Noise Generation Mechanisms of Marine Composite Pipelines Excited by the Internal and External Flows

The marine pipeline is an important energy transportation platform today. It can cause underwater noise pollution due to sound radiation caused by pipeline vibration during oil and gas transportation. This study focuses on the noise-generation mechanisms of marine composite pipelines under internal and external flow excitation. We investigated the impact of different materials, diameters, and wall thicknesses on the sound radiation characteristics of the pipeline under internal flow excitation. The relationship between radiated sound power and flow velocity was analysed. Additionally, we discussed the radiated sound field of marine composite pipelines under external flow excitation. Our results show that the radiated sound power under internal flow excitation is over 20 dB higher than that under external flow excitation in the 10 Hz to 2000 Hz frequency range. We conducted an experimental test near Zhoushan city, China, where the internal flow was driven by a pump and the external flow represented ocean currents. The results confirmed that the main noise-generating mechanism of marine composite pipelines is internal flow excitation. The numerical findings were validated by the experimental test. Overall, this paper provides a foundation for future investigations into the sound radiation of marine composite pipelines.


Introduction
The extensive development of offshore oil and gas resources in recent decades has been driven by increasing global energy demand, while onshore reserves have struggled to meet this demand.Consequently, the construction and maintenance of undersea pipelines for oil and gas have become highly significant both commercially and strategically.However, the construction of offshore pipelines has introduced environmental challenges, particularly concerning noise and vibration.The presence of noise and vibration induced by internal and external flow can disrupt marine life and ecosystem dynamics [1].Therefore, to minimize the noise and vibration generated by these pipelines has become a top priority.
Faal conducted an investigation on the vibration induced by internal flow in pipelines using isotropic materials and explored its vibrational stability [2].Kuiken examined the pipeline vibration caused by various fluids through theoretical analysis and numerical calculations [3][4][5].Guo analyzed the impact of two factors on flow-induced vibration in pipelines and concluded that turbulent fluctuation pressure serves as the primary source of vibration in pipeline systems [6].Li performed calculations to determine the surface sound pressure and characteristics of the radiated sound field in a hydraulic pipeline system, as well as discussed the influence of different system parameters on the sound field [7].Moreover, Guo examined the noise sources in a pipeline system, with particular focus on straight pipe and elbow pipe configurations.The study revealed significant effects of hydrodynamic noise on pipeline noise levels, and derived a relationship between radiated sound power and internal flow velocity [8].
Ganta conducted direct numerical simulations to investigate the sound radiation generated by laminar flow passing through a rotating cylinder.The study observed significant changes in frequency, amplitude, and directivity based on the rotation rate of the cylinder [9].Saxena employed the OpenFOAM flow field solver to study the hydrodynamic noise characteristics of a three-dimensional underwater cylinder.The research established a damping coefficient for noise reduction within a specific frequency range [10].Weinmann utilized a new numerical method to calculate the flow patterns around a cylinder and achieved satisfactory agreement with experimental findings [11].Cui effectively reduced hydrodynamic noise through controlling the flow field around a three-dimensional cylinder with a bionic saw-tooth design, which mitigated pressure fluctuations caused by turbulent vortices on the surface [12].Chen combined CFD and BEM for sound field simulation to investigate the tail vortices shedding characteristics around a cylinder [13].
The utilization of marine composite pipelines, which integrate polymer materials, presents numerous benefits including high strength-to-weight ratios, exceptional corrosion resistance, and flexibility.At present, the predominant type of marine oil and gas pipelines supplying China consists of composite flexible pipelines [14].Nevertheless, there is limited research available regarding the noise and vibration mechanisms of marine composite pipelines subjected to internal and external flow influences.
In this study, our objective was to explore the sound radiation characteristics of marine composite pipelines caused by internal and external flow excitations.By employing a combination of numerical simulations and experimental tests, we aimed to unravel the underlying mechanisms responsible for the noise generation in marine composite pipelines.The outcomes of this research are expected to offer valuable insights into the practical implementation of marine composite pipelines and their potential utilization in various engineering projects.

