Pipeline elbow based on bionic sharkskin and design method

Abstract

The present application relates to a kind of pipeline elbow based on bionic sharkskin and design method, based on the calculation of wall flow characteristic parameter at elbow of turbulent boundary layer theory;Introduce dimensionless groove spacing and dimensionless height, and combine wall flow characteristic parameter at elbow, determine the geometric parameter of micro groove;According to the curvature of elbow and / or viscous bottom layer thickness, the geometric parameter of micro groove is corrected;Verify the vibration reduction performance;The present application intervenes near-wall turbulent flow by sharkskin micro groove structure, lifts the streamwise vortex to above the tip of groove, effectively reduces the amplitude of wall pressure fluctuation, establishes the groove size calculation formula based on wall friction velocity and viscous bottom layer thickness, which can quickly adapt to different working conditions without relying on large-scale numerical simulation.For different flow characteristics of elbow outside, inside and top, respectively determine the geometric parameter of groove, realize "on-demand vibration reduction", avoid unnecessary resistance increase and manufacturing waste.

Description

Technical Field

[0001] This invention relates to the field of flow-induced vibration control technology for pipeline systems, specifically to a pipeline elbow based on biomimetic shark skin and its design method. Background Technology

[0002] Elbows are among the most common pipeline components in large pipeline systems such as main steam pipelines in nuclear power plants, chemical transmission pipelines, and ballast pipelines in ships. When high-speed fluid flows through an elbow, the change in flow direction causes the fluid to be subjected to centrifugal force, creating a high-pressure zone on the outside of the elbow and a low-pressure zone on the inside, inducing complex secondary flow vortices. Simultaneously, flow separation and turbulent pulsations at the elbow can generate strong wall pressure pulsations. When the frequency of these pressure pulsations approaches the natural frequency of the pipeline structure, flow-induced resonance can occur, potentially leading to pipeline fatigue fracture, support failure, and other safety accidents.

[0003] Existing elbow vibration reduction technologies mainly include passive control measures such as increasing wall thickness, adding reinforcing ribs, and installing dampers, as well as active control systems. These methods have the following shortcomings: (1) Increasing wall thickness and reinforcing ribs can improve structural stiffness, but it will significantly increase system weight and manufacturing cost; (2) The installation space for the damper is limited, and its effect on suppressing high-frequency pulsation is limited; (3) Active control systems require external energy and have low reliability.

[0004] Bionic research has discovered that shark skin is covered with micron-sized groove structures. This structure effectively suppresses the spanwise motion of turbulent quasi-ordered structures near the wall, lifting flow vortices above the groove tips, thereby reducing wall shear stress and pressure pulsation. Existing research shows that shark skin-inspired grooves can achieve drag reduction effects of up to approximately 10% in flat plates and straight pipes.

[0005] However, existing research mainly focuses on drag reduction, with insufficient attention paid to vibration suppression; and for elbows, which have complex secondary flow characteristics, a systematic design method for biomimetic shark skin-based pipe elbows has not yet been established. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides a biomimetic shark skin-based pipe elbow and its design method to solve the aforementioned problems.

[0007] This invention provides the following technical solution: A design method for a pipe elbow based on biomimetic shark skin includes the following steps: Calculation of wall flow characteristic parameters at bends based on turbulent boundary layer theory; Dimensionless groove spacing and dimensionless height are introduced, and the geometric parameters of the microgrooves are determined by combining the flow characteristic parameters of the wall at the bend. The geometric parameters of the microgroove are modified based on the curvature of the bend and / or the thickness of the viscous substrate; Verify vibration reduction performance.

[0008] Preferably, the vibration reduction effect of the microgrooves is characterized by the reduction of the root mean square value of the wall pressure pulsation.

[0009] Preferably, verifying vibration reduction performance includes: By comparing the wall pressure pulsation spectra of smooth elbows and elbows with microgrooves, the pressure pulsation suppression rate of each frequency band was calculated to verify the vibration reduction effect.

