Computational Fluid Dynamics - Science topic

FORTRAN is still being used to write CFD codes for a number of reasons, including:

While FORTRAN is a powerful language for writing CFD codes, it is also a relatively old language. It can be more difficult to learn and use than newer languages, such as Python and C++. However, for many CFD researchers and practitioners, the performance, maturity, portability, and libraries available for FORTRAN make it the best choice for writing CFD codes.

Simulating three-phase flows, such as liquid-liquid-gas systems in a bioreactor, can be complex but is achievable using ANSYS CFX. To define and simulate such systems, you typically need to utilize the Volume of Fluid (VOF) multiphase model or the Eulerian-Eulerian multiphase model, depending on the specifics of your problem. Here's a general guideline on how to set up and simulate a three-phase system in ANSYS CFX:

It's important to note that simulating three-phase systems can be computationally intensive, and convergence may be challenging in some cases. Therefore, it's crucial to have a well-refined mesh, suitable solver settings, and accurate physical properties to ensure the simulation's success.

As for documentation, ANSYS provides comprehensive user guides and tutorials that cover multiphase flow simulations using ANSYS CFX. These resources can be accessed through the ANSYS Customer Portal or by contacting ANSYS support. Additionally, ANSYS user forums and communities can be valuable sources of information and expertise for specific simulation challenges.

Optimizing the flow and consumption of fuel in the aerospace sector using Computational Fluid Dynamics (CFD) is a critical aspect of aircraft and spacecraft design and operation. CFD allows engineers and researchers to simulate and analyze the behavior of fluids (such as air or fuel) within aerospace components like engines, wings, and combustion chambers. Here are some ways CFD is used and the latest research trends in this field:

To stay updated on the latest research in CFD analysis in the aerospace sector, you can refer to academic journals, industry conferences, and research institutions specializing in aerospace engineering. Many aerospace companies and research organizations also publish whitepapers and reports on their ongoing work in this field. Additionally, keeping an eye on advancements in computational techniques and software tools for CFD can be valuable for staying informed about the latest developments.

First, Matlab is not a visualization software for CFD, yu should think to use Tecplot or similar.

However, for some 2D problems and structured grids, you can find useful commands like contour, mesh, surf, quiver.

HAve a look here:

In Matlab you can find also the CFDTool

First question: where could have ChatGPT taken the relative information for training? Honestly, if you're going to use that, I can't see how you could not want to know about that.

Second question: why ChatGPT? This is not some complex information we can't extract from complicated data. It's just exactly there for you to learn or to search trough. Also, it could be better done by making a new net from scratch just about that, than using a LLM that really has nothing to do with this.

My general comment is that engineers and scientists should not look for answers in a tool that is not even tought to be what they want. ChatGPT just spits out words following a prompt and a statistical model. That is not different from a bad student that just goes by memory without understanding. ChatGPT is actually worst, as it never assumes it could not know something. You wanna ask that about turbulence models? Oh my...

I don’t use RANS formulation, I can just address some general questions:

1) use at least 3-4 nodes within y+<1

2) Are you comparing your solution to experimental measurements? Are you sure to be congruent with the experimental inflow conditions?

3) have you assessed you are able to reproduce the results for the classic test-case of the backward facing step at high Re number?

I would first recommend you validate your numerical model with existing literature. Once you identify a good reference, you can use their boundary conditions, and numerical setup to generate the results. Make appropriate changes until you get within acceptable accuracy limits. Once you are done with validation you can use it for your own purpose. The validation will help you choose the right models, equations, etc for the evaporation-condensation process.

Turbulent flows occur for high Reynolds numbers. The energy cascade transfers turbulent energy from large to smaller eddies via inertial interactions with no energy dissipation. Viscous dissipation occurs in the smallest eddies for which the Reynolds number that characterizes their movement is of order 1 (Kolmogorov scale)

For more, I would recommend the book by Tennekes, H., & Lumley, J. L. (1972). A first course in turbulence. MIT Press (more than 12 000 citations). As suggested by its title, the book is a basis for a fundamental understanding of the physical mechanisms of turbulence and for the acquisition of the methodological elements related to the phenomenological approach applied to the analysis of basic turbulent flows. The book is Available on:

https://www.academia.edu/download/54563360/Henk_Tennekes__John_L._Lumley_A_First_Course_in_Turbulence.pdf (more than 12 000 citations) is essential for a fundamental understanding of the physical mechanisms of turbulence and for the acquisition of the methodological elements related to the phenomenological approach applied to the analysis basic turbulent flows.

