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# Turbulence Modeling - Science topic

Explore the latest questions and answers in Turbulence Modeling, and find Turbulence Modeling experts.
Questions related to Turbulence Modeling
Question
In fluid mechanics and computational fluid dynamics (CFD), the fluctuating velocity components u' v' w'′ represent the turbulent fluctuations in the velocity field. These components are used to model the turbulent behavior of fluid flow. The prime notation (u' v' w') denotes the deviation of the velocity from its mean value (U,V,W).
For laminar flow, turbulence is generally not considered, and the flow is assumed to be smooth and ordered. In this case, the fluctuating velocity components (u' v' w') are essentially zero.
In CFD simulations using software like FLUENT, you typically specify the turbulence model and relevant parameters to simulate turbulent flows. Common turbulence models include the k-epsilon model, the k-omega model, and the Reynolds stress model. These models provide equations for the turbulent kinetic energy (k) and the turbulent dissipation rate (ϵ), from which the fluctuating velocity components (u' v' w') can be derived.
In FLUENT, you will need to set up your simulation by defining the geometry, boundary conditions, and fluid properties. Additionally, you'll need to specify the turbulence model and provide initial conditions for the turbulence variables. The software will then solve the governing equations, including the RANS equations and turbulence model equations, to obtain the mean flow field and turbulence quantities, including the fluctuating velocity components.
The specific steps may vary depending on the version of FLUENT and the turbulence model chosen, so it's recommended to refer to the FLUENT documentation or user guide for detailed instructions based on your simulation setup.
We will get the value of turbulent kinetic energy (k) from fluent or cfd post. this value can be substituted into 𝑘=0.5(𝑢′2+𝑣′2+𝑤′2).
with isotropic turbulence condition 𝑢′= 𝑣′ = 𝑤′
hence the initial equation turns out to be 𝑢′ = sqrt (2k/3)
This way, you can calculate the fluctuations. If you want to know the fluctuations 𝑣′ and 𝑤′, one must use rsm turbulence model.
For better understanding, check the "Turbulence Modelling for CFD" by Wilcox.
Question
Some turbulence models such as DES and LES are incompatible with the overset mesh in fluent software. Is it possible to avoid or handle this problem vie UDF file and how?
Overset meshes for incompressible flows: On preserving accuracy of underlying discretizations
AsheshSharmaa,∗, ShreyasAnanthana, JayanarayananSitaramanb, StephenThomasa, Michael A.Spraguea
aNational Renewable Energy Laboratory, Golden, CO, USA
bParallel Geometric Algorithms, LLC, Sunnyvale, CA, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received in revised form 19 September 2020
Accepted 30 October 2020
Available online xxxx
Keywords:
Computational fluid dynamics
Unstructured meshes
Chimera grid
Overset
Wind energy
This study on overset meshes for incompressible-flow simulations is motivated by accurate prediction of wind farm aerodynamics involving large motions and deformations of components with complex geometry. Using first-order hyperbolic and elliptic equation proxies for the incompressible Navier-Stokes (NS) equations, we investigate the influence of information exchange between overset meshes on numerical performance where the underlying discretization is second-order accurate. The first aspect of information exchange surrounds interpolation of solution where we examine Lagrange and point-cloud-based interpolation for creating constraint equations between overset meshes. To maintain overall second-order accuracy, higher-order interpolation is required for elliptic problems, but linear interpolation is sufficient for hyperbolic problems in first-order form. Higher-order point-cloud-based interpolation provides a pathway to maintaining accuracy in unstructured meshes, but at higher complexity. The second aspect of information exchange focuses on comparing the approaches of overset single system (OSS) and overset Additive Schwarz (OAS) for coupling the linear systems of the overlapping meshes. While the former involves a single linear system, in the latter the discrete linear systems are solved separately, and solving the global system is accomplished through outer iterations and sequential information exchange in a Jacobi fashion. For the test cases studied, accuracy for hyperbolic problems is maintained by performing two outer iterations, whereas many outer iterations are required for elliptic systems. The order-of-accuracy studies explored here are critical for verifying the overset-mesh coupling algorithms used in engineering simulations. Accuracy of these simulations themselves is, however, quantified using engineering quantities of interest such as drag, power, etc. Consequently, we conclude with numerical experiments using NS equations for incompressible flows where we show that linear interpolation and few outer iterations are sufficient for achieving asymptotic convergence of engineering quantities of interest.
Question
Dear CFD Researchers,
Since AI tools are currently very popular, I am wondering if anyone use them to choose turbulence model for a CFD case.
So if you did, please share your experiences. Due to the answer, we can extend the boundaries of this discussion.
Kind regards,
Guven
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...
Question
Hello all,
I'm doing a 2D simulation of flow beneath a partially-submerged rectangular bluff body. The problem's geometry is shown in the figure attached. I'm using a fully structured hexahedral mesh with a y+ of below 1. When the draft (i.e. t in the figure) is small, the model predicts the reattachment length (i.e. Lr) quite accurately. However, it leads to excessive reattachment lengths for cases of large drafts.
As the draft increases, I wonder what turbulent mechanisms/characteristics become influential that SST fails to capture properly. Could it be the increasing curvature of the streamlines or probably turbulence anisotropy? I appreciate any insights.
Regards,
Armin
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?
Question
Hello everyone
I hope you are doing great, I am working on offshore VAWT, and trying to simulate a vawt in ansys fluent. I divided my meshing into two parts overset and background mesh, and want to use IDDES model for my simulation. But when I try to simulate my model the fluent give me an error that this model is not compatible with overset mesh. So, could anyone please tell me that I can’t use overset mesh technique with IDDES? or I can use this technique by changing any solver setting?
Thank you so much for your time!!
The IDDES (Improved Delayed Detached Eddy Simulation) turbulence model is a hybrid RANS-LES (Reynolds-Averaged Navier-Stokes - Large Eddy Simulation) approach that combines the benefits of both modeling techniques. While I cannot provide specific details about ANSYS Fluent or its compatibility with overset mesh technique, I can offer some general insights.
In most cases, the compatibility of turbulence models with overset mesh techniques depends on the specific implementation within the simulation software. Overset mesh, also known as chimera or overlapping grid, allows for simulations involving moving or deforming objects with relative motion. However, not all turbulence models may be compatible with this technique due to the complexities associated with the overlapping grids and grid interfaces.
To determine if the IDDES turbulence model can be used with overset mesh in ANSYS Fluent, you may need to consult the software documentation or reach out to ANSYS support for clarification. They can provide you with the most accurate and up-to-date information regarding the compatibility of specific turbulence models with the overset mesh technique in ANSYS Fluent.
Additionally, it may be worth considering alternative turbulence models that are known to be compatible with overset mesh simulations. Examples include the SST k-ω (Shear Stress Transport) model or the DES (Detached Eddy Simulation) model. These turbulence models have been widely used in conjunction with overset mesh techniques in various computational fluid dynamics (CFD) simulations.
Remember, it is essential to review the software documentation, consult experts in the field, or reach out to the software support team to obtain accurate and reliable information regarding the compatibility of turbulence models and meshing techniques in your specific simulation setup.
Question
Many researcher are using python all over the world. Few young researcher are interested to learn more to develop computation skills in application to planetary science, space science research and Helio-physics. If any experts who want to teach such interested researcher, I will make a common platforms in which we can learn together virtually.
