2020 Vol. 40, No. 9
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2020, 40(9): 092301.
doi: 10.11883/bzycj/2020-0016
Abstract:
To evaluate the fast cook-off safety characteristic of the big-size penetrator, an experimental device was designed. The penetrator with a mass of 290 kg was hoisted at a height of 0.4 m from the aviation fuel level for rapid heating. The surface temperature of the penetrator was collected in real time and the whole experimental process was recorded. The reflected shock wave overpressure at a distance of 7 m from the penetrator centroid was measured. The safety characteristic of the big-size penetrator was analyzed in detail in terms of the heating time, the surface temperature of the penetrator, the damage on experimental site, the peak value of the reflected shock wave overpressure, the reaction mechanism and the response type. The results show that the big-size penetrator starts to react violently at a temperature of 537 ℃ for 16 min and 4 s. The bottom explosive of the penetrator first responds to hot spots under continuous high temperature heating, gradually accumulating high-temperature and high-pressure gas in the shell and tearing the shell to quickly release high-pressure. The peak value of the reflected shock wave overpressure at 7 m is 33.622 kPa, which is much smaller than that caused by the penetrator totally detonated in the air. The reaction characteristic of the penetrator is deflagration, and its fast cook-off safety characteristic meets the standard requirement.
To evaluate the fast cook-off safety characteristic of the big-size penetrator, an experimental device was designed. The penetrator with a mass of 290 kg was hoisted at a height of 0.4 m from the aviation fuel level for rapid heating. The surface temperature of the penetrator was collected in real time and the whole experimental process was recorded. The reflected shock wave overpressure at a distance of 7 m from the penetrator centroid was measured. The safety characteristic of the big-size penetrator was analyzed in detail in terms of the heating time, the surface temperature of the penetrator, the damage on experimental site, the peak value of the reflected shock wave overpressure, the reaction mechanism and the response type. The results show that the big-size penetrator starts to react violently at a temperature of 537 ℃ for 16 min and 4 s. The bottom explosive of the penetrator first responds to hot spots under continuous high temperature heating, gradually accumulating high-temperature and high-pressure gas in the shell and tearing the shell to quickly release high-pressure. The peak value of the reflected shock wave overpressure at 7 m is 33.622 kPa, which is much smaller than that caused by the penetrator totally detonated in the air. The reaction characteristic of the penetrator is deflagration, and its fast cook-off safety characteristic meets the standard requirement.
2020, 40(9): 092302.
doi: 10.11883/bzycj-2019-0395
Abstract:
Nano titanium carbide (TiC) powder was synthesized by detonation shock utilizing octogen (HMX) as high temperature and high pressure source with titanium dioxide (TiO2) and activated carbon (C) as precursors. The samples were characterized by X-ray diffraction (XRD), energy dispersive spectrometer (EDS) and scanning electron microscope (SEM). At the same time, detonation shock synthesis mechanism of nanometer TiC was discussed in this paper. It was concluded that the test results of XRD and EDS were in good agreement with the theoretical values, and the sample contained both TiC and TiCx (x＜1). The particle size of both TiC and TiCx (x＜1) were less than 50 nm by SEM photos and micron-sized spherical agglomerates were found in the samples. The detonation shock synthesis of TiC belongs to a special solid-phase reaction, and its material diffusion rate and reaction rate are greatly improved.
Nano titanium carbide (TiC) powder was synthesized by detonation shock utilizing octogen (HMX) as high temperature and high pressure source with titanium dioxide (TiO2) and activated carbon (C) as precursors. The samples were characterized by X-ray diffraction (XRD), energy dispersive spectrometer (EDS) and scanning electron microscope (SEM). At the same time, detonation shock synthesis mechanism of nanometer TiC was discussed in this paper. It was concluded that the test results of XRD and EDS were in good agreement with the theoretical values, and the sample contained both TiC and TiCx (x＜1). The particle size of both TiC and TiCx (x＜1) were less than 50 nm by SEM photos and micron-sized spherical agglomerates were found in the samples. The detonation shock synthesis of TiC belongs to a special solid-phase reaction, and its material diffusion rate and reaction rate are greatly improved.
