An experimental investigation of the response of hot-wire X-probes in shear flows

The response of hot-wire X-probes in regions of strong velocity gradients (such as in the near wall region of boundary layer flows) is investigated experimentally. Although the wall-normal velocity component should be close to zero near the wall, one usually encounters an increased absolute value of this component when the wall is approached. Moreover some physically inconsistent behaviour in other measured quantities, for instance the Reynolds stresses, may be found. These effects can be due to a physical displacement of the wires (e.g. a wall-normal displacement of the two wires so that they do not cross at their mid-points), but also due to the influence of the probe on the local flow field. The latter might be an effect of blockage or wall interference and can be treated as a virtual displacement. The response equations of an X-probe with different wall-normal displacement of the wires are derived and applied in order to correct the measured data. A systematic experimental investigation of the effect of varying the physical displacement of the wires is also made, and it is shown that both the first and second order correction terms of the probe response equations can be estimated from this experiment. A correction procedure for measurements close to the wall is proposed and used to correct Reynolds stress profiles in a flat plate boundary-layer. It is also shown that the present experimental set-up can be used to estimate some turbulence correlations which otherwise are unaccessible with standard measurement techniques.