[kazemiakk]

Originally Posted by kazemiakk

Hi,
I modeling the same problem in fluent in Re=120 but I can not capture vortex shedding in my results. I changed time steps and number of time steps, but still have the same problem and everything is steady. I mean it start from zero and Vorticity profile starts to develop but not shed . what should I change?

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[Far]

we did this at Re 200 and 100. First you must calculate your frequency and time and then adjust your other parameters.

[flotus1]

Originally Posted by flotus1

What you observed in your simulation is more related to physics than to a numerical concept.
For the unsteady flow around a bluff body, the Strouhal number describes the frequency of the vortex shedding:
.
Here f is the frequency , D is the characteristic length (diameter in our case) and u is the velocity of the undisturbed flow.
The Strouhal number is in the order of 0.2 over a wide range of Reynolds numbers.
Conversely, the period time of the vortex shedding

will strongly depend on u since Sr is nearly constant.
So if you decrease your Reynolds number by decreasing u, the period time of the vortex shedding will increase.
The time it takes for the vortex street to develop is linked to the period time.
Thats why you need to simulate a physical time that is 10 times higher when you lower the Reynolds number the way you did it (through u).
And that is why i have been estimating time step sizes since the beginning of this thread.
Adjusting the time step size to a fraction of the period time allows you to simulate the flow with somewhat arbitrary Reynolds numbers with the same amount of time steps.

Wow nobody spotted the error I made

Of course its

[anuprdk]

Originally Posted by kazemiakk

Hi,
I modeling the same problem in fluent in Re=120 but I can not capture vortex shedding in my results. I changed time steps and number of time steps, but still have the same problem and everything is steady. I mean it start from zero and Vorticity profile starts to develop but not shed . what should I change?

When you say 'start from zero', do you mean to say you are initializing the fluid domain with zero velocity. There is nothing wrong with it but it will take a lot of time. Initialize with your velocity of 0.098 m/s and then run it.

Anup

[yzyang]

hey, i'm the beginner of ansys fluent and i'm trying to model a fluid flow over a sphere inside a hollow pipe. However, i don't know to separate the fluid flow and the sphere as i can't use the cut material function if i set the solid wall of the pipe and fluid flow as add frozen. Can anyone help in this problem? Thank you

[kazemiakk]

Originally Posted by anuprdk

When you say 'start from zero', do you mean to say you are initializing the fluid domain with zero velocity. There is nothing wrong with it but it will take a lot of time. Initialize with your velocity of 0.098 m/s and then run it.

Anup

No I initialize with 0.098 m/s
but I mean after converging, in the results, I make animation and vorticity profile starts from zero (in the first time step) to fully developed profile.

I got confused! because I used different time steps: 0.01, 0.08 1.8 and 2
but after some time everything is steady. Also the Reynolds number i tried was 120, 1540, 15400.
what should I do??

[snandhu23]

can anyone pls send me mesh files and case files for flow around circular cylinder?

[kazemiakk]

Originally Posted by kazemiakk

Yes I Did. but everything in my case is growing the vorticty to fully develop but no vortex shedding. I used the following parameters:
D=0.22 m
Re=1200
V=0.098 m/s
dynamic viscosity: 0.018 kg/ms
time step: 0.3
Number of time step:4000

What is the Reynolds number for laminar flow around cylinder?
Is it reasonable to consider laminar for a range of Re=1500 to Re=9000

[kazemiakk]

What is the Reynolds number for laminar flow around cylinder?
Is it reasonable to consider laminar for a range of Re=1500 to Re=9000

[jsinha1987]

It will be better if you use Strouhal number around 0.2. It will give good unsteady solution

[alimea]

Hi all

I have read a paper by the title of:
Method for time-dependent simulations of viscoelastic flows: vortex shedding behind cylinder
​I want to validate my research with this paper, but a problem have been occurred!
When I want to investigate time step independency, I will earn the attached results. it means that for dt=0.001 I can't get good in-dependency from time step in comparison with dt=0.0005. I'm working on viscoelastic fluid with these non-dimension numbers:
Re=100 - El=1 - Betta(etta_p/etta_0)=0.9 - epsilon=0.1 - zetta = 0.01 (EPTT model)
time derivative discretization scheme: CrankNicolson 0.9
In addition I added the energy eqn to my equations (Is it possible that solving this eqn need a very small time step?)
I guess it may because of high betta(the ratio of polymeric viscosity to total viscosity).
The problem properties:
Re=100 - El=1 - Pr=1000
residuals are set on 1e-8
Relaxatios are set on= 0.3
schemes:
- divergences, gradients and laplacians: central
- div(phi,tau): 1st order upwind
- div(phi,T): 2nd order upwind
solvers: for p : PCG , for others: BiCGStab
Algorithm: PISO
min Iteration in every time step=2

Could you please give me your idea?

[Aravind M]

hi everyone i am very much new to the cfd simulation field. As part of my new project am doing 2d analysis of flow over a circular cylinder.
The parameters i took is given below.

dia of cylinder - 0.1m
domain dimensions - 5m*4m
model - standard k-epsilon model

in reference values i gave the area that i got from surface intergral after initialization.

area-0.314161m2
velocity - 14m/s
rest of the parameter was left default in reference box.

residual absolute criteria was set to 1e-6 for all parameters.

no of iteration was 1000.

after all this am getting Cd value as 0.16649658 which is very low compared to theoretical value.. can someone help me

[vinerm]

Calculation of drag coefficient requires area, velocity, and density. So, you need to ensure that these values are set appropriately in the Reference Values Panel. Velocity and Density should be of the freestream. Area should be 0.1 because area used is supposed to be the frontal area of cylinder.

[anuprdk]

Hi Aravind,

At a high velocity of 14m/s, the flow separates at an angle less than 90 degrees subtended along the flow axis. Fomula for the same can be found from any Fluid mechanics text. That would be your arc length or in this case the area assuming unit depth. Use this area to determine Cd and not the whole area of the cylinder.

You haven't mentioned the fluid medium based on which the Reynolds number varies and hence the separation point as well