I am interested to learn if it is possible "Ram Chandra Pageni" since I am doing research in Space Science.
Question
How use y plus value with different types of turbulence models in cfd ?
In computational fluid dynamics (CFD), the y+ value is used to determine the appropriate treatment of the near-wall region in turbulence modeling. It represents the non-dimensional distance of the first grid cell from the wall, normalized by the viscous length scale.
Different turbulence models have different requirements and guidelines for the appropriate range of y+ values. Here's a general overview of how y+ values are used with different types of turbulence models:
1. Spalart-Allmaras (SA) Model: The SA model is widely used for boundary layer flows. For this model, a y+ value between 30 and 300 is recommended. It is typically advised to keep the y+ value below 200 to ensure accurate predictions. Near the wall (y+ < 1), wall functions are used, while in the outer region (y+ > 30), the turbulence model equations are directly solved.
2. k-epsilon Models: The k-epsilon models, such as the standard k-epsilon and the RNG k-epsilon, are widely used for a range of applications. For these models, a y+ value between 30 and 200 is typically recommended. In the near-wall region (y+ < 30), wall functions are used, while in the outer region (y+ > 200), the turbulence model equations are directly solved.
3. k-omega Models: The k-omega models, including the standard k-omega and the SST k-omega, are commonly used for complex flows. These models require a y+ value between 1 and 5 in the near-wall region. This ensures accurate predictions without using wall functions. In the outer region, the turbulence model equations are directly solved.
4. Reynolds Stress Models (RSM): RSMs provide more detailed predictions of turbulence by considering the Reynolds stresses. They are suitable for complex flows. In RSMs, it is recommended to use a low y+ value (< 1) in the near-wall region to resolve the flow accurately. Wall functions are not used, and the turbulence model equations are directly solved in the outer region.
It's important to note that these are general guidelines, and the appropriate y+ range can vary based on the specific turbulence model and the flow conditions. Additionally, advancements in meshing techniques and solver capabilities have reduced the sensitivity to y+ values in some cases. It's always recommended to consult the documentation or literature specific to the turbulence model and software you are using for more accurate guidance on y+ values.
Question
Hello! We pump the contour of a metal tube with a diameter of 3 mm with oil with a viscosity of 0.0031 kg / m * s (abt. the viscosity is 4 times higher than that of water). We are interested in the pressure for pumping this circuit (at the inlet).
When validating the model in Ansys CFX with k-epsilon default settings, the difference between the result and the experiment reaches 100%, although the mesh is adjusted according to the tested and validated model, only on the water (with a maximum deviation of 10%). The Reynolds number in a pipe with a diameter of 3 mm is Re=2500, that is, we are dealing with a transitional flow regime, and the pipe is not hydraulically smooth in terms of the critical number Re=20d/"roughness".
Tell me, please, maybe for this mode it is necessary to use another turbulent model, for example, "Reynolds stress" or "k-epsilon" set up somehow differently? Maybe there are works in which a similar problem is solved, or there are works with recommendations on the use of turbulence models?
Ilya Lichadeev What I mean that it depends how you verify your numerical model (point-wise, using integral values, etc.), and which experimental data is used. For some setups it is not possible to get a perfect quantitative match, but it is enough if your model follows the trends observed in the experiment.
Question
Hello all I am simulating flow over a sphere at Re = 881 where Drag coefficient must be almost Cd=0.5. I simulate it using both using laminar and Turbulent models(KOmegaSST). for laminar simulation, I get Cd ~0.5 but fort the Turbulent simulation, first of all it takes very long tome to converge and even after convergence, it gives Cd~0.65(Actually it has not fully converged yet, see attached figure). Now I have two questions; 1- what happens if I use turbulent model to simulate laminar flow? 2- when should I expect convergence based on the attached figure?
As you can see, the RANS solution produces an averaged field over the sphere also for laminar vortex shedding.
However, if you have a fully laminar flow, simply solve for it. If You think to have transitional conditions, Depending on you computational power, use LES or DNS.
Question
I need some urgent help . I have been simulating the steam cracking furnace from certain papers namely
1.) CFD simulations of steam cracking furnaces using detailed combustion mechanisms
2.) Impact of radiation models in CFD simulations of steam cracking furnaces.
In both these papers furnace geometry, reaction, composition of the fuel and the models applied are similar. But in both these papers , the maximum flame temperature was achieved at the height of 1.5m . But when I try to simulate these papers using the same flow , combustion , turbulence model in the ANSYS fluent , same as that of the papers my maximum flame temperature reaches at 2.1 m and has a longer flame when compared to those above paper. Can someone please help me why this is happening even though I have applied the same case as that of papers
I am attaching the papers for the reference
Secondly grid size was not mentioned but number of mesh was given but since adaptive grid refinement was used it automatically changes mesh size to minimize error
Question
Hello,
I am running a 3D density based transient problem with Spalart-Allmaras turbulence model. After few iterations I am getting Negative Nut in xxx cells. I tried to search the cause, but not successful in finding answers. Could anybody help me in understanding from where this problem arises?
Hi Naveen,
I have the same question; has your issue been resolved ?
Alin
Question
In openFOAM, there are multiple options for choose in LES, such as WALE model, Smagorinsky model and so on. Which one is suitable for dynamic mesh and why ?
I am a bigginner of LES simulation, as far as I know , the only difference between them is WALE model includes the rotation rate in the calculation of νsgs, and Smagorinsky not. What else should I take into account in simulation?
In general, for dynamic mesh simulations, models that involve additional terms in the turbulence kinetic energy equation, such as WALE (Wall-adapting Local Eddy-Viscosity) or dynamic Smagorinsky, may be more suitable. These models account for the effects of mesh deformation on the turbulence field and can improve the accuracy of the simulations.
WALE model includes additional terms in the eddy-viscosity calculation that account for the effects of rotation rate, strain rate, and dissipation rate. This model can be more accurate for complex flows with high strain rates and rotation rates, such as flows with vortices or swirling motions.
On the other hand, the Smagorinsky model is a popular choice for LES simulations in OpenFOAM and is widely used for a range of flows. This model uses a constant eddy viscosity coefficient that is proportional to the grid size, which means that it can be computationally efficient for large-scale simulations. However, it may not be as accurate for complex flows or when mesh deformation effects are significant.
When choosing an LES model for dynamic mesh simulations, it is important to consider the specific characteristics of the flow and the desired level of accuracy. It is also recommended to perform sensitivity studies to investigate the effect of different LES models on the results and to validate the simulations against experimental data or other established methods.
Question
In compressible flows, would improvements based on the SST K-Epsilon turbulence model , such as the use of AI，be a more promising direction in engineering. ten or thirty years from now, will the most commonly used model be the Reynolds stress model or its deformation, or will DES be the most commonly used simulation method.
Possibly you should have a look at
Baumert, H. Z., Universal equations and constants of turbulent motion,
Physica Scripta, T155, 014,001 (12pp), doi:10.1088/0031-8949/2013/T155/014001, 2013.
Also have a look at the attachments.
Feel free to ask specific questions!
Best,
Helmut
Question
Hello,
I am trying to simulate phase change during turbulent flow in comsol multiphysics, for that I require to add an additional sink term at the end of turbulent kinetic energy and energy dissipation rate equation which is 'k' and 'epsilon' equation in turbulent model to include the effect of solidification in the turbulent flow. Can somebody please suggest how to do that.
any suggestion will be greatly appreciated.
