[amsys]

Hello All,

I want to run a pulsatile simulation of flow through a pipe like structure. I need to set up a condition where flow/wall shear stress oscillates from -60 to +60 degrees deviating from its average, that is in the axial direction, following a sinusoidal function in time. I know that I will need to use expressions and set the expression at the inlet.

My question is how do I create this expression?

Any suggestions would be greatly appreciated!

[urosgrivc]

For transient simulations, you can use (t) in your expressions.
(t) will stand for time
So you are able to use it like this... fluctuating_expression = something1 * sin(t) + something2
this will make the value of your expression to fluctuate over time.
(something1 is probably 60 for your case, and something2 might be 0, you can also use "cos" of course)
(something2 will shift the sin function up or down)
(if you want diferent frequency of fluctuation you can also multiply time so... sin(t*something3) will work for speeding up or slowing down the sin function)

It is best to then check what your expression will be doing by plotting a graph from time 0 to some time, which can be done when writing that expression (check the tabs).

You are also able to use other variables like (x) for example, so sin(x) will fluctuate some variable according to the x direction.

I hope this helps

[amsys]

Hello!


Yes that is extremely helpful, thank you!


So my expression is:

60*sin(t/0.16[s])+sin(x/1[m]).



The attached plot is what I obtain



I am not clear on one thing - I want sin(x) to fluctuate some variable according to the x direction. As my flow/wall shear stress has to oscillate from -60 to +60 degrees deviating from it's mean that is in the AXIAL direction. Would the "+sin(x/1[m])" do so?
Apology for the rather basic questions I am not a mathematician by background!


Thanks again.

Originally Posted by urosgrivc

For transient simulations, you can use (t) in your expressions.
(t) will stand for time
So you are able to use it like this... fluctuating_expression = something1 * sin(t) + something2
this will make the value of your expression to fluctuate over time.
(something1 is probably 60 for your case, and something2 might be 0, you can also use "cos" of course)
(something2 will shift the sin function up or down)
(if you want diferent frequency of fluctuation you can also multiply time so... sin(t*something3) will work for speeding up or slowing down the sin function)

It is best to then check what your expression will be doing by plotting a graph from time 0 to some time, which can be done when writing that expression (check the tabs).

You are also able to use other variables like (x) for example, so sin(x) will fluctuate some variable according to the x direction.

I hope this helps

[urosgrivc]

Yes, this looks nice I can see you have quickly got the point.
Congrats on figuring out the units, by the way, I have left that one out
but I don't understand your boundary condition to the point I could help any further, I will reread it a couple of times maybe something will come to mind.
The thread is actually written quite nicely but I just don't get it yet.

[amsys]

Great thanks!
I was asking if the "+sin(x)" part of my expression would mean that the flow oscillates from -60 to +60 degree in the axial direction, if that clarifies it anymore!

Originally Posted by urosgrivc

Yes, this looks nice I can see you have quickly got the point.
Congrats on figuring out the units, by the way, I have left that one out
but I don't understand your boundary condition to the point I could help any further, I will reread it a couple of times maybe something will come to mind.
The thread is actually written quite nicely but I just don't get it yet.