[0010] Preferably, the finite volume method is used to calculate the transient flow field of the elbow pipe with microgrooves, and the SST turbulence model is selected. - The model corresponds to the dimensionless wall distance. SST must be met Turbulence model for The requirements are met, thus enabling accurate analysis of the velocity gradient within the viscous sublayer. The calculated wall pressure pulsation time history is then used to obtain the spectral characteristics through fast Fourier transform.

[0011] Preferably, the wall flow characteristic parameters at the bend include Reynolds number, wall shear stress, wall friction velocity, and viscous sublayer thickness.

[0012] Preferably, the geometric parameters of the microgrooves include the microgroove spacing, height, and tip thickness; wherein, the dimensionless groove spacing... Value 12 15; height Spacing between microgrooves ratio ; Tip thickness Spacing between microgrooves ratio .

[0013] Preferably, the correction of the geometric parameters of the microgrooves includes: Height is corrected using a curvature correction factor; and / or The physical spacing and height of the grooves at different locations along the circumference of the elbow are adjusted by circumferential scaling based on the thickness of the viscous underlayer.

[0014] Preferred curvature correction factor :

[0015] in, This is an empirical coefficient, ranging from 0.2 to 0.5; This refers to the inner diameter of the elbow. Let be the radius of curvature of the bend.

[0016] Preferably, taking the bending plane of the elbow as a reference plane, any cross-section of the elbow perpendicular to the pipeline axis is defined. Pointing towards the inside of the bend, Pointing to the outside of the bend, Pointing to the top of the bend, or Pointing to the bottom of the bend, a correction factor for the thickness of the viscous sublayer is defined based on this. By correction factor Circumferential differential scaling correction is applied to the microgroove spacing and height: : = ; : = ; The remaining positions at the top and bottom of the elbow: = .

[0017] A pipe elbow based on biomimetic shark skin is designed using the aforementioned design method for a pipe elbow based on biomimetic shark skin.

[0018] The present invention has the following beneficial technical effects: This invention intervenes in near-wall turbulence through a sharkskin-like microgroove structure, lifting the flow vortex above the groove tip and effectively reducing the amplitude of wall pressure pulsation. Theoretical estimation and numerical verification show that the pressure pulsation suppression rate can reach 5%–8%, corresponding to a reduction in flow-induced vibration response of approximately 6%–10%.

[0019] A system based on wall friction velocity was established and viscous underlayer thickness The method provides a formula for calculating the groove size, which can be quickly adapted to different working conditions without relying on large-scale numerical simulation.

[0020] Circumferentially differentiated layout: The geometric parameters of the groove are determined separately for the different flow characteristics of the outer side, inner side and top of the elbow to achieve "on-demand vibration reduction" and avoid unnecessary resistance increase and manufacturing waste.

[0021] Highly practical for engineering applications: Shark skin microgrooves are in the micrometer range (10 μm-50 μm) and can be manufactured using high-precision 3D printing or laser etching technology without changing the main structure of the pipeline, making it easy to modify existing pipeline systems.

[0022] Dimensionless design: Introduction , The dimensionless parameters allow this method to be extended to different media (water, oil, gas) and different operating conditions (flow rate, temperature changes), making it widely applicable. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of the sharkskin microgroove structure of the present invention; Figure 2 This is a schematic diagram of the circumferential microgroove arrangement of the elbow of the present invention; Figure 3 This is a schematic diagram of the sharkskin microgroove structure and geometric parameters of the present invention. Detailed Implementation

[0024] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0025] The study found that the outer wall of the elbow has high turbulence intensity and a thin boundary layer, making it the main excitation source region for flow-induced vibration; while the inner wall exhibits flow separation, and its pressure pulsation characteristics are significantly different from those on the outer side.

[0026] In turbulent flow, wall shear stress (drag) can be decomposed into viscous stress and Reynolds stress:

[0027] in It is the Reynolds stress, which directly characterizes turbulent fluctuations ( , Momentum transport caused by ). For fluid density; The velocity of the flow pulsation; Normal pulsation velocity; This is the outer boundary of the near-wall region; Indicates viscous stress; Indicates the dynamic viscosity of a fluid; This indicates that the value is taken at the wall surface; Indicates the average flow velocity; Indicates the distance from the wall; Drag reduction means total stress The reduction in Reynolds stress is usually accompanied by a reduction in Reynolds stress in the near-wall region; the reduction in Reynolds stress directly implies a decrease in turbulence intensity (…). , The weakening of wall pressure pulsation. These velocity fluctuations are generated by Poisson's equation. Therefore, mathematically, drag reduction (reduction of Reynolds stress) is necessarily related to a reduction in the intensity of pressure fluctuations.