-Akshay
Forget this k-eps model. While k is a so-called conserved variable, eps ist not. Eps is a colored mix of variables. I recommend for turbulence the k-Omega-model. It allows in my interpretation (2009) e.g. the derivation of the von-Karman constant as 1/sqrt(2*pi)~0.3989... as well as also other constants of this model. Cheers, Helmut
Question
I tried writing a udf for the turbulence model needed to validate a study. The wall boundary conditions (epsilon and f) specified for the v2-f model are available in the code I wrote. While I get consistent results for velocity and cp values in my validation, I do not get the correct result for v2 and f values. Is there a problem in the code or am I not transferring the boundary conditions to fluent correctly? I would be glad if you help me in this regard.
Here is a sample UDF code for the k-e-v2-f turbulence model in ANSYS Fluent:
This code defines a UDF for the k-e-v2-f turbulence model in ANSYS Fluent. The UDF computes the turbulent kinetic energy (k), the turbulent dissipation rate (eps), and the second moment of the velocity fluctuations (v2). It also computes the turbulent viscosity (mu_t) and the constants C1, C2, and C3, which are used to compute the wall function values of epsilon and f. The UDF then sets the values of k, epsilon, and f in the turbulent and laminar regions of the flow, and updates these values in the solver.
#include "udf.h"
/* User-defined function to define the k-e-v2-f turbulence model */
DEFINE_K_EPSILON(k_e_v2_f, c, t, dS, eqn) { real k, eps, f; real v2, S; real mu; real rho; real Cmu, C1, C2, C3; real sigma_k, sigma_e, sigma_f; real f_wall, eps_wall;
/* Compute the turbulent kinetic energy (k) and the turbulent dissipation rate (eps) */
k = C_K(c, t); eps = C_EPSILON(c, t);
/* Compute the second moment of the velocity fluctuations (v2) and the turbulent strain rate (S) */
v2 = 2.0k/3.0; S = sqrt(2.0k)/C_MU(c, t);
/* Compute the turbulent viscosity (mu_t) */
mu = C_MU(c, t); rho = C_R(c, t); Cmu = 0.09; mu_t = rhoCmukeps/(muS);
/* Compute the constants C1, C2, and C3 */
C1 = 1.44; C2 = 1.92; C3 = 1.0;
/* Compute the wall function values of epsilon and f */
eps_wall = C2sqrt(k)pow(C_DIST(c, t), 2.0/3.0); f_wall = C3(1.0 - exp(-C1pow(C_DIST(c, t), 1.5)));
/* Compute the constants sigma_k, sigma_e, and sigma_f */
sigma_k = 1.0; sigma_e = 1.3; sigma_f = 1.0;
/* Set the values of epsilon and f at the wall */
if (C_DIST(c, t) < 1.0e-10) { eps = eps_wall; f = f_wall; }
/* Set the values of k and epsilon in the turbulent region */
else { k = sigma_kmu_tfv2/(epsC_MU(c, t)); eps = sigma_emu_tfv2/(kC_MU(c, t)); }
/* Set the value of f in the turbulent region */
f = sigma_fmu_tfv2/(kC_MU(c, t));
/* Set the values of k and epsilon in the laminar region */
if (S < 1.0e-10) { k = 0.0; eps = 0.0; }
/* Update the values of k and epsilon in the solver */
dS[eqn] = 1.0; C_K(c, t) = k; C_EPSILON(c, t) = eps; }
Question
Hi
A 3D numerical simulation project involves a turbulent flow of water in a tube with a constant heat flux entering the surface.
This numerical simulation needs to be validated by a similar article that has simulated the K-Omega SST Turbulence model.
Please guide me if there is an article I can refer to.
Thanks
Hi,
In my reasearch project, I have completed recently; a research for a typical turbocharger compressor optimisation. It has been studied air turbulent flow simulated the K-Omega SST turbulence model. My numerical simulations has been validated and I wrote some papers. Please have a look into my papers, hopefully it can help.
Regards,
Question
Hello everyone
I have attached the integral length scale definition where upper limit goes up to infinity, but in some references practically they integrate from 1 to where autocorrelation function becomes zero (and naturally negative part is not included), references like;
1- Davidson, L., 2018. Fluid mechanics, turbulent flow and turbulence modeling. Chalmers University of Technology, Goteborg, Sweden (Nov 2011).
2- Tritton, D.J., 2012. Physical fluid dynamics. Springer Science & Business Media.
which one is correct,
1 - Integrating up to where R11 goes to zero
OR
2 - Should I include negative part too?
Thanks,
It is possible to calculate Taylor microscale and macroscale. Taylor microscale can be calculated using the fitting of the parabola at the origin. One may find details within this paper:
Chuychai, P., Weygand, J. M., Matthaeus, W. H., Dasso, S., Smith, C. W., & Kivelson, M. G. (2014). Technique for measuring and correcting the Taylor microscale. Journal of Geophysical Research: Space Physics, 119(6), 4256-4265. Available on:
Question
Hello, I want to publish a research paper on the assessment of CFD turbulence models, I need an airfoil geometry to start with. where can I find airfoil geometry or profile? and its experimental data, cd and cl values or cl alpha values? I have searched for experimental data, but unfortunately, I did not find any.
what type of airfoil should I choose for the assessment of different turbulence models?
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.
Question
Hello everyone,
In my research project i have air flow velocity and temperature distribution simulation from supply and return air duct in non furnishing control zone, which turbulence model is better ?
k- epsilon model poorly resolves the viscous layer unline K-Omega model. Furthermore, k-ω model is good in resolving internal flows, separated flows and jets and flows with high-pressure gradient and also internal flows through curved geometries.
Question
Hello all,
I've got a 2D simulation case in which the flow separates from the sharp leading edge of rectangular bluff body and reattaches to the wall some distance downstream. The main goal is a accurate prediction of pressure distribution along the body's face parallel to the flow.
I'm doing a transient simulation using SST model in conjunction with gamma-Re transition model. The time- cord-averaged y+ is less than 2~3 and the inflation layer around the face of interest contains 10 prism layers. The Re number based on the body's width (perpendicular to the flow) is 1.7e+4.
The problem is that my model overpredicts the reattachment length, which in turn leads to delayed pressure recovery.
I have a suspicion that longitudinal decay of turbulence values specified at the inlet might be to blame. Consulting the Ansys CFX-solver Modeling Guide, I learnt that one solution is to prescribe appropriate turbulence values at the inlet based on the desired values at the body. An alternative approach also suggests some additional source terms for k and w transport equations in order to preserve the inlet values up to some distance upstream the body, from where decay is allowed.
Here are my questions:
1- Is my suspicion valid in the case of my problem?
2- Is the decay of turbulence of physical basis or a numerical artifact?
3- which of the two methods works better? Are there any attempts in the literature?
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.
Question
Hi all,
I am going to use either Ansys CFX or Ansys Fluent solver to simulate flow past a semi-submerged rectangular cylinder, as shown in Fig. 1. The main goal here is to achieve an accurate prediction of pressure distribution over the cylinder, which seems to be particularly reliant on the capability of the CFD model to predict the separation and the reattachment of the flow correctly. I would appreciate any tips regarding the following questions.
Q1. How important is the choice of the turbulence model? Which models are superior? Could the flow be modelled as being laminar at all?
