Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session H7: Flow Control: Vortices, Coherent Structures and Surfaces |
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Chair: Robert Breidenthal, University of Washington Room: B115 |
Monday, November 21, 2016 10:40AM - 10:53AM |
H7.00001: Active Control of Stationary Vortices. Giovanni Nino, Robert Breidenthal, Aditi Bhide, Aditya Sridhar A system for active stationary vortex control is presented. The system uses a combination of plasma actuators, pressure sensors and electrical circuits deposited on aerodynamic surfaces using printing electronics methods. Once the pressure sensors sense a change on the intensity or on the position of the stationary vortices, its associated controller activates a set of plasma actuator to return the vortices to their original or intended positions. The forces produced by the actuators act on the secondary flow in the transverse plane, where velocities are much less than in the streamwise direction. As a demonstration case, the active vortex control system is mounted on a flat plate under low speed wind tunnel testing. Here, a set of vortex generators are used to generate the stationary vortices and the plasma actuators are used to move them. Preliminary results from the experiments are presented and compared with theoretical values. [Preview Abstract] |
Monday, November 21, 2016 10:53AM - 11:06AM |
H7.00002: Modifying formation and merging of shear-layer vortices using local periodic heating Chi-An Yeh, Phillip Munday, Kunihiko Taira The flow physics of a thermally forced shear layer downstream of a finite-thickness splitter plate is examined with 2D compressible DNS. Unsteady forcing is introduced at the tip of the plate with an oscillatory heat flux boundary condition. We observe that the forcing can introduce small-level oscillatory surface vorticity flux and generates volumetric baroclinic vorticity at the actuation frequency in the vicinity of the tip, which in turn is able to modify the vortex dynamics of the shear layer downstream. When using forcing frequency near the first subharmonic of the baseline flow, the strength of each roll-up vortex appears to have greater fluctuation, with its mean remaining unchanged from that of the baseline. The fluctuation added to each vortex leads to a wider shear layer by either encouraging the vortex deviating from the centerline while convecting downstream, or encouraging the merging process to take place earlier upstream. When forcing excites the roll-up and lock the roll-up frequency onto that of actuation, the mean strength of the vortices can be accordingly modified by controlling the amount of vorticity fed into each formed vortex. Consequently, the modified strength alters the shear layer thickness while vortices convecting along the centerline. Steady forcin [Preview Abstract] |
Monday, November 21, 2016 11:06AM - 11:19AM |
H7.00003: Controlled Flow Distortion in an Offset Diffuser using Hybrid Trapped Vorticity T.J. Burrows, B. Vukasinovic, A. Glezer Trapped vorticity concentration engendered by deliberate modification of the internal surface of an offset diffuser is coupled with a spanwise array of surface-integrated fluidic-oscillating jets for hybrid flow control of streamwise vorticity concentrations that dominate the base flow and give rise to flow distortions at the engine inlet. 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 $=$ 0.7, using surface oil-flow visualization and pressure distributions, and particle image velocimetry. It is shown that two sources of streamwise vorticity dominate the base flow distortion, namely, corner and a central pair of counter-rotating vortices. The present investigations demonstrate that the actuation affects the topology, strength and scale of the trapped vorticity and thereby its coupling to and interaction with the counter rotating streamwise vortices, where the central vortex pair becomes fully suppressed. As a result, the actuation significantly alters the evolution of the flow within the diffuser, and leads to significant suppression of pressure distortion at the engine inlet (by about 80{\%}) at actuation level that is less than 0.7{\%} of the diffuser's mass flow rate. These findings indicate the utility of hybrid trapped vorticity actuation for mitigating adverse effects of secondary vorticity concentrations formed by local separation and corner flows. [Preview Abstract] |
Monday, November 21, 2016 11:19AM - 11:32AM |
