Effects of Free-Stream Turbulence and Reynolds Number on the Aerodynamic Characteristics of a Semicylindrical Roof

Abstract

The influences of free-stream turbulence and Reynolds number on the aerodynamic characteristics of a semicylindrical roof have been investigated experimentally under uniform flow conditions. Grids were used to generate homogeneous turbulent flows with turbulence intensities varying from 5.7 to 12.2%. The diameters, D, of the semicylindrical roof models were 0.2 and 0.6 m. The Reynolds numbers based on D ranged from 6.90×104 to 8.28×105 in smooth flow and from approximately 6.06×104 to 4.79×105 in grid-generated turbulent flow. Measurements of the surface pressure indicated that, in smooth flow, the mean and RMS pressure distributions became Reynolds number independent when R>4.14×105. The introduction of turbulence has caused a premature transition of the separated shear layer from laminar to turbulent. Increasing free-stream turbulence helps to remarkably reduce the mean and fluctuating drag force at lower Reynolds number, R<1.46×105. There is also a relatively significant reduction in the mean lift force as the turbulence intensity increases when R>1.46×105. The data also suggested that the effects of turbulence essentially depend on the turbulence intensity and the ratio of turbulence length scale to cylinder diameter. The small-scale turbulence was better able to interact with the surface boundary layer before and after separation and thus dramatically increased the pressure fluctuations in both the top and wake regions.