validating simpleFoam for mixinig of NonNewtonian fluids
Hi,
I was wondering if anyone has validated OpenFOAM’s simplefoam solver for simulating mechanically agitated mixing of NonNewtonian fluids, i.e. combination of MRF and HerschelBulkley model. I’ve been trying to compare the results against the following article (http://www.sciencedirect.com/science...5527010700387X) which I think serves as a very good validation test case as includes both experimental and numerical results (using Fluent), but unfortunately the results are not adding up, both with regards to power consumption (evaluated from computed impeller torque) as well as velocity profile. In the following link I’ve included the case for Re=21.5. http://we.tl/IrtS3CQZI9 I used openfoam 2.4 for the computations, so simplefoam with MRF in system/fvOptions and HerschelBulkley in constant/transportProperties. I initially performed computations assuming a Newtonian fluid with an effective dynamic viscosity computed from available correlation equations (using equations 4 and 5 in the above article). Then switched to nonNewtonian model. The velocities more or less converge to 1e6, but pressure stagnates earlier around 1e4. The mesh was generated using snappyHexMesh. For the power number (Np=P/(rho*N^3*D^5), where P=2*pi*N*T is power, N impeller speed in rev/sec, T is torque (N.m), D is impeller diameter) I obtain a value of Np=5.2 while it should be 0.57. The axial velocity (fig 4.b) does show a similar profile but it is way off, so instead of being [0.036 0.036] (after taking into account that the results are scaled by vtip=D/2*omega =0.18/2*8= 0.72 m/s in the figure), it is [0.005 0.005]. I’ve gone over anything that I could think of that may cause this, but just can’t figure out where things are going wrong. I very much hope someone with experience would give their thoughts on this. Thanks, Ali 
Hi,
Just a few more comments. I checked the definition of the HerschelBulkley model in OF 2.4 and it has been modified from the old "Fluent" like one (which has actually been commented out). Now if I take the converged solution from the new definition and carry iterations with the old definition, the simulations will diverge! why? Also I took into account that the model coefficients are relative to the density. So that's taken care of. In addition for measuring the force/torque, I included the fluid density in the force functionObject. So the combined values from viscous and pressure don't need to get multiplied by density. Having said all that, again I would greatly appreciate it if some one would respond and if one has any suggestions as to why such discrepancies are present relative to the power number and velocity profile from the article (http://www.sciencedirect.com/science...5527010700387X). I'm sure there are quite a few of you OpenFOAM users out there which use MRF and nonnewtonian models. If not HerschelBulkle, at least the power law model. Even in that case I haven't seen any validated test case. So I think this is a very good test of the validity of the implementation in OpenFOAM. Thanks Ali 
Come on fellows :(
no one has any comments/suggestions? Best Ali 
MRF and Herschel Bulkley
I'm also working with simpleFOAM, MRF and Herschel bulkley models (for fruit purees). I wanted to have a closer look at your case but it is not available anymore. Would you mind making it available one more time and then I can have a closer look? Thank you!

Hi Jonas,
Thanks for the reply. Attached is the case: https://we.tl/qBwqVNMJrd I still haven't managed to resolve the issue. I've tried a case of a turbulent mixing of a power law fluid with Rushton impeller as well, since I thought there is perhaps something wrong with the Herschel bulkley model. But the results don't seem to be promising either. I've reduced the problem all the way down to a agitating newtonian flow in laminar range using a helical ribbon. But again results are off at least by a factor of 2. if you agree, I think it is best if i send you the simpler case as well (laminar newtonian flow), and we start from there. I'll post the case sometime later this week. Thanks for the help Best, Ali 
Validation Case Coeffs (MRF HerschelBulkley simpleFOAM)
Hi Ali,
thank you for posting the case again! I also think it will be a great paper to validate the model with. I cannot figure out how you derived the Herschel BulkleyCoeffs for the transportProperties file. Where did you get rho from? Did you use the rheological properties from the paper? Have you looked at radial and tangential velocity profiles, yet? I think it's a little trickier with OF... Best, Jonas 
Hi Jonas,
Ya, the paper doesn't give all the property details, I found them in a master thesis from the same group: http://digital.library.ryerson.ca/is...ect/RULA%3A844 page 56 (68 of pdf) table 5.2 gives the respective rho and page 58, table 3 gives the mu0 for 1.5 wt % solution. They are of course not from the same sample, but if you check other quantities such as K and n and tauY, both report quite similar values. So I think it is safe to use the rho and mu0 from the MSc thesis. Concerning the radial and tangential components, yes it doesn't seem to be a direct/quick way to get them. But in any case, the provided axial component at (r = 145 mm and θ = −45°) already is sufficient for comparison, specially that it features a more complex profile than the radial and tangential. Best Ali 
Dear all,
!!please!! ... this subforum is not for questions. Its for comparisons between OpenFOAM solvers (or new shared solvers) with analytical solutions or measurements as denoted by the sticked post: Quote:

I have worked a lot with mixing and OpenFOAM. If this is still an issue, send me your case and I can take a look at it ( I realize it has been a year now)
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