Re: flow over a cylinder, turbulent and laminar
 

A low-order discretisation scheme may be stabilising the solution (nonphysically) somewhat. Try using a higher order scheme.

Re: flow over a cylinder, turbulent and laminar
 

I use second order schemes.

After some reading and discussions, it seems that the vortex shedding will not be as obvious for a turbulent solution. And due to the added randomness, steady cycles may not be achieved. So my solution may be more accurate than i once thought, but I still need o understand the physics here.

Steve has given some valuable info about flow over cylinder. I have a question for all. When you say drag value at Re=10^6, what is it ? Is it average drag? Because the flow is turbulent the average value of drag can change with time(Correct me if I am wrong,please). How do you calculate this average drag. and how to we calculate the mean flow.

I have one more question. I am currently doing a Re=800 simulation without using any turbulence models. I have the flow solutions at different times. Can anybody tell me how to post process a turbulent flow (turbulence in wake in this case).

What do I need to do to calculate TKE and how do I get the E(k) vs k plot.
Thanks in advance . I think this is one of the best threads for flow over a cylinder

Hello all,

I agree; this really is a good thread. I have a general question regarding the unsteady wake of the 3D circular cylinder in cross flow problem I'm interested in:

- The mesh I'm using is very fine, worked on it for quite a while. Also, the boundary conditions I set and the domain size is accurate and sufficient. My problem lies in the unsteady solution. I first iterate the case for 4000 steps using steady state, and make sure I reach convergence. Then I run it unsteady for 300 time steps using a time step about 1/50 the time step obtained from inverting the frequency obtained by using the definition of the Strouhal number (St = f*L/V). The problem is that the unsteady oscillations of the wake disappear and I get a symmetric wake once I get convergence. I wait until all the residuals level out and stay constant for a good while.

Any ideas why this is happening? Is it because of the turbulence model (k-omega SST) being diffusive and getting rid of the vortex shedding, or is it the time step that is too large? And one more thing, how many pseudo-time steps are required for each time step?

Cheers,

Ozgur

To reply to some of your questions, cfdcfd123, here are my opinions:

- I haven't done calculations with Re magnitudes around a miliion, but I have worked on Re values around 10000 for quite a while.
- Fluent calculates TKE on its own; so you should be able to get TKE contour plots easily in post-processing.
- As for the plot (is it epsilon vs. k that you're looking for), you can take the average of any parameter you're interested in over the solutions obtained at different time steps. I take data at time steps placed really close to each other and take an arithmetic mean of the properties I'm looking at. Although, if you take an arithmetic mean with too large a time step between steps, this may be inaccurate.
- I am modeling 3D cross flow past a circular cylinder placed on a flat wall, but I'm not focused on drag that much; but I'll let you know if something comes to mind.

Best of luck,

Ozgur

Ozgur_,

I am also currently running a similar problem at Re = 90,000. I am attempting to predict noise, and as such have based my time step on the max resolvable frequency that I desire to have. I picked 4000Hz (close to upper limit of human range) and then decided that for each wavelength I wanted to resolve it with 10 points.

So delta t = 1/(4000 x 10) = 2.5E-5 s

I am not sure this will help you, but could be of some use in the future maybe. TO reduce computational time I am currently only running 15-20 iterations per time step.

I am relatively new to CFD and have been using the K-omega SST model, however after reading this thread I am considering trying Spallart.

Matt

I am trying to simulate a flow over a cylinder at Re= 10000 and using K-w SST as the turbulence model. I dont know how to select the BC for turbulence ???? Can anyone help me out.

Thank you

flow over a circular cylinder.. @ Re=1000
 

hiii..

i m very new to in CFD area...
i have worked on flow over a single cylinder, two clinder in tandem and tow cylinder in side by side @ diff Re with diff ... i got good results....

but when i started working for Re=1000 .. for single cylinder...
i didnt get gud result.... :confused:
i have use used mesh wid y~min <1 to Y+max<5.. wid Kw-sst model...
Cd wich i m getting is 1.4 and as per journals and different thesis, i went through, Cd should be 1~ or < 1.

i hv tried 0.5% turbulence @inlet and hydraulic dia equal to dia of cylinder.... and @ outlet 0, 0.5 and 0.7 back flow turbulence and same hyd dia...

but still... :confused: