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December 30, 2015, 06:16 
Pulsatile flow at inlet of pipe model

#1 
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HS
Join Date: Sep 2015
Posts: 30
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Hello All,
I have a tube which is rigid and a patch on the surface of this tube which is deformable/flexible, the entire model is made from silicone. I want to investigate the interaction of this flexible part with fluid that is pumped through the pipe, so I can see how the flow of the fluid is affected by the deformation of the patch and vice versa (fluid structure interaction). An experimental study has already been carried out and now I am trying to validate the results using ANSYS by coupling the CFX analysis system of ANSYS with the transient structural analysis system. The boundary conditions are not complex. The inlet of the the tube which is an opening has a relative pressure of 100 Pa with a pulsatile flow where the flow direction is normal to the boundary condition, the outlet of the tube which is also an opening has a relative pressure of 0 Pa where the flow direction is also normal to the boundary condition. The boundary conditions of the walls (fluid structure interaction interface) are no slip walls where the walls are smooth and their velocity is relative to the mesh motion. For the pulsatile flow at the inlet I believe that I need to specify it at the inlet boundary so a sine wave function is produced by creating an expression and using it as velocity at the inlet. From the experimental data I have the following information: Pulse period = 4, pulse frequency = 0.25, Womersley number = 7.5 and Mean Reynolds number = 70. I have 60 time steps for the experimental data. I am unsure of what to enter for the expression of the pulsatile flow from this information. Any suggestions would be very welcome. Thanks! 

January 2, 2016, 00:09 

#2 
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Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
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You need to select boundary conditions which best match whatever it is you are modelling. You can model pulsing flows by velocity or pressure boundaries at the inlet or outlet. So choose the one which best matches the flow you are modelling.


July 19, 2016, 21:43 

#3 
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sajiree
Join Date: Jul 2016
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@amsys,
Hey, I am working on a similar analysis. Did you find information on the correct form of expression at the inlet? Even I am struggling to find the right form of expression to be assigned at the inlet. I am considering a trigonometric pressure equation as an inlet boundary condition. However, I am unable to decide whether it should be a sine function or cosine. And also there is some confusion related to mean pressure value added to the harmonic component of pressure. for eg. P (inlet)= P(mean)+ cos (wt) (equation is time dependent) Your help is appreciated. 

July 20, 2016, 06:44 

#4 
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
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sin and cos are the same except for phase. So a better equation might be
pinlet = pmean + Amplitude*sin(wt+phase) And you set w, Amplitude and phase to give the waveform you like. That gives you control over frequency, amplitude and phase. 

July 20, 2016, 07:55 

#5 
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urosgrivc
Join Date: Dec 2015
Location: Slovenija
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You are also able to [evaluate expresion] from time1 to time2.
it is quite usefull, it plots a graph in the cfx pre so you can see the waveform your function creates. This will show you what is going on with your pressure threw time, so you can than change the expression to make sure it is correct before you start the solver. 

July 20, 2016, 13:18 

#6 
New Member
sajiree
Join Date: Jul 2016
Posts: 5
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@urosgrivc,
Thanks, this is a useful command. I will try plotting my function. @ghorrocks, Thank you! I will define my problem more clearly. I am solving FSI problem and in my case flowing fluid deforms the solid. There is a transfer of fluid velocity to solid boundary at the fluidstructure interaction. I have pressure values measured on the left and right side of solid structure. (fluid pressure around the inlet section and outlet section of the pipe). Outlet condition is simple Dirichlet condition to be 0 Pa pressure at the outlet. It is a laminar flow with Womersley number around 7. I have a fixed frequency value. The fluid flow is oscillatory in nature. I calculated mean pressure and pressure amplitude by using the experimental values. The formula that I used for this calculation is stated below; P(t)= P (mean) + P(amplitude) *cos(wt) However, my calculated value of mean and amplitude of pressure is not satisfying the equation and hence the pressure value is always positive. Hence, there is no reversal of fluid direction and the fluid flow direction is always forward. Please guide me with the right method of calculation for mean and amplitude of pressure. I thank you for your help. Last edited by sajiree; July 20, 2016 at 18:46. 

Tags 
boundary condition, cfx & fluent, fluid flow, fsi 2way coupling, pulsatile flow 
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