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The dependence of uniform momentum zones on mean shear and turbulence intensity

R. Jason Hearst
Department of Energy and Process Engineering Norwegian University of Science and Technology Trondheim, Norway; Engineering and the Environment University of Southampton Southampton, United Kingdom

Bharathram Ganapathisubramani
Engineering and the Environment University of Southampton Tizard, Highfield, Southampton, SO17 1BJ, UK

Sinopsis

The influence of mean shear and turbulence intensity on the existence and population of uniform momentum zones (UMZs) in shear flows is investigated experimentally with hot-wire anemometry and particle image velocimetry. UMZs have typically been investigated in wall-bounded flows (Adrian et al., 2000; Kwon et al., 2014; de Silva et al., 2016) and have been associated with the hierarchy of eddy packets that often present as hairpin-like structures. Similar structures have also been observed in uniform shear turbulence (Vanderwel & Tavoularis, 2011) and homogeneous shear turbulence (Dong et al., 2017). Thus, it is likely that UMZs exist in these flows as well. Investigating UMZs away from the wall provides benefits in that a wide range of parameters can be tested. For instance, in a boundary layer, the turbulence intensity and mean profile are relatively fixed. However, in a wind tunnel shear flow the shear and turbulence intensity can be varied independently of one another. This is achieved here with an active grid. Four test cases are produced. Two have comparable turbulence intensity, but different linear shear. A third profile has similar turbulence intensity and centreline shear parameter, but the profile is non-linear. The final profile has linear shear within the range of the other profiles, but substantially different turbulence intensity. UMZs are detected in all four flows using modal velocities detected from instantaneous probability density functions of the streamwise velocity (Adrian et al., 2000; de Silva et al., 2016). For the cases where the turbulence intensity was comparable, but the shear was varied, it was found that the distribution of modal velocities and the number of UMZs did not vary significantly. In contrast, when the turbulence intensity is increased, then the distribution of modal velocities becomes wider and it becomes more likely to observe a higher number of UMZs in each velocity field. The present results indicate that the population of UMZs is a function of turbulence intensity rather than shear for the cases investigated here. This observation supports the finding of de Silva et al. (2016) that the number of UMZs increases with Reynolds number because the local turbulence intensity in the log-layer of wall-bounded flows also increases with Reynolds number.