Re: [ATM] Wind resistance & golf balls

[Daniel Reinders <danreind@shaw.ca>]

My understanding is that the dimpels on a golfball serve to prevent flow
separation, by inducing a turbulent region at the base of the boundary
layer, or something like that.  I seem to recall that it's only really
effective over a certain range of air speeds (specifically Reynolds'
numbers), from which you might expect not to see a result if the wind speed
is too low to cause the flow separation.  Probably not worth the effort...

On the other hand, a symmetrical airfoil shape could significantly reduce
wind resistance if oriented correctly.  Golf balls need to be round, but not
telescopes.  Would this help?  Well here are some cursory thoughts.  A nice,
smooth, laminar air flow shouldn't, of itself, cause vibration.  Or at least
in theory, since all your forces balance and are constant.  Changes in the
windspeed would cause vibrations oriented in the direction of the wind, and
here an airfoil should vibrate less since the change in force would be less
(or you could just reduce the reaction to the force by making things heavier
and/or stiffer to compensate).  Things get very different if the wind
changes direction, in which case we're providing a larger cross-sectional
area and the scope would likely be more affected by wind speed changes.  If
the flow is turbulent, as it could well be, then you'd also expect off-axis
pressure variations also causing vibrations, although whether these would be
worse than a round tube I'm not sure.  Probably not too likely to be worth
it.  As Alan says, better to just set the scope up behind the car.

Cheers,

Dan Reinders
----- Original Message ----- 
From: "Alan Scott" <alanne1956@hotmail.com>
To: <hermit@outofoptions.org>; <atm@atmlist.net>
Sent: Thursday, March 31, 2005 12:51 PM
Subject: Re: [ATM] Wind resistance & golf balls


> I suspect that the reason that this works with golf balls has to do with
the
> size of the indentations.  I suspect that you need to keep the size of the
> indentations the same.  So, you would need gazillions of little tiny
divits
> - NOT round hammer head dings.
>
> Also, realize that is happening with the golf ball.  There is less
> resistance at 100 miles/hour, which says NOTHING about vibration.  You
want
> to change the vibrational characteristics of the scope with a wind of 10
or
> 20 miles per hour, and how the wind is causing that vibration.  Generally,
I
> believe that this means changing the harmonics of the scope (maybe make it
> stiffer or add/subtract weight), and/or dampen the vibration (some folks
> hang chains that seem to do this).
>
> You would also almost definately do better by putting your car upwind of
the
> scope, thus picking up a bit of shelter from the wind.  Better yet would
be
> a wall or building or wind screen.  I use the car upwind with the trunk
open
> method all of the time.
>
>
> Alan Scott
> http://www.xmission.com/~alanne/Astronomy.html
>
> >From: Ken Lowther <hermit@outofoptions.org>
> >To: atm@atmlist.net
> >Subject: Re: [ATM] Wind resistance & golf balls
> >Date: Thu, 31 Mar 2005 15:33:11 -0500
> >
> >
> >
> >Quoting Roger Chisholm-Batten <rogecb@tiscali.co.uk>:
> >
> >>At a recent viewing session the wind got up a bit and started shaking
> >>my 8.75 inch sonotube scope.  A friend who was observing with me
> >>commented adversely. He is a keen golfer and suggested that I think
> >>laterally and take the example of a golf ball.
> >>
> >>To cut resistance golf balls have regular indentations all over. The
> >>suggestion was that I do the same with my tube.
> >>
> >>Advice please.  How many indentations per square inch would you
> >>recommend and would hitting a round hammer head against the tube be
> >>sufficient to create permanent dents?
> >>
> >>I appreciate that objective views may be a little thin on the ground
> >>as this is such a giant leap in ATM thinking.so few ATMers would have
> >>contemplated let alone performed the proposed.  All ideas appreciated.
> >>
> >Golf balls are designed to be smacked at what?  Average Golfer: 82
mph -95
> >mph
> >
> >Tour Professionals: 110 mph â?" 127 mph
> >
> >Long Drive Professionals (like Pinnacleâ?Ts and RhodyGolf.comâ?Ts Dan
> >Boever) 135 mph â?"145 mph
> >
> >It is a little round sphere that is expected to travel about 300 yards
> >average max?  And it rotates adding another dimension.
> >
> >This relates to your scope design how?  It was suggested years ago that
> >baseball players should do this to their bats.  Hasn't happened. Why?
> >Probably no practical gain in return for the effort.
> >
> >I would think the same would be true here.  Adding a little mass would
> >probably be easier and more effective.
> >
> >But I am not an engineer.  I could be all wrong.  As usual. ;-)
> >
> >Ken
> >
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