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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

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

internal flow over cylindrical, rectangular and ellipse shapes.

outside is rectangular shape.

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 1
**Three 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!

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.

I'm using Ansys Fluent to model a rotating cage setup to monitor Flow Accelerated Corrosion.

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 )

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?

Thank you for your time in advance,

Majid

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?

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?

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

Turbulence model for blood flow simulation

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)

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.

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.

I'm reading this paper.

This paper is claiming there is a problem in k-epsilon model.
Because in k-epsilon model, we assume that eddy viscosity(

*ν*) is isotropy. But actually in real world, eddy viscosity(_{T}*ν*) 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._{T}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(

*ν*) is isotropy(like k-ε model)._{T}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(

*ν*) 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._{T}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 :(

Please help me.

Thanks :)

can one use turbulent models for studying quasi-steady flows in pipeline

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.

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?Its a

**steady state Analysis****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.**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?

Thanks in advance

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.

Thank you for your interest.

Best regards,

Güven

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.

Thank you for your interest.

Best regards,

Güven

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

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)?

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?

**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.**

in turbulent flow calculation is it acceptable to use an y+ of 15 with a wall function in k-e turbulence model?

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 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📷

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?

Thanks in advance

Regards

Johnny

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.

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,

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

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.

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.

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)

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:

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.

Hello All,

I have 2D mesh defined in x and y directions.

Now I would like to extend just once cell in Z direction so that it is 3D Mesh due to requirement of 3D mesh for turbulence model and type of input mesh.

How can I do this in Ansys Fluent Mesh Modular?

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

Green gauss node based gradient

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

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.

Thanks in advance.

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.

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.

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.

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

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

I am running a simulation for a turbocharger compressor fluid domain with turbulence model k-omega SST.

I am going to refine the mesh as the solution does not converge. What I know is that for a turbulence model k-omega, the Y plus value has to be <5.

I am looking to calculate the Y plus value before running the simulation.

Currently, I have a model of 3 Million of mesh elements.

Can someone explain what total mesh element number approximately should I refine to reach the Y plus value of <5?

I have problem in making my turbine simulation work with SST turbulence model

the other models: k-e, k-omega, BSL converge without problems

To analyze the thermal stratification of solar hot water storage tanks using numerical simulations, is it possible to apply the turbulence model? What are the parameters of using the model?

In air-water multiphase flow CFD simulation's it is common to use a VOF = 0.5 to track or identify the position of interface or free surface.

**I would like to know why this criteria is used and if it can always be assumed the same. Otherwise, I would like to know if volume fraction can be taken different to 0.5 and, if so, ask them about a methodology to find this volume fraction**.Below I describe a case that I am simulating in Ansys Fluent:

-Sewer pipe with Length L=6 m (aligned with the x axis), Diameter D=200 mm (8 in) and longitudinal slope So = 0.005 m/m.

-Simulation in steady state of sewer pipe considering biphasic flow air-water flow.

-Model Volume of Fluid (VOF) with implicit formulation and Open Channel's submodel.

-Boundary conditions: Inlet as mass flow rate = 26.51 kg / s; outlet = Pressure outlet. The mass flow entered is such that the ratio y / d = 0.61 (that is, occupation of 61%).

-Turbulence model: k-epsilon RNG with Enhanced Wall Treatment function (y + <5).

-Pressure-Velocity Coupling method: Coupled + Pseudotransient solver.

-Spatial discretization: Least Squared Cell Based; PRESTO!; Momentum, k and epsilon with 2nd order schemes; Volume Fraction = Compressive.

-Run Calculation: Time Step Method = Automatic with Length Scale Method = User-Specified; Length Scale = 0.064 (Hydraulic Radius); Time Scale Factor = 0.3.

To verify convergence I did the following:

* Residuals at 10-4 for all variables.

* Mass balance between input and output.

* Pressure drop between inlet and outlet.

* Velocity at Inlet and Outlet.

* VOF fraction for various orthogonal planes to the pipe at x = 0.5m, 1.0m, 1.5m, 2.0m, ..., 5.5m. I configured them as Surface report-Area Weighted Average, for the Field Variable=Volume fraction and Phase=water.

Convergence is good in terms of residuals, mass flow rate (1x10-6), drop pressure, velocity. In VOF monitors the trend also stabilizes towards values between 0.40 and 0.75.

**I know a flow profile along the pipe. In my initial simulation with refined mesh, with elements of 0.02mx0.005mx0.003m, [this is (dx) (dy) (dz)], I noticed that the theoretical profile is closer when I choose VOF = 0.7 and not VOF = 0.5 as is usual in the practice.**

**From the above my doubt arises about the VOF fraction to choose.**

Thanks!

