Flow around a cylinder

This is the analysis of a very classical problem, we’ve encountered it in fluid dynamics class (for those of you who had proper lecturers, I salute you). it’s one of the first stepping stones we place when building our fluid dynamics knowledge. it’s so fundamental in our education that it even appears in the equation sheets in our exams. For me, I’ve already put fluid mechanics course way in the past, but not long ago I revisited this elementary problem from a new perspective, that of CFD. specifically Fluent.

For me, this is a way to get to know the basic tools in ANSYS, and what better way is there than holding everything the same, except for changing one variable, which is the platform of the solution.

the theory says, very crudely, that this is what happens when a flow of air meets a cylinder:

stagcyl
Figure 1 – Simplified flow around a cylinder

which is good enough when you want to just get to know the general feel of the problem. But what if you want to dig a little deeper? the bigger picture so to speak.

A4 Flow around a Cylinder
Figure 2 – Velocity profile across a cylinder

That’s better, it’s giving me a lot of information that I can’t see from simplifications. For example, a really cool thing to see is the trail behind the cylinder, let me give you a close up (it’s the thumbnail as well)

A4 Flow around a Cylinder 2
Figure 3 – Zoomed in, Velocity streamlines

so, we see that in the center (as in, when facing the cylinder from the X axis) of the cylinder the flow does indeed split off, there’s zero flow on the borders, what we call the no-slip condition, you can see the fine blue line around the cylinder, this is something we expect and would be suspicious if it didn’t exist.

 

A4 Flow around a Cylinder 2 - Pressure
Figure 4 – Pressure contour

 

 

As expected, the high-pressure concentration is at the front of the cylinder, and we have lower pressure where the velocity is higher, through Bernoulli

Final thoughts

First I’d like to tell a little anecdote. When I was a child, I would often have to wait somewhere for, I don’t know friends, or family or whatever. The city I grew up in was windy at times, and I would try to shield myself from the wind because usually, it would be on a cold day (at least, waiting on a hot day is not that remarkable). So I would hide behind one of these circular columns that would support buildings around my neighborhood. I remember realizing that standing behind a column (behind referring to where the wind is coming from of course)  reduced how strong I felt the wind, however it never stopped it completely. And here, after 20 or so years, I have a physical representation as to what happens exactly.

The results match what I would expect initially, however, I am not a good metric because I already know what happens around a cylinder. One good use of the principles exemplified here is this:

https://en.wikipedia.org/wiki/Rotor_ship

The propulsion of the ship is done by the direction of the wind flowing, and the rotation of the columns on the ship (also called rotor sails). So it is practical to understand this basic problem, to reach cool inventions for one.

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s