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 A25: Flow Control I: Coherent Structures and Vortices |
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Chair: Sven Schmitz, Pennsylvania State University Room: 320 |
Sunday, November 24, 2013 8:00AM - 8:13AM |
A25.00001: Large-scale coherent structures in fractal-generated, axisymmetric wakes Jovan Nedic, Outi Supponen, Bharathram Ganapathisubramani, John Christos Vassilicos The coherence and energy of large-scale structures in turbulent axisymmetric wakes are known to play a role on the drag coefficient of the body. Specifically, there is an expectation that drag can be reduced by reducing the energy of the vortex shedding. We use fractal plates which have been shown to have higher drag coefficients than square plates and disks with the same frontal area (Nedic, Ganapathisubramani \& Vassilicos FDR 2013), yet show that the energy of the large-scale vortices shed from these plates is reduced by 15\% to 60\% compared to non-fractal plates. Fractal plates can reduce wake size and alter dissipation scalings [see DFD13-2013-000126] and the relation $C_D = C_VC_{\bar{\epsilon}}$ between the drag coefficient and coefficients of wake volume and average turbulent dissipation rate can be used to explore consequences on drag. Furthermore, the azimuthal mode associated with the vortex shedding ($m=1$) is still found to be dominant for all plates, however its coherence is slightly altered by the fractals, whilst mode $m=2$ has been dramatically altered. [Preview Abstract] |
Sunday, November 24, 2013 8:13AM - 8:26AM |
A25.00002: Plasma Streamwise Vortex Generators in an Adverse Pressure Gradient Christopher Kelley, Thomas Corke, Flint Thomas A wind tunnel experiment was conducted to compare plasma streamwise vortex generators (PSVGs) and passive vortex generators (VGs). These devices were installed on a wing section by which the angle of attack could be used to vary the streamwise pressure gradient. The experiment was performed for freestream Mach numbers 0.1--0.2. Three-dimensional velocity components were measured using a 5-hole Pitot probe in the boundary layer. These measurements were used to quantify the production of streamwise vorticity and the magnitude of the reorientation term from the vorticity transport equation. The effect of Mach number, pressure gradient, operating voltage, and electrode length was then investigated for the PSVGs. The results indicate that the PSVGs could easily outperform the passive VGs and provide a suitable alternative for flow control. [Preview Abstract] |
Sunday, November 24, 2013 8:26AM - 8:39AM |
A25.00003: Suppression of Wake Vortices Using Periodic Cross-Section Variations A. Bouabdallah, H. Oualli, A. Benlahnache, Y. Menad, M. Gad-el-Hak Vortices in the wake of blunt bodies are responsible for significant portion of the drag. An active flow control strategy is designed to inhibit the shedding of such vortex structures. A numerical study is conducted to investigate the effect of periodic cross-section variations on the shed vortices. We use an LES scheme with a Smagorinsky--Lilly subgrid model. The two-dimensional body sinusoidally changes its cross-section from circular to elliptic. The amplitude varies in the range of 5--100\% of the nominal cylinder's diameter, and the oscillation frequency varies in the range of 0.2--10 times the cylinder's natural shedding frequency. The von K\'arm\'an vortex street is most sensitive to the cross-section variations at a Reynolds number of 3,740. At this Re, the boundary layer is subcritical, and the wake is predominately bidimensional. The flow exhibits a cascade of bifurcations identified by the shifting of the shedding mode. When the flow control strategy is optimized, as much as 65\% drag reduction is achieved, which is a direct result of the shedding mechanism inhibition. An experimental validation of this result is forthcoming. [Preview Abstract] |
Sunday, November 24, 2013 8:39AM - 8:52AM |
A25.00004: Effect of the cross sectional aspect ratio on the flow past a twisted cylinder Jae Hwan Jung, Hyun Sik Yoon The cross-flow around twisted cylinders of cross sectional aspect ratio (A/B) from 1 to 2.25 is investigated at a subcritical Reynolds number (Re) of 3000 using large eddy simulation (LES). The flow past a corresponding smooth and wavy cylinder is also calculated for comparison and validation against experimental data. The effect of twisted surface assessed in terms of the mean drag and root-mean-square (RMS) value of fluctuating lift. The shear layer of the twisted cylinder covering the recirculation region is more elongated than those of the smooth and the wavy cylinder. Successively, vortex shedding of the twisted cylinder is considerably suppressed, compared with those of the smooth and the wavy cylinder. The maximum drag reduction of up to 13{\%} compared with a smooth cylinder is obtained at a certain cross sectional aspect ratio. The fluctuating lift coefficient of the twisted cylinder is also significantly suppressed. We found that the cross sectional cross sectional aspect ratio (A/B) plays an essential role in determining the vortical structures behind the twisted cylinder which has a significant effect on the reduction of the fluctuating lift and suppression of flow-induced vibration. [Preview Abstract] |
Sunday, November 24, 2013 8:52AM - 9:05AM |
