In this paper, the effect of radial positioning of the measurement plane on the measured velocity profiles in a stationary valve gap under unsteady flow conditions is discussed for three velocity components. As shown, positioning of the measurement plane significantly influences all measured velocity component distributions across the valve gap, as well as the temporal velocity behaviour, particularly in the upper part of the valve gap where a high radial gradient of velocity was observed. This effect relates to the azimuthal position of the measurement point around the valve axis. A generalization of the LDA measurement control volume positioning problem and practical considerations are also discussed in the paper. In the case of high-velocity gradients in areas remote from the wall, minimization of the LDA measuring control volume dimensions is not sufficient, and several well-defined positions of the measurement plane must be chosen. This choice has to take into account the object geometry, flow conditions and the methodology.
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