A method for vibration reduction assessment of a pre-propeller energy saving device

Abstract

The present application belongs to the technical field of vibration performance evaluation of pre-propeller energy-saving device, and particularly relates to a pre-propeller energy-saving device vibration simplified evaluation method, which comprises the following steps: performing parameter sensitivity analysis on the component structure of the pre-propeller energy-saving device and determining the component structure playing a leading role; establishing a simplified model for pre-propeller energy-saving device vibration evaluation; applying a simply supported boundary condition to the unit node connected with the hull of the pre-propeller energy-saving device; adjusting the unit normal, starting the fluid domain, defining the single / double-sided immersed unit, setting the VMOPT parameter; performing finite element modal calculation, and outputting the first three order vibration modes of the pre-propeller energy-saving device. The present application is based on the finite element modal analysis method, and establishes a simplified vibration evaluation method, which can be quickly implemented and achieve a certain accuracy, so as to realize the rapid evaluation of vibration performance in the pre-propeller energy-saving device scheme design stage.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of vibration performance evaluation of pre-propeller energy-saving devices of ships, and particularly relates to a vibration simplified evaluation method for pre-propeller energy-saving devices by using a modal analysis method. BACKGROUND

[0002] As shown in Figure 1 , a pre-propeller energy-saving device (also referred to as a "pre-positioned duct") is a device installed in front of a propeller to improve the flow field and improve the energy efficiency of a ship. It has a simple structure, is easy to install, has excellent energy-saving effect, and is widely used by shipowners at home and abroad. It mainly includes a flow guide pipe, a flow guide fin (if any), a connecting elbow plate or a flow guide plate and other appendage structures. The flow guide pipe is annular in structure and is simply referred to as a duct. The flow guide fin is radial in structure and is simply referred to as a fin.

[0003] From the structural aspect, the cross section of the duct (fin) is usually airfoil-shaped, as shown in Figure 2 . The duct (fin) is welded by transverse and longitudinal partitions and inner and outer shell plates, and the end part is generally a circular pipe or a round steel. The pre-positioned duct belongs to an elongated structure, and the length-width ratio can reach 14.5. Due to the weak stiffness, the first-order natural frequency of the duct is also relatively low. In particular, the pre-rotation duct is prone to low-frequency resonance with the propeller, causing vibration fatigue damage. Once the duct falls off, it may even damage the propeller. There was a case of cracks found at the fixed end of the stern pre-positioned duct of an oil tanker in China. It was analyzed that the cracks were caused by the resonance of the duct in water and the ship body.

[0004] The pre-propeller energy-saving device has a certain vibration risk, and the vibration performance needs to be evaluated by a finite element method during product development. This method is mature and reliable, but the modeling workload is large and time-consuming. SUMMARY

[0005] The purpose of the present application is to solve the technical problems in the background art. To this end, a pre-propeller energy-saving device vibration simplified evaluation method is provided. In order to reveal the vibration characteristics of the pre-propeller energy-saving device, the parameter sensitivity of the component structure of the pre-propeller energy-saving device is analyzed, including the thickness of the stern, the elbow plate, the internal partition plate and the outer plate, so as to determine the structures that play a leading role in the modal of the pre-propeller energy-saving device: the thickness of the duct outer plate, the internal transverse partition plate and the elbow plate.

[0006] In order to achieve the above purpose, the technical scheme adopted by the present application is as follows:

[0007] A pre-propeller energy-saving device vibration simplified evaluation method, comprising the following steps:

[0008] Step S1: performing parameter sensitivity analysis on the component structure of the pre-propeller energy-saving device;

[0009] Step S2: determining the component structure which plays a leading role in the modal of the pre-propeller energy-saving device;

[0010] Step S3: based on the component structure determined in step S2, a simplified model for vibration evaluation of the pre-propeller energy-saving device is established;

[0011] Step S4: a simply supported boundary condition is applied to the unit node connected with the hull of the pre-propeller energy-saving device;

[0012] Step S5: adjusting the unit normal, starting the fluid domain, defining the single / double-face immersed unit, and setting the VMOPT parameter;

[0013] Step S6: performing finite element modal calculation, and outputting the first three order vibration modes of the pre-propeller energy-saving device.

