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Has anyone tried modelling multiphase flow (interFoam solver) using LRR or SSG turbulence closure in OpenFOAM? I want to discuss about it.
Subhojit
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Yes, and it does not show good precision.
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Using COMSOL software to realize the numerical simulation of multiphase flow problem, in the field of microfluidic research, how to realize the simulation of the dynamic contact angle of droplets?
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Ziyi Wu, Thank you, I know what you said. But my question is about how to simulate dynamic contact angle instead of static contact angle.
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I am performing a simulation in Ansys fluent using multiphase flow settings for two phase flow problem and with out multiphase flow settings for single phase flow. The results differs heavily and huge enhancements in multiphase flow.
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THE ENHANCEMENT OF HEAT TRANSFER BY TWO PHASE OR MULTIPLE IS MORE THAN SINGLE PHASE
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Hi dears I encounter some problems in growth rate udf and I share them. I hope someone help me out. I have a growth rate in population balance which depends on the diameter: G(L)=(Rp*L0*L0*L0)/(3*ro*di*di*di) Which L0 is initial diameter and di is next diameter due the growth. I set initial diameter, but next step time initial diameter will change and I don't know how udf can recognize new diameter. Moreover, how I can set di?
bests
Mohammadhossein
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Without more details, I only may speak about the phenomenological aspects that may be verified. (i) When one applies User Defined Function (UDF) this should not contradict the mass conservation of the dispersed phase (ii) the initial diameter would depend on many factors: surface tension, turbulence, injection device...References on dispersed turbulent flows are available on the following research projects: https://www.researchgate.net/project/Single-Phase-and-Multiphase-Turbulent-Flows-SMTF-in-Nature-and-Engineering-Applications https://www.researchgate.net/project/Interfacial-Transfer-Closure-for-Eulerian-Eulerian-Two-Fluid-Modeling https://www.researchgate.net/project/Turbulence-Closure-for-Eulerian-Eulerian-Two-Fluid-Models
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Hello all,
I intend to perform a CFD analysis of the well-known bridge pier scour problem. I am considering two software packages for this purpose: Ansys Fluent and Flow 3D. I am trying to explore the pros and cons of each package for my case.
I would appreciate your comments on any of the following.
1- In order to make a sound comparison, I need to understand the modelling details implemented in Flow 3D, especially a clear description of the way sediment transport is modelled and coupled with the hydrodynamic solver, and how the interphase interaction is realized. Despite lots of research, I have not been able to find detailed information on this.
2- Any study on comparing the performance of these two packages for bridge pier scour problem.
Regards,
Armin
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First difference (discretization method): FLOW-3D implements finite differences while Ansys Fluent solves using finite volumes.
Second ("speed"): FLOW-3D is numerically "faster", due to the discretization method and because it has the possibility to simulate a single fluid (Water). Fluent resolves both fluids (Air-Water), at least with VOF.
Third (sediment transport): FLOW-3D allows coupling semi-empirical models of sediment transport to flow hydrodynamics (somewhat simplified, but better than nothing!). Until the version I have handled in Fluent (2019R2), there is no way to model sediment transport. You need to make a UDF for this purpose or try with particle models, for different grain sizes (field unknown to me!).
In my opinion, FLOW-3D is much more suitable CFD software for solving river hydraulics problems. In that aspect it seems to me that it has a great advantage over Fluent,
Good Luck,
Leonardo
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The pressure boundary in multiphase simulations is important. A widely used pressure boundary may be to enforce the normal gradient of the pressure to be zero. However, this is not always physical and the accuracy is poor. How to define the pressure boundary more physically, in particular, the partially wetting boundary condition is involved together. Have any suggestions?
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More information is needed to answer this question. Basically, even in single-phase flows, the normal gradient of the pressure is almost zero only in parallel or almost-parallel vertical laminar flows. The average pressure is "permeated" by turbulence even in 2D parallel flows. When it comes to two-phase flows and within the framework of RANS modeling using Eulerian-Eulerian two-fluid models closure is needed in order to express the pressure in one phase as a function of that of the second phase. The simplest considers that the two pressures are equal in the absence of surface tension. See references within the research projects: https://www.researchgate.net/project/Single-Phase-and-Multiphase-Turbulent-Flows-SMTF-in-Nature-and-Engineering-Applications https://www.researchgate.net/project/Interfacial-Transfer-Closure-for-Eulerian-Eulerian-Two-Fluid-Modeling https://www.researchgate.net/project/Turbulence-Closure-for-Eulerian-Eulerian-Two-Fluid-Models
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Hello all,
Source terms have been known to cause reliability issues in numerical methods affecting therein convergence and accuracy. I am currently facing a similar challenge when trying to solve a Poisson equation with a non-zero divergence velocity field. The source term that I am working which is a cavitation source term dependent on local value of pressure.
For the most part, linearizing that source term seems to solve the issue in the literature however even with linearization my Poisson equation does not converge, and even when it does, the solution is inaccurate and often oscillatory.
Any input from the experts would be helpful
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Noted professor Stam Nicolis , I will keep that in mind. Thanks for your time and help!
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In Ansys Fluent, there is an option to solve your multiphase flow problem using either Eulerian, Mixture or VOF models. Can we analyze phase change of water to steam using VOF?
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VOF is used where surface tracking among the phases is desired. In rest of the cases scholar mostly used Eulerian Model for steam water direct interaction problems.
VOF:DOI:
Eulerian Model : Numerical study of flow and direct contact condensation of entrained vapor in water jet eductor
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Hello,
I'm currently working on modelling multiphase flow with phase field, I'm using the PDE toolbox to simulate the phase field equation.
As described in this article: https://www.comsol.com/paper/adaptive-mesh-refinement-quantitative-computation-of-a-rising-bubble-using-comso-64111 AMR should lead to to massivley improved performance. But I'm not able to reproduce the results presented in the article. Does anyone has experience in time dependent AMR for multiphase flow in Comsol and could give me a hint what are the proper paramater in the AMR solver of COMSOL?
Thanks a lot.
Best regards,
Lukas
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We have tried AMR for multiphase flow with phase-field in the past. Usually, refinement is dependent on the phase-field parameter and you can control the number of refinements in a given time scale of simulations. You can refer to the following articles for more information on the implementation of AMR in the Comsol phase field.
1) Yue, P., Zhou, C., Feng, J. J., Ollivier-Gooch, C. F. and Hu, H. H., "Phase-field simulations of interfacial dynamics in viscoelastic fluids using finite elements with adaptive meshing," Journal of Computational Physics, 2006, v. 219, n. 1, pp. 47-67.
2) Zhou, C., Yue, P., Feng, J. J., Ollivier-Gooch, C. F. and Hu, H. H., "3D phase-field simulations of interfacial dynamics in Newtonian and viscoelastic fluids," Journal of Computational Physics, 2010, v. 229, n. 2, pp. 498-511.
