In this video we will learn how to solve the complex Navier-Stokes equations, within power bound of your PC. Here concepts of RANS and FVM are introduced in a logical manner.Detailed description of video lecture is given below.
Actual Vs Averaged Solution
As we discussed in previous article, exact solution of N-S equations is too much of accuracy.It captures every minute details of turbulent flow. But an engineer is not interested in such a solution, what he needs is an averaged solution as shown below.This is in fact averaged solution of actual solution.
|Fig.1 Actual Vs Averaged solution|
|Fig.2 Averaging operation done on a turbulent flow|
How to obtain averaged solutions ?
To obtain this averaged values, instead of solving actual N-S equations we can solve something called, averaged N-S equations. Navier Stokes equations generated after averaging operation is known as, Reynolds averaged Navier Stokes equations.
|Fig.3 Conversion of Navier-Stokes equations to Reynods Averaged N-S equations using averaging operation|
Reynolds Stress - Turbulence Modeling
But RANS is not purely in terms of mean values. The last term of RANS is in terms of fluctuating components. This term is known as Reynolds stress.
Solving RANS numerically
RANS is not so difficult to solve numerically. Current CFD packages use mainly 3 numerical methods. They are FEM, FDM and FVM. Some commercial packages using these methods are listed here.
|Fig.4 Different numerical schemes used in CFD and some commercial CFD packages using them|
Finite Volume Method
The fundamental flow equations are derived in FVM, using integral approach. Here instead of considering a differential volume we will consider a finite volume of arbitrary shape. And we will say that when flow passes through it, rate of increase of quantity inside the volume will be same as flux in minus flux out, plus generation of quantity.
|Fig.5 Basic philosophy of FVM : Conservation of quantity|
Now the challenge is to solve these equations throughout the control volume numerically. Here there are surface and volume integrals. We will execute numerical integration of these equations on small non overlapping cell volumes, of arbitrary shape. So before executing numerical integration, actual control volume is split into small cells, as shown below.
|Fig.6 Conversion of Control volume in to small non-overlapping cells : Meshing|
Now we need to execute surface and volume integrals on these cells. Such a cell is shown here. The method of surface and volume integrals are also shown pictorially.
|Fig.7 Numerical approximation of volume and surface integrals in FVM|
CFD at a Glance
The steps we have explained so far are summarized here in step by step manner.
|Fig.8 Step by step CFD procedure|
|Fig.9 A sample CFD problem|
How to build a career in CFD?
Hope you got a good introduction to CFD from here. Now its time to get hands on experience on CFD packages. Most used CFD packages in industry and its relevance are given below.
- ICEM CFD - A perfect software to do meshing
- Gambit - Another meshing software, but has become obsolete nowadays
- Fluent - Most preferred CFD solver in industry
- CFX - Another good solver
- Icepak - Electronics thermal management
Now it is clear from range of CFD packages available in market that, it is not possible to learn all of them at a stretch. So first thing you have to do is to find out your area of interest and select a good CFD package which will suit your need. You will be able to produce colorful results from CFD, but the real challenge is in understanding physics behind the problem ( make sure you are good in fluid mechanics and mathematics ). Please keep in mind that most of the time CFD produces rubbish results. Reasons might be poor quality of mesh, wrong physics used in CFD solver, wrong solver selected or wrong boundary condition applied. So it is imperative to spend lot of hours running simulation and analyzing results of simulation in order to become a good CFD engineer.