Newsgroups: comp.robotics
Path: brunix!sgiblab!sgigate.sgi.com!odin!sgi!wdl1!mail!cps233!dombrows
From: dombrows@lds.loral.com (Brian Dombrowski, 5424)
Subject: Re: Anemometer speed - help
Message-ID: <1994May25.203100.25723@lds.loral.com>
Sender: news@lds.loral.com
Reply-To: dombrows@lds.loral.com
Organization: LORAL Data Systems
Date: Wed, 25 May 1994 20:31:00 GMT
Lines: 185


Jaroslaw:

Someone replied earlier to my follow-up.  I'll send you a copy for
your information.

For the record, I understand what a hypotenuse is and what you
and this other fellow are trying to say.  However, I don't
think the diagonal motion concept applies to the cup based
anemomter.  Read on if you want to know why.

An anemometer I have is 6" in diameter, give four pulses
per revolution, and the slope equation to convert pulses
per second to MPH is 1.07 * pulses per second ( this is
a commercial anemometer.)  Solving the math gives the
tangential velocity of the cups to be 2.5 MPH in a
10 MPH breeze.

Bd


Anemomter speed dialogue:
.
.
.


> 
> In article <1994May23.124819.15434@lds.loral.com> you wrote:
> : Diagonal speed ???!!!  The anemometer is basically cups mounted to a
> : shaft.  The motion is rotation centered on the shaft.  This is radial
> : velocity.  From a theoretical stand point, any basic knowledge of
> : high school trig. will show there is no way the thing will turn
> : faster than the actual wind speed.
> 
> You are right in one. Anything moved by wind can't move faster than wind.
> Well, to say the true, moving in wind direction cannot be faster than 
> wind speed.
> Imagine a carriage on rail (small friction assumed) with sail. if the
> direction of rail and wind are the same, carriage can reach only wind speed.
> but what about difference in direction? Top view:
> 
>                                     / /
>                                    / /
> --->                              / /
>                                  /|/ cariage with sail here
>                                 / /
>                                / /
>                               / /
> 
> Basic knwledge of trigonometry says, that when cariage has the same speed
> as wind in wind direction, its speed in rail direction is greater than wind.
> 
> 
> I'll repeat my drawing. It is top view. Sorry, its only ASCII
>  
>  
>                   __
>                     |             __|
>                      \           /
>                        \       /
>                          \   /
>  ---->                     O
>  wind                      |
>                            |
>                            |
>                           /
>                           \
>  
> Note, that in above position, if top cups have wind velocity IN WIND DIRECTION,
> lower (on the drawing, in real all have the same height) will have greater
> speed.
> 
> 
> 
> Jaroslaw Lis
> 



> From rstevew@armory.com Tue May 24 19:20:28 1994
> To: dombrows@mail
> Subject: Re: Anemometer speed - help
> Newsgroups: sci.electronics,comp.robotics
> Organization: The Armory
> Cc: 
> Date: Tue, 24 May 94 16:05:50 PDT
> From: Richard Steven Walz <rstevew@armory.com>
> Content-Length: 763
> 
> Dear Bd,
> 
> And I suppose that those sailboats that go 55 mph can't really do that in a
> twenty mph wind, right???? Wrong. Even sailboats can sail faster than the
> wind, because they are on a course which is restricted by their keel and
> rudder. They don't get something for nothing, but they do have a mechanical
> advantage. That wind could have pushed a much greater weight through the
> water in the direction of the wind, but a lighter vessel can go at greater
> velocity with lower mass against water pressure if the path is restricted to
> one somewhat off the wind direction. It must travel the hypotenuese!!!!!
> Same with any cup or vane anemometer. Even a propeller exceeds the speed of
> the wind that blows it!!!
> -Steve Walz   rstevew@armory.com   physics UCSC 1982
> 
> 

Your making one major oversight in comparing anemometers to sails
and propellers.  You said that the sail boat gets 'pushed' by the
wind.  I'm not ignorant to the fact that sail boats travel faster
than the wind.  I've sailed for over 12 years.  But both sails and
propellers utilize a property called 'lift' to achieve an 
additional advantage - dig out your physics books.

The only time a sailboat is solely 'pushed' by the wind is when it
runs directly downwind.  In this case the lift is zero and the 
speed of the sailboat is that of the wind minus factors resulting
from water friction.

Even if you must view a propeller as being pushed ( excluding lift
generated on the opposite side of the prop ) the prop surface is
ALWAYS diagonal to the force of the wind no matter what its
radial posistion is:

		
	 WIND 			prop surface
			        /
	----------->           /
			      /

If you use vector math or even geometry and assume the prop surface transfers
100% of the wind's force that contacts it into rotational speed, then it
will travel faster than the wind driving it.  So your right for a 
vane anemometer.

But, for a cup anemometer...

An anemometer's cups are not 'wings' that provide lift like sails
or props.  They catch wind and only 100% of it ( ideally ) when the
cup's base is orthogonal to the wind.  It is only orthogonal at two
discrete points along its rotational path - one with the wind
and one against it.  Considering only the range of the path that
has a positive contribution to the speed of the anemometer
( a one cup version just to keep things simple):


		     _
	Wind vector  W
	------------->


	Position 1:	/\____

	At this position the wind is contributing |W|sin( 0 ) or zero
	to the speed of the ane.

	Position 2:	>
			|
			|

	At this position the wind contributes |W|sin( 90 ) or |W| to
	the speed of the ane.

	Position 3:

			_____
			     \/				

	At this position the wind contributes |W|sin( 180 ) or zero
	to the speed of the ane.


Along the other 180 degrees of motion the wind will have a negative
effect on the speed of the ane.  But, the negative effect is less
than the positive effect because the 'pointed' end ( which is
more aerodynamic than the cupped end)  is directed into the wind.
It's this differential that results in the motion of the anemometer.

Comparing cup based anemometers with sails and props is like comparing
apples to oranges. 


Bd 



