[Rhodes22-list] Fluid Dynamics

[Ronald Lipton]

Bill,

    In general there is a boundary layer between a solid object and
fluid flowing around it.  Typically this is analyzed to understand the
sources of drag on a hull, which are dependent on the relative
velocities, the viscosity of the fluid, and whether the flow is laminar
or turbulent.  The actual smoothness of the hull has only a minor
effect.  The molecular layer adjacent to the hull is considered as
moving at the hull velocity, there is then a boundary layer where the
velocity of the the water increases to the velocity of the stream.
For a hull moving through water at ~5 knots the boundary layer is
a few millimeters.  It may be a bit larger in a river, but if the bed 
has
few large obstacles it is hard to see how there would be a large
enough turbulent boundary layer the tumble an anchor.  the problem
is different if there is a bend or obstruction in the river with changes
the direction of flow.

By the way one of the nice applications of the boundary layer concept
is the "tea leaf" problem, which analyzes the reason tea leaves 
congregate
at the center of a teacup after stirring in terms of boundary layer 
effects.


Ron



On Jan 16, 2006, at 7:06 PM, Bill Effros wrote:

> Here's the fluid dynamics part.  As I understand eddies, they occur 
> because the water is not one solid mass flowing at a constant speed.  
> I think the water can move faster at the surface than the bottom.  So 
> if you drop an anchor it encounters turbulence which may cause it to 
> tumble or spin on the way to the bottom, but the water may be 
> traveling far more slowly at the bottom allowing the anchor to set at 
> a much slower speed than the boat is moving.
>
> I could be completely wrong about this.