ANSYSCFX础训练课程8.pptVIP

That first thing to consider is whether or not you need to consider turbulence modeling at all. Basically, the types of flows can be classified as either external, internal, or natural convection, The criteria for transition to turbulent flow is different depending on the type of flow you are considering. For external/internal flows common criteria are based on Reynolds number where the length scale varies depending on the flow. For flows along a surface, the Reynolds number is based on the distance along the surface. For flows about some object the Reynolds number is based on the diameter of the obstruction. Internal flows have the Reynolds number based on the hydraulic diameter. These criteria are not steadfast and can be affected by the other factors listed. Flows involving natural convection have been observed to transition from laminar to turbulent flow over a range of Rayleigh numbers. These are the basic equations that are solved for fluid flow problems. More equations must be solved for more complex flows involving combustion, radiation, etc. For DNS, you may mention the scaling argument; where R is turbulent Reynolds number Therefore, the minimum number of grid points per integral scale is Including time discretization, the computational work Boussinesq hypothesis is reasonable for simple turbulent shear flows---see “Turbulent Flows,” S.B. Pope, p. 361-362 --A large number of models have been developed that can be used to approximate turbulence --The choice of turbulence model determines how the unsteady behaviour affects the meanflow. --Some have very specific applications, while others can be applied to a wider class of flows with a reasonable degree of confidence. Simulation Models --The high end models actually simulate the transient nature of large scale eddy fluctuations. --for certain applications, simulation is needed. E.g. buffet hood of vehicle * The most widely used turbulence model is the k-epsilon turbulence model. It is