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