Direct Numerical Simulations of an Impinging Plane Jet

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SESAME Newsletter #3 (March 2018) 

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Direct Numerical Simulations of an Impinging Plane Jet

Visualisation of instantaneous fields in a plane: velocity magnitude |u|/Ujet (top) and normalised temperature (T-Tw)/ (Tjet-Tw) at the three Prandtl numbers (middle and bottom)
Visualisation of instantaneous fields in a plane: velocity magnitude |u|/Ujet (top) and normalised temperature (T-Tw)/ (Tjet-Tw) at the three Prandtl numbers (middle and bottom)

As part of the SESAME H2020 project, UCL (Université catholique de Louvain) is performing DNS simulations of a complex developing wall-bounded flows: an impinging plane jet. In that configuration, a hot plane jet is blown from a slot at the top wall of a channel and impinges on the cooler bottom wall. The jet is periodic in the spanwise direction. Impinging jets have been widely investigated both experimentally and numerically for Pr ≈ 1. The present DNS simulations were first carried out at Re = 4000, based on the jet width and velocity, and for Pr down to 0.01. Two cases are investigated: the case of a laminar uniform jet profile and the case of a fully turbulent inlet profile coming from an auxiliary channel flow simulation running in parallel. The latter case being more suitable for RANS model validation since it avoids the problem of the transitional character of the flow.

The DNS results show how the heat transfer in this configuration evolves from the turbulence-dominated case at Pr = 1 to the molecular diffusion-dominate case at Pr = 0.01. This can also be observed in the Figure where the instantaneous temperature field at Pr = 0.01 is much smoother than at higher Prandtl because the much larger heat diffusivity quickly diffuses the temperature fluctuations. The comparison between the laminar and the fully-developed turbulent inflows also show different heat transfer characteristics in the vicinity of the stagnation point, depending on the presence of velocity fluctuations in the bottom-wall boundary layers.

The results obtained so far corresponds to a low Reynolds number inlet channel (Reτ ≃ 133) which may be difficult to reproduce by several RANS models not very well adapted to low Reynolds flows. Therefore, currently at UCL more efforts are devoted in obtaining DNS results for a higher Reynolds number (Reτ ≃ 180) which will be better suited for RANS model validation.