2020, 40(9): 093101.
doi: 10.11883/bzycj-2020-0015
Abstract:
Research of concrete materials subjected to tensile stress wave at high strain rates is currently based on splitting experiments and spalling experiments with a split Hopkinson pressure bar device, however, they are not appropriate to study the stress-strain relationship of concrete materials subjected to one dimensional tensile stress wave. Therefore, the large diameter split Hopkinson tensile bar (SHTB) is urgently needed to perform direct dynamic tensile study of concrete materials. Mechanical analysis of a new type of SHTB apparatus was performed in numerical simulation method, then corresponding incident tensile stress wave was studied and optimize improvement measures for partial components were also proposed. The partly improved SHTB apparatus reconciled the demands of glued connect mode, hooked connect mode and so on. At last, concrete was considered as a two-phase composite material which composed of coarse aggregates and cement matrix, the annulus three-dimensional concrete aggregate model was established and applied to SHTB simulation experiment. The comparison between numerical simulation results and experimental results verified the effectiveness of partly improved SHTB apparatus, which also provided research directions for dynamic tensile responses of mesoscopic concrete model.
Research of concrete materials subjected to tensile stress wave at high strain rates is currently based on splitting experiments and spalling experiments with a split Hopkinson pressure bar device, however, they are not appropriate to study the stress-strain relationship of concrete materials subjected to one dimensional tensile stress wave. Therefore, the large diameter split Hopkinson tensile bar (SHTB) is urgently needed to perform direct dynamic tensile study of concrete materials. Mechanical analysis of a new type of SHTB apparatus was performed in numerical simulation method, then corresponding incident tensile stress wave was studied and optimize improvement measures for partial components were also proposed. The partly improved SHTB apparatus reconciled the demands of glued connect mode, hooked connect mode and so on. At last, concrete was considered as a two-phase composite material which composed of coarse aggregates and cement matrix, the annulus three-dimensional concrete aggregate model was established and applied to SHTB simulation experiment. The comparison between numerical simulation results and experimental results verified the effectiveness of partly improved SHTB apparatus, which also provided research directions for dynamic tensile responses of mesoscopic concrete model.
2020, 40(9): 093102.
doi: 10.11883/bzycj-2019-0432
Abstract:
A 37 mm-diameter split Hopkinson pressure bar (SHPB) apparatus was used to conduct impact tests on two types of coral sand to investigate the effect of strain rate. One-dimensional compressive stress-strain curves with the strain rates ranging from 460 to 1300 s−1 were obtained under different compactness levels. Combining with the static compressive results (strain rate of 10−4 s−1), it is found that coral sand is affect with strain rate obviously. Comparison of physical properties of two types of coral sand indicates that the strain rate sensitivity is highly correlated with the proportion of inner pores of the particles and friction between the particles. The calculation models of dynamic intensification factor are proposed, which provide theoretical basis for the numerical calculation of coral sand under impact.
A 37 mm-diameter split Hopkinson pressure bar (SHPB) apparatus was used to conduct impact tests on two types of coral sand to investigate the effect of strain rate. One-dimensional compressive stress-strain curves with the strain rates ranging from 460 to 1300 s−1 were obtained under different compactness levels. Combining with the static compressive results (strain rate of 10−4 s−1), it is found that coral sand is affect with strain rate obviously. Comparison of physical properties of two types of coral sand indicates that the strain rate sensitivity is highly correlated with the proportion of inner pores of the particles and friction between the particles. The calculation models of dynamic intensification factor are proposed, which provide theoretical basis for the numerical calculation of coral sand under impact.
2020, 40(9): 093201.
doi: 10.11883/bzycj-2019-0448
Abstract:
In order to study the crack growth and crack arrest law of the brittle materials, a large-sized trapezoidal opening crack with arc bottom (TOCAB) configuration specimen was proposed. The impact tests were carried out on the TOCAB specimens with radians of 0°, 60°, 90° and 120° under the drop hammer impact device. The crack growth speed was obtained by using the distance between the two resistance wires divided by the break time of the resistance wire of the crack propagation gauge, and crack propagation gauge (CPG) was used to monitor the crack initiation time and expansion time. The crack growth behavior of the TOCAB specimen was numerically simulated by using the finite difference software AUTODYN. And the crack growth process and the crack arrest law were numerically studied. The critical dynamic stress intensity factor of the moving crack was calculated based on the experimental-numerical method and the finite element software ABAQUS. Both experimental and numerical results show that the three arc-bottom specimens have a crack-stopping effect on the moving crack, andthe TOCAB configuration specimen is suitable for studying the crack arrest problem. And the crack growth path obtained in the numerical calculation is basically consistent with the experimental results, which verifies the validity of the numerical model. And the critical dynamic stress intensity factor at the time of crack initiation and crack arrest is greater than that at the time of the crack growth.
