Learning Forum

[xing]

In FLUENT user guide for open channel flow model, it is stated in "determining the free surface level" that "Here a horizontal free surface that is normal to the direction of gravity is assumed." My questions:

1.  I want to model a open channel flow where there is a slope (bottom of the channel is not horizontal). Does the above guideline prohibits me from using the open channel flow model since the gravity is not normal to the free surface?

2. If I must use a horizontal channel, how will FLUENT open channel flow model consider the potential energy change due to the slope?

3. One approach we are trying is to still use a horizontal mesh but specify both horizontal and vertical components of gravity based on the slope angle.

4. Also even for a horizontal domain but with a bump at the bottom, the free surface will deform near the bump. Therefore, the free surface will not be normal to the gravity force any more.  Will that be a problem?

Thanks!

 

Tao XING, Ph.D., P.E. 
Associate Professor 
Mechanical Engineering | College of Engineering 
Office: Engineering/Physics 324F 
xing@uidaho.edu | https://www.taoxing.net 
208-885-9032  |  208-885-9031 (Fax) 
875 Perimeter Dr., MS 0902  |  Moscow ID 83844-0902   |  United States

Director of Computational Fluid Dynamics Laboratory

Co-Director of Experimental Fluid Dynamics Laboratory

Associate Editor of ASME Journal of Verification, Validation and Uncertainty Quantification

Executive Member of Editorial Board of Journal of Hydrodynamics (Springer)

 

 

[Rob]

It'll be how the depth calculation is done on the boundaries. Slopes & bumps etc on the bottom of the domain are fine: it's the vertical height from domain bottom/reference of the water surface that's important. 

 

 

[xing]

Dear Rwoolhou:

 

Question 1:  For approach 1 shown on the sketch above: we specify velocity at inlet and rotate the simulation domain with a small angle (theta-exaggerated in the sketch).  We also enable gravity force. When we use the open-channel-flow model in FLUENT, what will be the "free surface level” and “bottom level” for such case at both inlet and outlet? My understanding from your reply is to find the vertical distance. So

Inlet (free surface level h  and bottom level h+L)  outlet (free surface level h  and bottom level 0)?  Will the selection of where y=0 matters?

 

 

 

 

 

 

 

 

•Question #2

As shown in the sketch above, we figured another method (Approach 2) in which instead of making a slope to our domain, we divided the gravity into horizontal and vertical direction components with angle theta similar to the angle of slope above. Free surface is at the origin. Does this approach work with the open channel flow model in Fluent and will it be equivalent to Approach 1?  If it does, what will be the "free surface level” and “bottom level” for such case at both inlet and outlet?

 

 

 

•Question #3

Is there a way to specify pressure gradient (ΔP =- ρ*g*slope) (if used with different boundary condition) in open channel flow model ?

Thanks!

[Rob]

We can't go into too much detail on here (export rules), but for 1 & 2 I'd be looking to set the bottom position as y= -L and free surface as y=0 , the error is likely to be small. Use a simple model and see how it behaves.