POINT MEASUREMENT OF TURBULENCE QUANTITIES IN SEPARATED FLOWS - A COMPARISON OF TECHNIQUES

Accurate measurement of turbulence quantities in regions of high turbu lence intensity is notoriously difficult. In the particular context of turbulent separated flows, only two 'point-based' (rather than full-f ield) techniques are generally acknowledged to be viable: laser-Dopple r anemometry (LDA) and pulsed-wire anemometry (PWA). This paper presen ts the first serious comparison of data obtained in the same high-inte nsity, separated flow using both techniques. It is shown that, with su fficient care and understanding of the instrument responses, mean velo city and turbulence stress measurements yielding similar results can g enerally be obtained by the two techniques, although PWA data are in s ome circumstances inevitably less accurate. In addition, the results a re close to those obtained from standard crossed-hot-wire anemometry i n regions where the latter can sensibly be used. LDA and PWA are based on very different physical principles and hence there are very differ ent sources of error. Whilst the present results from either technique do not, on their own, necessarily imply high accuracy, taken together they therefore do suggest that the measurement errors can usually be made acceptably small in both cases. Consequently, they increase the c onfidence with which each technique can be used for such complex flows although there are some unavoidable drawbacks in PWA which impose add itional limits on the scale of flow which can be successfully probed a nd make accurate assessment of the errors rather difficult.