A Variable Stiffness Model Inside a Tube and a Stiffness Adjustment Method

Abstract

The present invention provides a variable stiffness model inside a tube and a stiffness adjustment method, comprising: a plurality of stiffness adjustment plates and a plurality of fixing screws; wherein the stiffness adjustment plate is a cylinder, cut in an "I" shape, and the outer diameter of the stiffness adjustment plate is close to the inner diameter of the test thin-walled tube, and the front and rear ends of the "crossbeam" of the "I"-shaped stiffness adjustment plate respectively have grooves and protrusions of equal size. The "vertical beam" of the "I"-shaped stiffness adjustment plate is a stiffness adjustment beam, and the stiffness of the model is adjusted by designing different widths. Two screw holes are designed at both ends of the "vertical beam". During the test, the stiffness adjustment plate is fixed to the test tube by using a combination of fixing screws and bolts. After the test is completed, the fixing screws are removed and the model can be disassembled and reused.

Description

Technical Field

[0001] The present invention belongs to the field of aerospace engineering and relates to a variable stiffness model inside a tube and a stiffness adjustment method. Background Art

[0002] When conducting some dynamic wind tunnel tests, it is necessary to design the model to be similar to the design prototype in dynamics. This dynamic similarity is usually achieved through mass distribution similarity and stiffness distribution similarity. For the conventional practice of stiffness distribution similarity: the stiffness distribution that the model should have is obtained by finite element calculation, and then relevant stiffness grooves are processed, and the stiffness distribution model is integrally processed to ensure the similarity of the stiffness distribution of the model.

[0003] The existing stiffness similarity models have the following problems:

[0004] (1) Difficult test design. Since complex model design work is required to obtain the accurate stiffness distribution at each position on the simplified model and design the stiffness structure by equivalent methods, it is often difficult to achieve complete similarity with the design prototype in the design process, thus bringing design errors.

[0005] (2) The stiffness distribution is non-adjustable. Once the stiffness is designed and processed, it cannot be adjusted, otherwise the test model will be damaged. Even if a design mistake is found, the design model cannot be modified by changing the model stiffness.

[0006] (3) Model design errors cannot be avoided. Since the errors generated in aspects such as finite element calculation, machining, installation, and design cannot be eliminated, the dynamic characteristics of the actual model may deviate greatly from the expected ones, but still cannot be changed. Even if problems are found in the design components during the ground process, the model still cannot be changed.

[0007] (4) The designed stiffness distribution models cannot be reused. For the next test, it is still necessary to redesign, machine, and manufacture, resulting in a certain degree of waste. Summary of the Invention

[0008] The technical problem solved by the present invention is: overcoming the deficiencies of the prior art, and proposing a variable stiffness model inside a tube and a stiffness adjustment method, which can simply and conveniently adjust the stiffness of the model after ground tests, so as to achieve the goals of reducing the model design difficulty, saving the model processing cost, and increasing the model design accuracy and success rate.

[0009] The technical solution of the present invention is: a variable stiffness model inside a pipe, including N stiffness adjustment plates, N>1; the stiffness adjustment plates are cylindrical, cut in an "I" shape, and the horizontal beams of the stiffness adjustment plates are defined as circular rings at both ends; the vertical beams of the stiffness adjustment plates are vertical arc segments connecting the circular rings at both ends, and two vertical beams are distributed 180 degrees apart on the circumference. The vertical beams of the stiffness adjustment plates are stiffness adjustment beams, and the model stiffness is adjusted by designing different vertical beam chord lengths c.

[0010] The front and rear ends of the crossbeam of the stiffness adjustment plate are respectively processed with grooves and protrusions of equal size. The two adjacent stiffness adjustment plates are spliced through the matching relationship between the grooves and the protrusions. There are screw holes at both ends of each vertical beam. At the screw holes, each stiffness adjustment plate is supported and fixed to the inner wall of the test tube by using fixing screws and bolts.

[0011] Furthermore, M stiffness adjustment plates are prepared as alternatives, and the outer diameter a1, inner diameter a2, horizontal beam width a and vertical beam length b of the M stiffness adjustment plates are all the same, and only the vertical beam chord length c is different, M≥N.

[0012] Furthermore, the outer diameter a1 of the stiffness adjustment piece is 0.1 mm to 0.2 mm smaller than the inner diameter of the test tube, and the inner diameter a2 of the stiffness adjustment piece is 4 mm to 20 mm smaller than the outer diameter a1.

