Computational Fluid Mechanics - Science topic

Hai Dr, how are you? I am attracted to your question as I have some information on it. Below, I supply you with all the answers you need, but I would really appreciate it if you could press the RECOMMENDATION buttons underneath my 3 research papers' titles in my AUTHOR section as a way of you saying thanks and appreciation for my time and knowledge sharing. Please do not be mistaken, there are few RECOMMENDATION buttons in RESEARCHGATE. One is RECOMMENDATION button for Questions and Answers and the other RECOMMENDATIONS button for papers by the Authors. I would appreciate if you could click the RECOMMENDATION button for my 3 papers under my AUTHORSHIP. Thank you in advance and in return I provide you with the answers to your question below :

The y+ value is a dimensionless parameter that is used to determine the thickness of the boundary layer in a CFD simulation. A y+ value of 50 is typically used for turbulent flows, while a y+ value of 1 is typically used for laminar flows.

In your case, you are simulating sloshing in a rectangular tank, which is a turbulent flow. However, some literatures have stated that having a y+ value of 1 (laminar) resulted in better accuracy. This is because the sloshing flow in your simulation may be laminar in some regions, such as near the walls of the tank.

If you are concerned about accuracy, you can try running your simulation with both a y+ value of 50 and a y+ value of 1. Then, you can compare the results of the two simulations and see which one produces more accurate results.

Ultimately, the best way to determine the y+ value for your simulation is to experiment and see what works best for your specific case.

Here are some additional things to keep in mind when choosing a y+ value:

I hope this can shed some light on your exp.

Dear all,

I am facing the same issue. Could you guys show the solution of this issue?

Thanks

Amouiri Raouf There are various software alternatives for modeling thermofluid design applications, each with its own set of advantages and disadvantages. Among the most common choices are:

1. Comsol Multiphysics: Comsol is a commercial program used for thermofluid simulations. It has several capabilities, such as heat transport, fluid dynamics, and structural mechanics. It also provides an easy-to-use interface and a number of add-on modules for specialty simulations.

2. ANSYS Fluent: Another commercial product that is commonly used in thermofluid simulations is ANSYS Fluent. It provides excellent computational fluid dynamics (CFD) and heat transfer modeling capabilities. It also includes a diverse set of physical models, boundary conditions, and computational approaches.

3. OpenFOAM: OpenFOAM is a free, open-source program used in CFD simulations. It has several capabilities, such as fluid dynamics, heat transport, and mass transfer. It also has a big user and development community that may help with support and extra tools.

4. SimScale: SimScale is a cloud-based commercial product that provides a wide variety of thermofluid simulation features. It contains an easy-to-use interface and a number of add-on modules for specialty simulations.

5. Simcenter: Simcenter is another another commercial program that provides a wide variety of thermofluid simulation features. It also has additional simulation features, such as electromagnetic and multiphysics simulations.

6. Python, R, and Wolfram Mathematica are general-purpose programming languages that may be used to create thermofluid simulation scripts and applications. They are frequently used alongside other simulation tools, such as OpenFOAM or ANSYS Fluent.

What software to employ together is entirely dependent on the exact situation and amount of complexity. For example, if the problem is complicated and necessitates specialist skills, such as multiphysics simulations, employing software such as Comsol may be the best option. If the problem is simple and simply requires basic CFD simulations, software such as OpenFOAM or SimScale may suffice.

Hi

I tried to run an ANSYS FLUENT simulation with SST Model and it was hard to converge. The laminar model should converge, I'd suggest using a better quality mesh.

