Span-wise length for 3D computer modeling of bridges

One of the main challenges in three dimensional CFD simulations is the efficient use of computational resources viz. processing power, memory and computational time. In this regard the z-direction/span-wise length of the computational domain plays a vital role. If this length is less than adequate, the flow features in the vicinity of the body is not captured and the results obtained are inaccurate. But, increasing the span-wise length increases the burden on computational resources. Therefore the span-wise length should be the minimum size required to accurately represent the flow.;This thesis attempts to address the issue of adequate span-wise length for incompressible 3D flows across a bridge section for a wide range of Reynolds Numbers (Re 500 --- Re 105). The vortices formed in the wake region of the bridge section are studied and the variation in their magnitude and dimension with increasing Reynolds number is reported. It is observed that similar to circular cylinder, the wavelength of these vortices decreases with increasing Re. Based on this observation it is proposed that as the Re increases the computational domain required to accurate model the flow features decreases. To realistically capture the 3D effect one should consider at least 1.5 times the wavelength for the Re in the span-wise direction. Thus for higher Re (∼105) flows one can safely choose small span-wise lengths because the span-wise vortices can be captured in the computational domain with smaller distance in the z direction. In fact 2D simulations would also yield sufficiently accurate results. This would lead to considerable savings in terms in computational time, memory and processing power.