Q2. How important is the approach to the free surface? Could the free surface be modelled as a free-slip wall to reduce computational costs? Is it necessary to precisely track the free surface using the VOF model for this study?
Q3. What are the most appropriate boundary conditions for the simulation?
Q4. Which is more suitable for this problem? CFX or Fluent?
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?
Question
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
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.
Hopefully that explains things a bit.
Question
internal flow over cylindrical, rectangular and ellipse shapes.
outside is rectangular shape.
I would not use k-epsilon for this kind of problem. It is most valid for free stream turbulence like atmospheric flows. Near the walls k-epsilon behaves quite poor according to my experience and I believe that is where your heat transfer happens.
But you can try it yourself of course.
Question
Hi all, I have been encountering this problem shown in the pic with simulations with parameters:
• RNG k-epsilon Transient
• Pressure-based Coupled scheme
• massflow inlet and pressure outlet BC with Intensity 5% and Hydraulic Diameter 0.012m for Turbulence Specification Method
• URF and COurant number 0.2 while Turbulent Viscosity 1Three boxes for Frozen Flux Formulation, Warped-face Gradient Correction, Higher order term relaxation
• Timestep size 0.002 and 25 iteration/timestep
The problem occurs when the three boxes are checked, the number of cells overlimit starts very low and gradually builds up to such level after around 20 timesteps. When the three boxes are unchecked, overlimit cells range between 1 and 9 and disappear soon after.
Online posts suggested me to drop timestep size, max iteration per timestep, and URF but they don't seem to work for me. Especially when URF is quite low already. Assuming I wish to keep those 3 boxes checked, does anyone has any ideas?
I have been bugged for long. The model is also attached. Thanks a lot!
In such cases, modifying the BC or mesh configuration may be beneficial.
Question
Can we plot u-plus (U/Utao) Vs y-plus (Y/Ytao) plot for a turbulence model in ANSYS-Fluent post processing? Basically, I want to be ensure that the model will take into account the gradients in the viscous sublayer.
Hi Somenath, It should be possible with ANSYS as I have made it possible in STARCCM+ for my flat plate simulations. I will let you know the procedure in STARCCM+ and probably the same steps can be followed in ANSYS too.
1. Make a line probe where vertical measurements can be taken at the location or position where you want to have U+ vs Y+ plot.
2. Go for an XY plot where Y-axis is U+ and X-axis is a log scale of Y+.
3. Now the input part to this plot should be the earlier line probe.
4. Most of the software has Y+ defined so you just need to recall the expression for Y+ from the software database and assign it on the x-axis.
5. U+ may not be defined in most software. Hence create a custom field scalar function defining U+. (You will get a standard expression for U+ from any Fluid Mechanics Text Book, I follow Frank M White and it has expressions for U+ too).
6. Finally assign this newly defined U+ to the Y-axis.
Hope you too will get the same on ANSYS.
Question
I'm using Ansys Fluent to model a rotating cage setup to monitor Flow Accelerated Corrosion.
In general, for internal flows with curvature and rotations, k-Omega SST model performs the best. While k-epsilon models are used widely, they are not able to capture the near-wall effects (performs weakly under adverse pressure gradients and along curvatures). While k-Omega-SST model uses a blending function and allows us to go to y+ ~ 1, without having the adverse problems like in k-epsilon. In essence, it functions like the k-Omega model near the walls, and like a k-epsilon model in the far-fields.
Question
Is kw sst suitable for analyzing the flow over an airfoil after the airfoil has been stalled? specifically if we want to model the effective body of a stalled airfoil, can I do it with Kw sst or is there any better turbulent model to choose.(computational power is also limited )
Hello Mr Jayathilake,
the kOmegaSSTt is suitable for this problem in my opinion, especially when computational limits are low. With adequate refinement in the critical regions you might achieve good results. I don't know which code you are using for your simulations but, if available, you can also try the kOmegaSSTSAS approach. This "Scale Adaptive Simulation" approach is able to resolve turbulent length scales more precisely. It falls back to kOmegaSST if the mesh is coarse. But if you apply some further refinement you should get a more detailed results. Timesteps should be limited to CFL<=1. Compared to LES or DES mesh and solution order requirements are still lower I guess.
Computational effort for this model is not really higher than for the kOmegaSST.
You may have a look e.g. at these publications of the model developer Mr. F. Menter:
and many more.
I hope this helps.
Best Regards
David
Question
Hi all,
I am modeling a high-velocity (200 m/s) flow of water vertically entering the air. The water is supposed to hit the wall after traveling a 0.5 mm distance. The flow is highly turbulent, and I am using the k-omega phase-field approach for modeling.
The problem is that once the jet approaches the wall, the problem stops converging (Error: maximum number of segregated iterations reached). Any idea what is the root of the problem and how to solve it?
Majid
it may be caused by grid.
Question
First I simulated scramjet problem using k-e model in fluent. The results are validated using literature after that necessary modification was done in boundary conditions using same mesh. The results shows expected trends. After getting this results I refined the mesh and changed the turbulence model to K-w. The results are showing similar trends as k-e models but equivalence ratio of critical phenomenas are higher than k-e model. I want to know what are the possible reasons of high differnce of equivalence ratios? How I can verify which models are giving correct results?
First - changing two things in a simulation at the same time is obvious not a good approach (mesh AND turbulence model).
Further I need to say, that first you would need to find out, where your simulation comes to a mesh-independente solution (this question can be anbswered with no regard to which turbulence model you are using, if properly implemented in the CFD solver), and only on a mesh level, where the CFD solution is independent from the mesh resolution you can start to make judgements about model errors or about which turbulence model is more suitable for your particular application.
In contrary, if your solution is not yet mesh independent, i.e. the mesh is still too coarse, you are looking on a not known mix of discretization errors and model errors. This can mislead you to a misjudgement about a particular physical model and on the next following refinded mesh the situation (with the two models in question) might be reversed in comparison to what you have observed on the coarser mesh.
This CFD methodology has been outlined almost 22 years ago in the so-called "CFD best practice Guidelines for Industrial CFD".
The methodology says, that the following hierachy of CFD error quantification and elimination should be observed:
1) round-off error
2) iteration error (cut-off error with respect of stopping a solution process at a certain iteration number or by matchig a certain quantitative convergence criterion)
3) spatial and time discretization error
...and only afterwards you can investigate model errors of physical models.
Any attempt to investigate more than just one error source at the same time or to change the order of investigation of the errors from this error hierarchy potential will lead to erroneous CFD result.
Best regards,
Dr. Th. Frank.
Question
Hi everyone,
I am trying to simulate turbulent flow over a sphere with the following conditions:
Freestream velocty= 100m/s
ambient density and viscosity
Sphere diameter = 0.82 m
Reynold's number = 5.6 x 10^6
experimental Cd = 0.195
Sphere enclosure = 42 x 28 x 28 m3
mesh Size 4.5 milllion
I tried using S-A model and K-omega SST model in fluent, using which I am getting Cd=0.14. which is almost 26% error.
Any suggestion as to how may I improve the result?
WMLES or SBES model with a well thought (and validated) mesh resolution.
Regards,
Dr. Th. Frank.
PS : Your question did not involve any limitations on the computational time of the proposed turbulence modeling approach. Therefore a scale-resolving simulation would clearly deliver the best possible results (I'm not proposing a diret numerical simulation, since you asked for turbulence modeling approaches; DNS is resolving turbulence, but not modeling it).
