A miniature three-dimensional anemometer for use within and above plant canopies

A miniature anemometer has been designed for the measurement of turbulent transport within canopies. The sensing element utilizes a relatively new concept in hot-film anemometry, in which the angular measurement is derived from the non-uniformity in heat transfer coefficient around the circumference of the cylindrical hot-film. The element is split along its length to form two separate conducting films and the relative magnitudes of the heat convected from each side are used to calculate the elevation angle of the wind. An electromechanical servosystem operated by a second split-film keeps the sensing head facing into the wind. The anemometer measures all three components of velocity over the complete solid angle without octant ambiguity and at velocities as low as 20 cm s^–1. It is a research instrument and because of its non-linear response characteristics, data handling is best accomplished by digital computer.The response of the split-film elements extends to high frequencies. The servo-system follows turbulent fluctuations up to approximately 5 Hz and keeps the probe within a few degrees of the wind at all times. In field tests, total wind speed and wind component measurements compared well with more conventional anemometers; eddy-correlation measurements of shear-stress with the split-film anemometer were in good agreement with measurements from a shear stress lysimeter and from a pressure-sphere anemometer.