MEASUREMENT AND NUMERICAL SIMULATION OF TURBULENT FLOW OVER A ROUGH SURFACE

Abstract

The effect of surface roughness on velocity distribution, turbulence intensity and friction factor in a rectangular duct with bottom wall having transverse wedge shaped ribbed surface was investigated. The aspect ratio of the rectangular duct investigated was kept as 4.The test section has a cross-section of 50mmx200mn1 and carried an about 600 mm long, 10 mm thick aluminium plate having transverse wedge shaped ribs on the upper side. The upper and the two side wooden walls of the duct were smooth surface. The range of parameters studied are: Reynolds number from 5000 to 22000; relative roughness height (e/Dh) of 0.021875; the relative roughness pitch (pie) of 4.5 and rib wedge angle of 150 . Turbulence intensity, friction factor and velocity distribution in the duct were experimentally determined for the smooth as well as the rough surface. Hot wire anemometer was used for the measurement ' of turbulence intensity and velocity distribution inside the duct. A depth gauge was used for the vertical traverse of the hot wire probe and readings - were noted at each 2 mm vertical height along the measurement section. The voltage readings from the hot wire anemometer were acquired -in a computer at a time interval of 0.010 sec with the help of PCI-1711 L data acquisition card. Experimental results for the smooth and rough surface were compared. It was found that the velocity distribution at measurement section at any Reynolds number is much uniform at the core of the flow for both the smooth and rough (ribbed) surface. Also the ribbed surface produces a much flatter velocity profile in the core region. The maximum turbulence intensity at Reynolds number of 21000 was found to be 26 % for the rough surface and 8 % for the smooth surface: The friction factor was found to be increased by 3.42 times for the rough surface as compared to the smooth surface at Reynolds number of 21000. The friction factor values obtained for the rough surface compared well with those obtained experimentally by Karwa al. (1994). Numerical results were also obtained for the rough surface by the CFD analysis using commercial software FLUENT 6.1. The standard k-w turbulence model was used for CFD analysis. Comparison of experimental and numerical results shows reasonable agreement for turbulence intensity and velocity distribution.