The 20p Question Thread

[Edwardoka]

If you put a generating turbine on the front of your bike you can use it to power a fan on the back to give you a nice tailwind.

 

[stephec]

If you put a generating turbine on the front of your bike you can use it to power a fan on the back to give you a nice tailwind.

The faster you go the more power you'll generate, so you'll get a bigger tail wind, then you'll go faster.

Then you'll generate more power so you'll get a bigger tailwind, so you'll go faster.

Does that mean you'll keep accelerating until you reach infinity?

 

[lazybloke]

Another question....does a fast cyclist get wetter or less wet in the rain than a slow cyclist when cycling from A to B?

I saw a similar question modelled by a Maths Prof during an Open Day at Kingston Poly in the 80s.
Something like, "it's pouring with rain, at what speed should you run to stay driest?"

What followed was two hours of complete tedium that didn't even give a single concise answer.
I find it best to carry a brolly.

 

[Edwardoka]

If a wind turbine takes energy from the wind to convert into electrickery, does that wind energy diminish or is it free?
If it diminishes could we build lots and lots and not have a headwind when we ride our bikes ?

Another thought, If the turbines steal energy from the wind does extra wind come in fast to make up for the hole the turbine has made making the place even windier? If so we should have a campaign of wind turbine destruction. "Cyclists against the wind". Join now.

I'm not remotely an aerodynamics expert, but since this thread could correctly be renamed "Ed doesn't know what he's talking about but has a go anyway", why not give it a shot? Ladies and gentlemen, start your "well, actually" posts now!

Again, without googling, because any fool can look it up on wikipedia and go ah yes, vortex shedding!

Question 1.

Wind is just another name for air with lots of kinetic energy, and when it hits a turbine's blade, it causes the turbine to spin, so yes, some of the kinetic energy of the moving air is converted into kinetic energy in the turbine (and thus into electrotrickery), and some will be lost to noise and heat, too.

So yes, you could theoretically build enough wind turbines (although this would require a new design that allows ~200 mile tall towers to be built and with turbines stacked on top of turbines) to suck all of the kinetic energy out of the air entirely, and you'd need them placed over the entire planet's surface to prevent any new large-scale wind patterns getting up to speed which would be correctly regarded as a Bad Move With Quite a Lot Of Issues.

Your second question is a lot more complicated, but the answer is no.

You're right that there's a "hole" behind the turbine; like aeroplane wings, the turbine blades are aerofoils which create pressure differentials, with high pressure in front and low pressure behind. Like in a plane, the pressure differential generates lift, which is actually what spins the blades.

However, the way that the air rushes in to fill the hole is difficult to describe without analogy.
I've tried writing explanations four times already but can't find the right combination.
- Water from a tap hitting a sink and then spreading out in a sheet, climbing up the side of the sink before falling back, creating a turbulent ridge where the water doesn't have enough energy to climb higher, but which can't fall back down because more water has already come to replace it)
- Being passed by a fast truck, getting buffeted initially and then pulled towards it after it's passed
- Riding behind another cyclist
- Planes

Turbine aerofoils are twisted not only to allow the turbine to catch wind from different directions but also to allow the air which has collided and is now much slower to move off in a specific direction. If they were shaped like aircraft wings you'd get wingtip vortices as you do in planes (if you imagine surf tubes, but with air instead of water) which would cause a lot of noise and vibration as they collapse. Wingtip vortices are not an issue in planes because the plane has already moved on by the time the vortex collapses, but a wind turbine doesn't have that luxury.

Does any of this actually answer the question? No.
Locally there will be a lot more buffeting in the area around the turbines, because the air is piling up in front and spilling over behind and going in all different directions, but there will be less kinetic energy in the air.

Also, the sheer mass of air in the atmosphere is truly enormous, as is the air's kinetic energy in on a windy day.
Turbines only scrape a bit off the bottom of the mass of air, so although it's messy on the ground, a couple of hundred feet up it's plain sailing, and you wouldn't have to go far downwind to find that the mass of moving air will have re-asserted its dominance over the turbulent air.

The real question is: do all the fans people are running while doing zwift convert electrotrickery back into wind?

Oh god it's midnight I've been writing this for 2 hours

 

[Edwardoka]

I saw a similar question modelled by a Maths Prof during an Open Day at Kingston Poly in the 80s.
Something like, "it's pouring with rain, at what speed should you run to stay driest?"

What followed was two hours of complete tedium that didn't even give a single concise answer.
I find it best to carry a brolly.

I feel personally attacked.

En garde!

 

[Mo1959]

Does that mean you'll keep accelerating until you reach infinity?

..........and beyond!

 

[newfhouse]

Does that mean you'll keep accelerating until you reach infinity?

No, you’d use your brakes. They might glow a bit.

 

[Ming the Merciless]

No, you’d use your brakes. They might glow a bit.

Ah but you’d reverse the polarity of the turbines creating an air brake.

 

[BoldonLad]

In a similar vane ( ) to the wind and wind turbine question, if you generate electricity using energy from the sun (ie sunlight), does than mean some energy, which would normally be "heat" on the surface of the earth is diminished, so, a degree of global cooling occurs?

 

[glasgowcyclist]

reach infinity

Reach infinity??

 

[Archie_tect]

... tend towards infinity but never quite get there...

If we had dynamos on both wheels creating 'free' electro-magnetic energy to directly power a rear electrically powered propellor....

 

[matticus]

If over time you replace parts on a car, at what point does it stop being the same car you bought? How many parts do you need to replace to make it a new car?

Boring answer:
The kit-car industry has already addressed this. If you take a shi ... rubbish old car with an old, cute number plate, then rebuild with a kit;
- if you replace too many parts (because you chose a kit to make it look like an AC Cobra, say) you have to re-register it with DVLA (thus losing old plate)
- there is a "system" that says a gearbox counts for 10%, engine for 20, panels for ... you get the idea.

 

[matticus]

It would basically act like a pendulum - and for broadly the same reasons - albeit in a straight line rather than through an arc?

Fun Fact - if you drill through to ANY other point on the surface, you will get this oscillation, and the frequency is the same as the hole-right-through-the-centre. (i.e. you'd reach higher, more fun speeds going thru the middle)

 

[Edwardoka]

Fun Fact - if you drill through to ANY other point on the surface, you will get this oscillation, and the frequency is the same as the hole-right-through-the-centre. (i.e. you'd reach higher, more fun speeds going thru the middle)

Instructions unclear: dug two holes in the garden and connected them at the bottom. Now I can't get back out. Send help.

If you dig in a STRAIGHT line to any other point on the surface at the same elevation then yes, although "Frictionless" is doing a lot of heavy lifting, in my answer that meant air friction, but yeah, you can sort of see the effect with skateboarders on half-pipes.

 

[Ming the Merciless]

Does that mean you'll keep accelerating until you reach infinity?

The question is, which infinity would you reach since there are more than one.