A double-tail fin end resistance-reducing inclined strut of a ship and a design method thereof

Abstract

The application relates to a ship double-tail fin end resistance-reducing diagonal brace and a design method thereof, which comprises the following steps: determining the cross-section thickness and area of the diagonal brace; determining the cross-section geometric shape of the diagonal brace; determining the side inclination angle between the central axis of the diagonal brace and the center line of the tail fin; determining the longitudinal inclination angle between the central axis of the diagonal brace and the center line of the tail shaft; taking the average value of the flow line angles of the diagonal brace at multiple waterline heights as the central line angle of the cross-section of the diagonal brace; and screening the central line angle of the cross-section of the diagonal brace with the minimum resistance by using fluid mechanics software. While ensuring the structural strength of the diagonal brace itself, the rigidity of the tail fin and the damping requirement, the average attack angle of the diagonal brace at each waterline height is the minimum, the interference of the diagonal brace on the surrounding flow field can be reduced to the maximum extent, and the resistance of the diagonal brace itself can be effectively reduced.

Description

Technical Field

[0001] This invention relates to the technical field of ship appendages, and more particularly to a drag-reducing diagonal brace at the end of a ship's twin tail fins and its design method. Background Technology

[0002] Compared to conventional twin-propeller ships, twin-fin hull designs offer higher hull efficiency and are widely used in amphibious ships and inland waterway vessels. However, twin-fin hull designs also suffer from drawbacks such as large and uneven wake at the stern, weak lateral and vertical stiffness of the tail fins, and susceptibility to vibration, significantly impacting propeller and shafting performance as well as the structural lifespan of the twin tail fins. To fully utilize the high hull efficiency of twin-fin hull designs and effectively control the adverse effects of tail fin vibration, a common approach is to add diagonal bracing structures to the ends of the twin tail fins. This rigidly connects the ends of the twin tail fins and the diagonal bracing structures to the hull structure, thereby improving tail fin structural stiffness and reducing vibration. Currently, the design of diagonal bracing at the ends of twin tail fins in the shipbuilding industry generally borrows the design method of stern shaft brackets. A typical characteristic is that the centerline angle of the diagonal bracing cross-section is designed based on the streamline angle at the bottom plate, and the centerline angle of the diagonal bracing cross-section is the same at different waterline heights. For non-twin-fin hull designs with relatively flat stern surfaces, the difference in streamline angle at different waterline heights is smaller, and the designed stern shaft bracket has less impact on drag and wake field.

[0003] However, for twin-fin hull designs, the significant differences in fin shape at different waterline heights result in substantial variations in streamline angles. If the aforementioned tail shaft bracket design method is still used to design the twin-fin end braces, with the streamline angle at the bottom plate serving as the centerline angle of the brace's cross-section at different waterline heights, it can easily lead to excessively large angles of attack between the brace's cross-section and the incoming flow at some waterline heights. This results in large pressure differences between the inner and outer sides of the brace, easily generating cavitation and causing a chaotic flow field around the brace, significantly increasing its own drag. Therefore, there is an urgent need to provide a twin-fin end brace for ships and its design method that comprehensively considers the differences in streamline angles at different waterline heights for twin-fin hull designs, ensuring that the brace's structural strength, fin stiffness, and vibration reduction requirements are met while controlling the brace's own drag to an acceptable range.

[0004] The design of the tail fin end braces for ships presents two major technical challenges. First, how to determine key parameters such as the cross-sectional shape, sideslip angle, pitch angle, and brace length to effectively improve the stiffness of the twin tail fins and control their vibration within acceptable limits while ensuring the structural strength of the braces themselves. Second, how to determine the centerline angle of the cross-section of the tail fin end braces to achieve, as close as possible, zero angle of attack with the incoming flow, significantly reducing the drag of the braces themselves. Summary of the Invention

[0005] In view of this, the purpose of this invention is to provide a new drag-reducing brace at the end of a ship's twin tail fins and its design method.

