In algebraic models, algebraic equations that depend on the velocity field - and, in some cases, on the distance from the walls - are introduced in order to describe the turbulence intensity. For one- and two-equation models, additional transport equations are introduced for turbulence variables, such as the turbulence kinetic energy (k in k-ε and k-ω). In this flow regime, we can use a Reynolds-averaged Navier-Stokes (RANS) formulation, which is based on the observation that the flow field (u) over time contains small, local oscillations (u’) and can be treated in a time-averaged sense (U). Steady-state and time-dependent laminar flow problems do not require any modules and can be solved with COMSOL Multiphysics alone.Īs the flow rate - and thus also the Reynolds number - increases, the flow field exhibits small eddies and the spatial and temporal scales of the oscillations become so small that it is computationally unfeasible to resolve them using the Navier-Stokes equations, at least for most practical cases. Note that the flow is unsteady, but still laminar in this model.
Such a situation is demonstrated in the Flow Past a Cylinder tutorial model. In this case, it is necessary to solve the time-dependent Navier-Stokes equations, and the mesh used must be fine enough to resolve the size of the smallest eddies in the flow. It is then no longer possible to assume that the flow is invariant with time. As the flow begins to transition to turbulence, oscillations appear in the flow, despite the fact that the inlet flow rate does not vary with time. An example of this is outlined in The Blasius Boundary Layer tutorial model. Let us first assume that the velocity field does not vary with time. In the laminar regime, the fluid flow can be completely predicted by solving Navier-Stokes equations, which gives the velocity and the pressure fields. The weakly compressible flow option for the fluid flow interfaces in COMSOL Multiphysics neglects the influence of pressure waves on the flow and pressure fields. Density can vary with respect to pressure, although it is here assumed that the fluid is only weakly compressible, meaning that the Mach number is less than about 0.3. This is true, or very nearly so, for a wide range of fluids of engineering importance, such as air or water. We will assume that the fluid is Newtonian, meaning that the viscous stress is directly proportional, with the dynamic viscosity as the constant of proportionality, to the shear rate. The transition between these three regions can be defined in terms of the Reynolds number, Re=\rho v L/\mu, where \rho is the fluid density v is the velocity L is the characteristic length (in this case, the distance from the leading edge) and \mu is the fluid’s dynamic viscosity. After some distance, small chaotic oscillations begin to develop in the boundary layer and the flow begins to transition to turbulence, eventually becoming fully turbulent. The flow in this region is very predictable. The uniform velocity profile hits the leading edge of the flat plate, and a laminar boundary layer begins to develop. Let’s start by considering the fluid flow over a flat plate, as shown in the figure below. It has since been updated to include all of the turbulence models currently available with the CFD Module as of version 5.3 of the COMSOL® software. This post was originally published in 2013.
In this blog post, learn why to use these various turbulence models, how to choose between them, and how to use them efficiently. These formulations are available in the CFD Module, and the L-VEL, algebraic yPlus, k-ε, and low Reynolds number k-ε models are also available in the Heat Transfer Module.
The COMSOL Multiphysics® software offers several different formulations for solving turbulent flow problems: the L-VEL, algebraic yPlus, Spalart-Allmaras, k-ε, k-ω, low Reynolds number k-ε, SST, and v2-f turbulence models.