Assessment of Dual Plane PIV Measurements in Wall Turbulence Using DNS Data

Experimental dual plane particle image velocimetry (PIV) data are assessed using direct numerical simulation (DNS) data of a similar flow with the aim of studying the effect of averaging within the interrogation window. The primary reason for the use of dual plane PIV is that the entire velocity gradient tensor and hence the full vorticity vector can be obtained. One limitation of PIV is the limit on dynamic range, while DNS is typically limited by the Reynolds number of the flow. In this study, the DNS data are resolved more finely than the PIV data, and an averaging scheme is implemented on the DNS data of similar Reτ to compare the effects of averaging inherent to the present PIV technique. The focus here is to identify vortex core distributions for which the twodimensional and three-dimensional swirl strengths are used. The studies are performed in the logarithmic region of a turbulent boundary layer at a location of 110 wall units from the wall. The dual plane PIV data are measured in a zero pressure gradient flow over a flat plate at Reτ of 1160, while the DNS data are extracted from a channel flow at Reτ of 934. Statistical tools suggest the presence of hairpin shaped vortex structures, and the effects of averaging are quantified. It is observed that the effects of averaging are small compared to the raw DNS data with excellent agreement for a variety of vortex core angle statistics. The results indicate that the present PIV technique is an accurate and very reliable method for the purposes of statistical analysis and identification of vortex structures.