# Problem with suspended sediment concentration (scour model)

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

November 12, 2014, 02:18
Problem with suspended sediment concentration (scour model)
#1
Member

Chris
Join Date: Jun 2014
Posts: 39
Rep Power: 5
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?

Attached Images
 Unbenannt.jpg (23.7 KB, 42 views)

 November 17, 2014, 13:00 #2 Senior Member   Jeff Burnham Join Date: Apr 2010 Posts: 204 Rep Power: 10 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?

 Thread Tools Display Modes Linear Mode

 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 OffTrackbacks are On Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post ashtonJ CFX 6 August 11, 2014 14:32 PauliusRap FLOW-3D 0 August 4, 2014 04:47 nileshjrane Main CFD Forum 7 September 14, 2010 04:57 nileshjrane OpenFOAM Running, Solving & CFD 1 September 7, 2010 17:48 Srinivas Main CFD Forum 1 November 7, 2005 15:16

All times are GMT -4. The time now is 02:33.