3) Yue, P., Feng, J. J., Liu, C. and Shen, J., "A diffuse-interface method for simulating two-phase flows of complex fluids," Journal of Fluid Mechanics, 2004, v. 515, pp. 293-317.
4) Yue, P., Zhou, C. and Feng, J. J., "Spontaneous shrinkage of drops and mass conservation in phase-field simulations," Journal of Computational Physics, 2007, v. 223, n. 1, pp. 1-9.
5) Yue, P., Zhou, C. and Feng, J. J., "Sharp-interface limit of the Cahn–Hilliard model for moving contact lines," Journal of Fluid Mechanics, 2010, v. 645, pp. 279-294.
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I benchmarked my Shan-Chen Multiphase LB model using the droplet test; further I am trying to shear deformation of the droplet under moving parallel plates (in opposite directions). I would like to plot Capillary number v/s Deformation for the validation, but I am bit skeptical about the deformation and Ca calculations and hence to check for the Re of my simulation.
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I doubt you gonna get reliable results studying droplet deformation in liquid-liquid dispersed flow by means of simulations with commercial codes. The phenomena involved is too complex. Droplets break and coalesce under simultaneous population and wall effects, also depending on the interfacial tension between phases as well as on the local pressure (assuming constant temperature). For a given pair of fluids, the relative velocity between droplets and continuous phase depends on size and shape of droplets. The relative velocity also induces pairs of vortices rotating in opposite directions inside droplets (conservation of angular momentum). These may cooperate for the droplet breakage.
Even though it may be well outside your research scope, experiments are required if you want reliable data.
I guess a little craft hability is required. Build a mechanical device where small parallel glass plates (same width and thickness as those used in microscopy) are pulled along in opposite directions, similar to your Couette rheometer. Clearly the experiment will be 2D. The small gap between plates would be filled with the liquid-liquid dispersion. Some sort of rail system will be required for guiding the sliding plates. The plates could be pulled with the help of an electrical DC electrical motor with variable rpm and a system of pulleys. Make photograph/film from above. Ah, include my name on the Patent application (laugh).
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Modeling multi-phase flow propagation using Ansys Fluent, OpenFOAM, Flow-3D, etc., how the force, such as Drag force and Surface Tension, apply to each of the phases?
What about other forces? Do the forces such as, Added Mass, and Basset Force apply in the all phases? How?
Is there any concise reference or instruction for the modeling of multicomponent fluids?
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Some math about your questions and useful references.
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Hello guys,
I would like to know whether I could simulate three phases or three liquids in the VOF model in Ansys Fluent. If we can, then how can we do it?
One is Air and the other two liquids are polymers. Any suggestions are highly appreciated!
Thank you
Rajesh
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Resources and academic tools on turbulent single-phase and multiphase flows in industrial facilities and in natural environments
SINGLE PHASE AND MULTIPHASE TURBULENT FLOWS (SMTF) IN NATURE AND ENGINEERING APPLICATIONS | Jamel Chahed | 3 publications | Research Project (researchgate.net)
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Hello experts, I am a new to ANSYS fluent. Currently, I am facing some issues in my problem which is on multiphase flows. I have used VOF method in Ansys fluent but during my simulation I am unable to incorporate the material properties of my inlet fluids. Can anyone help me to know the process of incorporating more than two fluids in such multiphase flow analysis in Fluent ? Here I am attaching the screenshot of the procedure of applying material properties I have been using. I am confused whether the procedure I am applying is correct.
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I refer you to two discussions which bear on the two problems of Eulerian two-fluid models closures: Turbulence and momentum interfacial transfer:
(6) Turbulence Closure for Two-Fluid RANS Modeling (researchgate.net)
(6) Interfacial Transfer Closure for Two-Fluid RANS Modeling (researchgate.net)
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Hi all,
The case I want to simulate includes a porous media baffle, with water on the left side and no water on the right. I want to simulate the process that water on the left flows into the porous media and then flows into the right side.
In my simulatioin, the seepage velocity (from the soil to the fluid domain) at the interface is calculated by solid part. Then, there should be more water at the interface in fluid domain. I’ve managed to couple the seepage velocity at the interface in OF, but how could I add the water due to the seepage according to the velocity at the interface? Could anyone please give me any hints?
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I refer you to two discussions which bear on the two problems of Eulerian two-fluid models closures: Turbulence and momentum interfacial transfer:
(6) Turbulence Closure for Two-Fluid RANS Modeling (researchgate.net)
(6) Interfacial Transfer Closure for Two-Fluid RANS Modeling (researchgate.net)
Look forward to exchanges on these topics
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Hello Everyone, I am trying to simulate a Y shaped channel using VOF method in Ansys Fluent. Discretization of the model has been done. Inlet velocities of both dispersed and continuous phase are calculated using Capillary Numbers. I have considered no slip boundary condition at domain wall. Hereunder is the method I have applied while performing the simulation: Method- a)pressure velocity coupling scheme= coupled with vof b) Discretization method- Green gauss cell discretization method c) Pressure-PRESTO d)Momentum-Second order upwind e) Volume fraction-compressive f) Transient formulation-Bounded second order implicit Provided a suitable Residual of 10^-6, Time step size- 100 and  Max iterations/step size-20. My objective is to form a Janus Droplet. But droplet is forming but no janus droplet formation took place applying this methodology. Can anyone please throw a light on where I am doing wrong or is there any other process to form a Janus droplet.
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I'm looking for publications regarding the Two-Relaxation Time Lattice Boltzmann Method's forcing schemes without passing through the Multi-Relaxation Time's full procedures
I'm specifically searching for the TRT implementation of Shan-Chen forcing scheme (ueq=u+tau*F/Rho)
Any contribution is highly appreciated
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Almost a year later, but anyways:
I wrote together with someone else an article that probably suits your needs:
Preprint is on researchgate.
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hi.
I have a problem in my simulation work. I use flow-3d for simulating a surface vortex with an air core in a pipe-tank system. I have been successful simulating the vortex with 1cm mesh size, but I haven't been able to see the air core. Due to this problem I had to had to use finer mesh with size of 0.6cm.
When using finer mesh ,my streamlines of the near surface particles change dramatically in comparison with the streamlines I had when using coarser mesh with 1cm mesh cell size.
In coarser mesh my streamlines are strongly sucked and driven into my pipe but when i reduce my mesh cell to 0.6cm, with exactly the same setup my streamline suddenly get shorter and incomplete. It seems that after changing the mesh size the flow of water in to the pipe is not enough strong to create the enough suction for near surface particles of water and due to this there i see no air entrainment afterward.
I will upload the photos of my streamlines in both conditions, I'll be so delightful if anyone can tell what is the cause of this problem I have and how I can solve it?
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Giuseppe Altieri thanks very much for your answer
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I need two model heat exhanger between air and water. But air will be admitted in liquid state at negative temperature and on leaving the heat exchanger it should be in gaseous state.
