November 13, 2018 at 2:51 pmmaliuzairSubscriber
I am simulating three phase flow using eulerian-multiphase approach. When I use algebraic temperature formulation for granular phase, simulations work fine, but whenever i switch granular temperature model to partial differential equation, the simulations dont converge. i change the wall boundary conditions accordingly to specularity coefficient of 0.5.
What can be the issue
November 13, 2018 at 4:59 pmRobAnsys Employee
We'll need a lot more information on the model to be able to answer that. Please can you post images of the mesh along with boundary conditions and what you're actually trying to achieve. As staff we can't download attachments so unfortunately can't review the case directly: we can look at images.
November 13, 2018 at 5:13 pmmaliuzairSubscriber
Dear Sir, I am simulating fluidization of binary mixture. My problem is as follows
Three phase (1 continuous+2 granular)
Drag: Gidaspow's drag law for air-solid
Symamlal O'brein Symmetric for solid-solid
Granular phase options:
Granular viscosity: Syamlal O'brein
Granular bulk viscosity: lun-et-al
Frictional viscosity: Johnson et al
Granular conductivity: Syamlal O'brein
Solids Pressure: Lun et al
Radial Distribution: Arastoopour
Packing limit: 0.6
Inlet: velocity inlet=air
outlet: pressure outlet
wall boundary conditions:
specularity coefficient: 0.1
particle-wall coefficient of restitution:0.9
Air: no slip
first image is Solid velocity, and the first one is solid volume fraction. These are after divergence.
November 13, 2018 at 5:40 pmDrAmineAnsys Employee
Quite hard to obtain convergence with transport equation for this funny solids fluctuation named "Granular Temperature" that is I want to ask: do you know the Dirichlet or Newmann conditions for it at the boundary?
Same debugging like any other case: start from scratch, lowe URF's for GT, save residuals for post-processing and check at first with single granular phase
November 14, 2018 at 9:58 amRobAnsys Employee
Can you post an image of the mesh: the contour in the first image doesn't look very smooth.
November 14, 2018 at 10:08 am
November 14, 2018 at 12:03 pmRobAnsys Employee
Part of the problem may be the larger particle size as it's quite close to the cell size: they must be smaller. Also, when you initialised the solution, what did you set for the solids phase? I'd suggest around 0.55 (packing limit 0.6) otherwise the bed will try and separate VERY quickly in the first few time steps.
November 14, 2018 at 12:48 pmmaliuzairSubscriber
The particle size should be smaller than the cell size. Right? I ll make the grid coarse but the problem is the size of other particle, which is much smaller.
I have two granular phases. So, the total initial volume fraction of solid phases v1 (=0.45)+v2 (=0.15)=0.6. and max packing limit of each phase is 0.6.
November 14, 2018 at 5:44 pmmaliuzairSubscriber
the problem with volume fraction and velocity magnitude is also observed in other cases even when granular temperature model is algebraic i.e. phase property instead of partial differential equation. and this problem is with one solid phase present in less quantity. there is no problem with other solid phase which is present in major quantity
November 14, 2018 at 9:28 pmDrAmineAnsys Employee
The velocity contour plot is weird. Please check it again and ensure deep convergence and oproper settings.
November 14, 2018 at 9:50 pmDrAmineAnsys Employee
And run transient.
November 15, 2018 at 11:01 ammaliuzairSubscriber
Sir the problem has been solved APPARENTLY...
the problem was in mesh size. it wasnt fine enough.
Now the issue is that there are two solids. sizes are 5 mm and 0.5 mm. previously average cell size was 6 mm ie greater than both solid sizes.
Now, the cell size is smaller than the size of large particle. the cell size is 3mm. and the contours are fine. I don't understand that. what i know is that cell size should be greater than particle size.
November 15, 2018 at 1:34 pmRobAnsys Employee
If you look through the model theory we assume the momentum, mass etc transfer between phases gets passed on a cell basis: if the cells are smaller than a particle the maths is theoretically incorrect and source terms can become too large. However, as you've seen this doesn't always mean it doesn't work.
