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Learn about various pressure measurements techniques and equipment, from mechanical gauges to cutting-edge transducers. Explore how manometers work, the intricacies of pressure-sensitive paints, and the applications of pressure probes for flow analysis. Discover the principles behind pressure/vacuum gauges, diaphragm tubing systems, and the use of pressure-sensitive and temperature-sensitive paints for accurate measurements in different environmental conditions.
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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 • Measures the oxygen concentration in a polymer paint layer Oxygen Partial pressure Static pressure Henry’s Constant x oxygen concentration
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
Piezoelectric Pressure Transducer Only for Unsteady Measurements Kulite Transducer Used in shock tube
Electronic Scanning Valve Array of inexpensive transducers (16 –32) Method to calibrate automated during test
Yaw Probes • Kiel Probes • 3 Hole • 5 Hole • 7 Hole
7 hole Probe Easy to construct • Measures • total pressure • static pressure • 3 velocity components
x z y v w q y u Seven-Hole Pressure Probe Coordinate System is Defined By Pitch and Yaw Angles
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:
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
q CpTotal CpStatic y
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:
Mass Conservation: Momentum Conservation: Integral Conservation Equation Analysis • Allows determination of lift and drag breakdown from flow field data
Total Drag Analysis: “Profile Drag” “Induced Drag” • Drag Breakdown Analysis: • Viscous Drag: • Inviscid Drag:
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
Total Pressure Loss and Crossflow Velocity in Vortex Flow Regions DPTOTAL (psi)
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
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
Pressure Sensitive Paint (PSP)andTemperature Sensitive Paint (TSP)
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
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
Pressure Sensitive Paint Oxygen Quenches Luminescence • Stern-Volmer Equation Using Henry’s Law Rewrite Iref -reference -accounts for nonuniformities in thickness, lighting and concentration
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