CFD Online Logo CFD Online URL
Home > Forums > Software User Forums > FLOW-3D

Problem with suspended sediment concentration (scour model)

Register Blogs Members List Search Today's Posts Mark Forums Read

LinkBack Thread Tools Search this Thread Display Modes
Old   November 12, 2014, 01:18
Default Problem with suspended sediment concentration (scour model)
Join Date: Jun 2014
Posts: 39
Rep Power: 12
chripasch is on a distinguished road
Hej all

Another question arised during the last weeks of workng with FLOW-3D (v11).
I have a very simple geometry, a straight rectangular channel with a constant bed slope of 0.0127 (=1.27%, quite steep). The width is 4 m, the flow depth is h = 0.4 m, Froude Fr = 2, mean velocity u = 3.96 m/s, hydraulic roughness ks = 0.002 m and discharge Q = 6.34 m3/s. It is a steady-state situation with uniform flow conditions. Turbulence model: RNG

I "added" an suspended sediment concentration at the boundary condition Xmin: particle diameter 0.00045 m(= 0.45 mm), 1 kg/m3. As I am already working on the problem soon described, I already disabled some parameters of the sediment scour model which should not be needed for my case. Richardson-Zaki = 0, Bed Load Coefficient = 0. Drag and entrainment coefficient stayed = 1 (default), density of particles = 2650 kg/m3.

Investigating the flow condition described above, the sediments should stay in suspension and should in no way settle. Now, please have a look at the attached picture (it is the channel from the side, showing the first 40 m; shown is the suspended sediment concentration in a range from 0 to 1 kg/m3). The question is: Why are sediments settling (linearly)? They should be kept in suspension or picked up due to the high flow velocities and turbulence. I already changed parameters like diameter, entrainment and drag coefficient. It changed the results slightly, but they should not be calibration parameters, right?

I would appreciate your help!
Attached Images
File Type: jpg Unbenannt.jpg (23.7 KB, 77 views)
chripasch is offline   Reply With Quote

Old   November 17, 2014, 12:00
Senior Member
Jeff Burnham
Join Date: Apr 2010
Posts: 204
Rep Power: 17
JBurnham is on a distinguished road
The Richardson-Zaki Coefficient Multiplier should be 1, not 0. It multiplies the Richardson-Zaki coefficient, which is a power in the R-Z equation. That equation controls drag and therefore relative separation velocity between particles and liquid. I think you have made the settling velocity very high by setting the multiplier = 0. Try it with 1. Also, since you are using RNG turbulence, make the 'maximum turbulent mixing length' TLEN = 10% of the depth of the flow. The 'dynamic option' is not theoretically reliable for sediment/water mixtures. Do these two changes fix the result?
JBurnham is offline   Reply With Quote


Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are On

Similar Threads
Thread Thread Starter Forum Replies Last Post
Overflow Error in Multiphase Modelling with Two Continuous Fluids ashtonJ CFX 6 August 11, 2014 14:32
Air-lift model with hot gases and water. Time step problem. PauliusRap FLOW-3D 0 August 4, 2014 04:47
Turbulence model for mixing problem??? nileshjrane Main CFD Forum 7 September 14, 2010 04:57
Turbulence model for mixing problem nileshjrane OpenFOAM Running, Solving & CFD 1 September 7, 2010 17:48
problem with MFR model for multiphase mixing tanks Srinivas Main CFD Forum 1 November 7, 2005 14:16

All times are GMT -4. The time now is 07:57.