[0014] The following is a further limited technical solution of the application, the factors which have parameter sensitivity to the component structure of the pre-propeller energy-saving device include: the stern, the knee plate, the internal partition plate, and the thickness of the duct outer plate.

[0015] The following is a further limited technical solution of the application, the stern is used for performing parameter sensitivity analysis on the component structure of the pre-propeller energy-saving device, so as to judge the influence of the stern on the modal of the pre-propeller energy-saving device, and the method comprises the following steps: setting two groups of pre-propeller energy-saving devices, and the two groups of pre-propeller energy-saving devices adopt independent models and an integral model respectively; the model range of the independent model only includes the pre-propeller energy-saving device itself, and simply supported constraints are applied at the connection between the pre-propeller energy-saving device and the main hull structure; the model range of the integral model is connected with the stern cast steel piece through the root of the fin, and is connected with the hull through the knee plate at the front end in the duct; and the modal calculation results of the two groups of pre-propeller energy-saving devices adopting the independent model or the integral model are compared.

[0016] The following is a further limited technical solution of the application, the first order natural frequency error value between the independent model and the integral model is within 13%.

[0017] The following is a further limited technical solution of the application, the knee plate is used for performing parameter sensitivity analysis on the component structure of the pre-propeller energy-saving device, so as to judge the influence of the knee plate on the modal of the pre-propeller energy-saving device, and the method comprises the following steps: setting two groups of pre-propeller energy-saving devices, and the two groups of pre-propeller energy-saving devices adopt integral models; the integral model of one group of pre-propeller energy-saving devices has a knee plate, and the integral model of the other group of pre-propeller energy-saving devices does not have a knee plate; and the modal calculation results of the two groups of pre-propeller energy-saving devices are compared.

[0018] The following is a further limited technical solution of the application, for the pre-propeller energy-saving device, the duct located above the axis is connected to the midship section through the vertical knee plate, in order to avoid transverse vibration of the duct, the upper front end of the duct is provided with a knee plate parallel to the outer side of the duct and connected with the hull plate.

[0019] The following is a further limited technical solution of the application, the internal partition plate is parameter sensitive to the composition structure of the pre-paddle energy-saving device, so as to judge the influence of the internal partition plate on the mode of the pre-paddle energy-saving device, comprising: setting two groups of pre-paddle energy-saving devices, both using the overall model; comparing the modes of the two groups of pre-paddle energy-saving devices containing different types of partition plates.

[0020] The following is a further limited technical solution of the application, the different types of partition plates include: no partition plate, only longitudinal partition plate, only transverse partition plate, original model.

[0021] The following is a further limited technical solution of the application, the thickness of the outer plate of the duct is parameter sensitive to the composition structure of the pre-paddle energy-saving device, so as to judge the influence of the thickness of the outer plate on the mode of the pre-paddle energy-saving device, comprising: setting two groups of pre-paddle energy-saving devices, both using the overall model; adjusting the thickness of the outer plate of the duct based on the thickness of the main plate of the duct, so as to compare the mode calculation results of the two groups of pre-paddle energy-saving devices under different thicknesses of the outer plate of the duct.

[0022] The following is a further limited technical solution of the application, the composition structure that plays a leading role in the mode of the pre-paddle energy-saving device includes: the thickness of the outer plate of the duct, the internal transverse partition plate, and the elbow plate.

