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Instructor: Lichuan Gui lichuan-gui@uiowa

This lecture focuses on wall and in-flow pressure measurement methods in fluid mechanics, covering wall-pressure measurement via static-pressure taps, their construction, and connections to transducers. It discusses various techniques such as pressure-sensitive paints and different types of static pressure tubes, including Pitot tubes. The lecture highlights the influence of tap diameter on measurement errors, approaches for achieving improved dynamic response, and addresses the effects of turbulence and vibration on measurement accuracy. Relevant examples are provided to illustrate the applications of these methods.

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Instructor: Lichuan Gui lichuan-gui@uiowa

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  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

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