The mysteries of aerodynamic lift • Hacker News Board

Re: The mysteries of aerodynamic lift

#121

Post by Plough_Jogger » Tue, Feb 04, 2020, 11:48 PM UTC

BerislavLopac wrote: post #78

munificent wrote: post #60

Earlier quoted context omitted.

> There's the big square sailed ones which always have to move downwind. Actually, that's not true. Square rigged "pirate-style" ships are more efficient when sailing downwind compared to fore-and-aft style sailboats, but they can still sail just fine close-hauled (into the wind) or on a reach (perpendicular). There are some efficiency pros and cons to both styles, but they all have the same aerodynamic capabilities.…

> The main reasons square-rigged ships fell out of fashion, as far as I know, are mostly logistically. True, but it gets more complicated than that. For fast long-distance commercial travel they were replaced by the steam and later diesel propulsion, which both a) were faster and b) required less crew (so were logistically superior). On the other hand, where sailing is still used (and that's mainly sport and leisure)…

Beyond the spinnaker, there are several yachts using a modern square rig: https://en.wikipedia.org/wiki/Maltese_Falcon_(yacht) https://en.wikipedia.org/wiki/Black_Pearl_(yacht)

Re: The mysteries of aerodynamic lift

#122

Post by BerislavLopac » Tue, Feb 04, 2020, 11:50 PM UTC

scooble wrote: post #97

BerislavLopac wrote: post #78

Earlier quoted context omitted.

> The main reasons square-rigged ships fell out of fashion, as far as I know, are mostly logistically. True, but it gets more complicated than that. For fast long-distance commercial travel they were replaced by the steam and later diesel propulsion, which both a) were faster and b) required less crew (so were logistically superior). On the other hand, where sailing is still used (and that's mainly sport and leisure)…

> preferred because it requires far less crew I think that only really applies when the sails were set from aloft. I'm familiar with square-rigged cargo vessels of around 60ft in the UK that were crewed by 2.

Yes, more modern versions of square rigs (using motors, or designed without the yards -- again, spinnakers) do have vastly reduced need for the crew. But I was focusing on traditional rigs.

Re: The mysteries of aerodynamic lift

#123

Post by knolan » Tue, Feb 04, 2020, 11:52 PM UTC

I lecture fluid mechanics.

There are actually two Bernoulli equations. Remember that Bernoulli’s work is derived for and is only valid along a streamline. A streamline is an imaginary curve that is tangent to the flow field. It’s not some invisible tube, it’s a mathematical representation of a vector field.

The first one is the well known one mentioned in the article. It relates pressure and velocity tangent to a streamline. So if the velocity is somehow increased the pressure drops.

The second equation deals with pressure changes due to curvature, i.e. force normal to a curving streamline.

On an airfoil there is an effective reduction in the area through which the air passes between the surface of the airfoil and a far field streamline unaffected by the airfoil. This increases the velocity (via the continuity equation) which in turn reduces the pressure. The curvature of the airfoil also curves the streamline close to the surface resulting in an additional pressure reduction normal to the surface. You can imagine how this affects a highly cambered airfoil.

If anyone is interested here are my lecture notes on the derivation.

https://nbviewer.jupyter.org/github/nolankucd/MEEN20010/blob...

Re: The mysteries of aerodynamic lift

#124

Post by namirez » Tue, Feb 04, 2020, 11:57 PM UTC

lutorm wrote: post #117

namirez wrote: post #101

This is a confusing take on lift. To explain lift intuitively we need two ingredients: the Laplace equation and the Kutta condition. Most people have an intuitive understanding of the Laplace equation. For example lightning usually hits the peak of mountains. The reason is that in solutions of Laplace equations, field gradient is proportional to curvature. In fluid dynamics, this field is called the stream function.…

A flat plate generates lift, though, so the curvature is incidental.

That's in interesting observation. Yes, the same thing happens at the leading edge of a flat plate due to curvature resulting in flow separation and a vortex bubble on the top surface. If the flow doesn't separate, the velocity at the front (leading) edge of the plate will be infinity.

The result is a vortex bubble over the flat surface which effectively changes its geometry and aerodynamic behavior. See figure 2-12 in the following link:

http://heli-air.net/2016/02/13/inclined-flat-plate/

Re: The mysteries of aerodynamic lift

#125

Post by mhh__ » Tue, Feb 04, 2020, 11:57 PM UTC

If you want to see something really mysterious yet mathematically modellable (ish) then look between your car and the road. Tyres are hard.