[0023] Compared with the prior art, the application has the following technical effects:

[0024] Based on the finite element modal analysis method, the application establishes a simplified vibration evaluation method, which can be quickly implemented and achieve a certain accuracy, so as to realize rapid evaluation of vibration performance in the pre-paddle energy-saving device scheme design stage;

[0025] The first-order natural frequency of the pre-paddle energy-saving device obtained by the simplified vibration evaluation method is very close to the calculation result of the traditional finite element method, but the efficiency is greatly improved, and has good practical value;

[0026] The parameter sensitivity analysis finds that the transverse partition plate and the elbow plate play a leading role in the first-order natural frequency of the duct, and the thickness of the outer plate has little effect on the first-order natural frequency, which can guide the structure design of the pre-paddle energy-saving device and help designers take more effective measures to avoid resonance;

[0027] The application will be further described below in conjunction with the drawings and examples. DESCRIPTION OF DRAWINGS

[0028] In order to more clearly illustrate the technical solutions in the embodiments of the application or the prior art, the drawings needed in the embodiments or the prior art will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the application, and other drawings can be obtained by those skilled in the art without creative labor.

[0029] Figure 1 is a schematic diagram of the installation of the pre-propeller energy saving device on a real ship;

[0030] Figure 2 is a duct airfoil section view of the pre-propeller energy saving device;

[0031] Figure 3 is a comparative diagram of the modal analysis results of the 177K-BC pre-propeller energy saving device;

[0032] Figure 4 is a comparative diagram of the modal analysis results of the 308K-VLCC pre-propeller energy saving device;

[0033] Figure 5 is a structural schematic diagram of the knee plate;

[0034] Figure 6 is a comparative diagram of the influence of the knee plate of the 177K-BC and 308K-VLCC pre-propeller energy saving devices on the natural frequency of the duct;

[0035] Figure 7 is a structural schematic diagram of the internal transverse / longitudinal partition plate of the duct;

[0036] Figure 8 is a comparative diagram of the influence of the partition plate of the 177K-BC and 308K-VLCC pre-propeller energy saving devices on the natural frequency of the energy saving device;

[0037] Figure 9 is a cloud diagram of the outer plate thickness of the 177K-BC and 308K-VLCC pre-propeller energy saving devices;

[0038] Figure 10 is a comparative diagram of the influence of the outer plate thickness of the 177K-BC and 308K-VLCC pre-propeller energy saving devices on the modal of the pre-duct;

[0039] Figure 11 is a method flowchart;

[0040] Figure 12 is a modal analysis result diagram of the 114K-OT pre-propeller energy saving device;

[0041] Figure 13 is a modal analysis result diagram of the 76K-BC pre-propeller energy saving device;

[0042] Figure 14 is a modal analysis result diagram of the 57K-BC pre-propeller energy saving device. DETAILED DESCRIPTION

[0043] In order to make the above objectives, characteristics and advantages of the present application more obvious and easy to understand, the specific embodiments of the present application are described in detail below with reference to the drawings. In the following description, a large number of specific details are set forth in order to facilitate a thorough understanding of the present application. However, the present application can be implemented in many other ways different from those described herein, and those skilled in the art can make similar improvements without departing from the concept of the present application, so the present application is not limited by the specific embodiments disclosed below.

[0044] As shown in Figure 11 , a pre-propeller energy-saving device vibration simplified evaluation method is provided, comprising the following steps:

[0045] Step S1: parameter sensitivity analysis is performed on the component structure of the pre-propeller energy-saving device;

[0046] Step S2: determine the component structure that plays a leading role in the modal of the pre-propeller energy-saving device;

[0047] Step S3: based on the component structure determined in step S2, a simplified model for evaluating the vibration of the pre-propeller energy-saving device is established;

[0048] Step S4: a simply supported boundary condition is applied to the unit node connected to the ship body of the pre-propeller energy-saving device;

[0049] Step S5: adjust the unit normal, start the fluid domain, define the single / double-sided immersed unit, and set the VMOPT parameter;

[0050] Step S6: perform finite element modal calculation, and output the first three order vibration modes of the pre-propeller energy-saving device.

