Unsteady aerodynamic forces estimation on a square cylinder by TR-PIV

Abstract

The unsteady aerodynamic forces acting on a square cross-sectional cylinder are investigated by means of time-resolved particle image velocimetry (TR-PIV) at Reynolds number 4,900. The objective of the investigation is to prove the feasibility of nonintrusive force measurements around two-dimensional bodies. The PIV measurements performed at a rate of 1 kHz enable a time resolved (TR) description of the vortex shedding phenomenon occurring at 10 Hz and to follow the time evolution of vortex dominated wake. The instantaneous aerodynamic force coefficients are obtained from the integration of the force equations within a control volume enclosing the object. The required instantaneous pressure distribution is inferred making use of two physical models: Bernoulli relation is adopted in the potential slowly-evolving flow region; in the turbulent wake, the Navier-Stokes equations are invoked to determine the pressure gradient spatial distribution, which integrated in space yields the pressure distribution. The spatial acceleration field is directly obtained from the temporal difference of the time-filtered velocity field. For a choice of the control volume approximately one model height away from the surface the contributions to the aerodynamic forces coming from the different terms of the force equation are individually examined. The convective term dominates the unsteady lift forces whereas the pressure term prevails for the drag. The temporal evolution of C L returns a clear periodic pattern in phase with the vortex shedding at a frequency of 10.1 Hz (Strouhal number St = 0.128) with oscillation amplitude of 0.9, whereas C D barely shows periodicity. The measurement uncertainties associated to the evaluation of all the terms in the force equation and especially in relation to TR-PIV measurements are discussed.

Load-estimation techniques for unsteady incompressible flows

“…To date, no robust studies on uncertainty estimation have been performed for instantaneous force estimations with noisy data (be its classical, DMT, or with vorticity formulations). However, the short temporal discrepancies in estimates on the order of 10% observed in the synthetic test case of Mohebbian and Rival (2012) can be attributed to significant error propagation across the wake, as discussed by Kurtulus et al (2007). These idealized results show that the discrepancies are strongest in the direction of drag due to the higher gradients of velocity across the rear control surface, associated with the crossing of wake vortices.…”

“…In all these examples, pain-staking measures were taken to access the full velocity field around the body of interest using transparent models, complex optical techniques, etc. These studies include analyses by Unal et al (1997), Kurtulus et al (2007), Jardin et al (2009), Rival et al (2011, and Villegas and Diez (2014). To illustrate the full potential of the pressure and load determination technique, Fig.…”

Navier–Stokes simulations in gappy PIV data

“…The presence of gaps in PIV data is an important issue in large experimental campaigns performed in a limited time-frame and in applications that aim to compute integral quantities. Two examples of the latter are the determination of unsteady aerodynamic loads (Kurtulus et al 2007) and the computation of acoustic pressure fluctuations in turbulent flows using aeroacoustic analogies (Violato and Scarano, 2011). Despite its relevance, the treatment of gaps has received little to no attention among the PIV community, the problem being regarded as a surrogate of spurious vector detection (e.g.…”

Low-Frequency Variations of Force Coefficients on Square Cylinders with Sharp and Rounded Corners

“…Additionally, there has been some inconsistencies in reported values for the mean drag coefficient on square cylinders with sharp corners. While most of the reported values are near 2.0, there have been other reported values near 1.5 ([6], [7]). As such, we will also discuss the effects of different simulation conditions on the mean drag values for square cylinders with sharp and rounded corners.…”