## Fluids

#### How to properly set pressure inlet boundary condition?

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• ansysuser
Subscriber
I understand that there are times when one does not know the inlet velocity, and in these cases one may wish to specify a pressure inlet boundary condition instead. The problem is that Fluent documentation says we should use the total pressure (I believe this is more commonly referred to as the stagnation pressure), p0, which is defined in the documentation as:np0 = ps + 1/2 * rho * V^2nOk. So, in this equation for p0 we see both the static pressure ps and also the velocity. If I don't know the velocity in the first place, and that is why I cannot use a velocity inlet boundary condition, then how can I calculate the total pressure value to use for the boundary condition??nFor definiteness, say I have a room with one inlet window and one outlet chimney. In the room is a combustion happening such that the combustion products are leaving the room through the pressure outlet chimney. The documentation says to use the static pressure ps for the pressure outlet boundary condition - in this case 0 Pa gauge. But what about the inlet value? Here using 0 Pa gauge would imply that p0 is equal to ps, so that the velocity is zero according to the equation above. Of course, the air that is coming into the room is not moving at zero velocity, so this assumption is false. In order to calculate p0 and assign a numerical value, I have to know both ps and also v according to the equation above. nBut if I know v, then why not just use a velocity inlet?? The problem is that I am not sure when to ever use a pressure inlet boundary condition because in order to assign a numerical value I have to know v (as in the equation above), and if I know v then I might as well use a velocity inlet boundary condition. How to get past this?.
• YasserSelima
Subscriber
I will go with you through your example at the end of this comment. But let's think together of a pipe connected to the bottom of large water tank. What boundary condition you know here.? Now let's go through some math together. The absolute atmospheric pressure is 100,000 Pa. Compare this value to the dynamic head of air flowing with 30 m/s? Now compare the value of the static pressure in the room, to the dynamic head in the chimney ... what do you think? How much error you expect in the results if ignored the dynamic head and entered only the static pressure? nDefinitely if I know the velocity, I will use it ... but what if I don't? n