In this problem two fluids are involved
Water
Air
Once again Air has enter in
Liquid state
On transferring heat from water it has to be converted to
Vapour.
I'm aware interphasechangefoam for phase change and CHTMultiregionfoam for two fluids. But in this case two fluids are involved. In this two fluid, one fluid has to undergo phase change.
Regards
Dr. Ijaz Fazil.
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Yes you can, if those different fluids are separated. Just add another fluid region to your case.Then you can define individual thermophysicalProperties.
Let's say you have your heat exchanger which is set as a solid region, then you can define water as one fluid region on the outside of the HX and and Air as another flui region on the inside of the HX. The two fluids will be interacting with eachother only thermally.
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Dear. all,
I performed a direct numerical simulation of multiphase flow in a porous medium (considering surface tension).
How to determine the relative permeability?
In Wikipedia, it was not mentioned which flux (inlet or outlet?), how to calculate the gradient pressure for each phase?
What if I have changing viscosity (e.g. due to the temperature)
BR,
Evgenii
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In my limited understanding, i utilized a steady state model to calculate the permeability of certain structures (resembling metallic foam). The simple way that I used to adopt was to apply a known pressure difference at the inlet and outlet, and obtain volume flow rate through post processing. This would simply give you the permeability. Ofcourse, the higher the volume flow rate, the more permeable the porous medium shall be. However, under transient conditions and using variable viscosity, the problem gets much more complicated. Nonetheless, steady state isothermal condition can be a good starting point and may give you a reasonably accurate ball park value.
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Hello everyone, hope all is good.
I want to analyse multi-phase fluid flow through pipe, which software is best to do so ?
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Go for Ansys CFX or COMSOL.
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Dear All,
I have been working on multiphase flow through a complex geometry having porosity 0.25. I have patched it with oil and high-salinity brine. At the inlet I am flowing low-salinity brine. It is a pressure based, laminar, species transport without chemical reaction problem.
I am opting for a VOF steady state simulation since I wanted to pressure drop at water breakthrough and volume fraction of residual oil.
Even after 3500 iterations the solution is not converging even after using a coupled solver.
Please guide.
Thanks in advance
Dr. Shilpa Nandwani
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Dear Shilpa,
the source of the problem can be varied from one model to another. However, overall, coupling process of solute transport with the phase flow behavior can cause such issue. The associated factors, however, can be changed from traditional factors (i.e. grid size and time step) to non-conventional factors ( e.g. Effective concentration and the shape of rel perms). Also the solution type of the numerical scheme can be another source of such issue (e.g. fully coupled schems causes convergence issue more than decoupled scheme). See SPE-192074-MS and
Al-Ibadi, H., Stephen, K., & Mackay, E. (2020). Pulse Generation and Propagation in the Numerical Solution of Low Salinity Water Flooding. Journal of Petroleum Science and Engineering, 108151.
Hasan
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The electrical tomography can be utilized to calculate the solid concentration or gas holdup in aqueous based multiphase flows. However, there usually exists a strong nonlinear relationship between the solid concentration and tomographic image. Is there any effective way to handle this problem?
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Within the classical framework, one can minimize the error between measurements and calculated values, in function of the multiphase electrical properties. There are different optimization algorithms which deal with non-linearity, with the most popular being Gauss-Newton, Simulated Annealing and Kalman filtering. Of these three, Simulated Annealing requires a large processing time and Kalman filtering may be suitable for real time (depending on other aspects).
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I have this problem when i  take geometry in fluent .Model Information is incompatible with incoming mesh. How to resolve it?
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Right click 'Setup' and press reset. This will force you to start everything after the updated mesh from scratch. Don't bother looking for the error.
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In my Simulation I have a Filter with a multiphase flow (Air and Oil). Now in my report the pressure drop (Area weighted Average) Static-Pressure-Mixture is higher than the total pressure drop (Air selected).
Because Total Pressure is the sum of static and dynamic pressure, the static pressure normally cannot be higher than the total. Does anybody have an explanation for that?
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Well did you got the answer ?
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Hello all!
I am trying to simulate a problem where a drop deforms under the given flowing conditions of ambient fluid. I want to have a coarse grid throughout the domain, but different levels of refinement should be present near the interface. I want the mesh to be refined after every few timesteps. Is there any predefined function in Fluent to implement this or do I need to use a UDF?
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Don't know with fluent but with Hypermesh mesh refinement can be achieved.
These videos will help to learn 2D and 3D mesh transition techniques.
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I have a class project where we are vacuuming wet sand and my team wishes to know how fast the vacuum container would fill with the wet sand. I am unsure how to compute the multiphase flow because I have no experience in multiphase flow. The calculation can be fairly rough, but I don't want to use a model that simply doesn't work. I have seen there are a number of methods but cannot decide which to pursue. Some sources state mixture model to be a good fit would this be the case?
Much Appreciated,
Lucas Clapp
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It is better to approach Eularian-Lagrangian Model in this regard. Most of the comparative research articles shows better fit with this model than any other models.
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In air-water multiphase flow CFD simulation's it is common to use a VOF = 0.5 to track or identify the position of interface or free surface. I would like to know why this criteria is used and if it can always be assumed the same. Otherwise, I would like to know if volume fraction can be taken different to 0.5 and, if so, ask them about a methodology to find this volume fraction.
Below I describe a case that I am simulating in Ansys Fluent:
-Sewer pipe with Length L=6 m (aligned with the x axis), Diameter D=200 mm (8 in) and longitudinal slope So = 0.005 m/m.
-Simulation in steady state of sewer pipe considering biphasic flow air-water flow.
-Model Volume of Fluid (VOF) with implicit formulation and Open Channel's submodel.
-Boundary conditions: Inlet as mass flow rate = 26.51 kg / s; outlet = Pressure outlet. The mass flow entered is such that the ratio y / d = 0.61 (that is, occupation of 61%).
-Turbulence model: k-epsilon RNG with Enhanced Wall Treatment function (y + <5).
-Pressure-Velocity Coupling method: Coupled + Pseudotransient solver.
-Spatial discretization: Least Squared Cell Based; PRESTO!; Momentum, k and epsilon with 2nd order schemes; Volume Fraction = Compressive.
-Run Calculation: Time Step Method = Automatic with Length Scale Method = User-Specified; Length Scale = 0.064 (Hydraulic Radius); Time Scale Factor = 0.3.
To verify convergence I did the following:
* Residuals at 10-4 for all variables.
* Mass balance between input and output.
* Pressure drop between inlet and outlet.
* Velocity at Inlet and Outlet.
* VOF fraction for various orthogonal planes to the pipe at x = 0.5m, 1.0m, 1.5m, 2.0m, ..., 5.5m. I configured them as Surface report-Area Weighted Average, for the Field Variable=Volume fraction and Phase=water.