[0013] Furthermore, the crossbeam width a is between 4mm and 8mm, and the vertical beam length b is between 30mm and 50mm, and the larger the selected crossbeam width a, the larger the corresponding vertical beam length b; the vertical beam chord length c ranges from 2mm to 31mm, and each millimeter is used as a gradient. It is processed according to an arithmetic progression, and M / 30 stiffness adjustment plates are produced for each gradient.

[0014] Furthermore, the specific processing positions of the grooves and protrusions at the front and rear ends of the horizontal beam are as follows: the grooves and protrusions are located in the extension direction of each vertical beam, and the centers of the grooves and the protrusions pass through the symmetry axis of the vertical beam and are symmetrical about the symmetry axis of the vertical beam.

[0015] Furthermore, the processing dimensions of the groove and the protrusion are the same, the protrusion height d is in the range of 1 / 2 to 2 / 3 of the beam width a, and the protrusion chord length e is between 8 mm and 20 mm.

[0016] Furthermore, the outer surface of the fixing screw is kept consistent with the outer surface of the test tube, and the diameter of the screw holes opened at both ends of the vertical beam is between M1 and M3.

[0017] Furthermore, the stiffness adjustment plate is made of metal and has an elastic modulus of not less than 70 GPa; the test tube is made of non-metallic material and has an elastic modulus of not more than 10 GPa while ensuring that the shape is not destroyed in the flow field.

[0018] Further, based on the variable stiffness model inside the pipe of the present invention, a method for adjusting the stiffness of the variable stiffness model inside the pipe is provided, which is characterized by including the following steps:

[0019] Step 1: Conduct a ground vibration test on the processed test pipe to obtain the vibration frequency of the test pipe. At this time, no stiffness adjustment pieces are installed on the inner wall of the test pipe.

[0020] Step 2: Compare the vibration frequency obtained in Step 1 with the corresponding design target frequency. At this time, the obtained vibration frequency is lower than the design target frequency. Assemble N stiffness adjustment pieces on the inner wall of the test pipe, and the total length of the assembled N stiffness adjustment pieces is less than or equal to the length of the test pipe.

[0021] Step 3: Conduct a ground vibration test on the test pipe assembled with the stiffness adjustment pieces. If the vibration frequency is lower than the design target frequency, replace the stiffness adjustment piece to increase the chord length c of the vertical beam. If the vibration frequency is higher than the design target frequency, replace the stiffness adjustment piece to decrease the chord length c of the vertical beam. Repeat Step 3 until the vibration frequency obtained in the test reaches the preset accuracy requirement, and then execute Step 4.

[0022] Step 4: Continue to conduct a ground vibration test to obtain the vibration mode of the test pipe.

[0023] Step 5: Judge according to the vibration mode obtained in Step 4: If the amplitude at a local position is higher than the design target, replace the stiffness adjustment piece corresponding to this local position with a stiffness adjustment piece with a smaller chord length c of the vertical beam. If the amplitude at a local position is lower than the design target, replace the stiffness adjustment piece corresponding to this local position with a stiffness adjustment piece with a larger chord length c of the vertical beam. Repeat Steps 4 to 5 until the vibration mode meets the design requirements.

[0024] Further, when adjusting the chord length c of the vertical beam of the stiffness adjustment piece in Step 3, follow the principle that the square of the frequency is proportional to the stiffness, that is where ω is the vibration frequency, k is the stiffness value, and m is the total mass of the test pipe and the stiffness adjustment piece.

[0025] The beneficial effects of the present invention compared with the prior art are:

[0026] (1) The fault tolerance rate of the model design of the present invention is higher. Traditional test methods often require the model design to be in place at one time and must be accurate in all aspects of accuracy, which requires a high demand for model design. However, the structure and method shown in the present invention can be adjusted by changing the stiffness adjustment piece during the test process even if there are design deviations in the initial model design stage, increasing the opportunities for model modification and adjustment, and greatly reducing the difficulty of model design.

[0027] (2) In the traditional design and processing process, there are often deviations between the model and the computationally designed model. Under previous conditions, such design deviations could not be adjusted, which would affect the accuracy of the model test. However, this invention can adjust the model by adjusting the stiffness adjustment piece, reducing or even eliminating the model design errors caused by factors such as processing and installation, making the model more accurate and the corresponding test results more accurate.

