Re: Name that force

[Brian Whatcott <betwys1@SBCGLOBAL.NET>]

At 11:57 AM 11/3/2003, Kari, you wrote:

> > On Tuesday, Oct 28, 2003, at 06:50 US/Pacific, Kari Uolevi Eloranta
> > wrote:
> >
> >> I think the viscosity of water plays a role  in acceleration, also. So,
> >> for the total effect, you have to consider both the viscosity and
> >> interactions with
> >> the walls.
> >>
> >> Terveisin, Kari Eloranta, Finland
>
> ...If I consider a single droplet, it
> takes time  to accelerate it in horizontal direction. Honey is stickier,
> so greater force  is excerted on a honey droplet than on a water
> droplet thus causing greater acceleration. Final situation remains
> the same in both cases (honey,  water),   but processess are
> different in their rates.
>
> Could you please explain to me what is wrong in my reasoning.
>  Probably I am just missing here something obvious, but I just
> cannot see it.
>
> Best regards, Kari Eloranta

Various players have visualized water tight wagons, with water sloshing,
or flat beds, with front and back walls onto which rain drop can impinge.
 The former never exist in practice (railroad companies taking exception to
freighting rainwater around the country side) - the latter certainly exist,
and add some complication to the discussion.

If you would imagine a flat bed with no walls at all, it is argued that no
matter the viscosity, the drops are accelerated to flat bed speed, before
dropping away.   Hence the flow is incremented by fluid at zero horizontal
 velocity, and decremented by fluid loss at flat bed velocity.
This momentum change is a braking force.

Viscosity can be shown to enter the equation at the fluid shedding stage.
 Viscous fluid escapes at a lower rate than inviscous fluid for other
parameters the same, so the attached fluid layer is thicker for a viscous
fluid in order to result in a balanced inflow and outflow. This model
variant leads to a  attached fluid mass which varies with
 its viscosity, and so leads to a greater retarding force with viscosity.

One takes some justification in feeling more drenched when soaked with
cold rain, than with warm rain: water viscosity does indeed vary with
temperature. This is at the whimsical limit, however....




Brian Whatcott    Altus OK    Eureka!