To People That Want To Start CFD But Are Affraid To Get Started
In my previous blog, I did tell few aspects of CFD, now I would like to share the second encounter with CFD also known by the name Computational Fluid Dynamics. So after understanding small aspects of RANS also known widely as Reynolds Averaged Navier Stokes Equation the concept of boundary conditions met the eye.
So there are 2 types of boundary condition:
1)Neumann Boundary Condition
2)Dirichlet Boundary Condition
To simplify I can summarise that
Neumann Boundary Condition refers to fixed heat flux
Dirichlet Boundary Condition refers to fixed values.
Boundary conditions are encountered by boundary cells which is the interface between the surface and the interior cells of the mesh.
Velocity profiles are an important element when determining the boundary conditions.
We see that the flow near the wall has zero velocity due to the no-slip condition as the shear stresses are dominant.
We have to analogies in CFD, the y+ and y* wherein they make use of the friction velocity and average kinetic energy respectively. The demerits of them are that y+ has a zero value in case there is a separation of the surface because the velocity is zero.
With y* it is only the aspect that the data required is difficult to procure.
The flow near the wall isn’t normal and we need to consider the wall effects and the wall function respectively.
We can find some boundary cells with multiple boundary faces and may have different boundary conditions.
For ease of simplicity, the summation method is used wherein no matter how many faces we have we can represent it with a simple formula.
The fact of profiles of different flows is that though the flows may have the same velocity profiles the temperature profiles may vary and to indicate this we use Prandtl Number which is the ratio of kinematic viscosity to thermal diffusivity, it also has two types which are:
1)Molecular Prandtl Number
2)Turbulent Prandtl Number
To conclude most of the CFD aspects are governed with the help of transport equations which may be extended to 1D, 2D, etc.