[0028] (3) Compared with the traditional one-time stiffness model, the variable stiffness model designed in this invention can remove relevant parts such as stiffness adjustment pieces and fixing bolts after the test, and the test components can be reused, which can greatly reduce the test cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 is the design of the existing stiffness groove model;

[0030] Figure 2 is the isometric view of the stiffness adjustment piece in the embodiment of the present invention;

[0031] Figure 3 is the cross-sectional view of the stiffness adjustment piece in the embodiment of the present invention;

[0032] Figure 4 is the top view of the stiffness adjustment piece in the embodiment of the present invention;

[0033] Figure 5 is the combined assembly drawing of the stiffness adjustment piece in the embodiment of the present invention;

[0034] Figure 6 is the schematic diagram of the vibration mode in the embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0035] The present invention will be further described below with reference to the accompanying drawings.

[0036] As Figure 1 shown, the existing stiffness groove model design includes a test model shell 1 and a traditional stiffness groove 2, which is a one-time integral stiffness model with non-adjustable stiffness distribution. Once the stiffness distribution is designed and processed, it cannot be adjusted. Moreover, the designed stiffness distribution model cannot be reused, and the next test still requires re-designing, processing, and manufacturing.

[0037] The present invention includes several stiffness adjustment pieces 8, and the stiffness adjustment pieces 8 are installed on the inner wall of the test tube through fixing screws.

[0038] As Figure 2As shown, the stiffness adjustment plate 8 includes a horizontal beam 3, a protrusion 4, a vertical beam 5, a screw hole 6, and a groove 7. The stiffness adjustment plate 8 is a cylinder cut in the shape of an "I". The horizontal beam 3 of the stiffness adjustment plate 8 is defined as a circular ring at both ends, and the vertical beam 5 of the stiffness adjustment plate 8 is a vertical segment connecting the two circular rings at both ends. Two vertical beams 5 are distributed 180° apart on the circumference. The vertical beam 5 of the stiffness adjustment plate 8 is a stiffness adjustment beam, and the model stiffness is adjusted by designing different vertical beam chord lengths c. The front and rear ends of the horizontal beam 3 of the stiffness adjustment plate 8 are respectively machined with equal-sized grooves 7 and protrusions 4. Two adjacent stiffness adjustment plates are spliced together through the matching relationship between the grooves 7 and protrusions 4. Screw holes are opened at both ends of each vertical beam 5. At the screw holes, each stiffness adjustment plate 8 is supported and fixed to the inner wall of the test tube by using a fixing screw and a bolt.

[0039] In this embodiment, a total of 150 stiffness adjustment plates 8 are designed. The outer diameter a1, inner diameter a2, horizontal beam width a and vertical beam length b of the 150 stiffness adjustment plates 8 are all the same, and only the vertical beam chord length c is different.

[0040] like Figure 3 As shown, the outer diameter a1 of the stiffness adjustment piece 8 is 0.1 mm to 0.2 mm smaller than the inner diameter of the test tube, and the inner diameter a2 of the stiffness adjustment piece 8 is 4 mm to 20 mm smaller than the outer diameter a1.

[0041] like Figure 4 As shown, the horizontal beam width a ranges from 4 mm to 8 mm, and the vertical beam length b ranges from 30 mm to 50 mm. The larger the horizontal beam width a, the larger the corresponding vertical beam length b. The vertical beam chord length c ranges from 2 mm to 31 mm, with each millimeter representing a gradient. Five stiffness adjustment plates 8 are fabricated for each gradient. The grooves 7 and protrusions 4 are machined at the front and rear ends of the horizontal beam 3 as follows: the grooves 7 and protrusions 4 are located along the extension direction of the vertical beam 5, with the centers of the grooves 7 and protrusions 4 passing through and symmetrically about the axis of symmetry of the vertical beam 5. The grooves 7 and protrusions 4 are machined along the extension direction of both vertical beams 5. The machined dimensions of the grooves 7 and protrusions 4 are identical. In this embodiment, the protrusion height d is 2a / 3, and the protrusion chord length e ranges from 8 mm to 20 mm. Two screw holes 6 are drilled at each end of each vertical beam 5 for the test tube. Screw holes 6 are preferably M1 to M3.

