Turbulent-flow interactions and drag reduction
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Some of the salient experimental observations in polymer drag reduction are reviewed, in light of basic concepts of turbulent flow. The areas common to drag reduction by polymers and surfactants, and drag reduction by fibers and suspensions in both liquids and gases are examined, and some general rules for obtaining friction reduction are suggested. Based on these ideas, the drag‐reducing properties of polymers, soap and surfactant aggregates, and fibers and solids are portrayed, noting the role of additive size and weight in producing the effect.Cite
For the experiment of the friction drag reduction by microbubble injection, a drag reduction water tunnel was specifically designed and made. Experimental apparatus and procedures were devised and developed for measuring the change of wall friction drag with microbubble injection. For fully-developed channel flows. the change of friction drag with important parameters of microbubble injection is investigated and the experimental data and results obtained are presented. The amount of friction drag reduction up to 25% is observed in the present study.
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Slippage
Zero-lift drag coefficient
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A micro-blowing technique (MBT) experiment was conducted in the Advanced Nozzle and Engine Components Test Facility at the NASA Lewis Research Center. The objectives of the test were to evaluate the pressure-drag penalty associated with the MBT and to provide additional information about the porous plates used for micro-blowing. The results showed that 1 of 12 plates tested could reduce the total drag (skin-friction drag plus pressure drag) below a solid flat plate value. The results of this experiment and prior data showed that a total drag reduction below a solid flat plate value was possible. More tests are needed to find an optimal MBT skin and to find a technique to reduce pressure drag.
Wave drag
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In the present study we investigate a possibility of reducing skin-friction drag in a turbulent channel flow with active wall motions. The wall is locally deformed according to two successful control strategies [J. Fluid Mech. 262, 75 (1994); J. Fluid Mech. 358, 245 (1998)]. Results show that overall 13–17% drag reductions are obtained with the active wall motions, and turbulence intensities and near-wall streamwise vortices are significantly weakened. It is remarkable that instantaneous wall shapes are elongated in the streamwise direction and resemble riblets in appearance. However, the mechanism of the present drag reduction is essentially different from that of riblets.
Flow Control
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In this study, a novel but simple biomimetic turbulent drag reduction topology is proposed, inspired by the special structure of shark skin. Two effective, shark skin-inspired, ribletted surfaces were designed, their topologies were optimized, and their excellent drag reduction performances were verified by large eddy simulation. The designed riblets showed higher turbulent drag reduction behavior, e.g., 21.45% at Re = 40,459, compared with other experimental and simulated reports. The effects of the riblets on the behavior of the fluid flow in pipes are discussed, as well as the mechanisms of fluid drag in turbulent flow and riblet drag reduction. Riblets of various dimensions were analyzed and the nature of fluid flow over the effective shark skin surface is illustrated. By setting up the effective ribletted surface on structure’s surface, the shark skin-inspired, biomimetic, ribletted surface effectively reduced friction resistance without external energy support. This method is therefore regarded as the most promising drag reduction technique.
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DURING the past year the increase in performance shown by certain American and Continental aircraft has merited the attention of all aircraft technicians. The performance is, of course, partly due to increase in horse‐power, but this is a small factor compared with the advance shown in the reduction of drag. Now that it has been shown that it is possible to build aeroplanes of low form‐drag and with little interference, the drag due to skin friction is becoming of great importance, and it will probably be of advantage to outline the methods of estimating skin friction, and discuss what information is now available on the effect of surface finish and protuberances.
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In human swimming, the total drag comprises the skin friction drag, form drag and wave drag. The relative contribution of each component to the overall hydrodynamic drag is controversy issue. PURPOSE: To analyse the relative contribution of the skin friction drag and the form drag for the total drag during the gliding, using computational fluid dynamics. METHODS: A 3-D domain was created to simulate the fluid flow around a swimmer model. The numerical simulation analysis consisted of the use of a three-dimensional mesh of cells that simulates the flow around the human body. Computational fluid dynamics methodology uses the finite volume approach, where the equations are integrated over each control volume. The k-epsilon turbulent model was applied to the flow around a three-dimensional model of a male adult swimmer in two gliding positions: in ventral position with the arms extended at the front and in ventral position with the arms aside the trunk. The swimmer model middle line was placed at a water depth of 0.90 m, equidistant from the top and bottom surfaces of the 3-D domain. The coefficient of drag (CD) was computed using a steady flow velocity of 2 m/s for both gliding situations. The CD was decomposed into form and skin friction drag. RESULTS: The position with the arms extended at the front presented a CD value of 0.43 whereas the position with the arms aside the trunk presented a CD value of 0.74. In the position with the arms extended at the front, form drag and skin friction drag represented, approximately, 87% (CD = 0.37) and 13% (CD = 0.06) of the total drag, respectively. In the position with the arms aside the trunk, form drag and skin friction drag represented, approximately, 92% (CD = 0.68) and 8% (CD = 0.06) of the total drag, respectively. CONCLUSIONS: The gliding position with the arms extended at the front produced lower drag coefficients than with the arms placed aside the trunk. Although form drag was dominant, skin friction drag was by no means negligible during the swimming gliding. Supported by FCT (SFRH/BD/25241/2005; POCTI/DES/58872/2004).
Drag equation
Position (finance)
Wave drag
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An experimental assessment has been made of the drag reducing efficiency of the outer-layer vertical blades, which were first devised by Hutchins. The drag reduction efficiency of the blades was reported to reach as much as 30%. However, the drag reduction efficiency was quantified only in terms of the reduction in the local skin-friction coefficient. In the present study, a series of drag force measurements in towing tank has been performed toward the assessments of the total drag reduction efficiency of the outer-layer vertical blades. A maximum 9.6% of reduction of total drag was achieved. The scale of blade geometry is found to be weakly correlated with outer variable of boundary layer. In addition, detailed flow field measurements have been performed using 2-D time resolved PIV with a view to enabling the identification of drag reduction mechanism.
Towing
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