Convergence is good in terms of residuals, mass flow rate (1x10-6), drop pressure, velocity. In VOF monitors the trend also stabilizes towards values between 0.40 and 0.75.
I know a flow profile along the pipe. In my initial simulation with refined mesh, with elements of 0.02mx0.005mx0.003m, [this is (dx) (dy) (dz)], I noticed that the theoretical profile is closer when I choose VOF = 0.7 and not VOF = 0.5 as is usual in the practice.
From the above my doubt arises about the VOF fraction to choose.
Thanks!
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Leonardo Henao; Use the HRIC Scheme and refine the mesh in the z-direction (the wave elevation direction).
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Hello everyone;
I m trying to model the mixing of two miscible liquids inside a circular tank by using Ansys-fluent and i m confused which model of multiphase flow i have to use to model such case (VOF-Mixture or Eulerian).
Best regards and thanks.
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I think that the "species transport model" is the most suitable for your simulations, it's more suitable for the mixing of miscible fluids but you must be careful because you need to disable the reaction process.
For the colleagues who recommended the VOF model, I inform them that it is suitable for multiphase flows where fluids are immiscible, and for the others like mixture and Eulerian models the problem arises that you can't introduce the diffusivity coefficient particularly to validate with previous work.
Good luck.
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In the first case (single phase case) liquid enters the tube and absorbs heat energy which is produced by the solid region. There is no phase change happening here. The following UDF is used to calculate the source term in single phase flow. The UDF works correctly.
#include "udf.h"
DEFINE_EXECUTE_AT_END(s_calculation)
{
Domain *d=Get_Domain(1);
int zone_id=6;
Thread *t = Lookup_Thread(d,zone_id);
cell_t c;
thread_loop_c(t,d)
{
begin_c_loop(c,t)
{
C_UDMI(c,t,0)=C_T(c,t)*0.25;
}
end_c_loop(c,t)
}
}
DEFINE_SOURCE(energy, c, t, dS, eqn)
{
real x[ND_ND];
real source;
source =1680000*C_UDMI(c,t,0);
dS[eqn] = 0;
return source;
}
In second case, liquid enters the tube and undergoes phase change due heat addition.
How to rewrite above UDF for this multiphase case.
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it basically should work the same, as in single phase flow, since your are not interacting with any of the phases?
But why to you use Thread *t = Lookup_Thread(d,zone_id); and then doing a thread_loop_c(t,d) where you loop over all threads in the domain. That makes no sense at all, but it also should not really matter, if you did not activate the source term in the fluid cell zone.
Best regards
Christian
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I am currently studying about non isothermal multiphase flows and I want to model with non isothermal lattice Boltzmann methods. Any relevant input is always welcomed. Thank you!
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A good starting point is the book of Krüger et al. as well as the open source code OpenLB and the LBM spring school, see www.openlb.net .
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I am trying to simulate pyrolysis of biomass in different reactors and I have been using multiphase flow capabilities on top of conjugate heat transfer, species transport and laminar flow. I want to know more about the steps in Fluent to include multiple reaction pathways and being able to include more complicated kinetics in the simulation.
Thanks in advanced!
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Thank you very much Swapnil! I
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We are doing RTD study (pulse input method) for the flow through a packed bed in case of single phase (water) as well as air-water multiphase flow. We are facing problem about the experimental result of the Peclet number calculation, sometimes it increases with increasing velocity of the phases and sometimes it decreases. Now, my question is what would be the actual trend of Peclet number when the velocity of water increases for single phase flow and when the velocity of air increases (water inlet velocity fixed) for the multiphase flow? Please help me with the answer if you have any idea about the problem.
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Please mention the direction of motion of both phases i.e., water and air and tracer inlet position.
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Hello all,
I am trying to study a very simple case of bubble rise ( air in water) using Phase field method in COMSOL. However, again and again I get this error " fFailed to find consistent initial values.
Last time step is not converged."
I see that there is some scaling setting that needs to be done. However, just to see of things work well, I took the exact parameters as given in example of capillary filling.
The only difference in my case is that I try to study the rise of bubble instead of capillary filling.
While the given example worked well, changing the boundary conditions and physics lead to an error mentioned above.
May some one help in this regard, please? How to deal with this issue. I looked across but not much was found.
Thanks
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Hi, I was able to to it. Thanks for your reply. It was really helpful.
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What's the meaning of the capillary entry pressure (Pd or Pe) in Brooks Corey model giving the capillary pressure as function of a fluid saturation Pc=Pd*S^(-1/lambda) and how it can be determined for a binary immiscible flow in porous media?
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You also need to consider contact angle; so the corrected version is as follows based on Franken's model:
Pe = -2*B*sigma*cos (theta)/ R_max
where B depends on the pore geometry shape: 1 for cylindrical pores and 0<B<1 for non-cylindrical ones. Note that for B =1, the equation converts to well-known Young-Laplace equation.
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I got one really good paper on "Two phase modelling in ANSYS FLUENT", in which they have used UDF and linked them to Governing differential equations of VOF Model in ANSYS FLUENT. The Paper I have attached below, in which terms to be added as source term is also given.
I am going through UDF manual of ANSYS FLUENT and all the materials that I have with me for understanding how to link the source terms though UDF with the Governing differential equation of VOF model.
If you have any program written in C for adding the source term to governing differential equation of VOF model, please share your inputs on this topic
Thanks regards,
Somnath Rangrej
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Please write your considering sources as formulas here. Maybe I could help.
Thank you.
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Dear all,
I am modelling a drying process using a Eulerian multiphase flow in Fluent. The rate of evaporation of water-liquid is changing depending on the volume fraction of water liquid in the porous media. Therefore, to calculate the mass transfer I should write a udf which calculate the mass transfer according to the volume fraction which is provided in each time step. When searching in udf guide, I face C_VOF for this but it is mentioned that this can be used to get volume fraction in VOF multiphase model. I will really appreciate if anyone help me how i can get the volume fraction in each timestep in Eulerian model.
Thank you in advance,
Elham
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You can also use the C_VOF. It works for both models.
For example, you can just simply write: v1=C_VOF(cell,liq1);
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I am currently doing a gas-liquid phase change simulation, but the false phase change near the gas-liquid interface occurs due to the spurious velocity. Is there any way to reduce or eliminate this non-physical phenomenon?
Thanks for your help
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I think you are using the Shan Chen model, whereas the Free Energy and Phase Field models can eliminate spurious velocity.
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We all know that VOF method in multiphase flow is conservative...
when using this with method in free surface or interfacial flow we ended with mass loss even with a a finest grid, is there any way to conserve the overall mass of liquid ?
