Compressive surface-normal velocity gradient at a free surface leads to high mass transfer across a free surface. Our research aims to directly measure this velocity gradient at the free surface by proposing an advanced Particle Image Velocimetry (PIV) technique and simultaneously evaluate its applicability. This technique, PIV/IG (Interface Gradiometry), was proposed by Nguyen et al. (2004) to directly measure wall velocity gradient with high S.N.R. Herein, we adapt this technique to measure the compressive surface-normal velocity gradient at the free surface of open channel flow with minimal fluctuation of water surface. We validate this technique in a two-component PIV configuration by synthetic PIV images corresponding to uniform compression, linearly-varying compression, and a velocity field based on DNS data of open channel flow at friction Reynolds number Reτ = 240 and zero Froude number. The results clearly show that this technique works much better than the velocity differentiation method. The effect of template size on the measured value is evaluated.

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