Comparison between Time Averaged and Instantaneous PIV and Hotwire Measurements Downstream of a Delta Winglet Pair

Vortex generators (VGs) have been studied extensively in the literature owing to their ability to enhance heat transfer at modest pressure drop penalties. VGs are incorporated in heat exchanger design and are more fundamentally studied on heated flat surfaces or in channel flow. The techniques used to investigate the flow phenomena associated with VGs are classically IR imaging, particle image velocimetry (PIV), five hole pressure probes and the naphthalene sublimation technique which relates mass transfer to heat transfer by evoking the heat and mass transfer analogy. Typically, the results presented using these techniques are in time-averaged form and this implies that the majority of information that relates to enhanced heat transfer, i.e. unsteady flow phenomena, is lost. Hotwire and PIV images were obtained downstream of a delta winglet pair placed on an unheated flat surface and comparisons are made between instantaneous and time-averaged flow features using both techniques. Reasonable comparisons were obtained between the time-averaged PIV and hotwire measurements demonstrating that useful information can be gained with the use of a 1D hotwire measurement in a complex 3D flow. Examining the instantaneous flow features reveals the true nature of the unsteadiness and also elucidates some of the more complex flow phenomena. Some of the key observations from the instantaneous PIV are that at any given time the vortices shed from each delta winglet can vary in size, strength, and location from wall. These variations cause the two counter rotating vortices to sweep across the lower wall while at the same time they move towards and away from the wall causing compression and extension of the underlying boundary layer. All of these complex unsteady flow phenomena may result in enhanced heat transfer.