[0028] The generation of wall pressure fluctuations is closely related to the generation and dissipation of turbulent kinetic energy (TKE). The turbulent kinetic energy transport equation is:

[0029] in, It is the turbulent kinetic energy generation term. Indicates time; Represents the time-averaged velocity component; Indicates the first directional spatial coordinate components; Indicates the first The pulsating velocity component in the direction; This represents the fluctuating velocity component in the j-th direction; the presence of the groove alters the average velocity gradient. This affects the turbulent kinetic energy generation term. Indicates pressure pulsation; This represents the pulsating velocity component in the j-th direction; Indicates the kinematic viscosity of a fluid; Represents the pulsating strain rate tensor; Indicates the turbulent kinetic energy dissipation rate; Drag-reducing trenches (such as) It is typically found that trenches can suppress the generation of turbulent kinetic energy, especially in buffer zones. This means that the energy used to generate pressure pulsations is reduced. If the trench is designed properly, the turbulent kinetic energy generation term is reduced, and the turbulent kinetic energy... If the pressure decreases, then the wall pressure pulsation driven by turbulent kinetic energy will... It will also decrease.

[0030] Wall pressure pulsation It is determined by the turbulent velocity field through Poisson's equation:

[0031] in, Indicates the first The pulsating velocity component in the direction; Indicates the first The spatial coordinate components of the direction; the "source term" on the right-hand side of the equation, which is entirely determined by the average velocity gradient. and pulsation velocity Composition. The horizontal line indicates the average; by designing and assembling grooves to change the average velocity gradient (drag reduction effect), the pulsating velocity can be directly suppressed by limiting the motion of the vortex. A weakening of the source term can lead to pressure pulsations. The amplitude decreases, which is the mathematical essence of vibration reduction.

[0032] In engineering practice, empirical formulas correlate wall pressure fluctuations with wall shear stress. For example, for turbulent boundary layers, the root mean square value of wall pressure fluctuations... The speed of friction with the wall surface is usually Proportional:

[0033] in It is a constant (usually around 2-3, depending on the Reynolds number and location).

[0034] Although this relationship is rough, it reveals a core trend: if the trench successfully reduces the average wall shear stress... (i.e., drag reduction), then it is also very likely to reduce wall pressure pulsation. (i.e., vibration reduction).

[0035] A pipe elbow based on biomimetic shark skin, such as Figures 1-3 As shown, sharkskin-like microgrooves are provided circumferentially on the inner wall of the pipe, with a spacing of [missing information]. (Tooth tip, tooth root, or tooth center distance are all acceptable), height is The tip thickness is .

[0036] For the entire pipeline, the core principle for determining the reasonable arrangement range of circumferential teeth is to judge the characteristic length of turbulence excitation, development, and eventual decay based on the flow field structure, rather than pre-setting a fixed value. Based on this principle, a reasonable and systematically validated solution is to extend the tooth arrangement by 5-10 times the inner diameter of the elbow on both sides of the elbow center. The distance between them is arranged. The excitation source of flow-induced vibration is not concentrated at the geometric center point of the bend, but rather follows a clear evolutionary process: Pressure pulsations begin to decay after a period of intense development: Studies indicate that the reattachment point of the jet shear layer is typically located 2-4 times the inner diameter of the elbow downstream of the elbow. This means the strongest pulsation will continue until approximately four times the inner diameter of the bend. After reaching its peak, the turbulent energy enters the dissipation phase. To ensure vibration reduction, the trench coverage area needs to be divided into upstream and downstream sections: the upstream section covers the stage where turbulence is excited and develops; the downstream section covers the intense development and dissipation zone of turbulence until the energy significantly decreases.

