The total pressure is the force per unit area that is felt when a flowing fluid is brought to rest and is usually measured with a pitot tube type instrument. The total pressure is the sum of the static pressure and the dynamic pressure.

Ptotal = Pstatic + Pdynamic

Static pressure is felt when the fluid is at rest or when the measurement is taken when traveling along with the fluid flow. It is typically measured with pressure gauges and transmitters attached to the side of a pipe or tank wall.

When measured relative to the atmospheric pressure, the static pressure is the same as the gauge pressure.

The difference between the total and static pressure is the dynamic pressure, which represents the kinetic energy of the flowing fluid. Dynamic pressure is a function of the fluid velocity and its density and can be calculated from:

Pdynamic = 1/2*rho*v^2

Static pressure is uniform in all directions, so pressure measurements are independent of direction in an immovable (static) fluid. Flow, however, applies additional pressure on surfaces perpendicular to the flow direction, while having little impact on surfaces parallel to the flow direction.

This directional component of pressure in a moving (dynamic) fluid is called dynamic pressure. An instrument facing the flow direction (pitot tube) measures the sum of the static and dynamic pressures; this measurement is called the total pressure or stagnation pressure.

Gauge pressure is zero-referenced against ambient air pressure. so, it is equal to absolute pressure minus atmospheric pressure.

To distinguish a negative pressure, the value may be appended with the word "vacuum" or the gauge may be labeled a "vacuum gauge".

Hi Ali

Static pressure is defined as the pressure measured or calculated in a plane parallel to the local fluid velocity (see, for instance, Perry´s Chemical Engineers’ Handbook). If it is a gage pressure, it can be above the local Patm (that is, positive), it can be zero (that is, equal to the local Patm) or below the local Patm (that is, negative = vacuum). The adjective “static” has to do only with the way it should be measured or calculated.

Dear Ali Sam

The classifications of pressure are generally of two kinds.

Kind-1: Atm. pressure, Gauge pressure, Vacuum pressure and Absolute pressure. (with ref. to datum)

P_abs=P_atm+P_gauge

or

P_abs=P_atm-P_vac

P_atm is used as operating pressure in fluent

Kind-2: Static pressure, dynamic pressure and total pressure

For incompressible fluid: P_total=P_static+P_dynamic (1/2*rho*v^2)

Hence, gauge pressure can be taken as static or total.

Fluent: In fluent, there are two types of pressure boundary conditions. i.e. pressure inlet B.C and Pressure outlet B.C.

Pressure inlet B.C. requires gauge (Total pressure) as input

Pressure inlet is treated as a loss-free transition from stagnation to inlet conditions and Fluent calculates the static pressure and velocity at

the inlet and the mass flux through the boundary varies depending on the interior solution and specified flow direction.

Pressure outlet B.C. requires gauge (Static pressure) as input.

It can be used as a free boundary in an external or unconfined flow because in pressure outlet boundary, flow can move out of the domain or in- to the domain (reverse flow or back flow) depending on inside pressure condition. If back-flow occurs, outlet acts like an inlet.

I think this is the reason you are getting negative value of gauge pressure in the solution (because of the back-flow).

Thank You...

http://www.afs.enea.it/project/neptunius/docs/fluent/html/ug/node239.htm

http://www.afs.enea.it/project/neptunius/docs/fluent/html/ug/node244.htm

Hi,

Solving the incompressible Navier-Stokes equations only requires the gradient of static pressure. Hence, any type of pressure boundary condition will be defined to a constant (or reference).

Lets assume that you are going to specify the static pressure at the outlet combined with inflow velocity. The "value" of the pressure at your outlet boundary will not change the flow field.

To interpret the static pressure field, you should know the pressure value at "a location" inside the domain or at the boundary.

For our imaginary case, if you set the value of static pressure to zero at the outlet and you except to have atmospheric pressure at the outlet of your domain, then the solved static pressure field of your numerical simulation represents the gauge pressure. However, setting any different value at your boundary only shift the static pressure field inside your domain and has no effect on your velocity field.

Unfortunately I don't know the details of the boundary conditions in the Fluent, however, I hope that this answers part of your question.

Regards

Fluent defines the 'total pressure' as a 'gauge pressure' with respect to the 'operating pressure'. Hence, 'total pressure' = 'static pressure' + 'dynamic pressure'. Same for the 'static pressure', which is a 'gauge pressure'. Only the 'absolute pressure' takes also the 'operating pressure' into account (e.g. 6.2. manuals, chapter 7.3.1).

Hi all

Regarding my previous message to Feichen, I conclude that for Fluent, “operating pressure” is a synonym for Patm (local atmospheric pressure). By the way, I remember giving to my Fluid Mechanics students a list of twenty adjectives most frequently coupled to the term “pressure”: static, piezometric, modified, total, dynamic, kinetic, velocity, impact, stagnation, atmospheric, gage, manometric, relative, reference, effective, absolute, thermodynamic, partial, viscous, and vacuum. They were asked to explain each one.

The total pressure is the force per unit area that is felt when a flowing fluid is brought to rest and is usually measured with a pitot tube type instrument. The total pressure is the sum of the static pressure and the dynamic pressure.

Ptotal = Pstatic + Pdynamic

Static pressure is felt when the fluid is at rest or when the measurement is taken when traveling along with the fluid flow. It is typically measured with pressure gauges and transmitters attached to the side of a pipe or tank wall.

When measured relative to the atmospheric pressure, the static pressure is the same as the gauge pressure.

The difference between the total and static pressure is the dynamic pressure, which represents the kinetic energy of the flowing fluid. Dynamic pressure is a function of the fluid velocity and its density and can be calculated from:

Pdynamic = 1/2*rho*v^2

Static pressure is uniform in all directions, so pressure measurements are independent of direction in an immovable (static) fluid. Flow, however, applies additional pressure on surfaces perpendicular to the flow direction, while having little impact on surfaces parallel to the flow direction.

This directional component of pressure in a moving (dynamic) fluid is called dynamic pressure. An instrument facing the flow direction (pitot tube) measures the sum of the static and dynamic pressures; this measurement is called the total pressure or stagnation pressure.

Gauge pressure is zero-referenced against ambient air pressure. so, it is equal to absolute pressure minus atmospheric pressure.

To distinguish a negative pressure, the value may be appended with the word "vacuum" or the gauge may be labeled a "vacuum gauge".