Measurements in Fluid Mechanics 058:180 (ME:5180) Time & Location: 2:30P - 3:20P MWF 3315 SC Office Hours: 4:00P – 5:00P MWF 223B-5 HL Instructor: Lichuan Gui lichuan-gui@uiowa.edu Phone: 319-384-0594 (Lab), 319-400-5985 (Cell) http://lcgui.net Lecture 13. Wall and in-flow pressure measurement 2 Wall-pressure measurement Static-pressure taps - simple and widely used method - small orifice (tap) at solid wall - steady or slowly vary static pressure - connected to manometer or pressure transducer - possible error: counter-rotating vortices create pressure in cavity solution: infinitesimal tap Construction of practical taps - clean small holes perpendicular to surface - hole size d ranging between 0.5 and 3 mm - length-to-diameter ratio l/d in the range of 5-15 - cavity of larger diameter d’ to connect pressure sensors & reduce l - removable plug to ensure surface and hole quality 3 Wall-pressure measurement Connections to transducers (a) Flexible tubing connected to transducer - simplest - plastic or metallic tubing - small space required - remote mounting - multiport measurement with single transducer - deterioration of dynamic response (b) Transducer in cavity - improved dynamic response - retained high spatial resolution (c) Transducer flush with wall - maximal dynamic response - reduced spatial resolution 4 Wall-pressure measurement Static-pressure taps Example: pressure taps on a turbine blade model 5 Wall-pressure measurement Static-pressure taps Example: used to measure pressure distribution around airfoil in wind tunnel p Flow around a wing in a wind tunnel x Distribution of pressure taps on the wing Pressure distribution on the wing 6 Wall-pressure measurement Static-pressure taps System error: āš = šš − š pm – measured pressure - usually ļp>0 Influence of tap diameter on measurement error: š+ = š¢š = š ā š¢š š šš¤ /š d+ – dimensionless tap diameter uļ“ – friction velocity Solid curve for flat plate Dashed curves for pipe flow ļ“w – wall shear stress Polynomial fit for d+<2500: 7 Wall-pressure measurement Pressure-sensitive paints (PSPs) - test surface coated with PSP for flows of M>0.3 - Illuminated with ultraviolet or blue light - light absorbed by photosensitive molecules in paint - undergo transition to unstable state - some unstable molecules return to original state and emit radiation of longer wavelength (yellow or red) - others convert energy to oxygen molecules - higher pressure increase oxygen number density in paint to reduce the fluorescence intensity 8 In-flow pressure measurement Static-pressure tubes - thin hollow tubes - sealed tip facing flow - holes on the side - measure static pressure in flow - disk-static probes for larger orifice openings Pitot tubes š0 - hollow cylindrical tubes - open-ended facing flow š šš - measure total pressure p0 for high Re and low M - insensitive to misalignment thin-wall cylindrical tube: ļ± ļ£ 20ļ° di/d0 =0.6 : ļ± ļ£ 12ļ° Kiel probes: ļ± ļ£ 45ļ° 9 In-flow pressure measurement Pitot-static tubes - open-ended tip to measure p0 - holes on the side to measure p - Flow velocity determined with Pitot probe in shear flow n V ļ¤ - displacement effect: Vm>V šš = š + šš/šš šæ Wall-proximity effect: V Vm<V 10 In-flow pressure measurement Turbulence and vibration effects: Vm > V - Turbulent effect - Vibration effect ļ” related to turbulent length scale f - frequency a - amplitude Viscous effect: Compressibility effect: 11 Homework - Read textbook 8.3-8.5 on page 188 - 203 - Questions and Problems: 7 on page 204 - Due on 09/26 12 Try to write a Matlab program • To cut a 64×64-pixel image sample from a 1280×1024-pixel image at i=200, j=400 64×64-pixel image sample http://lcgui.net/ui-lecture2012/hw/00/A001_1.BMP 13