[0051] (1) Influence of ship body tail on pre-propeller energy-saving device modal

[0052] The model range only includes the energy-saving device itself (referred to as "independent model"), and a simply supported constraint is applied at the connection between the energy-saving device and the main ship body structure. Generally speaking, the "overall model" can accurately reflect the natural modal of the pre-positioned guide pipe compared with the "independent model", the pre-processing time of the "independent model" is much less than that of the "overall model", and the modal calculation efficiency is higher, but the result accuracy, i.e. the influence of the model range on the modal of the pre-positioned guide pipe, needs to be further analyzed. The modal calculation results of the 177K-BC and 308K-VLCC two models are compared as shown in Figure 3 and 4 .

[0053] The first order natural frequency error value obtained by the two models is within 13%, and the first three order vibration modes obtained are close. The pre-whirl guide pipe is mainly connected to the ship tail cast steel through the root of the fin, and the front end is connected to the ship body through a knee plate in the guide pipe. The error mainly comes from the boundary difference of the knee plate in the modal calculation of the "independent model" and the "overall model", but the error value can be controlled within a reasonable range.

[0054] In summary, the independent model has the advantage of high computational efficiency, and the error value of the independent model is within an acceptable range.

[0055] (2) The influence of the elbow plate on the modal of the pre-propeller energy-saving device

[0056] For the pre-propeller energy-saving device, the duct located above the axis is connected to the midship section through the vertical elbow plate. To avoid transverse vibration of the duct, the upper front end of the duct is generally provided with an elbow plate (both sides of the midship vertical elbow plate are provided) parallel to the outer side of the duct and connected to the outer plate of the ship body, as shown in Figure 5 .

[0057] For the 177K-BC and 308K-VLCC energy-saving devices, the "overall model" is used, as shown in Figure 6 . Comparing the influence of the elbow plate on the natural frequency of the energy-saving device shows that the influence of the elbow plate on the first three natural frequencies of the duct is significant. The elbow plate will significantly increase the first three natural frequencies of the duct, especially for the first natural frequency, which increases by 83% for the 177K-BC energy-saving device and by 240% for the 308K-VLCC energy-saving device.

[0058] For structural designers, to prevent resonance between the pre-swirl duct and the propeller, the elbow plate can significantly change the vibration mode and natural frequency of the duct, achieving the purpose of avoiding the blade frequency and multiple blade frequency of the propeller. Therefore, during the vibration evaluation stage, the vibration evaluation model must consider the influence of the elbow plate.

[0059] (3) Analysis of the influence of internal partitions on the modal of the pre-propeller energy-saving device

[0060] The internal partitions of the pre-propeller energy-saving device duct and fin are longitudinally and transversely supported by partition members. The partition diagram is shown in Figure 7 .

[0061] Based on the 177K-BC and 308K-VLCC energy-saving devices, the "overall model" is used to compare the modal of the energy-saving device containing different types of partitions (no partition, only longitudinal partition, only transverse partition, and original model), as shown in Figure 8 . The internal partitions have a significant influence on the modal of the pre-duct. Increasing the transverse partition or the longitudinal partition will increase the first three natural frequencies of the energy-saving device. As can be seen from the figure, the duct containing only the outer plate directly undergoes vibration evaluation, and the first frequency will be within the range of the blade frequency or multiple blade frequency of the propeller, which can easily lead to resonance of the energy-saving device, posing a hidden danger to the safety of the energy-saving device and the propeller.

