Aerodynamic question

WWII U-boats had a bow like a surface ship because they did in fact operate primarily as surface ships.

 

Underwater you don't make waves, there is no surface to slice.

 

It's not surface tension or air that you are cutting, so why does it make such a difference?

 

I guess that makes hydraulics and pneumatics the same

 

All I'm saying is that there must be something very special about the transition plane/point between air and water. There some be a special word or area of study which deals with this - just guessing.

 

Re: Cars and canoes - aero and hydrodynamics.

Cars have to deal with ground effect - so they tend to move away from
a pure airfoil shape.
Also it was noticed that station wagons often gave better fuel economy
than the sedans they were derived from.
Because just like a transom, a sudden flat break, can " fool " the air
that the car is longer than it is.
Unless you can afford a long gentle incline, moving back from the windshield.
Like a canoe does. in plan view.
Gee - maybe those " primitive " boat builders knew what they were doing
after all.

 

If you make waves, like a boat does and like very fast airpplanes does, then a sharp nose is an advantage.

In canoe design, maybe a sharp bow at the waterline changing to a round shape further up may be good?

 

Not to mention torpedoes. Oh I just have.

They have a rounded nose with the rest of the body tapering down to the end. Although they have to push the water aside, the fact the water wants to go back to where it came from would squeeze into the sides of the torpedoe and that force would push the torpedoe forward countering to a large degree for the force required to push through the water.

Should hulls have a rounded bow tapering down to the stern. Although you would have to push the water away, the water returning quickly to fill the void would help to push the boat forward and also counter the bow wave.

Someone did say in another thread that the less wave the less fuel.

 

Kach- yes there is: free-surface fluid dynamics.
re Canoes: Aerodynamics are irrelevant to a canoe, it only goes 4 knots. Canoe shapes are based mainly on wave drag and seakeeping considerations.
Poida- what you say about torpedoes is true in the inviscid (fully idealized) model; it does not hold when the boundary layer flow is considered. The gradual reduction in cross-sectional area aft of the point of maximum area is intended to keep the adverse pressure gradient that must exist there from becoming so adverse as to cause flow separation. If a flow separates without leaving a free surface, horrendous drag is created. (Planing hulls cause the flow to separate at the transom, leaving a free surface, which is left behind the craft.)

 

Marshmat

I have so much to learn and so little time left to learn it.

 

inverse pressure

Marshmat
I know composition of flow is a specialism of yours. So I’ll take this opportunity to ask about ‘The gradual reduction in cross-sectional area aft of the point of maximum area’
Is there a prescribed method for draughting this? Will any smooth gradient of curved areas do? Is area to girth a factor?

Fred

 

Marshmat, I did a Google search on "free-surface fluid dynamics" and got some college course class descriptions. Not a lot else in the first couple of pages. I know with a title like that most of the information would be over my head anyway.

Can you recommend a book or webpage which describes in simple English or layman's terms the basic concepts of that topic?

My interest lay in Hovercraft and Surface Rffect Ships which do their best to avoid this interaction altogether. However nature has a way of breaching man's conceptual contracts with himself and drafting one into a position of confrontation. Which is just my fancy way of saying choppy waves are going to get cha!

 

The analysis of turbulent flows and adverse-gradient flows is an active research area at present. A full model of such behaviour does not yet exist.
Fred- The issue with a region of decreasing cross-sectional area (on a boat) is this. On the front half of the boat, below the waterline and out of the area where wave effects dominate, the flow accelerates as it passes over the curves of the hull. The pressure thus drops- a 'favourable' gradient, since it goes in the same direction as the flow. On the aft half of the boat, though, if the sectional area decreases, the flow decelerates and the pressure goes up- an 'adverse' gradient since the pressure gradient is now against the flow direction. If the adverse gradient becomes strong enough, the boundary flow separates from the hull, leaving a very low-pressure region against the hull that greatly increases drag. The exact point of transition is hard to calculate, but in general if tangents to the hull diagonals are less tha about 7-10 degrees from the direction of travel, the flow will probably stay attached.
Kach- Afraid I know of no such book. The best I can think of at present is "Fluid Mechanics" (5th ed.) by Frank White, a good fluids overview that is mostly at about a 2nd/3rd-year level. Its coverage of free surfaces is limited to open-channel flows though. There are many books and papers that discuss free surface issues (ie. boundaries between air, water) in more detail but they are mostly at a 4th-year or graduate level and mathematics-heavy. If anyone knows of a good one that doesn't involve differentials or tensors, I'd be glad to hear of it

 

Found this.

http://www.cfdthermo.hut.fi/freesurfaces.html
Last update Dec 14, 1996

 

Cool
And I'm sure everyone can appreciate how much fun it must be just to generate that time-dependent grid, let alone solve the three-dimensional partial differential equations governing the flow in each of those elements!