H7.00004: Analysis of vortical structure over sinusoidal riblet surface in turbulent channel flow by means of Dual-plane stereoscopic PIV measurement Hiroya Mamori, Kyotaro Yamaguchi, Monami Sasamori, Kaoru Iwamoto, Akira Murata We perform a dual-plane stereoscopic particle image velocimetry (DPS-PIV) measurement to investigate vortical structure over a sinusoidal riblet surface in the turbulent channel flow. In the sinusoidal riblet surface, its lateral spacing of the adjacent walls varies in the streamwise direction and $12$\% of the drag reduction rate has been confirmed in the turbulent channel flow. The DPS-PIV measurement system consists of four high-speed CCD cameras and the two laser sheets. In the flat case, the profile of the velocity statistics shows a good agreement with previous data. In the ribet case, the velocity statistics decrease in the region close to the wall as compared with that of the flat case. Since all velocity components are measured on adjacent laser sheets simultaneously, vortical structures can be obtained by a second invariant of the tensor i.e. the $Q$ value. According to an analysis for the $Q$ value, we found that the vortical structure is shifted up and attenuated owing to the riblet. Moreover, the riblet prevents the approaching of the vortical structure: the upward and downward flows in the region near the wall are generated by the riblet; if the vortical structure approaches the wall, it is shifted away from the wall due to the upward flow. [Preview Abstract] |
Monday, November 21, 2016 11:32AM - 11:45AM |
H7.00005: Vortex-Induced Vibration of a Circular Cylinder Fitted with a Single Spanwise Tripwire Ehsan Vaziri, Alis Ekmekci A spanwise tripwire can be used to alter the coherence and strength of the vortex shedding from cylindrical structures. While this has been well-documented for cylinders in stationary state, there exists a lack of understanding regarding the control induced by spanwise tripwires for cylinders undergoing vortex-induced vibration (VIV). The current experimental research investigates the consequences of spanwise tripping on VIV of a cylinder. Experiments are conducted in a recirculating water tunnel at a Reynolds number of 10,000. The test setup allows the rigid test cylinder to have one-degree-of-freedom vibration in the cross-flow direction as a result of fluid forcing. To measure the cylinder motion, a high-resolution laser displacement sensor is used. The tripwire diameter to cylinder diameter ratio is fixed at 6.1{\%}. Various angular positions of tripwire are studied ranging from 40 to 90 degrees. It is shown that the tripwire location controls the pattern, amplitude, frequency, and mid-position of oscillations significantly. Different oscillation modes are classified based on the observed oscillation pattern, amplitude and frequency. Oscillation amplitude can be reduced by 61{\%} with respect to the amplitude of a clean cylinder undergoing VIV under the same flow condition. [Preview Abstract] |
Monday, November 21, 2016 11:45AM - 11:58AM |
H7.00006: A single bubble in a turbulent channel flow: Towards understanding drag reduction Raghuraman N. Govardhan, Narsing Kumar Jha Two phase turbulent flows are ubiquitous in natural systems such as in clouds and oceans, besides in the chemical process industry. There is also interest in such flows from the drag reduction perspective of marine vehicles through the injection of bubbles into the boundary layer. In these flows, the bubbles interact with the turbulent structures present in the flow resulting in complex bubble paths and modification of the structures, the physical mechanisms of which are not well understood. In the present work, we experimentally study the interaction of a single air bubble with turbulent structures in a fully developed horizontal turbulent channel flow. The bubble path is tracked using high speed imaging in two perpendicular planes, while the vortical structures are tracked using time resolved particle image velocimetry (PIV). We observe different bubble paths even at the same incoming flow Reynolds number and bubble size, which is likely related to the interaction of the bubble with the incoming turbulent structures. The effect of bubble size on the bubble motion trajectories and its relation to structures will be presented in the meeting. This simplified study of interaction of a single bubble can help in understanding more complex interaction of multiple bubbles with multi-scale turbulent flows. [Preview Abstract] |
Monday, November 21, 2016 11:58AM - 12:11PM |
H7.00007: Solution to Shape Identification of Unsteady Natural Convection Fields to Control Temperature Distribution Eiji Katamine, Shinya Imai This paper presents a numerical solution to shape identification of unsteady natural convection fields to control temperature to a prescribed distribution. The square error integral between the actual temperature distributions and the prescribed temperature distributions on the prescribed sub-boundaries during the specified period of time is used as the objective functional. Shape gradient of the shape identification problem is derived theoretically using the Lagrange multiplier method, adjoint variable method, and the formulae of the material derivative. Reshaping is carried out by the traction method proposed as an approach to solving shape optimization problems. Numerical analyses program for the shape identification is developed based on FreeFem++, and the validity of proposed method is confirmed by results of 2D numerical analyses. [Preview Abstract] |
