GB2048788A - Preventing separation of flow over a vessel's stern - Google Patents

Abstract

A ship's hull is fitted on each side with vertically-spaced co-rotating vortex generators at a position two or three frame spaces forward of the waterline endings at the sternframe, to prevent local separation of flow. Horizontal splitter plates may also be fitted on the sternframe in the upper and lower regions of the propeller arch to suppress vorticity in the propeller arch. <IMAGE>

Description

SPECIFiCATION Improvements in ship's hulls This invention relates to ship's hulls, and more particularly to control of the waterflow over ship's hulls.

For various reasons the separation of flow over the stern area of modern vessels has become a serious problem and a co-rotating vortex generator system is the subject of our patent No.

1459425. This previous patent covers the use of vortex generators producing co-rotating vortices with axes perpendicular to the hull of the vessel, these controlling the separation of flow or the after body generally.

The present invention relates to a development of the system whereby additional local control in way of the sternframe area is exercised, preferably together with control of a transverse vortex which has been found to exist in the top of the propeller arch in certain vessels.

According to the invention, a ship's hull is fitted on each side with a set of vertically-spaced corotating vortex generators at a position two or three frame spaces forward of the waterline endings at the sternframe.

In many modern vessels the waterline endings, due to both design and production exigencies, have become very abrupt and considerable local separation of flow can occur in a narrow band immediately aft of the sternframe.

Each vortex generator fitted in this area is situated at the point where local flow would otherwise begin to separate from the shell of the vessel, and it is disposed normal to the shell which may mean that it is at a considerable angle to the fore and aft centreline of the vessel. It is arranged so that, as far as possible, inflow to it is at a zero angle of incidence and so that there is a considerable downwash angle from the exit of the generators, this being at an angle to the general flow in that region and producing considerable vorticity perpendicular to the surface of the vessel.

Because of the angle of each generator to the fore and aft line of the vessel, and due to. the tendency for the separated flow to stream fore and aft on leaving the converging surface of the vessel, it is desirable with these generators to fit tip plates which are at right angles to the span of each generator. These tip plates are therefore at an angle to the flow and themselves produce a downwash which has an axis of vorticity approximately parallel to the shell. Each tip plate is of the same order of size as the generator itself and is designed on hydro-dynamic principles to suppress local spanwise flow virtually entirely.

An embodiment of the invention will now be described by way of example, with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic plan showing a vortex generator fitted at the sternframe of a hull according to the invention, Figures 2 and 3 are, respectively, edgewise and end views of the vortex generator, Figure 4 is an elevation showing vortex generators and a splitter plate fitted to the stern area of the hull of the vessel, and Figure 5 is a plan view of the splitter plate.

Figure 1 shows a vortex generator 11 fitted at two or three frame spaces forward of the waterline endings at the sternframe 21 of the hull 12 of a vessel. The vortex generator is one of a series of such vortex generators 14 (Figure 4) fitted at vertical intervals on both sides of the hull at the region where the local water flow 13 would otherwise begin to separate from the hull shell.

Figure 1 shows how, with vortex generators fitted, the water flow, as indicated at 14, continues smoothly back to the propeller (not shown), whereas without vortex generators the flow will show considerable separation from the hull shell commencing at the region 1 6.

Figure 2 shows how the vortex generator 11 is set at right angles to the shell plating 12 to which it is fitted, and has a tip plate 17 at right angles to the span of the generator to suppress spanwise flow. Figure 3 shows how the vortex generator 11 is arranged to have substantially zero angle of incidence at its leading edge 1 8,but a considerable downwash angle at its trailing edge 19, the tip plate T7 following the same general contour.

The net result of the sternframe vortex generators with their tip plates 17 is to produce a mixture of vorticities with axes both perpendicular to the shell 12 of the vessel and parallel to it. It has been found that suchgenerators exercise a strong controlling effect on the flow leaving the sternframe area and, where it is tending to separate or would separate, are successful in reattaching the flow and ensuring a re-energised and smooth entry of the water to the propeller.

It has been found, however, in the type of vessel where this phenomenon is apparent that due to the low pressure area behind the sternframe and at the inflow to the propeller there is a strong tendency for a major circulatory component of flow to occur in the propeller arch 20 (Figure 4) about an axis transverse to the centreline of the vessel. This flow tends to be forward at the top of the arch and down' the aft wall of the sternframe 21, as indicated by the arrows 22. There is also a matching rotational flow or vortex below the propeller although this is usually much less strong. This rotation is normally in the same rotational sense as the top arch flow but may be reversed.

Such circulatory flow is undesirable in that it produces strong wake variations by virtue of removing energy from the lonyitudinal component of the wake and injecting it into a transverse component. This effect may produce a strong variation in wake in a narrow band in the vertical plane of the propeller and thus cause undesirable thrust and torque pulses which can result in vibration.

It is possible to suppress this vorticity by means of a horizontal splitter plate or plates on the sternframe 21, one or two such plates 23 being fitted in the region of the top part of the propeller arch and one or two more in the bottom part.

These plates extend aft of the sternframe and are set so that they are closely parallel to the inflow.

There is little or no effect upon the hull resistance but the rotational flow 22 in the propeller arch 20 is much reduced or suppressed. These plates 23 may be fitted in association with a known form of vertical propeller hull-vortex suppressor plate which is on the centreline of the vessel and extends from the top of the arch rather aft of the propeller along the centreline to about .7 of the propeller radius. By combination of the sternframe vortex generators 14 and the horizontal splitter plates 23, local flow separation aft of the stern frame 21 can be suppressed entirely on most vessels, thus removing or greatly reducing the propeller-excited vibration.

Claims (7)

1.A'ship's hull fitted on each side with a set of vertically-spaced co-rotating vortex generators at a position two or three frame spaces forward of the waterline endings at the sternframe.

2. A ship's hull according to claim 1, wherein each vortex generator is disposed normal to the hull shell and is set to have substantially zero angle of incidence at inflow and a considerable downwash angle at exit.

3. A ship's hull according to claim 1 or claim 2, wherein each vortex generator has a tip plate at right angles to the span of the generator.

4. A ship's hull according to claim 1 or claim 2 or claim 3, wherein one or more horizontal splitter plates are fitted on the sternframe in the region of the upper part of the propeller arch to suppress vorticity in the propeller arch.

5. A ship's hull according to claim 4, wherein one or more splitter plates are also fitted on the sternframe in the region of the bottom part of the propeller arch.

6. A ship's hull according to claim 4 or claim 5 wherein a vertical vortex suppressor plate is fitted at the top of the propeller arch on the centreline of the hull.

7. A ship's hull substantially as described with reference to the accompanying drawings.