Learning Forum

[simonhenn]

I am simulating an air flow through a two dimensional rectangular channel. The channel splits into two channels, one of them is followed by a 90 degree bend downwards and leads into a cylindrical volume through which the air flows out of the geometry. Here is a picture of the geometry:

[Karthik Remella]

Hello,nThis is a good question. There are problems where steady-state results are not physically possible and running a steady-state analysis will provide results that are not converged. These problems almost always require a transient analysis to obtain good physical results from your simulations. nRegarding your question on RANS model averaging out fluctuations - Turbulence has multiple scales and is broadly classified as large and small scales. Large scales are relatively steady whereas the smaller scales are typically unsteady. In RANS, we write each variable as the sum of its mean and fluctuating components, and then time average these. Here, the sampling time is large enough that we can assume that the fluctuating component averages to 0. The goal of this time averaging is to be able to solve for the mean flow field. But, what happens when the mean flow field itself is inherently unsteady? In such a case, we will need to rely on unsteady RANS to capture this unsteadiness. nI hope this answers your questions. nHere is a small course on Turbulence and its modeling from our Ansys Innovation Courses. This should give you a good overview of this topic. There are plenty of simulation examples in this course that will help you understand how to model such problems in Ansys Fluent. Just an FYI! nnThank you.nKarthikn

[simonhenn]

Hello, nthank you for your answer! So you mean when flow properties like for example the pressure oscillate around a certain mean value these oscillations will be averaged out but when the mean value of the oscillations itself is unsteady that will lead to problems ? Do you know some literature around this topic?nThank you. n

[Karthik Remella]

nYes, your understanding is correct. nUnfortunately, I don't have any literature readily available with me that I can point you to. Perhaps, you can start looking at unsteady problems solved in the open literature. Examples that come to mind are flapping airfoil or cylinder.nThank you.nKarthikn