Instructor: Lichuan Gui lichuan-gui@uiowa

1. Measurements in Fluid Mechanics058:180 (ME:5180)Time & Location: 2:30P - 3:20P MWF 3315 SCOffice 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

2. Lecture 13. Wall and in-flow pressure measurement

3. 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 - removable plug to ensure surface and hole quality - 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

4. 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

5. Wall-pressure measurement Static-pressure taps Example: pressure taps on a turbine blade model

6. 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

7. Wall-pressure measurement Static-pressure taps System error: pm – measured pressure - usually p>0 Influence of tap diameter on measurement error: Solid curve for flat plate Dashed curves for pipe flow d+– dimensionless tap diameter u– friction velocity w– wall shear stress Polynomial fit for d+<2500:

8. 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

9. 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 - hollow cylindrical tubes - open-ended facing flow - measure total pressure p0 for high Re and low M - insensitive to misalignment di/d0 =0.6 :   12 Kiel probes:   45 thin-wall cylindrical tube:   20

10. In-flow pressure measurement Pitot-static tubes - open-ended tip to measure p0 - holes on the side to measure p  Pitot probe in shear flow n V - displacement effect: Vm>V V Wall-proximity effect: Vm<V - Flow velocity determined with

11. In-flow pressure measurement Turbulence and vibration effects: Vm> V - Turbulent effect  related to turbulent length scale - Vibration effect f - frequency a - amplitude Viscous effect: Compressibility effect:

12. Homework - Read textbook 8.3-8.5 on page 188 - 203 • Questions and Problems: 7 on page 204 - Due on 09/26

13. 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