Twenty-Fourth Symposium on Naval Hydrodynamics (2003) / Chapter Skim
Currently Skimming:
Wave Patterns and Minimum Wave Resistance for High Speed Vessels
Wave Patterns and Minimum Wave Resistance for High Speed Vessels
Pages 23-39
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 23... ...
In contrast to the thin-ship case, the ability to compute accurate near-field flows is essential to the solution process for a prescribed planing surface, which requires solution of an integral equation to determine the equivalent pressure distribution. Such pressure distributions can also be optimised to minimise wave resistance, from which follows a formally simpler design process for planing surfaces, where the shape of the surface is output rather than input.
|
From page 24... ...
We summarise here new results for pressure patches of minimum resistance at fixed total load, and give examples illustrating the corresponding nearfield wave patterns, which are candidate hull shapes for planing surfaces. Interestingly, although optimal pressure distributions are necessarily fore-aft symmetric, this does not imply such symmetry of the near-field pattern, nor of the flat-ship hull, whereas the corresponding optimal thin-ship theory demands fore-aft hull symmetry.
|
From page 25... ...
for extreme diverging waves, and in practice it is best to avoid its direct computation. THIN SHIPS For a monohull thin ship with offsets y = ~Y(x, z)
|
From page 26... ...
Thin-ship theory also holds for multihull vessels, and again in most such cases the individual demihulls are thinner than conventional ships, so the linearization is more justifiable. However, now there is a further complication involving interactions between hulls.
|
From page 27... ...
This quantity z, is not really intended to represent molecular viscosity as such, but to model the damping effect of the turbulent shear layer in the wake, and hence must take a value comparable to the eddy viscosity in the wake. Choice of an appropriate value of zJ is a difficult matter; a value of between 100 and 10000 times the molecular viscosity seems to eliminate some of the most extreme short diverging waves (with ~~ close to ~r/2)
|
From page 28... ...
Again, Filon's quadrature plays an important role, and without it the diverging waves are poorly predicted. In addition, a special algorithm as in Tuck et al (1971)
|
From page 29... ...
The Froude number for the main hull is low enough for that part of the pattern to be mainly transverse, whereas the Froude number based on sail length is large, so that part of the pattern is mainly diverging. PRESSURE DISTRIBUTIONS The velocity potential for the flow induced by a unit delta-function pressure (Wehausen and Laitone 1962, p.
|
From page 30... ...
However, there are certainly pressure distributions which perform much better from the wave-resistance point of view. One such is a tandem pair of patches.
|
From page 31... ...
Minimum wave resistance patches The above simple examples of pressure distributions are not necessarily optimal from the point of view of low wave resistance. Although for actual hovercraft there may be severe limitations on achievable pressure variations, nevertheless it is of significant interest to seek pressure distributions yielding minimum or at least low wave resistance, at fixed total lift force balancing the weight.
|
From page 32... ...
Its edge smoothness results in a much lower envelope of the spectral peaks of the extreme diverging waves with ~ > 75°, but these carry little energy. The other pressure distributions shown in Figure 14 have a local minimum in their free-wave spectrum curves at wave propagation angles a little higher than that at which the constant-pressure patch reaches its peak.
|
From page 33... ...
to examine wave cancellation effects of multi-hulled displacement vessels. PLANING SURFACES Planing surfaces are flat ships, i.e.
|
From page 34... ...
In practice, with a fixed trailing edge, this is normally an adjustment in the location of the leading edge, and such significant leading-edge adjustments are natural and inevitable for 12 real planing surfaces. However, this is a computationally difficult task, and is not attempted in the present paper.
|
From page 35... ...
It is also relevant that our work on minimization of wave resistance suggests that optimal pressures tend to vanish at the sides. For a rectangular planform R with side boundaries parallel to the stream, it is also not entirely obvious whether the side boundaries should be considered as "leading edges" or "trailing edges"; if the latter, we are required to demand a Kutta condition of zero pressure on them in any case.
|
From page 36... ...
This is a notoriously difficult task, essentially because of the above-mentioned short diverging waves, and presents significant computational challenges. The present type of code shows considerable promise of yielding robust solutions, but we are not quite there yet.
|
From page 37... ...
and Tuck, E.O., "Free-surface elevation due to moving pressure distributions in three dimensions", Journal of Ship Research, 2002. Suzuki, K., Nakata, Y., Ikehata, M., and Kai, H., "Numerical prediction on wave making resistance of high speed trimaran", Fourth Int.
|
From page 38... ...
Tuck, E.O. and Lazauskas, L., "Free-surface pressure distributions with minimum wave resistance", ANZIAM Journal, Vol.
|
From page 39... ...
If so, did you optimize the arrangement for one design speed or several speeds? We have compared four different calculation methods, ranging from slender ship theory to fully nonlinear Euler solution, for the trimaran case and found that the slender ship approximation can be used for determining the optimal arrangement of the center hull and the outer hulls for a trimaran.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.