Turbulence intensity scaling in uniform and non-uniform smooth open channel flow

In the wall region, the logarithmic law of the wall coincides well with experimental values for open channel flow. However, accurate determination of friction velocity and log-law constants in open channels has always been a matter of discussion. The present study explores different scaling for the turbulence intensity (u(rms)) in the outer region of smooth bed open channel flow. This scaling is an extension of the diagnostic plot originally proposed by Alfredsson & Orlu (2010) for turbulent boundary layers. The diagnostic method predicts u(rms) value in the outer region using the mean velocity distribution and free surface velocity. In the outer region of uniform smooth open channel flow a universal linear decay of the streamwise turbulence intensity with increasing streamwise mean velocity is observed. It was shown that this decay is independent of flow Reynolds number. The diagnostic method was also applied for the cases of non-uniform smooth open channel flow with adverse and favorable pressure gradients. It was observed that the effect of the non-uniformity changes the trend of the diagnostic plot in the outer region.