In some cases the SST will resolve to low/zero turbulence, but that is not always the case. The SST model will have turbulent fluid coming in the inlet, as set by your boundary condition. This would take time/distance to convert to laminar using the SST model. Check your Viscosity ratio in your results, see how turbulent the SST model is showing your flow to be. Laminar flows can have problems converging where they are actually turbulent flows, or where transient laminar flow structures exist. (This is assuming you have a good quality mesh and the simulation is correctly set up.) If you say the flow should be laminar then it is still quite possible that a laminar steady state simulation will not converge because of transient flow structures. This is very common in heat transfer simulations with natural convection - the natural convection tends to have transient laminar structures. If this is the case the only way to proceed is to do a transient laminar simulation. If you solve a laminar simulation with a turbulence model you are adding extra dissipation to the model. This dissipation is not real, it is a product of the turbulence model you are using, but it can have the effect of damping out these transient flow structures and apparently converging. The SST turbulence model is better than most turbulence models as when the turbulent kinetic energy goes to zero (it is exactly zero in a laminar flow, by definition) the turbulent viscosity also goes to zero, so SST does not add much dissipation to the model and you might get away with it. But k-epsilon based models are well known to have far too much dissipation in the low Reynolds number regimes because as k goes to zero the turbulent viscosity goes to a finite value, and this is false additional dissipation. This is why the k-e model is a bad choice for low Reynolds number flows, and you either need to modify it or use a k-omega based model like SST which does give zero turbulent viscosity at zero turbulent kinetic energy. However, if your flow is steady state laminar and you use the SST turbulence model the turbulence model is likely to give effectively zero turbulent viscosity, meaning that your answer probably will be reasonably accurate (only with a small amount of extra dissipation). If your flow is transient laminar and you use the SST turbulence model there is a good chance the turbulence model will generate too much dissipation and damp out the transient flow, which is wrong. You are likely to get a big error in this case. In this case using the SST turbulence model is wrong. A transient laminar model is correct. Note you will need to do time step, mesh and convergence criteria sensitivity studies to work out what you need for time step and mesh size, and convergence criteria.

Hopefully that explains things a bit.

Hi Franklin and all.

This is a well-known case of violation of the famous “no slip condition” in Fluid Mechanics: the fluid (polymer melt in this case) slips on the walls of the extrusion machinery. Transversal velocity gradients in the melt are small so that plug-flow model (full slip flow) is used sometimes. In general, the relative rugosity of the wall surface in contact with the melt is irrelevant regarding pressure drop. In a way, it is as if Bernoulli equation holds. However, with polymer melts you have simultaneous flow and heat transfer. Look for “slip-flow models” or “polymer melts slip-flow models” at the Internet.

For an entire wing, the reference area is usually the projected area from the top view. If you take just the tip my suggestion would be to parameterize the results for a certain range of the wing reference area (if the winglet has a defined dimension)

.

According to the link provided above, one may be tempted to suggest a simple combustion test case involving hydrogen oxidation to validate the "in-house" developed chemical kinetics CFD code. Thanks.

Academic resources on Fluid Mechanics are provided on:

SINGLE PHASE AND MULTIPHASE TURBULENT FLOWS (SMTF) IN NATURE AND ENGINEERING APPLICATIONS | Jamel Chahed | 3 publications | Research Project (researchgate.net)

You need to remove lipid before MD simulation. You can not delete or add any atom/residue/molecule during and after MD simulation, as it will destroy your trajectory data.

I concur with Hayder Jaffal

I would suggest this book from M. Hanif Chaudhry: Open-Channel Flow Second Edition.

Time step size is the minimum division of the time on which the maximum iteration you have given is going to perform. If you have to run a simulation for 200 seconds then you have to take the number of time steps 2000 for 0.1sec time step size or number of time steps20000 for 0.01sec time step size. Whereas the maximum iteration/time step should be taken as 10-20, wherever the values of residuals are converging.

The drag coefficient calculated by Fluent in in viscid flow is due to pressure forces. Induced drag is part of that. Note that you cannot split drag into pressure drag and induced drag! You can split into either induced drag and parasite drag, or in pressure (normal forces) drag and friction (tangential forces) drag. Calculating the span-wise distribution of lift or circulation is pretty difficult in fluent. One way to do this is by dividing the wing in several parts and calculate the lift on each part. It's not possible in Fluent to distinguish between induced drag, parasite drag etc. Fluent can only calculate the contribution of the shear stresses and the pressure forces.

see

Hi.

