Existence of positive skewness of velocity gradient in early transition

Abstract

In the statistical point of view, the skewness of the longitudinal velocity gradient is usually related to energy transfer in turbulence, for example, the Kolmogorov forward energy cascade in fully developed turbulence implies negative skewness, while Gaussian random noise and infinitesimal perturbations lead to zero skewness. We show by two DNS cases that, in the early transition stage, the skewness can be positive, in contrast to the energy cascade in equilibrium turbulence.