[0006] The ship's twin tail fin end brace designed according to the design method of the present invention can not only meet the structural strength requirements of the brace itself, the stiffness of the twin tail fins and the vibration reduction requirements, but also significantly reduce the resistance of the brace itself.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] A drag-reducing brace at the end of a ship's twin tail fins includes twin tail fins and a brace. The brace connects the twin tail fins to a transverse strong structure above the ship's bottom plate. The brace has a side angle between its central axis and the center line of the tail fins, and a longitudinal angle between its central axis and the center line of the tail shaft. The average angle of attack between the center line of the brace's cross-section and the incoming flow at each waterline height is minimized, effectively reducing the brace's interference with the surrounding flow field and thus achieving better drag reduction.

[0009] Furthermore, the minimum lateral angle between the central axis of the diagonal brace and the center line of the caudal fin is not less than 50°.

[0010] Furthermore, the minimum pitch angle between the centerline of the diagonal brace and the centerline of the tail shaft is not less than 60°.

[0011] Furthermore, the thickness of the cross-section of the diagonal brace is 0.35d, and the area is 0.45d. 2 , where d is the diameter of the tail shaft at the connection between the brace and the tail fin.

[0012] A design method for drag-reducing braces at the ends of twin tail fins on ships comprehensively considers the structural strength of the brace itself, the stiffness and vibration of the tail fin, and the drag of the brace itself. The method includes: determining the thickness and area of ​​the brace cross-section; determining the geometry of the brace cross-section; determining the side angle between the centerline of the brace and the centerline of the tail fin; determining the pitch angle between the centerline of the brace and the centerline of the tail shaft; taking the average of the streamline angles of the incoming flow at multiple waterline heights as the centerline angle of the brace cross-section; and using fluid dynamics software to select the centerline angle of the brace cross-section that minimizes drag.

[0013] Furthermore, the above design method specifically includes the following steps:

[0014] Step S1: In 3D geometry software, complete the geometric modeling of the twin-fin hull shape, excluding the tail fin brace, as scheme 0;

[0015] Step S2: Using fluid dynamics software, set the incoming flow velocity according to the design speed to obtain the ship's resistance and the streamlines and angles of the incoming flow to the inclined support at different waterline heights;

[0016] Step S3: Referring to the relevant design requirements for the stern shaft support arm in the structural section of the China Classification Society's "Rules for Classification of Steel Seagoing Ships," and considering that the stiffness of the tail fin itself is greater than the stiffness of a single stern shaft support, the thickness of the diagonal brace cross-section is determined to be 0.35d and the area to be 0.45d. 2 , where d is the diameter of the tail shaft at the connection between the diagonal brace and the tail fin;

[0017] Step S4: Select a symmetrical streamline shape as the geometry of the cross-section of the diagonal brace according to the thickness and area of ​​the cross-section of the diagonal brace;

[0018] Step S5: Within the minimum side angle range, based on the position of the longitudinal strong structure above the bottom plate near the outer side of the tail fin and the upper end of the brace, determine the side angle between the brace and the center line of the tail fin, that is, the minimum side angle between the center line of the brace and the center line of the tail fin is not less than 50°.

[0019] Step S6: Within the minimum trim angle range, determine the trim angle between the brace and the centerline of the stern shaft based on the position of the transverse strong structure above the bottom plate near the outer side of the tail fin and the upper end of the brace. That is, the minimum trim angle between the centerline of the brace and the centerline of the stern shaft shall not be less than 60°.

[0020] Step S7: Along the aforementioned side angle and longitudinal angle, starting from the outer side of the end of the twin tail fins, stretch the cross-section of the diagonal brace to the bottom plate to determine the geometry of the entire diagonal brace, thereby determining the length of the diagonal brace, wherein the stretching line is the central axis of the diagonal brace;

[0021] Step S8: Extract the streamline angles of the incoming flow at multiple waterline heights of the diagonal brace in Scheme 0, calculate the average value of each streamline angle, and use it as the centerline angle of the diagonal brace cross-section; rotate the diagonal brace cross-section at each waterline height counterclockwise on the port side and clockwise on the starboard side around the centerline of the diagonal brace to the centerline angle of the diagonal brace cross-section, and smoothly transition the outer contour of the diagonal brace to obtain the diagonal brace;

[0022] Step S9: In the 3D geometry software, complete the geometric modeling of the diagonal brace; install the geometric shape of the diagonal brace according to the side slope angle and longitudinal tilt angle at the end of the twin tail fins and the position of the strong frame of the bottom structure, as Scheme 1;

[0023] Step S10: Use structural finite element software to check the structural strength of the diagonal brace and the stiffness of the double tail fins to ensure that the structural strength of the diagonal brace, the stiffness of the double tail fins, and the vibration are within acceptable ranges.

