Wind tunnels

1
Wind tunnels

• Purposes
• To provide a uniform velocity field (e.g.
  aeronautic, automotive testing) and low
  turbulence intensity
• To provide suction or blowing and measure flow
  rate (e.g. airflow impedance testing, AC
  experiment, heat exchanger testing)

2
Wind Tunnel Design

• Proper shape of the inlet is important several
  journal papers offer guidance.
• Flow straighteners (honeycomb) and screens used
  to straighten flow and provide minimal turbulence
  intensity.
• Test section should not be too far downstream
  from inlet otherwise boundary layers will be
  large for slow flows.
• If used, orifice plate or venturi tube must be
  design per strict ASME standards.

3
Wind tunnel Examples
blower
Test section Uniform velocity
Inlet Designed for uniform inlet flow and
minimal boundary layer formation
Wind tunnel and test section model at NASA-Ames
Source Electronic Laboratory Design
4
More Wind tunnels
5
Velocity Measurement
Pitot-static tube
Hot-wire anemometer
Senses stagnation pressure (at tip) and static
pressure (at holes on periphery) Po P can be
measured. Velocity calculated from Bernoullis
equation.
Uses electricity to heat wire connecting two
prongs to a constant temperature. Required
current and/or voltage is calibrated to velocity.
Orifice plates and venturi tubes are often used
as well (discussed during AC experiment).
6
Hot Dog Anemometer

• Using a cylinder of known properties, a
  thermocouple, and stopwatch, fluid velocity can
  be determined. If a warm cylinder is placed in a
  wind tunnel, the faster the air flows around the
  cylinder the faster the temperature of the
  cylinder changes.
• You will use the concepts of transient heat
  conduction and convection around a cylinder in
  this lab to calculate the air temperature.
• In the past, a hot dog was used as the warm
  cylinder. However, a thermocouple must be placed
  right in the center, and its tough to make sure
  of that with a hot dog, so a plastic cylinder is
  now used.