# Vortex flow from a fan

1. Nov 14, 2004

### lucy_b14

I am doing an investigation into the air flow from a fan in my physics class. I need to know why the reason why air flows in a vortex shape from the fan (in physics terms), and what equations I can investigate (anything, e.g thrust etc).
Also, I believe the airflow from a fan can be straightened using airfoils. Could anyone tell me how this works?

Thanks!

2. Nov 15, 2004

### Clausius2

Do you mean behind or beyond the fan? The flow behind the fan is not necessary vortex shaped. One can lay out a blade design such that the air swirl is eliminated. Upstream the fan, the flow starts to be vortex shaped due to the proper rotation of the blades. But the very small dynamic viscosity of the air makes almost imperceptible that shape.

With the airfoils maybe you are referring to the special design I mentioned above in order to eliminate swirl. The swirl behind a fan enhances noise and turbulence. So that, designers usually employ some special configurations of the blades or add stator blades (airfoils as you named) to deflect into a straight flow the stream. Try to consult some book of Turbomachinery, where you will see the blade schemes for swirl elimination.

3. Nov 18, 2004

### Astronuc

Staff Emeritus
The fan blades are rotating and due to friction with the air, they will impart a rotational component to the flow, as well as an axial flow parallel to the axis of rotation. Think of momentum transfer.

Airfoils and ducts can reduce rotational flow, but as the air leaves the fan, the rotational flow will interact with the static air around it and the rotational flow will dissipate. Think of shear forces.