Flow Past a Circular Cylinder

Topics: Fluid dynamics, Viscosity, Reynolds number Pages: 7 (1609 words) Published: November 3, 2013
Fluid Mechanics
Flow Past a Circular Cylinder
The behaviour of flow around and past bodies that are circular in the crosssection is very important, mainly because there are so many matching scenarios out there. Golf balls, over head cables and underwater supports are just a few examples of circular cross-sections and how studying flow in this case is important. Let us take golf balls for example, they have been engineered in such a way from them being perfectly round, to having small indents in them to reduce drag, allowing for maximum distance after the ball has been struck.

Early techniques such as potential flow did not produce particularly useful results, especially in what is now known as D‟Alembert‟s Paradox, who predicted that the lift and drag on any closed body was to be zero due to the assumption made that fluid could be treated as “inviscid” meaning the viscous effects can be negligible. However it soon arose that this was not the case and in fact, viscosity plays a very large role in finding flow behaviour and resulting forces on a body in a flow. The main way in which viscosity affects the behaviour of flow is through the “boundary layer” meaning a thin layer of fluid next to the surface of any body in a moving fluid. The increase and behaviour of the boundary layer destroys the predictions of the potential flow theory by means of it distorting the pressure distribution. With the result that the pressure forces over the body are no longer zero, the body has pressure or “form drag”. Viscosity also produces its own kind of drag, that being “skin friction drag” the name essentially explaining itself. Inviscous flow over a circular cylinder, below is a pictorial explanation of D‟Alemberts paradox, if this was possible boats and other floating items would be able to go much faster, as no drag is produced and no wake is made.

Unfortunately, as mentioned earlier this is not possible, at least not yet and this is a pictorial explanation of Viscous flow over a circular cylinder. Think of pushing a ping pong ball in a sink full of water, It starts fast due to the force applied then suddenly stops due to the drag and floats gracefully across the surface producing a small channel behind it know as the wake.

Experimental Apparatus:
A cylindrical model with a diameter of 110mm has been installed in the working section of Wind Tunnel No.2. The model has 36 pressure tappings spaced evenly around its circumference, each of which is connected to one of the tubes of a multi-tube manometer. Airspeed or rather dynamic pressure will be measured by the Betz manometer installed in the tunnel. An alternative method of doing this uses two more tubes of the multi-tube manometer that are connected to the same tappings in the tunnel used by the Betz.

Method
First you must check that the Betz manometer is set to zero, if not adjust accordingly. Now record
i)
ii)
iii)

the angle of the multi-tube manometer
the specific gravity of the manometer fluid
the calibration factor, K1, of the Betz manometer

Once they have been noted turn on the wind and increase the fan speed carefully to between 15 and 20 units on the Betz scale. Allow the conditions to stabilise, determined when the fluid in the manometer stop moving. Dependant on whether of not it is possible to lock off the manometer, to “freeze” the current readings. If it is possible to lock the manometer and read the Betz, you may now shut down the fan, and take down the reading from each tube, from 1-36 38 and 39 of the multi-tube manometer.

If it is however not possible to lock the manometer, multiple readings are necessary to get an average to allow for drift in fan output during the test. If this is the case, then;
i)
Read the Betz and record the value
ii)
Two students must read the values of the tubes at the same
time. One starting from tube 1 and ascending and the other
starting from tube 39 and descending.
iii)
Once both sets of readings have been taken,...
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