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# Computational Fluid Mechanics - Science topic

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I have a specific case about internal pipe flow with constant heat flux. Although the inlet boundary condition is laminar, the flow is a passing transition (a significant part of the tube) and turbulent regime along the tube (because of the change of thermophysical properties depending on implied heat). SST models with intermittency term (For fully laminar flow, γ = 0 and the model reverts to a laminar solver. When γ = 1, the flow is fully turbulent.) can catch laminar/transitional and turbulent flow regimes. These models were designed for turbulent inlet boundary conditions (models solve intermittency term, so it needs extra boundary conditions such as turbulent intensity). Can Transitional SST Models be used for laminar inlet / turbulent outlet boundary conditions? If so, what is the approach?
Regards,
EB
Dear Dr. Youyun Shang thank you for your answer. I tried "enhance wall" in many different ways but couldn't achieve it yet. The transition model (by Menter) predicting transition was formulated for external flows. This model actually should be adapted to flow in pipes. J. P. Abraham conducted some studies in this context and presented modifications to the original transition model. Although I have adjusted my model considering these modifications, I couldn't take a proper solution (which is compared with experimental results).
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Hello dear Researchers :
Does anyone have modeled, or have literature on how to mathematically model a :
Fluid Flow of a :
- Non-Newtonian Fluid
- Non-Isotermal Flow
- Compressible Flow
- Polymer.
Thank you for any help, I'll appreciate it !
Best Regards !:)
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.
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I'm simulating a 3D wing with winglet (on the tip) using ANSYS Fluent. I'm confused about how to calculate the area in reference values section. Please check the attached pictures of my wing model.
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)
.
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I guess my question is quite simple: you know how for compressible CFD codes we have the Sod Shock tube problem that we can use as a benchmark. What if I now want to expand my code and add chemical reactions to it, what would be a good/similar test problem that I could use to validate how well my code runs?
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.
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This Discussion is to help people that are both experts and newbies in the field of CFD and simulation to help each other and to give links to websites and drop materials that are invaluable to push us to the frontiers.
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)
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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 would like to study the apo form (lipid-free) of a protein that only has been crystallized with lipids. I want to explore if it is possible to generate with a molecular dynamic a reasonable structure, making subtraction of lipids in several steps until obtaining the apo form. Likewise, I don't know if, during the molecular dynamic trajectory, it is possible to disappear lipids. I am thinking of using programs like GROMACS, AMBER, etc.
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.
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In an article, the authors obtained the Nusselt number like 6.487. When I apply the same operating conditions for the same geometric model, I obtained the Nusselt number like 3.741. How can ı found the nearest value to authors? Thanks for your interest.
Note: I use the (h*Dh/k) for Nusselt number calculation. For the calculation, I obtained surface heat transfer coefficient from Fluent in Excel and I took the average values at thermally developed region for heat transfer coefficient values. In article, thermally developed region was used for calculations.
I concur with Hayder Jaffal
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I just want to find out a good book on fluid flow through channel which is having mathematical modelling of fluid flow through different type of channels eg vertical channel, inclined channel, wavy channel etc.
I would suggest this book from M. Hanif Chaudhry: Open-Channel Flow Second Edition.
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Any explanation for time step size definition for fluent ?
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.
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Hello Everyone ,
I am simulating a 3D airfoil in fluent . The results I am getting for lift coefficient are within 5% error limit of the experimental values .
However , the results for drag coefficient are significantly lower than the experimental results .
I have read in literature that this under-prediction of drag coefficient is due to the lift induced drag in experimental results and that CFD does not account for lift induced drag . Is it true ?
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.
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For the 2D NS equation of the stream-vorticity formula:
$\partial w/ \partial = J(\psi,w) + (1/Re)\Delta w+f(x,y,t); w = -\Delta \psi$.
Using the Fourier-Fourier basis Exp(I(k1*x+k2*y)), I got the evolution in spectral space:
$dW/dt = LW+N(W,\psi_hat)$,
where W, \psi_hat are complex. It has the solution W(t;W0) if the initial values W0 is given.
To search the periodic orbits of this problem with a period T, I tried to solve the equation
$F(W0,T) := W(T;W0)-W0=0$,
Using the Newton search, I can get:
$\partial W/\partial W0 * dW0 + \partial W/\partial T *dT = -F(W0,T)$. (*)
Now the problem I meet is: both dW0 and dT are complex since the coefficient matrix of (*) is complex.
