Spectrum analysis of fluid flow in a rotating two-pass duct with detached 90° ribs

Laser-Doppler velocimetry measurements were performed in a rotating two-pass square duct with 90degrees ribs detached from the leading and trailing walls. The ribs were square in cross-section and their detached-distance/height ratio was 0.38. The rib-height/duct-height ratio and the pitch/rib-height ratio were 0.136 and 10, respectively. The duct Reynolds number was varied from 1 x 10(4) to 4 x 10(4) and rotation number (Ro) ranged from 0 to 0.2. Results are presented of the evolutions of mean velocity components, spectra of fluctuating velocity components at various locations and Re, decay rates of autocorrelation coefficients, and variations of integral time and length scales with Ro. It should be pointed out that no previous work was reported on the spectral analysis for rotating duct flows. The effectiveness of wall jet on removing the rib-duct corner recirculating bubble (whose existence resulting in heat transfer deterioration) for stationary ducts is found to remain for rotational ducts. However, vortex shedding behind the rib top or bottom prevailing in the case of stationary ducts is absent in the case of rotational ducts according to the measured spectra. The influences of Coriolis force upon the decay rates of autocorrelation coefficients on the leading and trailing wall sides of the first radially outward pass are documented. Simple expressions are obtained to correlate integral time scale with rotation number. The information gathered is believed to be valuable for CFD development and engineering reference. (C) 2003 Elsevier Science Inc. All rights reserved.