Keith Ross, of Taumaranui, asks :-

At the gym my excercycle has a mesh cage over the fan on the wheel which cools the rider. In cooler weather I put my towel over the cage to stop the draught. It is a lot less effort when the cage is covered by the towel. Why is this?

Reg Dunlop, a UniServices research engineer at the University of Auckland, responded.

The fan throws out a lot of air and brings in air to replace that lost. The replacement air is accelerated before being thrown out and that requires some force. If the fan cage is totally enclosed, then the air inside cannot be thrown out and is soon moving at the speed of the fan and not much force is required to keep it moving.

Partially covering the fan cage with a towel is an intermediate condition where not all of the air is being thrown out, and hence less force is required than for the unshrouded fan.