SO how can I get a REAL dT?
THANK YOU VERY MUCH!
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Dear all Professors & fellows,
Respected to you all.
I saw few published manuscripts & found that, authors have evaluated TAWSS & OSI for rigid case of CFD simulations.
My question is that, actual meaning of TAWSS is - Avgd WSS over one cardiac cycle, which is used to determine the shear stress magnitude applied on vascular wall surfaces during one cardiac cycle.
So, how & for what reasons TAWSS & OSI parameters were calculated when they have done CFD simulations where they don't have vascular wall surfaces or absence of wall.
As of my understanding I believe that, these parameters can be only evaluate if you are doing FSI simulations (fluid structure interaction studies) but not for CFD (Rigid wall) cases because that give incorrect results as CFD cases doesn't have surfaces or walls.
SO, can you please tell me how much I am correct here .
I am saying that, TAWSS, OSI should be evaluated for Fluid structure interaction cases only but not give correct results for CFD (rigid wall cases) so not necessary to calculate in rigid cases of artery.
is that I am correct??
Thanking you in anticipation,
Regards,
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.
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Dear all;
I am trying to simulate non-Newtonian fluid flow in porous media with the help of Palabos software. Palabos is a C+++ open-source library that solves flow problems with the lattice Boltzmann method. I am not familiar with object-oriented programming and I want to extract a value from a template that is used in every iteration. To be more precise; I want to print Omega (which is an (Nx-1)*(Ny-1) dimensional matrix) in every 250 iterations in the form of gif or data to validate my result. I added pcout to print variable in this function and it works but it is repeated every iteration which generates too much data ( imagine 200*200 lattice and 10000 iterations!).
I will be really glad if you could help me through this. the code is provided below. also, I uploaded
.h and .hh file to this message
Many thanks;
Elahe;
template<typename T, template<typename U> class Descriptor,int N>
T CarreauDynamics<T,Descriptor,N>::
getOmegaFromPiAndRhoBar(Array<T,SymmetricTensor<T,Descriptor>::n> const& PiNeq, T rhoBar) const
{
T nu0_nuInfoverCs2 = (global::CarreauParameters().getNu0()-global::CarreauParameters().getNuInf())*Descriptor<T>::invCs2;
T nuInfoverCs2 = global::CarreauParameters().getNuInf()*Descriptor<T>::invCs2;
T nMinusOneOverTwo = (global::CarreauParameters().getExponent() - (T)1)/(T)2;
T lambdaOverCs2sqr = global::CarreauParameters().getLambda()*Descriptor<T>::invCs2;
lambdaOverCs2sqr *= lambdaOverCs2sqr;
T piNeqNormSqr = SymmetricTensor<T,Descriptor>::tensorNormSqr(PiNeq);
T alpha = lambdaOverCs2sqr * piNeqNormSqr *(T)0.5
*Descriptor<T>::invRho(rhoBar)*Descriptor<T>::invRho(rhoBar);
T omega = carreauDynamicsTemplates<T,N>::fromPiAndRhoToOmega(alpha, nu0_nuInfoverCs2, nuInfoverCs2, nMinusOneOverTwo, this->getOmega());
T tau=(T)1/ omega;
T visco=((T)2 * tau - (T)1)/(T)6;
pcout << "" << visco << std::endl;
return omega;
}
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 );
Regards;
Elahe
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Discussion of the state of art on the application of the Ertel's potential vorticity theorem in atmospheric physics & physical oceanography.
Prof. H. Ertel generalized Rossby's work proposal of 1939. Prof. Rossby firstly proposed that instead of the full three-dimensional vorticity vector, the local vertical component of the absolute vorticity is the most important component for large-scale atmospheric flow.
Via an independent paper published in 1942, Prof. Ertel identifying a conserved quantity following the motion of an air parcel proved that a certain quantity called the Ertel potential vorticity is also conserved for an idealized continuous fluid.
A very complete review on Ertel potential vorticity theorem, thank you for suggesting its reading, Prof. Aref Wazwaz .
Best Regards.
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What are the advantages of Finite volume method (FVM) over Finite difference Method (FDM) for particularly flow simulation (CFD) ?
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.
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Hi
I'm trying to simulate thermosyphon. But when the UDF read, and I want to run, the following error was printed!
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.
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Dear all,
In my simualtion domain, I need to use mixture of gas ( N2 gas with 20% CO2) at INLET in ANSYS Fluent.
