Wind Tunnel Calibration

Wind Tunnels

  • Systems for experimental data on external flows (aircraft, vehicles, buildings)

  • Collect data on Drag/Lift Force, Surface Pressures, Flow Visualization

Parts of a Wind Tunnel

  • Settling Chamber: Straightens airflow, reduces turbulence

  • Contraction Section: Reduces volume while increasing air velocity

  • Test Section: Measures forces on models (wings, planes)

  • Diffuser Section: Slows air before exhausting, reduces costs

  • Fan Section: Provides airflow through the tunnel

Airflow Speed

  • Expressed relative to speed of sound (761 mph at sea level)

  • Mach Number (Ma): Ratio of object speed to sound speed

    • Subsonic: Ma << 1.0

    • Transonic: Ma ≈ 1.0

    • Supersonic: 1.0 < Ma < 5.0

    • Hypersonic: Ma > 5.0

Types of Wind Tunnels

  • Open Circuit: Air enters and exits into the environment

  • Closed Circuit: Air is continually recirculated

  • Suction Tunnel: Inlet open to the atmosphere with post-test section fan

  • Blow Down: Air is introduced at the inlet by a blower

Flow Assumptions

  • Steady: Flow does not change with time

  • Inviscid: Negligible viscous effects

  • Incompressible: Density is constant

Calibration Process

  • Adjust fan speed to change airspeed

  • Determine airspeed from fan speed using measurements

  • Use Bernoulli's Equation for calculations:
    P + \frac{1}{2} \rho V^2 + \rho gz = C

Types of Pressure

  • Static Pressure: Actual pressure of fluid

  • Dynamic Pressure: Pressure rise when fluid stops

  • Hydrostatic Pressure: Pressure change with elevation

Stagnation Points

  • Where velocity is zero, streamline is perpendicular to surface

Pitot-Static Tubes

  • Measures velocity via fluid pressure

  • Consists of stagnation and static ports for readings

  • Equation for velocity:
    V=\sqrt{\frac{2(P_1-P)}{\rho a}}=\sqrt{\frac{2(\rho m)}{\rho a}}

Calibration Steps

  • Set fan speed

  • Measure dynamic pressure using a manometer

  • Calculate velocity using Bernoulli's equation