[0024] Step S11: Using fluid dynamics software, set the incoming flow velocity according to the design speed to obtain the ship's resistance and the streamline of the incoming flow to the diagonal brace; the resistance difference between Scheme 1 and Scheme 0 is the resistance of the diagonal brace itself.

[0025] Step S12: Extract the streamline angles of the incoming flow at multiple waterline heights of the inclined brace in Scheme 1, calculate the average value, and use it as the centerline angle of the cross-section of the inclined brace at each waterline height. Smoothly transition the outer contour of the inclined brace to obtain a new inclined brace scheme, which is Scheme 2. Repeat step S11 to obtain the inclined brace resistance of Scheme 2.

[0026] Step S13: Compare the bracing resistance of Scheme 1 and Scheme 2, and select the scheme with the smaller bracing resistance as the new scheme;

[0027] Step S14: Repeat steps S12 and S13 until the centerline angle of the diagonal brace cross section and the diagonal brace scheme with the least resistance are selected.

[0028] Furthermore, in step S3, from the perspective of ensuring the structural strength of the diagonal brace itself, a design method for the thickness and area of ​​the diagonal brace cross section is given.

[0029] Furthermore, in step S5: from the perspective of ensuring the stiffness of the double tail fins and reducing vibration, a design method for the minimum side angle between the central axis of the brace and the center line of the tail fin is given.

[0030] Furthermore, in step S6, from the perspective of ensuring the structural strength of the diagonal brace itself, a design method for the minimum longitudinal tilt angle between the centerline of the diagonal brace and the centerline of the tail shaft is given.

[0031] Furthermore, in step S12, in order to reduce the disturbance effect of the inclined brace on the surrounding flow field, the average value of the streamline angle of the incoming flow at multiple waterline heights is taken as the centerline angle of the cross-section of the inclined brace.

[0032] Furthermore, in steps S11 and S15, fluid dynamics software is used to select the angle of the cross-section of the diagonal brace with the least resistance from multiple diagonal brace cross-section centerline angle schemes, with the goal of minimizing the resistance of the diagonal brace itself.

[0033] The present invention, by adopting the above-described technical solution, has the following beneficial effects compared with the prior art:

[0034] The ship's twin tail fin end brace and its design method of the present invention ensure the structural strength of the brace itself, the stiffness of the tail fin, and the vibration reduction requirements, while minimizing the average angle of attack of the brace with the incoming flow at each waterline height. This can minimize the interference of the brace on the surrounding flow field and effectively reduce the drag of the brace itself. Attached Figure Description

[0035] Figure 1 This is a linear stern view of the stern portion of the twin-fin boat-shaped structure of the present invention;

[0036] Figure 2 This is a linear side view of the stern portion of the twin-fin boat-shaped structure of the present invention;

[0037] Figure 3This is a top view of the stern portion of the twin-fin boat-shaped structure of the present invention.

[0038] Figure 4 This is a cross-sectional view of the diagonal brace of the present invention;

[0039] Figure 5 This is a schematic diagram of the incoming flow streamline in front of the diagonal brace of the present invention;

[0040] The accompanying figures are labeled as follows:

[0041] 1-Double tail fins; 2-Caudal fin centerline; 3-Tail shaft centerline; 4-Tail shaft diameter d; 5-Dip brace at the end of the double tail fins; 6-Central axis of the brace; 7-Side angle α; 8-Heel angle β; 9-Cross-section of the brace; 10-Minimum thickness of the brace cross-section; 11-Minimum area of ​​the brace cross-section; 12-Cental fin centerline; 13-Angle θ of the brace cross-section centerline; 14-Streamline of the incoming flow from the brace; 15-Angle of the incoming flow from the brace; 16-Angle of attack γ; 17-Propeller radius R; 18-Vertical plane passing through the tail shaft centerline; A1-Cross-section of the brace at the connection with the tail fin; A2-Cross-section of the brace at 0.7R; A3-Cross-section of the brace at R; A4-Cross-section of the brace at 1.25R; A5-Cross-section of the brace at the connection with the bottom plate. Detailed Implementation

[0042] The present invention will now be described in detail and specifically through specific embodiments to enable a better understanding of the invention. However, the following embodiments do not limit the scope of the invention.