Mixture model, and species model, may be these models work. Actually, I need your support to notify the model and how to put the above specifications on that model too.
Regards
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.
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I have a case, which is about internal flow with constant heat flux. Although the inlet boundary condition is laminar, the flow is passing transition and turbulent regime along the tube. As known, the intermittency term is 1 (so, admitted as turbulent inlet BC) for freestream velocity for external flow, I would like to learn that whether using the transitional SST model by laminar inlet boundary condition in the pipe is the corrects way or not.
Best 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.
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I am trying to simulate a heterogeneous reaction between two phases in Eulerian multiphase model. I have written a UDF and it compiles and runs well. The reaction is: C+2H2--> CH4. Although my solution shows reasonable values for reactions rates, but I do not see any changes in Hydrogen concentration from the inlet gas. Can anyone where the problem is arising from?
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...
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I need the following tutorial zip files to learn the basics of the software:
• Using sliding and dynamic meshes
• Modeling solidification
• Using the Eulerian Granular Multiphase Model with heat transfer
• Postprocessing
• Using the Adjoint Solver – 2D Laminar Flow Past a Cylinder
• Simulating a Single Battery Cell Using the MSMD Battery Model
• Simulating a 1P3S Battery Pack Using the MSMD Battery Model
Youssef Hmayed Walaikum assalam warahmatullahi wabarakatuhu.
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• How to find a relationship between velocity and weir landing length (X') as shown in figure?
That seems more like a homework... I suggest to have a look to this textbook: Fluid Mechanics by White.
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Hello all,
I am looking for an method / algorithm/ or logic which can help to figure out numerically whether the function is differentiable at a given point.
To give a more clear perspective, let's say while solving a fluid flow problem using CFD, I obtain some scalar field along some line with graph similar to y = |x|, ( assume x axis to be the line along which scalar field is drawn and origin is grid point, say P)
So I know that at grid point P, the function is not differentiable. But how can I check it using numeric. I thought of using directional derivative but couldn't get along which direction to compare ( the line given in example is just for explaining).
Ideally when surrounded by 8 grid points , i may be differentiable along certain direction and may not be along other. Any suggestions?
Thanks
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”.
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.
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Dear all,
I'm doing a two way FSI problem with Static Thermal and Fluent as my systems. Something like the video attached. While solving the system coupling threw the following error: "Update failed for the Solution component in System Coupling. The coupled update for System, Fluid Flow (Fluent), threw an exception."
In total there were four errors. See the image attached.
Anyone with any suggestions or advice is appreciated.
Regards,
Nikhilesh
"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.
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If POD or DMD is applied to flows which are not turbulent in a broad sense, which means that there is not much chaotic mixing, will they carry any significance? Because there will be not many spatial and temporal scales like in a turbulent flow.
Thank you Prof. Sengupta.
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An aircraft is consist of many sub-parts , while estimating the Reynolds number and y+ value first layer height which length do we consider as the characteristic length to be used in the respective formulas ?
The characteristic length is used in determining dynamic similitude. The ratio between inertia and viscous forces called Reynolds number depends on a characteristic length.
1. In general, the smallest available length scale is the characteristic length scale.
2. Sometimes (e.g. in dynamic systems) there is no fixed length scale to choose as a characteristic length scale. In such cases often a dynamic length scale can be found.
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.
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I am solving a transinet convection-diffusion equation
u_t + a*u_x - nu*u_xx = 0 in (0,1)
Taking,
Convection coefficient a = 1
Diffusion coefficient nu = 0.005
Number of elements = 100
I got Peclet number Pe = 1
For same case now,
Time step = 0.01
End time = 0.6
I got Courant number C = 1
I need following 4 conditions to fulfill.
1). C =1 and Pe = 5
2). C =1 and Pe = 100
3). C =3 and Pe = 5
4). C =3 and Pe = 100
My question is:
What should be the value of
"Convection coefficient a",
"Number of elements",
"Diffusion coefficient nu",
"Time step" and
"End time"
to get these 4 results separately.
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
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Dear all:
I need help, I hope there is someone out there who can help me.
I finish drawing the 2D geometry of an Air Solar Collector with an undulated absorber plate (in contrast with a flat plate absorber). This is a ~~~~~ shape plate to capture the sun radiation.
