# A first time CFD user's queries

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 November 17, 2016, 00:38 A first time CFD user's queries #1 New Member   Ken Join Date: Dec 2015 Posts: 15 Rep Power: 4 Hi I have a few questions about CFD which I was hoping to clarify here. (I am extremely new to this so pardon the 'stupidity' of my questions) I understand that in CFD, we begin with a mathematical problem (Navier Stokes - to model fluid motion etc). Followed by breaking this Navier Stokes equations into simpler numerical models for the discretised control volumes. The control volumes are broken down into cells and the values are obtained by means like finite difference, finite volume or finite element methods. My questions are... 1. In CFD, how is the step from the Navier Stokes -> Numerical modelling done? Like in packages like Fluent for instance, we just key in the boundary conditions but I dont remember seeing this step of Navier Stokes -> Numerical modelling. Or are we supposed to do this step ourselves before passing the numerical model into the CFD package to solve for all the cells? 2. How is the FDM/FEM/FVM conducted in the packages? I know how they are done theoretically (FDM from taylor series simplifications, FVM from conservation of mass/momentum etc and values at cell centres) but how is this 'seen' in the packages? I dont recall seeing this during my previous simulation on the CFD package. 3. Stability criteria such as the CFL etc, since we include boundary conditions into the CFD package, how do we tell what are the equations to look at for the stability? Or does it depend on quantitiesrequired from the simulation (Eg: velocity, pressures etc) and look at the formulae for these quantities to see what happens when the n-th cell approaches infinity (to see if the value will explode etc)? Thank you very much in advance. Once again very sorry if they sound silly.

November 17, 2016, 01:32
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Quote:
 1. In CFD, how is the step from the Navier Stokes -> Numerical modelling done? Like in packages like Fluent for instance, we just key in the boundary conditions but I dont remember seeing this step of Navier Stokes -> Numerical modelling. Or are we supposed to do this step ourselves before passing the numerical model into the CFD package to solve for all the cells?
1. Typically, a CFD package performs the numerical modeling internally. It has been a while since I used Fluent, but I remember specifying the order of accuracy for convective and diffusive terms, as well as the method of interpolation for cell face values. These are just two basic examples of the numerical modeling techniques you can specify during pre-processing.

In some CFD codes, you can specify things such as implicit or explicit integration, finite volume or finite difference discretization, linear solver methods, preconditioners, etc. These all vary based on the code.

So yes, you typically specify numerical modeling options before submitting a job to the CFD solver. This goes along with a discretized domain (mesh) if your code requires this. Generally, you can refer to the solver manual for information on these modeling techniques.

Quote:
 2. How is the FDM/FEM/FVM conducted in the packages? I know how they are done theoretically (FDM from taylor series simplifications, FVM from conservation of mass/momentum etc and values at cell centres) but how is this 'seen' in the packages? I dont recall seeing this during my previous simulation on the CFD package.
2. Again, the CFD software typically includes a solver manual which includes details of the discretizations and algorithms involved for solving the governing equations. This information may be in general terms, because some low-level details may be proprietary to the software. Since OpenFOAM is open-source, you can crack open the source code in conjunction with studying the manual to get a good perspective of the solution methods.

Quote:
 3. Stability criteria such as the CFL etc, since we include boundary conditions into the CFD package, how do we tell what are the equations to look at for the stability? Or does it depend on quantitiesrequired from the simulation (Eg: velocity, pressures etc) and look at the formulae for these quantities to see what happens when the n-th cell approaches infinity (to see if the value will explode etc)?
3. I am not sure about this, but I think the codes likely examine local cell quantities at each time-step/iteration step to determine and maintain stability. Again, details are likely in the solver manuals as the methodology is probably unique to the implementation.

November 17, 2016, 03:58
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Filippo Maria Denaro
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Quote:
 Originally Posted by kennyboy Hi I have a few questions about CFD which I was hoping to clarify here. (I am extremely new to this so pardon the 'stupidity' of my questions) I understand that in CFD, we begin with a mathematical problem (Navier Stokes - to model fluid motion etc). Followed by breaking this Navier Stokes equations into simpler numerical models for the discretised control volumes. The control volumes are broken down into cells and the values are obtained by means like finite difference, finite volume or finite element methods. My questions are... 1. In CFD, how is the step from the Navier Stokes -> Numerical modelling done? Like in packages like Fluent for instance, we just key in the boundary conditions but I dont remember seeing this step of Navier Stokes -> Numerical modelling. Or are we supposed to do this step ourselves before passing the numerical model into the CFD package to solve for all the cells? 2. How is the FDM/FEM/FVM conducted in the packages? I know how they are done theoretically (FDM from taylor series simplifications, FVM from conservation of mass/momentum etc and values at cell centres) but how is this 'seen' in the packages? I dont recall seeing this during my previous simulation on the CFD package. 3. Stability criteria such as the CFL etc, since we include boundary conditions into the CFD package, how do we tell what are the equations to look at for the stability? Or does it depend on quantitiesrequired from the simulation (Eg: velocity, pressures etc) and look at the formulae for these quantities to see what happens when the n-th cell approaches infinity (to see if the value will explode etc)? Thank you very much in advance. Once again very sorry if they sound silly.

I think you should distinguish between questions concerning general CFD issues and questions concerning implementation in commercial CFD codes.
What CFD textbook have you used to start with?

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