# Question regarding surface integrals.

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 February 24, 2015, 14:02 Question regarding surface integrals. #1 New Member   Join Date: May 2014 Posts: 16 Rep Power: 5 Hello, If a surface integral value (such as pressure) is negative, what does it imply? If I generate a force report in the positive Y direction for two horizontal walls, the vector value of one is positive and the other is negative. But the surface integral values of both (which are scalar values) are negative. This has me confused, any clarification provided would be appreciated.

 February 26, 2015, 17:47 #2 Senior Member   JSM Join Date: Mar 2009 Location: India Posts: 168 Rep Power: 13 Hi, You are comparing pressure and force that are entirely different. Don't compare the signs of these properties. Pressure is scalar quantity and it has no signs. In general, fluent will show gauge pressure and it will not add the operating pressure. If it is confusing you, then plot the absolute pressure instead of static pressure. And the most possible reason for negative pressure is re-circulation nearer to the walls. In other hand, force is vector quantity and what ever the sign you are getting in Fluent is the force acting direction. techctrl likes this. __________________ With regards, JSM

February 27, 2015, 01:44
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 Originally Posted by jsm Hi, You are comparing pressure and force that are entirely different. Don't compare the signs of these properties. Pressure is scalar quantity and it has no signs. In general, fluent will show gauge pressure and it will not add the operating pressure. If it is confusing you, then plot the absolute pressure instead of static pressure. And the most possible reason for negative pressure is re-circulation nearer to the walls. In other hand, force is vector quantity and what ever the sign you are getting in Fluent is the force acting direction.
Thank you for the explanation.

You are correct that I shouldn't compare a scalar and vector quantity. I was mostly just curious as to why the pressure values end up being negative, which you clarified for me.

One more thing; does the pressure force on a wall surface and the pressure distribution on that same wall act in the same direction? If I were to draw the pressure profile on a wall surface, it should be in the same direction as the force vector, yes?

 February 28, 2015, 11:15 #4 Senior Member   JSM Join Date: Mar 2009 Location: India Posts: 168 Rep Power: 13 It is difficult to answer without considering the shape of wall and that is the reason we are using software to understand it. techctrl likes this. __________________ With regards, JSM

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