Windage loss of flywheel

[Kajunbee]

Is windage loss greater with spoked flywheel or smooth disc of same dimensions. i have a small spoked flywheel and it appears to have considerable loss through air resistance. Could I reduce this loss by covering both sides with a smooth metal cover. Or would this increase the air resistance.

 

[FvM]

I would expect spokes to create additional windage. Flat disc should be better.

 

[ads-ee]

Just look for professional racing track bikes many of these are for velodrome and many of them have carbon fiber disks as the rear wheels and sometimes even the front wheel. I believe that having both front and rear as disks is harder to control as any wind perpendicular to the bike will cause more stability issues. Mostly you'll see only the back wheel and the front is either standard spokes or 3 or more blade "spokes".

 

[Kajunbee]

Something I wasn't expecting was the suction that the flywheel creates. I held a piece of cardboard to the side of the flywheel and it sucked it towards it. It wasn't pulling in one side and discharging out the other side. It would draw it inwards from either side. It appears that the greatest inward suction is near the hub. Is this true or is the inward draw of air the same across the whole diameter of the flywheel.

 

[ads-ee]

Air getting sucked in has to go somewhere so it's has to exit somewhere else. Maybe it leaves the flywheel closer to the outside of the spokes. My best guess would be the spokes accelerate the air to the outside as the motion of the spokes is higher the further from the hub you get, the accelerated air causes lower pressure (e.g. it's how planes can fly).

Adding a smooth disk with no gaps will likely reduce the amount of air motion and hence the amount of that suction effect you were seeing.

I don't know enough about aerodynamics to be able to help, beyond the experience with bicycles and knowing that spokes cause drag.

 

[Kajunbee]

Air getting sucked in has to go somewhere so it's has to exit somewhere else.

I don't know enough about aerodynamics to be able to help, beyond the experience with bicycles and knowing that spokes cause drag.

And I imagine I understand much less than you do. I like the idea of the bike wheel but the prices are astronomical. Yes, whatever goes in must come out. But I could not understand where the air was coming out. It seemed like there was a suction across the whole side of the wheel except at the outside the perimeter. What I did was place a short piece of tape on the end of a screwdriver. When the flag is near the center it points straight in as this seems to be the direction of airflow. As I move the flag outwards it tends to bend towards the direction of rotation. Maybe upwards also but hard to tell with such a crude instrument. Until you reach the perimeter where it bends in line with the rotation. To me it behaves somewhat like a centrifugal pump. If so this leads to my next question. Could this effect be used to create a lower pressure against the side of the wheel. And would this lower pressure correspond to less drag. I imagine two plates closely spaced to the sides of the wheel. Much the same as a pump housing.
My thought process was the way a vacuum cleaner accelerates when the suction is blocked. My wet vac draws 12 amps but if you block the suction it speeds up and it drops to 5 amps. But I'm not sure if the windage loss is any less. The load drops because it's not moving air. But is the actual windage loss any less.

 

[c_mitra]

Let us see the basics: there are certain flaws in the conventional flywheels that are not really well-designed.

At any given rotational velocity, the linear velocity near the edge (periphery) will be higher. Air coming in contact with the peripheral regions will be thrown out in a direction perpendicular to the rotation axis. This flow will take away the energy from the flywheel but cannot be avoided.

Like a centrifugal fan, the pressure will be higher near the periphery and air will be sucked in from the central region.

Spoked designs are popular because of mechanical considerations; the stress produced by the centrifugal force is not uniform and can cause failure. Modern idea is to have lighter mass and higher rotational velocity; because the energy stored is proportional to the square of the angular velocity, higher speed produces greater energy storage (that is the principal job of a flywheel).

To reduce stress, it is better to have a number of light disks stacked together; the centre one will be the largest and the outer one will be smaller in size; they can be stuck together with glue (and the edges nicely bevelled). You will end up with a flywheel that is fat at the centre but thinning out near the edges.

The airflow will still be there but the flow will not be turbulent and less energy will be lost due to friction with the air. In short, no spokes!

My suggestion: fill up the flywheel with foam (no stress for all practical purposes) and then cover the surface with a smooth foil. You may be able to reduce the loss by half!

 

[Kajunbee]

Thanks to all for the information. I decided to attempt making a housing. I needed the metal anyway to cover the wheel. So why not at least try. I did see a small reduction in current draw. At 89 vdc the current was 1.76 amps with the housing. And without it was 88.8 vdc at 1.96 amps. I don't know if this is conclusive enough but it does seem to make a difference. I wasn't able to make a proper seal on the hub so air was able to be drawn in. Maybe a will try again later. For now I will fix the plates to the wheel. If can cut the drag by half as Cmitra suggest that would be a big improvement.