Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session H20: Flow Control: Drag Reduction |
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Chair: Jens Fransson, KTH Royal Institute of Technology Room: 2008 |
Monday, November 24, 2014 10:30AM - 10:43AM |
H20.00001: Cylinder Drag Reduction Using Cross-Section Variation From Circle to Ellipse A. Bouabdallah, H. Oualli, M. Mekadem, H. Chetitah, C. Boulahbal, 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. Last year, we presented a numerical study to investigate the effect of periodic cross-section variations on the shed vortices. We extend the research to experiment using a cylinder with finite length. The controlling frequency range is extended up to 40 times the natural shedding frequency $f_0$. Amplitude and frequency modulations are the key parameters directly affecting the efficiency of the system and the topology of the flow, which permanently adjusts in response to the superimposed pulsatile motion exhibiting a cascade of bifurcations accompanied by shedding modes shifting from the natural mode 2S to 2P, 2T, and 2C. Optimal operational conditions are identified and the results show that drag drastically drops to zero then negative, i.e. thrust, with complete suppression of the vortex shedding. Von K\'arm\'an vortices are no longer shed, but rather pulled out in small-scale, weakened vorticity packets released from the lateral cylinder wall. For a deforming amplitude of 100\% and an exciting frequency of $20f_0$, the negative drag reaches 14 times its value for an uncontrolled cylinder. [Preview Abstract] |
Monday, November 24, 2014 10:43AM - 10:56AM |
H20.00002: Disc actuators for turbulent drag reduction Daniel J. Wise, Claudia Alvarenga, Pierre Ricco The response of a turbulent channel flow to flush-mounted steadily rotating discs is investigated numerically. The effect on drag reduction of the discs arrangement is studied at a Reynolds number of $R_b$=5600, based on the bulk velocity and channel height. The flow exhibits complex dependence on the positioning of the discs. For low disc-tip velocity the drag reduction scales linearly with the percentage actuated area, whereas for higher tip velocity the drag reduction may be higher than predicted from this coverage scaling. Therefore by halving the number of discs increased drag reduction can be found. The departure from linear scaling is found to relate to the presence of stationary-wall regions upstream of discs. This is explained by the impingement of the disc boundary layer onto the areas of unactuated wall. For some arrangements tubular, streamwise-elongated structures occur between discs. The criteria for their creation are elucidated through the employment of the Fukagata-Iwamoto-Kasagi identity and flow visualisations. Improvements in the performance of the disc actuators are found with inspiration from the laminar solution to the disc flow. Through the introduction of novel half-disc and annular configurations, a maximum drag reduction of 26\% is obtained. [Preview Abstract] |
Monday, November 24, 2014 10:56AM - 11:09AM |
H20.00003: ABSTRACT WITHDRAWN |
Monday, November 24, 2014 11:09AM - 11:22AM |
H20.00004: Utilization of transient growth disturbances for drag reduction in boundary layers Jens H.M. Fransson Over the last decade wind tunnel experiments\footnote{Fransson et al. PRL {\bf{96}}, 064501, 2006.}$^{,}$\footnote{Shahinfar et al. PRL {\bf{109}}, 074501, 2012.} have shown that steady streamwise elongated streaks, produced by the lift-up mechanism, are able to reduce skin-friction drag by delaying transition to turbulence in flat plate boundary layers. Steady streaks may be generated by passive devices such as circular roughness elements or miniature vortex generators (MVGs), the latter being the more effective device. The optimal streak amplitude to accomplish the stabilizing boundary-layer effect is around 30\% of the free-stream velocity (considering an integrated amplitude definition). On the basis of a parametrical study, by varying boundary layer as well as geometrical parameters of the MVGs, a streak amplitude scaling founded on empiricism has been proposed, which is necessary when applying the control strategy in new flow configurations. Different types of disturbances have successfully been damped and the possibility of extending the laminar boundary layer even further by mounting a second array of MVGs downstream of the first one has been accomplished. A review of the AFRODITE program results and future work will be presented. [Preview Abstract] |
Monday, November 24, 2014 11:22AM - 11:35AM |
H20.00005: Direct numerical simulation of a compressible turbulent channel flow with uniform blowing and suction through isothermal walls Yukinori Kametani, Koji Fukagata High-speed transports such as aircrafts and bullet trains support human activity in the modern society. In such applications, the turbulent friction drag is the major contributor to the energy loss. Kametani and Fukagata (J. Fluid Mech., 2011) investigated by means of direct numerical simulation (DNS) the drag reduction effect by blowing and the turbulence stabilization effect by suction in an incompressible spatially developing turbulent boundary layer, and quantitatively discussed different contributions to those effects. In this study, DNS of a compressible turbulent channel with uniform blowing and suction through the isothermal walls is performed. The Reynolds number based on the bulk mass flow rate, the viscosity on the wall and the channel half width is set to be 3000. The bulk Mach number is set to be 0.8 and 1.5 to compare the results in subsonic and supersonic cases. The drag reduction (enhancement) effect was confirmed on the blowing (suction) wall. As the Mach number increases, however, the control efficiency of blowing is found to be deteriorated because of the increased density near the wall. [Preview Abstract] |
Monday, November 24, 2014 11:35AM - 11:48AM |
