General > General Technical Chat
Mess with your minds: A wind powered craft going faster than a tail wind speed.
electrodacus:
--- Quote from: bdunham7 on August 31, 2021, 06:10:19 pm ---
--- Quote from: electrodacus on August 31, 2021, 06:06:13 pm ---I know it may look non intuitive to you but please think more about this or do a test.
--- End quote ---
It's not about intuition, it's about logic and math. In each case the two surfaces are moving at the same rate in the same direction. What other characteristic could that surface have that would change how it interacts with the car?
--- End quote ---
The paper moves relative to the table while the treadmill is not moving relative to the table.
The vehicle rolls on to the paper while the vehicle will be pushed back by the treadmill.
Not sure how to better describe this but if something else come to mind I will make sure to let you know.
Question will be: If you will understand the large difference between the sliding paper and a treadmill will this do anything to convince you that blackbird uses stored energy to exceed wind speed ?
Alex Eisenhut:
I am but a humble amputated brain ape, but what on Earth do you mean by "The paper moves relative to the table while the treadmill is not moving relative to the table"?
Obviously the paper is the belt in the treadmill? Would you prefer there to be another sheet of paper between the table and the first sheet, moving in the opposite direction?
bdunham7:
--- Quote from: electrodacus on August 31, 2021, 06:27:53 pm ---The paper moves relative to the table while the treadmill is not moving relative to the table.
--- End quote ---
Does the top surface of the treadmill not move relative to the table? If not, at what point in the 5 steps I outlined above does this change occur?
electrodacus:
--- Quote from: Alex Eisenhut on August 31, 2021, 06:50:15 pm ---I am but a humble amputated brain ape, but what on Earth do you mean by "The paper moves relative to the table while the treadmill is not moving relative to the table"?
Obviously the paper is the belt in the treadmill? Would you prefer there to be another sheet of paper between the table and the first sheet, moving in the opposite direction?
--- End quote ---
I'm sorry I do not have the ability to properly explain this but that dragged paper will not have the same effect on vehicle as a treadmill.
My analogy was if you power OFF the treadmill so the belt of the treadmill is not moving and then push the treadmill towards the vehicle then that will be analog to the dragged paper. But the fixed powered ON treadmill will not be the same with powered OFF treadmill that is pushed.
What actually happens is that in case of my diagram the treadmill represents the road moving underneath the generator wheels while vehicle is stationary the vehicle and road where switched and then the ground can represent the wind at same speed a vehicle speed so zero.
Now once you power off the treadmill so no rotation (basically no longer a treadmill) and you drag that from right to left then this becomes the wind (input energy source) and the ground is the road so this will be a vehicle powered from the wind (moving paper or turned OFF treadmill) traveling in the opposite direction to the wind direction at lower speed.
So basically everything is completely changed when you decide to replace a treadmill with a piece of paper.
IanB:
--- Quote from: dunkemhigh on August 31, 2021, 12:17:51 pm ---Now, I accept that the terms being used are incorrect, in that 'suction' is perhaps confusing in a literal sense. However, from my point of view there is high pressure across the top of that pipe - if a sheet of something was placed there, at right angles to the flow, it would be pushed away from the hose outlet. Now, you might say that this is merely air flowing, but if it is flowing then it is being pushed against something. Either the sheet of whatever or just ambient air - there is a restriction against which the flowing air is being pushed, and that must surely be increasing the pressure. A fan is taking air from one side and pushing it to the other despite there already being air there, so the pressure must increase.
--- End quote ---
There is a directional component when you measure the pressure of a flowing air stream. If you measure the pressure head-on, you will see a higher pressure than if you measure the pressure at right angles. The flowing air has mass and momentum, so when you measure the pressure head-on it "pushes" against the measurement device and increases the effective pressure. The following Wikipedia article talks about this:
https://en.wikipedia.org/wiki/Pitot_tube
This is the reason that low pressure air can flow out of the vacuum cleaner against the higher pressure of the surrounding atmosphere. The static pressure may be lower than the surroundings, but the total pressure (including the effect of velocity and kinetic energy) is higher.
The key equation is this one:
$$p_t = p_s + \frac{\rho u^2}{2}$$
Along a stream tube, \$p_t\$ doesn't change, so whenever \$u\$ (velocity) goes up, \$p_s\$ goes down.
Of course, when the air passes through the compressor or blower, then energy is added to the system, so here \$p_t\$ is increased. But a typical blower changes the air velocity much more than the air pressure.
--- Quote ---The way I understood there being an area of low pressure, which the air in the pipe will flow to fill, is because the air moving across the end of the pipe drags molecules of air at the pipe entrance along with the flow.
--- End quote ---
This is a real effect, but it is relevant only in special kinds of vacuum pump. It is not applicable here.
--- Quote ---So I am suggesting that the chap is measuring an artifact caused by the act of measuring. Instead of the pipe, what if he put, say, a flap of paper in the airstream. Would that bend toward the blades? I doubt it. But if he placed the paper edge-on, would it be pulled into the stream? It likely would (if it could retain it's shape) but only because the surface of the paper is causing the low pressure that pulls it in.
--- End quote ---
The paper experiment is a good one to do. Get two sheets of paper and hold them in a V shape around the air jet from the vacuum nozzle (blow the air into the V shape between the parallel sheets of paper). You will see the two sheets of paper get squeezed tightly together. The harder you blow, the more tightly the paper will get squeezed. This is another way of demonstrating there is a vacuum in the space between the two sheets of paper. The dragging of air molecules cannot apply here, as the paper does not have any free molecules to get dragged.
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