Improvements of the performance of triple hot wire probes

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Abstract
Two specific problems associated with a triple hot wire probe are discussed and analyzed here, namely, the problem of nonorthogonality of the three wires and the variation of the directional sensitivity coefficients k2 and h2 with the pitch and yaw angles. Both problems can cause significant errors in the measured turbulence quantities. In particular, v̄2 and w̄2 can be drastically affected. The present results confirm that v̄2 and w̄2 can be measured rather inaccurately with a triple wire, if ‘‘overall’’ values of k2 and h2 are used, where ‘‘overall’’ denotes a mean integral value over a wide range of angles. It was found here that if these coefficients, in particular h2, are left to vary with the angles, v̄2 and w̄2 can be measured more accurately. It is strongly recommended to take this effect into account when high-velocity fluctuations are expected. Two numerical schemes have been proposed as remedies to these problems. They were found to work effectively although the data processing time is significantly increased. It is believed that these algorithms improve the performance of a triple wire, and can be applied to any three-dimensional flow with high turbulence level for which the mean velocity vector is unknown beforehand.