So we know that one of the advantages of a disc is lower power to rotate because you do not have spokes creating rotational drag as they spin. But I have seen no analysis of the difference between a solid disc like a Zipp Super 9 vs. a Hed Jet Disc which has spokes under what is essentially a very fancy carbon disc cover. I would suspect that the power to rotate would be less than a Jet 9 or 808, but more than a solid disc because:
There is always 0 yaw inside the wheel (I assume - seems logical), and
You do not have the component of wind speed from your forward motion adding to rotational speed… though this would be somewhat balanced out by the increased apparent wind speed at the spokes on top vs. the decreased apparent wind speed at the spokes on the bottom in a normal wheel… so maybe not a huge benefit
But it is unlikely that the air inside the wheel or under the cover moves in perfect harmony with the spokes. So there must be some disadvantage in power to rotate, right? But how big is the difference? Anyone have any idea?
There’s no airflow flux going into the inside of a disc wheel. Therefore, no external force isenacting upon the disc spokes to create resistance to rotation. The air at the top of the rotation inside the disc pushes the opposite direction of the air at the bottom of the inside of the disc causing an equal and cancelling force.
The drag on exposed spoke’s comes from external air passing through the wheel. The Flux of the air external to the system is what imparts the drag force.
Plus, I think the internal air would quickly begin to rotate at the same speed as the wheel. Wouldn’t take long.
Not sure what you mean by “air flow flux”. Sounds redundant.
The air in there will definitely be moving in relationship to the spokes. The air molecules cannot be made to just travel in circles without being constantly acted upon by an outside force. Think Newton’s Laws of Motion. That outside force is supplied by the spokes, the “skin” of the wheel and the rim. You will likely build up higher pressures inside the wheel near the rim and lower near the hub (the more I think about this pressure gradient the less sure I am about it). But exactly how air will move around in there I’m not 100% sure.
I am 100% sure that the air pushing on the spokes at the top of the wheel inside the disc will be pushing against the direction of rotation, as will the air at the bottom of the inside of the disc, so both forces serve to slow the wheel down, not cancel each other out.
There’s no flux into the “system”. It’s all internal to the interior of the disc. There may be some small amount of resistance to rotation at the very beginning of rotation, but once it’s all moving, the air will be close to the rotation of the wheel.
But air molecules will not just move in a circular motion without a continuous force being applied to them. They can continue to move in a straight line once momentum is imparted to them, but not in a circle.
And people wonder why triathlete are perceived as minutia-focused, data-focused dweebs…
/pink
Oh I fit that description perfectly! It does not mean that it doesn’t matter! Actually, that’s the answer that I want… is the effect big enough to matter?
There’s no flux into the “system”. It’s all internal to the interior of the disc. There may be some small amount of resistance to rotation at the very beginning of rotation, but once it’s all moving, the air will be close to the rotation of the wheel.This is correct.
But this made me realize that taping the wheel covers might be slightly faster than not taping them…
There’s no flux into the “system”. It’s all internal to the interior of the disc. There may be some small amount of resistance to rotation at the very beginning of rotation, but once it’s all moving, the air will be close to the rotation of the wheel.This is correct.
But this made me realize that taping the wheel covers might be slightly faster than not taping them…
Think of it this way: A wheel with a disc cover or a Hed Jet Disc is like the resistance unit of a water rower
No water is moving in or out, just like no air is moving in or out of the wheel cover. The spinning paddles are like spokes - just designed to create more drag, not less.
But air molecules will not just move in a circular motion without a continuous force being applied to them. They can continue to move in a straight line once momentum is imparted to them, but not in a circle.This is correct.
It doesn’t mean that there is a loss of energy though, and therefore no extra ‘aero drag’.
Force is not the same as power… read up on your Newtonian.
But air molecules will not just move in a circular motion without a continuous force being applied to them. They can continue to move in a straight line once momentum is imparted to them, but not in a circle.This is correct.
It doesn’t mean that there is a loss of energy though, and therefore no extra ‘aero drag’.
Force is not the same as power… read up on your Newtonian.
Uhh… Power=Work/Time = (Force * Distance)/Time = Force * Velocity
But the boundary layer of the stationary outside acts to stop rotation while a turbine tries to start it. Opposite forces that require force. In the wheel scenario, the whole system rotates. Think of the whole fluid resistance unit rotating, you wouldn’t get much of a resistance workout on that case.
No water is moving in or out, just like no air is moving in or out of the wheel cover. The spinning paddles are like spokes - just designed to create more drag, not less.
Clearly this system creates drag.
Clearly you don’t understand Newtonian physics or basic fluid mechanics.
The brake unit on the Waterrower has spinning paddles in a stationary housing.