November 15, 2018 at 4:02 pmDrAmineAnsys Employee
You know for granular flow all drag models will behave well if one has at least a cell size not larger then 10 times of particle diameters. For very coarse mesh one requires some high end treatment. This is pronounced for Geldart Particle A or A/B. What rwoolhou wrote is true. But what rwoolhou said is true.
So you are in-between and now it is working for you. Eulerian-Eulerian and mesh refinement is always an issue and very academic.
However still encouraging to run transient.
November 16, 2018 at 4:03 pmmaliuzairSubscriber
i didn't understand, what do you mean by transient? the simulations are transient
November 16, 2018 at 4:44 pm
November 16, 2018 at 4:50 pm
December 10, 2018 at 1:49 pmmaliuzairSubscriber
The final situation is (eulerian multiphase model, transient simulation)
There are two solid granular phases, with different densities, and with different sizes.
solid 1: density: 500 kg/m3
solid 2: density: 2600 kg/m3
The cases I checked are:
SAME size particles, same densities of solids, partial differential equation of granular temperature... works
DIFFERENT size particle, same densities of solids, partial differential equation of granular temperature... works
same size particle, same densities of solids, ALGEBRAIC GRANULAR TEMPERATURE... works
same size particle, DIFFERENT densities of solids, partial differential equation of granular temperature... DOES NOT work
This density difference is causing trouble in convergence for GRANULAR TEMPERATURE, despite using v low under-relaxation factors for density.
December 10, 2018 at 4:43 pmDrAmineAnsys Employee
Different mass of particles>Different Weights>Different buoyancy effects>Different Geldart Groups== Different behavior.
December 10, 2018 at 5:40 pmmaliuzairSubscriber
Understood sir, but the issue with granular temperature is numerical, instead of physical (in my opinion). Certainly the behavior of different geldart particles is different. but here the simulations don't converge. the residuals of granular temperature (particularly of heavier particle) and hence of the continuity diverge.
December 10, 2018 at 6:11 pmDrAmineAnsys Employee
Yes off course that is numerical reasoning behind it but do not forget that the "granular temperature" is an approximation of the real world and is not really famous when it comes to particle kinetics.
Please share with us last settings and models of the "non-converging case"
December 10, 2018 at 6:31 pmmaliuzairSubscriber
2D, Eulerian Eulerian multiphase model
number of phases 3
air, solid 1 (granular), solid 2 (granular)
Pressure-based solver, Transient, gravity...y=-9.8 m/s2
Geometry: 0.25 m * 2.3 m
Average cell size: 7 mm* 7 mm
particle sizes and density:
solid 1: 0.8 mm, 2600 kg/m3
solid 2: 2 mm, 500 kg/m3
Velocity inlet: air: 0.25 m/s
pressure outlet: atmospheric
Walls: no slip for air,
partial slip for solids (specified shear, x and y components = 0),
Granular conditions: Johnson and Jackson; wall-particle coefficient of restitution = 0.9
Least Squares cell based,
momentum: 2nd order upwind,
volume fraction: quick,
Granular temperature: 2nd order upwind,
transient formulation: 2nd order upwind
SOLUTION CONTROLS: Default
SOLID PHASES PROPERTIES
Granular temperature model: partial differential equation
Granular viscosity: Syamlal
Granular bulk viscosity: lun et al
Frictional viscosity: none
Granular conductivity: syamlal
solids pressure: lun et al
radial distribution: lun et al
Elasticity modulus: Derived
packing limit: 0.63
Particle: particle restitution coefficient: 0.9
Air-solid Drag: Gidaspow
Solid-Solid Drag: Syamlal Obrien symmetric
initial volume fraction:
solid 1: 0.52, solid 2: 0.1
December 10, 2018 at 7:27 pmDrAmineAnsys Employee
You need to account for granular friction(you are starting from incompressible regime), use Gidapow as granular viscosity and a blended Guidaspow for Drag and symmetric syamlal for particle-particle interaction.
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