Principle of numerical calculation
In recent years, CFD has become a crucial method for simulating hydrodynamic noise.The process involves calculating the flow field around the marine composite pipeline using CFD techniques to capture time-domain information.Fourier transform is then applied to convert the data into the frequency domain.This frequency-domain data is used to calculate the pipeline sound field.The RNG k-ε turbulence model is commonly used in these simulations to account for turbulent vortices' influence.CFD coupled with the RNG k-ε model enables more accurate predictions and analysis of hydrodynamic noise in marine composite pipelines.
LES (Large Eddy Simulation) is a turbulence model that uses average calculations in the simulated space.LES captures the influence of different scale vortex separation by filtering larger-scale vortices and approximating the impact of smaller vortices.This reduces computational resources compared to other methods.
In this study, LES is employed to calculate the complex flows inside and outside the marine composite pipeline.To calculate the hydrodynamic noise, the FW-H (Ffowcs Williams-Hawkings) equation is used.The combination of LES and the FW-H equation allows for accurate calculation of the flow field and assessment of hydrodynamic noise in marine composite pipelines.This research aims to provide insights into the noise generation mechanisms and improve predictions and analysis of hydrodynamic noise.

The noise mechanism of the internal flow excitation
In our experimental test, we investigated the marine composite pipeline with the length of approximately 200 m.The perceived noise due to internal flow excitation decrease as the hydrophones used in the experiment are away from the surface.To calculate the flow field and sound field of practical marine pipeline, we applied periodic boundary conditions to a unit-length pipeline.The pipeline's length is more than ten times greater than its inner diameter (D = 0.05m), so the flow field is fully developed.The unit-length marine pipeline has a total length of 20 D and consists of multiple layers, with each layer's thickness of 0.005m.The three-dimensional model of the pipeline is shown in Figure 1.While the unit-length model can be seen as a unit within an infinitely long pipeline, some modal effects may still be present in the sound field calculation.Therefore, this paper focuses on the sound radiation analysis of a finite-length pipeline.
Figure 1.The marine pipeline model.The flow direction is along the negative z-axis.After the boundary conditions definition, the steady-state flow field is computed by the RNG k-ε turbulence model with second-order upwind spatial discretization for pressure and momentum.Stability is then obtained for the subsequent transient-state calculation.Turbulent kinetic energy and turbulence dissipation rate are discretized with second-order upwind spatial and temporal discretization using the Pressure-Implicit with Splitting of Operators algorithm for velocity-pressure coupling.This approach accurately captures flow dynamics and turbulent characteristics in both steady-state and transient-state conditions.Sound propagation areas inside and outside the pipeline are modelled as water-filled acoustic parts.Infinite elements are used for boundaries.Flow excitation is introduced through point-load on the inner wall, and both ends of the pipeline simulate an infinite boundary by periodic modal bases based on Hui's research [15].

Sound radiation analysis with different ratios
Marine pipelines can be categorized into 2 types: rigid pipelines and composite pipelines.Rigid pipelines are typically made of steel, while composite flexible pipelines consist of three layers.The inner layer is high-density polyethylene (HDPE), the middle layer is steel, and the outer layer is chlorinated polyvinyl chloride (CPVC).Composite pipelines, as highlighted by Wang [16], offer a desirable combination of physical properties, including lightweight construction, ease of installation, and excellent mechanical strength.These characteristics make composite pipelines an appealing choice for various marine applications.
Figure 2 shows sound radiation power curves for rigid and composite pipelines at different diameter-to-thickness ratios (12:1, 7:1, and 5.3:1) with an internal flow velocity of 1.0 m/s.The blue curve is steel, green is HDPE, and purple is CPVC.Overall, increasing Young's modulus decreases the sound power level.The three materials exhibit similar trends in the low-frequency range, peaking near 50 Hz.Lower frequencies result in larger pressure fluctuations and higher sound power levels.Steel pipelines have the lowest sound power among the three materials with the same wall thickness, while CPVC pipelines have the highest due to pressure variations in the low-frequency range.
Variations in pipeline thickness impact its mode structure.Thicker pipelines can change resonance frequency and amplitude.The decrease of diameter-to-thickness ratio reduces overall sound power.Polymers have higher internal damping than steel and hard to achieve resonant peaks.This leads to differences in resonant characteristics between polymer-based and steel pipelines.From a functional perspective, marine composite pipelines can be divided into two primary components: the steel section and the polymer materials.The steel component provides the strength and structural integrity, while the polymer materials serve to seal and protect the steel from corrosion.Unlike traditional steel pipelines, the steel section of the marine composite pipeline is not continuous.