Description by the authors "This is the first book specifically designed to offer the student a smooth transitionary course between elementary fluid dynamics (which gives only last-minute attention to turbulence) and the professional literature on turbulent flow, where an advanced viewpoint is assumed.

Hi

please say how solve it. I choose the Mach number from data quantity files in fluent but it does not exist in cfd post

Thaank you Alin Sonny

specific changes that you can make to your dynamic mesh settings:

Thank you Mostak Ahamed for your answer. Do you have any reference that you can give me?

What software do you want to solve the problem? Fluent? CFX?...?

Hello! I would be interested in collaborating on a research of such project

You can solve this problem by Ansys Fluent, you can also do meshing with Gambit or ICEM CFD which is in the Ansys software package.

Do you have these software?

Hi

It may say that you do the simulation with your own code, or with Fluent or CFX?

You can search for sites that offer 3D models, these models can be obtained for free or paid depending on their accuracy.

Type 3dmodel in Google and many sites will be brought up. Of course, the suffix of the file must be such that you can enter it in your fluid analysis software. So first, see what suffixes this fluid analysis software accepts!

You may find some information regarding different airfoil sections characteristics in the book "Theory of wing sections by Abbot and von Doenhoff".

Milne Ando, understanding the concept of Galilean invariance can indeed be beneficial in the field of fluid dynamics, especially if you plan to pursue a career in computational fluid dynamics (CFD) or similar areas. While the term may not appear explicitly in some introductory fluid dynamics textbooks, it is a fundamental concept in physics and is particularly relevant in CFD.

Galilean invariance, also known as Galilean relativity, states that the laws of physics should be the same in all inertial frames of reference. In the context of fluid dynamics and CFD, this means that the predictions of a model should remain consistent when the frame of reference is changed. In other words, the model should provide the same results regardless of whether you analyze the flow from a stationary or moving reference frame.

As you progress in your studies and delve deeper into fluid dynamics and CFD, you may encounter various turbulence models with different levels of Galilean invariance. Understanding this concept will help you make informed decisions when choosing the appropriate turbulence model for a particular simulation.

So, if you plan to work with CFD or fluid dynamics in the future, it would be worthwhile to spend some time learning about Galilean invariance. You can start by exploring classical mechanics or introductory physics textbooks, as well as online resources, to build a strong foundation in this concept.

Good luck with your studies, and feel free to reach out if you have any further questions! Regards,

A floating point exception is an error that occurs when you try to do something impossible with a floating point number, such as divide by zero.

In fluent floating point error can be caused by many factors such as, improper mesh size, defining some property close to zero. However, floating point exception is very specific case that is being solved, the actual reason can be determined by changing one quantity (can be mesh, model, parameters, boundary conditions etc.) at the time.

Yes. OpenFOAM can be used for flow and fluid structure interactions encountered in hemodynamic flow problems.

OpenFOAM and ANSYS Fluent are mainly based on finite-volume method. Whereas, COMSOL is mainly built based on finite-element method.

The notable difference between OpenFOAM and the other two commercial softwares will be the user-interface. OpenFOAM is inherently built on a TUI. Whereas, ANSYS and COMSOL have a very user-friendly GUI.

But, OpenFOAM is very powerfool for research purposes due to its easy customization possibilities.

Meshing using blockMesh of OpenFOAM can be tedious for complex geoemtries. Hence, you have to depend on some third-party meshing applications and snappyHexMesh, etc.

Whereas, ANSYS and COMSOL have better stand-alone meshing capabilities.

Hi Armin Ramezanzad

If I understand correctly, you divided your simulation domain into four regions and tried to solve a PDE in each one separately.

From a numerical perspective, this might give you inaccurate results since FEM evaluates the function at the nodes and estimates the solution between them using the sum of unique basis set. If the points are closer to each other (denser mesh), the simulation is likely to converge faster with reliable results.

I recommend the use of geometrical properties instead. You can divide your domain into many parts in each direction using the "Layers" tab in the geometry definition. Then, you can select each region and solve your PDE separately or solve all of them at once using proper boundary values at the interfaces.

Hello Syed, it's hard to say which tool works best for you as you did not mention your application.

If you want to carry out fluid or thermal simulation, then it might be worth checking out SimScale. Its automatic mesher and default simulation controls have been specifically designed to be very robust, accurate and user-friendly. That way you can spend most of your time analysing results instead of optimizing your mesh, relaxation factors, Courant number, etc.

If you're looking at niche applications, then you can't go wrong with ANSYS, Siemens or Dassault as they cover areas currently missing in SimScale. If these three don't have your application, you can start looking at OpenFOAM (you might have to create your own solver in OpenFOAM :)

Understanding what "laminar regime" stands for is a topic of basic fluid dynamics.

