A comparison of velocity measurements using a laser anemometer and a hot‐film probe, with application to grid‐stirring entrainment experiments

A laser‐Doppler anemometer has been used to measure the spatial decay of velocity fluctuations generated by a vertically oscillating grid in a laboratory tank. These velocity data were found to be in agreement with earlier studies using hot‐film probes, though a comparison of the length scales of the motions obtained with the two different probes is difficult. Normalized entrainment rates follow similar trends using values obtained either with the laser or with the hot film, though quantitatively some differences are seen. It is concluded that true turbulence does not exist for distances less than about two mesh spacings from the grid.