Question
I would like to start a discussion of this specific topic.
Here I would like to discuss the list of possible techniques helpful for performing the simulation of oscillating bodies in quiescent fluid.
This discussion is open to all the students, teachers, and researchers.
I request you to reply here if you are familiar with code development in OpenFoam, IBPM, NEEK1000, lilypad and CFX
Pradyumn Chiwhane I am posting here a previous answer I provided on submerged oscillating bodies in quiescent fluid. The total force exerted by the fluid on the cylinder, you should consider besides the drag the added mass force. Academic references are provided within the following research projects:
Question
In my study, I have two cases for CFD which I performed on Ansys Fluent,
1) oscillating flow over a solid cylinder (blunt face perpendicular to flow direction)
and
2) solid cylinder (blunt face perpendicular to flow direction) oscillating in the stationary fluid. It involves dynamic meshing.
In both of the above cases, I am using the SST k-w turbulence model for this simulation.
[ ie what should be the input value for Turbulent Kinetic Energy and Specific dissipation rate? ]
What does it means if both these values are equal to 1.0?
I have attached a graph representing drag force as a function of time [calculated from the case (2)]. I want to know what might be the possible reason for this behavior in the initial time.
My UFD (equation for velocity of moving cylinder): V = 1.0 * cos(2*3.1415*1.55*t) m/s.
The initial spike? why does this occur?
The decreasing amplitude?
What would be the expected output if this was calculated for longer time values (amplitude behavior after 5 secs)?
Fluid: incompressible (water)
calculation settings:
time-steps = 500, time-step size = 0.01 sec, maximum iterations = 500.
Meshing (element size = 1.0 mm, element shape = triangles)
Pradyumn Chiwhane I am not used to performing calculations with CFX. Researchers in my group do.
If this is the total force exerted by the fluid on the cylinder, you should in my opinion deduce the added mass force to obtain the net Drag. You may consult academic references within the following research projects:
Question
In case of turbulent flow over an irregular shape, how can we find the first layer thickness near the walls of the irregular boundary.?
For flow over a flat plate, the skin friction co-efficient can be found by the direct formula, for irregular shapes how can we find the skin friction coefficient.
Academic resources on fluid Mechanics are provided on the project references:
SINGLE PHASE AND MULTIPHASE TURBULENT FLOWS (SMTF) IN NATURE AND ENGINEERING APPLICATIONS | Jamel Chahed | 3 publications | Research Project (researchgate.net)
Question
Some commonly used turbulence models include k-epsilon, k-omega and shear stress transport (SST) models. The k-epsilon model is a best candidate for flow away from the wall (e.g. free surface flow region), while the k-omega model is best suited for near the wall flow region (e.g. adverse pressure gradient flow region). The choice of most appropriate turbulence model for any turbulent flow problem may be extremely difficult for young computational fluid dynamists, hence the need for this germane question.
The SST k-ω model is a hybrid of the best of k-ω and k-ε. This model uses the k-ω model near walls, and transitions to k-ε model in the open flow field. It’s popular in aerospace and turbo-machinery applications.
So SST is the most effective model for young computational fluid dynamists.
Question
This paper is claiming there is a problem in k-epsilon model. Because in k-epsilon model, we assume that eddy viscosity(νT) is isotropy. But actually in real world, eddy viscosity(νT) is anisotropy for high Reynolds number and is isotropy for low Reynolds number. It means there can be error when we consider flow with high Reynolds number.
And I have a question in uploaded figure. How he can calculate fluctuation velocity?
What I know is fluctuation velocity can be calculated only when we assume that eddy viscosity(νT) is isotropy(like k-ε model).
When we use k-ε model, we can find k(Turbulence Kinetic Energy) by T.K.E transport eqation and k is same with '½(u'2+v'2+w'2)'.
Then if k(T.K.E) is found, we can calculate u', v' and w'
because u', v' and w' are same each other by assumption of isotropy.
But in this paper, author claims that we should consider flow as anisotropy and he suggests new Eddy viscosity(νT) with new Cμ. (I've uploaded expression of Cμ by picture.) So I think it is impossible to calculate fluctuation velocity because flow is considered as anisotropy.
But there is a fluctuation velocity profile that is calculated by CFD. And I think author calculated fluctation velocity using root mean square and T.K.E
Because we can find desription in figure that means he calculated fluctuation velocity by root mean square. (Figure 8: Profiles of rms velocities perpendicular (v) and parallel (u) to the wall in the impinging jet)
So it looks like contradiction to me.
How fluctuation velocity is calculated in anisotropic flow?
Actually I've thought that assumption of isotropy can be possible in the impinging jet sometimes. Because impingement occurs nearby wall, so there is a low Reynolds number nearby wall by No slip condition(dominant molecule viscosity). But, eventhough my deduction is right, I can't understand why there is a difference between v' and u'. In the case that r/D=2.5, there is a difference between v' and u' that I've marked in uploaded picture. Difference between v' and u' means this flow is anisotropy and this is contradiction against author's claim also.
Also I've infered about the one more reason why calculation of fluctuation can be possible in this paper.
I don't know well but what I've heard is
In RSM(Reynolds Stress Model) is suggested by the same claim of this paper.
I've heard RSM is suggested because flow with high Reynolds number is anisotropic in real world
and this is different with original k-ε model.
So in RSM, it is possible to calculate the each fluctuation velocities(u', v' and w') in anisotropic flow.
So RSM and this author's model has same purpose that pursue to consider anisotropic flow.
And RSM can calculate the the each fluctuation velocities(u', v' and w').
So I think this author's model can also calculate fluctuation velocity in the same reason.
Eventhough I don't know how RSM calculate each fluctuation velocities(u', v' and w'), I've tried to infer.
Summary
Anisotropic flow by using new eddy viscosity that has direction - Calculated fluctuation velocity
: I think there is a contradiction.
Left side: Anisotropic
Right Side: Should be isotropic
→ ???
Calculated fluctuation velocity - Difference between u' and v'
: I think there is a contradiction.
Left side: Isotropic
Right Side: Anisotropic(Different fluctuation velcity.)
→ ???
I'm not good at Turbulent.
I'm just Senior.
I don't have bachelor's degree yet.
But I'm interested in Turbulence.
But I'm confused now :(
Thanks :)
Flows with greater anisotropy in the turbulence stress tensor render standard one equation and two equation RANS models not great as they make assumptions about the turbulent shear stresses which turn out to be untrue for specific cases. This anisotropy is commonly present in flows with large separations etc. Reynolds stress models remedy this by solving transport equations for each component of the stress tensor but these too have limitations and haven't proven to be of much advantage. If high accuracy is desired, LES or DNS are the only way. But for most engineering flows of interest, one equation and two-equation eddy viscosity models provide sufficient engineering accuracy with results in a reasonable time.
Question
Hello everyone.
I'm working on a flow simulation in an axial pump. in the literature, they recommend that y + should be kept below 1. however, my results have shown that y + is greater than 10.
how can I keep y + below 1?
Note 1: I am using TurboGrid for the mesh and I am using SST as the turbulence model.
Note 2: I am using TurboGrid as a mesh generation tool.
Mohamed Bounouib please follow the links attached herewith and follow the steps for the flow simulation in an axial pump. That will be easier for you
1. https://youtu.be/aY2HRp6zq2g (CFturbo turbomachinery design software.)
