Hi!

I am trying to develop a cfx model to calculate the sediment transport around structures(scour around bridge piers) in a river. But I'm having some difficulties to find the right approach. I have tried to make a multiphase model in a box with just sand and water, but then you have to define sand particles as a fluid? I couldnt make it work because there obviously isn't any viscousity for a solid. Any sugestions on a way to approach the problem will be most appreciated.

Roger

Hi,

Particle deposition and picking-up (there are probably high-tech names for these processes) cannot be modelled in CFX currently. You would need to consider coupling CFX with a discrete element model such as EDEM. Also there may be some basic functionality in this area in CFX12. But you will have to wait until next year for that.

Glenn Horrocks

Hi thx for the reply... hurry up Ansys!

Hi,

I recommend you look into discrete element models anyway as they will allow you much more physics. Look into coupling CFX with EDEM or some other DEM software.

Glenn Horrocks

Hi Thanks for the advice on the edem modelse. I have read a little about them, but I think it will be too computing intensive to have any practical use. Anyway I got the eulerian-eulerian approach to work, but the chalenge is still the change in state from when the particles are part of the river bed to becoming dispersed solids. What is bothering me is that I have seen something like this done in fluid, but I think it could be done in cfx aswell.

kind regards

CFX can be used for modelling sediment transport *within* the flow. It doesn't (as Glenn mentions) solve for the dynamics of every particle exactly, but suitable approximations to the transport of particles *within* the fluid can be made, and the Eulerian-Eulerian approach you mention can certainly be used to do this.

However, as you note, this is not the real problem.... what CFX (or for that matter any Navier-Stokes solver) *cannot* do is to tell you the conditions under which particles leave the river bed and enter the flow, or the conditions under which particles leave the flow and adhere to the river bed. The reason is that this is governed by much more detailed physics (such as electrostatics, capillary forces, flow forces on tiny individual particles, particle roughness/shape) that CFX just cannot resolve.

So, in order to proceed, you will need an empirical correlation which relates the rate of particle resuspension to the flow conditions and particle properties. This relation then needs to be implemented in CFX as a mass source term for the sand phase at the river bed boundary, probably using some CEL.

The big problem is it can be very hard to identify an appropriate model - really you need to conduct an experimental study based on the particular mud which forms your river bed. Whilst some correlations exist in the literature, they vary greatly and it is hard to identify the correct one.

I got to know this the hard way, as I was asked to finish off a study someone else had proposed, and I was missing this crucial piece of information.... It was very hard work to make convincing arguments in the absence of firm data.

So, finally I can't offer you much help, except to emphasise that this is not a pure CFD problem. You need extra data about the resuspension of sand to complete your model which CFD cannot provide. So your next, and most important, step is to identify a suitable model in the literature (backed up by experimental tests if possible). Once you've found a suitable model, then you can return to the CFD....

Good luck. Andy

Thx very much for the reply Andy. Your idea with the source term will surely be valuable. But at first I will try to make the model fit to a scour experiment with even distributed grain sizes whitch make it a little easier.

I have defined the riverbed as porous domain and the water phase as fluid. Did you use a similar approach? At the moment i'm a little sceptical to it...

best regards

my work was for thin layers of adhesive sediment in a metal floored tank - so i didn't need a porous medium model. my work certainly has differences from yours, but the need for extra information about particle resuspension to complete the model is similar.

i remember coming across many publications about river beds which were not appropriate for my study, but might help your work - so i hope you are able to find the necessary information more easily than me.

good luck andy