of waters and winds

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of waters and winds

Post by nikos » 2021 Mar 07, 07:28

here's the comment area for today's blog post found at
https://www.zabkat.com/blog/wind-power-riddles.htm

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Re: of waters and winds

Post by johngalt » 2021 Mar 07, 17:36

Interesting blog. I like it.

A couple of points that intrigue me:
If wind speed is the average speed of the individual particles, how exactly can it be measured independently from the number of said air molecules?
An average is just that - an average. An average of 5 pts of data and an average of 50,000 pts of data is still just the average of them. Averages don't take into account the number of points of data, other than in the calculation, because it is a mathematical calculation taken across all points of data being available (until you start to bring in statistics and remove outliers and whatnot).

A better illustration of this is from this article: https://www.thoughtco.com/definition-of ... p2-2312349
Like the median and the mode, the average is a measure of central tendency, meaning it reflects a typical value in a given set. Averages are used quite regularly to determine final grades over a term or semester. Averages are also used as measures of performance. For example, batting averages express how frequently a baseball player hits when they are up to bat. Gas mileage expresses how far a vehicle will typically travel on a gallon of fuel.

In its most colloquial sense, average refers to whatever is considered common or typical.
So, the concept of central tendency comes into play and is the focus of an average (aka the mathematical mean).

So, is the wind speed the average of speed of any given particle (sum of all the speeds it exhibits over the time that it was observed) or the sum of all the speeds of *all* particles divided by the number of particles? Or, is it more complex - maybe the average wind speed is the average of all the average speeds of every particle? Food for thought.

Now, as to your demonstration of the cup anemometer - in *my* understanding, the force exerted by the wind is, indeed, on both cups (in a 2 cup system like you've drawn) but the physics of it comes into play. The concave cup will redirect the *force* of the air pushing against the interior concave surface in directions other than the original direction - again, *my* understanding is not all gets directed perfectly back in the exact opposite direction, but enough does that it causes the cup to move. By contrast, the convex surface of the other cup also causes redirection of force - but almost none of it is 'reflected' back in the exact opposite direction. Hence, that convex size has an imbalanced force. Both sides experience the force of the wind in the direction of the arrows that you've drawn - but only one of the also experiences a reflection of the force of the air back against the original direction of force. Therein lies the imbalance - thus the cause of the rotation. I'm sure there is a lot I'm glossing over, but that is the gist of how I understand it.

So - if it is force, versus velocity, is it really speed?
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Re: of waters and winds

Post by nikos » 2021 Mar 07, 18:10

the orientation of the "wind cups" is such that no matter the wind direction, the anemometer turns the same way. We really need a fluid dynamics PhD to tell us the real story

I asked this question before in yahoo answers where a "know it all" guy came up with a quick answer but on further probing he remained silent, which shows that he doesn't know. Typical "expert" :)

sometimes the simplest questions are the hardest to answer

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Re: of waters and winds

Post by otlaolap » 2021 Mar 08, 00:45

Not a physics person, me, but I had thought that the reason that sails work is not that air is blowing against them, pushing the boat along, but that air is flowing around them from one side, creating an airfoil, and the vacuum on the leeward side is sucking the boat along. If one is wondering, as the blog might be, about relative differences in density in winter and summer, and the number of particles banging in to the sail and the energy they have as they bang into the sails, then I get confused and am lost at sea. I suppose one could image a boat that is square-rigged (if that is the right term -- essentially flat panels for sails) sailing exactly downwind and then think about pellets of air. But that someone would be someone else.

Anyway, this user of file explorer software of a superlative kind enjoys the blogs.

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Re: of waters and winds

Post by dunno » 2021 Mar 08, 04:17

A sail functions exactly like a aircraft wing but vertical and floppy, same dynamics and formula apply. water is just more dense than air hence the drag coefficients are vastly different but the principles are the same.

