Bulletin of the American Physical Society
70th Annual Meeting of the APS Division of Fluid Dynamics
Volume 62, Number 14
Sunday–Tuesday, November 19–21, 2017; Denver, Colorado
Session D16: Flow Control: Passive IIControl

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Chair: Mahmoud Hussein, University of Colorado Boulder Room: 603 
Sunday, November 19, 2017 2:15PM  2:28PM 
D16.00001: Passive control of fully developed turbulent flow by subsurface phonons. Mahmoud Hussein, Sedat Biringen, Alan Hsieh, Clemence Bacquet, Mary Bastawrous Flow control is a central problem in fluid dynamics where the goal is to alter a flow's natural state to achieve improved performance, such as delay of laminartoturbulent transition or reduction of drag in a fully developed turbulent flow. Meeting this goal promises to significantly reduce the dependence on fossil fuels for global transport, impacting air and sea vehicles as well as longrange pipelines. In earlier work, we have shown that phonon motion underneath a surface interacting with a flow may be tuned to cause the flow to stabilize, or destabilize, as desired. This concept was demonstrated by simulating a fully developed plane Poiseuille (channel) flow whereby a small portion of an otherwise rigid wall is replaced with a onedimensional phononic crystal. A TollmienSchlichting wave was introduced to the flow as an evolving disturbance. Upon tuning the frequencydependent phase and amplitude relations of the surface of the phononic crystal that interfaces with the flow, an artificially introduced instability was shown to stabilize, or destabilize, as needed. In this work, we demonstrate the applicability of the phononic subsurface paradigm to the suppression of turbulence production events in fully developed turbulent flow. [Preview Abstract] 
Sunday, November 19, 2017 2:28PM  2:41PM 
D16.00002: Lubricant retention in liquidinfused microgrooves exposed to turbulent flow Matthew Fu, TingHsuan Chen, Craig Arnold, Marcus Hultmark Liquid infused surfaces are a promising method of passive drag reduction for turbulent flows. These surfaces rely on functionalized roughness elements to trap a liquid lubricant that is immiscible with external fluids. The presence of the lubricant creates a collection of fluidfluid interfaces which can support a finite slip velocity at the effective surface. Generating a streamwise slip at the surface has been demonstrated as an effective mechanism for drag reduction; however, sustained drag reduction is predicated on the retention of the lubricating layer. Here, a turbulent channelflow facility is used to characterize the robustness of liquidinfused surfaces and evaluate criteria for ensuring retention of the lubricant. Microscale grooved surfaces infused with alkane lubricants are mounted flush in the channel and exposed to turbulent flows. The retention of lubricants and pressure drop are monitored to characterize the effects of surface geometry and lubricant properties. To improve the retention of lubricant within grooved structures, a novel laser patterning technique is used to scribe chemical barriers onto grooved surfaces and evaluated. [Preview Abstract] 
Sunday, November 19, 2017 2:41PM  2:54PM 
D16.00003: Adaptivepassive control of flow over a sphere at high Reynolds numbers Seokbong Chae, Jooha Kim, Jae Hwa Lee Controlling flow over a bluff body by simple shape modification is the most common strategy of passive flow control. It has the advantage that no energy input is required, but has the limitation in that its flowcontrol effectiveness depends on Reynolds number (Re). That is, a fixedsized surface modification that successfully controls flow in a certain Re range does not work for other Reynoldsnumber ranges. In the present study, a new passive control method (AMR: adaptive moving ring) is devised to reduce drag on a sphere with no Reynoldsnumber dependence at high Reynolds numbers of $0.4 \times 10^{5}  4.6 \times 10^{5}$, and tested through windtunnel experiments. AMR uses an elastic spring as both sensor and actuator, and adaptively changes its size depending on the wind speed (i.e. Reynolds number) without power input. We measure the drag on a sphere with AMR and compare it with that of a smooth sphere. The spring constant is manually tuned for optimal performance, showing up to 64\% drag reduction in the tested Reynoldsnumber range. Some more details will be discussed in the presentation. [Preview Abstract] 
Sunday, November 19, 2017 2:54PM  3:07PM 
D16.00004: The effects of interface deformation of superhydrophobic surface on wall turbulence ShaoChing Huang, John Kim Superhydrophobic surfaces have shown the possibility of achieving substantial skinfriction reduction in wallbounded turbulent flows. Under the right conditions, the air pockets stably entrapped within the microgrates create an airwater interface, causing an effective slip to the external liquid flow. When the effective slip length reaches a certain range of viscous length scale of the nearwall flow, significant drag reduction is observed in prior studies. We have performed direct numerical simulations of a turbulent channel flow over superhydrophobic surfaces. A masspreserving immersedboundary method is developed to resolve the gasliquid interface and to handle the mixed slip and noslip surfaces within the Cartesian grids. The computer program is fully parallelized using MPI optimized for execution on supercomputers. Turbulence statistics at various Reynolds numbers obtained from curved interface configurations is compared to those obtained from previous studies using the flat interface assumption. Simulations of a spatially developing boundary layer over superhydrophobic surface will also be presented. [Preview Abstract] 
Sunday, November 19, 2017 3:07PM  3:20PM 
