Lagrangian time auto-correlation of strain-rate tensor in channel turbulence

It is confirmed that the dimensionless time for evolution of velocity gradient tensor (VGT) is local Kolmogorov time scale in homogeneous isotropic turbulence. The channel flow at Reynolds number 7 000 was calculated using large-eddy simulation in this paper. The flow field was divided into different regions according to the size of the dimensionless distance to the wall and the auto-correlation functions of different regions were normalized by local Kolmogorov time scale. The decline curves of auto-correlation functions in different regions were found not really the same. In logarithmic layer, the decline curves of auto-correlation functions in different regions almost overlapped, while the similar phenomenon did not exist in viscous bottom layer near the wall and buffer layer. The results show that local Kolmogorov time scale is not the universal dimensionless time of evolution of VGT in channel flow.