A aircraft's Air Speed Indicator measures "Indicated Air Speed" this is the number of molecules that a aircraft or wing is flying through, this is vital as a aircraft's stall speed is predicated by the number of air molecules flowing over the wing.

The Instruments which do the measuring/sampling of air molecules are, Pitot probes, Static ports, and Ambient Temperature sensors, this air/analogue data is sent to the CADC's Central Air Data Computers which then calculate the following. TAS - True air speed, CAS - Calibrated Air speed, IAS - Indicated Air speed, and Ground Speed. Ground speed is calculated by the navigation computers using Trigonometry, heading, track, drift angle etc.

At high altitudes where air is thin, i.e. low density of air molecules, the Indicated Air Speed is vastly different to the TAS True Air Speed of the aircraft. True air speed will equal Ground speed in zero wind conditions and in perfect 90 degree cross winds.

There's also Mach Number which is used in high speed aircraft civilian and military. Mach number can be mathematically derived given TAS and local temperature.

A good book for the basics of aerodynamics is A. C. Kermode's The Mechanics Of Flight.

Local winds is a classic study of thermodynamics with the Sun, earths rotation, land and sea temperatures being the primary drivers as it entails the movements of a gas/air subjected to many variables, heat, Coriolis force, local topographical features, High and low pressure systems, hadley cells, so many variables that the worlds largest super computers are employed in trying to solve these thermodynamic problems, and they're still chugging away with no end in sight calculating trying to predict the movement of air.

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Re: of waters and winds

Post by nikos » 2021 Mar 08, 06:25

dunno are you a pilot? Say for example an aircraft is flying at 500 km/hr (ground relative speed) and there's no wind other than the one generated by the plane's motion. Is the lift felt on the wings temperature (density) related?

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Re: of waters and winds

Post by pj » 2021 Mar 08, 15:51

dunno wrote:
2021 Mar 08, 04:17
A sail functions exactly like a aircraft wing but vertical and floppy, same dynamics and formula apply. water is just more dense than air hence the drag coefficients are vastly different but the principles are the same. ...
Sorry but sails are driven by direct pressure, not via Bernoulli’s Principle. Anyone that's hung sheets on a line knows these forces very well.

-----------------------
PJ in (windy) FL

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Re: of waters and winds

Post by nikos » 2021 Mar 08, 16:55

what direct pressure? unless you go straight downwind, the spoon effect is what moves a boat

however my origin is kitesurf where the kite gets more or less blown by the wind in a brute force fashion rather than aerodynamics

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Re: of waters and winds

Post by johngalt » 2021 Mar 08, 18:47

nikos wrote:
2021 Mar 07, 18:10
the orientation of the "wind cups" is such that no matter the wind direction, the anemometer turns the same way. We really need a fluid dynamics PhD to tell us the real story

I asked this question before in yahoo answers where a "know it all" guy came up with a quick answer but on further probing he remained silent, which shows that he doesn't know. Typical "expert" :)

sometimes the simplest questions are the hardest to answer
Correct - when you use actual anemometers (aka 3 or 4 cups, NOT 2 lol).

There are a few videos out about Pelton Wheel Turbines, and one of them, by Tom Stanton, really shows the fluid dynamics of a very small one that he used in conjunction with his air powered device series of videos:

https://www.youtube.com/watch?v=zXEnXEfVBA8

After seeing that video a month or so ago, I had a better understanding about the fluid dynamics in those little buckets, and I extrapolated the idea to the hemispherical cups in the anemometer.

And then I find this extremely short video which *seems* to be supporting of what I thought might be happening:

https://www.youtube.com/watch?v=m6RV6POHo0Q

But, that also led me to this video:

https://www.youtube.com/watch?v=BhBariUDwP4

which addresses one of your concerns directly.
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Re: of waters and winds

Post by nikos » 2021 Mar 08, 18:54

you need a phd to follow this guy's arguments... and it's too late at night over here :)

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Re: of waters and winds

Post by johngalt » 2021 Mar 08, 19:26

Lol. Since I only have a Masters....ah, well.