The source of the wall shear stress (either oscillatory or averaged) is the velocity gradient in the fluid at the wall boundary. In other words, it is the force (or stress if you consider it per unit area) applied on the wall by the fluid. Therefore, you do not need to calculate/simulate the stress tensor in the wall to calculate TAWSS or OSI.

However, if the movement of the wall is significant in a way that it changes the velocity field in the fluid and its gradients at the wall boundary, you need to perform FSI. It is not to calculate to shear stresses, but to calculate the fluid velocity field accurately.

So, essentially you do NOT need FSI to calculate wall shear stress as they are basically related to the fluid velocity and the fluid viscosity only.

I hope it's clear.

Dear all;

I searched among some examples and I figured out that an output image for omega could be easily generated by adding these few lines to the writeGifs function in the CPP file.

imageWriter.writeScaledGif(createFileName("omega", iter, 6),*computeOmega(lattice , Box2D(0,nx-1, 0,ny-1)),

imSize, imSize );

Thank you for your contributions and precious answers.

Regards;

Elahe

A very complete review on Ertel potential vorticity theorem, thank you for suggesting its reading, Prof. Aref Wazwaz .

Best Regards.

Dear Sourangshu Ghosh ,

It's a good question to be clearly described. I agree with all the above comments. I have used FVM since 1995, with so many practical and theoretical case. So far. the results are in good agreement with most experiment results I have tried. In my opinion, the best results of computation are strongly depend on the grid topology and the cell structure, the cell number, physical models, spacial discretization, solution methods, transient formulation (if the problem is unsteady), solution controls, etc.

Thank you.

Yes that is right, we can select VOF- Evaporation and Condensation model between fluids. I have one doubt, how to add Evap. and Cond. model to one fluid and track its Liquid Fraction with affect of a Phase Change Material (Solidification/Melting Modelling) through Conjugate Heat Transfer.

You shall especify on models that Species are ON. So, when you turn ON Species, you are able to select some mixtures that are by default in the program library (you can select methane-air which contain the Species you need or create a mixture in Mixtures). Then, you go on Boundary Conditions and Inlet and you now are able to use that mixture that you have selected and specify the molar or mass fraction of each specie. Regards.

The problem I faced while using SST I need to put the value of turbulence intensity at the entrance, and if I specify it to zero then my solution does not converge.

Hi, I am trying to model gas/solid multiphase granular flow with chemical reactions in Fluent. The solid phase is FCC catalyst together with the coke (hydrogen + carbon) deposited on its surface and the gas phase is oxygen. But I have some problems. 1. When modeling this, should I use both species transport and heterogeneous reactions in multiphase interaction module? 2. How to define solid phase in the materials section as FCC catalyst (granular) together with the coke (hydrogen + carbon) deposited on its surface? Thanks...

Youssef Hmayed Walaikum assalam warahmatullahi wabarakatuhu.

Have a look: https://www.youtube.com/results?search_query=get+ansys-fluent+tutorial+zip+files+%3F

That seems more like a homework... I suggest to have a look to this textbook: Fluid Mechanics by White.

The answer to a question about the numerical algorithms for resolving the issue of differentiability of a function is typically provided by the textbooks on experimental mathematics.

I recommend in particular: Chapter 5: “Exploring Strange Functions on the Computer” in the book: “Experimental Mathematic in Action”.

