STRAIN-GAGE ANEMOMETER FOR ANALYZING VARIOUS CHARACTERISTICS OF WIND TURBULENCE

Abstract

This article describes an apparatus composed of a strain-gage assembly, electrical amplifier, and recording oscillograph for measuring characteristics of wind turbulence of velocity-fluctuation frequencies up to 120 cy per sec. All parts but the strain-gage assembly are available commercially. Fluctuations of wind pressure and velocity were recorded along the x, y, and z axes for different wind velocities and various heights above surfaces of certain degrees of roughness in a wind tunnel and in the field, and the recorded data were analyzed to check the adaptability of the apparatus. Analyses indicated the presence of uniformly turbulent flow in the wind tunnel, but records from the field indicated a variably turbulent flow in which the larger scales of turbulence were superimposed on the smaller ones. No matter what type of turbulent flow, analyses indicated that the velocity fluctuations along the x, y, and z axes some distance above the ground were statistically distributed to fit the normal curve. The fit was reasonably good for various drag velocities, axes, and magnitudes of surface roughness. Close to the ground, on the other hand, both pressure and velocity curves for x and y axes did not fit the normal curve but velocity fluctuations along the y axis reasonably fit the normal curve. Records indicated that the relative intensity of turbulence, which is a measure of the velocity fluctuation at some particular height in proportion to the mean velocity at that height, was greatest nearest the surface and decreased in arithmetic proportion with height. The relative magnitude of turbulence, which is a measure of fluctuation of velocity at a particular height in proportion to the drag velocity, on the other hand, was lowest nearest the surface, increased with height depending on roughness of surface, then decreased with height. The scale of turbulence, which denotes the relative size of eddies, was smallest nearest the ground and increased as the logarithm of height. The turbulence factor, which denotes how much greater the maximum impulse forces of turbulent flow are over those of uniform flow of equal mean velocity, was greatest and had a mean value of 2.71 very close to the surface and decreased rapidly with height. It remained virtually constant for different degrees of surface roughness and drag velocities of the wind.