Okay, got a question regarding "steady state" versus time dependent simulations. My understanding of flow is that it solves every problem as time dependent. If you set it as a time dependent it simply means that it will save data along the way.

This is a simulation of a gas exiting a slot and impining a plate - colors represent concentrations of various gases on the impinged surface.

In the attached, I have a video. Frame 1 shows the result of a steady state simulation that was taken as the starting point (0seconds) of a time dependent simulation. The entire video represents a time dependent 10 second simulation. This suggests that my "steady state" may have been steady as far as the overall model (95% unshown in these images), but obviously not stabilized for this region. This 10 seconds took about 2200 iterations. And as it stabilizes about 2 seconds in I would expect under 1000 iterations to stabilize.

Based on this, I added more goals locally to ensure this was accounted for in my steady state simulation. I have captured screen shots of the steady state along the way recently. There are oscillations at the ends of the slot which result in the wild goal graphs (as the data shows, the oscillations are in mm/sec).

Assuming my opening statement (steady state is solved the same but only saves the end result), then why does continuing the steady state simulation for an extended period not produce the same result? Why doesn't adding 2000 iterations to the end of a simulation produce the same end result as a transient simulation that consumes 2000 iterations?

My problem is, actual testing verifies that the steady state solution presented is accurate. So why might the time dependent simulation converge at a different condition? And how do I know in the future which answer is accurate?

Has anybody else run into a discrepancy like this in any models?

Chris,

The steady-state solver does not advance through the time-steps until the system comes to a steady state; this would be very wasteful. Instead, it sets the time dependent terms in the Navier Stokes equations to zero before setting up the systems of equations.

The transient study, on the other hand, iterates until it converges in space, then steps in time, then iterates until it converges in space...

The reason you get different answers, I'm guessing, is that there are fluctuations in the actual solution. These cannot be captured by the steady state solution, because they depend on time, and you have artificially imposed time-independence.

Think of vortex shedding, or a flickering lighter. It is impossible to capture unsteady phenomena with a steady-state solution.