[0043] like Figures 1 to 5 As shown, this embodiment provides a drag-reducing brace at the end of a ship's twin tail fins, including twin tail fins and a brace. The brace connects the twin tail fins to the transverse strong structure above the bottom plate. There is a side angle between the central axis of the brace and the center line of the tail fins, and a longitudinal angle between the central axis of the brace and the center line of the tail shaft. The average angle of attack between the center line of the cross section of the brace and the incoming flow at each waterline height is the smallest, which can effectively reduce the interference of the brace on the surrounding flow field, thereby achieving a better drag reduction effect.

[0044] Preferably, the minimum lateral angle between the central axis of the diagonal brace and the center line of the caudal fin is not less than 50°.

[0045] Preferably, the minimum pitch angle between the centerline of the diagonal brace and the centerline of the tail shaft is not less than 60°.

[0046] Preferably, the thickness of the diagonal brace cross-section is 0.35d, and the area is 0.45d. 2 , where d is the diameter of the tail shaft at the connection between the brace and the tail fin.

[0047] This embodiment provides a design method for drag-reducing diagonal braces at the ends of twin tail fins on ships, such as... Figure 1-5As shown, the specific steps include the following:

[0048] Step S1: In 3D geometry software, complete the geometric modeling of the twin-fin hull shape (e.g., Figure 1 As shown in ,2,3 (excluding the tail fin brace), this is scheme 0;

[0049] Step S2: Using fluid dynamics software, set the incoming flow velocity according to the design speed to obtain the ship's resistance and the streamlines 14 and angles 15 of the incoming flow towards the brace at different waterline heights (e.g., Figure 5 (as shown);

[0050] Step S3: As Figure 4 As shown, referring to the relevant requirements for stern shaft brackets in the structural section of the China Classification Society's "Rules for Classification of Steel Seagoing Ships," and considering that the stiffness of the stern fin itself is greater than the stiffness of a single stern shaft bracket, the thickness 10 of the diagonal brace cross-section is determined to be 0.35d and the area 11 to be 0.45d. 2 , where d is the diameter of the tail shaft at the connection between the diagonal brace and the tail fin;

[0051] Step S4: As Figure 4 As shown, according to the thickness 10 and area 11, the symmetrical streamlined NACA0018 is selected as the geometry of the cross section 8 of the diagonal brace.

[0052] Step S5: As Figure 1 As shown, the minimum side angle α between the central axis 6 of the diagonal brace and the center line 2 of the tail fin is 50°; within the range of the minimum side angle α, the side angle 7 between the central axis 6 of the diagonal brace and the center line 2 of the tail fin is determined to be 60° based on the position of the longitudinal strong structure above the bottom plate near the outer side of the tail fin and the upper end of the diagonal brace.

[0053] Step S6: As Figure 2 As shown, the minimum pitch angle β between the centerline 6 of the diagonal brace and the centerline 3 of the stern shaft is not less than 60°; within the range of the minimum pitch angle β, the pitch angle 8 between the centerline 6 of the diagonal brace and the centerline 3 of the stern shaft is determined to be 70° based on the position of the transverse strong structure above the bottom plate near the outer side of the tail fin and the upper end of the diagonal brace.

[0054] Step S7: As Figure 1 As shown in Figure 2, starting from the outer side of the end of the double tail fin along the side angle 7 and the longitudinal angle 8, the cross section 9 of the diagonal brace is stretched to the bottom plate to determine the geometry of the entire diagonal brace, thereby determining the length of the diagonal brace. The stretching line is the central axis 6 of the diagonal brace.

