The measurement of Reynolds stresses with a pulsed-wire anemometer

An investigation of the errors arising in pulsed-wire anemometer measurements of the Reynolds stresses in turbulent flows is described. Attention is concentrated first on a theoretical approach, in which an idealized yaw response and an assumed form for the joint velocity probability-density distribution are used to determine the errors in measurements of, principally, UV and 3 when the probe is used like an ordinary single slanted hot wire. Actual pulsed-wire measurements in a range of turbulent shear flows are then compared with crossed-hot-wire results and with the theoretically simulated pulsed-wire response obtained from calculations and the crossed-wire data. It is shown that whilst pulsed-wire measurements of lateral intensity and shear stress are inevitably rather unsatisfactory in regions of low intensity (less than lo%, say) they agree reasonably well with crossed-wire measurements in flows where the intensities are higher, but do not exceed those for which sensible corrections to cross-wire data are possible (up to, say, 30 yo). In this medium-intensity range, however, the pulsed-wire errors are found to be critically dependent on the finite limit of the pulsed wire's yaw response; it seems that acceptable measurements can only be made if this exceeds about 75". Beyond an intensity of about 30 the errors in 3 measurements (which are usually much higher than those for UV) become less dependent on the exact nature of the yaw response and invariably decrease with increasing intensity. They can, with care, be made as low as 15 yo. It is concluded that pulsed-wire measurements of the Reynolds stresses can be made with an accuracy similar to that of crossed-wire measurements in medium-intensity flows. Such measurements are certainly adequate for many practical purposes in high-intensity flows where hot-wire techniques are useless.