[0042] like Figure 5 When assembling the stiffness adjustment plates 8 on the inner wall of the test tube, bolts are used to secure the stiffness adjustment plates 8 to the interior of the test model housing 1 at the locations of the screw holes 6 and then tightened with fixing screws. During the securing process, the protrusions 4 and grooves 7 of the front and rear stiffness adjustment plates 8 mate with each other. An appropriate number of stiffness adjustment plates 8 are assembled, so that the total length of the stiffness adjustment plates 8 is less than or equal to the length of the test tube.

[0043] The described stiffness adjustment piece 8 is manufactured by metal processing, and its elastic modulus is not less than 70 GPa; the test tube is made of non-metallic materials such as plastics and rubbers. Under the condition of ensuring that the outer shape is not damaged in the flow field, the lower the elastic modulus, the better, and the elastic modulus is not higher than 10 GPa.

[0044] The present invention provides a stiffness adjustment method for a variable stiffness model inside a tube, including the following steps:

[0045] Step 1, perform a ground vibration test on the processed test tube to obtain the vibration frequency of the test tube. At this time, the stiffness adjustment piece 8 is not installed on the inner wall of the test tube;

[0046] Step 2, compare the vibration frequency obtained in Step 1 with the corresponding design target frequency. At this time, the obtained vibration frequency is lower than the design target frequency. Assemble the stiffness adjustment piece 8 on the inner wall of the test tube, and the number of assemblies is determined according to the length of the test tube;

[0047] Step 3, perform a ground vibration test on the test tube assembled with the stiffness adjustment piece 8. If the vibration frequency is lower than the design target frequency, replace the stiffness adjustment piece 8 to increase the chord length c of the vertical beam. If the vibration frequency is higher than the design target frequency, replace the stiffness adjustment piece 8 to decrease the chord length c of the vertical beam; repeat Step 3 until the vibration frequency obtained in the test reaches the preset accuracy requirement, and then execute Step 4;

[0048] Step 4, continue to perform a ground vibration test to obtain the vibration mode of the test tube;

[0049] Step 5, judge according to the vibration mode obtained in Step 4: If the amplitude at a local position is higher than the design target, replace the stiffness adjustment piece 8 corresponding to this local position with a stiffness adjustment piece 8 with a smaller chord length c of the vertical beam. If the amplitude at a local position is lower than the design target, replace the stiffness adjustment piece 8 corresponding to this local position with a stiffness adjustment piece 8 with a larger chord length c of the vertical beam; repeat Steps 4 to 5 until the vibration mode meets the design requirements.

[0050] Among them, when adjusting the chord length c of the vertical beam of the stiffness adjustment piece 8 in Step 3, according to the principle that the square of the frequency is proportional to the stiffness, that is where ω is the vibration frequency, k is the stiffness value, and m is the total mass of the test tube and the stiffness adjustment piece 8.

[0051] As Figure 6 shown, it is a schematic diagram of the obtained vibration mode. The dotted line is the design target vibration mode, and the solid line is the vibration mode obtained from the actual test. When specifically replacing the stiffness adjustment piece 8, the stiffness adjustment piece 8 near the zero position of the target vibration mode and the actual vibration mode can be preferentially replaced, that is Figure 6 the positions marked with asterisks in.

[0052] Although the present invention has been disclosed above in preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make possible changes and modifications to the technical solution of the present invention by using the methods and technical contents disclosed above without departing from the spirit and scope of the present invention. Therefore, any simple modifications, equivalent changes and decorations made to the above embodiments based on the technical essence of the present invention without departing from the technical solution of the present invention shall fall within the protection scope of the technical solution of the present invention.

Claims

1. A variable stiffness model inside a tube, characterized in that Including N stiffness adjustment pieces, where N > 1; The stiffness adjustment piece is a cylinder, cut in an "I" shape. Define the cross beam of the stiffness adjustment piece as the two end rings; the vertical beam of the stiffness adjustment piece is the vertical arc section connecting the two end rings. Two vertical beams are distributed at intervals of 180° on the circumference. The vertical beam of the stiffness adjustment piece is the stiffness adjustment beam, and the stiffness of the model is adjusted by designing different chord lengths c of the vertical beam; Equal-sized grooves and protrusions are processed at the front and rear ends of the cross beam of the stiffness adjustment piece. Adjacent two stiffness adjustment pieces are spliced through the matching relationship of the grooves and protrusions. Screw holes are opened at both ends of each vertical beam. At the screw holes, each stiffness adjustment piece is supported and fixed on the inner wall of the test tube by using a fixed screw in cooperation with a bolt.