[0055] Step S8: Extract the streamline angles 15 of the incoming flow at multiple waterline heights of the inclined brace in Scheme 0, including: the connection between the inclined brace and the tail fin (A1), 0.7R (A2), R (A3), 1.25R (A4), and the connection between the inclined brace and the bottom plate (A5), where R is the propeller radius 17. Calculate the average value of each streamline angle as the centerline angle 13 of the inclined brace cross-section. Rotate the cross-section 9 of the inclined brace at each waterline height around the centerline 6 of the inclined brace, counterclockwise on the port side and clockwise on the starboard side, to the centerline angle 13 of the inclined brace cross-section, and smoothly transition the outer contour of the inclined brace to obtain the inclined brace.

[0056] Step S9: In the three-dimensional geometry software, complete the geometric modeling of the diagonal brace; install the geometric shape of the diagonal brace according to the side slope angle 7 and longitudinal tilt angle 8 at the end of the twin tail fins and the strong frame position of the bottom plate structure, as Scheme 1;

[0057] Step S10: Use structural finite element software to check the structural strength of the diagonal brace and the stiffness and vibration of the double tail fins to ensure that the structural strength of the diagonal brace and the stiffness and vibration of the double tail fins are within acceptable ranges.

[0058] Step S11: Using fluid dynamics software, set the incoming flow velocity according to the design speed to obtain the ship's resistance and the streamline of the incoming flow to the diagonal brace; the resistance difference between Scheme 1 and Scheme 0 is the resistance of the diagonal brace itself.

[0059] Step S12: Extract the streamline angle 15 of the incoming flow at multiple waterline heights of the inclined brace in Scheme 1, including: the connection between the inclined brace and the tail fin (A1), 0.7R (A2), R (A3), 1.25R (A4), and the connection between the inclined brace and the bottom plate (A5), where R is the propeller radius 17. Calculate the average of the five streamline angles as the centerline angle 13 of the inclined brace cross-section, and smoothly transition the outer contour of the inclined brace to obtain a new inclined brace scheme, which is Scheme 2; repeat step S11 to obtain the inclined brace resistance of Scheme 2;

[0060] Step S13: Compare the bracing resistance of Scheme 1 and Scheme 2, and select the scheme with the smaller bracing resistance as the new scheme;

[0061] Step S14: Repeat steps S12 and S13 until the center line angle 13 of the diagonal brace cross section with the least resistance and the diagonal brace scheme are selected.

Claims

1. A drag-reducing diagonal brace at the end of a ship's twin tail fins, characterized in that: The system includes dual tail fins and a diagonal brace. The dual tail fins are connected to the transverse strong structure above the bottom plate by the diagonal brace. The diagonal brace has a side angle between its central axis and the center line of the tail fin, and a longitudinal angle between its central axis and the center line of the tail shaft. The average angle of attack between the center line of the diagonal brace cross section and the incoming flow at each waterline height is minimized, which can effectively reduce the interference of the diagonal brace on the surrounding flow field, thereby achieving better drag reduction effect. The minimum side angle between the central axis of the diagonal brace and the center line of the tail fin is not less than 50°, and the minimum longitudinal angle between the central axis of the diagonal brace and the center line of the tail shaft is not less than 60°.

2. The drag-reducing diagonal brace at the end of the ship's twin tail fins according to claim 1, characterized in that: The thickness of the cross-section of the diagonal brace is 0.35d, and the area is 0.45d. 2 , where d is the diameter of the tail shaft at the connection between the brace and the tail fin.

3. A design method for a drag-reducing diagonal brace at the end of a ship's twin tail fins as described in claim 1 or 2, characterized in that, Taking into account the structural strength of the brace itself, the stiffness and vibration of the tail fin, and the drag of the brace itself, the following requirements were considered: determining the thickness and area of ​​the brace cross-section; determining the geometry of the brace cross-section; determining the side angle between the centerline of the brace and the centerline of the tail fin; determining the longitudinal angle between the centerline of the brace and the centerline of the tail shaft; taking the average of the streamline angles of the incoming flow at multiple waterline heights as the centerline angle of the brace cross-section; and using fluid dynamics software to screen the centerline angle of the brace cross-section with the least drag.