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Okay I made a systematic study recently with OpenFOAM about the issue that you were talking about. And it seems not surprisingly, you will over all lose mass/vol. while you track the whole mass in the system. And the reason for this is that the equation which is solved for the interface (alpha advection) is of hyperbolic type and once you try to solve it with a dissipative discritization (Upwinding) it'll make minor errors in your alpha field resulting in overall misbalance of mass/vol. in your enclosed medium. Of course this is not noticed once you try to simulate an open case. Anyways, what is important is that you cannot avoid this dissipation in the solution of alpha because it has also a good side and that is "stabilization". The good news is refining the cells at the interface would make this effect less vibrant.
BTW check the implementation of VOF in OF as well (interFoam solver) regarding the interface compression...
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We want to apply the solid surface by Shao et al. 2014 model [Free-energy-based lattice Boltzmann model for the simulation of multiphase flows with density contrast].
How can terms of density, chemical potential and order parameter gradient for solid surface be defined?
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Dear Fellow colleagues,
I'm trying to simulate a metallic powder flow under the effects of gravity only in a circular tube in 2D. I'm using the eulerian multiphase model, fluent doesn't seem to like that the density of the secondary phase (the powder phase which is 5500 kg/m3) is higher than the density of the primarty phase (air), up to 5-8 kg/m3, there's no divergence but higher than that, the divergence starts from the early iterations!
Any advices on how to solve that?
Thank you in advance
Karim ZAYNI
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Volodimir Brazhenko
Sorry for the late reply, I didn't receive a notification for your last reply for some reason.
How can I set a pressure drop exactly?
Gravity is on.
For the geometry, it's a 2D rectangular tube 20 mm long and 0.7 mm wide placed vertically, I patched the whole tube with powder.
DPM is working fine but the eulerian granular model is more interesting for me.
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If so, why are they defined as "suspensions" in the majority of literature, including high-ranked journals?
Although both are heterogeneous two-phase systems, as far as I know, in suspensions, particles settle on standing. While in colloids, the solid phase remains dispersed and does not separate on standing.
I am no expert, but this truly confuses me. Am I missing something?
Your feedback is appreciated!
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with a bit sense of humor I would say, Omar see what you get, several conflicting views on the matter, depending on how many experts you ask as much expert views you will get. I am thankful to you putting forward this question.
I highly value the knowledge of Bernhard and John, however, I have to disagree to some degree.
Based on how I understand classical German Colloid science, here is my version -
Dispersions are inhomogeneous systems characterized by having at least one interface where molecules feel a different environment than same molecules in the bulk, there are always molecules at interface but also in bulk. Based on this, suspensions (nothing else as solids dispersed in liquid, which may settle under given circumstances or not) and emulsions (liquids dispersed in liquids) are both dispersions beside other ones, e.g. fog.
Term colloid is unfortunately historically not clearly defined, something between characterized by slow diffusion and microscopically not resolved. Importantly properties of colloid dispersions are interface determinated. For more confusion there are also colloidal solutions (no dispersed bulk phase). If colloidal dispersions get flocculated, they are still interface determinated opposite to possible result of coalescence or Ostwald ripening, when dispersed phase grows beyond colloidal size.
In this sense nanofluids are colloidal dispersions and of course colloidal suspensions same time. And they settle with slow or slower velocity. When settling and diffusion velocity get to the same order you will get a so call 'isobaric' equilibrium. Even solutions (e.g. salt solutions) or mixtures of gases exhibit enrichment of higher density components towards the bottom, even at earth gravity.
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how to add a wall flux boundary condition of the primary species onlu?
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I would have a look at the newly introduced expression language capabilities in ANSYS Fluent 2019 R1 and R2. I could imagine, that using expressions, you do not have to write a UDF anymore? In an expression, the defined wall flux could be just multiplied with an expression value, which is always 1 for the primary phase of your multiphase flow and always zero for all other phases. I have not yet tried this by myself in ANSYS Fluent, since usage of expressions in this solver is rather new, but that is how I would implement it in e.g. ANSYS CFX using CCL language. New expressions in ANSYS Fluent should allow to do that at some point as well.
Regards,
Dr. Th. Frank.
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I am using Fluent for simulating multiphase flow (gas-liquid) inside micro channels (0.1-16 microns). I have a problem in making water (as wetting fluid/hydrophilic) flowing as a film on the insid the wall of the channel. Please could anyone help with this one?
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I would suggest trying to define wall slip in the wall boundary condition and see where that leads you.
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I have simulated a multiphase flow in Ansys fluent and now want to transfer the pressure load from the fluent to the ansys mechanical to do a fluid structure interaction.
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Simply plug and play the (ijes) or parasolid model into the ansys workbanch and then try to resolve it
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If so, how can we define the Knudsen number for solid nanoparticles dispersed in a liquid carrier? And how can the mean free path of liquid molecules be calculated?
Conventionally, Knudesn number is used in the framework of the kinetic theory of gases to judge the levels of gas rarefaction and slip-boundary effects. Is the same concept somehow applicable to particulate flows (such as nanofluid flows) with a liquid carrier phase?
Please note that in my approach, the particle phase is already treated from a discrete perspective and only the fluid phase is treated as a continuum. Therefore, my purpose is to judge if this treatment of the fluid phase is correct.
Also please note that I am aware of the popular use in the nanofluid literature of a Kundsen number defined as MFP/D, where MFP if the mean free path of the liquid molecules (?) and D is the nanoparticle diameter. However, I am not sure about the applicability of this criterion for treating the fluid phase as a continuum in a particulate flow system.
Your feedback is much appreciated.
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The Knudsen number is defined via the mean free path l_fp. In liquids the mean free path hardly can be introduced. Though liquids differ greatly from the solids, the marked molecule in liquid actually is moving under the impact of the self-consistent-field caused by the other molecules. Introducing the concept of the quasi-particles we can introduce the mean free path for those quasi-particles. The last is of the order of the mean distance between the molecules. Knudsen number as Kn_p=l_fp/D, where D is the nanoparticle diameter formally can be introduced, but how can it be used for the continuous model justification? The Knudsen number arises as a dimensionless parameter in the kinetic equation. Small Knudsen numbers correspond to the continuous model, even taking into account, that for the dense gas, all the more for the liquids, much more complicated kinetic equation compared to the Boltzmann equation should be used. That parameter has no practical use without considering of any type of the kinetic equation. For the nanoparticles Kn_p may not be small, so I see no application field for such parameter, specially taking into consideration the long-range interaction of the particles via the liquid.
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maybe the small amount of the DataSource
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Hi Weijia Lu,
Great point. Machine learning uses algorithms to parse data, learn from that data, and make informed decisions based on what it has learned. Deep learning structures algorithms in layers to create an “artificial neural network” that can learn and make intelligent decisions on its own.
Best,
Moh.
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How can we calculate capillary pressure in multiphase flow system in a porous reservoir with relative permeability and saturation as the given values. What I require is data on capillary pressure with change in relative permeability of gas or water saturation. This is in context of CO2 sequestration in deep saline aquifers.