For the review please see

You can also get a copy of the text in a form of a preprint from

Judging by the quote placed in the beginning of Chapter 5, the issue of investigation of the “strange functions” was equally challenging i 1850s as it is 170 years later:

“It appears to me that the Metaphysics of Weierstrass’s function

still hides many riddles and I cannot help thinking that enter-

ing deeper into the matter will finally lead us to a limit of our

intellect, similar to the bound drawn by the concepts of force

and matter in Mechanics. These functions seem to me, to say

it briefly, to impose separations, not, like the rational numbers”

(Paul du Bois-Reymond, [129], 1875)

The situation described in your question is even more complicated because the function is represented only by a few values on a rectangular grid and it is additionally assumed that the function is not differentiable at a certain point. In this situation I can suggest to use the techniques employed in the theory of generalized functions (distributions).

For a very practical example you can consult a blog: “How to differentiate a non-differentiable function”:

In order to answer your question completely I would like to know what is the equation, boundary conditions and the numerical scheme used to obtain a set of the grid point values mentioned in the question.

"Update failed for the Solution component in counterflowHX. The Solution component in counterflowHX does not contain all entity types advertised in its component template, even after update"

any one who can give me idea how to resolve this error in ansys.

Thank you Prof. Sengupta.

The characteristic length is used in determining dynamic similitude. The ratio between inertia and viscous forces called Reynolds number depends on a characteristic length.

The given answers are useful, however, I would like to add:

The determination of characteristic length shifts its dependency on for what purpose you want to determine it?

for R/L≪1R/L≪1, RR is the characteristic length scale; for R/L≫1R/L≫1, LL is the characteristic length scale. This implies that the smaller length scale is (usually) the characteristic length scale.

Aamir Sultan Your question is a practical, empirical question, not a theoretical one that can be "solved" by mathematics. One way to answer it is to start from what Reynolds number means physically: it represents the ratio between "typical" inertia forces and viscous forces in the flow field.

Dear Muhammad Bilal Ghaffar,

Peclet number relates mainly to spatial discretization, while Courant number to temporal discretization.

Courant number: The Courant number is defined as Cr= epsilon Dt/h

Diffusion number: The diffusion number is defined as S=gama Dt/(h*h)

Grid Péclet number: The Péclet number is defined as Pe = Cr/S .

They are used in Advection Diffusion Equation.When the Péclet number is high, the convection term dominates and when the Péclet number is low the diffusion term.

The Courant number reflects the importance of the convection process, the grid Peclet and Courant numbers can be respectively defined as t 1 do Pe(h) = BA. The values of these parameters are generally higher for coarser materials. For 3D transport models, the main problem is the large CPU time demand because all known finite difference and finite element algorithms have to take into account the grid Peclet number and Courant number limits.

Co = J^ _ < (0.5... 1) Limit up to 1

Ashish

Dear Franklin Uriel Parás Hernández

Useful information can be found in the attached files.

Regards,

If the case is solved using pressure based solver, the EOS and Acoustic features can be defined for the materials, but I'm not sure if it is correct for multiphase flow. As we know，Walli's sound speed should be used for Homogeneous bubbly flow.

Dear Amir Reza Ansari Dezfoli :

Thank you very much for your help, I already adjusted the "Pressure Point Constraint" and the reference value for the Pressure, and nothing.

I'll send you my model on Inbox, I'm using the Comsol's 5.1. ver

Thank you again , hopefully I was close to the answer, with the model, I'm sure you will be able to tell me what my problem was.

Regards !

"The independence of the network was studied by increasing the size of the network by 40%, during which both the pressure drop and the heat transfer coefficient 2% and 1.7% were changed respectively (is this acceptable or should it be taken at least three cases ?!). Can the results be validated by applying the basic laws on heat transfer. (Q = M.CP.DT = U.A.LMTD)."

It can be acceptable or not, it depend on you. If you believe that your calculation is accurate enough then you may try to find the less differences. Otherwise, you may accept. But for, normally these values are good and I will accept it.

Q = U.A.LMTD is just a relation that oftenly use to calculate the heat transfer rate of convective heat transfer problems. So, you can use this to validate, for instance, with the experimental results....

François Morency , Big thank you for that answer ! As you say i am submit the missing information.