- Single Phase FLuid Flow - Laminar Flow
- Heat Transfer in Solids
- Heat Transfer in Fluids
But when I run the simulation, even though I have the Heat Transfer in Solids and the Heat Transfer in Fluids modules included in my model, the Results doesn't show whatsoever the Graphs for the Temperature Distribution, or contrours. Also the Pressure Results Graph doesn't shows anything
I'am attaching two images: on of my model's geometry, and one with the solution of the Velocities distribution, but this is as much as I can arrive to.
I'm sure I am doing something wrong with the Boundary Conditions I'm entering.
These are all the Boundary Conditions I'm trying to use :
i. Constant Radiation Heat Flux at the top of the glass cover.
ii. Constant Convection Heat Flux as Losses from the top of the glass cover to the ambient.
iii. Tried to establish a Constant Conduction Heat Flux from the bottom of the collector to the surroundings (to represent the Heat Conduction Losses from the bottom of the Collector to the roof and environment).
iv. Isothermal Walls at both of the sides of the air cavity.
v. The No-slip Boundary Condition (u=v=0) at the inside of all the walls of the air cavity.
vi. An entering Air Mass Flow from the left to the right of the bottom air cavity.
vii. I wanted to establish also and exiting Mass Flow at the right side of the bottom cavity, but couldn't. So instead, I established this Condition as a Pressure at the Outlet, as the Boundary Condition.
So I'm pretty sure I am doing something wrong with one or several of the BC.
I hope someon can give me some help me. I'll appreciate it
Thank You !
Useful information can be found in the attached files.
Regards,
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Hi everyone ,
I am implementing Acoustic model in FLUENT. Geometry is 2D. By using FW-H model, I am facing difficultly to understand these things. ( I have already read FLUENT manuals and one tutorial )
1) It is very important and necessary to define accurately Source Correlation Length in 2D geometry. I do not know this parameter. how to calculate it. my geometry is very simple, just a rectangular to 10*20 mm.
2) What should be the source zone and type ? In 2d-cylindrical tutorial, I read it is a cylindrical wall. My geometry have 0.5 mm inlet a bottom. Is it the source zone and type will be Velocity INLET ?
3) Where will be the receivers position ? I have to monitor the the time history when gas passes through from INLET ? I read one paper there was one monitoring point which is away form the inlet.. I could not understand.
4) What is FW-H integral surface ? I think, the under investigation portion has very fine mesh. this is called FW-H integral surface. But I am not sure about it..
Any type of advise, guideline or tutorial will be highly appreciated. Thanks in advance..
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.
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Dear all :
I'm trying to solve the following problem using COMSOL Multyphysics.
I have an enclosed geometry (please see the attached images) with a top flat surface (the celing) and an undulated shaped bottom surface (the floor). And I have Air as a medium enclosed inside this cavity.
So, I want to solve for fluid Velocities (u and v) distribution of the air inside the cavity, and for its Temperature distribution too.
So, I already included the physics modules: Laminar Flow (for single-Phase Fluid Flow) and Heat Transfer in Fluids.
However, I'm sure I'm doing something wrong witht he Boundary Conditions (it can be observen from the first attached image, that I don't get any velocities distribution for the air mass).
The BC I am trying to stablish are the following :
i. Isothermal Walls in all the interior walls of the cavity.
ii. No-slip Boundary Condition (u=v=0) in all the walls (on the inside) of the cavity.
iii. A constant Temperature at the upper flat surface
iv. A constant Temperature at the bottom (wavy floor) surface of the cavity.
Where the problem could be at ? The Velocity Stream Lines and Temperatures Contours should show a kind of oval spiral pattern at the top of each of the 'valleys' of the wavy bottom surface. But as depicted in the image, I don't get any valocities distribution.
I will really appreciate if someone can give some help.
Thank you all !
Best Regards !
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 !
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I want to model a part of a river in Ansys-Fluent. The discharge and the water surface level at downstream (or outlet) are known. The model is multi-phase VOF (water+air) and Open Channel option is also selected. My question is what boundary conditions best suit for this model.
Is it proper to use "mass-flow-inlet" for inlet and "pressure-outlet" for outlet? Are any special considerations required? I am relatively new to this kind of problems (CFD, I mean).
I have experience in modelling the two-phase flow (water+air) through an emergency gate in a dam. I use the same BCs (Mass flow inlet (or velocity) for the inlet and zero pressure outlet for the outlet). Also, I guess in your case, since it is a river, you need to specify the top boundary with pressure outlet. Cheers
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I am doing a numerical study of a new model of a heat exchanger and there are no previous studies on the same geometry that the study was conducted on. Is there a way other than conducting experiments to ensure the validity of the numerical study?
