Numerical Simulation of Unsteady Flows using Unstructured Grid Method

1. Introduction A great variety of flows with importance to practical engineering applications are inherently unsteady and virtually all of the Newtonian fluid flows in nature are turbulent. Therefore, it is natural and unavoidable to deal with unsteady turbulent flows when Computational Fluid Dynamics (CFD) techniques are applied to engineering design and industrial use. CFD for general fluid dynamics problems is in mature state for steady flow simulations and its outlook is brighter than ever thanks to the availability of high performance computing. However, CFD for unsteady turbulent flow is still an ongoing work and a great deal of efforts are being undertaken. Although there exist several alternatives for unsteady turbulent flow simulations, the Reynolds averaged Navier-Stokes (RANS) methods remain still the only viable means for complex computations. Another issue that CFD developers and users struggle with everyday is grid generation. As CFD is now being used as a practical engineering design tool, the task of grid generation around complex geometry is one of the main obstacles in real world applications. Grid generation requires a great deal of experience and tedious manual work, which are not beneficial to field designers. This is particularly true when the traditional structured grids based on regular arrays of hexahedral cells are used. Recently, unsteady RANS methods are employed in various engineering problems such as flow in turbomachinery [1], forced and natural unsteady flow around two-dimensional (2D) bodies [2], oscillating or pulsating or swirling flow for a nozzle, channel, jet, or cylinder [3], and boundary layer and wake response to unsteady external flows [4], etc. Most of the studies show promising results of the unsteady RANS methods applied to fundamental unsteady turbulent flows. However, three dimensional (3D) flow calculations are relatively rare and the flow around a complex geometry such as ship hull has hardly been tried by solving the unsteady RANS equations. Also, it is difficult to make a clear judgment on the performance of each method because the results of