Local shear stress measurements on an axisymmetric body in a

Local shear stress measurements on an axisymmetric body in a microbubble modified flow fieldS. Deutsch and S. Pal Citation: Physics of Fluids A 2, 2140 (1990); doi: 10.1063/1.857800 View online: http://doc.guandang.net/10.1063/1.857800 View Table of Contents: http://doc.guandang.net/content/aip/journal/pofa/2/12?ver=pdfcov Published by the AIP Publishing Articles you may be interested in A comparison of shear stress fluctuation statistics between microbubble modified and polymer modified turbulent boundary layers Phys. Fluids A 1, 1360 (1989); 10.1063/1.857311 Microbubble skin friction reduction on an axisymmetric body Phys. Fluids 29, 3590 (1986); 10.1063/1.865786 Stress Measurements at the Start of Shear Flow: Comparison of Data from a Modified Weissenberg Rheogoniometer and from Flow Birefrigence J. Rheol. 24, 517 (1980); 10.1122/1.549597 Magnetohydrodynamic Flow Past Axisymmetric Bodies with Aligned Magnetic Field Phys. Fluids 12, 2083 (1969); 10.1063/1.1692314 A Method for the Measurement of Normal Stresses in Simple Shearing Flow Trans. Soc. Rheol. 5, 133 (1961); 10.1122/1.548891This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://doc.guandang.net/termsconditions. Downloaded to IP: 219.217.241.8 On: Sat, 27 Feb 2016 02:30:13local shear stress measurements on an axisymmetric body in a microbubble modified flow fieldS. Deutsch and S. PalThe Applied Research Laboratory and The Department of Mechanical Engineering, Pennsylvania State University, University Park, Pennsylvania 16804(Received 18 May 1989; accepted 7 August 1990) An array of flush-mounted hot film probes has been used to measure the local shear stress reduction as a result of microbubble injection over an axisymmetric body at the four discrete, free-stream speeds of 4.6, 10.7, 13.2, and 16.8 m/sec. Visualization of the bubble flow pattern supplement these results at intermediate free-stream speeds. At speeds of 10.7 m/sec and above, a circumferential gradient in skin friction, with skin friction reduction larger at the top than at the bottom of the model occurs at some distance downstream of injection. For these speeds, the gradient is stronger at the lower speeds and higher gas injection conditions. Higher speeds tend to drive the axial location of the gradient farther from the injection location. At speeds below 10.7 m/sec, the flow is dominated by a double vortex structure that entrains the bubbles at the bottom and sides of the model and transports them to the top. At sufficiently high gas flow rates a cavity, large enough to be observed visually, is formed just upstream of the vortices, centered near the body midline. The axial position of the cavity is roughly independent of flow speed and gas flow conditions. The transport of bubbles by the vortices, to the top of the body, is the cause of the poor skin friction reduction performance of microbubble injection at low speeds on an axisymmetric shape. Integrati

on of the current local skin friction results gives good agreement with earlier drag balance measurements. The persistence of the drag reduction phenomenon with axial distance as well as the statistics of the shear stress fluctuations are quite similar to what was observed earlier on fiat plates.I. INTRODUCTIONThe reduction of skin friction drag through the injection of gas, to form microbubbl.es into a liquid turbulent boundary layer, has received a good deal of experimental !--4 and analyticaI/numerica1 5- 7 attention over the last 10 years. The decade's work was built on earlier studies by McCormick and Bhattarcharyya, g who used electrolysis to generate bubbles that reduced drag on a towed body of revolution, and by a group of Soviet researchers,9-12 who observed microbubble drag reduction at low speeds on both a flat plate and in a pipe. The most recent experimental studies l -4 have characterized both the integrated and local skin friction reduction with microbubble injection for a flat plate l , 2 and the integrated skin friction reduction for an axisymmetric geometry4 at speeds as high as 18 m/sec. Both the trajectory of the microbubble cloud as well as some bubble size characteristics have also been studied. 3 The analytical/numerical studies,5-7 which are all based on an assumption that the microbubble-laden boundary layer is a homogeneous medium with a spatially varying density and viscosity, have provided a first-order model for explaining the skin friction reduction phenomenon. A more detailed discussion of these studies may be found in a comprehensive review,13 which has recently become available. In the following, we shall only briefly discuss our current understanding of this drag reduction phenomenon. We note that this understanding is based, in general, on experiments and calculations made in zero pressure gradi2140Phys. Fluids A 2 (12), December 1990ent, high-speed, turbulent boundary layers of typical laboratory dimensions.13 It is dear from all the experimental studies that the injection of microbubbles can reduce the skin friction drag substantially for both flat plate and axisymmetric geometries. The primary nondimensionaI variable appears to be the ratio of gas to liquid volume flow rate in the boundary layer. 13 That is, in general, increasing the amount of gas volume relative to the amount of liquid volume in the boundary layer will increase the amount of drag reduction. Although useful, this normalization is less effective then we would like it to be in normalizing the data between different experiments and, moreover, it cannot explain the observed discrepancies between the effects of different gases. 4 Bubble sizes, as measured in a single study;' range from some ten times the sublayer thickness to a tenth of the boundary layer thickness. These values are apparently consistent with either a hypothesis that treats the bubble sizes as fixed by the injection process or one that treats bubble sizes as fixed by the t…… 此处隐藏：21576字，全部文档内容请下载后查看。喜欢就下载吧 ……