[0037] A design method for pipe elbows based on biomimetic shark skin, such as... Figures 1-3 As shown: This method, based on turbulent boundary layer theory, first calculates flow characteristic parameters such as wall friction velocity and viscous sublayer thickness at the bend, and then determines geometric parameters such as microgroove spacing, height, tip thickness, and aspect ratio. Next, based on the bend curvature and secondary flow distribution, it determines the circumferential arrangement area of ​​the microgrooves and differentiated design schemes. Finally, it verifies the vibration reduction performance by numerically solving for the wall pressure pulsation suppression effect. Specifically, it includes the following steps: Step 1: Calculate the characteristic parameters of wall flow at the bend. average flow velocity Below, fluid medium density Dynamic viscosity Kinematic viscosity The inner diameter of the elbow is... The radius of curvature is .

[0038] First, calculate the Reynolds number:

[0039] For turbulent flow in a smooth circular pipe, the Darcy friction coefficient The solution can be obtained iteratively using the Kohlbrook formula, or approximated using the Blasius formula (applicable to...). ):

[0040] Wall shear stress for:

[0041] Wall friction speed for:

[0042] viscous underlayer thickness for:

[0043] For the outer wall of the elbow, the boundary layer is compressed due to centrifugal force, and the actual thickness of the viscous sublayer is about 0.5 to 0.7 times that of the straight pipe; the boundary layer on the inner wall is thickened, about 1.2 to 1.5 times that of the straight pipe.

[0044] Step 2: Determine the basic geometric parameters of the microgrooves Let the spacing between the microgrooves be... The height is The tip thickness is Introducing dimensionless groove spacing and dimensionless height :

[0045] Dimensionless groove spacing The optimal dimensionless spacing range for blade-shaped microgrooves is: ,high Spacing between microgrooves The optimal aspect ratio is For flow through a circular pipe, considering the effect of curvature, the optimal spacing is too small; a smaller spacing is recommended. = .

[0046] Therefore, the microgroove spacing and height Determined by the following formula:

[0047] Tip thickness The vibration reduction effect of the trench is affected; it is recommended to take... Minimize as much as possible within the limits allowed by the manufacturing process.

[0048] Step 3: Consider the correction of the elbow curvature. The curvature of an elbow affects the effectiveness of microgrooves. When the radius of curvature... When the radius of curvature is small, the centrifugal force is enhanced, the outer boundary layer is thinner, and the groove height needs to be appropriately reduced to avoid entering the "k-rough" region; when the radius of curvature is large, the flow is close to that of a straight pipe, and it can be designed according to the parameters of a straight pipe.

[0049] Introducing curvature correction factor :

[0050] in This is an empirical coefficient, with values ​​ranging from [value missing]. The corrected trench height is:

[0051] Among them, the place Indicates the height after curvature correction. ; Step 4: Consider the circumferential correction of the groove dimensions under the influence of secondary flow at the elbow. However, the straight pipe formula described above yields basic dimensions applicable to straight pipes or elbows in the circumferential neutral position. The thickness of the viscous sublayer differs significantly between the outer and inner sides of an elbow (see step one), thus requiring circumferential differential correction.

[0052] Circumferential angle Definition: Taking the bending plane of the elbow as the reference plane, and taking any cross-section of the elbow (perpendicular to the pipeline axis), the definition is... Pointing to the inside of the bend (center of curvature side). Pointing to the outside of the bend (i.e., the direction of centrifugal force, the opposite side of the center of curvature). and (or ) pointing to the top and bottom of the bend respectively (in the direction of the neutral layer), as shown in the attached figure. Figure 3 As shown.

[0053] Based on this definition, the viscous sublayer thickness correction factor is defined. : outer wall surface ]: ; inner wall surface ]: ; The remaining areas at the top / bottom are neutral zones. ; The spacing between microgrooves at each circumferential position and height It should be revised to:

[0054] in, Indicates the circumferential angle The corresponding corrected microgroove spacing ; Indicates the circumferential angle The corresponding microgroove spacing before correction ; Indicates the circumferential angle The corresponding corrected height ; Indicates the circumferential angle The corresponding height before correction This correction is related to the curvature correction coefficient in step three. They can be used independently or in combination – that is, first correct the overall height according to the curvature, and then scale according to the circumferential position differences.

