Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session L26: Flow Control: Jets |
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Chair: Robert Martinuzzi, University of Calgary Room: 608 |
Monday, November 25, 2019 1:45PM - 1:58PM |
L26.00001: Influence of the obstacle after-body on actuation effectiveness for wake control. Robert Martinuzzi, Alex Li The dynamic response of the turbulent wake to leading-edge synthetic jet actuation was investigated experimentally for a two-dimensional normal flat plate and square cylinder. The obstacles are immersed in a uniform stream of turbulent intensity less than 0.5{\%}. The actuators are placed directly behind one of the obstacle leading edges. The leeward surface pressure and wake velocity (PIV) measurements are synchronized. For actuation frequencies approaching those of the Kelvin-$=$Helmholtz shear layer instability, mean drag and wake velocity fluctuation levels can be reduced for both geometries. The flat plate wake is generally insensitive to lower frequency actuation. In sharp contrast, for the square cylinder the vortex shedding frequency can lock-on to subharmonics of the actuation frequency. Most striking is that the wake can be manipulated to increase or decrease drag. It will be shown that two different mechanisms underlie the low-frequency response. First, actuation results in a pulsed vortex pair which can interfere with the shear layer to trigger shedding. Second, the after-body can act to alter the vorticity flux along the shear layers and thus modify the strength of the shed vortices. These mechanisms are mitigated when the after body is removed, such as for the flat plate. [Preview Abstract] |
Monday, November 25, 2019 1:58PM - 2:11PM |
L26.00002: The effects of geometrical modifications on flow characteristics of a sweeping jet actuator. Abdul Raouf Tajik, Bartosz Slupski, Vladimir Parezanovic This work studies the effects of modifications of the internal geometry of a sweeping jet (SWJ) actuator, such as the feedback channel diameter (D$_{\mathrm{1}})$ and the size of its exit nozzle (D$_{\mathrm{2}})$. The effects on the jet frequency and the overall pressure drop in the actuator are investigated using time-resolved flow fields from 2D-URANS simulations and local pressure data from experimental measurements. Similarly to [1], it is observed that the variation of the feedback channel diameter D$_{\mathrm{1}}$ has little impact on the sweeping jet frequency magnitude. However, the spectrum of the sweeping jet yields a much better definition of the frequency peaks for larger D$_{\mathrm{1}}$ diameters of the feedback channel. This is a surprising result, considering that the enlarged diameter could be expected to diffuse the feedback flow momentum and thus reduce the quality of the peak definition. On the other hand, the reduction of diameter D$_{\mathrm{2}}$ of the feedback channel exit yields up to 50{\%} increase in the SWJ oscillation frequency for the same pressure drop along the actuator axis. This could be a significant optimization of the SWJ geometry, given that reduced energy investment is required to produce the same oscillation frequency. [Preview Abstract] |
Monday, November 25, 2019 2:11PM - 2:24PM |
L26.00003: Computational Analysis of Turbulence and Interactions in an Array of Fluidic Oscillators for Flow Control N. Koukpaizan, D. Heathcote, C.J. Peterson, B. Vukasinovic, A. Glezer, M. Smith The interactions between a spanwise array of fluidic oscillating jets and a separation bubble that is formed over a 2-D curved surface modeling the suction surface of a VR-12 airfoil are investigated numerically to capture the suppression of the separation with increasing actuation momentum. The high-fidelity simulations first examine the fully-resolved spatio- temporal evolution of the oscillating jets within the actuator cavity in a quiescent ambient. These findings are used to develop a boundary condition model for the actuator array in the cross flow. It is shown that the boundary condition model developed including turbulent variables provides good agreement with the experimental measurements of the flow control effects at a significantly reduced computational cost, and thereby enables additional insight into the flow physics in the vicinity of the actuators’ exit planes where the measurement techniques are limited by adequate access or resolution. The effect of turbulence on the interactions of the oscillating jets with the separated outer flow are also assessed. [Preview Abstract] |
Monday, November 25, 2019 2:24PM - 2:37PM |