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Well,
q=kA (P1- P2)/ µL---------------------- (Equation ) where
q = Volume flow rate
k = Permeability
A = Core area perpendicular to flow direction
P1 = Upstream pressure at location 1
P2 = Downstream pressure at location 2
µ = Dynamic viscosity of flowing fluid
L = Distance over which pressure drop (P1- P2) occurs.
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What are the differences between capillary pressure and interfacial tension in the context of multiphase flow in porous media? I understand that capillary pressure is the pressure at the interface between two phases in contact. Then what precisely is the interfacial tension?
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Hi Ankita
Interfacial tension is the work (J or N.m) that is required to increase the surface area of an interface between two phases, expressed per unit surface area (m2). The units of interfacial tension is therefore J/m2, or N/m.
Given two immiscible liquids in a thin tube, capillary pressure is the difference in pressure accross the boundary between the two phases. It results from the interfacial tension between the two phases.
All the best
Johan
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Hi
I am simulating a kind of electrolyzer in Fluent. Consider a 2D-box, the bottom boundary is opening, top boundary is degassing, gas is generated from a part of left side boundary and right boundary is wall. I did the mesh dependency test and got dy=1mm is an optimum size. Now, if I set the vertical size of cells close to the degassing boundary equal to 1mm the total amount of gas in the domain will be A%. But, if I change the cell size close to the degassing boundary to a smaller value (e.g. dy= 0.5mm) then the total amount of gas in the domain in the steady-state condition will change. Note that I only change the cell size close to the degassing boundary to see the effect of cell size of the performance of the degassing boundary.
I know that Fluent specifies a mass sink for the gas phase in the cells adjacent to the degassing outlet which is related to the cell size (i.e. mass sink=integral (e_g * rho_g* u_n*dA) ), but I do not know how I can get similar results with degassing boundary when changing the cell size. any idea?
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Thanks a lot Ricardo
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I need to compare results for a simulation with nanofluids in a coiled tube using the single-phase and two-phase methods in FLUENT. The single-phase method works perfectly well. However, I am confused with the development of the two-phase Mixture model. The pertinent literature states that the fluid properties need to be defined through the use of the effective properties (as in the single-phase simulations). Hence, I am not sure on how to define the model in FLUENT. 
Therefore, if we define the mixture with two phases, my understanding is that the base fluid properties (water) will be defined with the UDFs for the effective nanofluid properties. Then how can we define the properties of the nanoparticles i.e. the second phase? 
Am I right to assume that for the second phase, the granular option needs to be activated?  
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Its calculated based on effective mixture properties.
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Hello all
I am simulating hydrogen generation in an electrolyzer compartment using Eulerian model in Fluent. The final amount of gas in the domain when the solution reaches steady-state condition is about 25% and 14% for Explicit and implicit solvers respectively. What is the reason for this difference? I have checked even very small values for the time-interval to see if two solutions become similar, but they did not. Any idea about the reason for this difference between the results? How can I get similar results?
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I think you should check mesh independency for your solution, changing the solver, will result in change in the mesh dependency criteria.
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The multi-phase fluids are widely used for annular hole cleaning, but I am not sure what advantages do they offer compared to single-phase fluids in overall removal of cuttings for the annulus.
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hi
used for non Newtonian flow
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How can we model multiphase flow of DEM (discrete element modelling) and SPH (smooth particle hydrodynamics)/ lattice boltzman method using comsol. Also if there is any other good software to perform similar problem
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You can check below mentioned article:
Article A LBM-DEM solver for fast discrete particle simulation of pa...
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Hi,
I am simulating a multiphase flow through a geometry having a cylindrical passage then nozzle then an orifice. I am interested in calculating the Reynolds number. I have gone through few literature. Different people have used different correlation. I am not sure which one to use? some people have used Re= 4mtotal/pi*Do*muavg
Where Do is the exit dia. M total is the total mass flow rate which includes both the phases.
mu avg is the mass average coefficient viscosity
Some people has used Re = rhol*Va*D/mul
where rhol ,mul are liquid property and Va is the gas velocity
Both the correlation is giving me pole apart results. Kindly suggest me the proper one.
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Dear Chinmoy,
Using Re as such is NOT wise as we fundamentally have a different physical domain for single and multi-phase fluid flows. In case of a single phase fluid flow, we have a solid-fluid boundary, where the concept of "NO-SLIP" boundary condition can be applied comfortably,
whereas,
in case of multi-phase fluid flow,
what we have is a fluid-fluid (i.e., an immiscible liquid-liquid boundary) interface at the fluid boundary. Here, we cannot apply the conventional "No-Slip" boundary condition and hence, we may need to apply a "free-slip" boundary condition as the fluid interface is NOT rigid and it is deformable and this deformability depends on the intensity of the interfacial tension. And it requires a knowledge of Laplace number, which is the ratio between the fluid pressure and the Laplace pressure (Sigma/L). Determining the value of L (the capillary length) remains a real challenge.
Good Luck.
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Hello everyone,
I want to simulate a multiphase - flow (eulerian - eulerian) through a porous media in Ansys Fluent without the effect of diameter. In the eulerian model, by default only drag is active for the phase interaction between the two phases. So the effect of diameter should no more be present, if I disable drag. If I disable drag (from schiller-naumann to none), my simulation gets unstable and I get the message "floating point exception". Can anyone help what I can do, or give me an alternative to delete the effect of diameter in my simulation?
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Tristan Davenne is entirely right. Essentially what you mentioned about your current solution points in the same direction. It seems to me, like ANSYS support has recommended to you to switch from a disperse multiphase flow formuölation to a VOF-like (VOF = volume-of-fluid) formulation. VOF exists in the ANSYS software in two flavours - homogeneous and inhomogeneous. The homogeneous model with just 1 set of momentum equations follows the assumption, that both phases move with the same velocity field. In that case no assumptions or knowledge about interfacial momentum transfer is needed. ANSYS has implemented an inhomogeneous VOF model as well. In that case from the local ratio in volume fractions of both phases an inherent drag term is deduced, but in that case you don not have to specify it yourself. It assumes, that separatign interfaces between the two phases are rather macroscopic and resolved by the simulation like typically done in free surface flows.
Regards,
Thomas Frank.
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Dear All,
Greetings.
I would appreciate it if anyone can recommend a book (or any other related reference) for miscible multiphase displacement flow especially if it is for Non-Newtonian fluids in Oil well cementation.
Thanks in advance.
Best Regards.
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Dear Professor Suresh,
Thank you for your reply.
Much appreciated.
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I have a thin porous material whose saturation needs to be determined. The relative permeability for a multi-phase flow is to be determined through the porous material and in order to express relative permeability as a function of saturation, I need to determine the saturation. The sample is of the dimension of 10mmX5mm and thickness of approximately 500 microns. How can I effectively measure the water saturation. Also, I think the internal methods of saturation determination like X-Ray method, neutron scattering would be not very effective considering the small dimensions of the sample. Correct me if I'm wrong.
