Response of a turbulent pipe flow to a change in surface roughness

Measurements were made in the transition region between fully-developed smooth and rough wall flows in a circular tube. Measurements included mean velocities, static pressure, turbulence quantities, and Reynolds shear stress. The rough tube was formed by placing rectangular cross-sectioned rings inside a smooth tube. The effect of various ring spacings was investigated. Phenomenological and predictive models were developed to describe the flow in the transition region. The growth of the effects of the roughness change was found to be highly dependent on the positioning and height of the initial roughness element. In this investigation the first element protruded well above the smooth wall and the growth rate was proportional to the square root of the axial distance. The growth rate was unaffected by element spacing, provided the spacing was sufficient to allow a separation bubble to form between the elements. For these configurations, the mean longitudinal velocity midway between the elements can be accurately described by a three part semilogarithmic profile.