Sleep well.
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Re: of waters and winds

Post by dunno » 2021 Mar 09, 04:32

nikos wrote:
2021 Mar 08, 06:25
dunno are you a pilot?
Yes I had three qualifications in aviation, Aviation Technician, Flight Engineer, and a Pilot. (retired now).
As a Aviation Technician I was certified to work on any Aircraft weighing more than 5700 kg's, my speciality was the B747 series Classic aircraft.
nikos wrote:
2021 Mar 08, 06:25
Say for example an aircraft is flying at 500 km/hr (ground relative speed) and there's no wind other than the one generated by the plane's motion. Is the lift felt on the wings temperature (density) related?
That is why Indicated Air Speed is used as it measures the number of molecules the aircraft is flying through. Less air speed (less molecules), fly faster, either push nose down or add more thrust, if no excess thrust is available push nose down, quickly :-)

Anything that uses air for energy has to deal with how the air is affected by temperature AND pressure, these are described as Density Altitude and Pressure Altitude. e.g. the higher the runways elevation the longer it has to be for the same weight at sea level (Pressure Alt), however both of these need to be adjusted for ambient temperature, The hotter it is the longer the runway has to be (Density Alt). Both engines AND wings are affected. On a cold day your car will have a tiny bit more vroom than on a hot day at the same elevation.

A aircraft at altitude is continuously adjusting its altitude, CADC's are very clever black boxes. The aircraft's vertical flight path if plotted would resemble a wavy line, its continuously adjusting for the variances in temperature and pressure, that's why ALL aircraft use standard altimeter baro settings after a certain height called the transition level, this ensures that all aircraft in the vicinity are reacting the same way to LOCAL variations in air thermodynamics.

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Re: of waters and winds

Post by dunno » 2021 Mar 09, 04:37

johngalt wrote:
2021 Mar 08, 18:47
There are a few videos out about Pelton Wheel Turbines
I would suggest a bit of research into Impulse and Reaction turbine wheels as some turbines only use one type of reactionary force whilst others use another, a jet engine utilises both, it is a Impulse and Reaction Turbine.

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Re: of waters and winds

Post by nikos » 2021 Mar 09, 06:38

dunno sometimes you're more verbose than kilmatead :)
can you, in under 10 words, tell us if my main article thesis is right or wrong, according to your fluid dynamics excellence? :)

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Re: of waters and winds

Post by johngalt » 2021 Mar 09, 15:15

dunno wrote:
2021 Mar 09, 04:37
johngalt wrote:
2021 Mar 08, 18:47
There are a few videos out about Pelton Wheel Turbines
I would suggest a bit of research into Impulse and Reaction turbine wheels as some turbines only use one type of reactionary force whilst others use another, a jet engine utilises both, it is a Impulse and Reaction Turbine.
Sure, I can - not sure how they affect a discussion of wind speed measurements and the relevance of pressure and temperature on those measurements, but sure.

The idea here was that the Pelton turbine uses a very specific aspect of fluid dynamics to run the turbine itself, which is pretty similar to how the hemispherical cup-anemometers seem to work to me.
nikos wrote:
2021 Mar 09, 06:38
dunno sometimes you're more verbose than kilmatead :)
can you, in under 10 words, tell us if my main article thesis is right or wrong, according to your fluid dynamics excellence? :)
Lol! I'm honored.

As to your request: "I don't know - yet." I'm not an expert on fluid dynamics myself, (and neither do I play one anywhere) - but I do have a decent understanding of the basics of physics, and I've read a thing or two and seen a thing or two on various phenomena in physics (and in other scientific realms). As with anything, the answer is not simple at all. I'm currently looking at a variety of scientific papers' abstracts from Science Direct after searching for the phrase 'why are anemometers hemispherical" - as I've yet to find a really good, solid explanation as to the mechanics of the particles of air from start to finish when striking the anemometer cup.
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