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# mapping CFD pressure data

Sir

I am having a CFD pressure data in bellow format in a txt file

X axis   Y axis  Z  Axis Pressure(pa)

I have created a field by importing it, after that in loads i a have used the pressure load type and and selected the surface where it will be applied, and in the magnitude place i have selected the existing field,

The  display of the data points are exactly matching to the surface where i have applied the pressure

But now i am having a confusion that when i plot the contour by right clicking on the pressure load the contour scale are showing from positive to negative value of pressure, in my data the pressure values are all positive values onlywith out absolute value ticked

secondly when i plot the contour with a absolute values which can be got from banded plot window then the pressure values are as per the data, with absolute value ticked

Now my question is that while solving the solution whether it will solve with the absolute value or the contour plot which i got without the selection of absolute value if so my result will vary because i don't have any negative pressure values in my data.

plz clarify

thank you

karthik

3 REPLIES
Solution
Solution
Accepted by Ganesh_S (Community Manager)
‎06-29-2017 05:16 PM

## Re: mapping CFD pressure data

Karthik,

The negative values may be coming from the interpolation of the field data onto the element faces / nodes that the pressure is applied to. Study the results with various interpolation methods. I agree that it is strange to have only positive values in the field show up as some negative values in the contour plot, particularly when your most negative value is really your most positive value.

Since the sign of a pressure indicates direction, I wonder if your geometry wraps around on itself, or if you have a narrow passage where the sign is flipping? For example if you have a region of the geometry represented as a U, maybe the data on the left hand side is influencing or being applied to the right hand side. If direction is taken into account during the interpolation (I don't know if it is and I don't think it should be), then that would explain the change in sign.

I'd be interested in taking a look if you can share the data. Otherwise, you may want to submit an IR through GTAC with the data going to GTAC also.

Regards,

Mark

Mark Lamping

Simulation Product Management

Simulation and Test Solutions

Siemens Industry Sector

Siemens Product Lifecycle Management Software Inc.

mark.lamping@siemens.com

www.siemens.com/plm

## Re: mapping CFD pressure data

Thank you Mark

After going through the reply of yours i have checked my model, where i am having mid surface and solid parts, the negative values are belonging to the surface elements and the positive value are from the solid elements to conform them i have taken a simple cylinder with sell (green)element and solid elements(pink)

now i am applying a constant  pressure of 2000pa an the top surface of the solid and surface also

now i am plotting the contour hear the difference

solid surface is showing positive values and the shell elements are showing the negative values

if i give absolute value then all are fine

i have tried changing the surface normal also the result is same as i stated above

now plz clarify  me that it will have any effects in the results.

thank you

regards

karthik

## Re: mapping CFD pressure data

Karthik,

The sign difference stems from behavior related to Simcenter and NX Nastran convention differences. In Simcenter, a positive pressure points inward. This means that on a solid, the positive pressure's direction points into the solid. For a sheet body, inward is the direction from the top face to the bottom face. It's in the OPPOSITE direction of the shell element normal. NX Nastran PLOAD4 pressure load has that same convention for solid elements. It has the opposite convention for shell elements. For shells, the PLOAD4 pressure is positive in the direction of the shell element normal. This is described in the NX Nastran Quick Reference Guide's description of PLOAD4. Remark 4 pertains the the direction of the load on 2D elements.

So, to get the same pressure direction in Simcenter and NX Nastran, the sign of the pressure has to be opposite one another.

Regards,

Mark