Investigation Of The Reynolds Number Effects On The Aerodynamic Characteristics Of Large Span Cylindrical Roofs

Reynolds number effects of engineering structures have always been a fundamental problem in wind engineering. Large span roofs represent an innovative form that are increasingly used for modern building. With the increasing of roof span, whilst the decreasing of structural weight, the wind-resistant design tends to be more precise, which makes more and more researchers turn their attention to the Reynolds number effect. As is well known, for arch-roof structures, there is a strong dependence of the aerodynamic behaviour on Reynolds number, and there have been no systematic studies focusing on this specific issue. In actual practice, the flow patterns around large span roofs are quite complex due to the influence of turbulent flow fields in which the structures are immersed, and corresponding Reynolds number effects depend on three critical points: geometric features, surface roughness and free-stream turbulent flow with high turbulence intensity.The experimental method of this work was validated based on the experimental data from the flow past a cylinder. The most relevant aerodynamic parameters for representing the Reynolds number effect were then proposed and corresponding identification methods were also investigated; Systematic experiments were carried out in a wind tunnel to study the Reynolds number effects of cylindrical roofs, under different geometric feature(including length-to-span ratio, rise-to-span ratio), free-stream turbulence and surface roughness conditions; Based on the above ideas, the wind load estimation methods for the design of cylindrical roofs were proposed with consideration of the Reynolds number effect, and some recommendations for future studies were also put forward. The main content of this work can summarized as follows:(1) The Reynolds number effects can be attributed to the differences among the flow state, surface boundary-layer separation and vortex structures at different Re numbers, which will directly affect the aerodynamic behaviour of bluff bodies. Thus, the analysis of the recorded surface pressures may reveal precious information on the qualitative behaviour of the flow field. The flow transition and surface boundary-layer separation can be well recognized by analyzing the surface pressure distributions. Besides, a stochastic decomposition technique(spectral proper transformation, SPT) for analyzing the vortex structures was proposed, which can detect the advection features of the dominant vortex and evaluate their contribution to the overall pressure field, as a result some recommendations for wind-resistant design may be offered.(2) Compared with a cylinder, cylindrical roof can be characterized by the ground boundary conditions. To clarify their influence on the Reynolds number effect, the ground boundary condition of cylindrical roof can be approximated as a cylinder with attached splitter plates placed at the front and rear of the body. It is found that the upstream splitter plate through increasing the turbulence intensity in free-stream flow has caused the premature transition of the separated shear layer, while the vortex shedding can be suppressed by a splitter plate in the wake. The results reveal that, the differences of the Reynolds number effects between the cylindrical roof and cylinder essentially depend on the additional turbulence in free-stream flow generated by the ground boundary condition.(3) The geometric features of cylindrical roof have a significant influence on their Reynolds number effect, and consequently systematic experiments were conducted in a wind tunnel to study the Reynolds number effects of cylindrical roofs with different length-to-span and rise-to-span ratios. Based on the variations of aerodynamic parameters with Re, it is found the transitional Re number ranges from about 6.9×104 to 4.14×105. Studies suggest that the transition of the separated shear layer from laminar to turbulent will be postponed with a decrease in the length-to-span ratio, however, opposite results can be observed in cases with the rise-to-span ratio decreasing.(4) A series of wind tunnel tests, with surface pressure measurements on a cylindrical roof at different Re numbers, have been undertaken in homogeneous turbulent flows generated by grids. The results indicate that the influences of free-stream turbulence on the Reynolds number effect essentially depend on the turbulence intensity and the turbulence length scale. It is found that increasing turbulence intensity in free-stream flow has caused a premature transition of the separated shear layer, while turbulence length scale mainly affects the Reynolds number effect of fluctuating wind loads. Under the same conditions of Re number and turbulence intensity, the small-scale turbulence with 0.18≤Lx/D≤0.33 is better able to interact with the surface boundary layer before and after separation, and consequently increase the pressure fluctuations in both the top and wake regions; The effects of surface roughness(the relative roughness ks/D= 3.0×10-4~7.5×10-4) on the Reynolds number effect of a cylindrical roof were also investigated. It is possible to make the lift-Re and drag-Re curves move towards low Re number level with surface roughness increasing. However, it is found the introduction of surface roughness has significantly reduced lift force. These findings suggest that the measured data should be further corrected, to simulate the aerodynamic behaviours in at high Re numbers through increasing the surface roughness of test model, and future work will address these specific issues.(5) Focus on the mean wind loads, a wind pressure model was proposed based on the potential flow theory, which can provide reliable wind loads estimates for both cylindrical and spherical roofs, considering the influence of Reynolds number; whilst an extensive data base has also been developed to construct the wind force models with consideration of the turbulence intensity and rise-to-span ratio. Finally, a fuzzy neural network approach was employed for the prediction of the fluctuating wind loads on cylindrical roofs at different Reynolds numbers, which can be efficiently applied in a wind-resistant design of the roof cladding and its fixings.