## Wind-US: One CFD Code, a lot of Physics

Finding a single CFD code that meeds your needs can be difficult if your company does a lot of different types of work. The same thing applies if a single product or application encompasses a wide range of physics. The Wind-US CFD solver, while it cannot do everything, can be applied to a wider range of physics than many other codes I have used.

Wind-US solves the governing equations in compressible (density-based) form. It has been successfully applied to flows with Mach numbers ranging from the low subsonic (less than M=0.1) to hypersonic. As long as your fluid medium is not completely incompressible (we don't do liquid flows at this point) and can be treated as a continuum (with or without particles and droplets), then Wind-US can take a shot at it.

### Viscous? Turbulent? Steady or Unsteady?

Depending on your needs, you can model inviscid fluids, laminar flow, or turbulent flow. For the latter, a selection of algebraic, one-, and two-equation RANS models are available, along with a Rumsy-Gatski algebraic Reynolds stress model. If you need a time-accurate simulation (as opposed to a steady-state solution), you may want to use one of several hybrid RANS/LES turbulence models can be used.

**Spatial Discretization Algorithms**This CFD code has a lot of different options governing the numerical algorithms used in a solution. You don't have to worry about them if you don't want to, as the defaults are usually pretty reasonable for most cases, but it's nice to know they are available if you need them. For example, there are multiple algorithms for the treatment of the explicit advection terms.

The default scheme is a Roe scheme, but several others, including HLLC, HLLE, and Rusanov, are available. These methods come with spatial differencing algorithms that range from first order accurate in space (for when you have really strong discontinuities in the flow) to fifth order upwind-biased schemes (for when you need more accuracy with less numerical dissipation).

**Time Marching Algorithms**While Wind-US can be run in explicit mode using various Runge-Kutta methods, by far the most common usage is to employ one of the implicit operators. Among these are an Approximate Factorization method, a Gauss-Seidel iterative solver, and MacCormack's modified first-order approximate factorization method.

For unsteady (time accurate) flows, a global Newton time stepping algorithm can be used with versions of several implicit schemes that are second order accurate in time to obtain good temporal accuracy with fairly large time steps.

### Unsteady (in a Good Way)

For an example of what this CFD code can do, see the figure below, which shows Mach number contours on the centerline of the Weapons Internal Carriage and Separation (WICS) bay (based on some wind tunnel experiments). The freestream Mach number for this case was 0.95, and the length to depth ratio of the bay was 4.5. This case demonstrates the ability of Wind-US to solve a time-accurate, transonic, perfect gas case.

This run employed a second order spatial differencing scheme with the HLLE algorithm. Newton iterations were used with a Gauss-Seidel iterative solver to achieve second order accuracy in time. The Nichols and Nelson hybrid RANS/LES turbulence model was used, along with wall functions, to model the effects of turbulence for this case.

### Fun with Chemistry!

In addition to conventional ideal gas simulations, Wind-US can model several more advanced approaches to the fluid chemistry and thermodynamics. One option is to use the Liu-Vinokur equilibrium air curve fits.

Another possibility is to specify arbitrary frozen (non-reacting) chemistry. The composition can vary throughout the domain, which allows for studies of mixing and diffusion effects without the added cost and complication of chemical reactions.

For fully reacting flows, this CFD code supports several variations of finite rate chemistry, and you can add your own chemical reaction mechanisms simply by creating an appropriately formatted text file. Recent improvements to the code have dramatically increased its viability for this class of problem.

### But Wait! There's More...

In addition the above, Wind-US has the ability to compute more sophisticated real gas effects for a limited number of species, and, if needed, additional species could be added relatively painlessly.

A limited magneto-fluid dynamics capability has also been developed, and efforts are ongoing to improve the multi-phase (particle tracking) algorithms. One recent improvement to the latter is the addition of droplet evaporation.

### But Will it Run Your Applications?

All of this is well and good, but everything comes down to: will it work for you? This page gives you a broad overview of the models available in this CFD code, but, obviously, there isn't space to go into all the strengths and weaknesses of the various options. If you would like to discuss your particular needs, and whether or not Wind-US is (could be made to be) appropriate for them
contact me.

Ready to move on? The next page covers
the various types of computational meshes
supported by Wind-US.

Or, you can return to the
introduction to the Wind-US CFD code page

Alternatively, go back to the
free CFD solvers
page or the
Innovative CFD home page

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