# How to achieve an oscillatory fluid motion inside a pipe

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 July 28, 2016, 17:44 How to achieve an oscillatory fluid motion inside a pipe #1 New Member   sajiree Join Date: Jul 2016 Posts: 5 Rep Power: 7 I am trying to solve a FSI (fluid-structure interaction) problem in COMSOL. Fluid flowing through the pipe is deforming the solid attached inside the pipe. It has a womersley number of 7. For this model, my inlet BC is a pulsating pressure wave defined by the formula- P(inlet)= P_mean+ P_amp*cos (wt) (time dependent analysis) My outlet BC is zero pressure (0 Pa) condition. With this set of BC's, I was hoping to get a oscillatory motion of fluid which will travel from inlet to outlet and reverse it's direction and start flowing from outlet to inlet. I am calculating p_mean and p_amplitude values by using two pressure values found out experimentally using the same formula as stated above : P(inlet)= P_mean+ P_amp*cos (wt) But due to p_mean > p_amplitude, the overall value of pressure at all time steps is always positive and there is no flow reversal from outlet to inlet. So I am guessing my pressure equation is wrong (calculation method for P-mean and P_amplitude) or my BC's are wrong (especially outlet BC to be 0 Pa). Hence I wanted to know how can I calculate p_mean and p_amplitude for such a case and how can I achieve oscillatory motion of the fluid? Your help is appreciated. Thanks.

 July 28, 2016, 17:52 #2 Senior Member   Filippo Maria Denaro Join Date: Jul 2010 Posts: 5,819 Rep Power: 62 I think that you can obtain such effect simply using a periodical pipe and a driving force (the forcing pressure gradient) that changes in time its sign.

 July 28, 2016, 17:57 #3 Senior Member   Filippo Maria Denaro Join Date: Jul 2010 Posts: 5,819 Rep Power: 62 see for example: https://www.researchgate.net/profile...df36000000.pdf I personally used the setting illustrated in 5.1 here https://www.researchgate.net/publica...ection_methods

 August 19, 2016, 13:11 #6 New Member   sajiree Join Date: Jul 2016 Posts: 5 Rep Power: 7 Hi Dr.Filippo, Thank you for your help. I followed your advice and ran my analysis with pressure equation as the forcing term in the momentum equation. I am assigning this pressure equation at the inlet of the pipe. However, to get the oscillatory profile, I had to use same equation with sin term in it to introduce a phase difference in between inlet and outlet. See below- P(inlet)= P_mean+ P_amp*cos (wt) and P(outlet)= P_mean+ P_amp*sin (wt) I used this outlet condition so that there will be pressure difference in between inlet and outlet which will govern the flow in the forward and backward direction. Although, I am not sure if the outlet pressure condition is right even when it is giving me oscillatory fluid flow. Please share your opinion on this. Also, I am still struggling to find the P_residual term of the pressure equation given in your paper. I apologize for not understanding it correctly. Can you please tell me how can I find out P_residual? I highly appreciate your help. -Sajiree

August 19, 2016, 15:31
#7
Senior Member

Filippo Maria Denaro
Join Date: Jul 2010
Posts: 5,819
Rep Power: 62
Quote:
 Originally Posted by sajiree Hi Dr.Filippo, Thank you for your help. I followed your advice and ran my analysis with pressure equation as the forcing term in the momentum equation. I am assigning this pressure equation at the inlet of the pipe. However, to get the oscillatory profile, I had to use same equation with sin term in it to introduce a phase difference in between inlet and outlet. See below- P(inlet)= P_mean+ P_amp*cos (wt) and P(outlet)= P_mean+ P_amp*sin (wt) I used this outlet condition so that there will be pressure difference in between inlet and outlet which will govern the flow in the forward and backward direction. Although, I am not sure if the outlet pressure condition is right even when it is giving me oscillatory fluid flow. Please share your opinion on this. Also, I am still struggling to find the P_residual term of the pressure equation given in your paper. I apologize for not understanding it correctly. Can you please tell me how can I find out P_residual? I highly appreciate your help. -Sajiree

the residual pressure is nothing that the solution of the elliptic pressure equation that allows to ensure the divergence-free velocity field

 Tags fsi 2-way coupling, fsi simulation, oscillatory flow