Imagine a spinning housing with stationary to the housing paddles. No resistance (after a quick spin-up), stationary to the housing water, but spinning water to the ouside observer.
No water is moving in or out, just like no air is moving in or out of the wheel cover. The spinning paddles are like spokes - just designed to create more drag, not less.
Clearly this system creates drag.
**Clearly you don’t understand Newtonian physics or basic fluid mechanics. **
The brake unit on the Waterrower has spinning paddles in a stationary housing.
Imagine a spinning housing with stationary to the housing paddles. No resistance (after a quick spin-up), stationary to the housing water, but spinning water to the ouside observer.
No water is moving in or out, just like no air is moving in or out of the wheel cover. The spinning paddles are like spokes - just designed to create more drag, not less.
Clearly this system creates drag.
Clearly you don’t understand Newtonian physics or basic fluid mechanics.
The brake unit on the Waterrower has spinning paddles in a stationary housing.
Imagine a spinning housing with stationary to the housing paddles. No resistance (after a quick spin-up), stationary to the housing water, but spinning water to the ouside observer.
Perhaps you end up with the rim providing centripetal force to the air molecules - a pressure gradient forms to provide centripetal force to air molecules further toward the center of the wheel - this provide for circular motion. Then perhaps only a minor force is required to keep the angular velocity of the air molecules moving. But the power consumption will not be zero. It has some quantity. I’m curious as to what that is.
But air molecules will not just move in a circular motion without a continuous force being applied to them. They can continue to move in a straight line once momentum is imparted to them, but not in a circle.This is correct.
It doesn’t mean that there is a loss of energy though, and therefore no extra ‘aero drag’.
Force is not the same as power… read up on your Newtonian.
Uhh… Power=Work/Time = (Force * Distance)/Time = Force * VelocityI will say it again, clearly you don’t understand Newtonian physics…
“In physics, a force is said to do work if, when acting on a body, there is a displacement of the point of application in the direction of the force.”
Your air molecules move perpendicular to the centripetal forces acting on them. But you knew that…
And if my above postulation is correct, to require continued force to keep the air molecules angular velocity up, there needs to be a drag force to slow them down. During accelerations and decelerations, those forces will exist, but at steady state, they could be small enough to be negligible. So the rim of the wheel is providing the force required to impart circular motion, but since the radius of the wheel remains constant, the power requirement for that is 0. So the power to rotate a well-sealed wheel cover could well be about the same as a solid disc.
There’s no flux into the “system”. It’s all internal to the interior of the disc. There may be some small amount of resistance to rotation at the very beginning of rotation, but once it’s all moving, the air will be close to the rotation of the wheel.
I second this solution, and for visual solutions search videos for “candles in centrifuge”. Since those videos demonstrate density distributions, the relevant take-away for this conversation is noticing that the flames have no tangential component - not that the flow is heading towards the axis.
The walls impose zero relative speed, and longitudinal acceleration is temporary. A centrifuge with a superimposed additonal linear velocity will behave identically to a static one. So the steady state solution is that the flow spins with no relative motion to the boundaries, as a solid would, but it would have shear with itself (vorticity will not be zero).
TL;DR don’t worry about it, much as you don’t worry about the air in your inner tubes.
But air molecules will not just move in a circular motion without a continuous force being applied to them. They can continue to move in a straight line once momentum is imparted to them, but not in a circle.This is correct.
It doesn’t mean that there is a loss of energy though, and therefore no extra ‘aero drag’.
Force is not the same as power… read up on your Newtonian.
Uhh… Power=Work/Time = (Force * Distance)/Time = Force * VelocityI will say it again, clearly you don’t understand Newtonian physics…
“In physics, a force is said to do work if, when acting on a body, there is a displacement of the point of application in the direction of the force.”
Your air molecules move perpendicular to the centripetal forces acting on them. But you knew that…
I’m sorry but name one single thing I typed in the quotes above that is incorrect with respect to Newtonian physics?
I’m waiting.
And actually, the key to there being low/no extra power to rotate is precisely that the air molecules do NOT move in a direction perpendicular to the centripetal force. They are in equilibrium. Otherwise work would be done.
There’s no flux into the “system”. It’s all internal to the interior of the disc. There may be some small amount of resistance to rotation at the very beginning of rotation, but once it’s all moving, the air will be close to the rotation of the wheel.
Maybe a pressure gradient builds up. Then if the seal is imperfect at both ends of the gradient you’d have an induced airflow from hub to rim. That would absorb some energy from the system.
If the seal is perfect then you’d just have higher density air near the rim, and I agree, the skin friction of the system would dominate the very low mass fluid.
Just my guess. Sometimes reality confounds theory.