The flow over an obstacle can be laminar even in case it is unsteady, as happpens in the classic case of laminar vortex shedding.

Turbulence is charactertized by a wide range of vortical structures.

Quiero monitorear múltiples gráficos de convergencia ya que múltiples problemas se resuelven simultáneamente. Pero solo puedo monitorear un solo gráfico de convergencia... Por favor, dame un consejo.@

Have you got the solution. If yes please share with us.

Dears Mohamad Afifi , , Mohsen Ghadyani , @

A true time accurate solution is obtained by keeping the Courant number below 1. The explicit time integration methods requires this condition to be true for stability and accuracy. One benefit is that you don't need to solve a large linear system of equations, but 1 iteration per time step is sufficient..

Some techniques such as transient SIMPLE, SIMPLEC (or fully coupled solvers/ dual time integration) ... can be used with a larger time step, even CFL = 20, 100 etc... but you will need to put enough inner iterations in each time step to reduce the residuals to near zero values for that particular time step..

Now the question is how to choose a time step..? At high Reynolds numbers and for aerodynamic flows this becomes a problem due to having large stretched cells in the boundary layer... So if you were to go with CFL < 1 the simulation becomes very expensive.. For implicit systems with some iterations per time step can be effectively used with time step estimates based on the Convective time scale, I have seen papers where people have used 0.01 to 0.1 of c/U. Another approach is to put at least 50-100 time steps in each oscillation of the vortex shedding time period.. This way you will filter out the high frequency oscillations (which I believe you are not interested in)...

Hope this helps..

Best

n ANSYS Fluent, "Finite Rate" and "Eddy Rate" are two different models that can be used to model chemical reactions. The main difference between the two is the way they calculate the reaction rates.

Finite Rate: The finite rate model calculates the reaction rate based on the actual chemical reactions taking place in the fluid. It considers the chemical kinetics of the reactions and the local thermodynamic conditions. This model is more accurate but requires more computational resources and detailed information about the chemical reactions.

Eddy Rate: The eddy rate model calculates the reaction rate based on the mixing of the fluid. It assumes that the reactions are controlled by the mixing of the fluid and not by the actual chemical kinetics. This model is less accurate but requires less computational resources and detailed information about the chemical reactions.

Another difference between the two is that the finite rate model is more suited for high-temperature reactions, whereas the eddy rate model is more suited for low-temperature reactions.

In practice, the choice of which model to use depends on the specific situation. The finite rate model is generally used when more accurate results are needed, while the eddy rate model is used when computational resources are limited.

I am old school CFD researcher; hence, my view point on this issue is old school. For those persons not well acquainted with coding that is closer to machine level, i.e., those not very familiar with memory management and more complicated data structures, I recommend regular FORTRAN. FORTRAN memory and data structure management is easy to grasp, and if you utilize a compiler flag like check bounds, the compiler will identify many memory problems for you. When you code equations in FORTRAN (in either the 77, 90 or 95 releases), your coded equations will look a lot like those in your notes. Debugging is rather easy either by the use of old fashioned print statements or by the use of debugging software. Reading other person's code is an entirely different topic. Unless you are familiar with the compiler or interpreter used to develop the code, you are in for a lot of work. Good luck.

Mass weighted averages are more meaningful. For combustion simulations, local density and mass depend on temperature... Area weighted average is based on the mesh...

Narjes Malekjani So, is it okay to have the same values of center points?

Can we report results without having a Lack of Fit value?

In general, a solution can be non-simmetric also for a symmetric geometry.

But that depends on the adopted formulation.

Are you solving steady RANS equations?

How do you check the convergence ?

Are the BCs congruent to a symmetric solution?

Chinedu Nwaigwe Great, Interested, In fact, 'm pleased to learn you're working on open-source programs to solve integral equations. Publish your code to make your study more publicly available and to allow others to build on your work.

A partner with skills in code development and numerical analysis would be ideal. As you indicated, knowledge of Git and Python would be a must. However, further knowledge of numerical techniques for solving integral equations would be advantageous.

The "reversed flow in 274 faces on pressure-outlet 18" error message indicates that there is a problem with the boundary conditions at pressure outlet 18. This error can occur if the pressure outlet boundary condition is not properly defined, or if the flow direction at the outlet is opposite to the expected flow direction.

The "Divergence detected in AMG solver: k temperature limited to 1.000000e+00 in 8519 cells on zone 2 in domain 3" and "turbulent viscosity limited to viscosity ratio of 1.000000e+05 in 18252 cells" messages indicate that the solver is encountering numerical instability during the simulation. This can be caused by a variety of factors, including an inappropriate grid, inaccurate boundary conditions, or a poorly defined model.