"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....
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I am trying to calculate flow parameters of the multi stage subsonic axial compressor using Ansys CFX. But it turned out that the solver cannot find the solution. In all cases an "OVERFLOW" error has appear. I am trying to solve with following boundary conditions: 1) total pressure and temperature at the inlet and massflow rate at the outlet; 2) total pressure and temperature at the inlet at the inlet and static pressure at the outlet. On the other hand when i performing calculations of the separate stages, the Ansys CFX solver easily find the solution. As I understand it, when i try calculate all stages in one calculation, the initial parameters in the solver are too wrong and because the solver cannot find the solution.
What can I try do to find a solution?
François Morency , Big thank you for that answer ! As you say i am submit the missing information.
1.I get "OVERFLOW"after 15 iterations
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?
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Is it possible to take into account connecting wire(fig.1) in the fluid flow calculation of a turbine stage in the Ansys CFX?
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.
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While doing 2D simulation of underexposed and over-expanded supersonic jets my results is getting converged with low NPRs (2-4) with choosing Zero Supersonic/Initial Gauge Pressure at the inlet of nozzle in the Fluent solver. But  it get diverged with high NPRs (4-10) value. However results are converging when putting nearer value of Supersonic/Initial Gauge Pressure to inlet nozzle pressure. Kindly help me to resolve this problem ?
I am sorry sir, would you please tell me,
what do you mean by both?
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I need to simulate the transition of Flow through Coarse Porous Media such as Rock-fill dams, to investigation of Water level profile in each distance from up-stream and determination of discharge of fluid seepage from the body of these media. Notice, I want to simulate a Single-Phase Flow.
How can I simulate this project? Please suggest and introduce a useful software for this issue to me...
Dear Dr. Majid Heydari ,
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
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I want to know if Ansys Fluent is an appropriate tool for modeling river bed sediment transport which is a frictional particulate flow regime rather than a collisional regime such as fluidized bed. Fluent has a DDPM-DEM model that can be used, however, I couldn't find any studies that have been used Fluent for this purpose. Thanks
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
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I make calculation of my model of finned heat exchangers by Ansys Fluent. Model is somewhat complicated and have abut 5 millions elements and 8 inflation's layers. I chose k-epsilon method for solving and choose pressure outlet and velocity inlet for air (1 m/s).
On outlet is appeared reversible flow on great amount (see atachment). What is reason?
How to eliminate this reversibilke flow?
Did i make some mistake in input data?
See my question
How to calculate hydraulic diameter (for finned heat exchanger)-debate?
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When the buoyancy effect is significant in supercritical fluids passing in the tube, some eddies can form in any location of tube, especially at the vicinity of supercritical temperature. So, what is the numerical method to solve like the compressible problem? As my experience and literature, there is no way to calculate this type of problem by using RANS models, so I have just started using LES models. Although it was used a very low time step size (about E-5) and tried all of the subgrid models, I couldn't solve this problem. I would like to be grateful if you could share your recommendations. Thanks
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
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I had to calculate parameters of the technical oil flow in Ansys CFX.
To solve this problem, I used "the Turpentine" (from Standard Ansys CFX material library) as the working body.
Is it right ?
I know the actual density of the oil. The Turpentine has similar density. Is it enough to use"the Turpentine" as working body?
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.
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In our experiments with shear-thinning supensions, should we consider the viscosity change with change in shear rate? E.g., capillary number (Ca) and Reynold's number (Re) both contain viscosity as well as characteristic velocity for viscous forces. We are varying the flow rate and so the shear rate changes. Should I change the viscosity accordingly? Or is using a single value is fine?
I'm guessing this is an issue with experiments and not with computational work where the formulation is usually non-dimensionalized and one can simply vary the Ca or Re directly.
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)/{8n-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.
1. Aroon Shenoy, Heat Transfer to Non-Newtonian Fluids: Fundamentals and Analytical Expressions, Wiley-VCH, Weinheim, Germany (2018).
2. Aroon V. Shenoy, Rheology of Filled Polymer Systems, Kluwer Academic Publishers, Netherlands (1999).
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I was trying to solve channel flow with fully developed flow at inlet as well as outlet using simpleFoam. I would like to know how to implement periodic/cyclic boundary condition (translational) in inlet as well as outlet.
