Drag resolution of a PIV wake rake for transiting models

Abstract

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.

On-site cycling drag analysis with the Ring of Fire

“…The velocity distribution prior to the passage is taken into account for the drag computation via Eq. (2) as it contributes to the overall momentum budget, as also discussed by Terra et al (2018). Furthermore, unsteady effect may influence the interaction of the wake with the initial velocity field, resulting in variations of the measured drag.…”

“…The recent works of Terra et al (2017Terra et al ( , 2018 with tomographic PIV in combination with helium-filled soap bubbles to determine the drag of a towed sphere can be seen as preliminary to the current study. The use of HFSB offers the potential to upscale the measurement region up to several square meters as demonstrated by Bosbach et al (2009).…”

“…A recent review of the problem has been provided by Rival and Van Oudheusden (2017). The formulation of the problem is simplified from its unsteady form to the steady condition when applying a Galilean transformation (Arnold 1989), whereby the frame of reference moves with the object and the fluid is considered in uniform motion upstream of the object (Terra et al 2018). If the control surfaces S 1 and S 2 are sufficiently far from the object surface, it can be shown that the viscous stress is negligible (Kurtulus et al 2007).…”

“…Assuming uniform and quiescent conditions prior to the passage would largely simplify the problem formulation. However, even in scaled experiments, the disturbances in the air motion induced by the environment and the seeding generation are reported not to be negligible (Terra et al 2018).…”

“…The uncertainty of the estimated C d A values is analyzed a posteriori, based on the standard deviation of the instantaneous drag area estimates, and the number of independent samples (considering both the number of passages of the cyclist, and the number of independent flow measurements in the wake of a cyclist during one passage). A detailed analysis of the measurement uncertainty and drag resolution of the Ring of Fire system for small-scale applications is reported in a recent work of Terra et al (2018), where the effect of simplifications in the conservation of momentum equation is considered.…”