A25.00005: Multi-Point Velocity Correlations in the Wake of a Three-Dimensional Bluff Body Patrick Shea, Mark Glauser Three-dimensional bluff-bodies known as turrets are commonly used for housing optical systems on airborne platforms. These geometries generate highly turbulent wakes that decrease the performance of the optical systems and the aircraft. The current experimental study used dynamic suction in both open and closed-loop control configurations to actively control the wake turret. The experiments were carried out at a Reynolds number of 5x10$^5$, and the flow field was characterized using stereoscopic PIV measurements acquired in the wake of the turret. These data were processed using traditional single-point statistics which showed that the active control system was able to significantly alter the wake of the turret. Using multi-point correlations, turbulent characteristics such as the integral length scale can be calculated. For the turret wake, estimates of the integral length scales were found to be highly dependent upon the region of the flow that was evaluated, especially when comparing the shear layers to the center of the wake. With the application of the active control, the integral length scales were generally found to increase. [Preview Abstract] |
Sunday, November 24, 2013 9:05AM - 9:18AM |
A25.00006: Dynamic modeling of a turbulent axisymmetric bluff-body wake Georgios Rigas, Aimee Morgans, Jonathan Morrison En route to chaos the stable laminar wake past axisymmetric bluff bodies undergoes two well-documented transitions by increasing the Reynolds number: a steady bifurcation of the $m=1$ azimuthal mode followed by an unsteady bifurcation with $m=\pm1$, the latter giving rise to periodic shedding of vortices with opposite signs, known as vortex shedding. In this study we present experimental evidence that these structures persist far from the critical points at high Reynolds numbers ($Re=2\times10^5$). We show that a low-order model based on the normal form describing the codimension-two bifurcation captures accurately the dynamic behavior of the large-scale coherent structures associated with the destabilized modes, if noise is appropriately accounted for in the model. The model is validated based on simultaneous aerodynamic force measurements on the base of an axisymmetric bullet-shaped body and Time-Resolved Stereo PIV in the near wake. Finally, we extend the model to include external forcing when periodic blowing and suction is applied at the base below the point of separation. [Preview Abstract] |
Sunday, November 24, 2013 9:18AM - 9:31AM |
A25.00007: Suppressing vortex shedding behind a circular cylinder via a tangential standing wave Nansheng Liu, Xiyun Lu Flow over a circular cylinder with a tangential standing wave imposed on the leeward surface has been numerically studied using the lattice Boltzmann method. The modulating effects of the Tangential Standing Wave (TSW) on the vortex shedding and the hydrodynamic forces have been investigated. Three vortical flow regimes in the wake have been identified, namely, Natural Vortex Shedding (NVS) dominated, NVS/TSW competing and TSW dominated regimes. Specifically, in the TSW dominated regime, alternative vortex shedding found behind an uncontrolled circular cylinder is fully suppressed so that elimination of von Karman vortex street occurs; More interestingly, an inverse von Karman vortex street leading to a net thrust is demonstrated under some certain cases of TSW control. [Preview Abstract] |
Sunday, November 24, 2013 9:31AM - 9:44AM |
A25.00008: Spanwise correlation in the wake of circular cylinder and normal plate placed inside a pipe Amit Agrawal, Arumuru Venugopal, S.V. Prabhu The spanwise correlation of a circular cylinder and normal plate placed inside a pipe in fully developed turbulent regime is studied using hotwire anemometer. The present configuration possesses complex fluid structure interaction owing to the following features: high blockage effect, low aspect ratio of the body, upstream turbulence and interaction of axisymmetric flow with a two dimensional bluff body. Three different blockage ratios (0.14, 0.19 and 0.28) are considered. Correlation coefficient was observed to be improved with increase in blockage ratio. Compared to a circular cylinder, a normal plate possesses high correlation length. The near wall effects tend to increase the phase drift, which is reflected in low correlation coefficients. The results show that the simultaneous effect of curvature, low aspect ratio and upstream turbulence reduces the correlation coefficients significantly as compared to unconfined and confined (parallel channel) flows. The three dimensionality of vortex shedding for normal plate with a blockage ratio of 0.28 was observed to be lower compared to circular cylinder and all other blockage ratios. Low frequency modulations were found to be responsible for weak vortex shedding from a circular cylinder compared to a normal plate. [Preview Abstract] |
Sunday, November 24, 2013 9:44AM - 9:57AM |
A25.00009: Experimental Investigation of a Helicopter Rotor Hub Wake David Reich, Brian Elbing, Sven Schmitz A scaled model of a notional helicopter rotor hub was tested in the 48'' Garfield Thomas Water Tunnel at the Applied Research Laboratory Penn State. The main objectives of the experiment were to understand the spatial- and temporal content of the unsteady wake downstream of a rotor hub up to a distance corresponding to the empennage. Primary measurements were the total hub drag and velocity measurements at three nominal downstream locations. Various flow structures were identified and linked to geometric features of the hub model. The most prominent structures were two-per-revolution (hub component: scissors) and four-per-revolution (hub component: main hub arms) vortices shed by the hub. Both the two-per-revolution and four-per-revolution structures persisted far downstream of the hub, but the rate of dissipation was greater for the four-per-rev structures. This work provides a dataset for enhanced understanding of the fundamental physics underlying rotor hub flows and serves as validation data for future CFD analyses. [Preview Abstract] |
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