Can any one help me how to implement this boundary condition?
I recently documented implementing streamwise periodic boundary condition in OpenFOAM.
Hope this helps!
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Dear
I am simulating the natural convection in circular annulus through magnetic field. For that I am writing udf for x direction magnetic, kindly share your valuable experience in doing such a task?
How can I debug the code written in UDF File?
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.
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Can some one give me details for the application of QUICK scheme to 2d lid driven cavity?
Best Regards
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
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Trying to find any benchmark on the drag coefficient for a cylinder in axial flow. There are many on the flow from the side, also several on axial flow for Re over 10000. What about the near-Stokesian regime?
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Hello
I am solving sub-cooled boiling flow with CFX. I do not want to use single phase wall function. I want to solve the viscous sub-layer. Is it possible? If so which turbulence model is suitable? Which points should I take into account?
Thank you
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.
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Hello Everyone ,
Hope you are doing great .
I am simulating a 3D wing of aspect ratios
a) 0.5
b)1
c)1.5
d) 2
Why is it so that as I move from AR=0 to AR=2 the difference between CFD and experimental results increases .
Regards
Aamir Sultan
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 .
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Hello Everyone !
I am simulating a 3D airfoil at higher angle of attacks (post stall regime) . I wanted to know that is RANS formulation capable enough to resolve tip vertices or not .
Regards
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.
Tapan K. Sengupta
it is instructive to explore this issue.
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Hello Everyone ,
I am running a 3D simulation of an airfoil . The results I am getting for drag or lower than that of experimental results . I wanted to know could I calculate the value of lift induced drag from drag polar ?
With respect
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I am simulating (CFX) a pipe with a turbulence promoter in it. I am using RNG k-e turbulence model yplus is lower than 1. For mesh independency analysis I am using turbulence kinetic energy as a output. The problem is, as I increase the mesh size, relative difference is getting higher up to certain mesh size (solution is getting mesh dependent). After that mesh size the relative difference is decreasing. For example from 4 to 12 million, relative change increases and from 12 to 25 million, it decreases. This values are coming because of the region where is just after the turbulence promoters (dominant anisotropic flow).
What could be the reason?
Horimek Abderrahmane I will thank you very much
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Hello Researchers',
I am working in computational fluid mechanics problems that investigate the effect of dialysate concentration and flow rate.
Q1: what is the maximum net osmotic pressure (MPa) that we can apply on membrane BC as driving force to the mass transfer along the membrane surface or maximum solute mass fraction for the dialysate (k_gsolute/kg_solvent)?
Q2: why the dialysis flow rate limited by 800 ml/min and some publishable journal used 1000 ml/min and not more than this flow rate?
Q3: what is the clearance of urea or uraemic toxin during one session of hemodialysis in (kg/m^2.hour)?
Thanks in advance for you and if you can provide me with a study/studies that have these pieces of information.
Interesting
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I am working on enhancement of heat transfer in a square channel through broken ribs. I need to calculate friction factor f so I need pressure drop across the periodic duct. I am using FLUENT 6.3.26 for simulation with periodic and symmetric conditions. The upper and lower walls are heated walls where constant heat flux of 2500 W/m2 is applied and other two walls are adiabatic, and adiabatic ribs were provided on heated walls. I am using the k-ep turbulence model. Is there any other method to calculate friction factor in this case?
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
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I am trying calculate a steam turbine stage which works in wet steam zone. But i dont know how define boundary conditions: in Ansys CFX, because i can define only Temperature and Pressure.
Temperature and Pressure are not enough data for wet steam, because Temperature and Pressure are equivalent in wet steam zone.
I need additionally define enthalpy or wetness,but i stuck to do it.
Can anyone tell me how do it ?
(P.S. I am use Steam1 from IAPWS-97 library)
In the pdf that is inside the package open chapter 25. There is an example of wet steam case.
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i am simulating a pulsatil blood flow inside abdominal aortic aneurysm AAA witch is unsteady (cardiaque cycle ) , Fluent gives differents parametres ( velocity , pressure , wall shear stress WSS... ) at each time steps ,but i want to calculate OSI oscilating shear index witch depends on the mean and the magnitude of WSS wall shear stress in the cardiaque cycle ... is there a way i can do to calculate OSI in Fluent ?
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...