H20.00006: The impact of superhydrophobic surface texture on channel-flow turbulence and drag Thomas Jelly, Seo Yoon Jung, Tamer Zaki Fully-developed turbulent channel flow past streamwise-aligned superhydrophobic surface (SHS) textures is simulated at a fixed bulk Reynolds number, $Re = 2{,}800$ (Jelly et al., Phys. Fluids, 2014). The influence of the spanwise-repeated surface pattern is examined using phase-averaged statistics of the flow which is decomposed into mean, periodic and stochastic motions. Relative to a reference no-slip channel flow, the mean skin-friction coefficient is reduced by $21.6\%$. At particular phases, however, the skin friction far exceeds the reference value. The contributions to drag are examined and are attributed to changes in the primary flow, the presence of a secondary flow of Prandtl's second type, and changes in the Reynolds stresses. Each of these contributions is quantified, and the largest performance penalties are examined in detail. Finally, an eduction algorithm is used to identify near-wall turbulence structures and to quantify the changes in their strength and population density by the SHS texture. [Preview Abstract] |
Monday, November 24, 2014 11:48AM - 12:01PM |
H20.00007: Reinforcement of steady streamwise streaks for consecutive transition delay Sohrab S. Sattarzadeh, Jens H.M. Fransson Miniature vortex genrators (MVGs) are recently proven efficient as passive control devices to delay the turbulence transition on a flat plate boundary layer by modulating the base flow in the spanwise direction, through generating steady streamwise elongated streaks, and hence reducing the skin-friction drag\footnote{Shahinfar, S., Sattarzadeh, S. S., Fransson, J. H. M., Talamelli, A. {\emph{Phys. Rev. Lett.}} {\bf{109}}, 074501, 2012.}. As the MVGs are localized in the streamwise direction, a shortcoming of the passive laminar control is the recovery of the two-dimensional boundary layer which force the control effects to fade away. In the present study we show that by placing a second array of MVGs downstream of the first one the streamwise extent of the control can be prolonged by reinforcing the steady streaks in the streamwise direction. The reinforced passive control strategy results in consecutive turbulence transition delay with obtaining a net skin-friction drag reduction of 65$\%$, for the present measurement conditions, compared to the smooth plate boundary layer. [Preview Abstract] |
Monday, November 24, 2014 12:01PM - 12:14PM |
H20.00008: Flow Through Surface Mounted Continuous Slits A. Tariq, M.A. Ali, M. Gad-el-Hak Ribs are used inside certain gas-turbine blades as passive devices to enhance heat transfer. Slits in those ribs are utilized to control the primary shear layer. The role of secondary flow through a continuous slit behind a surface mounted rib is investigated herein in a rectangular duct using hotwire anemometry and particle image velocimetry. Changing the open-area-ratio and the slit's location within the rib dominate the observed shear layer. The behavior of discrete Fourier modes of the velocity fluctuations generated by different configurations is explored. Two distinct flow mechanisms are observed in the rib's wake. Both mechanisms are explained on the basis of large-scale spectral peak in the shear layer. The results show the successful impact of changing the open-area-ratio by manipulating the small-scale vortices at the leeward corner of the rib, which is suspected to be the potential cause of surface ``hot spots'' in a variety of engineering devices with heat transfer. Eventually, the size and location of the slit are seen to be an additional parameter that can be used to control the fluid flow structures behind rib turbulators. [Preview Abstract] |
Monday, November 24, 2014 12:14PM - 12:27PM |
H20.00009: A Study of Laminar Drag Reducing Grooves A. Mohammadi, Jerzy M. Floryan The performance of grooves capable of reducing shear drag in laminar channel flow driven by a pressure gradient has been analyzed. Only grooves with shapes that are easy to manufacture have been considered. Four classes of grooves have been studied: triangular grooves, trapezoidal grooves, rectangular grooves and circular-segment grooves. Two types of groove placements have been considered: grooves that are cut into the surface (they can be created using material removal techniques) and grooves that are deposited on the surface (they can be created using material deposition techniques). It has been shown that the best performance is achieved when the grooves are aligned with the flow direction and are symmetric. For each class of grooves there exists an optimal groove spacing which results in the largest drag reduction. The largest drag reduction results from the use of trapezoidal grooves and the smallest results from the use of triangular grooves for the range of parameters considered in this work. Placing the same grooves on both walls increases the drag reduction by up to four times when comparing with grooves on one wall only. The predictions remain valid for any Reynolds number as long as the flow remains laminar. [Preview Abstract] |
Monday, November 24, 2014 12:27PM - 12:40PM |
H20.00010: The momentum balance in upward turbulent channel flow laden with microbubbles Yoichi Mito, Shota Ikeda The influence of the addition of microbubbles as dispersed gas phase on fully-developed turbulent flow in a vertical channel in which the liquid is flowing upward with a constant pressure gradient or a constant rate has been examined by using direct numerical simulation to calculate the liquid velocities seen by the microbubbles and the point force method to consider the influence of the microbubbles on the liquid. The microbubbles are represented by solid spheres and are released from uniformly distributed point sources. The streamwise momentum balance shows that the influence of the addition of the microbubbles on the drag of the liquid flow appears as a function of volume fraction, Froude number and friction velocity that results from distribution of the microbubbles. The experimental conditions are chosen such that zero to two hundred percent of the pressure gradient of the single-phase flow is added by the addition of the microbubbles. The drag decreases by the addition of the microbubbles, whereas the frictional drag increases with the increases in the accumulation of the microbubbles on walls, which attenuates the effect of reducing drag. Changes in the liquid turbulence are not clearly seen except for what are due to the changes in the bulk Reynolds number. [Preview Abstract] |
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