hydrostatic pressure boundary problem

cth_yao · February 23, 2012, 01:02

I am simulating a open channel flow with wave coundary condition. I set the out let bc as a hydrostatic BC, which is defined as p=rou*g*h (CFX expression as: waterDen*g*(waterD-y) where waterD is the water depth and the waterDen is the water density) but since the free surface in the channel is moving up and down, the water depth is changing according to the wave profile as a function of time. How can I define the waterD correctly and avoid back flow?

Thanks!

evcelica · February 29, 2012, 20:57

I've never attempted this, so this might be stupid. Just a thought.
Define an expression for instantaneous water height:

Instantaneous Outlet Water Height = (areaAve(VolumeFractionWater)@Outlet) / (Total Outlet Height)

You probably already have, but if not, look at the "flow over a hump" tutorial, it may help.

monkey1 · March 2, 2012, 08:46

I would just replace your " waterDen*g*(waterD-y)" by the original formula
rho*g*height in terms of CFX variables it would be "density*g*y" or whatever your height coordinate is.
I hope someone will correct me if I am wrong. But to my knowledge writing it like this using the "density" variable would force CFX to use the density existing at the considered height...so you wouldn't have to mind about varying water levels...and having the same BC on the "air side" shouldn't cause any problem....

February 23, 2012, 01:02	hydrostatic pressure boundary problem	#1
cth_yao New Member Join Date: Jan 2011 Posts: 10 Rep Power: 15	I am simulating a open channel flow with wave coundary condition. I set the out let bc as a hydrostatic BC, which is defined as p=rough (CFX expression as: waterDeng(waterD-y) where waterD is the water depth and the waterDen is the water density) but since the free surface in the channel is moving up and down, the water depth is changing according to the wave profile as a function of time. How can I define the waterD correctly and avoid back flow? Thanks!

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
natural convection	mehrdadeng	CFX	10	February 25, 2011 05:25
total pressure boundary problem ==> flow from outlet to inlet!!	mrshb4	OpenFOAM	0	November 20, 2010 12:41
Pressure Velocity boundary condition problem	deepsterblue	OpenFOAM Running, Solving & CFD	3	September 24, 2008 07:19
Hydrostatic pressure in 2-phase flow modeling (CFX4.2)	HB &DS	CFX	0	January 9, 2000 13:19

February 29, 2012, 20:57		#2
evcelica Senior Member Erik Join Date: Feb 2011 Location: Earth (Land portion) Posts: 1,167 Rep Power: 23	I've never attempted this, so this might be stupid. Just a thought. Define an expression for instantaneous water height: Instantaneous Outlet Water Height = (areaAve(VolumeFractionWater)@Outlet) / (Total Outlet Height) You probably already have, but if not, look at the "flow over a hump" tutorial, it may help.

March 2, 2012, 08:46		#3
monkey1 Senior Member Join Date: Jul 2011 Location: Berlin, Germany Posts: 173 Rep Power: 14	I would just replace your " waterDeng(waterD-y)" by the original formula rhogheight in terms of CFX variables it would be "densitygy" or whatever your height coordinate is. I hope someone will correct me if I am wrong. But to my knowledge writing it like this using the "density" variable would force CFX to use the density existing at the considered height...so you wouldn't have to mind about varying water levels...and having the same BC on the "air side" shouldn't cause any problem....