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
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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
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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
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Wall-pressure measurement
Static-pressure taps
Example: pressure taps on a turbine blade model
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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
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Wall-pressure measurement
Static-pressure taps
System error:
∆𝑝 = 𝑝𝑚 − 𝑝
pm – measured pressure
- usually p>0
Influence of tap diameter on measurement error:
𝑑+ =
𝑢𝜏 =
𝑑 ∙ 𝑢𝜏
𝜈
𝜏𝑤 /𝜌
d+ – dimensionless tap diameter
u – friction velocity
Solid curve for flat plate
Dashed curves for pipe flow
w – wall shear stress
Polynomial fit for d+<2500:
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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
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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
𝑑0
- hollow cylindrical tubes
- open-ended facing flow
𝜃
𝑑𝑖
- measure total pressure p0
for high Re and low M
- insensitive to misalignment
thin-wall cylindrical tube:   20
di/d0 =0.6 :   12
Kiel probes:   45
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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

- displacement effect: Vm>V
𝑉𝑚 = 𝑉 + 𝜕𝑉/𝜕𝑛 𝛿
Wall-proximity effect:
V
Vm<V
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In-flow pressure measurement
Turbulence and vibration effects: Vm > V
- Turbulent effect
- Vibration effect
 related to turbulent length scale
f - frequency
a - amplitude
Viscous effect:
Compressibility effect:
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Homework
- Read textbook 8.3-8.5 on page 188 - 203
- Questions and Problems: 7 on page 204
- Due on 09/26
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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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