Video of “Flow attachment, wakes, and blowing a candle out around a cup”

Here is animated GIF of the “Flow attachment, wakes, and blowing a candle out around a cup” demonstration. The full video is here.

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 (nbkaye@clemson.edu). I also welcome comments (through the comments section or via email) on improving the demonstrations.

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Flow attachment, wakes, and blowing a candle out around a cup

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.

Equipment

1. A candle
2. Matches
3. A cylinder such as a drinking glass

Demonstration

1. Light the candle
2. Place the glass a few diameters away from the candle
3. With your mouth a similar distance from the glass as the glass is to the candle, blow out the candle.

Explanation

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 (nbkaye@clemson.edu). I also welcome comments (through the comments section or via email) on improving the demonstrations.

Lift, Boundary layer separation, and curve balls

Getting a ball (e.g. a cricket ball, soccer ball, tennis ball, or baseball) to move laterally is somewhat difficult (though I find it all too easy with a golf ball). The fluid mechanics of this phenomenon is quite interesting and provides insight in to how boundary layer separation influences both drag and lift. Here is a simple demonstration for showing how to get a ball to move upward after release by putting backspin on it.

Equipment

1. A ping-pong ball
2. Some sort of track. I used a 30 inch wall mounted shelving frame piece that you can get at a hardware store (I used this one http://www.lowes.com/ProductDisplay?productId=3006188)

Figure 1. Equipment.

Demonstration

1. Hold the bottom of the track and rest the ball on top of your hand (see figure 2 below).
2. Rotate your wrist rapidly so that the ball is forced up the track and forwards (see figure 3 below)

With a bit of practice you can get the ball to fly off the end of the track roughly horizontally and then rise up due to the lift force

Figure 2. Initial setup.

Figure 3. Launching the ball by rapidly rotating the track.

Explanation

The key to the explanation is that the rotation of the ball leads to asymmetric boundary layer separation. As you flick your wrist and rotate the track the ball is accelerated and forced up the track. Friction with the track imparts a backspin to the ball such that, as the ball leaves the end of the track, it is rotating in a clockwise direction as shown in figure 3. The backspin on the ball means that the lower side of the ball is moving faster than the upper part of the ball (see figure 4 below). As a result, provided you are in the right Reynolds number regime such that the separation point is  Re dependent, the boundary layer on the top of the ball will stay attached over a greater distance than the boundary layer on the lower side (see figure 4 below). As a result the airflow will be deflected downward by the ball.

Figure 4. The rotation of the ball causes the underside to move faster and the underside boundary layer to separate further upstream than on the top.

If you draw a control volume around the ball with the control volume moving with the ball then the C. V.  inflow is in the direction of flight and the outflow is deflected downward. Therefore, the ball must be applying a downward force on the airflow to create the downward component of the outflow momentum. As a result, the airflow must be applying a force vertically upward in reaction. This lift force drives the ball upward.

Figure 5. Control volume moving with the ball showing the inflow, the deflected outflow and the force that the ball applies to the flow to generate the downward momentum.

The demonstration requires very little equipment but is a little fiddly. You need to use a ping-pong ball as it is light enough that the lift force can overcome the balls weight and move it upward. An alternate way to do this is to spin it sideways and get it to move laterally. The implications of this physics are discussed with actual numbers in “The physics of baseball” by R. K. Adir. The origin of this particular demonstration is unknown though I have seen basic descriptions of it in a few different books.

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 (nbkaye@clemson.edu). I also welcome comments (through the comments section or via email) on improving the demonstrations.

Videos of flow separation and lift forces on houses

Here are animated GIFs for the “Flow separation and lift forces on houses” demonstration.

The first shows the case of the door being on the downwind side of the building, therefore, no internal pressurization (full video here).

The second shows the  door on the upwind side of the house such that the building is pressurized (full video 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 (nbkaye@clemson.edu). I also welcome comments (through the comments section or via email) on improving the demonstrations.

Flow separation and lift forces on houses

This is a demonstration that Professor Sparks here at Clemson used to use in his wind engineering course. It is a simple way of demonstrating the role of flow separation on the uplift force on a flat roof. It can also be extended to examine the effect of building internal pressurization on the roof uplift force.

Equipment

1. A fan
2. A shoe box
3. A trash bag or plastic shopping bag
4. A sheet of plywood to mount the shoe box on or a brick to place in the shoe box to prevent it blowing away. .

Glue the shoe box to the board or place the brick inside it. Cut out a rectangle of the plastic and tape it over the open top of the shoe box so that it is sealed though not taut (in the image shown the plastic is mounted on a frame that is inserted into the shoe box so that different roof angle models can be swapped in and out). You can also cut a door in one of the long sides of the box if you want to talk about internal pressurization.

Demonstration

Place the fan so that it blows over the long side of the box (not the side with the door). Turn on the fan and the plastic at the front of the roof should lift up due to the flow separation at the building leading edge.  Depending on the fan placement the flow should re-attach at the downstream end of the building shown by the plastic being depressed at that end.

Turn around the building so that the door is facing toward the fan. In this case, the internal pressure in the building increases to the stagnation pressure of the flow. As a result, the entire roof will lift up.

Cornering flows are more complex.

There are case studies in the literature of buildings collapsing as a result of small building envelope failures. In these cases a window or door fails and the pressure in the building increases. The resulting uplift causes the roof to liftoff. If the roof provides significant bracing for the building walls then the entire building can collapse under the wind loads

Qualitative analysis

This is a little simplistic, but I draw a schematic diagram of the building and a streamline curving away from the leading edge.  I then draw a normal acceleration vector down toward the roof top. The flow will only accelerate in that direction if there is a pressure gradient (high to low) in the direction of the normal acceleration vector. Therefore, given atmospheric pressure in the free stream, there must be a vacuum pressure on the roof.

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 (nbkaye@clemson.edu). I also welcome comments (through the comments section or via email) on improving the demonstrations.