Seminar announcement

Prof Paul Linden
Department of Applied Mathematics and Theoretical Physics, University of Cambridge
Conditional sampling of a high Péclet number turbulent plume and the implications for entrainment
Thursday, 24 Nov 2016
Room C2.18, Craig Building 35A, ANU Campus
We present simultaneous two-dimensional velocity and scalar concentrations measured on a central vertical plane in a pure turbulent plume. We use an edge-detection algorithm to determine the plume edge, and compare the data obtained in both a fixed Eulerian frame and relative to local coordinates defined in terms of the instantaneous plume edge. In an Eulerian frame we observe that the time-averaged distributions of vertical and horizontal velocity are self-similar, the vertical velocity being well represented by a Gaussian distribution. Measurements in the plume coordinate show that there is significant vertical flow outside the edge of the plume, contrary to the classical notion of purely horizontal velocities at the plume edge. We observe large variations in the plume width and reason these to be due to the passage of large coherent structures (eddies). The flow within the plume and in the nearby ambient fluid depends on whether an eddy is present or absent. When an eddy is present and the plume is wide, the vertical velocities near the plume edge are small, but in regions where the plume is narrow and there is no eddy, large vertical velocities are observed outside the plume. This suggests that pressure forces associated with the eddies accelerate ambient fluid which is then engulfed into the plume. This is supported by our measurements that show that around 15% of the volume flux consists of ambient fluid that has been engulfed into the plume but is yet to be mixed. Finally, we show that observing significant vertical velocities outside the plume does not suggest that the characteristic width of the scalar distribution is narrower than that of the velocity; on the contrary, we show our observations to be consistent with a buoyancy distribution that is up to 20% wider than that of the velocity.

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