Cap 11 Flashcards
Sketch the principal difference between a wakes from a V-shaped afterbody and a
bulbous one. What are the effects on performance?
The v-shap aftbody have a lower resistance but dos not create any vortex in the aft and therefore the propeller efficency is reduced.
The bilbous aftbody have more resistance but create a stronger vortexdue to that the bilge radius are smaller in the region in front of the propeller. This is a well known and safe way to op- timize the stern from a hull efficiency and vibration point of view.
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What is a goose-neck bulb?
What is it good for?
A goose- neck shape is characterized by a maximum cross-section ahead of the FP and with lines sloping down aftward. It is very well integrated with the hull lines.
The downslope of the upper surface and the convex curvature preced- ing it will help to draw the flow down, thus reducing the wave height. Further, the longitudinal inclination of the lines may reduce the viscous resistance.
As pointed out previously, the streamlines in the bow region point down- ward, and with this bulb shape, the flow will experience a smaller curvature of the surface when passing along the bulb and the forebody. This minimizes the boundary layer growth and the risk of vortex separation.
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When is a twin skeg stern especially beneficial for a full-body ship?
Any particular problems when designing the gondolas?
The shape is particularly advantageous for beamy and shallow hulls, especially if the block coefficient is large. Savings in delivered power of a few procent up to 10%.
A careful design of the gondolas is however required because the upper part has to follow the inviscid stream- lines, whereas the lower part necessarily must be aligned with the shafts in the axial direction. This calls for tilted gondola sections, as seen in Fig. 11.18.
Which are the advantages and disadvantages of barge type full-body sterns?
This type is characterized by straight and flat buttocks and a central gondola in which the machinery is located.
Advantages
Such hulls have proven to have a very low resistance and vortices can be avoided entirely by care- ful design of the bilge region. Hard chines may be utilized as well, often with little or no increase in resistance.
Disadvantages
If the bilges are not well designed, so that vortices are created, the positive effect described does not materialize because they will pass the propeller plane far outside of the propeller disk.
Because the hull boundary layer will be spread over the large girth of the stern sections, there will be no concentration in the propeller disk, which will only collect the much thinner boundary layer from the gondola. The hull efficiency is therefore much lower than for a con- ventional stern, and there is also a tendency for a larger suction effect of the propeller at the stern (larger thrust deduction) so the advantage of the small resistance is lost.
The reason why the barge shape is still of interest is that the flat stern sections are less prone to separation,
How can a designer make use of the stern bilge vortex to create a good wake?
By going from a v-shap stern to a more of a bulbous stern the larger curvature of the bilge will create a stronger vortex. The vortex will redistrubes the velocity conture in a favorble way.
Furthermore the line above the propeller shaft become more slender and reduces the deep wake peak at the top of the propeller disk. The flow becomes more even and the contour is more cirkuler.
Sketch the principal difference between a wakes from a V-shaped afterbody and a bulbous one. What are the effects on performance?
The sketch indicate that the flow in to to the v-shape are unequal and thereby the load working on the propeller will also be un equal. This will not only give a low effeciency but alson create vibrations.
The bulbous-shap have a better flow pattern in to the propeller. The flow is more evenly spredd and the load on the propeller is more evenly. This will result in a higher efficiency and less vibration.
Use velocity contours to display the boundary layer development on the aftbody of a tanker hull.
Explain peaks and valleys. Why is there a large region on low speed flow in the propeller disk?
A stern bilge vortex hits the propeller disk and redistribute the velocity conturs. This is utelized by designers to move conturs from the central and lower parts of the disk and squize them together at the top of the disk. Aftbodys goes from v-shape to u to bulbous shape and make the vortex stronger and stronger because the bilge radium decrases.
B:L will be spead over the larg girth of the stern section.
=> no consentration in the propeller disk
How can a bulb reduce the viscous resistance of bluff forebodies?
The streamlines in the bow region point downward, and with this bulb shape, the flow will experience a smaller curvature of the surface when passing along the bulb and the forebody. This minimizes the boundary layer growth and the risk of vortex separation.
Which is the main concern when designing the forebody of a slow ship.
The largest resistance factor is the viscous resistance. This is the done by minimaze the wetted aria and by doning this the growth of B.L is slowed down.
But by changing the forbody its importent to find the right balance between the bluntness of the bow and the sharpness of the shoulders. This is to reduce the wave resistance.
What is hull efficiency?
How can it be optimised?
The hull efficiency takes into account both the suction effect from the propeller on the flow around the stern and the reduction in inflow velocity to the propeller due to the boundary layer/wake from the hull.
For exaple the hull efficiency can be optimised by decrease the suctioneffect from the propeller on the flow around the stern.
It turns out that the efficiency factor is larger than one for many ships, which may seem impossible for an efficiency factor but it is related to the fact that the momentum loss in the boundary layer/wake may be utilized to increase the propulsive efficiency.
Explain, without formulas, the basic principles of the Holtrop-Mennen method for estimating the resistance of displacement hulls.
The resistance is split into wave resistance and viscous resistance.
The viscous resistance (CF, K) is received from modeltest, whilst the wave resistance is theoretically calculated where the ship is replaced by two pressure zones.
Define the velocity potential in an irrotational flow and the assumptions for this to be a valid approximation.
Discuss the benefits of introducing the velocity potential
compared with studying the full Navier-Stokes equations.
The velocity potential #(x,y,z), where u(x, y, z)=€#
As thecurl of the gradient always vanish this simplification guarantees that the flowis irrotational.
With the introduction to velocity potential N-S can be expressedas Bernoulli and Laplace insted. This make calculationeasier since Laplace is liniar and the epuations are uncoupled. The result can therby be superposed and final results are found quicker.
What is a “limiting streamline”?
How is it normally visualized in towing tanks?
The streamlines on the surface that are limeted by the friction to develop are called limeted streamlines.
They are visulazed in towing tanks test by painting lines vertecly on the hull and then do the test before the paint is dry. The paint will then be draged out over the direction of the limeting stremline.
What expectations can we have on accuracy when it comes to
i) local flow predictions and
ii) global predictions like resistance and wake fraction, using RANS?
Around 5% according to a workshop held in Gothenburg.
RANS method works well if the integrated wave is considered but for bluff bodies the wake can not be predicted.
