in the attached figure there two shapes of the same main dimensions but a slight difference is made to the nose of the first to be more like a dolphin nose, imagine the revolved 3D shape, and i want to know the shape of higher drag if both of them are put into water stream at 10 m/s for example.
I'd say there is no significant difference. Drag will be caused by friction along the cylindrical part of the body and by the shape of the rear end. The round nosed body will be the preferred option for a body moving in water. Just because of the drag in case the body is not exactly aligned with the flow.
Which leads to the question why the dolphin's nose is shaped as it is.
The drag is likely to be smaller with the dolphin nose because less pressure drag is generated. But I agree with Ries that this holds for perfect alignment, i.e. in a small range of incident angles. For non-small incidence angles there will be massive separation, more than with a fully rounded nose, like of a typical submarine. But your second setup has a lower sharp edge, and that is also bad at incidence.
Generally, the sharp leading edge configuration is typical of a body that must work in supersonic condition, the rounded shape is typical of subsonic condition. The explanation of why is a typical issue of the aerodynamics field.
I'd say the dolphin shape is better in general and the whale shape may be better for big bodies. Though I think you'd do better to choose a symmetrically rounded smoother shape for better all-around hydrodynamic drag qualities, like a teardrop ….. JFH
yes, but this is not the case of compressible or rarified flows, the shape for water...
Filippo's first comment is quite interesting. The question is about a velocity of 10 m/s. And if the body is immersed in water, there is a difference between the situation in deep water or close to the surface. For shallow open water, there is an analogy to compressible flow. So, close to the water surface, the dolphin nose may actually provide less drag than the blunt body. But hey, there should be specialists round here, right?
i have done some ANSYS simulation and found dolphin shape is better but i need the physics involved.
dolphins have some byological requirements that are more strict than aerodynamics one...in case of air/water interaction the issue of drag is also quite difference due to the difference character of the governing equations.
Some old and new references:
http://www.dtic.mil/dtic/tr/fulltext/u2/a369158.pdf
http://jeb.biologists.org/content/217/2/252
The lower shape i.e. hemispherical nose will have higher drag. In dolphin shaped nose a vortex will be formed near the nose with reduced pressure in front of the body and hence a reduced drag.
My guess is that they are nearly identical when fully submerged. But possibly different when travelling near a free surface - like a dolphin or a ship. Many ships also have a submerged bulb like this on the bow. The bulb does not affect pressure drag or viscous drag significantly - but it reduces wave drag (the energy expended making surface waves). Wave drag dominates for ships and is probably significant for a large marine animal.
This might explain why you see this design on a dolphin and not on a fully submerged fish/shark. It is also quite possible that the dolphin nose has more to do with what it eats - and little to do with drag reduction.
Ibrahim, is the difference in drag bigger than the numerical error in the ANSYS/Fluent solution? I design CFD codes for a living. You should not think they are magic. Because they are not. Try some more refined meshes and see how much both answers change. Then see if the difference is shape is bigger than the difference in meshes.
I think the dolphin nose will produce less drag at low speeds where as at supersonic speeds, the blunt nose experiences high drag.
- Badges
- Science topic
- Similar topics
- Geoscience
- Atmospheric Sciences
- Atmosphere
- Air