On the analytical solution of transient friction in channel flows

Abstract

[Abstract] The transient friction in channel mean flows is the sum of two contributions, i.e., the underlying laminar flow (ULF) and the purely turbulent component (PTC), and the contributions are analyzed separately by theoretical experiments. It is found that, the transient friction may be higher or remarkably lower than that in equal-Reynolds number steady-state flows. The universal time constant for plane-parallel laminar flows is reported, and the role of the time constant in a turbulent mean flow is examined. It is shown that the time constant is related to the turbulence’s frozen time. Finally, a study of the logarithmic layer during the transient flow is accomplished, which shows that the logarithmic layer is destroyed.