Sound radiation analysis under different internal flow excitation
Instead, the marine composite pipeline is configured as a tape-like structure, which imparts flexibility to the overall pipeline design.This flexible nature allows for easier installation, alignment adjustments, and adaptability to varying seabed conditions.To ensure accurate turbulence simulation, the flow state inside the pipeline must be turbulent, which is determined by the Reynolds number (Re): (1) where, represents the flow density, is the velocity, is the pipeline diameter, and is the viscosity coefficient.
The flow regime inside the pipeline is categorized based on the Reynolds number (Re).When Re is below 2300, the flow is laminar.For Re between 2300 and 4000, the flow is transitional turbulent.When Re exceeds 4000, the flow is fully turbulent.In our simulations, the Re consistently exceeds 4000 for all working conditions, showing fully turbulent flow in the pipeline.
Figures 3 and 4 show a positive correlation between overall sound power and increasing flow velocity.The total pipeline sound radiation power gradually increases with higher flow velocities, but there is a minor step-change at the flow velocity of 1.6 m/s, possibly due to the sharp transition in the flow regime.On average, an increase of 0.1 m/s in internal flow velocity leads to approximately a 2.22 dB increase in the total sound radiation power.For acoustic fault detection in marine pipelines, we do not recommend to place sensors close to the pipeline below 200 Hz.Internal flow excitation generates high noise levels in the low frequency, which can overwhelm the sensors.In the high frequency range (200-2000 Hz), resonant frequencies of composite flexible pipelines exhibit lower amplitudes compared to steel pipelines.This makes the composite flexible pipeline more compatible with fault detection sensors in this frequency range.The use of marine composite pipelines may offer advantages in terms of fault detection and acoustic monitoring due to reduced resonant amplitudes in the high frequency.6 shows the sound radiation power from steel pipelines with different ratios under external flow excitation.Thicker walls result in lower radiated sound power levels due to reduced wall vibrations.Below 400 Hz, consistent changes exhibit, while higher frequencies show multiple peaks that increase with wall thickness.Thicker walls lead to reduced radiated sound power and resonance phenomena at higher frequencies.Figure 7 shows the sound radiation power of HDPE pipelines with different wall thicknesses under external flow excitation.Thicker walls result in lower radiated sound power levels, attributed to reduced wall vibrations, while maintaining the same external flow excitation force.Across the frequency range of 10-2000 Hz, small-amplitude peaks show the influence of fluctuation pressure from vortex shedding on the HDPE pipeline.This influence is determined by the material's Young's modulus and damping ratio.In summary, increasing the wall thickness of an HDPE pipeline reduces sound radiation power, and small-amplitude peaks show the influence of vortex shedding-induced pressure fluctuations, which depend on the material properties of high-density polyethylene.Figure 8 shows the sound radiation power of CPVC pipelines with different thicknesses under external flow excitation.Thicker walls lead to lower radiated sound power levels, attributed to reduced wall vibrations while maintaining a constant external flow excitation force.In the 10-2000 Hz range, no significant peaks are observed in the radiated sound power curve for CPVC.This lack of distinct peaks can be explained by the higher damping ratio of CPVC compared to the other materials.The increased damping characteristics of CPVC result in greater energy absorption in the higher frequency band, leading to the absence of prominent peaks in the sound radiation power.In summary, increasing the wall thickness of a CPVC pipeline reduces sound radiation power.The higher damping ratio of CPVC contributes to the absence of noticeable peaks in the sound radiation power within the analysed frequency range.

The noise mechanism of the external flow excitation
In our sound field simulation, both rigid and composite pipelines are modelled using the material parameters representing steel, HDPE, and CPVC.When subjected to external flow excitation, the marine pipeline primarily generates sound radiation in the low frequency due to vortex shedding occurring at lower frequencies.At high frequencies, the excitation from external flow can cause resonance peaks in pipelines made of different materials.These peaks correspond to the characteristic resonant modes and damping factors of each pipeline material.To summarize, our simulation have considered both rigid and composite pipelines, and we have observed that external flow excitation primarily contributes to low frequency sound radiation.Moreover, different resonance peaks occur at high frequency due to the unique characteristics and damping properties of the specific pipeline materials.

