[ufechner]

Hello,

I have an air heating with an inlet and an outlet.

For the outlet I want to define the pressure.

Can anyone explain to me the difference between:

- environmental pressure
- static pressure and
- total pressure

Which type should I use?
(I am definitly in the undersonic range).

Regards:

Uwe Fechner

[qihongming]

§There are two ways to measure pressure in fluid flow: Static Pressure, P, and Total (Stagnation) Pressure, Pt.
–Static pressure is the pressure indicated by a measuring device moving with the flow or by a device that introduces no velocity change to the flow.
§The usual method for measuring static pressure in a flow along a wall is to drill a small hole normal to the surface of the wall and connect the opening to a manometer or pressure gage. In the region of the flow away from the wall, static pressure can be measured by introducing a probe, which in effect creates a wall.
–Total pressure is the pressure measured by bringing the flow to rest isentropically (without loss).
§A device for measuring total pressure is the Pitot tube, an open-ended tube facing directly into the flow.

–Dynamic pressure = ½* Density*Velocity^2
–Dynamic pressure can also be defined as the difference between the Total pressure and Static pressure.
§Ptotal = Pstatic + Pdynamic

[matheusguzella]

Originally Posted by qihongming

§
§A device for measuring total pressure is the Pitot tube, an open-ended tube facing directly into the flow.

–Dynamic pressure = ½* Density*Velocity^2
–Dynamic pressure can also be defined as the difference between the Total pressure and Static pressure.
§Ptotal = Pstatic + Pdynamic

Hi,

You sure that the Pitot tube measure the total pressure? Don't you mean dynamic pressure? Sorry if it's a dumb question!

[murat]

I


If you see the picture, you can see where the total and static pressure is measured. Stagnation point means total pressure point( Pressure is max and velocity "0"" Total pressure includes Dynamic pressure and static pressure also. If you create a boundary condition for outlet always you can use static pressure. Environment pressure in FloEFD has two meanings. If you want to create inlet pressure by Environment pressure in Floefd it means total pressure. If you want to create outlet pressure by Environment pressure in FloEFD it means static pressure.

[will321321]

Hi,

Is it possible to specify outlet pressure instead of pressure opening?

thank you.

-will

[Boris_M]

As Murat said correctly, the environment pressure distinguishes only between inlet beeing total pressure and outlet beeing static pressure. So if the flow enters the model through such a boundary condition then the specified value is considered as total pressure and if it leaves, it is considered as static pressure.

There is no way of defining an "outlet" pressure. The flow direction is either given by the flow rate direction if specified as a boundary condition or the pressure gradient in the model.
If you have just two openings, one is inlet and the other outlet then you need at least one pressure opening and the other can be either flow or pressure opening.
For the flow opening it is clear by the name if it is an inlet or outlet flow. For the pressure opening the higher pressure is the inlet and the lower pressure is the outlet of course.

Boris