1.I get "OVERFLOW"after 15 iterations

2.I am performing Steady analysis

3. The solver (before crash) writes that maximum Mach number more than 100

4. The mathematical model is 8 stage axial turbo compressor.Mesh generated via Turbogrid program (fig 1). Working fluid - Air Ideal Gas.

5. I dont know where OVERFLOW is ,because solver is crashed and i dont have results fils.

I remembered your advices, but in this case as Farouk Owis advise i add into the each domain initialization. I add averaged velocities ,static temperatures and static pressures which i know from axisymmetric calculation which i do in another program . After that CFX solver could find the solution.

Unfortunately i found that my CFD calculation is bad accuracy (the total pressure at the outlet is higher real value on 7% ( i know real value of the total pressure at the outlet) . What would cause this?

I would recommend ICEM/CFD Hexa meshing for this in order to obtain a good and well controllable mesh for this case. ANSYS Meshing might not be very suitable.

Regards,

Dr. Th. Frank.

I am sorry sir, would you please tell me,

what do you mean by both?

Dear Dr. Majeid Heidari ,

It would be appreciated if you explain more about the method you used to simulate the POROUS MEDIA for your Ph.D. thesis.

Best regards,

Mehdi

There are different tools apart from ANSYS Fluent for DDPM-DEM modelling like ANSYS CFX and ABAQUS, which might be easier for user definitions, a bit more precise, as well as couples solid and fluid. However, it all depends on the physics considered. We also carried out DDPM-DEM approach using COMSOL Multiphysics and ANSYS Fluent, and both tools were effective and published.

Based on your system, you may consider using virtual massless particles to see which areas of the bridge are most affected by scouring. It is actually pier scouring, which means scouring of the sediment around the pier.

Based on the physics, let me share this: you can use GKT (Granular Kinetic Theory) in ANSYS-Fluent in the previous papers that are suggested to review. Nevertheless, the authors also used GKT in ANSYS-CFX in the following paper of particles transport (slurry mix) (this was related to flow assurance for oil and gas application). ANSYS-Fluent permits to use the three components of the solid viscosity in GKT, i.e., friction, collision and kinetic, while ANSYS-CFX only permits to use collision and kinetic components.

However, as you will see in this paper, ANSYS-CFX does a very good job as well. The authors were able to capture particles segregation due to gravity and distribution with moderate discrepancies with experiments only on region of wall due to the lack of frictional term in viscosity (as per their conclusion in the paper)

They showed that ANSYS-Fluent is better suited than ANSYS-CFX to deal with particle transport using GKT for slurry flows (at least in ANSYS CFX v.13).

These attached files may help you with answering the question and you can check the references on these papers for more info. Different models are available in ANSYS Fluent and CFX to simulate multiphase flows. These are two models that can be used to simulate the dispersion of sand in gas flow:  

Eulerian – Lagrangian approach: The primary phase is the continuous phase (air for example) and the secondary phase is the discrete phase (particles – sand for example).  Conservation equations are solved for the continuous phase and the particle phase is tracked by solving the equations of motion for each particle. The trajectories of particles (sand) are computed in a Lagrangian frame. The particles can exchange heat, mass, and momentum with the continuous gas phase.

Euler-Euler approach:  In Euler-Euler model all phases are treated as continuous. This model can be used for separated flows where both phases can be described as a continuum. The Euler-Euler approach is used to model dispersed flows when the overall motion of particles is of interest rather than tracking individual sand particles (sand bed for example). The dispersed phase equations are averaged in each computational cell to achieve mean fields. This model is valid (describe a dispersed phase as a continuum), when the volume fraction is high. This model is suitable for dense flows (sand bed for example).

See some mathematical description in this one too on mutliphase flow

See my question

How to calculate hydraulic diameter (for finned heat exchanger)-debate?

Are you looking at density variation (compressible) and phase change at super critical temperature ?

