# 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.

# Videos of Vorticity and walking in circles

Here are the GIFs of the demonstration “Vorticity and walking in circles“. The headings link to the full videos.

(1)    Rotational flow around a circle

(2)    Irrotational flow around a circle

(3)    Rotational flow along a straight line

(4)    Irrotational flow along a straight line

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.

# Vorticity and walking in circles

This is a really simple demonstration that requires no equipment at all, though it is slightly enhanced if you have a stool that you can place at the front of the room. The demonstration shows the difference between rotational and irrotational flow for both straight and circular streamlines.

Equipment

Ideally you would have a stool or chair to walk around for the first two parts of the demonstration. If you do not, then you just need a space on the floor to walk around. The demonstration is exactly the same with or without the stool. You just don’t have the visual reference to walk around if you don’t have a stool.

Demonstration

The demonstration has four parts covering rotational and irrotational flows for both circular and straight streamlines.

(1)    Rotational flow around a circle

Place the stool in the front of the room with space all around it. Start off facing the class with the stool in front of you. Simply walk around the stool while facing the stool the whole time. Stop after one trip around the stool and point to the back of the room. Point out to the class that your body (the model fluid particle) is facing the back of the room. Start walking around the stool again and stop half way around, you should have your back to the class. Point to the front of the room and point out to the class that you are now facing in the opposite direction and have, therefore, rotated. Hence, this is a rotational flow with circular streamlines.

(2)    Irrotational flow around a circle

Repeat the first demonstration, only this time always face the back of the room as you walk in a circle around the stool. Again, stop after one trip around the stool and point to the back of the room. Continue for another half circle and stop. The stool should be behind you and you should be facing the back of the class room. Point to the back wall and point out that, while you were walking in a circle you were not rotating (you always faced the same direction). Hence, this is an irrotational flow with circular streamlines.

(3)    Rotational flow along a straight line

Start at one side of the room with a clear path across the front of the room. Roll yourself across the front wall of the room. That is, walk across the front of the room while rotating as if you were a wheel rolling along the wall. Stop a third of the way along while you are facing the back of the room and point to the back wall telling the class which way you are pointing. Continue rolling and stop a bit further on when you are facing the front wall. Point out to the class that you are now facing the front wall and must therefore have rotated. This is a rotational flow with straight streamlines.

(4)    Irrotational flow along a straight line

This is the easiest part. Simply walk across the front of the room in a straight line while always facing in the same direction. The degree of difficulty can be raised (though only slightly) by facing the students while walking sideways. This is an irrotational flow with straight streamlines.

Discussion

For each of the components of the demonstration you can give an example of such a flow. Examples might include (1) solid body rotation, (2) the bath tub vortex, (3) laminar flow in a pipe, and (4) Wind above the atmospheric boundary layer where there is negligible shear. You can also use examples from outside of fluid mechanics. For example, (1) is analogous to how the moon rotates around the earth such that we only ever see one side of the moon. Example (3) is analogous to a car tire as it drives along a flat road.

# Video of Measuring specific gravity with a U-tube manometer

Here is an animated GIF of the tube filling from the “measuring specific gravity with a U-tube manometer” demonstration. The full video is here.

Don’t try this alone, it really takes two people to do the filling.

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. If you do not wish to register with twitter or wordpress to get updates then send me an email and I will add you to the list I send update notifications to.