Noise: Air Turbulence
Blade noise occurs because of vortices, swirling air commonly known as turbulence. Vortices occur along the length of the fan blade and at the blade tip as the blade moves through the air. Increasing the speed and pitch of the blades will correspondingly increase noise levels due to the increased vortex intensity.
The key to noise reduction is better blade design and construction, however given the cost constraints of supplying a product that is functional and affordable, fan blades are generally made of wood or pressed metal. Of these two options, the wooden blades perform better through a wider range of speeds than their metal alternatives. This is due to the thicker rounded leading edge of the wooden blade being more efficient at reducing the blade’s leading edge vortices.
What are vortices?
Blade vortices can be broken into two categories: leading edge vortices that initiate along the blade’s front edge due to blades’ shape and angle of attack; and tip vortices from the air that spirals off the end of the blade.
Leading edge vortices occur on the upper face just behind the leading edge in turbulent flow (assumes normal downdraft operation). Blade leading edge vortices occur because the low pressure created above the blade from its movement through the air is being filled by the higher ambient pressure air above it moving in to fill the depression. At low operating speeds and low blade pitch angles the pressure differential is not enough to accelerate the air fast enough to hear it, but as the fan is turned up, so too the pressure differential and associated air noise increase.
Tip vortices are created as the high pressure below the blade (in normal downdraft operation) tries to escape around the blade tip to the low pressure created above it. This air movement, combined with the blade’s movement through the slow moving air around it, creates a spiralling effect of the air spilling off the tip of the blade. The centre of this spiral is low pressure and as the higher pressure around it accelerates in to fill the vacuum the rushing air creates the crackling noise you hear.
What can be done?
There are several ways to reduce blade tip noise, assuming we are trying to sustain the volumetric flow rate of the fan.
- Smooth and sculpt fan blades for minimum turbulence by avoiding rattan style blades with rough surfaces or blades with designer sculpted holes and irregularities.
- Increase blade size and reduce blade velocity to reduce the veracity of low pressure air in the vortices.
- Design and manufacture specialised aerodynamic blades for specific fan speeds and air movement requirements similar to aircraft wings, thus reducing the leading edge and blade tip vortex effects.
Why aren’t blades designed better?
Some of them are; the Velo ceiling fan has an aerodynamic blade design that produces more air flow with reduced noise, but in the end it is more costly to produce these advanced blades. Pressed metal and wooden blades are the norm because these are cost effective ways to produce a blade that is elegant and yet still functional.
The standard metal and wooden blades could be sculpted and shaped to perform at an optimum efficiency at a specific blade speed, but as most fans have variable speeds, it is the thicker wooden blade with rounded leading edge that succeeds in reducing the vortices the most at these variable speeds.
If you would like more information on the causes and solutions of ceiling fan noise or the please contact us. The Spinifex team includes a yacht design guru who spent more then 10 years in Europe and the US designing racing saling yachts and he is always happy to discuss the physics and engineering of what we do.