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 Phone: (Lab), (Cell)

2 Lecture 13. Wall and in-flow pressure measurement

3 Wall-pressure measurement Static-pressure taps - small orifice (tap) at solid wall - steady or slowly vary static pressure - connected to manometer or pressure transducer - simple and widely used method - possible error: counter-rotating vortices create pressure in cavity Construction of practical taps solution: infinitesimal tap - 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 cavity of larger diameter d’ to connect pressure sensors & reduce l - removable plug to ensure surface and hole quality

4 Wall-pressure measurement Connections to transducers (a) Flexible tubing connected to transducer (b) Transducer in cavity (c) Transducer flush with wall - remote mounting - multiport measurement with single transducer - simplest - deterioration of dynamic response - improved dynamic response - retained high spatial resolution - maximal dynamic response - reduced spatial resolution - small space required - plastic or metallic tubing

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

6 Wall-pressure measurement Static-pressure taps p x Flow around a wing in a wind tunnel Distribution of pressure taps on the wingPressure distribution on the wing Example: used to measure pressure distribution around airfoil in wind tunnel

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

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 - disk-static probes for larger orifice openings - measure static pressure in flow Pitot tubes - hollow cylindrical tubes - open-ended facing flow - measure total pressure p 0 thin-wall cylindrical tube:   20  - insensitive to misalignment d i /d 0 =0.6 :   12  Kiel probes:   45  for high Re and low M

10 - open-ended tip to measure p 0 - holes on the side to measure p - Flow velocity determined with In-flow pressure measurement Pitot-static tubes Pitot probe in shear flow V n  - displacement effect: V m >V Wall-proximity effect: V m <V V

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

12 Homework - Questions and Problems: 7 on page Read textbook on page Due on 09/26

13 To cut a 64×64-pixel image sample from a 1280×1024-pixel image at i=200, j=400 Try to write a Matlab program 64×64-pixel image sample