[0062] It is worth noting that although the increase of transverse / longitudinal bulkhead will make the fundamental frequency increase, but the transverse bulkhead and longitudinal bulkhead on the front of the duct base frequency there are differences, for example, the first order frequency, the ratio of the longitudinal bulkhead energy saving device and the original model is: 177K-BC (43%), 308K-VLCC (67%); The ratio of the transverse bulkhead energy saving device and the original model is: 177K-BC (90%), 308K-VLCC (94%). The energy saving device containing only the transverse bulkhead is more than 90% of the first order frequency of the original model, compared with the longitudinal bulkhead duct, more close to the modal results of the original model, indicating that the transverse bulkhead has a dominant effect on the overall modal of the front duct, and the longitudinal bulkhead has a limited effect on the modal. Because the number and arrangement of the transverse bulkhead can be quickly determined according to the main dimensions of the duct, therefore, in the initial design stage of rapid vibration evaluation, it is recommended not to consider the longitudinal bulkhead, and only the transverse bulkhead can obtain more accurate vibration results.

[0063] (4) The effect of the thickness of the duct outer plate on the modal of the pre-propeller energy saving device

[0064] In the structural design stage of the pre-propeller energy saving device, the outer plate thickness of the duct and the fin needs to meet the requirements of the specification for the outer plate of the ship body tail, and also needs to meet the requirements of the specification for the plate thickness under the tail slamming pressure. Taking the 177K-BC and 308K-VLCC energy saving devices as examples, Figure 9 The thickness of the duct outer plate is shown in the figure. Generally, the duct plate thickness is slightly larger than the fin outer plate thickness, and the outer plate of the connecting part with the ship body is also locally thickened (mainly at the end of the fin), but the main plate thickness of the outer plate changes little.

[0065] Due to the change of the thickness of the duct outer plate, the weight distribution of the energy saving device will be affected, and the modal of the energy saving device will be affected. Based on the 177K-BC and 308K-VLCC energy saving devices, the main plate thickness of the duct is taken as the benchmark to adjust the plate thickness of the duct, and the effect of the plate thickness on the modal of the energy saving device is analyzed, as shown in the following Figure 10 With the gradual increase of the plate thickness, the first three order natural frequencies of the four types of front ducts show an increasing trend.

[0066] For the structural designer, the change of the first order frequency after the local plate thickness modification is more concerned. Take the calculation results of the type 2 duct as an example, the initial design plate thickness of the 177K-BC duct is 15mm, and the first order frequency is 15.1Hz. The thickness of the fin outer plate is reduced to 14mm in the detailed design stage, and the first order frequency is reduced to 15Hz, with a frequency change of 0.6%. The initial design plate thickness of the 308K-VLCC duct is 20mm, and the first order frequency is 13.8Hz. The thickness of the fin outer plate is reduced to 14mm, and the thickness of the end outer plate is increased. The first order frequency is increased to 13.5Hz, with a frequency change of 2%. Since the local adjustment of the plate thickness has little effect on the first order frequency, the simplified vibration evaluation according to the recommended plate thickness in the initial design stage can obtain more accurate results.

[0067] In order to verify the correctness of the method, as shown in Figures 12-14 The first three order natural frequencies of the "simplified model" and "overall model" of the three propeller front energy-saving devices 114K-OT, 76K-BC and 57K-BC are compared, and the results of the "overall model" are taken as a reference to evaluate the error value of the results of the "simplified model".

[0068] It is found that the error value of the first order frequency calculated by the simplified model compared with the overall model is 5%(114K-OT), 1%(76K-BC) and 8%(76K-BC) respectively, which meets the calculation accuracy requirement. Compared with the traditional vibration evaluation method which needs to evaluate the vibration of the overall model, the pre-processing time of the simplified model is shortened by an average of 60%, which greatly shortens the vibration evaluation period.

[0069] The above is only the preferred embodiment of the present application, and does not limit the present application in any form. Any skilled person in the art can make many possible changes and modifications to the technical solution of the present application, or modify it into equivalent embodiments with equivalent changes, without departing from the scope of the technical solution of the present application. Therefore, any equivalent changes made according to the shape, structure and principle of the present application, without departing from the technical solution of the present application, should be covered by the protection scope of the present application.

