Mean-flow scaling of turbulent pipe flow

Abstract

Measurements of the mean velocity profile and pressure drop were performed in a fully developed, smooth pipe flow for Reynolds numbers from 31×10³ to 35×10⁶. Analysis of the mean velocity profiles indicates two overlap regions: a power law for 60<y⁺<500 or y⁺<0.15R⁺, the outer limit depending on whether the Kármán number R⁺ is greater or less than 9×10³; and a log law for 600<y⁺<0.07R⁺. The log law is only evident if the Reynolds number is greater than approximately 400×10³ (R⁺>9×10³). Von Kármán's constant was shown to be 0.436 which is consistent with the friction factor data and the mean velocity profiles for 600<y⁺<0.07R⁺, and the additive constant was shown to be 6.15 when the log law is expressed in inner scaling variables.

A new theory is developed to explain the scaling in both overlap regions. This theory requires a velocity scale for the outer region such that the ratio of the outer velocity scale to the inner velocity scale (the friction velocity) is a function of Reynolds number at low Reynolds numbers, and approaches a constant value at high Reynolds numbers. A reasonable candidate for the outer velocity scale is the velocity deficit in the pipe, U_CL−Ū, which is a true outer velocity scale, in contrast to the friction velocity which is a velocity scale associated with the near-wall region which is ‘impressed’ on the outer region. The proposed velocity scale was used to normalize the velocity profiles in the outer region and was found to give significantly better agreement between different Reynolds numbers than the friction velocity.

The friction factor data at high Reynolds numbers were found to be significantly larger (>5%) than those predicted by Prandtl's relation. A new friction factor relation is proposed which is within ±1.2% of the data for Reynolds numbers between 10×10³ and 35×10⁶, and includes a term to account for the near-wall velocity profile.