[0055] Step 5: Establish a model for evaluating the effect of wall pressure pulsation suppression. The vibration reduction effect of microgrooves can be measured by the root mean square value of the wall pressure pulsation. The decrease is characterized by this. The pressure pulsation suppression rate is defined as... :

[0056] in, This represents the root mean square value of pressure pulsation on a smooth wall surface. This represents the root mean square value of pressure pulsation on the wall of the microgroove; According to classical theory, there is an approximate relationship between pressure pulsation and wall shear stress:

[0057] in This is an empirical constant, with a range of values. Therefore, the pressure pulsation suppression rate It can be approximated as:

[0058] in, Represents the shear stress on a smooth wall surface; This represents the wall shear stress on the microgroove wall surface; drag reduction rate of micro trenches with dimensionless spacing The relationship can be obtained by fitting classical experimental data, where, This indicates the reduction in the coefficient of friction (for smooth walls and for grooved walls). This represents the coefficient of friction resistance for a smooth wall surface. For blade-shaped grooves, when... At that time, the drag reduction rate can reach .

[0059] Step Six: Numerical Solution and Verification The transient flow field of a bend with microgrooves was calculated using the finite volume method, with the SST turbulence model selected. - The model corresponds to the dimensionless wall distance. ,in, This represents the normal distance from the wall to the center of the calculated mesh cell; it must satisfy SST. Turbulence model for This meets the requirements, thus enabling accurate analysis of the velocity gradient within the viscous sublayer. The calculated wall pressure fluctuation time history is used to obtain the spectral characteristics through Fast Fourier Transform (FFT).

[0060] Define dimensionless frequency:

[0061] By comparing the wall pressure pulsation spectra of smooth elbows and elbows with microgrooves, the pressure pulsation suppression rate of each frequency band was calculated to verify the vibration reduction effect.

[0062] The invention is verified using a typical working condition as an example: the inner diameter of the elbow. The radius of curvature of the elbow average flow velocity The medium is water. The calculation results are as follows:

[0063] The calculation results show that the microgroove size determined by this method is in good agreement with the theoretical optimal value, and the estimated pressure pulsation suppression rate can reach 5%-8%.

[0064] Example 1 Step 1: Determine operating parameters The inner diameter of the main steam pipe bend in a nuclear power plant. Elbow curvature radius average flow velocity The medium is water.

[0065] Step 2: Calculate flow characteristic parameters Reynolds number:

[0066] The friction coefficient is estimated using the Blasius formula:

[0067] Wall shear stress:

[0068] Friction speed:

[0069] Thickness of viscous underlayer:

[0070] Step 3: Determine the geometric parameters of the microgroove Find the optimal dimensionless spacing Optimal aspect ratio :

[0071] Tip thickness

[0072] Given the inner diameter of the elbow radius of curvature Take the empirical coefficient. (range of values) ), calculate the curvature correction factor:

[0073] The corrected trench height is:

[0074] The final geometric parameter used is the microgroove spacing. ,high Tip thickness .

[0075] The thickness of the outer viscous sublayer is that of a straight pipe. Times (take the middle value), inner side is Times, neutral zone The actual trench dimensions at each location are:

[0076] Note: In actual manufacturing, different groove molds or 3D printing parameters can be used in the circumferential partitions to achieve differentiated layouts.

[0077] Step 4: Verification and Evaluation Using SST - The turbulence model is used for CFD transient calculations, with a time step of [missing information]. The total calculation time was 0.1 s (approximately 2000 time steps). The pressure pulsation time history of the outer wall of the elbow was extracted, and after FFT transformation, the pressure pulsation spectra of the smooth elbow and the elbow with microgrooves were compared.

[0078] Expected results: In the low-frequency range (10Hz-100Hz), the pressure pulsation amplitude is reduced by about 6%-8%, indicating that the structure of the present invention has a good vibration reduction effect.