4. The design method according to claim 3, characterized in that, Specifically, the steps include the following: Step S1: In 3D geometry software, complete the geometric modeling of the twin-tail fin hull shape, excluding the tail fin brace, as scheme 0; Step S2: Using fluid dynamics software, set the incoming flow velocity according to the design speed to obtain the ship's resistance and the streamlines and angles of the incoming flow to the inclined support at different waterline heights; Step S3: Referring to the relevant design requirements for the stern shaft support arm in the structural section of the China Classification Society's "Rules for Classification of Steel Seagoing Ships," and considering that the stiffness of the tail fin itself is greater than the stiffness of a single stern shaft support, the thickness of the diagonal brace cross-section is determined to be 0.35d and the area to be 0.45d. 2 , where d is the diameter of the tail shaft at the connection between the diagonal brace and the tail fin; Step S4: Select a symmetrical streamline shape as the geometry of the cross-section of the diagonal brace according to the thickness and area of ​​the cross-section of the diagonal brace; Step S5: Within the minimum side angle range, based on the position of the longitudinal strong structure above the bottom plate near the outer side of the tail fin and the upper end of the brace, determine the side angle between the brace and the center line of the tail fin, that is, the minimum side angle between the center line of the brace and the center line of the tail fin is not less than 50°. Step S6: Within the minimum trim angle range, determine the trim angle between the brace and the centerline of the stern shaft based on the position of the transverse strong structure above the bottom plate near the outer side of the tail fin and the upper end of the brace. That is, the minimum trim angle between the centerline of the brace and the centerline of the stern shaft shall not be less than 60°. Step S7: Along the aforementioned side angle and longitudinal angle, starting from the outer side of the end of the twin tail fins, stretch the cross-section of the diagonal brace to the bottom plate to determine the geometry of the entire diagonal brace, thereby determining the length of the diagonal brace, wherein the stretching line is the central axis of the diagonal brace; Step S8: Extract the streamline angles of the incoming flow at multiple waterline heights of the diagonal brace in Scheme 0, calculate the average value of each streamline angle, and use it as the centerline angle of the diagonal brace cross-section; rotate the diagonal brace cross-section at each waterline height counterclockwise on the port side and clockwise on the starboard side around the centerline of the diagonal brace to the centerline angle of the diagonal brace cross-section, and smoothly transition the outer contour of the diagonal brace to obtain the diagonal brace; Step S9: In the 3D geometry software, complete the geometric modeling of the diagonal brace; install the geometric shape of the diagonal brace according to the side angle and longitudinal angle at the end of the twin tail fin and the strong frame position of the bottom structure, as Scheme 1; Step S10: Use structural finite element software to check the structural strength of the diagonal brace and the stiffness of the double tail fins to ensure that the structural strength of the diagonal brace, the stiffness of the double tail fins, and the vibration are within acceptable ranges. Step S11: Using fluid dynamics software, set the incoming flow velocity according to the design speed to obtain the ship's resistance and the streamline of the incoming flow to the diagonal brace; the resistance difference between Scheme 1 and Scheme 0 is the resistance of the diagonal brace itself. Step S12: Extract the streamline angles of the incoming flow at multiple waterline heights of the inclined brace in Scheme 1, calculate the average value, and use it as the centerline angle of the cross-section of the inclined brace at each waterline height. Smoothly transition the outer contour of the inclined brace to obtain a new inclined brace scheme, which is Scheme 2. Repeat step S11 to obtain the inclined brace resistance of Scheme 2. Step S13: Compare the bracing resistance of Scheme 1 and Scheme 2, and select the scheme with the smaller bracing resistance as the new scheme; Step S14: Repeat steps S12 and S13 until the centerline angle of the diagonal brace cross section and the diagonal brace scheme with the least resistance are selected.

5. The design method according to claim 4, characterized in that: In step S3, from the perspective of ensuring the structural strength of the diagonal brace itself, a design method for the thickness and area of ​​the diagonal brace cross section is given.

6. The design method according to claim 4, characterized in that: In step S5: From the perspective of ensuring the stiffness of the double tail fins and reducing vibration, a design method for the minimum side slope angle between the central axis of the brace and the center line of the tail fin is given.

7. The design method according to claim 4, characterized in that: In step S6, from the perspective of ensuring the structural strength of the diagonal brace itself, a design method for the minimum longitudinal tilt angle between the centerline of the diagonal brace and the centerline of the tail shaft is given.

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