Development of a Global Interferometer Skin-Friction Meter

Abstract

An improvedmethodformeasuringskinfrictionintwo-dimensionale owsisdescribed.Theinstrument,termeda globalinterferometerskin-frictionmeter,isusedtomeasuretheshapeofathinoile lmplacedonatestmodelsubject to aerodynamic shear. The oil e lm shape is then related to the applied shear through lubrication theory. Through acquisition ofa singleimageoftheoile lm interferencepattern,theskin-friction distribution can beobtainedat any pointcovered by thee lm. Enhancementsto themethod arepresented that may extend theinstrument' sapplication to fully three-dimensional e ows with time-variant model temperatures. Results from tests performed on e at plate boundary layers along with numerical simulations show the instrument can accurately measure wall shear over relatively large areas in a single test. Results show the method provides numerous advantages over other shear measurement techniques. The instrument' s cone guration, the theoretical background, and the parameters that ine uence its accuracy are presented. CCURATE measurement of the wall shear stress distribution is important for understanding many types of e owe elds. Such data are crucial to understanding aerodynamic drag and for evalu- ating the performance of proposed drag reduction strategies. More- over,skin-frictiondataareextremelyvaluableforvalidationofcom- putationalsolutionsduetothedirectlinkbetweenthepredictedshear stress and the turbulence model. However, despite the importance of accurate skin-friction data, obtaining such, either experimentally or computationally, can be extremely dife cult, costly, and time con- suming. The level of dife culty increases considerably for measure- ments in complex, three-dimensional e ows with separation. Theprimarydife cultywithmeasuringwallshearliesinthelackof a universal measurementtechniquethat canprovideaccurate data in awide rangeof e ows.Winter 1 providesagood reviewof thevarious wallshear measurementtechniques. Nearly all of the existing meth- odsfor wall shear measurements, e.g., Preston tubes, Stanton tubes, sublayer fences, electrochemical techniques, and e oating element balances, have very limited ranges of application. In general, these methodsareintrusive,arerestrictedtocertaintypesofe ows,arelim- ited to e ows with modest pressure gradients, and require detailed calibration.Additionally,thesemethodsprovideonlypointwisedata and can be time consuming to implement on large surfaces. A more recent method of shear measurement that has shown promise is the laser interferometer skin-friction (LISF) meter. The LISFmeter wasoriginally invented byTanner and Blows 2 forusein low-speed e ows.The basic principle of operation of the LISF meter isto optically measure the time rate of thinning of an oil e lm placed on a test surface subject to aerodynamic shear. The rate of thinning of the oil e lm is determined using optical interference and can be related to the applied shear through lubrication theory. Although the LISF meter was originally used for low-speed aire ows, further development of the method 3-6 has resulted in an instrument that is capable of measurements over a broad range of e ow conditions, including supersonic e ows, 7-12 shock- wave/boundary-layer interactions with strong pressure gradients and e ow separation, 9-12 and more recently two-phase (air-water) and single-phase (water) pipe e ows. 13 Despite its relatively broad range of applicability, the LISF tech- niquesuffersfromseveral shortcomings. Foremost,themethod pro- videsonlypointwisedata.Hence,measuringsheardistributionsover