2. https://youtu.be/H07b2pXpCCM (ANSYS)
Regards
Question
I am designing a turbine and predicting the Performance of the Vertical Axis Turbine. I have the experimental results of the same turbine with me. The problem is after the optimum TSR, the performance of don't go down its continuously increasing and goes above the Bitz limit. May be its not concluding stall due to high TSR. How do i resolve it?
Geometry -- Mesh -- CFx
Doubt i have but need help to resolve,
* I unable to capture the correct boundary layer, if it so then how could i capture it?
*Maybe i am using Turbulence model which is not correct, But i have chosen shear stress transport which is be better choice, mabe?
* Also share if you any any other.
I will be very thankful, and please make it urgent.
Ahmed Gharib Yosry Thanks for your assistance. i tried to reverse the boundary conditions, replace the inlet and outlet with each other. but the torque doesn't goes down the optimum value.
Question
Hi.
I'm trying to simulate a fluid through a vertical channel, width = 0.2 cm - height = 60 cm - fluid == water - inlet v = 1, using the 2D model in COMSOL. Due to the characteristics mentioned a turbulent model is needed. When choosing Algebraic YPlus method the no-slip condition in the vertical walls states (u,v) = (0,0), which is what I thought, but when using the k-epsilon or k-omega model for de NO-SLIP condition (u,v).(nx,ny) = 0 [u = 0 and v is free] which i don´t understand why. Can anyone explain why in k-e or k-omega don'y use velocity=0 on the walls?
In the COMSOL documentation the no-slip BC is indicated to be (u,v)=(0,0).
What you described is slip condition. You can consult the COMSOL team for more clarification. You can also run a test simulation to see how the code performs.
Question
Dear CFD Fellows,
Is there a decision table, diagram or software you use for turbulence model selection? What I'm looking for is not a table with information for each model, but rather a visual with decision steps such as "if the flow contains a-b-c, these models can be used". In the relevant case, after following the characteristic factors of your flow, I expect turbulence models to be proposed in the last section that can best define the flow.
Best regards,
Güven
Dear Fellows,
We are all choosing a Turbulence Model for our flows. We are already creating decision chart in our heads. So, why not put these decision steps together and create a real chart? Please do not think so complicated. At the end of the chart there won't be only one Turbulence Model for relevant case, there will be list (at end of different decision steps same model could be listed too). After that you can do some literature search to select the best model in the list.
Best regards,
Güven
Question
Dear CFD Fellows,
Is there a decision table, diagram or software you use for turbulence model selection? What I'm looking for is not a table with information for each model, but rather a visual with decision steps such as "if the flow contains a-b-c, these models can be used". In the relevant case, after following the characteristic factors of your flow, I expect turbulence models to be proposed in the last section that can best define the flow.
Best regards,
Güven
Question
Hi all,
I'm planning to simulate flow past a floating body using CFD method with the main purpose of investigating its stability against hydrodynamic forces. A sketch of the problem is presented the the figure attached.
It seems that an accurate estimation of pressure field, and therefore hydrodynamic forces, is heavily dependent on correct prediction of flow topology, particularly separation and reattachment of the flow.
I'm wondering what turbulent models would best handle this problem. I would appreciate it if you provide details and specific reasoning.
Regards,
Armin
The Reynolds Stress Model is the most complete turbulence model with regards to representing turbulent flow.
Question
I have watched videos about wall function on youtube but still confused about understanding viscous sublayer, logarithmic region, and wall functions. I couldn't find relevant material. Where do I find these topics? Can someone suggest some material regarding viscous sub-layer/logarithmic region and to understand y+ (wall functions)?
I would add that the meaning of y+ is very very simple. It is nothing but that the local Reynolds number measured along a normal-to-wall direction. That is, y+=0 is the wall position and then the value increases according to y+=(u_tau/vi)*y.
Question
Turbulence model for blood flow simulation
The Reynolds number is not high, and accuracy can mean saving of lives. Do not use a turbulence model, perform a direct simulation (DNS) resolving all temporal and spatial scales with an appropriate function for the viscosity (shear thinning character). This can be done with any numerical code solving the Navier-Stokes equations, and the numerical costs will be affordable.
Question
Hi everyone
I am working on a shell and helically coiled tube heat exchanger with laminar flow through the shell and turbulent flow through the coil tube. I have performed iterations for coil by selecting different types of turbulence models but in each case, energy starts to diverge after some iterations (images attached).
What is the possible reason for this?
I think it may be due to BCs. Can you please your problem in detail, so that I may help you in diverg. Issue
Question
Can you suggest where can I find video lectures on turbulent flows or turbulence modelling? Are there any video lectures on understanding turbulence modeling? Or any books to understand turbulence modelling? Can someone please help!
i have a project on how different turbulent models can be used on Naca aerofoil. and i am not knowing how to find material to understand turbulent flows.
Dear Vinay,
I would recommend you to follow the video lecturer on Computational Fluid Dynamics by Prof. Suman Chakraborty according to me it is one of the best.
You can also follow Prof. Fillipo Maria Denaro's technical report/ppt on CFD, available in the ResearchGate.. He is one the famous researcher in computational physics and CFD.
Question
in turbulent flow calculation is it acceptable to use an y+ of 15 with a wall function in k-e turbulence model?
Dear Prof. Th. Frank
Thank You very much for your explanation and suggestions on my error.
Question
I am performing CFD simulation upon NACA 4418 airfoil at 45 m/s free steam velocity and Reynolds number of 3 million. I am following a particular paper where the experimental results of NACA airfoils are shown. The turbulence model I am using is k-omega SST turbulence model and the transition is ticked on. The mesh is also properly refined and since my computational power is limited, therefore I am choosing my y+ value to be greater than 30. But the following problems are observant from my simulation
01. Lift coefficient seems to be around the mark but the error percentage is still a bit above 10%
02. Drag coefficient is the real issue as it is exceeds the supposed value and produces.much higher value. Almost double the value that I want
What is the issue here and what should I do? I have been going through the theory behind drag and lift coefficient and the Computational process used behind but it's not helping me so far and I really need help regarding this right now.
Thank you very much
Hi Nehal,
If you have refined the mesh and used the appropriate turbulence model, then I suggest you to please check the convergence of the result.
Question
hi everyone I need a little help here; Im new to fluent and recently I am trying to simulate a two phase flow in a pipe-tank system where I want to see and analyse the intake vortex with the air core. i am using vof+cls and les as turbulance model, I don't have problem with convergence but after simulating 100s of the flow, I can't see the formation of vortex and air core of in getting sucked in to the pipe. can any body help? has anyone experienced this problem too? I really need help at this point📷
Hi,
this is not an easy to setup simulation, also it may look like this on the first glance.
Why?
1) Vortices leading to air entrainment are rather small flow structures of thin diameter and having substantial velocity gradients over the diameter of the vortex. This velocity gradients need to be fairly good resolved in order to resolve the low pressure occuring in the center of the vortex leading to the air entrainement. For that a very fine mesh resolution is required.
2) Usually such vortices - if they are not artificially and by geometric design measures stabilized - are not stationary in space. So the vortex is moving around / precessing and that makes it even harder to establish a well resolved numerical mesh with fine mesh resolution at the vortex center. It might be suitable to use adaptive mesh refinement, but without a coarsening algorithm there is the danger, that the geometrical space more and more fills with refined mesh cells, making the simulation rather very expensive.