How can calculate the value of Y+ for each turbulent models in Fluent CFD?

I am working on an open-channel multi-phase (water+air) flow with k-epsilon turbulence model. How can the computational cost of the model be minimized, while the accuracy and convergence are ensured?

Hi,

I am simulating a turbine in Fluent that it has curve. Mesh is structure completely. but there is high Skewness in tip clearance and some parts of around of the blade:

When i use SIMPLEC Method, one factor (Skewness correction) appear under the SIMPLEC method in ANSYS Fluents method.

How i must adjust this factor? i dont know which number must be set?1< or 1 , 2, 3,

I used K-e turbulence model and when I set Skewness correction to 5 or 20, the following error is appeared:

"Error: Divergence detected in AMG solver: k

Error: Divergence detected in AMG solver: k

Error Object: #f"

Why does this happen?

Please suggest a way to reduce the skewness in ansys fluent and how can i check Skewness in Fluent?

Thanks.

Best.

Ali

Hi

in ansys fluent's dacuments it has been said that : (( ANSYS Fluent has taken steps to offer more advanced wall formulations, which

allow a consistent mesh refinement without a deterioration of the results. Such -independent formulations are the default for

all omega-equation-based turbulence models. For the epsilon-equation-based models, the Menter-Lechner and Enhanced Wall Treatment (EWT)

serve the same purpose. A -insensitive wall treatment is also the default for the Spalart-Allmaras model and allows you to run this model

independent of the near-wall resolution. ))

so, is it true that for models mentioned above like k-omega or k-epsilon(EWT,Menter-lechner)there is no need to check y+ value to be in a certain range ?

for example can i use k-epsilon model with enhanced wall treatment and y+ value changes between 0.5 to 600 ?

if my assumption is'nt true , what is the certain range of y+ value for all turbulence models like k-epsilon with wall functions or k-omega

without wall functions ? please give me a reliable source ...

thanks...

Hello everyone,

I have problem on settings of farfield boundary condions. I want to increase turbulence intensity on the airfoil as inlet turbulence intensity but if i enter %2 , it decreases to 0.9% on the point front 1 meter from leading edge. I want to know if there is any equation of Turbulence intensity for C shape mesh domain on airfoil to have needed turbulence intensity.

Waiting for your comments.

Thank you so much...

Hello, I am doing a 3D simulation of a wind turbine using S-A turbulence model. Without wall function, I need to make y+ in the order of 1. So, I must use a finer boundary layer grids, but when I decrease the value of the first layer thickness, the solution diverge.

Can anyone please define me Linear Eddy Mixing model, and how can it be implemented in a quasi 1d combustion model for a pulsative combustor ?

To me, a CFD is solve a fluid mechanics problem using mathematical approaches with the help of a computer. In other words, it is like a marriage between a mathematical model and numerical methods for solving a fluid mechanics problem.

According to the above, I ask the following questions and answer what I think:

**1). Is a CFD a tool to solve only 3D problems?**

*According to the definition, No. A CFD is not restricted to dimension.*

**2). Can any software tool that solves a fluid mechanics problem be considered a CFD?**

*No. I would say that the mathematical model to be solved has to come at least from a simplification of the Navier-Stokes equations.*

**3). Based on what we understand as a cfd, please mention the CFD softwares you know. For now, exclude the computational algorithms programmed by you.**

*In 3D: Ansys Fluent; Ansys CFX; Flow3D; OpenFOAM.*

*In 2D: Iber; Hec-Ras 2D.*

*In 1D: EPA Swmm.*

I would appreciate if you can share your opinions about it.

- I'm trying to simulate a turbulent flow around an airfoil. I'm using Ansys software. The problem is my concern focus on the near wall region and I want to know which turbulence model is successful in such case?

Hello Everyone,

Im performing a CFD study using ANSYS 18.1 on a 3D inverted Ahmed model to investigate the effect of an active underbody rear diffuser (Re > 1.4 e+6), my study was supposed to be performed using a wind tunnel, but due to the corona pandemic, i had to change the method to CFD, which is new to me, so i have some questions if anyone can help.

1- is there a specific rule for the enclosure (domain) dimensions? i made it 10-12 times the model L,H,W.

2- im using the k-omega SST turbulent model as it predicts well near walls, what value for y+ should i aim for? and is there a redcommended limit for the number of inflation layers?

3- how can i perform a local refinement for the area around the model in ANSYS 18.1?

Appreciate your help guys

I'm currently working on a research of thermal-hydraulic performance of artificially roughened pipes using Ansys Fluent.