Sound radiation analysis under different external flow excitation
Figure 9 shows the total sound radiation power of the composite pipeline under different external flow velocities (0.6, 0.8, and 1.0 m/s), measuring 70.08 dB, 89.57dB, and 92.56 dB, respectively.The sound radiation power for the three pipelines exhibit similar trends, with low frequency higher levels.This is attributed to periodically shedding vortices on the pipeline sides, generating alternating impulses that excite structural vibrations and result in sound radiation.According to the Karman vortex theory, the vortex shedding frequencies for the flow velocities considered are 1.5, 2.0, and 2.5 Hz, respectively.However, it is noted that the simulation of sound radiation power in this study spans from 10-2000 Hz.In this range, no significant peaks are observed in the low-frequency region.Instead, the peak value occurs at 295 Hz, primarily due to the vibration characteristics of the pipeline.To summarize, the sound radiation power of the composite pipeline under different flow velocities demonstrate consistent trends, with elevated levels at low frequencies.However, the absence of low frequency distinct peaks suggests that the pipeline's vibration characteristics dominate the sound radiation power, with the peak occurring at 295 Hz.

Figure 9. Sound radiation power at different external flow velocities
The sound radiation characteristics of the composite pipeline under different Re are analysed by examining sound pressure levels (SPL) at selected field points within the sound field propagation domain.These field points are positioned on the yOz plane, with the positive pipeline centre located at x=0 and 12.The measurement points distribution is shown in Figure 10, where the points are symmetrically arranged around the pipeline's center-line.Field points P1, P2, P3, and P4 are equidistant from the centre-line, situated at a distance of 3.125 D. Points P5, P6, P7, and P8 are located 7.5 D away from the centre-line, while points P9, P10, P11, and P12 are positioned 12.5 D away.This arrangement allows for the evaluation of SPL at different positions relative to the centre-line of the marine composite pipeline.By the measurement of SPL at these various field points, a comprehensive analysis of the sound radiation characteristics based on different Reynolds numbers can be conducted.1, 2, and 3 shows that the SPL at each field point increases with the flow velocity.At a certain velocity, field points located on both sides of the pipeline and perpendicular to the flow exhibit similar sound pressure.Similarly, field points parallel to the flow and at the same distance from the pipeline also have comparable sound pressure.These findings show a symmetric sound field generated by the flow around the pipeline, where corresponding positions on either side exhibit similar magnitudes.However, it should be noted that the overall SPL in the perpendicular direction is generally higher than in the parallel direction.This difference is attributed to periodic vortex shedding occurring on the pipeline sides.In summary, the sound field analysis demonstrates the symmetric nature generated by the flow around the composite pipeline, with higher SPL observed perpendicular to the flow due to periodic vortex shedding.
Figure 11 displays the directivity at four frequencies and three flow velocities.The blue curves are 0.6 m/s, green curves are 0.8 m/s, and purple curves are 1.0 m/s.From the data analysis, we can conclude that the directivity of the composite pipeline under external flow excitation exhibits a dipole pattern.In this pattern, the maximum SPL is observed perpendicular to the incoming flow direction.This shows that the strongest sound radiation occurs orthogonal to the flow direction.In summary, the sound field directivity of the composite pipeline under external flow excitation demonstrates a dipole characteristic, with the highest SPL occurring perpendicular to the flow direction.

The test environment.
The sound field of the composite pipeline were tested in the northwest of Diaoshan, Zhoushan City, Zhejiang Province, China.To ensure accurate experimental data, noise measurements emitted by the test platform ship were conducted initially.Corrections were made during each specific experimental procedure.The ship was anchored during the experiment, and efforts were made to minimize background noise on the ship to maintain data accuracy.
Figure 12.The overall experimental system.In the test, the composite pipeline used has an inner diameter of 0.05 m, which is consistent with the numerical simulation conducted in this study.To mitigate any finite length effects, the pipeline total length exceeds 200 m, with the submerged section being more than 100 m.
Figure 12 shows the diagram of the experimental system.A short section of the pipeline serves as a seawater intake on the centrifugal pump's intake side.The internal flow is monitored by an electromagnetic flow meter, and a double-layer damping base isolates the pump vibrations.Over 100 m of the pipeline is immersed on the outlet side, allowing for controlled flow velocity.This setup aims to replicate real-world conditions and obtain accurate sound characteristic data for the marine composite pipeline.
The data acquisition system used in the experiment consists of accelerometers (Type JZC2), hydrophones (Type RHS-20), a data collector (Type Pulse 3660D), and a computer.Accelerometers are used to measure surface vibrations, while hydrophones measure sound radiation.The signals from the accelerometers and hydrophones are conditioned and recorded by the data collector.A computer controls the data collector.To analyse the frequency domain characteristics, a fast Fourier transform is applied to the collected signals.Each group of data is sampled for 10s and the average value of 10 samples is obtained to mitigate the impact of ocean currents on the measurements.A significant distance between the sensors and the composite pipeline allows for validation of the numerical simulation results.These measurement designs and procedures are implemented to ensure data accuracy and reliability.

