Introduction to CFD - Part V : Solution

Solutions

Once the preprocessing is done, means once we have good mesh, we can import the mesh in specific solver are solve the desired governing equations. This step is called as solution step. The tools which take mesh as input and carryout the solution of selected governing equations are called as solvers. 

In typical solver we will be doing following steps:

a) Selecting the appropriate solvers

i) Pressure based or density based

b) Selecting the mathematical equations to be solved

i) Steady state or unsteady state equations

ii) Compressible of incompressible equations

iii) Laminar or turbulent flow equations

iv) Energy equation

v) Multiphase flow equations

vi) Radiation equations 

           And many more based on the physics you want to simulate

 c) Selecting the material properties

i) Fluid Material (Water, air etc...)

ii) Solid Material (Copper, Aluminum etc...)

d) Assigning appropriate boundary conditions

i) Inlet Boundary:

- Example 1 : Velocity Inlet with 10 m/s and normal to boundary

- Example 2: Total pressure at inlet = 101325 Pa (Gauge)

- Example 3: Mass flow rate at inlet = 12 Kg/s

ii) Outlet Boundary

- Example 1 : Pressure at outlet = 0 (Gauge)

- Example 2 : Mass flow at outlet = 10.2 Kg/s

- Example 3 : 50 % flow is going out of the outlet

iii) Wall Boundary

- Example 1 : Adiabatic and no slip boundary

- Example 2 : Constant temperature 400 K and no slip boundary

iv) Fluid domain conditions

- Example 1 : Working media is air and domain is rotating with 1000 rpm

- Example 2: Working media is water

v) Solid domain conditions

- Example 1 : Heat generation rate in solid = 20 W/m³

           Any many more depending on the problem that you are simulating

e) Selecting the Solutions controls

i) Selecting discritization schemes

- Example : 1^st order for pressure and 2^nd order for density

ii) Selecting under relaxation factors

iii) Selecting pressure correction methods

- SIMPLE/SIMPLEC/PISO

iv) Selecting the CFL number 

f) Providing the initial guess of the solution

g) Putting some monitors for getting idea about if my solution is converged or not

h) Carrying out the solution

 

You may not be in position to appreciate most of the above steps. But all the above steps will be explained in detail in chapters to come. You will come to know the meaning and importance of each and every step written above but the only conditions is you should keep on reading.

The overall idea of task carried out in solver is to solve the selected set of equations with specified boundary conditions for selected materials using selected numerical schemes

The type of numerical scheme and solution strategy will depend on which type of equations we will be solving, which type of boundary conditions we will be using and what type of geometry is involved in the problem.

 

Following chart will give an overall idea about inputs needed for any solver