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Forced-convection heat transfer in a heated working fluid at a thermodynamic state near its pseudocritical point is poorly predicted by correlations calibrated with data at subcritical conditions. This is primarily due to the influence of large wall-normal thermophysical property gradients that develop in proximity of the pseudocritical point on the concentration of coherent turbulence structures near the wall. The physical mechanisms dominating this influence remain poorly understood. In the present study, direct numerical simulation is used to study the development of turbulence structures within a turbulent spot, which is a more controlled turbulence environment than a fully-turbulent boundary layer, with large wall-normal property gradients. It is found that during improved heat transfer, wall-normal density gradients accelerate the growth of the Kelvin-Helmholtz instability in the shear layer enveloping low-speed streaks through baroclinic vorticity generation. This causes hairpin vortices to form at a faster rate and to mutually interact more frequently.

Numerical Study of Coherent Structures of Turbulence with Large Wall-Normal Gradients in Thermophysical Properties