Monday, November 21, 2016 12:11PM - 12:24PM |
H7.00008: Experimental study of turbulent structures over hairy poro-elastic surfaces Marie Couliou, Jonas Hansson, Wouter van der Wijngaart, Fredrik Lundell, Shervin Bagheri Flows over slender, deformable and dense structures are ubiquitous in both nature and technological applications, ranging from the atmospheric flow over trees to the flow over the over the skin of organisms. In order to create a fundamental understanding of how poro-elatic surface can be used for flow control purposes, our work focuses on the behaviour of wall-bounded turbulent flows over fibrous poro-elastic surfaces. We fabricate the coatings using Off-Stoichiometry-Thiolene-Epoxy (OSTE+) polymers and multidirectional UV-lithography which enables us to design arrays of flexible pillars with various geometrical parameters (aspect ratio, pitch, inclinaison, etc.). We assess the effects of these coatings on an overlying low-Reynolds number turbulent flow using a water-table facility and PIV measurements. In particular, we focus on the modification of near wall turbulent structures in both space and time due to the presence of the poro-elastic coatings. [Preview Abstract] |
Monday, November 21, 2016 12:24PM - 12:37PM |
H7.00009: Manipulation of flow around bluff bodies by flexible slender filaments mohammad omidyeganeh, Alfredo Pinelli Manipulation of bluff bodies wakes to control the intensity of fluid forces and the induced solid vibrations is of paramount importance. A biomimetic passive control based on the use of flexible slender appendages protruding from the body into the separated region has shown promising achievements in drag reduction and moderating force fluctuations. The present research aimed at understating and optimizing the physical properties and the arrangement of elongated flexible filaments to delay the 3D transition of the wake in terms of Reynolds number, mean drag reduction, and mitigation of the force fluctuations. The numerical campaign unveiled the role of flexural stiffness of the filaments: matching the natural frequency with the vortex shedding frequency enhances the mixing at the lee side. However, softer filaments (i.e. larger time scales) lock-in on either side of mid plane breaking the symmetry of the flow field (inducing a net lift force). In addition to 2D effects, the presence of filaments can interfere with the 3D bifurcation process resulting in a delay of the spanwise destabilization of the wake. The most effective parameter for this transitional interference is the spacing between filaments that should be smaller than the wavelength of the dominant 3D unstable mode. [Preview Abstract] |
Monday, November 21, 2016 12:37PM - 12:50PM |
H7.00010: Control of Flow Structure on Non-Slender Delta Wing: Bio-inspired Edge Modifications, Passive Bleeding, and Pulsed Blowing Mehmet Metin Yavuz, Alper Celik, Cenk Cetin In the present study, different flow control approaches including bio-inspired edge modifications, passive bleeding, and pulsed blowing are introduced and applied for the flow over non-slender delta wing. Experiments are conducted in a low speed wind tunnel for a 45 degree swept delta wing using qualitative and quantitative measurement techniques including laser illuminated smoke visualization, particle image velocimety (PIV), and surface pressure measurements. For the bio-inspired edge modifications, the edges of the wing are modified to dolphin fluke geometry. In addition, the concept of flexion ratio, a ratio depending on the flexible length of animal propulsors such as wings, is introduced. For passive bleeding, directing the free stream air from the pressure side of the planform to the suction side of the wing is applied. For pulsed blowing, periodic air injection through the leading edge of the wing is performed in a square waveform with 25{\%} duty cycle at different excitation frequencies and compared with the steady and no blowing cases. The results indicate that each control approach is quite effective in terms of altering the overall flow structure on the planform. However, the success level, considering the elimination of stall or delaying the vortex breakdown, depends on the parameters in each method. [Preview Abstract] |
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