The influence analysis of the centrifugal pump
The centrifugal pump's shaft and impellers are the mechanical energy primary sources in the system.However, the noise signals generated by the axial excitation and blade excitation of the pump can overlap with the other excitation sources frequency spectrum of the pipeline.This can have a significant impact on the observed vibration and sound radiation.To address this issue, adjustments are made to the motor's frequency converter.By varying the frequency converter settings, we have found that signals associated with the pump's axial excitation and blade excitation become distinct and identifiable in the accelerometer readings when the frequency converter is set to 15 Hz or above.This adjustment helps isolate and analyse these specific excitation signals, enabling a more accurate assessment of their influence on the pipeline's vibration and sound characteristics.
Based on the information provided, we have concluded that the marine composite pipeline has a dampening effect on the vibration and noise produced by the pump.This is evident from the observation that the line spectrum associated with the pump's axial excitation and blade excitation is not detected by the hydrophones during the experimental test.Instead, the hydrophones primarily capture noise data resulting from internal flow excitation.Therefore, the composite pipeline attenuates the vibration and noise generated by the pump to some extent, indicating its effectiveness in reducing the overall vibration.
In the presence of background noise at an overall SPL of 123.94 dB (10 Hz to 2 kHz), the pump is started while the ship's main engine remains operational.The marine composite pipeline, filled with seawater, allows the pump to return the seawater flow back to the sea using a three-gate valve, resulting in no internal flow within the pipeline.The radiated SPL of the pipeline is measured under these conditions.Next, the pump's frequency converter is set to 20 Hz, creating an internal flow velocity of 1.0 m/s.The noise from the composite pipeline measures a SPL of 124.01 dB.When the pump is further set to 50 Hz, generating an internal flow velocity of 3.0 m/s, the noise from the pipeline measures a SPL of 123.57dB.Compared the noise from the composite pipeline under pump excitation to the background noise, we have observed that the difference between them is small.This suggests effective attenuation of the pump's axial excitation and blade excitation at the hydrophone positions where the sound signal is received.Consequently, the vibration and noise generated by the pump do not significantly interfere with the noise from the composite pipeline.Based on these findings, we have concluded that the high internal damping factor of the pipeline inhibits the vibration and noise caused by the centrifugal pump's excitation.

The sound radiation analysis under internal flow excitation
Afterwards, the three-gate valve is opened to enable the internal flow to occur at specific velocities.The resulting data is recorded, and the radiated SPL of the composite pipeline under different internal flow velocities is depicted in Figure 13.For enhanced clarity and detailed analysis within the frequency range, the figure has been divided into two sections.The combined measurement uncertainty under internal flow excitation is shown in Table 4.The combined measurement uncertainty obtained at different flow velocities from various hydrophones does not exceed 1.83 dB.This indicates that the measurements taken by the hydrophones can be considered accurate and reliable.5 presents the SPL of the composite pipeline at different flow velocities.The results show that the SPL increases as the internal flow velocity increases, which aligns with our numerical calculation.On average, there is an increase of 1.09 dB in the SPL of the composite pipeline for every 0.1 m/s increase in the internal flow.

The sound radiation analysis under external flow excitation
During measurements, the ship's main engine is shut down.An ultrasonic velocity sensor is to measure the external flow velocity.Real-time changes in the velocity data are observed to be less than 0.01 m/s for each test.The minimum velocity is 0.055 m/s, while the maximum velocity is 0.160 m/s.At the velocity of 0.055 m/s, the SPL of the marine composite pipeline measures 117.89 dB. Figure 14 shows the SPL at different external flow velocities, with a maximum level of 118.97 dB at the flow velocity of 0.160 m/s.Considering the relatively low external flow velocities, we can infer that the sound radiation of the pipeline under external flow excitation is generally negligible compared to the background noise.Therefore, the primary noise source is internal flow excitation for the pipeline.Additional measures should be taken to reduce the sound radiation caused by internal flow excitation.

