yoavraz said:
Forget about the wind. The only place you have a (external) wind in a DWFTTW on a treadmill is in the name DWFTTW (the W's)... I thought we have cleared this.
Wind: the cart needs a source of power to work. It uses the wind as that source. The amount of power available from the wind is measured by the wind speed. That always assumes (and rightly so) that the ground is stationary.
Yoavraz, let's say we want to check a wind turbine to see how much power it can harness from a 10 mph wind. We mount the wind turbine on a trailer and drive around until we find a place that is experiencing a 10 mph wind. We stop, point the wind turbine into the wind and start taking our measurements. Everything is good, we get our measurements before the wind goes calm. We pack up but forget to secure the wind turbine. As we get to 10 mph in the still air, we notice that the wind turbine is turning. For the fun of it, we again hook up our test equipment and find that when the wind turbine is being towed through the air at 10 mph, it acts exactly the same as when it is sitting still in a 10 mph wind. Therefore, from now on we decide that instead of wasting time looking for the right wind, we generate our own wind by moving the ground reference at 10 mph.
The wind turbine doesn't know the difference. When the air is still and the ground is moving, the wind turbine behaves exactly the same way as when the ground is still and the air is moving. Those are equivalent frames of reference, and both are perfectly satisfactory for testing. According to the wind turbine and the output that is measured, the air is flowing past the wind turbine. To the wind turbine, that is wind. To the cart, air flowing past is also wind and generates the same results. You can call it fake wind (most would call it relative wind which is the correct term - look it up) but the results are the same as a wind moving across the ground.
The treadmill allows us to "tow" the cart along the ground at 10 mph. If we had a really long treadmill that was moving at 10 mph, we could step onto the treadmill surface holding our cart, set it down facing "downwind" and let it go.
What are the possible outcomes of this?
1: The cart doesn't move.
2: The cart moves and reaches a speed of less than 10 mph in reference to the treadmill surface
3: The cart moves and reaches 10 mph in reference to the treadmill surface.
4: The cart moves and reaches a speed above 10 mph in reference to the treadmill surface
Since we only have a short treadmill (I hope that you now accept that the treadmill is a valid substitute for a wind - if not read the frames of reference again and ask questions), we can only test a short portion of the full scale outdoor test. That segment is with the cart at 10 mph, with only a few feet either way to indicate a trend. If we place the cart on the treadmill surface moving at 10 mph, the outcomes that I listed above would look like this:
1: The cart would quickly move to the back of the treadmill when it is released
2: The cart would slowly move to the back of the treadmill when released (may be hard to see the difference between this and #1 but in either case this would be a failure)
3: The cart would stay in position on the treadmill (this would be downwind at wind speed)
4: The cart would move forward on the treadmill ( this would indicate moving directly downwind faster than the wind)