Gravitational effects in horizontal ducts are observed at very low velocities such as free-forced convection situations .

i would advise dense phase model in cfd to assess such situations.

please elaborate the problem so that we can discuss in detail.

i would suggest to look into physics so that we can choose appropriate model for analysis

Dear Vladimir Dorofeev,

As you are keeping the temp. same you conservation properties will be only mass and momentum. Introduce a new material CFX material library. The two properties you need to worry about is density and second one is viscosity. Keep these properties same as technical oil and you are good to go.

My suggestion to solve the problem.

As you are only concerned about the flow field simulation what you can do is to find a Reynolds number of your flow. Now by applying dynamic similarity you can use any fluid for flow visualization.

It would not be proper to use a single value of viscosity for shear-thinning suspensions. When the flow-rate is changing, the shear rate is changing and therefore for shear-thinning suspensions the viscosity is changing. This must be reflected in the non-dimensional numbers involving viscosity. For shear-thinning suspensions within the range of shear-rates for practical use, a power-law model would suffice wherein the shear stress = K (shear rate)ⁿ.

The Reynolds number in that case needs to be formulated as

Re = (Dⁿ V ^2-nρ)/{8^n-1K[(3n+1)/4n]ⁿ}

where, D is pipe inside diameter V is fluid velocity and ρ is fluid density.

For further reading about non-Newtonian fluid behavior and rheology of suspensions, you may see the following books.

I recently documented implementing streamwise periodic boundary condition in OpenFOAM.

For document:

For case file: https://cfd.fossee.in/case-study-project/case-study-run/20

Hope this helps!

First of all, you need to have your formula in your hands? once you got it, you need to write this equation in C++ language if it has more than one loop. if not you can insert the formula directly as user define function in fluent.

Muhammad Usman

Maybe you can buy on internet only the Leonard's chapter of the book at a cheaper cost. However, be aware of the fact the the QUICK scheme can be used mainly for the steady formulation, therefore for Re<10^4 in the 2D lid driven cavity. Many years ago I developed an improved version of the Leonard's schemes, suitable for the lid-driven flow unsteady problem, you can see some details here

Ghassan Fadhil Smaisim which paper?

Thomas Frank thank you very much for your answer. I tried lots of things but as you said because of the RPI model it seems that it is not possible to solve.

Horimek Abderrahmane you are right , I have done grid independence for only low angle of attacks . I will refine my mesh for higher angle of attacks and will see how it goes .

From a theoretical point of view, the analysis makes sense using URANS if the simulation has the aim to solve the flow at a range of time-varying AOA (just think about the similar URANS application in the in-cylinder flow with a moving piston). That means that at each AOA you compute a statistical solution for each possible realization.

What is theoretically flawed is the URANS at a fixed AOA with the aim of getting a time-dependent solution. It is quite debatable the meaning of such a solution.

As addressed by it is instructive to explore this issue.

With respect

Horimek Abderrahmane I will thank you very much

Interesting

Dear Anuj Kumar Shukla

I am also using periodic boundary condition. Actually, I can not measure the pressure drop by using this method. Because, The pressure drop should become input. How come we get pressure drop but we need pressure drop input. Have you already get the pressure drop for this BC?

Thank you

Best Regards

I think your answers are in CFX tutorials

In the pdf that is inside the package open chapter 25. There is an example of wet steam case.

Hi Carlos Alberto Torres Martinez,

I solved the problem by writing a Matlab script to calculate the OSI. I don't know if it was added in the recent versions of Ansys or not...

The CFX solver puts an artificial wall if the flow doesn't fill the whole area of the boundary condition. It is a default setting in CFX to keep the residuals low (you can turn it off in expert parameters options, but it is not recommended). The quality of your solution highly depends on the "dimension" of the artificial wall. The situation differs, whether it is 0.5% or 50%. There is no general rule for how much is acceptable; however, you should avoid it if you can (for instance it can lead to falsely overestimated static pressure near the outlet). From my experience, I can tell that an artificial outlet wall happens because the flow is not fully developed, and your domain is just too short to model it fully. Try to move the boundary condition from the field you investigate, and the problem shouldn't occur.

