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

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 $6.90\times {10}^4$ to $8.28\times {10}^5$ in smooth flow and from approximately $6.06\times {10}^4$ to $4.79\times {10}^5$ 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 $\mathsf{R}>4.14\times {10}^5$. 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, $\mathsf{R}<1.46\times {10}^5$. There is also a relatively significant reduction in the mean lift force as the turbulence intensity increases when $\mathsf{R}>1.46\times {10}^5$. 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.