wind meter

"Ron Hardin" wrote in message
...
Philip Holman wrote:
Alternatively put, the forward component of the vector drag
increases.
v^2 isn't linear.

If I'm traveling north at 8mph and there is a 6mph straight crosswind
from the east, my pathway through the air is at10mph but in a
direction
that is N 36.87 E. Drag force will be proportional to 10^2 = 100 in
this
direction. The forward component of this will be proportional
to100cos36.87 = 80. This is greater than when travelling at 8mph with
no
croswind i.e 8^2 = 64. Is this what you mean?

Yes. that's the effect. So you're riding along at 8mph, and your
airspeed is
8mph, and a sudden gust hits from the east. Both your actual speed
and your
forward airspeed fall together. That's a little unexpected, is all.

If the wind meter were mounted to swivel into the wind, the airspeed
would
go up, which is what you expected.

Well, if it points at N36.87E through weather vaning, it will show
10mph. The equivalent windspeed is Sqrt 80 = ~9mph which would be a
little harder to determine.

Phil H

Philip Holman wrote:

A side-wind increases the amount of air you have to move, without
increasing the forward wind speed. So of course the forward speed
drops at constant power, and so does the indicated airspeed as a
result.

Ron Hardin wrote:

I'm not sure what you are getting at here Ron. Work is done against the
road and is force times distance moved along the road. Unless forward
direction drag increases, in theory there should be no difference. Any
side vector force is reacted by tire traction. In practice, bike
handling and power output can be disrupted in strong side winds.


You bring all the air you hit up to your forward speed, and your path
through the air is larger, ie. you hit more air, even with a straight
crosswind. It will be felt as a quartering headwind, of course.

Alternatively put, the forward component of the vector drag increases.
v^2 isn't linear.

Jobst Brandt has done a very thorough analysis of this, which I have
hosted on my Website at: http://sheldonbrown.com/brandt/wind.html

Frankly I don't see why this is of such interest, since it is of no
practical import, the wind being outside of human control.

Sheldon "Graphs" Brown
+---------------------------------+
| Is ambivalence a bad thing? |
| Well, yes and no. |
| -- Garrison Keillor |
+---------------------------------+
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Phone 617-244-9772 FAX 617-244-1041
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Sheldon Brown wrote:
Frankly I don't see why this is of such interest, since it is of no
practical import, the wind being outside of human control.

It's of interest to people interested in physics, a large segment of
males. There you are, pedalling along happily measuring your ground
speed with your GPS and your forward airspeed with the wind meter
at the top of this thread, knowing that since you're a top experienced
cyclist you're running at exactly constant power, when whoa! along
comes a gust of wind, and BOTH your ground speed and your air speed
drop. What is a physicist to make of that? That the wind meter is
broken? It may be of no practical import, but it's an urgent problem.

It comes from the nonlinearity of air drag, and all the meters are
working fine.
--
Ron Hardin


On the internet, nobody knows you're a jerk.

On Sat, 12 Feb 2005 10:35:20 GMT, Ron Hardin
wrote:

wrote:
In liquids, which are enormously denser, you get a bow wave
at even low speeds. (The spoon is an example of a
displacement hull.)

But in gases, as far as I know, you don't get a bow wave of
any consquence at similar speeds.

I don't know if you're thinking of the surface wave (a gravity wave)
as in the wake of a boat, or not. I'm not talking about a propagating
wave but just motion in getting around the obstacle.

What the fluid decides to do about the obstacle as it approaches depends
on the reynolds number of the flow. Water speed has to be about 1/20
of air speed to duplicate the flow. So think of water at 1 mph.

At low speeds, the fluid moves out of the way well in advance of
the object. Viscous forces give it ample warning and have time to act.

Or think of a wind meter, and you put your hand a foot downstream
of it, blocking the flow somewhat. How far downstream would you
expect to see an effect from?

Dear Ron,

No, I don't think that either of us is talking about a
propagating wave.

I like your idea about the hand behind the wind meter, but
I'm not sure that the effect (which is noticeable about six
inches or so behind the meter at 20 mph) is quite what
you're thinking of.

The problem is that the wind meter works by constraining a
column of already moving air through a four-inch long tunnel
less than half an inch square, which is right next to the
comparatively enormous mass of the rest of the wind meter.

This creates a surprisingly long low-pressure plume of
turbulence behind the meter. If we start filling that zone
in with our hand, the pressure rises. This is not so much a
bow wave of air as a complicated interference.

What we really need is to separate the sensor tunnel from
the interfering (but convenient) body of the meter with some
thin wires and then mount the tunnel on some sliding stick
stick arrangement that lets us extend it forward.

If you're right about the bow wave effect being significant,
then the meter should show the wind speed increasing as the
meter gets further from the rider.

But most of what I see in wind tunnel testing is a slight
sideways deflection, usually greatly exaggerated in diagrams
and not actually large enough to be even visible in
low-speed tests. Again, try a hand test. The hairs on your
forearm are quite sensitive. Stick one arm out and move your
other palm toward it briskly. You'll feel the trailing plume
of turbulence washing over your forearm, but not a
preliminary pressure wave.

Cockroaches have far more delicate hairs, so they can sense
the extremely faint increase in air pressure of a rapidly
approaching object and evade your palm, but I don't think
that this is anywhere near the kind of bow wave effect that
you have in mind.

