Drag resolution of a PIV wake rake for transiting models

The accuracy of the “PIV wake rake” method to measure the drag of transiting objects is evaluated. Tomographic particle image velocimetry measurements are conducted on a sphere towed at different speeds, within a Reynolds number range where the drag coefficient is constant. In contrast to PIV wake rake application on steady models in wind tunnels, where the upstream conditions can be accurately controlled and known a priori, measurement of the flow field prior to the passage of the model is essential for an accurate estimation of the drag for towed models when control of the undisturbed conditions is more challenging. The drag resolution of the technique is estimated to prospect the use of the technique in large-scale applications. A resolution of approximately 20 drag counts is obtained which is coarser than wind tunnel experiments but comparable to techniques used for field measurements. Graphical abstract: [Figure not available: see fulltext.].