velocity magnitude and direction!!

asal · April 30, 2014, 17:12

Hello everybody.

I have a problem to set the velocity inlet boundary condition in form of "velocity magnitude and direction".

I want to introduce 2m3/s to my domain. So by having the inlet area I did the following:
2/inlet area = inlet velocity.
When this value assigned in the form and velocity, normal to boundary, I got the correct volume flow rate at the inlet.
But since the inlet velocity is not normal to boundary, I set the direction:

X-component of Flow Direction: 0.5627
Y-component of Flow Direction: 0.8267

then I got 1.65 m3/s in the inlet!!

Does anyone know what is the problem? why it is so? and what is the solution?
thanks.

CFD-fellow · April 30, 2014, 18:44

Hi
According to your data i think your inlet face normal vector is in y direction.
When you say for normal velocity it is correct but for non normal velocity its not, this shows that Q/inlet area=inlet velocity is not a scalar equation its a vectorial equation.
For having 2m3/hr at face, you should set a volume flow rate so that its y (normal vector direction of your face) component equals 2m3/hr.
Regards

LuckyTran · May 1, 2014, 06:14

As the person before me suggested, the x,y,z-component of flow direction are the scalar components of the flow vector. Each of those components carry the units (default is m/s).

Depending on which plane your inlet lies in, you need to specify the right x or y component velocity to get the desired volumetric flowrate. And then specify the other component to get the desired direction.

asal · May 2, 2014, 04:12

Hello and thanks for your reply.

@ LuckyTran: Would you please advice how can I set the correct x and z component of velocity to have 2m3/s flow rate on the inlet? degree = 30

Tarantino · June 20, 2017, 18:59

Hello all,

I have the same problem, Asal. did you fix the problem?

Consider the following example:
Inlet is normal to -Y with total area of 0.1 m2.
1 m3/s should be imposed to the inlet with 65-degree deviation from the inlet normal.

In this case:

velocity = 1 [m3/s]/0.1 [m2]= 10 [m/s]

therfore:

y-component = 10*cos(65)
z-component = 10*sin(65)

However, this will not give the desired volume flow rate (1 m3/s)

Can anyone explain this?

thanks.

April 30, 2014, 17:12	velocity magnitude and direction!!	#1
asal Senior Member Astio Lamar Join Date: May 2012 Location: Pipe Posts: 186 Rep Power: 14	Hello everybody. I have a problem to set the velocity inlet boundary condition in form of "velocity magnitude and direction". I want to introduce 2m3/s to my domain. So by having the inlet area I did the following: 2/inlet area = inlet velocity. When this value assigned in the form and velocity, normal to boundary, I got the correct volume flow rate at the inlet. But since the inlet velocity is not normal to boundary, I set the direction: X-component of Flow Direction: 0.5627 Y-component of Flow Direction: 0.8267 then I got 1.65 m3/s in the inlet!! Does anyone know what is the problem? why it is so? and what is the solution? thanks.

April 30, 2014, 18:44		#2
CFD-fellow Senior Member Behrooz Jamshidi Join Date: Apr 2013 Posts: 110 Rep Power: 13	Hi According to your data i think your inlet face normal vector is in y direction. When you say for normal velocity it is correct but for non normal velocity its not, this shows that Q/inlet area=inlet velocity is not a scalar equation its a vectorial equation. For having 2m3/hr at face, you should set a volume flow rate so that its y (normal vector direction of your face) component equals 2m3/hr. Regards asal likes this.

May 1, 2014, 06:14		#3
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,754 Rep Power: 66	As the person before me suggested, the x,y,z-component of flow direction are the scalar components of the flow vector. Each of those components carry the units (default is m/s). Depending on which plane your inlet lies in, you need to specify the right x or y component velocity to get the desired volumetric flowrate. And then specify the other component to get the desired direction. asal likes this.

May 2, 2014, 04:12		#4
asal Senior Member Astio Lamar Join Date: May 2012 Location: Pipe Posts: 186 Rep Power: 14	Hello and thanks for your reply. @ LuckyTran: Would you please advice how can I set the correct x and z component of velocity to have 2m3/s flow rate on the inlet? degree = 30 Last edited by asal; May 6, 2014 at 12:31.

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June 20, 2017, 18:59		#5
Tarantino Member Tarantino Join Date: Feb 2013 Posts: 46 Rep Power: 13	Hello all, I have the same problem, Asal. did you fix the problem? Consider the following example: Inlet is normal to -Y with total area of 0.1 m2. 1 m3/s should be imposed to the inlet with 65-degree deviation from the inlet normal. In this case: velocity = 1 [m3/s]/0.1 [m2]= 10 [m/s] therfore: y-component = 10cos(65) z-component = 10sin(65) However, this will not give the desired volume flow rate (1 m3/s) Can anyone explain this? thanks.