Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session G15: Flow Control: Wakes and Internal Flows |
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Chair: Miki Amitay, Rensselaer Polytechnic Institute Room: Georgia World Congress Center B302 |
Monday, November 19, 2018 10:35AM - 10:48AM |
G15.00001: An Artificial Neural Network trained through Deep Reinforcement Learning achieves Active Flow Control of the 2D Karman Vortex Street behind a Cylinder at moderate Reynolds number Jean Rabault, Nicolas Cerardi, Ulysse Reglade, Miroslav Kuchta, Atle Jensen The Karman vortex street has attracted much attention for over a century. However, it still offers a topic of investigation for flow stability and control. These are critical for industrial applications and outstanding scientific questions. Active flow control remains mostly inaccessible due to the combination of non-linearity, high dimensionality, and time dependence implied by the Navier Stokes equations. Here we report that Artificial Neural Networks trained through Deep Reinforcement Learning can achieve active flow control of the Karman vortex street behind a cylinder in 2D simulations at Re = 100. In particular, the Neural Network manages to reduce drag by around 8% through increasing the size of the recirculation area by 125% while decreasing the pressure drop. This active flow control is achieved through synthetic jets normal to the surface of the cylinder blowing perpendicular to the flow. The jets have very low mass flow rates, in average no more that 0.5% of the incoming mass flow rate intersecting the cylinder diameter. These results show that Deep Reinforcement Learning is a promising tool for attacking the still largely unsolved problem of optimal flow control. |
(Author Not Attending)
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G15.00002: Rear Passive Control of Accelerating Bluff Bodies Manuel Lorite-Díez, José Ignacio Jiménez-González, Candido Gutiérrez-Montes, Carlos Martínez-Bazán We investigate the transient dynamics of the wake induced by a constantly accelerated D-shaped bluff body, starting from rest and reaching a permanent regime. We explore the effect of acceleration and rear geometry: a blunt base, a straight cavity and an optimized, curved cavity, obtained by adjoint optimization. TR-PIV was performed in a towing tank to characterize the near wake development in the early stages. Firstly, the wake is symmetrically developed with a pair of primary eddies attracted toward the body base. Eventually, the interaction between the upper and lower shear layers provokes the flow destabilization and the symmetry breaking, giving rise to a transitional vortex shedding regime. This process is sped-up when the curved cavity is used. In particular, the optimized geometry has been shown to limit the growth of the primary eddies, decreasing the recirculation region and providing with a more regularized transient vortex shedding. Finally, numerical simulations have been performed to evaluate this passive control in terms of drag evolution. Thus, the curved cavity produces a smaller averaged drag, together with less energetic fluctuations, regardless of acceleration. |
Monday, November 19, 2018 11:01AM - 11:14AM |
G15.00003: Flow around a circular cylinder with axially or helically arranged holes Jihee Kim, Jooha Kim In this study, we investigate the characteristics of flow around a circular cylinder with axially or helically arranged holes. The experiment is performed in a wind tunnel at Re = 80,000 while varying the angle of attack (α; the angle between the reference hole center axis and the streamwise direction) from 0° to 90°. We directly measure the drag forces on a circular cylinder with axially or helically penetrated holes and compare them with that on a circular cylinder with no hole. Axially arranged holes reduce the drag at low α but rather increase the drag at high α, as compared to that with no hole. On the other hand, the drag is reduced by helically arranged holes, regardless of the angle of attack. The velocity fields around a circular cylinder with and without holes are measured by particle image velocimetry, and the flow mechanisms responsible for the drag reduction are to be suggested in the presentation. |
Monday, November 19, 2018 11:14AM - 11:27AM |
G15.00004: Control of the Forebody Vortices Over an Inclined Axisymmetric Body Edward Lee, Daniel Heathcote, Bojan Vukasinovic, Ari Glezer The aerodynamic loads on an inclined cylinder are regulated in wind tunnel experiments by controlling the counter-rotating vortices formed near the leading edge of the cylinder’s conical forebody segment and the near-wake at its aft end. Fluidic actuation is effected by independent azimuthal arrays of synthetic jets at the transition sections between the conical forebody and aft segments of the modular cylinder (L/D = 4). The model is supported by a 6-DOF eight-wire traverse, whose motion is controlled by a dedicated servomotor and load cell for each wire. Vortex pairs formed over the forebody by adjusting its azimuthal position and their interactions with the wake of the cylinder are manipulated at pitch angles up to 25° and yaw within ±10o. The effects of these interactions on the global flow field are assessed using stereo PIV measurements in the wake of the cylinder. It is shown that actuation at the forebody juncture can enhance or restore the symmetry of the vortex pair, providing bi-directional control of the aerodynamic side forces, and that actuation at the fore- and aft body junctures alters the axial and normal forces. Spectral analysis of the aerodynamic loads indicate that the actuation stabilizes the model and alters the shedding of vorticity concentrations. |
Monday, November 19, 2018 11:27AM - 11:40AM |
