NIST’s wind tunnel has two interchangeable test sections. (See Fig. 1) The upper test section has a uniform cross section; the lower test section has a contracted zone in its center
Upper section: 2.1 m high × 1.5 m wide × 12 m long; speeds to: 45 m/s (100 mi/hour)
Contracted zone: 1.2 m high × 1.5 m wide × 2 m long; speeds to: 75 m/s (165 mi/hour)
The upper test section is used to study the effects of wind on model buildings. It contains a remotely-controlled gantry, turbulence-generating spires, and a rotatable instrument platform. The lower test section is used to calibrate customers’ anemometers and to conduct research directed towards improving air speed measurements. It contains an automated stage that changes the pitch and yaw angles between the wind speed and any anemometer mounted on a rod.
In normal use, the turbulence intensity in NIST’s wind tunnel is approximately 0.001.
(We define the turbulence intensity as , where is the mean velocity.) When needed, the turbulence intensity is increased by inserting grids or “spires” upstream of the test sections.
Calibrations are traced from a rotating disk to a laser Doppler anemometer to working standards (pitot tubes). The fractional uncertainty (at 95% confidence level) of the working standards is (0.0044 + 0.0016/u 2), where u is the air speed in m/s. (see Shinder et al.,2013)
NIST developed the capability for automated calibration of anemometers as a function of airspeed, pitch angle, and yaw angle.
NIST demonstrated that the calibration of a multi-hole (so-called “3-dimensional” or “3-D”) pitot tube has hysteresis in certain ranges of airspeed, pitch angle, and yaw angle. (see Crowley et al., 2013.) The hysteresis was caused by a flow instability associated with flow separation. As the turbulence intensity was increased from 0.25% to 1% the hysteresis gradually vanished; however, the calibration function is turbulence-dependent up a turbulence intensity of at least 2%. Therefore, multi-hole pitot tubes should be calibrated and used at the same turbulence levels.
Ongoing: Studies of calibration functions of 1-D, 2-D and 3-D pitot tubes for flue-gas measurements according to EPA-approved protocols.