So far, I can't think of an easy way to mount the meter on a
stick to let it slide back and forth, much less how to
separate the meter from the display body, but I'll ponder it
some more.

Carl Fogel

On Sat, 12 Feb 2005 02:42:44 -0500, "Boyle M. Owl"
wrote:

RonSonic wrote:


I am appalled by your speciist bigotry which has produced a staggeringly
incurious and ascientific outlook. Your refusal to test the windspeed of a
basset hound has denied the world of science a brilliant opportunity for
research. Nevermind the chew testing that will remain undone.

The measurement of basset hound windspeed would entail several scenarios.

1. Attach the wind meter to the basset hound's back. Play "fetch" with
an old slipper. That is, if the hound is so inclined.

[snip]

Dear Boyle,

Bah! Typically impractical theory! Obviously ivory-tower
thinking!

As has been discussed, basset hounds lose interest in
"fetch" by the age of one when the objects are tennis balls,
puppy toys, slippers, and so forth. Even squirrels and cats
are ignored. The only objects pursued are liver treats,
porcupines, and rabbits.

Facing into a steady 15 mph wind, the ears of a basset will
begin to spread and rise. Since the Windscribe is portable,
I expect to confirm this within a week or two.

Carl Fogel

On Sat, 12 Feb 2005 13:09:32 -0500, Sheldon Brown
wrote:

[snip]

Frankly I don't see why this is of such interest, since it is of no
practical import, the wind being outside of human control.

Sheldon "Graphs" Brown

Dear Sheldon,

I hear that the slope of the road is also outside human
control, but many riders seem to be interested in that, too.

Possibly much of what we do with bicycles is only faintly
related to practicality?

Carl Fogel

On Thu, 10 Feb 2005 19:51:05 -0700,
wrote:

[snip greatest story ever told]

Ron Hardin and I have been wondering elsewhere in this
thread about bow waves from the rider confusing the wind
meter's reading.

Ron foolishly believes in such bow waves in the face of my
disbelief, presumably relying on such weak reeds as theory
and experiment.

While I'm not yet convinced that this will be a tough spot
to lie my way out of, I'm beginning to wonder if Ron might
be right.

After three rides with the Windscribe, I think that I'm
noticing a pattern of the wind speed being noticeably lower
than the road speed on a long downhill.

That is, I'm seeing the Windscribe that was within a mile or
two per hour of my road speed at about 20 mph on an
unusually calm series of days lagging 6 mph behind road
speeds of 34-39 mph.

It's hard to be sure about the wind on the downhill (it's
open and has a fifty-mile view), but my experience suggests
that 39 mph is my normal maximum speed on a calm day at the
very bottom--which consists of a well-sheltered curve
dropping to the river bottom. I think that a 6 mph tailwind
would push me into the low 40 mph range.

A possible explanation is that a bow wave from the rider
expands as speed rises, which leads to the Windscribe
showing less and less of the true wind speed.

(Not that I believe in the bow wave yet.)

A scheme involving a broom stick clamped to the top tube and
protruding past the handlebars like the needle on the nose
of supersonic jet is taking shape.

Carl Fogel

wrote:

Dear Boyle,

Bah! Typically impractical theory! Obviously ivory-tower
thinking!

Hey, you forgot the CYA disclaimer at the end!

Facing into a steady 15 mph wind, the ears of a basset will
begin to spread and rise.

Obviously since the ears rise, they have enough aerodynamic lift for
themselves...which leads to: What are the lift capabilities of a
basset's ears and can the hounds be bred to be heavy lift aircraft?

--
BMO

On Sat, 12 Feb 2005 21:24:35 -0500, "Boyle M. Owl"
wrote:

wrote:

Dear Boyle,

Bah! Typically impractical theory! Obviously ivory-tower
thinking!

Hey, you forgot the CYA disclaimer at the end!

Facing into a steady 15 mph wind, the ears of a basset will
begin to spread and rise.

Obviously since the ears rise, they have enough aerodynamic lift for
themselves...which leads to: What are the lift capabilities of a
basset's ears and can the hounds be bred to be heavy lift aircraft?

Dear Boyle,

Unfortunately, legal considerations prevent me from pursuing
this matter.

Let's just say that copies of Disney's "Dumbo the Flying
Elephant" will soon be recalled and replaced with more
accurate information if there is any justice.

Carl "Class Action" Fogel

wrote:
A possible explanation is that a bow wave from the rider
expands as speed rises, which leads to the Windscribe
showing less and less of the true wind speed.

As far as I can see, such a bow wave would get smaller at
higher speed rather than bigger. However, it might be mostly
unchanged, which means the same percentage of speed drop, which
of course would get bigger in absolute terms.

Putting it off to the side is better than ahead, if you want to
try to figure out what's going on. I was going to do two experiments
when the weather got nicer, with my three of them, an broomstick
across the bars (and compare the average speeds of all 3) on
a calm day, swapping the devices halfway to cancel any differences
among them (and not letting them recalibrate).

And on some day with a good headwind, do the same with a vertical
broomstick, to see how much the wind depends on height. After all,
the wind way down by the handlebars may be childs' play compared to
what's working against your chest.
--
Ron Hardin


On the internet, nobody knows you're a jerk.