## Fluids

#### Residulas in Transient flow

• Nicolo
Subscriber

Hi,

I am currently working a transient  two pahse flow simulation of water and air and in the residual I have the following pattern.

basically x and y velosity get a steady value very fast and and z velosity and continouty need much higher number of itterations to reach steady values.

it enough to choose the number of itterations so that the first two parameter (red and blue line) get steady like in first part or Should I choose the number of itterations high so all of them reach the steady value (like in the second part).

Also if based on these immages you can tell that there is something wrong with my simulation, please let me know.

• Rob
Ansys Employee

Does anything change in the z-direction between time steps?

• Nicolo
Subscriber

If you mean in reality, the flow is inside a pipe and there is very little movement in z direction (which is prepedicular to flow), but if you are refering to changes in residulas, it changes (it goes a bit high and comes down a bit).

• Rob
Ansys Employee

So z-flow is near enough zero?  Residuals are a function of the error: if the starting solution was z=0 and the converged solution is z=0 then the residuals may not change much.