I have a 2D model of pipe section (upper boundary is wall with rectangular juts which are supposed to be elements of artificial roughness, their height and step are widely varied). I also created a structured mesh in Ansys Mesher. The first layer's height is chosen so y+ is equal to 1 at the highest Reynolds number I'm working with.

The problem is that Fluent does not give any proper results no matter what turbulence model I'm choosing. The heat flux is 2 times lower than in plain pipe, which is physically impossible.

That may be something wrong with the mesh or with the turbulence model settings. Could you help me?

I am working on the turbulence simulation. The scaled residuals of x, y & z velocity before going upward 1 e-5. The solution seems not to be converging. I am using K-E turbulence model to capture turbulence characteristics. Same issue is occurred in RSM model.

I used under relaxation factors as follows.

1. mass flow rate = 0.5

2. Pressure = 0.1

3. Density = 1

4. Body Forces = 1

5. Momentum= 0.5

6. Turbulent Kinetic Energy = 0.5

7. Turbulent Dissipation Rate = 0.5

8. Turbulent viscosity = 1

SIMPLE scheme is used for pressure coupling velocity, with least square cell based gradient , standard for pressure and all discretization are first order upwind . I hereby attached the sample picture of residual plots.

Kindly suggest what are the possible issues.

I am trying to calculate friction factor and heat transfer coefficient for flow in pipe, I am using Ansys Fluent as a solver when k-e model was used with 32 as Y+ but after analysis Y+ computed by solver was 23. Same was repeated for with k-w model in which Y+ was initially taken as 0.7 and the computed Y+ was 0.31.

Each time the used and computed Y+ values are different as a result friction factor and heat transfer coefficients don't match with the computed friction factor and heat transfer coefficient from cool-brook and Dittus-Boelter respectively.

Ansys ICEM was used to create geometry and mesh.

what concept I am missing here please guide me.

What the differences between the SST k-omega and LES turbulence models for Pumps ?

I'm modelling a combustion simulation for a liquid rocket, regarding chemical equilibrium, radiation and gas phase instead liquid phase for propellants. I would like to know what is the best turbulence model for this case.

I am working to simulate single and cavitation model in an axial flow pump can anybody provide me any tutorials in this field please. Thanks

The instantaneous kinetic energy k(t) of a turbulent flow can be categorized to the sum of the mean kinetic energy K and the turbulent kinetic energy k.

Question 1:

How the mean kinetic energy will be applied in engineering?

Question 2:

And for turbulent kinetic energy, it's obviously much more important in instantaneous kinetic energy when we try to understand the engineering principles and catch the effects of turbulence. So, after solving the k-equation, ε-equation along with 6 partial differential equations for Reynolds stresses, should we just focus on turbulent kinetic energy k and ignore the mean kinetic energy K?

Question 3:

The rate of deformation sij(t) can be split into two components as well: a mean Sij and a fluctuating component sij'. The question is elicited from the use of Sij in equations of K and k. Why the rate of destruction of K in the equations for mean kinetic energy K and the rate of production of k in the equation for turbulent kinetic energy k use the same term ρui'uj'*Sij but an opposite operating symbol? Won't that term be neutralized?

Hello all.

I am simulating a centrifugal pump. Before I started simulating, I ran few test cases on a simple setup to look into the y+. In the test case (which is a simple pipe flow), I could see that the yplus value specified for inflation matches the post-processing results. I am using SST turbulence model with automatic wall treatment.

When I am taking the same yplus to pump, which consists of rotating flows, the yplus fails. Now, as I did the sensitivity analysis using (10-15 layers in the boundary layer), y+ <1 and yplus 20-200 (where SST behaves like k-epsilon),l. However, I could not find the yplus specified for inflation to be matching with that of the post processing results. The difference is atleast an order near the blade walls. Now, I assume that it might be due to the large separation zone in the pressure side. Besides, the effect of y+ variation is not significant to compute the performance for the pumps.

My question is: Has any researcher found yplus value to be fully satisfied in case of turbomachines in CFX? Is it important to verify yplus (for Journal Papers) value even when we know that the wall function is not affecting our parameters of study?

Hello Everyone !

It is alway said that SA model is apporopriate for adverse pressure gradients then how come it lacks the ability to accurately predict separation ?

Hello Everyone

I am simulating an airfoil at Low Reynolds Number flow Regime Re=10^5 , in this particular case a laminar boundary layer separates , forming a laminar separation bubble , then transition to turbulent and reatching to the airfoil . Is SA model capable of simulating such kind of flow in which three 'different behavior of flow exists ?

Regards

Which turbulence model is most suitable for numerical calculations of flow and heat transfer in annular finned tubes geometry?