2. The variable stiffness model inside the tube according to claim 1, characterized in that Prepare M stiffness adjustment pieces as alternatives. The outer diameter a1, inner diameter a2, cross beam width a, and vertical beam length b of the M stiffness adjustment pieces are the same, only the chord length c of the vertical beam is different, and M ≥ N.

3. A variable stiffness model inside a tube according to claim 1, characterized in that The outer diameter a1 of the stiffness adjustment piece is 0.1 mm to 0.2 mm smaller than the inner diameter of the test tube, and the inner diameter a2 of the stiffness adjustment piece is 4 mm to 20 mm smaller than the outer diameter a1.

4. A variable stiffness model inside a tube according to claim 2, characterized in that, The cross beam width a is between 4 mm and 8 mm, and the vertical beam length b is between 30 mm and 50 mm. Moreover, the larger the selected cross beam width a, the larger the corresponding vertical beam length b; the chord length c of the vertical beam ranges from 2 mm to 31 mm, with each millimeter as a gradient, processed in an arithmetic progression, and M / 30 stiffness adjustment pieces are made for each gradient.

5. A variable stiffness model inside a tube according to claim 1, characterized in that The specific processing positions of the grooves and protrusions at the front and rear ends of the cross beam are: the grooves and protrusions are located in the extending direction of each vertical beam, and the centers of the grooves and the centers of the protrusions pass through the symmetry axis of the vertical beam and are symmetric about the symmetry axis of the vertical beam.

6. The variable stiffness model inside the tube according to claim 5, characterized in that, The processing dimensions of the grooves and protrusions are the same. The value range of the protrusion height d is 1 / 2 to 2 / 3 of the cross beam width a, and the protrusion chord length e is between 8 mm and 20 mm.

7. A variable stiffness model inside a pipe according to claim 1, characterized in that The outer surface of the fixed screw is shaped to be consistent with the outer surface of the test tube, and the aperture of the screw holes opened at both ends of the vertical beam is between M1 and M3.

8. A variable stiffness model inside a tube according to claim 1, characterized in that, The stiffness adjustment piece is manufactured by metal processing, and the elastic modulus is not less than 70 GPa; the test tube is made of non-metallic materials. Under the condition of ensuring that the shape is not damaged in the flow field, the elastic modulus is not higher than 10 GPa.

9. A stiffness adjustment method for the variable stiffness model inside the tube according to claim 1, characterized in that Including the following steps: Step 1, conduct a ground vibration test on the processed test tube to obtain the vibration frequency of the test tube. At this time, no stiffness adjustment piece is installed on the inner wall of the test tube; Step 2, compare the vibration frequency obtained in Step 1 with the corresponding design target frequency. At this time, the obtained vibration frequency is lower than the design target frequency. Assemble N stiffness adjustment pieces on the inner wall of the test tube, and the total length of the assembled N stiffness adjustment pieces is less than or equal to the length of the test tube; Step 3, conduct a ground vibration test on the test tube assembled with the stiffness adjustment piece. If the vibration frequency is lower than the design target frequency, replace the stiffness adjustment piece to increase the chord length c of the vertical beam. If the vibration frequency is higher than the design target frequency, replace the stiffness adjustment piece to decrease the chord length c of the vertical beam; repeat Step 3 until the vibration frequency obtained in the test reaches the preset accuracy requirement compared with the design target frequency, and then execute Step 4; Step 4: Continue with the ground vibration test to obtain the vibration mode of the test pipe; Step 5: Judge according to the vibration mode obtained in Step 4: If the amplitude at a local position is higher than the design target, replace the stiffness adjustment piece corresponding to this local position with a stiffness adjustment piece with a smaller chord length c of the vertical beam; if the amplitude at a local position is lower than the design target, replace the stiffness adjustment piece corresponding to this local position with a stiffness adjustment piece with a larger chord length c of the vertical beam; repeat Step 4 to Step 5 until the vibration mode meets the design requirements.

10. The stiffness adjustment method according to claim 9, characterized in that, When adjusting the chord length c of the vertical beam of the stiffness adjusting piece in Step 3, according to the principle that the square of the frequency is proportional to the stiffness, that is where ω is the vibration frequency, k is the stiffness value, and m is the total mass of the test tube and the stiffness adjusting piece.

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