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-   -   Pressure Inlet VS velocity Inlet difference (http://www.cfd-online.com/Forums/fluent/94875-pressure-inlet-vs-velocity-inlet-difference.html)

 Mohsin November 28, 2011 23:57

Pressure Inlet VS velocity Inlet difference

Dear Fluent users

In my geometry of spray Dryer, there are 2 inlets and 1 outlet. The outlet's BC is "Pressure outlet" and for both the inlets "Velocity Inlet" BC is used. When the flow field is calculated, the contours of static pressure at the inlets is giving inappropriate values. Static pressure values are being referenced to outlet's pressure values.

To get insight into this issue, rather than "velocity inlet" BC, I used "Pressure inlet" BC for the calculation of flow field. However this time, the velocity was calculated to be very high (as opposed to experimental results at the specified pressure).

My question is: Why is there such a difference?? While using "velocity Inlet" BC Static pressure values are being referenced to pressure outlet (Means the contours are showing the range of values used in the Pressure outlet. It is also not taking account the 'operating pressure' value given by me). On the other hand, while using "pressure inlet" BC, Static pressures at inlet come out be accurate but Velocity values are not appropriate....Could anyone tell me How to get the correct values of Static Pressure while using "velocity inlet" BC.

 santiagomarquezd December 9, 2011 09:48

Mohsin, respect of reference pressure, it is used only if no pressure BC is set in order to not obtain a "floating pressure" (https://www.sharcnet.ca/Software/Flu...ug/node375.htm). In addition setting pressures both at inlet and at outlets leads you to have to set total pressure at the inlets (https://www.sharcnet.ca/Software/Flu...ug/node219.htm), with

p_T=p_S+1/2*rho*V^2

Usually, using Velocity Inlet at inlets and Pressure Outlet at outlets behaves well, but if you want to compare with experimental values sometimes is useful to set the same values you had measured. Nevertheless in case you had measured velocities in all points, is it not suitable for BC's. You have to set V at inlets and P at outlets and then to check the velocity at the outlets.

Finally, in systems where fluxes are being mixed probably you need to set the BC's far from the mixing point in order not to have influence in the mixing process.

Some ideas, maybe you can post some pics about the results on the geometry.

Regards.

 shk12345 December 9, 2011 10:37

HI

Hello
I am doing the same simulation and also in my case the problem is same.
I also need the solution to this problem.
In my case even after using the pressure boundary conditions i am not getting that desired pressure profile in the computational domain.

Kindly help me as soon as it is possible.

Thanks
SHK

 Mohsin January 3, 2012 01:23

3 Attachment(s)
Dear Santiago Marquez

I have 2 inlets and 1 outlet. The lower inlet is having a "velocity inlet" BC with a velocity magnitude of 4.678m/s. The upper inlet is also having a "velocity inlet" BC with a velocity magnitude= 0.3 m/s. The operating pressure is 297458 Pa (as the flow is considered incompressible and this value is used to compute the density of the flow, hence, this value is set according to mean flow pressure of the inlets). The outlet is creating a suction and it has a Pressure outlet" BC with a guage pressure value of -196623.3 Pa (i-e Absolute pressure=100834.7 Pa, as guage pressure will be added to operating pressure for absolute pressure). After simulation: The solution gives appropriate velocity contours but the pressure contour plots for the inlets are strange.

Picture 1: (Contours of Static Pressure at Lower inlet)
The contours of static pressure are giving negative value (similar to the outlet Pressure values)
Picture 2: (contours of static pressure at Upper Inlet)
Here, also the contours of static pressure are giving negative value (similar to the outlet Pressure values)
Picture 3: (contours of static Pressure at outlet)
Here the contours are giving negative values as specified pressure outlet Boundary condition.
Question 1: Shouldn't the contours of static pressure be similar to operating pressure value (which is 297458 Pa-mean flow pressure)? What is actually operating pressure? Why is the contour plots at the inlets are giving pressure values relative to outlet pressure values and not the one specified in operating pressure.

To check this confusion, I calculated the solution with "pressure inlet" BC at inlets and "pressure outlet" BC at outlets. In that case, pressure values were given at inlets and static pressure values came out to be appropriate but the velocity values at the inlets were calculated very high (i-e 100m/s, actually it should be 4.678 m/s).
Question 2: how is FLUENT calculating velocity magnitude for Pressure inlet case? through Bernouli equation?

I'll be grateful for your guidance.
Mohsin

 shk12345 January 3, 2012 10:18

hi

The operating pressure influence the density of the fluids and not the inlet pressure.
You can try with operating pressure at the inlet and outlet boundary condition at the outlet.
In that case that i guess you can get some results. I have done some work consistently using VOF model and got some good results .But in my case i had only once inlet.
You can try that and tell me .

shk

 shk12345 January 3, 2012 10:38

hi

also you can try to use higher order descretization schemes for pressure and lower the under relaxation factors for it

 opop002 January 15, 2015 18:15

can anyone tell me how to assign a udf temperature profile to an outflow BC in fluent or apply a udf velocity profile to a pressure outlet

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