Energy cascade in a turbulent boundary layer over spanwise heterogeneous roughness

One of the most attractive features of turbulences is the inter-scale energy transfer from the large energy-containing scales towards the dissipation scales. Recently, Johnson (2020, PRL, 124, 104501) found that the inter-scale energy transfer can be precisely divided into three terms: vorticity stretching, strain self-amplification, and the interaction between strain and vorticity. This brilliant finding makes it possible to characterize the cascade from the contribution of each component. Johnson focused on the homogeneous isotropic turbulence (HIT). Inspired by his work, the decomposition is applied to a heterogenous anisotropic turbulence flow: a turbulent boundary layer over spanwise heterogeneous roughness. By doing so, we try to address the following issues. First, strain self-amplification has been found to be the main cause of the cascade in HIT. Here we verify that this conclusion still hold for such a complicated turbulent flow. Second, both the instantaneous locations of intense strain self-amplification and intense vorticity stretching events are found to be highly related with the local dynamics of the large-scale low-speed motions (L-SMs). Understanding these behaviours could shed new light on the modelling of wall-bounded turbulence.