In order to study the crack growth and crack arrest law of the brittle materials, a large-sized trapezoidal opening crack with arc bottom (TOCAB) configuration specimen was proposed. The impact tests were carried out on the TOCAB specimens with radians of 0°, 60°, 90° and 120° under the drop hammer impact device. The crack growth speed was obtained by using the distance between the two resistance wires divided by the break time of the resistance wire of the crack propagation gauge, and crack propagation gauge (CPG) was used to monitor the crack initiation time and expansion time. The crack growth behavior of the TOCAB specimen was numerically simulated by using the finite difference software AUTODYN. And the crack growth process and the crack arrest law were numerically studied. The critical dynamic stress intensity factor of the moving crack was calculated based on the experimental-numerical method and the finite element software ABAQUS. Both experimental and numerical results show that the three arc-bottom specimens have a crack-stopping effect on the moving crack, andthe TOCAB configuration specimen is suitable for studying the crack arrest problem. And the crack growth path obtained in the numerical calculation is basically consistent with the experimental results, which verifies the validity of the numerical model. And the critical dynamic stress intensity factor at the time of crack initiation and crack arrest is greater than that at the time of the crack growth.
2020, 40(9): 093301.
doi: 10.11883/bzycj-2019-0439
Abstract:
With the development of hypervelocity kinetic energy weapons, the mechanism of long rod hypervelocity penetration into concrete target was a research highlight. To study the penetration mechanism and the crater law of long rod hypervelocity penetration into concrete, two kinds of long rods, TU1 and Q235, hypervelocity penetration into concrete with initial velocity of 1.8−2.4 km/s were experimented. Dimensional analyses of crater diameter and crater volume were performed based on the experiment data from this paper and references. Prediction formula of crater depth was derived from the bowl shape contour of crater section. The crater size of hypervelocity penetration was obviously larger than that of low and medium velocity penetration, and so the crater phase was non-negligible during the penetration mechanism researches. The length of the long rod was severely shortened until the long rod was completely eroded, the radius of the hole was obviously larger than that of long rod, and these results can be used to verify that the mechanism of long rod hypervelocity penetration into concrete was semi-fluid penetration. At the same time, it can be seen from the experimental results that the length of the long rod was the most important parameter affecting the penetration depth. The penetration depth increased with the increase of the length and density of the projectile, but was not affected by the strength of the long rod.
With the development of hypervelocity kinetic energy weapons, the mechanism of long rod hypervelocity penetration into concrete target was a research highlight. To study the penetration mechanism and the crater law of long rod hypervelocity penetration into concrete, two kinds of long rods, TU1 and Q235, hypervelocity penetration into concrete with initial velocity of 1.8−2.4 km/s were experimented. Dimensional analyses of crater diameter and crater volume were performed based on the experiment data from this paper and references. Prediction formula of crater depth was derived from the bowl shape contour of crater section. The crater size of hypervelocity penetration was obviously larger than that of low and medium velocity penetration, and so the crater phase was non-negligible during the penetration mechanism researches. The length of the long rod was severely shortened until the long rod was completely eroded, the radius of the hole was obviously larger than that of long rod, and these results can be used to verify that the mechanism of long rod hypervelocity penetration into concrete was semi-fluid penetration. At the same time, it can be seen from the experimental results that the length of the long rod was the most important parameter affecting the penetration depth. The penetration depth increased with the increase of the length and density of the projectile, but was not affected by the strength of the long rod.
2020, 40(9): 095101.
doi: 10.11883/bzycj-2020-0011
Abstract:
In order to study in depth the structural response of the bottom protective component of the vehicles under blast loading and improve the blast resistant performance of the protective vehicles, a finite element model of the bottom protective component of a vehicle under blast loading was established, and the reliability of the finite element simulation was verified by the explosion impact bench test; the concave hexagonal negative Poisson’s ratio honeycomb material was used as the core layer of the protective component, the deformation mode of the negative Poisson’s ratio honeycomb material under blast loading was analyzed, and the blast resistant performance was compared with the other three protective components of the same mass. The results show that the protective component containing negative Poisson’s ratio honeycomb core has better resistance to blast loading. A mathematical model was established for multi-objective optimization problems with the cell size parameters of the honeycomb material as design variables, and the multi-objective genetic algorithm was used to obtain the optimal solution of the cell geometric parameters, which effectively reduces the maximum deflection and maximum kinetic energy of the protective component substrate.