G15.00005: Controlled Dynamics of Streamwise Vorticity Effected in an Offset Diffuser Travis Burrows, Bojan Vukasinovic, Ari Glezer The flow in offset diffusers of modern propulsion systems is dominated by streamwise vorticity concentrations that advect low-momentum fluid from the flow boundaries into the core flow and thereby give rise to flow distortion and losses. As formation of these vortices is strongly coupled to trapped vorticity concentrations within locally-separated flow over diffuser bends, this coupling is exploited for controlling their streamwise evolution and thereby significantly reduce the flow distortion and losses. The effectiveness of fluidic actuation for distortion suppression is demonstrated at Mthroat = 0.64 by implementing actuator arrays at the downstream diffuser turn that leads to over 60% reduction in the average circumferential distortion. Spectral and POD analyses of high-speed, time-resolved total-pressure measurements at the exit plane indicate that these streamwise, counter-rotating vortex pairs are unstable within a narrow frequency band centered about 1 kHz. The actuation alters the spectral content of the pressure fluctuations and leads to broadband suppression that includes the unstable frequency band of the secondary vortices. This is further reflected in suppression of the time-averaged total-pressure distortion and its instantaneous fluctuations. |
Monday, November 19, 2018 11:40AM - 11:53AM |
G15.00006: Modeling of Active Flow Control in an Aggressive Diffuser with Comparison to Experiment Ryan Skinner, Jeremy Gartner, Michael Amitay, Kenneth E Jansen This computational work explores active flow control for separation mitigation in an asymmetric, aggressive diffuser of rectangular cross-section at inlet Mach ∼ 0.4 and Re ∼ 1.3M. Unsteady tangential blowing is used to control separation on the single ramped face. Different methods of modeling the unsteady jet are evaluated, and the corresponding Spalart-Allmaras DDES and RANS simulations are compared to experimental PIV and pressure data. |
Monday, November 19, 2018 11:53AM - 12:06PM |
G15.00007: Abstract Withdrawn
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Monday, November 19, 2018 12:06PM - 12:19PM |
G15.00008: Modal Decomposition Analysis of Controlled Internal Flow Separation Hemanth Sarabu, Curtis Peterson, Bojan Vukasinovic, Ari Glezer The effects of fluidic actuation on the dynamics and evolution of flow separation within a diffuser are investigated using modal decomposition (EMD, POD) of the velocity field. Actuation is effected using a spanwise array of fluidic oscillators placed upstream of the natural separation. The significant effects of the actuation strength (as measured by the flow rate coefficient Cq) on the global streamwise pressure gradient, flow constriction, flow rate and losses within the diffuser are characterized (M = 0.4). The flow dynamics in the immediate vicinity of the unsteady separation are investigated in detail in the absence and presence of actuation using high speed particle image velocimetry (up to 5 kHz) with specific emphasis on the effects of the actuation on the temporal and spatial migration of the local separation front. Local measurements of turbulent kinetic energy indicate that while in the presence of actuation (Cq = 0.8%) the scale and dynamics of the flow surrounding the separation front change drastically, the scaled flow structure remains nearly invariant. In particular, the underlying spectral contents of the flow centered locally about the incipient separation are compared in the absence and presence of actuation using EMD and POD. |
Monday, November 19, 2018 12:19PM - 12:32PM |
G15.00009: Flow Separation in Bends in a 2-D Closed Channel Thomas Samper, Thomas Charles Corke Experiments were conducted in a turbulent 2-D channel flow leading to a range of curved bends that result in an inner-radius flow separation. The ratio of the bend centerline radius to channel height was 1.125. The bend angle was adjustable and ranged from 0 to 150 degrees. The approaching channel flow to the bends was approximately fully developed, and did not vary with the bend angle. The flow conditions were adjusted to maintain a constant centerline velocity to account for the different pressure losses associated with the different bend angles. The flow field within the bend was documented using smoke wire flow visualization, a traversable Pitot probe, and particle image velocimetry over a range of channel Reynolds numbers. The objective was to identify characteristics of flow separation and reattachment, as well as approaches to flow separation control. Flow separation control by both passive and active means are investigated. Active control involves a diaphragm-driven unsteady tangential wall jet, and a pulsed-DC plasma actuator. An optimum frequency was found in each case to minimize the reattachment length. The scaling of the optimum frequency to the separated flow characteristics is presented. |
Monday, November 19, 2018 12:32PM - 12:45PM |
G15.00010: Passive Control of the Counter-Rotating, Vortical Structures on a Backward Facing Ramp Daniel J Simmons, Flint O Thomas, Thomas C Corke The formation of twin, counter-rotating, vortical structures on the surface of an adverse pressure gradient backward facing ramp geometry is a well-known phenomenon. Many groups have looked into delaying separation for the purpose of improving pressure recovery via active or passive flow control. However, very few have looked into directly controlling these structures, which were believed to be caused by the presence of the sidewall boundary layers. This presentation will discuss a series of surface oil film flow visualization experiments conducted on a 0.9 meter chord, backward facing, two-dimensional ramp geometry of 0.91 meter span, complete with an adjustable internal wind tunnel ceiling for streamwise pressure gradient control. A detailed description of the surface topology of the three-dimensional separating flow will be presented along with results from the various passive control schemes, directed at controlling these structures. The results will be used to elucidate the inherent sensitivity of the counter-rotating, vortical surface flow patterns and show that although the sidewall boundary layers may feed into their development and influence their sign of rotation and overall spatial extent, they do not appear to cause their formation. |
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