Interpreting laser and hot-film anemometer signals in a separating boundary layer

In a previous paper (Simpson et al., 1974) measurements of mean streamwise velocity, mean square fluctuation, and the fraction of time the flow was moving in the downstream direction (gamma-p) were carried out in a separating turbulent boundary layer by two different instruments: a directionally sensitive fringe-type laser anemometer with a Bragg cell, and a directionally insensitive, constant-temperature, single-sensor cylindrical hot-film probe. The present work discusses the interpretation of signals from these two instruments in a separation region (gamma-p less than 1.0). A typical histogram of the laser data samples is shown, which could be fairly well represented by a Gaussian distribution centered about the mean velocity. Transit-time broadening was found to be less than or equal to the maximum system resolution. To account for gradient broadening when mean streamwise velocity was below the rms fluctuation, a top-hat type of particle location probability distribution had to be assumed. Biasing correction of Tiederman et al. (1973) is argued to be unjustified in the present experiment. Hot-wire data on mean velocity were found to agree with the laser data only for gamma-p above 0.8. Rectifying the Gaussian probability distribution and computing apparent mean velocity and mean square fluctuation values without regard for certain cooling components was not effective for correlating hot-film data.