In order to study in depth the structural response of the bottom protective component of the vehicles under blast loading and improve the blast resistant performance of the protective vehicles, a finite element model of the bottom protective component of a vehicle under blast loading was established, and the reliability of the finite element simulation was verified by the explosion impact bench test; the concave hexagonal negative Poisson’s ratio honeycomb material was used as the core layer of the protective component, the deformation mode of the negative Poisson’s ratio honeycomb material under blast loading was analyzed, and the blast resistant performance was compared with the other three protective components of the same mass. The results show that the protective component containing negative Poisson’s ratio honeycomb core has better resistance to blast loading. A mathematical model was established for multi-objective optimization problems with the cell size parameters of the honeycomb material as design variables, and the multi-objective genetic algorithm was used to obtain the optimal solution of the cell geometric parameters, which effectively reduces the maximum deflection and maximum kinetic energy of the protective component substrate.
2020, 40(9): 095201.
doi: 10.11883/bzycj-2019-0471
Abstract:
In order to solve the problem of poor performance of EMD (empirical mode decomposition） filter de-noising for vibration signal, an adaptive orthogonal decomposition signal de-noising method PEMD (principal empirical mode decomposition) is proposed. This algorithm combines the self-adaptability of EMD decomposition and the complete orthogonality of principal component analysis (PCA), eliminates the phenomenon of mode aliasing in the process of signal EMD decomposition, and obtains the best de-noising effect. The results showed that compared with EMD and EEMD (ensemble empirical mode decomposition）, PEMD (principal component analysis) improved 1.15 dB and 0.38 dB respectively in the simulation test, and the root-mean-square error was the smallest. In frequency domain, PEMD has the highest sensitivity to the frequency of simulation signal (30 Hz), and the noise filtering effect is the best outside 30 Hz. In the blasting vibration test, PEMD and EEMD had better performance in removing burrs, and PEMD had the best performance in preserving medium and low frequency vibration signals at 0−300 Hz, and the best performance in filtering high frequency noises above 300 Hz.
In order to solve the problem of poor performance of EMD (empirical mode decomposition） filter de-noising for vibration signal, an adaptive orthogonal decomposition signal de-noising method PEMD (principal empirical mode decomposition) is proposed. This algorithm combines the self-adaptability of EMD decomposition and the complete orthogonality of principal component analysis (PCA), eliminates the phenomenon of mode aliasing in the process of signal EMD decomposition, and obtains the best de-noising effect. The results showed that compared with EMD and EEMD (ensemble empirical mode decomposition）, PEMD (principal component analysis) improved 1.15 dB and 0.38 dB respectively in the simulation test, and the root-mean-square error was the smallest. In frequency domain, PEMD has the highest sensitivity to the frequency of simulation signal (30 Hz), and the noise filtering effect is the best outside 30 Hz. In the blasting vibration test, PEMD and EEMD had better performance in removing burrs, and PEMD had the best performance in preserving medium and low frequency vibration signals at 0−300 Hz, and the best performance in filtering high frequency noises above 300 Hz.
2020, 40(9): 095202.
doi: 10.11883/bzycj-2020-0003
Abstract:
In order to study the influence of weak dynamic disturbance on rockburst, test on cubic medium-coarse grained granite specimen with a circle hole was conducted to simulate the rockburst ejection process in circle tunnel subjected to weak dynamic disturbance. Three loading paths, namely, no disturbance, weak dynamic disturbance starting at high-stress level, and weak dynamic disturbance beginning at a low-stress level were considered. The testing process was recorded using AE and video monitoring system. The ejection failure process, characteristics of rockburst pit, acoustic emission signal characteristics and rockburst intensity were investigated. The testing results show that the weak dynamic disturbance can reduce the stress level at the occurrence of rockburst and increase the range of rockburst. The applied dynamic disturbance at a high-stress level leads to a rapid occurrence of rockburst. In contrast, when the weak dynamic disturbance is applied at a low-stress level, rockburst will occur in a gradual manner. In addition, compared with that without dynamic disturbance, rockburst with the weak dynamic disturbance starting at high-stress level has a higher intensity, while that with weak dynamic disturbance starting at low high-stress level has a lower intensity. This is because that weak dynamic disturbance starting at high-stress level is capable of stimulating and amplifying the energy release process, while weak dynamic disturbance starting at a low-stress level is only capable of stimulating the energy release process.