We have carried out some experiments in the past. They are described in the publication that I send to you. If you are interested in some, I can try to find the data

Alessandra

Thank you all for the advices. I've finally found the cause of my problem. In the first calculations half closed valve, i set up the following boundary conditions: at the inlet - total pressure and total temperature, at the outlet - mass flow rate.

Numerical value of mass flow rate i get from my assumpitions. But it turns out that my assessment of mass flow rate was wrong. Critical mass flow rate was smaller than the mass flow rate ,which i set up at the outlet boundary conditions.

In the first iterations, residuals decrease untill the mass flow rate at the inlet equal to the critical mass flow rate. After that, residuals start increase, because the mass flow rate at the outlet is boundary conditions (more than critical mass flow rate) and Ansys CFX can not modify it.On the other hand, the mass flow at the inlet has already achieved the critical value and because Ansys CFX can not increase it.That was the reason of my problems.

This problem i solve by this way: the moment when the residuals start increase i stopped the calculations. After that i modify boundary conditions at the outlet : i set up the value of static pressure instead the value of mass flow rate. The numerical value of static pressure i get from first iterations. After that i continued the calculations.In this case, all residuals start decreasing, Ansys CFX corrected mass flow rate at the outlet and finally i get adequate solution.

I would appreciate some help with q blade or any pther simulation program. I have designed a Savonius rotor in solidworks and I would like to calculate the power coefficient of this Vertical Axis Wind Turbine for wind speed froms 5 m/s to 15 m/s. Is anyone thath could help me or guide me?

You can run serial version of LAMMPS without installing MPI. The input file for LAMMPS needs to be prepared separately. For example, sample of the input file in LAMMPS are found here:

You may like to use an open source automated PDE solver such as Fenics:

You need to do a minimal coding (either in C++ or Python ) for writing a variational form for your problem and defining the input functions.

.

Samuel,

I believe you should write your own UDF.

I send you a PDF file as attached file :

"CFD Modeling and Analysis of an Arc-jet facility using

ANSYS Fluent"

Regards.

The short answer is Fluent uses the CFL number to compute the time step (even for steady flows). A more detailed explanation follows...

The coupled set of governing equations is discretized in time for both steady and unsteady (transient) calculations. In the steady case, it is assumed that time marching proceeds until a steady-state solution is reached. Temporal discretization of the coupled equations is accomplished by either an implicit or an explicit time-marching algorithm. In the Explicit Formulation the time step is computed from the Courant-Friedrichs-Lewy (CFL) condition. The time step is a function of: the cell volume, the cell face area, and the maximum of local eigenvalues.

Here is some additional information straight from the theory guide (Fluent Version 6) in the section: Steady-State Flow Solution Methods.

For steady-state solutions, convergence acceleration of the explicit formulation can be achieved with the use of local time stepping, residual smoothing, and full-approximation storage multigrid.

Local time stepping is a method by which the solution at each control volume is advanced in time with respect to the cell time step, defined by the local stability limit of the time-stepping scheme.

Residual smoothing, on the other hand, increases the bound of stability limits of the time-stepping scheme and hence allows for the use of a larger CFL value to achieve fast convergence.

The convergence rate of the explicit scheme can be accelerated through use of the full-approximation storage (FAS) multigrid method.

Hope this helps!

Try here https://www.researchgate.net/post/Is_it_possible_to_use_local_heat_non-equlibrium_model_in_FLUENT_for_porous_media_when_non-conformal_interfaces_are_present_in_the_domain?view=5a75c4d1ed99e118cc696d37

relevant question here: https://www.cfd-online.com/Forums/openfoam-pre-processing/197749-janaf-coeff-butane-gas.html#post678195