The "floating point exception" error message indicates that there is a problem with the numerical computation being performed by the solver. This can be caused by a division by zero, an invalid operation, or an overflow.

There are a few things you can try:

I hope this helps!

I recommend you the previous links because you can easily copy and paste values from there, but you find theory, airfoil geometry and experimental data here: https://aeroknowledge77.files.wordpress.com/2011/09/58986488-theory-of-wing-sections-including-a-summary-of-airfoil-data.pdf

Also I recommend to search for experimental data here in RG.

You can find it in the Ansys theory guide. p-645

If you want a better book to learn the basics of the scheme used you can check the following link.

The CFD simulations is importance to both students and educators. This is because it is able to open the inside and view the behaviours.

For this case, the use of multiple CFD programs can be useful, such as coupling the towing models with one program and meshing with the simulation using another program.

If you are working with modal bases, it is more straightforward to change the order of an element. This means that the existing basis functions do not change. Some people enjoy this because it is easier to work with. Some antialiasing techniques require modal bases to be filtered directly, but this is not a performance bottleneck. Modal commands can also highlight sparsity within an element for special operators. This means that the savings are small for the modest order typically used in 3D. Nodal bases simplify the definition of element continuity and simplify the implementation of boundary conditions, contact, and the like. They are also easier to plot, leading to better ḥh-ellipticity in discretized operators. This, in turn, allows the use of less expensive smoothers/preconditioners. This means that people who create ways of solving problems can more easily use terms like "rigid body modes" and "multigrid methods," and they do not have to change how they think about things when they are using these methods. Discretization is a way to turn an ongoing problem into a discrete one that a computer can solve. An embedded discretization is one where the discretization is done in a way that makes it easy to access and use later on.

Dear Armin Hajighasem Kashani

trying to achieve a match with experimental measurements is not a simple task. In your formulation you should match the statistics of the experimental inflow and that is not straightforward also for a laminar flow problem as detailed in Sec.4 here

A further issue could be also due to the fact your grid resolution is not sufficient to resolve the BL. You should be able to set at least 3-4 nodes at y+<1 along the surface of the body.

Again, I strongly suggest to try your setting for the well studied problem of the backward facing step turbulent flow. You can find a lot of results in literature.

Are you applying compressed ideal gas law? Also, set operating pressure zero and operating density also zero. Don't use outflow boundary condition.

You can find the answer on your question in the treatise of Schlichting, chapter Thermal boundary layer.

What Nemati said is wrong. If a flow is irrotational, a velocity potential exists in 3d, while independent of irrotational condition) the stream function is defined just in 2D flows, and (in 3D) only for axisymmetric flows. Moreover, the stream function is not ∇xF, but it is the one dimensional vector F, oriented in the invariant z direction orthogonal to a flow in the (x,y) plane.

If the size t is less than 8-12 distances to the bottom surface, the drag force will be determined, among other things, by the added masses of the liquid reflected from the bottom surface. If this is not important, then in the model you need to specify a distance of more than 12.

The number of wall layers is enough 5.

From above on a free surface, the wind has a speed and direction?

Just a personal opinion, based on 60 years experience.

From the 1950s, with analogue computers and calculating machines, there have been attempts at physical modelling of cardiovascular hydrodynamics and to a lesser mathematical extent respiratory fluid dynamics and gas exchange.

These have usually been motivated by inter disciplinary collaborations or in some rare cases by individual medical trained people who were also well educated in mathematics and physics ..

The topics chosen were usually the most amenable to mathematical modelling rather than the most clinically relevant.

With the development of digital computers and easy to use software packages requiring much less mathematical and physical knowledge, modelling has become more doable in medicine and especially in biomedical engineering. This has enabled more relevant problems to be addressed and has attracted a wider class of "mathephysicians", medical biophysicists and medical computing people and created its own academic niche.

However , many of the most important medical research developments have been due to mathematics/physics carried out by non medical people eg CT scanning, MRI, DNA sequencing, PCR, prosthesis design, neural net theory, drug design.

See Nobel prize winners.

Dear Souri,

Good luck

You cannot use an arbitrary discretization! You have to reply exactly the same formula used by Fluent.

Do not pay too much attention on the shape of the regions, that can be misleading since of the 1D spatial problem.

The best way to start understanding the hyperbolic character is to use the simple linear problem dphi/dt+u dphi/dx=0. Then, consider the exact solution phi(x,t)=phi(x-u*t,0) that is phi is constant along the characteristic line dx/dt=u. Compare that to the CFL=u*dt/h to see what happens in case of upwind. You will see when you get an interpolation or an extrapolation of the values.