In order to study the influence of weak dynamic disturbance on rockburst, test on cubic medium-coarse grained granite specimen with a circle hole was conducted to simulate the rockburst ejection process in circle tunnel subjected to weak dynamic disturbance. Three loading paths, namely, no disturbance, weak dynamic disturbance starting at high-stress level, and weak dynamic disturbance beginning at a low-stress level were considered. The testing process was recorded using AE and video monitoring system. The ejection failure process, characteristics of rockburst pit, acoustic emission signal characteristics and rockburst intensity were investigated. The testing results show that the weak dynamic disturbance can reduce the stress level at the occurrence of rockburst and increase the range of rockburst. The applied dynamic disturbance at a high-stress level leads to a rapid occurrence of rockburst. In contrast, when the weak dynamic disturbance is applied at a low-stress level, rockburst will occur in a gradual manner. In addition, compared with that without dynamic disturbance, rockburst with the weak dynamic disturbance starting at high-stress level has a higher intensity, while that with weak dynamic disturbance starting at low high-stress level has a lower intensity. This is because that weak dynamic disturbance starting at high-stress level is capable of stimulating and amplifying the energy release process, while weak dynamic disturbance starting at a low-stress level is only capable of stimulating the energy release process.
2020, 40(9): 095203.
doi: 10.11883/bzycj-2019-0367
Abstract:
In the process of freezing vertical shaft blasting, the baseline drift and noise in the near area monitoring signal have significant influence on the fine extraction of local characteristics. On the basis of effective acquisition of shaft lining vibration signals in near field of blasting, complementary ensemble empirical mode decomposition (CEEMD) method, baseline estimation and de-noising with sparsity (BEADS) method and hidden Markov model de-noising (HMMD) method and so on are used to solve the problem of baseline drift and random noise elimination in the signal, and the correlation evaluation of correction and noise elimination effect is carried out by cross wavelet transform (CWT). The analysis results show that: the slowly changing baseline component in the signal exists the whole process of each modal component, and it is mainly concentrated in the low frequency component, while the noise is concentrated in the high frequency component. The combined analysis method can deal with low frequency baseline drift and high frequency noise effectively. It is an efficient and relatively amplitude-preserving signal analysis method, and can be used to preprocess of batch blasting vibration signal data.
In the process of freezing vertical shaft blasting, the baseline drift and noise in the near area monitoring signal have significant influence on the fine extraction of local characteristics. On the basis of effective acquisition of shaft lining vibration signals in near field of blasting, complementary ensemble empirical mode decomposition (CEEMD) method, baseline estimation and de-noising with sparsity (BEADS) method and hidden Markov model de-noising (HMMD) method and so on are used to solve the problem of baseline drift and random noise elimination in the signal, and the correlation evaluation of correction and noise elimination effect is carried out by cross wavelet transform (CWT). The analysis results show that: the slowly changing baseline component in the signal exists the whole process of each modal component, and it is mainly concentrated in the low frequency component, while the noise is concentrated in the high frequency component. The combined analysis method can deal with low frequency baseline drift and high frequency noise effectively. It is an efficient and relatively amplitude-preserving signal analysis method, and can be used to preprocess of batch blasting vibration signal data.