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If I understand correctly, it is an elongated tube that can serve as a waveguide. The electromagnetic waves can not penetrate the wall of the pipe, even at very high power. You have to couple the energy on one side of the wet diaphragm and measure the electromagnetic energy on the other side with a similar probe. Suitable coupling elements can be found here:
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In a case where there is a multiphase flow through a heating pipe (heat flux is provided), if I use (say 0.3kg/s) mass flow rate as tube inlet boundary condition at a known temperature, can I use same mass flow rate (0.3kg/s) as tube outlet boundary condition?
Or will there be change in mass flow rate at outlet.
Actually, I need the BC at outlet to be like 'Outflow' (as we use it in Fluent). I don't know the property of fluids at outlet. Rather, it is what which is to be find out (like temperature, vol. fraction, etc.).
Also, how can I allow backflow (if needed) around outlet?
In 'Opening' type BC, I do not understand how I can specify Opening Pressure and Temperature if they are yet to be calculated by the solver?
Thank You.
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What would you say about the 'Bulk mass flow rate' available for Outlet boundary condition.
For that, no volume fraction information is asked in CFX-Pre.
Should I conclude that, if I use that option: the liquid+vapor=bulk; it would mean that it is necessarily kept constant (0.3kg/s both for inlet and outlet)?
If so, is it Physically possible as well?
P.S.: my fluids are liquid water and water vapor (wall boiling case).
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Hi guys,
I recently ran into a paper simulating pore-scale porous media with FVM-VOF model and then validated it with another numerical approach based on LBM named Shan-Chen LBM model and received more or less the same results; however, it did not mention anything about the pros and cons of these two approaches except that LBM is more flexible in dealing with complex geometries and boundary conditions than traditional FVM-VOF.
Has anyone got any clear idea and explanation of the difference between these two approaches, e.g, computational time or accuracy in capturing interface, etc?
Any helps or comments would be appreciated in advance
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Dear Sir,
For multiphase flow inside porous media, you can use the mathematical formulation of TPMM along FVM instead of VOF. For further information, please see my articles in my page and also you can contact with me at any time.
best regards
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Why does the imbibition process need a much higher pressure than the drainage process in the two-phase fluid flow in porous media?
Principally, because capillary pressure plays a positive role to drive the flow of wetting phase (e.g., water), the flow in an imbibition process (e.g., water displacing oil) should be easier (less pressure cost) than in a drainage (e.g., oil displacing water) process.
However, as shown by many laboratory core-scale test results, an imbibition usually needs a much higher pressure (e.g., 100 kPa) to drive the flow than a drainage (e.g., 20 kPa) under the same injection rate (e.g., 0.5 mL/min).
The high pressure during imbibition should be not due to the viscosity difference between fluids. When injecting either one of the phases alone in the media, the pressure drop is quite low.
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I fully agree with previous answers. But your question is not clear enough.
the distribution of fluids (previous history) and wettability (if the viscosities are similiar) are the controlling factors for the pressure needed to flow. involving capillary pressure muddles the issue.
For example, if your water saturation is 40 % and all the other variables are the same. Not all the pores have 40%! some will have 100% and others 20%. Therefore, the pressure drop will be different for different pore size distributions, mainly if they are not unimodal.
Also, during imbibition and drainage at the same saturation you will have different fluid distributions and as a consequence different pressures.
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Hallo fellow Researchers,
I need to update my knowledge on Multiphase Flows and Fluidization processes. Especially with Continuum and Kinetic Theory Descriptions.
Does anyone know if something changed about understanding the Continuum and Kinetics during last 20 years?
Are there some newer books with updated data or Gidaspow's work is complete for now?
Thanks in advance and Best Regards,
Stan
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Multiphase Flow: Theory and Applications
by P. Vorobieff, C. A. Brebbia
Publisher: WIT Press / Computational Mechanics (April 18, 2018)
Multiphase Flow Handbook, Second Edition
Efstathios Michaelides, Clayton T. Crowe, John D. Schwarzkopf
September 1, 2016 by CRC Press
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Hello all,
May some one suggest references wherein some relation has been proposed ( and proved ) between grid size and cahn-number.
In my understanding, it is known that one should consider about 3 grid points ( minimum as proposed by Jacqmin) in the interface.
Well then as per that relation, Cn= int_thickness/L ( L being characteristic length of the domain).
Thus, we have Cn= 3*grid_size/L . But is this correct. I read somewhere ( can't remember where) that its best to have grid size varying from Cn-Cn/4.
May some one help in this regard.
Thanks
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Thanks a lot for your kind references.
Deewakar
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What are the recent advancements to dissolve hydrogen in water?
I couldn't find any recent (2014-2015) articles on dissolving hydrogen in water. It would be great if someone could help me in this regard.
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As long as you have low mixing, a sufficiently high water column and volume (to provide buffer against liquid/gas phase contact) and a membrane/module which can inject the gas (smaller pores work better; "pore-free" silicone is also effective), and you can control the gas input pressure/flow, you should be able to inject high amounts before the liquid phase becomes fully saturated and bubbles start to form. The Henry's constant and mass transfer characteristics (which would severely limit the amount of H2 you could inject (i.e. 1.6mg/L) if the liquid phase is open to atmosphere) are then not limiting in this case. In a similar vein, if you have high H2 partial pressures in the gas phase and your reactor is well-sealed, you can dissolve more as well.
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Is someone aware of a method for simulating embedded boundaries on a Cartesian grid (cut-cell or IB) which conserves mass exactly in volume-of fluid framework for the direct numerical simulation of multiphase flows?
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Not a VOF expert here, but for what concerns the Immersed Boundary (IB) there are several possible routes to fix the conservation problem, depending from what method you actually use.
Actually, I expect you to be working in a finite volume (FV) context, otherwise you have additional sources of non conservation, besides IB itself.
The general idea to fix the conservation problem of IB with a FV method is to track the mass flux on your boundary (but it can be also an energy flux, etc.). This is trivial in a FV context, as you know all the cell faces that make up your immersed boundary and you apply the boundary flux there.
Once you know the amount of your leak (and hopefully it is just proportional to the interpolation error on the boundary) you can cancel that leak by applying a global correction.
One possible way is to apply the correction to all the cells of your domain as a volume source of mass (or energy, etc.). This is done, for example, here:
but I don't like it as it acts as an instantaneous propagation of a local information (boundary) the the whole domain without a specific physical meaning.
A better approach, the one I currently use in my code, is to apply the correction as an additional boundary flux on the same boundary where the leak is created. This is not different from correcting the outflow mass flow in an incompresible flow in order to preserve the divergence free condition.
A possible implementation is here:
Note that these two implementations in the papers above are typical of unstructured settings, where the "solid" part of the grid is not actually present in the computation. But similar treatments have been developed in the more common setting where the whole cartesian block grid is part of the computation.
I don't know if VOF in these settings would require additional attention.
