Static or Dynamic pressure

Hamed&honey · September 11, 2014, 11:27

Hi everybody
I have a question about meaning of pressure in Flow-3D
Is that is static or dynamic(due to velocity)?
See the attached picture
If the pressure is static in flow-3D, so what is the high-pressure region in near the wall or right boundary?
best regards
Dyna.jpg

Hamed&honey · October 2, 2014, 05:14

Please somebody help me
Thanks

JBurnham · October 2, 2014, 20:43

Pressure is not static in FLOW-3D, and hydrostatic pressure is never assumed to be true, except at the boundaries (when hydrostatic is set as the initial pressure condition), or if you're using the shallow water equations.

If you're going to specify pressure at the top and pressure on the side, then make sure they match. The pressure at the top is assigned at the center of the first row of "ghost cells" which are cells outside of the domain used for boundary conditions. Pressure is always calculated at cell centers, while velocities are always calculated at grid lines between cells.

You can find the cell center coordinate either by math (if all the cells are the same size and there's no stretching) or from the "mesh generated in z" section of the prpout.xxx file.

Once you know where that pressure is assigned, then use that information to set the pressure (and free surface elevation, if used) at the side boundary so that the pressures match at the elevation of the ghost cell centers. Then you won't have the drafts which you are seeing at the side.

Hamed&honey · October 3, 2014, 06:29

Dear Jeff Burnham
So many thanks for your reply
my main problem is: in free surface model, when flow impact to the solid body, Flow-3d calculate pressure near that solid body in cells.Is This pressure (P) considered effect of velocity (1/2*ro*V^2)?
best regards

Allenlin · October 9, 2014, 02:24

The pressure drawn in flow-3d is total pressure, including static pressure (due to water level) and dynamic pressure (due to fluid motion). In your case, the flow impact the wall would increasing the pressure due to the decreasing of fluid velocity.

September 11, 2014, 11:27	Static or Dynamic pressure	#1
Hamed&honey New Member Ham Ta Join Date: May 2010 Posts: 18 Rep Power: 16	Hi everybody I have a question about meaning of pressure in Flow-3D Is that is static or dynamic(due to velocity)? See the attached picture If the pressure is static in flow-3D, so what is the high-pressure region in near the wall or right boundary? best regards Dyna.jpg

October 2, 2014, 05:14	Help	#2
Hamed&honey New Member Ham Ta Join Date: May 2010 Posts: 18 Rep Power: 16	Please somebody help me Thanks

October 2, 2014, 20:43		#3
JBurnham Senior Member Jeff Burnham Join Date: Apr 2010 Posts: 204 Rep Power: 17	Pressure is not static in FLOW-3D, and hydrostatic pressure is never assumed to be true, except at the boundaries (when hydrostatic is set as the initial pressure condition), or if you're using the shallow water equations. If you're going to specify pressure at the top and pressure on the side, then make sure they match. The pressure at the top is assigned at the center of the first row of "ghost cells" which are cells outside of the domain used for boundary conditions. Pressure is always calculated at cell centers, while velocities are always calculated at grid lines between cells. You can find the cell center coordinate either by math (if all the cells are the same size and there's no stretching) or from the "mesh generated in z" section of the prpout.xxx file. Once you know where that pressure is assigned, then use that information to set the pressure (and free surface elevation, if used) at the side boundary so that the pressures match at the elevation of the ghost cell centers. Then you won't have the drafts which you are seeing at the side. Hamed&honey likes this.

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October 3, 2014, 06:29		#4
Hamed&honey New Member Ham Ta Join Date: May 2010 Posts: 18 Rep Power: 16	Dear Jeff Burnham So many thanks for your reply my main problem is: in free surface model, when flow impact to the solid body, Flow-3d calculate pressure near that solid body in cells.Is This pressure (P) considered effect of velocity (1/2roV^2)? best regards

October 9, 2014, 02:24		#5
Allenlin New Member Join Date: Oct 2014 Location: Taiwan Posts: 5 Rep Power: 12	The pressure drawn in flow-3d is total pressure, including static pressure (due to water level) and dynamic pressure (due to fluid motion). In your case, the flow impact the wall would increasing the pressure due to the decreasing of fluid velocity.