Claims

1. A method of vibration reduction assessment of a pre-propeller energy saving device, characterized in that, The method comprises the following steps: Step S1: parameter sensitivity analysis is performed on the component structure of the pre-propeller energy-saving device; Factors that have parameter sensitivity on the component structure of the pre-propeller energy-saving device include: stern, knee plate, internal partition, thickness of the duct outer plate; The parameter sensitivity analysis is performed on the component structure of the pre-propeller energy-saving device, so as to determine the influence of the stern on the modal of the pre-propeller energy-saving device, which comprises the following steps: two groups of pre-propeller energy-saving devices are set, and the two groups of pre-propeller energy-saving devices respectively adopt independent models and overall models; the model range of the independent model only includes the pre-propeller energy-saving device itself, and a simply supported constraint is applied at the connection between the pre-propeller energy-saving device and the main ship body structure; the model range of the overall model is connected with the stern cast steel piece through the root of the fin, and the front end in the duct is connected with the ship body through the knee plate; the modal calculation results of the two groups of pre-propeller energy-saving devices adopting the independent models or the overall models are compared; The parameter sensitivity analysis is performed on the component structure of the pre-propeller energy-saving device, so as to determine the influence of the knee plate on the modal of the pre-propeller energy-saving device, which comprises the following steps: two groups of pre-propeller energy-saving devices are set, and the two groups of pre-propeller energy-saving devices respectively adopt independent models and overall models; one group of pre-propeller energy-saving devices has a knee plate, and the other group of pre-propeller energy-saving devices does not have a knee plate; the modal calculation results of the two groups of pre-propeller energy-saving devices are compared; The parameter sensitivity analysis is performed on the component structure of the pre-propeller energy-saving device, so as to determine the influence of the internal partition on the modal of the pre-propeller energy-saving device, which comprises the following steps: two groups of pre-propeller energy-saving devices are set, and the two groups of pre-propeller energy-saving devices respectively adopt independent models and overall models; the modal of the two groups of pre-propeller energy-saving devices containing different types of partitions is compared; The parameter sensitivity analysis is performed on the component structure of the pre-propeller energy-saving device, so as to determine the influence of the thickness of the duct outer plate on the modal of the pre-propeller energy-saving device, which comprises the following steps: two groups of pre-propeller energy-saving devices are set, and the two groups of pre-propeller energy-saving devices respectively adopt independent models and overall models; the thickness of the outer plate of the duct is adjusted based on the thickness of the main plate of the duct, so as to compare the modal calculation results of the two groups of pre-propeller energy-saving devices under different thicknesses of the duct outer plate; Step S2: determine the component structure that plays a leading role in the modal of the pre-propeller energy-saving device; Step S3: based on the component structure determined in step S2, a simplified model for vibration evaluation of the pre-propeller energy-saving device is established; Step S4: a simply supported boundary condition is applied to the unit node connected with the ship body; Step S5: adjust the unit normal, start the fluid domain, define the single / double-sided immersed unit, and set the VMOPT parameter; Step S6: perform finite element modal calculation, and output the first three order vibration modes of the pre-propeller energy-saving device.

2. A paddle front energy device vibration reduction assessment method as claimed in claim 1, wherein, The error value of the first order natural frequency between the independent model and the overall model is within 13%.

3. A paddle front energy device vibration reduction assessment method as in claim 1, wherein, For the pre-propeller energy-saving device, the duct located above the axis is connected to the midship section through the vertical knee plate, in order to avoid transverse vibration of the duct, the upper front end of the duct is provided with a knee plate parallel to the outer side of the duct and connected with the ship body outer plate.

4. A paddle front energy device vibration reduction assessment method as claimed in claim 1, wherein, Different types of partitions include: no partition, only longitudinal partition, only transverse partition, and original model.

5. A paddle front energy device vibration reduction assessment method as claimed in claim 1, wherein, The component structure that plays a leading role in the modal of the pre-propeller energy-saving device includes: the thickness of the duct outer plate, the internal transverse partition, and the knee plate.

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