What are relaxation factors and how do they affect our flow field?

Adam

Dear Adam,

Relaxation schemes are iterative procedures that are employed to solve system of algebraic equations. This appears in CFD when you make use of implicit schemes for time stepping . The update looks like U^(n+1) = U^n + alpha*delU, where U is the variable being updated. The term alpha is the relaxation factor. When alpha<1, we refer to it as underrelaxation, mostly used to prevent the solution procedure from diverging and when alpha>1 we refer to it as overrelaxation, often used to accelerate convergence. The use of these factors affect the convergence of the iterative procedure, and to that extent the solution update and hence the solution.

Hope this helps

Regards,

Ganesh

Hi Ganesh

Thank you so much for your answer. What i have understood from your answer is that if we tamper with relaxation factor we are not letting the flow field take its natural course. As a result it affects the out come of our results. Then why do we even have to have such a tool because if we tamper with this factor we are bound to get wrong results right?

Regards Adam

At steady state (or convergence of an inner iteration for time-accurate results) the effect of under-relaxation disappears since at convergence u^n = u^n+1. Thus under-relaxation does not lead to wrong results as long as it is applied correctly.