes with no preferential selection, reaching saturation at approximately 5 times the shear layer frequency. This proportionality is a direct result of reduced coupling between the jet perturbation and both the convective and global wake instabilities. The entrainment interface is examined in detail. [Preview Abstract] |
Sunday, November 24, 2013 3:20PM - 3:33PM |
D25.00006: Flow Interactions of a Finite-span Synthetic Jet near a Wing Tip Joseph Vasile, Michael Amitay An experimental investigation was performed to study the three-dimensional flow structures and interactions of a finite-span synthetic jet located near the tip of a sweptback finite wing (NACA 4421, AR $=$ 4, $\Lambda =$ 30$^{\circ}$) at Re $=$ 10$^{5}$ and at three angles of attack, 0$^{\circ}$, 9$^{\circ}$ and 15.5$^{\circ}$. Three blowing ratios were investigated; Cb $=$ 0.8, 1.2 and 2. Stereoscopic Particle Image Velocimetry data were collected at multiple 2-D planes in the vicinity of the jet's orifice. The effect of the jet's blowing ratio was analyzed using time-averaged and phase-averaged statistics. The study showed that the flow field in the vicinity of the synthetic-jet orifice becomes highly three-dimensional and is governed by the streamwise structures that are associated with the finite span of the jet (edge vortices). Due to the close proximity of the jet to the wing tip, the baseline (i.e., unactuated) flow field is highly three-dimensional with a non-uniform spanwise boundary layer that becomes more pronounced with increasing angle of attack. Consequently, the formation and advection of the secondary flow structures are altered. [Preview Abstract] |
Sunday, November 24, 2013 3:33PM - 3:46PM |
D25.00007: Feedback Control of the Wake of a Three-Dimensional Blunt Bluff Body Thibault Flinois, Aimee Morgans When cars or trucks drive on motorways, more than two thirds of their fuel consumption is due to aerodynamic drag, a significant part of which is caused by the large scale separation that takes place near their trailing edge. We tackle this problem using Large Eddy Simulations and use feedback control of synthetic jets to reduce the losses associated with large-scale structures in the wake. The geometry is a long surface mounted block, whose leading edge is not modelled for computational efficiency and the structure of the unforced flow field around this body is similar to the flow over a surface mounted block or hump. Considering this flow field as a control system, the base pressure force was used as the system output and the input is a slot jet actuator located near the trailing edge. Using open-loop forcing, a form drag reduction of about 7.5\% was obtained. Open-loop system identification also allowed a transfer function that models the system's response to actuation to be found. Finally, a set of feedback controllers were applied to the plant and their performance was analysed. These controllers successfully reduce the fluctuations in the near wake, with only a small control effort. However, more significant mean drag reductions are expected at higher Reynolds numbers. [Preview Abstract] |
Sunday, November 24, 2013 3:46PM - 3:59PM |
D25.00008: Aerodynamic Flow Control of a Moving Axisymmetric Platform Thomas J. Lambert, Bojan Vukasinovic, Ari Glezer Active fluidic control of induced aerodynamic forces and moments on a moving axisymmetric platform is investigated in wind tunnel experiments. Actuation is effected by controlled interactions between an azimuthal array of integrated synthetic jets with the cross flow to induce localized flow attachment domains over the aft end of the model and thereby alter the global aerodynamic forces and moments. The axisymmetric platform is wire-mounted on a 6 DOF traverse such that each of the eight mounting wires is connected to a servo motor with an in-line load cell for monitoring the wire tension. The desired platform motion is controlled in closed-loop by a laboratory computer. The effects of continuous and transitory actuation on the induced aerodynamic forces of the moving platform are investigated in detail using high-speed PIV. The time-dependent changes in the forces are explored for model maneuvering and stabilization. It is found that the actuation induces forces and moments that are on the order of the forces and moments of the baseline flow. These measurements agree with preliminary results on the stabilization of a model moving in a single DOF demonstrating the effectiveness of the actuation for trajectory stabilization. [Preview Abstract] |
Sunday, November 24, 2013 3:59PM - 4:12PM |
D25.00009: Supersonic Jet Noise Reduction Using Flapping Injection and Pulsed Injection Haukur Hafsteinsson, Lars-Erik Eriksson, Daniel Cuppoletti, Ephraim Gutmark Aircraft are in general noisy and there is a high demand for reducing their noise levels. The jet exhaust is in most cases the main noise source of the aircraft, especially for low bypass ratio jet engines. Fluidic injection affecting the shear layer close to the nozzle exit is a promising noise reduction technique as it can be turned of while not needed and thus the negative effect on the engine performance will be minimized. In the presented work, LES is used to compare steady-state mass flow injection with steady-state mass flow flapping jet injection. The work is a direct continuation of a previous LES study on pulsed injection which showed that the pulsed injection induced pressure pulses in the jet which caused increased tonal noise in the downstream directions. The injection system considered in the presented work consists of eight evenly distributed injectors at the nozzle exit plane with a $90^\circ$ injection angle relative to the flow direction. Flapping jet injection is believed to minimize the creation of these pressure pulses since it provides steady-state mass flow. [Preview Abstract] |
Sunday, November 24, 2013 4:12PM - 4:25PM |
D25.00010: Drag reduction in a turbulent channel flow using a passivity-based approach Peter Heins, Bryn Jones, Atul Sharma A new active feedback control strategy for attenuating perturbation energy in a turbulent channel flow is presented. Using a passivity-based approach, a controller synthesis procedure has been devised which is capable of making the linear dynamics of a channel flow as close to passive as is possible given the limitations on sensing and actuation. A controller that is capable of making the linearized flow passive is guaranteed to globally stabilize the true flow. The resulting controller is capable of greatly restricting the amount of turbulent energy that the nonlinearity can feed back into the flow. DNS testing of a controller using wall-sensing of streamwise and spanwise shear stress and actuation via wall transpiration acting upon channel flows with $Re_\tau=100-250$ showed significant reductions in skin-friction drag. [Preview Abstract] |
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