Most undergraduate engineering textbooks have a schematic diagram of the flow around a cylinder at different Reynolds numbers. The one below is styled on the diagram from Munson et al. However, the implications of the diagram can seem a little abstract. Here is a simple demonstration that illustrates how, at the right Reynolds number and positioning, you can blow out a candle with a glass or cup between your mouth and the candle.
Schematic diagram of flow regimes for flow around a cylinder for increasing Reynolds numbers. From left to right and down Laminar fully attached wake, steady separation bubble, von Karmen vortex street, wide turbulent wake, flow reattachment with narrow turbulent wake (drag crisis). Adapted from Munson et al.
- A candle
- A cylinder such as a drinking glass
- Light the candle
- Place the glass a few diameters away from the candle
- With your mouth a similar distance from the glass as the glass is to the candle, blow out the candle.
The main point of the demonstration is that the glass does not create and endless wind shadow but rather there is a wake behind the glass that gradually decays as the flow moves downstream. Therefore, provided the glass is not too close to the candle, the velocity in the wake regains sufficient velocity to extinguish the candle. The only exception to this would be if the candle were a low flow portion of the steady separation bubble that exists for low to moderate Reynolds numbers. However, it is difficult to achieve this Reynolds number while still blowing hard enough to blow out the candle. I found that my peak air speed is around 25 mph (11 m/s). The glass diameter was approximately 3” (7.5 cm) which gives a Reynolds number of 55,000. This places the flow squarely in the wide turbulent wake (pre drag crisis) regime (or it would if it were a uniform flow as opposed to a round air jet coming from a small opening). The demonstration takes a little practice.
An index of all the demonstrations posted on this blog can be found here. Don’t forget to follow @nbkaye on twitter for updates to this blog. If you have a demonstration that you use in class that you would like to share on this blog please email me (firstname.lastname@example.org). I also welcome comments (through the comments section or via email) on improving the demonstrations.