3) Standard 2-equation turbulence models are by default not capable of capturing the strong velocity gradients in flows with strong streamline curvature / rotation. A SST k-omega-based turbulence model with curvature correction is at least recommended.
4) It has to be made sure, that the turbulence conditions at the free surface between liquid and air are capturing the physical valid conditions. Usually a fine mesh resolution at the free surface is required for that in addition with turbulence model damping terms at the free surface, i.e. the turbulent kinetic energy at the free surface needs to be dampended to zero or small values.
Regards,
Dr. Th. Frank.
Question
Hello guys,
I am running a simulation of a mixing tank in unsteady-state( transient) with turbulence model SST K-omega and multiphase model VOF. And I gave a number of time steps as 10000 and time step size as 0.0003 and max iteration as 50 and the solution is converging at each time step. So my question is how do I know my simulation is completed ? or I need to wait till 10000-time steps to complete?
Regards
Johnny
I try to address better the question. If the flow problem admits a statistically steady state you start from an initial condition and run until you flow variables are statistically steady, that is you see them oscillating in time around a constant value. From that moment the solution is physically correlated.
Conversely, if you have a problem that has an unsteady behavior also in the mean values, you need to simulate at least the largest time period of your problem.
This approach is typical for DNS/LES formulations, sice you are using a URANS approach the real physical meaning of your unsteady simulation can be debated. If your flow problem is statistically steady, your URANS shouldn't be somehow different from a RANS solution.
The key is that you cannot fix arbitrarily a number of time steps without knowing the nature of your flow problem.
I suggest to monitor in time the mean values
Question
I am performing CFD simulation over a NACA 4418 airfoil on Ansys FLUENT and I have collected a data chart of NACA (from 1945) for the purpose of validation.
The Reynolds Number chosen for this CFD simulation is 3,000,000 and the free stream velocity is 45 m/s. Since the simulation for my case is 2D simulation both the characteristics length and area is 1m. The value of y+ considered is 1 and that is ensured by calculating the wall spacing and putting that on First Wall thickness and I have even gone through Report in Ansys FLUENT to check the maximum facet value is below 1. I have used both Spalart Allmaras and K-omega SST Turbulence model. But the following problems are prevalent:
01. For Spalart Allmaras Turbulence model,my lift coefficient is within the range i.e less than 5% difference in between but drag coefficient is way too much high in value sometimes even 200% more
02. For K-omega SST Turbulence model, my lift coefficient is significantly higher in value and as Angle of attack increases this difference gets higher but the highest it gets is below 30% however drag coefficient is also over predicted but in this case the difference is around 50% higher.
I have tried it again for both Turbulence model with a bit more refined mesh but the value of lift coefficient increases in both the cases and but the difference for drag coefficient decreases for K-omega SST Turbulence model
I have been trying this for months now and haven't come to a proper solution. I tried everything at my disposal. Read aerodynamic books and gone through understanding how it is represented on CFD but so far I am not able to find the solution. If anyone can explain it to me what is going on for my case and how can I find a solution for this, that will be really helpful and appreciated.
Best
Question
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.
Question
I am interested in broadening my understanding of the physical assumptions needed to simplify its mathematical description. From these assumptions i will to choose a suitable turbulence model to run the simulation in Ansys.
The problem is fairly basic;
Inlet flow conditions: Velocity in= 44.2 m/s, Mach number inlet = 0.128, atmospheric total pressure and temperature. Turbulent boundary layer thickness @ 4H upstream of the step is 1.9 cm.
Outlet flow conditions: Fully developed flow.
Any advice would be much appreciated
Kind regards
Anton
If you want to use the RANS formulation you should assume your flow field is totally developed. That means you can set the inflow profile according to a statistically mean velocity in a channel
Question
I want to determine the aerodynamic coefficients of a 2D model (solid boosters). To be clear with the model, It is a 2D axisymmetric model, with blunt nose and flared aft body. There is a plume exiting from the nozzle. The jet plume interacts with the freestream flow, to form plume induced flow separation (PIFS). The interaction will affect the aerodynamic coefficients.
Experimentally the axial force coefficient was found to be decreasing at increased jet pressure ratio (Jet pressure/free stream pressure). I've solved it using pressure based solver with standard, K-e turbulence model (I've tried using other turbulence model too). But results from Ansys Fluent showed no change with change in jet pressure ratio.
Am I missing something or is there any other method to find axial force coefficient.
Question
I am trying to run a transient simulation of a stirred tank reactor. Initially, I am simply trying to using water as the only fluid with the k-epsilon turbulence model. I ran the steady-state simulation first and then in the same workbench's fluent file, I simply changed the option of steady-state to transient and set the simulation to run using for 0.0008-sec timestep, 2000 timesteps, 25 iterations per time step (in order to use that steady-state data as the initial condition for the transient case). I had also used report definition function for plotting turbulence kinetic energy as a function of time and had used the option of autosave for every 5 steps along with export data of certain parameters for every 5 steps. (pl find the ss) But after around 200 timesteps, I am getting the error of (Error: GUI-domain-label: no domain selected Error Object: 1522351816) and the simulation stops at the next iteration. I am able to click ok and the simulation runs for another timestep but the error crops up again every timestep.
simply you can save your case and data file at the current time step , and then reopen it and procced the calculation again . the error at each time step will disappear.
Question
Both PANS (Girimaji, 2006) and TFLES (Pruett et al, 2003) provide a self-consistent formulation that allows one to recover RANS versus DNS at opposite limits. Are the two approaches fundamentally different (e.g., in the sense that RANS vs LES employ different ansatz and thus yield different types of closures to be modeled: the Reynolds stress vs the subfilter stress)? Or are they conceptually the same approach, independently developed and thus mainly differing in preferred modeling choices/perspectives?
(note: I am unfamiliar with PANS and only marginally familiar with TFLES; apologies for any misinterpretations)
Ok, I can't stop making some practical comments.
To "Refinement in this scenario should give a converged result for the chosen filter width etc": Theoretically maybe, but not in numerical practice. If you do not fully resolve, without modelling, you have to be lucky for a simple physical behavior of the filtered/averaged part. Sometimes this holds, and/or the error is not significant, and modelling works well for some aspects of the flow.
"... admit different types of modelling strategies?": Formally yes in my view, but eventually it is always about how to view/sell the result. There are many LES modellings (SGS, or with explicit or implicit filtering, or with designing the truncation error to 'exactly' model the missing parts, or ...), and still, no one is definitely superior in numerical practice other than for the cases shown by the designers for which they know what to do. Also, the numerical implementation may play a role especially for compressible flow.
So you see, I am a DNS guy, and methods with less resolution need
be stable and can be more or less acceptable; the reasonings for the behavior varies with the method.
The late J. Ferziger (Stanford) once said: "FDs of second order with Smagorinsky is the best", at a time where higher order was used already, so he was convinced of the method he used as a well reputated specialist. The same was said by a German group one decade later, comparing 2nd and higher-order FDs in a LES journal paper. At the same it turned out however that switching-off the model yielded virtually the same results in the latter case. Oops, but this is not untypical.
Question
I want to add the penalty term as you suggested in the paper "Topology optimization method with finite elements based on the k-ε turbulence model" Could someone guide me on how to add the penalty terms for Eq. (11) and (12) as shown:
Question
Hi everyone,
I'm simulating a 3D turbulent pipe flow using the SST K-W turbulence model in ANSYS Fluent. Is there any quantity among the postprocessing quantities that represent "eddy size" or "eddy length scale"? If there is not such a quantity, How can I define it in Fluent?