[0079] Example 2 Variable operating condition adaptability verification To verify the adaptability of this method under different operating conditions, the average flow velocity was changed to... and The geometric parameters of the microgrooves were calculated separately. The curvature correction factor is independent of the flow velocity and is still taken as... (Calculations are the same as in Example 1), corrected trench height At the same time, the outer and inner sides need to be... , The scaling result is as follows:

[0080] Calculation results show that the microgroove size decreases with increasing flow velocity, which is consistent with the physical law that the thickness of the turbulent boundary layer decreases with increasing Reynolds number. This method can adaptively determine the optimal groove size according to actual working conditions and has good adaptability to different operating conditions.

[0081] The embodiments described above are merely illustrative of specific implementations of the present invention, and while the descriptions are detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention.

Claims

1. A design method for a pipe elbow based on biomimetic shark skin, characterized in that, Includes the following steps: Calculation of wall flow characteristic parameters at bends based on turbulent boundary layer theory; Dimensionless groove spacing and dimensionless height are introduced, and the geometric parameters of the microgrooves are determined by combining the flow characteristic parameters of the wall at the bend. The geometric parameters of the microgroove are modified based on the curvature of the bend and / or the thickness of the viscous substrate; Verify vibration reduction performance.

2. The design method of a pipe elbow based on biomimetic shark skin according to claim 1, characterized in that, The vibration reduction effect of microgrooves is characterized by the reduction in the root mean square value of the wall pressure pulsation.

3. The design method of a pipe elbow based on biomimetic shark skin according to claim 2, characterized in that, Verification of vibration reduction performance includes: By comparing the wall pressure pulsation spectra of smooth elbows and elbows with microgrooves, the pressure pulsation suppression rate of each frequency band was calculated to verify the vibration reduction effect.

4. The design method of a pipe elbow based on biomimetic shark skin according to claim 3, characterized in that, The transient flow field of a bend with microgrooves was calculated using the finite volume method, with the SST turbulence model selected. - The model, in the near-wall region, must satisfy SST. Turbulence model for The requirements are met, thus enabling accurate analysis of the velocity gradient within the viscous sublayer. The calculated wall pressure pulsation time history is then used to obtain the spectral characteristics through fast Fourier transform.

5. The design method of a pipe elbow based on biomimetic shark skin according to claim 1, characterized in that, The characteristic parameters of wall flow at the bend include Reynolds number, wall shear stress, wall friction velocity, and viscous sublayer thickness.

6. The design method of a pipe elbow based on biomimetic shark skin according to claim 1, characterized in that, The geometric parameters of microgrooves include the microgroove spacing, height, and tip thickness; among which, the dimensionless groove spacing... Value 12 15; height Spacing between microgrooves ratio ; Tip thickness Spacing between microgrooves ratio .

7. The design method of a pipe elbow based on biomimetic shark skin according to claim 6, characterized in that, The correction of the geometric parameters of the microgrooves includes: Height is corrected using a curvature correction factor; and / or The physical spacing and height of the microgrooves at different locations along the circumference of the elbow are adjusted by circumferential scaling based on the thickness of the viscous underlayer.

8. The design method of a pipe elbow based on biomimetic shark skin according to claim 7, characterized in that, Curvature correction factor : in, This is an empirical coefficient, ranging from 0.2 to 0.5; This refers to the inner diameter of the elbow. Let be the radius of curvature of the bend.

9. The design method of a pipe elbow based on biomimetic shark skin according to claim 7, characterized in that, Using the bending plane of the elbow as a reference plane, take any cross-section of the elbow perpendicular to the pipeline axis and define... Pointing towards the inside of the bend, Pointing to the outside of the bend, Pointing to the top of the bend, or Pointing to the bottom of the bend, a correction factor for the thickness of the viscous sublayer is defined based on this. By correction factor Circumferential differential scaling correction is applied to the microgroove spacing and height: ： = ； ： = ； The remaining positions at the top and bottom of the elbow: = .

10. A pipe elbow based on biomimetic shark skin, characterized in that, The pipe elbow was designed using the design method based on biomimetic shark skin as described in any one of claims 1-9.

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