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Pressure Measurements

Pressure Measurements. Measurable pressures. Absolute pressure Gage pressure Differential pressure Atmospheric/barometric pressure Static pressure Total Pressure. Pressure Measurement. Mechanical Pressure Measurement Manometer Mechanical deflection Pressure Sensitive Paint

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Pressure Measurements

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  1. Pressure Measurements

  2. Measurable pressures • Absolute pressure • Gage pressure • Differential pressure • Atmospheric/barometric pressure • Static pressure • Total Pressure

  3. Pressure Measurement • Mechanical Pressure Measurement • Manometer • Mechanical deflection • Pressure Sensitive Paint • Measures the oxygen concentration in a polymer paint layer Oxygen Partial pressure Static pressure Henry’s Constant x oxygen concentration

  4. Pressure/Vacuum Gauge Ranges

  5. Manometers

  6. Manometer Resolution • Main use for Manometers is calibration of electronic pressure transducers • 1.0 inch of H2O = .036 psi • .001 inch of H2O = .3.6 x 10-5 psi

  7. Bourdon Tube Pressure Gauges

  8. Strain Gauge Pressure Transducer

  9. Capacitance Pressure Transducer

  10. Resonant Wire Pressure Transducer

  11. Piezoelectric Pressure Transducer Only for Unsteady Measurements Kulite Transducer Used in shock tube

  12. Magnetic Pressure Transducer

  13. Optical Pressure Transducer

  14. Dead Weight Calibrator

  15. Electronic Scanning Valve Array of inexpensive transducers (16 –32) Method to calibrate automated during test

  16. Scanning Valve Systems

  17. Pitot Probe

  18. Yaw Probes • Kiel Probes • 3 Hole • 5 Hole • 7 Hole

  19. Kiel Probe

  20. 3 Hole Probes

  21. 5 Hole Probes

  22. 7 hole Probe Easy to construct • Measures • total pressure • static pressure • 3 velocity components

  23. x z y v w q y u Seven-Hole Pressure Probe Coordinate System is Defined By Pitch and Yaw Angles

  24. 4 0.035" 5 3 0.109" 7 2 6 1 Velocity Invariant, Non-Dimensional Seven-Hole Probe Pressure Coefficients Flow Angle Coefficients: Total and Static Pressure Coefficients:

  25. Probe V¥ q y Dy = 5° Pitch Dial Dq = 4° Computer Controlled Rotary Table Stepper Motor Seven-Hole Probe Angular Sensitivity Must be Calibrated in a Uniform Flow Field

  26. q CpTotal CpStatic y

  27. Measure Probe Pressures (P1, P2, P3, P4, P5, P6, P7) Interpolate on probe calibration data to find total and static pressure coefficients: CPtotal = ¦(q , y) CPstatic = ¦(q , y) [Figures 3.5 & 3.6] Compute: CPa = ¦(Pi)i=1,7 CPb = ¦(Pi)i=1,7 [section 3.1.2] Compute total and static pressure: Compute magnitude of flow velocity: Interpolate on probe calibration data to find flow angles: q = ¦(CPa , CPb) y = ¦(CPa , CPb) [Figures 3.7 & 3.8] Compute velocity components:

  28. Mass Conservation: Momentum Conservation: Integral Conservation Equation Analysis • Allows determination of lift and drag breakdown from flow field data

  29. Total Drag Analysis: “Profile Drag” “Induced Drag” • Drag Breakdown Analysis: • Viscous Drag: • Inviscid Drag:

  30. Constant Area Duct (6' x 4') Vertical Axis Wind Tunnel Contraction x z Horizontal Axis Upstream Measurement Station (¥) y v (c=48”) Probe Arm V ¥ Y w Z X Inflow Plane x/c = -0.71 q y Plane 1 x/c = 1.25 7-Hole Pressure Probe Domain for Heat Transfer Measurements Plane 2 x/c = 2.25 u

  31. Total Pressure Loss and Crossflow Velocity in Vortex Flow Regions DPTOTAL (psi)

  32. Diaphragm Tubing Cavity Pressure System Response Time Flush Mounted Transducer Maximum performance • Time response depends on • Volume Change • Friction in Tube • Acceleration of • fluid and diaphragm • Compressibility of fluid

  33. Diaphragm L Tubing -dt pi suddenly increasesThe pressure differential will cause the fluid to flow into the cavityConsider steady laminar Incompressible flowCharacteristic of transducer V – average velocity in tube

  34. Pressure Sensitive Paint (PSP)andTemperature Sensitive Paint (TSP)

  35. Photo-physical process: -absorb a photon -transition to excited state -Oxygen quenching (PSP) or thermal quenching (TSP) => Pressure and/or temperature dependent luminescent intensity and luminescent lifetime TSP -Temperature Sensitive Paint PSP - Pressure Sensitive Paint

  36. Temperature Sensitive Paint • High temperature causes non-radiative decay • “thermal quenching” • Obeys Arrhenius relation: • For limited temp. range • Similar molecules to • PSP, but in oxygen • impermeable binder

  37. Pressure Sensitive Paint Oxygen Quenches Luminescence • Stern-Volmer Equation Using Henry’s Law Rewrite Iref -reference -accounts for nonuniformities in thickness, lighting and concentration

  38. Jablonski Diagram

  39. PC with digitizing board TV Monitor painted wind tunnel model long-pass filter CCD camera short-pass filter Measurement Systems CCD based Temperature or pressure  ratio wind off/wind on images lamp SS or Flash

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