For a deeper understand of such problems, have a look at the tectbook of Leveque.

Dear Momen,

Thank you very much, I appreciate it!

When can I call you, what do you prefer?

Regards, Rob

i actually meant to say.. dont use hexa mesh.. i could not go back and correct it.. sorry about that..

I've looked at the polynomials that you have noted: none of them make sense, whether using actual temperatures or reduced temperatures.

I suggest going back to the original data and making your own, (lower order) curve fit using Excel.

One further comment, you would be mis-calculating enthalpy if you used your last equation as it assumes a constant specific heat over the temperature range: the correct form is to integrate Cp.dT over the range, I would integrate the secific heat equation (if it was correct).

hi Nikos,

Have you got the answer of "How can I add FLOTRAN CFD in ANSYS together with the elements 2D FLOTRAN 141 and 3D FLOTRAN 142?"

Please share the solution if you got it as i am also interested in doing this.@Nikos

The Environmental Fluid Dynamics Code (EFDC+) is an open-source 2/3D model that is quite powerful for such applications. It includes advanced features for hydrodynamic simulations, wetting and drying, and sediment transport simulations.

You can download the source code and compiled executables from GitHub: https://github.com/dsi-llc/EFDCPlus

The package from GitHub also includes a simple grid generation tool.

GUI software, EFDC+ Explorer Modeling System, is also available to support EFDC+ model development, calibration, post-processing, etc. at: www.eemodelingsystem.com

Licenses range in price depending on if you are an academic or commercial user, so please reach out to sales@eemodelingsystem.com with any questions.

However, even if you choose not to use the GUI you will still have full access to all the model features, and can make use of example demonstration models and test cases to learn how to build the input files.

can anyone give boundary condition code explanation in keller box

Hello Jagan Adithya Elango ,

Pls do refer to the Comsol blogs on natural convection available on the comsol website. You'll get a clear idea of how to proceed.

Let me know if you have any other doubts.

Regards

Gautam

Hi,

ffmpeg is your friend here. Googeling for 'ffmpeg image to video' or something like that produces a lot of material. This one, e.g., seems decent

Please take a look at Delaunay and Trisurf functions in matlab. Extract your vertices data in a matrix. Following is the code that I used in my, You can tailor it according to your needs:

vel_files = dir('*.csv'); %Only velocity data

velnum = length(vel_files);

vel_data = cell(1, velnum);

for k = 1:velnum-1

filename = vel_files(k).name;

vel_data{k} = readmatrix(filename);

end

Tvert = readtable('vertices.csv'); %vertices data

x = Tvert.Var1;

y = Tvert.Var2;

vertices= cat(2,x ,y);

DT = delaunay(vertices);% Delaunay Triangulation

VELALL=cell2mat(vel_data); trisurf(DT,vertices(:,1),vertices(:,2),VELALL(:,141),'EdgeColor','none'); axis tight; daspect([1 1 1]); shading interp; colormap jet; view(2);title('Ux t=100s')

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.

Ravi Babu Sajja

Hi, I am an engineering doctor (expected) at the Institute of Mechanics, CAS and I major in Fluid Mechanics. I am currently engaged in Hypersonic aerodynamics, Compressible flows, and Complex aerodynamic interference associated with shock waves in multi-body dynamic separation at high speed, CFD simulation, and wind tunnel tests. As my experience, the CFDers' work is more concentrating on better CFD methods, higher accuracy, faster calculating speed, and more reliable simulation results for complex and possible fluid flow in engineering problems. However, the aerodynamicist's work is to solve one of the specific or simply aerospace engineering problems, to study the flow mechanism associated with the considered problem, using the fluid mechanic's theory, CFD tools or numerical analysis, sometimes, the wind tunnel tests. Nevertheless, the fundamental knowledge of CFD, fluid mechanics, data analysis, etc., are essential for us to become qualified aerodynamicists. Whichever master or doctor to be, we are not just limited in it. The skills and expertise in aerospace engineering or aerodynamics are overlapping, cross and interdependence. When we choose to become a master, unknown things are approaching on the way. So, we would not be afraid of the thing we are not good at, we have the learning skills, and we can overcome something. By the way, the wind tunnel experiment is interesting, at least for me, and the experimental results are precious. Maybe, we could be more comprehensive by engaging in aerodynamics or aerospace engineering. Each master has its advantages and characteristics. Believe us, we can make something, and we can enjoy it. Just a little experience and feeling, hope that can help you.

Do you mean some particles trapped and some pass through? You may need UDF.

Hi Kun Dou ,

Thank you so much for your suggestion.