2020, 40(9): 095401.
doi: 10.11883/bzycj-2020-0009
Abstract:
To analyze the effect of porous materials on the explosion characteristics of premixed gas, a self-built explosion experiment platform was used to investigate the behavior of porous materials with different porosities and thicknesses on premixed methane/air gas explosion with a stoichiometric ratio of 1. Experimental studies have shown that porous materials with different porosities can either promote or suppress the explosive flame and overpressure. When the porosity was low, the propagation speed of the deflagration flame decreased with the increase of material thickness, and when the thickness was large, the flame had a short propagation delay. When the porosity was high, the quenching effect occurred when the premixed flame impacted the porous material. However, in the following period, due to negative pressure suction, diffusion combustion occurred on the surface of the material towards the side of the exploded area, and the degree of diffusion combustion was inversely proportional to the thickness of the material. The solid phase structure of porous materials could reduce the efficiency of pressure release and absorb energy, resulting in increasing the rate of explosion overpressure rise and reducing the overpressure peak. When the porous material with δ=10 were used to promote flame propagation, compared with premixed gas explosions with a stoichiometric ratio of 1, the peak overpressure could be increased by about 2 times at most, causing more serious consequences. Quenching occurred when flame impacted the porous material with δ=20, and the maximum overpressure attenuation was 47.17%. The maximum overpressure decreased by 24.62% at the porous material with δ=30.
To analyze the effect of porous materials on the explosion characteristics of premixed gas, a self-built explosion experiment platform was used to investigate the behavior of porous materials with different porosities and thicknesses on premixed methane/air gas explosion with a stoichiometric ratio of 1. Experimental studies have shown that porous materials with different porosities can either promote or suppress the explosive flame and overpressure. When the porosity was low, the propagation speed of the deflagration flame decreased with the increase of material thickness, and when the thickness was large, the flame had a short propagation delay. When the porosity was high, the quenching effect occurred when the premixed flame impacted the porous material. However, in the following period, due to negative pressure suction, diffusion combustion occurred on the surface of the material towards the side of the exploded area, and the degree of diffusion combustion was inversely proportional to the thickness of the material. The solid phase structure of porous materials could reduce the efficiency of pressure release and absorb energy, resulting in increasing the rate of explosion overpressure rise and reducing the overpressure peak. When the porous material with δ=10 were used to promote flame propagation, compared with premixed gas explosions with a stoichiometric ratio of 1, the peak overpressure could be increased by about 2 times at most, causing more serious consequences. Quenching occurred when flame impacted the porous material with δ=20, and the maximum overpressure attenuation was 47.17%. The maximum overpressure decreased by 24.62% at the porous material with δ=30.
2020, 40(9): 095901.
doi: 10.11883/bzycj-2019-0399
Abstract:
The dynamic response of buried high density polyethylene (HDPE) bellows under blasting seismic load was studied. First, the blasting test of buried pipeline was carried out by combining the blasting seismic test and dynamic strain test. Secondly, the dynamic response characteristics of buried pipeline under blasting seismic load were analyzed. Then, the characteristics of vibration velocity and dynamic strain distribution were studied. Finally, the pipe safety was evaluated based on the von Mises yield criterion, and the blasting vibration velocity control standard was proposed. The experimental results show that the vibration velocity of pipeline and ground and the dynamic strain of pipeline increase with the decrease of core distance and the increase of explosive quantity. The dominant frequency of blasting seismic wave is higher. The dominant frequency of pipeline is higher than the surface. Under the same blasting condition, the ground vibration velocity above the pipeline is generally higher than that of the pipeline. The peak axial strain on the back explosion side of the pipeline section is mainly tensile strain, and the peak circumferential strain on the front explosion side is mainly compressive strain. The vibration velocity of the pipeline can be safely controlled by 20 cm·s−1, and the pipeline is in a safe state.
The dynamic response of buried high density polyethylene (HDPE) bellows under blasting seismic load was studied. First, the blasting test of buried pipeline was carried out by combining the blasting seismic test and dynamic strain test. Secondly, the dynamic response characteristics of buried pipeline under blasting seismic load were analyzed. Then, the characteristics of vibration velocity and dynamic strain distribution were studied. Finally, the pipe safety was evaluated based on the von Mises yield criterion, and the blasting vibration velocity control standard was proposed. The experimental results show that the vibration velocity of pipeline and ground and the dynamic strain of pipeline increase with the decrease of core distance and the increase of explosive quantity. The dominant frequency of blasting seismic wave is higher. The dominant frequency of pipeline is higher than the surface. Under the same blasting condition, the ground vibration velocity above the pipeline is generally higher than that of the pipeline. The peak axial strain on the back explosion side of the pipeline section is mainly tensile strain, and the peak circumferential strain on the front explosion side is mainly compressive strain. The vibration velocity of the pipeline can be safely controlled by 20 cm·s−1, and the pipeline is in a safe state.