Conclusions
This study investigated the sound radiation of marine composite pipelines using numerical methods and experimental tests.The following conclusions were drawn: First, internal flow excitations, particularly low-frequency fluctuations caused by vortex migration, result in vibration and noise emission from the pipelines.However, steel construction is more effective in suppressing low-frequency sound radiation.Comparing the sound radiation between steel and marine composite pipelines, the composite construction only shows a slight increase in amplitude at resonant frequencies.The materials like HDPE and CPVC demonstrate significant suppression of higher frequency sound radiation due to high internal damping ratios.
Second, decreasing the ratio of diameter to wall thickness reduces the sound radiation power of the pipeline when the length remains constant.On average, the sound radiation power increases by 2.22 dB for every 0.1 m/s increase in internal flow velocity.
Third, the primary noise sources of the pipeline under external flow excitation are concentrated in low frequency.The sound radiation power decreases with a lower ratio of diameter to wall thickness.SPL is highest perpendicular to the flow and lowest in the parallel direction, decreasing with distance between the pipeline.The directivity generated by external flow exhibits dipole-like pattern.
Fourth, the sound radiation power from internal flow excitation is over 20 dB higher compared to external flow excitation at the same velocity.Internal flow is the main source of sound radiation from the marine composite pipeline.Noise generated by external flow excitation can generally be disregarded in numerical calculations.Experimental tests show that SPL of the marine composite pipeline increases by an average of 1.09 dB for every 0.1 m/s increase in internal flow velocity.Due to the low velocities of ocean currents, sound radiation induced by external flow excitation is relatively low and can be neglected.
These findings provide valuable insights for studying noise properties, optimizing multi-layer designs, and implementing noise control strategies for marine composite pipelines in engineering applications.They serve as a useful reference for researchers and engineers aiming to improve acoustic performance and mitigate noise issues associated with marine composite pipelines.

Figure 2 .
Figure 2. The sound radiation power curve of different materials with different ratios.

Figure 3 .
Figure 3.The sound power at the internal flow velocity from 0.6 to 1.2 m/s.From a functional perspective, marine composite pipelines can be divided into two primary components: the steel section and the polymer materials.The steel component provides the strength and structural integrity, while the polymer materials serve to seal and protect the steel from corrosion.Unlike traditional steel pipelines, the steel section of the marine composite pipeline is not continuous.

Figure 4 .
Figure 4.The sound radiation power at the internal flow velocity from 1.4 to 2.0 m/s.To ensure accurate turbulence simulation, the flow state inside the pipeline must be turbulent, which is determined by the Reynolds number (Re):(1)

Figure 5 .
Figure 5. Flow field computation domain.When the spanwise length of composite pipeline exceeds πD , accurate calculation of the pipeline three-dimensional characteristics can be achieved.Figure 5 shows a diagram of the computational domain, which is set up as 16.25 D × 5 D × 12.5 D.

Figure 7 .
Figure 7.The sound radiation power of HDPE pipeline with different ratios.Figure7shows the sound radiation power of HDPE pipelines with different wall thicknesses under external flow excitation.Thicker walls result in lower radiated sound power levels, attributed to reduced wall vibrations, while maintaining the same external flow excitation force.Across the frequency range of 10-2000 Hz, small-amplitude peaks show the influence of fluctuation pressure from vortex shedding on the HDPE pipeline.This influence is determined by the material's Young's modulus and damping ratio.In summary, increasing the wall thickness of an HDPE pipeline reduces sound radiation power, and small-amplitude peaks show the influence of vortex shedding-induced pressure fluctuations, which depend on the material properties of high-density polyethylene.

Figure 8 .
Figure 8.The sound radiation power of CPVC pipeline with different ratios.Figure8shows the sound radiation power of CPVC pipelines with different thicknesses under external flow excitation.Thicker walls lead to lower radiated sound power levels, attributed to reduced wall vibrations while maintaining a constant external flow excitation force.In the 10-2000 Hz range, no significant peaks are observed in the radiated sound power curve for CPVC.This lack of distinct

Figure 11 .
Figure 11.The directivity of the composite pipeline at different frequencies.

Figure 13 .
Figure 13.The SPL of the pipeline at different internal flow velocities.

Figure 14 .
Figure 14.The SPL of the pipeline at different external flow velocities.
Sound radiation power from steel pipeline with different ratios Figure

Table 1 .
The SPL of the point P1 to P4

Table 2 .
The SPL of the point P5 to P8.

Table 3 .
The SPL of the point P9 to P12.

Table 4 .
The combined measurement uncertainty at different internal flow velocities

Table 5 .
The SPL of the composite pipeline at different internal flow velocities.