# Drag force on a cylinder

1. Sep 5, 2008

### VooDoo

Hey guys,

If I were to have a large cylindrical building and in front of that building I was to place a smaller square building. Would this reduce the drag on the larger cylindrical building.

Here is what I know, just to show that I have tried to understand. Now I understand that the square building will possibly provide the slip stream effect. However, as I understand it...if the flow is turbulent there will be a reduced pressure at the rear of the square building, but if its laminar there will be a higher pressure?

Now if the square building reduces the pressure behind it, doesnt that mean that the velocity of the fluid reaching the cylinder building will be reduced? hence the fluid particles will not have sufficient energy to travel along the pressure gradient on the cylinder and separation will occur thus increasing drag on the cylinder.
Also what effect does the wake have on the drag of the cylinder? and the vortex shedding of the square

Im just struggling to understand whether placing a small square building will decrease the drag on a larger cylindrical building.

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2. Sep 5, 2008

### minger

It really depends, but I would initially say that it would increase the drag. Flow around cylinders are a well-known flow, and they produce what's known as a Von-Karman vortex street. Essentially, rather than "splitting" the flow like I presume you 'think' will happen, what happens is that the flow whips back and forth producing vortices, see:

http://en.wikipedia.org/wiki/Von_Karman_vortex_street

for a good animation. Basically though, my intial thought is that the vortices impinging on the building would cause quite unwanted results.

3. Sep 5, 2008

### VooDoo

Hi Minger,

The building at the front is a square building, can these vortices still occur for square buildings cos wiki seems to state that it only occurs for blunt bodies?

Last edited: Sep 5, 2008
4. Sep 5, 2008

### FredGarvin

A square face is about as blunt as you can get. The VK vortex street natural frequencies are usually low which is exactly what you don't want since a natural frequency of a building will be very low as well. Situations like this are exactly why builders make models and put them in wind tunnels. Honestly, I would be more worried about the dynamic forces, i.e. induced vibrations than drag forces. The statics will most likely come out in the wash when you deal with the dynamics IMO.

5. Sep 5, 2008

### minger

You could kind of think of the square body in front as a vehicle driving, and the cylinder behind "drafting" behind it. I did a quick CFD analysis of a semi and car before, and IIRC the distance needed for a small drafting effect was around 12 feet. Now, I'm not sure what you can scale that by, but you can get an idea of relative distances needed.

As Fred and I said though, those vibrations could be dangerous.

6. Sep 5, 2008

### VooDoo

Cheers guys!

Quick question, I understand that the first building that is producing the vortices can vibrate if the frequency of the shedding is close to the natural frequency. If the second building is close to the first building can this potentially vibrate as well?

Also if the buildings were to be spaced further apart, would the viscous effects consume the energy of the vortices thus reducing the effect of the vortices on the second building?

7. Sep 5, 2008

### rbeale98

i think it would definitely decrease drag on the building. For this to increase drag, you would have either higher pressure behind the square structure, or increased velocity of flow directed at the cylinder. The recirculation region between the structures should have a reduced pressure. pressure drag and skin drag on the cylinder should both go down.

If the shedding frequency is near a natural frequency of the round structure, that may excite it. from a material strength standpoint, it could be worse to have a vibratory loading in the structure than a constant drag even if the constant drag is higher than the max vibratory. im not sure how significant shedding would be in this case. depends on a lot of variables.