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I am modeling multiphase flow and i counter this error: In Analysis 'Flow Analysis 1' - Domain 'fluid': The following materials require Viscosity to be defined: 'Aluminium'
i try to use alminium as my dispresd solid phase and water as my countinus phase. How can i solve this error?
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Currently, the fluid drop impact on solid walls is an important research area in the scientific community. The droplet spreads out on the wall after impact. In some cases, the tip of the spreading lamella levitates and travels along the surface without touching the wall. Quite interestingly, this is observed in drop impact on both hydrophobic and hydrophilic surfaces. What are the main reasons for this phenomena? How can we categorize them?
The image is from
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Following!
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Hi,
I am trying to renovate old laboratory apparatus for the study of the hydrodynamics of countercurrent liquid-liquid flow through a packed column. Not for research, but rather education and didactics, as part of general course on various aspects of basic unit processes (Chemical Engineering lab).
While it was once operated using water and tetrachloromethane, since that time new legal regulations were introduced and the former was deemed too toxic to use in such configuration (it isn't standing in the fume hood and is rather impossible to move it there). New question arose - whether replace this apparatus with something else,, or maybe just change the second liquid. We have quite a bit of ideas, but I decided it wouldn't be bad if I will also move the discussion here, and maybe somebody will catch up.
After that general introduction to a topic, the final question is just the same as above: can anyone recommend some non-toxic liquid, that is not miscible with water in considerable proportions and with comparable dynamic viscosity (or just quite low)?
I am open to various suggestions.
PS.
We thought about, for example, some kind of vegetable oil as non-polar phase, as they are cheap and readily available. However, they are also quite viscous in comparison with water and prone to ageing. While first isn't big problem, we could just buy the pump well-adjusted for handling oils, the second issue is quite a nuisance.
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Krzysztof,
I believe there are alternatives which can be safer.
Regarding liquid-liquid emulsions, (Matthias Kraume ) has tested different oils when he was looking for Toluene alternatives.
See attached file. There are different oils & their physical properties. I hope it will help you.
Best regards.
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I am trying to simulate the gap resonance between two floating bodies and with a gap of 3m under the effects of ocean wave (direction longitudinal of the ships body) . I am trying to put surge, sway and yaw as fixed. There should be a damping effect on floating bodies due to viscous effect and radiation . How do I determine the linear and damping coefficient of such bodies. This is my first time simulating a multiphase flow and needed advice on this.
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You shoud add function ovject to your controlDict in order to read forces etc.
From the forces probably you can calc what u need.
Franco
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As in multiphase flow it is said that surface tension plays a role in mini channels rather than gravity, I am unable to understand, is there any mathematical equation that supports or experimental study?
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Hi
You can simply understand it by " Surface area to volume ratio" (SAV) concept. From dimensional analysis, SAV=1/L. so when a channel characteristic length is 1mm, SAV=1000 , that is surface effect are 1000 times higher than volume effect. Gravity is a body force which acts on the volume so its effect compare to lets say friction is 1/1000 smaller in a minichannel with characteristic length 1 mm. For heat transfer surface area is important so mini or microchannels are better heat exchangers.
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Some thermodynamics confusion.
Assume ideal gases for simplicity.
Are kinetic energy (K) and internal energy (U) for gases, different from one another? As I understand,
dU = n Cv dt
And if some work is done by a gas adiabatically, then
heat change dQ = 0
Now we can have two cases:
1) Expansion occurs at the cost of internal energy of the gas and so gas temperature falls as the gas expands. So, U decreases.
Or,
2) As the temperature falls, the velocity of the gas molecules deceases too which then consecutively lowers the K of the gas.
So, are K and U the same physical quantity? Do they imply the same physical parameter of a system or something different? Or, may be both K and U undergo change in such a process?
Another issue is if work (classically) done = P V
then dW = P dV is seen more often. I am aware that W is not an exact differential, still shouldn't it be dW = P dV + V dP, as one can intuitively guess that during an adiabatic expansion of a gas, as volume increases, pressure will change (fall?) too? Why V dP is often left out?
Any comment will be appreciated.
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Dear Mr. Pandey,
let me try to help you out and clarify your doubts in a simple way.
A gas is constituted by many many molecules, each one having its own velocity. However, while studying a gas flowing in a given domain, we cannot follow each molecule so we introduce the concept of particle.
A particle is constituted by a bunch of many molecules (say millions) in the neighbourhood of a point, which however has dimensions much less than the domain to be studied. So the studied domain is composed of a very large number of particles. This is called the continuum assumption which is not valid, e.g. in rarefied flows.
More precisely, the particle must have a volume that is much larger than the cube of the gas mean free path and this volume must be much smaller than the volume of the studied domain. For air at STP, a cube having its side equal to a mean free path contains about 10,000 molecules.
The velocity of a particle of a single component gas is generally defined as the vectorial average of the several molecules contained in it. Of course, in a multi-component gas, a weighted vectorial average has to be performed. So, the velocity of a particle can be regarded as the velocity of the center of mass of the particle itself. Besides, the velocity of a particle depends on the adopted reference frame.
The kinetic energy of a gas particle is the one which is evaluated with the velocity of said particle.
But we have to remember that each molecule has its own velocity so, we may define the peculiar velocity of a molecule as its actual velocity minus the velocity of the particle it belongs to. The peculiar velocity of a molecule does not depend on the adopted reference frame because it is always measured with respect to the center of mass of the particle.
In this simple model, the internal energy of the particle is represented by the average kinetic energy of all the molecules contained in the particle evaluated with the peculiar velocities.
It is like if we split the total kinetic energy of the molecules in two parts, a macroscopic one (the particle kinetic energy) and a microscopic one (the particle internal energy).
Going to your example, in the gas expansion, the particle velocity increases ond so does the kinetic energy, while the internal energy (and so the gas temperature) decreases.
I hope to have been clear enough even not using equations.
Best regards and good luck for your work
Carlomagno
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I am working on a multiphase flow through a porous medium. in my system I am introducing water at a constant flow rate in a porous medium filled with oil. There is outflow only when pressure in the system has exceeded 400 psi. please help me in how to put this condition in gambit.
Thanks.
Regards,
Shilpa Nandwani
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Are you having back flow issue in defining the outlet pressure in FLUENT? You have to be selective in inlet velocity also to avoid the back flow.
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which is better for temp measurement of fluid flowing inside tube in upward direction. there are an options by use thermocouples or PT100, which way is more acceptable for such case? I hope to hear from you if any one have an experience. for information the flow is multiphase flow.
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Dear Mohammed,
I'a agree with James Garry. In my thesis preparation, i tested the position of temperature sensor in cylidrical tube. The accuracy of results depends on the position of the sensor and the flow regime which must have a high Reynolds number value.
I f you need more details about this, i'am ready to help you.
With my best regards!
Dr. Adel OUESLATI
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