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Right solve or not, if during all iterations Ansys CFX Solver manager writes, that he set up artifical wall on the outlet (in my case) ? Howewer, all residuals has attained the level required and mass flow rate at the inlet equal to the mass flow rate at the outlet.
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.
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I'm looking for experimental data about the free surface elevation downstream of a horizontal cylinder in shallow water flow. Ideally for different Froude- and Reynolds numbers in a 2D setting. I'm not interested in the detailed flow structure, only the free surface shape is relevant.
Does anybody know some literature/papers?
For a submerged hydrofoil (Duncan 1981, 1983) such data is available.
Thank you
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
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I'm trying to calculate valve of steam turbine in Ansys CFX (i'm using single phase material - water ideal gas).I'm getting that the mass flow rate in the inlet zone is not equal to the mass flow rate in the outlet zone when calculating half closed valve. This difference amounts to 20%. Howewer, the mass flow rate in the inlet zone is practically equal to the mass flow rate in the outlet zone when calculating fully open valve.
By comparing the displays of CFX-Solver-Manager,I noticed that , unlike when calculating fully open valve, residuals are not declining when calculating half closed valve (fig.1).Therefore, i think that calculation is not right in this case.
Also, by comparing flow structures, I noticed that ,unlike when calculating fully open valve, there are vortex zones (fig.2). I think, that this vortex zones are cause of error.
Please can you tell me how to raise accuracy of calculation of flow with vortex zones?
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.
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I want to optimized my basic design of a new kind of VAWT blade.
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?
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By applying the Buckingham Pi theorem, dimensionless parameters such as Reynolds number (Re) and Froude number (Fr) can be indicated for the two-phase flow.
Can I use the CFD FLUENT approach to investigate and examine these dimensionless groups for the two-phase flow patterns predictions and How? Is there any suggestion please.
yes, you can
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Can somebody help me in Installing and starting of LAMMPS? I have completed all the steps in online guide of LAMMPS for windows but stuck? I am installing LAMMPS for DPD simulations.
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:
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Which programming language is the best to use in CFD?
FORTRAN, MATLAB or any other?
Is there difference between the performance of these codes and applications such as OpenFOAM?
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.
.
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Are the equations of Fay and Riddell embedded in Fluent or do they have to be imported using a UDF?
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.
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As usually the trend is a Courant number (CFL no) can only be used for a Transient( Unsteady) flow condition.
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.
By default, FLUENT uses a 3-stage Runge-Kutta scheme for steady-state flows that use the density-based explicit solver.
Hope this helps!
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Dear Researchers,
I am working on simulating shell and tube heat exchangers using ANSYS Fluent. I have the huge number of tubes and it is not recommended to model it. Therefore, I am trying with alternative approaches like porous medium and heat exchanger models.
*In the porous medium approach, the non-equilibrium thermal model is recommended for heat exchanger type of problems.
*In heat exchanger model, dual cell method is used to simulate shell side and tube side flow and heat transfer.
I request you guys to help me in this regard.
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Hi all,
I want to simulate a compressible sonic process of butane gas, also with transaction phase (from gas to liquid) if temperature goes too low using OpenFOAM. About the solver I chose sonicFOAM but other suggestions are welcome.
Mainly I'm looking for a good definition for the thermophysical properties data to simulate accurately butane. It is not a perfect gas. Any suggestions are welcome!!
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Modeling a sediment transport phenomena using Volume of Fluid (VOF) method for Multiphase problem and Eulerian-Larangian particle transport method for Discreet Phase Modeling (DPM). My mesh maximum skewness is around 0.5. For solver settings pressure-velocity coupling was introduced with Green-Gauss Cell Based gradient, PRESTO! pressure and Second Order Upwind momentum. I tightened all URF (Under-Relaxation Factors) to 0.1~0.2 and used 0.001 time step size.
After running the solution for some time everything looks fine in the animation but the solver fails showing "Error: Divergence detected in AMG solver: epsilon". Anyone has any idea on how it can be solved or the error can be removed?
" In the name of God, the Compassionate, the Merciful"
1- check your mesh. Is your skewness, mesh orthogonal quality, ... acceptable? 2- Initialization. Try the hybrid (FMG) initialization. 3- Under-relaxation Factors. try using 0.5 for both k & e, then ramp them up to 0.9 after 50~100 iterations. Also decrease time step or Currant Number (dependent on solver)
4- Boundary Conditions. k & e values suitable?
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