Experimental investigation of the effects of intermediate length topography on gravity-driven film flow in an inclined circular pipe

Gravity-driven film flow was experimentally investigated in circular pipes that include topography over an intermediate length of the substrate to examine the transition from isolated topography response to that for fully corrugated pipes. Three flow rates, two inclination angles, and two topography shapes were examined for substrates with integer variations of two to eight troughs. The steady free surface was comprised of features shorter than, similar to, and longer than the substrate wavelength depending on flow condition and topography. Amplitude of steady features was larger while wavelength-averaged film thickness was similar to fully corrugated pipe film flow. Transient free surface fluctuations had local amplitudes similar to and wavelength-averaged magnitudes lower than fully corrugated behavior. Distinct steady features with large amplitude and axial length approximately three-quarters of the substrate wavelength produced intermittent overhangs in the free surface not observed in fully corrugated pipes. Periodicity in transient free surface motion formed over substrates with as few as three triangular troughs and trended toward the frequency of periodic traveling waves in fully corrugated pipes for triangular topographies with six or more troughs. For intermediate length topography, periodic traveling waves were only observed over triangular topography with eight troughs. A preliminary examination of pipe exit shape reveals an effect on the location of flow features with a reduced impact on steady and transient property magnitude.