Dynamic Anemometers

Abstract

In eddy-flux measurements of wind stress, heat exchange, and evaporation over the sea a knowledge of the instantaneous wind vector is essential. Dynamic anemometers measure wind velocity by sensing either pressure or drag force on an object placed in the flow. Three-axis thrust anemometers sense the wind thrust, or drag, on a spherical object. Pressure-sensing dynamic anemometers are seen to have similar characteristics. These anemometers, designed for measurements of turbulence and eddy fluxes, are normally mounted on stable platforms. Dynamic anemometers which measure horizontal wind speed and direction and require minimal maintenance may be suitable for general use on buoys, ships, etc., where rotating anemometers (see Busch, Chapter 1) are more often used. In many cases the anemometer is used to determine the wind stress on the sea surface,

$$ \tau = - \overline {\rho {u_1}'{u_3}'} \simeq - \overline {\rho {u_1}'{u_3}'} $$

(1)

where u’₁ and u’₃ are wind velocity fluctuations along the mean wind direction and in the vertical direction, respectively. Since the rms downwind and vertical fluctuations over the sea are typically 10% and 5%, respectively, of the mean wind speed, the anemometer must have good resolution and low noise in both wind speed and tilt angle, relative to these figures.