Reynolds critic number
 

Hello,

I have a doubt about the critical Reynolds number and how I can know in ANSYS when it goes from laminar flow to turbulent flow.

In my case I have a flow over a cylinder and I have simulated a laminar flow changing te Reynolds number from 10 to 200 every 10 numbers. My question is how I can now that the flow is turbulent seing the residuals of the simulations of the laminar flow?

Thanks beforehand,

If you use only the laminar flow model and you are using the steady solver, you'll never see the transition to turbulence. You can only see turbulence if you are running a transient simulation with a super fine grid (i.e. doing a DNS).

So, I have simulated with a Reynolds number of 127 turbulent and I have obtained the results that can be seing in the first two images.

Attachment 73736

Attachment 73737

Also I have simulated with 254 and I have obtained the last two images results.

Attachment 73738

Attachment 73739

How I can know where is the Reynolds critical number?

DNS is not as simple as running laminar transient. Without perturbations, the flow can remain laminar for as high of a Reynolds number as you like.


The way to determine the critical Reynolds number is to do a binary search like approach. Keep running cases and show that there is a point where it goes from laminar to turbulent. What you need to do is, given a flow field, determine/prove that it is laminar or turbulent.


To show that one of them is turbulent... You can do this by placing monitors inside the boundary layers and getting the velocity as a function of time, then doing spectral analysis on the fluctuations and show that it looks like a turbulent spectrum (broadband).

You can try using a transitional model and looking to see if the intermittency factor changes. However, there is a (very) tunable model parameter in these models which may or may not give you the accuracy you want in determining the critical Reynolds number.

And witch is this parameter that may give me the accuracy in determining the critical Reynolds number?

For example, for a Reynolds number of 80 I have the folowing chart.

Attachment 73856
Attachment 73857
Attachment 73858

This would be turbulent?