L26.00004: Active control of large-scale structures in turbulent boundary layers using wall-normal jets Zhoushun Ruan, Charitha de Silva, Ivan Marusic, Nicholas Hutchins This experimental study uses a spanwise array of nine wall-normal jets to actively perturb large-scale structures in a high Reynolds number turbulent boundary layer. The footprints of passing large-scale structures are sensed by an upstream spanwise array of hot-film sensors, which provides the input to the real-time controller. Various real-time large-scale control strategies can be tested with this facility. Downstream of the jet array, a further sensing array probes the efficacy of the control strategy. A large field of view streamwise / wall-normal plane PIV experiment also provides additional opportunities to investigate the interaction between the jet actuators and the large-scale structures. Results demonstrate that the degree of drag reduction attained is strongly correlated to how effectively large-scale positive fluctuations have been targeted by the control scheme. In addition, there is a strong correlation between the reduction of large-scale energy and the observed drag reduction. Such control effects persist to 2$\delta$ (approximately 28000 wall units) downstream of the control input. Further analysis of the near-wall small scale turbulence reveals that the near-wall cycle has been modified due to the altered footprint from the controlled large-scale events. [Preview Abstract] |
Monday, November 25, 2019 2:37PM - 2:50PM |
L26.00005: Vortex ring bifurcation in a moderate aspect ratio, rectangular orifice synthetic jet Joseph Straccia, John Farnsworth Vortex ring dynamics play an important role in setting the shape, entrainment rate, and near field unsteadiness in synthetic jets. In this study multi-planar stereo particle image velocimetry (SPIV) data obtained for an AR$=$13 rectangular orifice synthetic jet is used to volumetrically reconstruct the coherent structures of the jet. Analysis of this data reveals the axis switching dynamics of the primary vortex ring in addition to identifying several types of related secondary structures. Most significant however is the confirmation that the vortex rings undergo a viscous interaction called vorticity reconnection which causes the vortex rings, and the momentum of the jet, to bifurcate. The structure of the vortex ring during and after the vorticity reconnection event is reported at different phases of the jet and compared with studies of isolated vortex rings. To understand how this phenomenon is affected by the unique environment inside of the synthetic jet the Reynolds and Strouhal numbers of the jet were varied independently and centerline SPIV data was acquired. From these results conclusions are drawn regarding which conditions allow for more complete vortex ring bifurcation. [Preview Abstract] |
Monday, November 25, 2019 2:50PM - 3:03PM |
L26.00006: Synthetic jet-based control of wing tip vortices. Carlo Salvatore Greco, Andrea Piccolo, Mirko Zaccara, Gerardo Paolillo, Tommaso Astarita, Gennaro Cardone Wing tip vortices are coherent structures generated by the roll-up process developing in the wake of a finite lifting wing. Currently, winglets are the unique passive control device employed in the general aviation to weaken these undesired vortices, reducing their related problems, although active control devices (ACD) are studied in the literature. ACD try to weaken the wing tip vortices by triggering their inherent instabilities. Although these instabilities are caused by the mutual induction of the vortices, all the experimental works in this field employ only half-wing models and report only time-averaged results. The purpose of the present experiment is testing the effects of rectangular synthetic jets, placed at the wing tips and actuated at the wing-tip vortex instability frequencies, on the wing tip vortices generated by a finite span airfoil placed at the exit of an open jet wind tunnel. Phase-locked Stereo-PIV measurements of the flow field in the wing extended near-wake are carried out. The main observed effects of the control are the reduction of the wing tip vortex vorticity and induced rotational velocity. [Preview Abstract] |
Monday, November 25, 2019 3:03PM - 3:16PM |
L26.00007: Constraints of scaling synthetic jet trajectories in crossflow Girish Jankee, Bharathram Ganapathisubramani Synthetic jet actuators remain coveted components in flow control applications as the convection of vortex rings allows distribution of momentum in a boundary layer although the net mass flux remains zero. The ability to predict the trajectory of these vortex rings is critical for efficient and targeted usage of such actuators. In this investigation, a synthetic jet is issued into a crossflow from rectangular orifices with aspect ratios 3, 6 and 12 over a range of operating frequencies and blowing ratios, with the flowfield being captured through PIV measurements. An assessment of the trajectories culminates in scaling characteristics which encapsulate the aspect ratio, momentum ratio and the Strouhal number. We observe synthetic jets to follow identical trajectories provided the total momentum ratio between the jet and the crossflow remain the same, irrespective of the actuation frequency. However, the universality of such scaling is subjected to certain constraints with a lower bound defined by the jet formation criterion and an upper bound related to successive vortex pair interactions. [Preview Abstract] |
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