Odd residuals plot using NITA

Jehosh · January 7, 2016, 08:24

Hello everyone, I am modeling a simple system of water flowing down a vertical surface. (I think it is funny what some people in some forums call a simple system...) I am tracking the wavy gas/liquid interface using VOF. Recently I start using Non-iterative Time Advancement (NITA) which solves faster than without, even with time steps that are small by an order of magnitude. However, it give odd residuals plots sometimes and I don't know if there is something wrong. The interface looks sharp, shaped as I expected, the solution progresses and most of the time it looks like:

NITA normal.png

However, every once in a while I look at the residuals and they look like:

NITA odd.png

the solution doesn't crash and progresses as normal. Do I need to be worried? Is it just that Fluent is only storing a certain amount of residuals and clears it every once in a while?

Any help is much appreciated.

hwet · January 8, 2016, 00:02

IF it is transient, that is what the residuals will look like.

January 7, 2016, 08:24	Odd residuals plot using NITA	#1
Jehosh New Member Mark Schulte Join Date: Dec 2015 Posts: 19 Rep Power: 10	Hello everyone, I am modeling a simple system of water flowing down a vertical surface. (I think it is funny what some people in some forums call a simple system...) I am tracking the wavy gas/liquid interface using VOF. Recently I start using Non-iterative Time Advancement (NITA) which solves faster than without, even with time steps that are small by an order of magnitude. However, it give odd residuals plots sometimes and I don't know if there is something wrong. The interface looks sharp, shaped as I expected, the solution progresses and most of the time it looks like: NITA normal.png However, every once in a while I look at the residuals and they look like: NITA odd.png the solution doesn't crash and progresses as normal. Do I need to be worried? Is it just that Fluent is only storing a certain amount of residuals and clears it every once in a while? Any help is much appreciated.

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January 8, 2016, 00:02		#2
hwet Senior Member Join Date: Mar 2014 Posts: 375 Rep Power: 13	IF it is transient, that is what the residuals will look like.