Thank you.
You have to define for example your desired Kolmogorov scale through UDF.
Cheers,
Question
The methodology I am using was
Type- 2D Axis-symmetric behaviour
Flow domain is divided into finite number of unstructured quadrilateral cells
2D double precision, implicit, density based solver of flux type Roe-FDS
2 equation SST k-w turbulence model and RANS energy equations are solved
Air of ideal gas density as fluid with 3 coefficient Sutherland’s viscosity
Pressure far-field, stationary wall and ensured no slip condition
Free stream conditions
Mach number 6
Pressure 1064 Pa
Temperature 234 K
Thermal condition for wall : constant wall temperature of 300k
Turbulence intensity of 5% and Viscosity ratio of 10
First order upwind discretization scheme for first few iterations and followed by second order upwind for further till convergence
Higher Order Term Relaxation, Convergence Acceleration for Structured Meshes and Warped face gradient correction were enabled
All the residuals were set to 1e-06
You should decrease the residuals. How many residuals did you have when you used the transient calculation. If you can not decrease the residuals you should increase the number of elements. If you have a separation zone then the residuals periodicity will remain.
Question
Hello
Just before I start I am new to CFD modelling and I am trying to simulate a numerical wave tank with a wave of amplitude 0.1m and wavelenght of 1.561m using stokes third order. I am using the VOF method Implicit model with 2 Eulrian phases. The turbulence model is K-omega SST and I am using a water as my second phase where I have selected compressible so it is able to interact with the air. I have set up numerical beach at the end of my domain and the BC I am using are as follows.
Inlet-Velocity inlet
outlet-pressure outlet
Atmosphere-pressure outlet
bottom and side walls-Wall(no slip)
cylinder-wall(no slip)
I am using the PISO algorithm and have decreased the relaxation factors for momentum,k and omega. For some reason I am not able to reach similar results to an experiment. I am not to sure where I am going wrong. If anyone could help I would be extremely grateful.
My answer is a bit late. But might be of help to others.
I am not a user of Ansys. I cannot tell you whether you need a used defined routine to obtain the wave kinematics at inflow.
However, I have experience with generating waves in CFD and recently published a paper in which I also compare a generated CFD wave with the Stokes 5th order theory and I also computed wave loads on a vertical surface piercing circular cylinder , but both with incompressible fluids. The results of the latter are also compared against experiments.
Your inflow BC should include wave kinematics (e.g. orbital velocity vectors for a regular wave) and the volume fraction according to the wave you are using (Stokes 3rd order). Some codes allow also coupling with potential flow codes that include several irregular wave spectra. The wave theory should also be chosen according to their applicability ranges. See e.g. Lé Méhauté.
Depending on the domain, you might see reflections at outflow, sides, objects, etc. Therefore, you need to tune your damping algorithm to your case. Further details see .
As Mohammad Mehdi Baseri pointed out, the grid/mesh is important. Usually, people try to relate the cell size in vertical and propagation direction to the wave height and length, respectively. However, this also depends on the steepness of the wave. A steeper wave might need different resolution. And it also depends on the structure you investigate, i.e. the diameter of the cylinder or any other difficult areas.
Furthermore, the time discretisation is also very important. Time discretisation can also be related to the wave period. Time discretisation is important because it influences the iterative convergence. Some interface capturing schemes like HRIC depend on the CFL (Courant-Friedrichs-Lewy) number. They switch to lower order methods for larger CFL numbers for robustness but this depends also on the CFD code you use. Thus, you will need to investigate different time step sizes. If you see irregular waves for regular inputs, the iterative convergence might be the problem. When you do the grid and time step sensitivity study, try to follow ITTC guidelines for refinement. If you want to read more about this, you are welcome to read my paper: Towards credible CFD simulations for floating offshore wind turbines . I believe test case 2 is similiar to yours. I will add a couple of further reading material below.
The iterative convergence is also important. To achieve convergence is usually difficult for simulations with VOF, see . Residuals in the order of Linf<1e-3 might be okay for a first investigation. Then, you can check the influence for 1e-4, 1e-5 etc. But this comes once your grid and time step are acceptable and the results in the same order as the experiment. The iterative convergence is also influenced by the relaxation parameters, the number of iterations per time step, the numerical schemes, the grid quality, etc.
Good luck!
Cheers, SImon
09377255.2015.1119921
Question
I'm trying to develop a code to solve the stream function-vorticity equations using the Finite Element Method in order to simulate a 2D incompressible flow problem. I was wondering what the pros and cons are, whether coupling a turbulence model is possible, whether formulating the boundary conditions may face difficulty and whether the evaluation of the pressure field is flawed possibly due to decoupling of the pressure variable from the governing equations. Note an accurate evaluation of the pressure field is particularly important for my case of study.
I very much appreciate helping me out.
Good topic
Question
Hi everyone
I'm doing a steady state simulation using ANSYS fluent and k-epsilon turbulence model to investigate thermal mixing in a T-junction and I need to get a temperature contour at a specific time steps. How can I achieve that?
Any help will be much appreciated. Thanks in advance.
Dear Md Nuruzzaman,
Use ANSYS FLUENT 12.0 Tutorial Guide to develop temperature contour data at specific time set up. Use the solution animation feature to save contour plots of temperature every five time steps. .ANSYS FLUENT to update the animation sequence at every time step and contour plot in Fluent at specified intervals of a calculation.
Step Condition
1. Geometry and boundary conditions.
2. Overall mesh generation
3. Independence studies. (a) Grid Independence; (b) Time Independence
4. Temperature contour for different time step sizes (top = 0.5 s, middle = 0.01 s, bottom = 0.005 s)
5. Animation
6. Post solution and save
Ashish
Question
I'm currently working on a project that is using the SST k-w turbulence model. This model, reads velocity flow on the near wall region of a bluff body as well as the far wall region which is the combination of k-w and k-e turbulence model. The SST k-w model can be the best approach in solving this type of flow problem. The only issue is defining this model onto the CFX pre process.
???
Just go to flow domain --> turbulence model and select the Shear-Stress Transport (SST) model. So what is here the issue?
Regards,
Dr. Th. Frank.
Question
Hi all. I am running simulations on a compressor fluid domain (Turbocharger compressor for a passenger car) in Ansys fluent 19.3. I am using tetra element and k-omega SST turbulence model. But y+ is still high after refinement. How to reduce to get y plus value <5 ? Could someone give me any suggestion.
Hi,
you could use a combination of refinement on the wall surfaces and then apply inflation layers of your choice ( you may input the first layer height), thus you could generate a mesh of good quality in terms of skewness as well as aspect ratio.
hope it helps.
Regards,
Rajesh.
Question
Hi
I am trying to model an impeller rotating at a certain angular speed using MRF technique in Ansys fluent. Overall i expect clean attached flows and I am not really interetsted in resolving the boundary layer as I am more interested in studying the mixing of 2 liquids by the impellers. As i understand K-omega reliazable model is computationallly expensive and more for resolving the boundary layers. Which turbulence model is better for my flow as I want to keep my model light weight so that it is easier for convergence and computationally less expensive. Your advise will be really appreciated
Thanks/Regards
You can begin by trying different k-epsilon models (standard, RNG, realisable ) and compare it with experimental data.
Question