Re: The mysteries of aerodynamic lift

#126

Post by knolan » Tue, Feb 04, 2020, 11:59 PM UTC

munificent wrote: post #24

This is a somewhat confusingly written article about a famously confusing topic. It directly parallels arguments about how sailboats are able to sail. Sails are also airfoils so similar mechanics come into play. Interestingly, because a sail has effectively no thickness, both sides of the sail always have the same length, which immediately calls the Bernoulli argument into question. Sailboats are also interesting bec…

Viscosity is responsible for drag via the boundary layer and flow separation. You can absolutely account for lift with an ideal inviscid fluid, tools like Xfoil with inviscid solvers accurately match experimental data but ignore stall. It’s a pressure effect, see my other post in this thread. The viscous solvers can handle drag, separation and stall.

Re: The mysteries of aerodynamic lift

#127

Post by salawat » Wed, Feb 05, 2020, 12:01 AM UTC

luckyscs wrote: post #52

WalterBright wrote: post #38

Earlier quoted context omitted.

Take a drafting triangle that has 30-60-90 degree corners. Place it between two objects, and squeeze the triangle between them. It'll move to the side faster than the two objects move together. Or you can just think of it like squirting toothpaste. The wind pressure on the sail and the water pressure on the keel form the two "objects" being pushed together and the sailboat "squirts" out the side. Edit: The angle betw…

It's like our brains aren't meant to handle resolving that not only is it pushed through, it's sucked into a thin and ever moving void. It's pushed and pulled at the same time, in otherwords, part of the continuum.

That's exactly how I mentally model it. You have to imagine it like a stone skimming the surface of the fluid on one side, and a bubble "sucking" the other side into it.

It's two concurrent visualizations of the same force.

Re: The mysteries of aerodynamic lift

#128

Post by glofish » Wed, Feb 05, 2020, 12:29 AM UTC

Rury wrote: post #54

munificent wrote: post #24

This is a somewhat confusingly written article about a famously confusing topic. It directly parallels arguments about how sailboats are able to sail. Sails are also airfoils so similar mechanics come into play. Interestingly, because a sail has effectively no thickness, both sides of the sail always have the same length, which immediately calls the Bernoulli argument into question. Sailboats are also interesting bec…

>Interestingly, because a sail has effectively no thickness, both sides of the sail always have the same length, which immediately calls the Bernoulli argument into question Bernoulli's principle is often misunderstood. The principle itself doesn't say anything as to why different speed effects are observed around airfoils. It only states that within a steady state of fluid flow, increases in speed are associated wit…

my understanding is that Bernoulli's principle also involves equal transit times. Meaning that the same two positions need to rejoin later.

When the sail has no thickness the outer flow cannot go faster and still be a valid Bernoulli effect, because the outer and inner paths have the same distance.

That being said, there might still be another similar principle at work, though technically speaking it should not be called Bernoulli.

Re: The mysteries of aerodynamic lift

#129

Post by rtkwe » Wed, Feb 05, 2020, 12:37 AM UTC

munificent wrote: post #24

This is a somewhat confusingly written article about a famously confusing topic. It directly parallels arguments about how sailboats are able to sail. Sails are also airfoils so similar mechanics come into play. Interestingly, because a sail has effectively no thickness, both sides of the sail always have the same length, which immediately calls the Bernoulli argument into question. Sailboats are also interesting bec…

I've always wondered since learning slightly more in college physics. What is supposed to make the air going over the top of the wing suddenly go faster than the air going over the bottom of the wing in the Bernoulli model?

Re: The mysteries of aerodynamic lift

#130

Post by glofish » Wed, Feb 05, 2020, 12:39 AM UTC

mike_ivanov wrote: post #109

phdp wrote: post #98

Earlier quoted context omitted.

A solar sail doesn't work via aerodynamic lift. It works on conservation of momentum. Also, the biggest contributor to force on a solar sail is not solar wind, but radiation.

Right, radiation -- thank you. Nevertheless, you think that conservation of momentum is not the ultimate source of aerodynamic lift? It's not an electromagnetic phenomenon, obviously, neither gravitational -- so it has to be mechanical. Where that energy is otherwise coming from? Or are you claiming that aerodynamic lift is a fundamental force?

conservation of momentum and conservation of energy both take place at the same time, but are independent concepts altogether

momentum has nothing to do with energy and vice versa.

For example, energy can be stored (transformed into different forms, short or long term) and released later, momentum cannot.