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.
- A fan
- A shoe box
- A trash bag or plastic shopping bag
- 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.
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
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.
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