D16.00005: Effects of superhydrophobic surface on the propeller wake Hongseok Choi, Jungjin Lee, Hyungmin Park This study investigates the change in propeller wake when the superhydrophobic surface is applied on the propeller blade. The propeller rotates in a quiescent water tank, facing its bottom, with a rotational Reynolds number of 96000. To measure the threedimensional flow fields, we use stereo PIV and a water prism is installed at the cameraside tank wall. Two cameras are tilted 30 degrees from the normal axis of the tank wall, satisfying schiempflug condition. Superhydrophobic surface is made by coating hydrophobic nanoparticles on the propeller blade. Measurements are done on two vertical planes (at the center of propeller hub and the blade tip), and are ensemble averaged being classified by blade phase of 0 and 90 degrees. Velocity fluctuation, turbulent kinetic energy, and vorticity are evaluated. With superhydrophobic surface, it is found that the turbulence level is significantly ($2030\%$) reduced with a small penalty (less than $5\%$) in the streamwise momentum (i.e., thrust) generation. This is because the cone shaped propeller wake gets narrower and organized vortex structures are broken with the superhydrophobic surfaces. More detailed flow analysis will be given. [Preview Abstract] 
Sunday, November 19, 2017 3:20PM  3:33PM 
D16.00006: Turbulence Enhancement by Fractal Square Grids: Effects of the Number of Fractal Scales Alexis Omilion, Mounir Ibrahim, Wei Zhang Fractal square grids offer a unique solution for passive flow control as they can produce wakes with a distinct turbulence intensity peak and a prolonged turbulence decay region at the expense of only minimal pressure drop. While previous studies have solidified this characteristic of fractal square grids, how the number of scales (or fractal iterations N) affect turbulence production and decay of the induced wake is still not well understood. The focus of this research is to determine the relationship between the fractal iteration N and the turbulence produced in the wake flow using wellcontrolled watertunnel experiments. Particle Image Velocimetry (PIV) is used to measure the instantaneous velocity fields downstream of four different fractal grids with increasing number of scales (N = 1, 2, 3, and 4) and a conventional singlescale grid. By comparing the turbulent scales and statistics of the wake, we are able to determine how each iteration affects the peak turbulence intensity and the production/decay of turbulence from the grid. In light of the ability of these fractal grids to increase turbulence intensity with low pressure drop, this work can potentially benefit a wide variety of applications where energy efficient mixing or convective heat transfer is a key process. [Preview Abstract] 
Sunday, November 19, 2017 3:33PM  3:46PM 
D16.00007: On the origin of the drag force on golf balls Elias Balaras, Nikolaos Beratlis, Kyle Squires It is well establised that dimples accelerate the dragcrisis on a sphere. The result of the early dragcrisis is a reduction of the drag coefficient by more than a factor of two when compared to a smooth sphere at the same Reynolds number. However, when the drag coefficients for smooth and dimpled spheres in the supercritical regime are compared, the latter is higher by a factor of two to three. To understand the origin of this behavior we conducted direct numerical simulations of the flow around a dimpled sphere, which is similar to commercially available golf balls, in the supercritical regime. By comparing the results to those for a smooth sphere it is found that dimples, although effective in accelerating the drag crisis, impose a local dragpenalty, which contributes significantly to the overall drag force. This finding challenges the broadly accepted view, that the dimples only indirectly affect the drag force on a golf ball by manipulating the structure of the turbulent boundary layer near the wall and consequently affect global separation. Within this view, typically the penalty on the drag force imposed by the dimples is assumed to be small and coming primarily from skin friction. The direct numerical simulations we will report reveal a very different picture. [Preview Abstract] 
Sunday, November 19, 2017 3:46PM  3:59PM 
D16.00008: An experimental investigation of the efficacy of perforated holes on unsteady aerodynamic force reduction for a 2D cylinder in uniform incoming flow Vignesh Sudalaimuthu, Xiaofeng Liu A series of wind tunnel aerodynamic force measurements have been conducted on a 2D hollow cylinder with perforated holes uniformlydistributed on its surface to evaluate the efficacy of perforation as a means of passive flow control in reducing unsteady aerodynamic forces. Both smooth and perforated cylinders were tested for comparison at Reynolds numbers ranging from 50,000 to 200,000 corresponding to free stream velocities varying from 5 to 20 m/s (at an increment of 5 m/s) and a cylinder diameter of 0.152 m. The aerodynamic forces acting on the testing model were measured using a 6component load cell. For each tunnel speed, the test has been repeated for 10 runs at a sampling rate of 10 kHz for 60 seconds each, with a total of 6,000,000 samples acquired for each test. Both mean and r.m.s. values of the lift and drag coefficients were calculated. Power spectral density distributions of the unsteady aerodynamic force loading was analyzed to investigate the effect of the perforation on the frequency composition. Comparisons indicate that the perforated cylinder with a 8{\%} porosity and a hole diameter of about 2{\%} of that of the cylinder gives both substantially less unsteady drag and lift than those of the smooth cylinder for the entire Reynolds number range tested, with the r.m.s. force reduction from 8{\%} to 82{\%} for the drag and 64{\%} to 85{\%} for the lift, confirming a corresponding beneficial reduction in flowinduced cylinder vibration as observed during the experiments. [Preview Abstract] 
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