Physics Dept. Demonstration Manual

DF07 - Roof Model / Bernoulli's Principle

Objective:

To demonstrate that air flow (wind) over a roof can cause the roof to lift off of the building.

 
Apparatus:  
  • DF7 roof board (lucite board on which a model of a peaked roof and of a rounded roof are mounted. Each roof is mounted such that it can hinge upward from the "upwind" edge (see Figure 1)).
  • air source such as: - "Home and Shop Dust Collector" (vacuum cleaner), BCIT number 9155, with a 2.5 inch diameter hose connected to the exhaust port and a 1 inch reducer attached to the output end of the hose, or
  • laboratory compressed air outlet with a flexible hose attached.
  • lab jack

Method:

Set up the apparatus as shown in Figure 1. Turn on the air source and direct the air flow over first the peak roof model and then the rounded roof model. The air flow should start at the hinged edge. With some experimentation one can show that the rounded roof model has a slightly greater tendancy to lift. (less turbulance for the rounded model). Note that the difference is not marked, the demonstrator needs to sharpen their technique before making the class presentation. The strength of the air flow can be modified by increasing the distance between the end of the hose and the models. Further, if the lab supply with flexible hose is used, pinching the end of the hose enhances the lifting effect.

Comment: A couple of years ago the aluminum cover over my sundeck was destroyed by the effects illustrated above. Figure 2 shows the design. I happened to be outside to observe the sequence of events. First a strong gust of wind over the roof and across the canopy rotated the canopy to a vertical position. With the canopy now perpendicular to the air flow the force direction was reversed and the canopy slammed down to the
deck. Crunch!