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Static or Total Pressure for FFR?

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Old   November 9, 2020, 04:39
Default Static or Total Pressure for FFR?
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Hi all!

I was hoping that anyone could help me out with the problem I am currently facing.

I am using Fluent for fluid dynamic simulations in coronary stenoses (narrowings). Currently, I am using 2D axisymmetric models for these simulations, but eventually I will be using 3D patient-specific meshes derived from image data. The aim of the CFD model is to derive a measure for the Fractional Flow Reserve (FFR), for which I need to compute the pressure drop over the stenosis. In the clinic, the FFR is measured using a pressure wire that is inserted into the artery. I have read and understood Fluent's Theory Guide regarding the different pressure definitions (static, dynamic, total, absolute) Fluent uses. I have read several papers on the Computed FFR and everyone just mentions 'pressure'. So now I am not sure if I need to use the static or total (dynamic+static) pressure to estimate the pressure drop for the FFR. Does anyone have experience with this or any other suggestions?

More details about my CFD case: I am currently using a 2D axisymmetric case of a stenosis. The domain has a length of 10cm and a radius of 2mm. The narrowing is placed in the center of the tube and does have a width equal to the diameter of the tube. At the inlet a constant velocity is prescribed, to the wall a no-slip condition and the outlet is pressure-free. (Later on, the outlet will be coupled to a 1D model or 0D Windkessel model) The viscous model is laminar and blood is simulated as a Newtonian fluid with constant viscosity.

I hope I have given you enough details about my problem to help me out. Thank you!
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Old   November 10, 2020, 16:41
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Lorenzo Galieti
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Hi, did you give an operating pressure in your set up? If not, work with total pressure. If yes, create a new function that sums total pressure and operating pressure, then use that one.
DO not work with static pressures.

Also remember to calculate inlet and outlet pressures as area averages, not single values. Should not change that much because I think the flow is fairly incompressible, but that is how it should be done
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Old   November 11, 2020, 08:22
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Hi Lorenzo,

Thanks for your suggestions!

I was wondering why you would use the area-averaged values? Is it to easily get from a field to one representative value for the pressure at the inlet and outlet or is there another (additional) reason why you would do so?
Besides, why do you suggest to use the area-averaged values instead of the mass-averaged values?
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Old   November 11, 2020, 09:32
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Hi, it is because if the total pressure is not uniform at your outlet/inlet for any reason, your quantity of interest is the average total pressure, not the point one.

Concerning area averaged, sorry I meant mass averaged, because total pressure is a quantity transported by the flow. (i.e. total pressure = static pressure + "kinetic energy", from bernoulli equation if the flow is incompressible).
Should you want the static pressure (and you don't want to), use area averaged instead, because pressure force has to do with area, not with mass
Nevertheless, I really don't think in your case it will change that much, so long as you take some form of average
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