High-speed Camera Observation Research Articles

Methodology for Surface Reconstruction and Prediction Based on the Electrical Discharge Machining Removal Mechanism of Cf-UHTC Materials.

Cf-UHTC is an ideal aerospace material because of its exceptional properties, but its machinability is facing great challenges. Electrical discharge machining (EDM) offers a potential solution, but its removal mechanism remains unclear, lacking reliable prediction tools to guide the actual production. This paper deeply explores the EDM removal mechanism of Cf-ZrB2-SiC through single-pulse experiments, high-speed camera observations, and thermal-fluid coupling simulations, revealing key processes like heat transfer, phase transformation, molten pool dynamics, crater formation, and reinforcing phase effects. And the prediction of single-pulse removal with different parameters is also realized. Based on experimental and simulation results, a random continuous discharge model is developed, which deeply studies the dynamic erosion process, reconstructs EDM surfaces, and accurately predicts surface roughness. Furthermore, the thickness of the recast layer can be predicted based on the equivalent temperature method. Undoubtedly, this model provides an ideal approach for efficient production.

New Insights for Improving Low-Rank Coal Flotation Performance via Tetrahydrofurfuryl Ester Collectors

With the advancement of large-scale coal development and utilization, low-rank coal (LRC) is increasingly gaining prominence in the energy sector. Upgrading and ash reduction are key to the clean utilization of LRC. Flotation technology based on gas/liquid/solid interfacial interactions remains an effective way to recover combustible materials and realize the clean utilization of coal. The traditional collector, kerosene, has demonstrated its inefficiency and environmental toxicity in the flotation of LRC. In this study, four eco-friendly tetrahydrofuran ester compounds (THF-series) were investigated as novel collectors to improve the flotation performance of LRC. The flotation results showed that THF-series collectors were more effective than kerosene in enhancing the LRC flotation. Among these, tetrahydrofurfuryl butyrate (THFB) exhibited the best performance, with combustible material recovery and flotation perfection factors 79.79% and 15.05% higher than those of kerosene, respectively, at a dosage of 1.2 kg/t. Characterization results indicated that THF-series collectors rapidly adsorbed onto the LRC surface via hydrogen bonding, resulting in stronger hydrophobicity and higher electronegativity. High-speed camera and particle image velocimeter (PIV) observation further demonstrated that THFB dispersed more evenly in the flotation system, reducing the lateral movement of bubbles during their ascent, lowering the impact of bubble wakes on coal particles, and promoting the stable adhesion of bubbles to the LRC surface within a shorter time (16.65 ms), thereby preventing entrainment effects. This study provides new insights and options for the green and efficient flotation of LRC.

Combustion characteristics of Ammonia/Air mixture ignited by flame jet with and without hydrogen injection in Pre-Chamber

Ammonia is one of the important fuels that can promote the achievement of carbon neutrality, but its combustion characteristics are not conducive to its application in engines. Injecting hydrogen in the pre-chamber to form a flame jet to ignite the ammonia/air mixture is a method to improve the combustion of ammonia. This ignition method was investigated with the constant volume combustion chamber, high-speed video camera observation and the main-chamber pressure analysis. The flame behavior and combustion characteristics were compared with those ignited by the ammonia flame jet. Hydrogen flame jet significantly enhanced the combustion of the mixture and extended the lean flammability limit. Under conditions of hydrogen injection, the pressure rise delay and combustion duration became shorter, the average heat release rate became higher, and the combustion process was more stable. The hydrogen flame jet rapidly penetrated the main-chamber and impinged on the lower surface, generating intense turbulence. As a result, the combustion pressure took less time to rise from 10% to 50% than it did to go from 50% to 90%. This was different from the situation without hydrogen injection, where time required for both was similar. The hydrogen flame jet was shuttle-shaped when touching the lower surface owing to the rapid combustion speed of hydrogen, while the ammonia flame jet was spindle-shaped with the flame kernel in the center. The combustion process of the flame jet igniting the mixture exhibited two peaks in the heat release rate, reflecting two combustion stages dominated by the flame jet and flame propagation.

Fractal dimension analysis of lightning discharges of various types based on a comprehensive literature review

In this study, photographic records of natural lightning flashes obtained from high-speed video camera observations are analyzed based on an extensive literature review. The purpose of this work is to estimate the fractal dimension of lightning discharges and to analyze and evaluate the effect of lightning type (downward and upward flashes during their propagation as well as return strokes) and lightning polarity (negative and positive). Three methods of fractal dimension estimation are employed namely the: (i) box-counting, (ii) sandbox, and (iii) correlation method, utilizing algorithms developed in MATLAB software. Appropriate image processing techniques are employed to guarantee precision in fractal dimension estimation. A thorough discussion is conducted by comparing the estimated fractal dimension values with those previously documented in the relevant literature based on field observations. The mean fractal dimension of downward negative leaders for the three methods (1.18, 1.31, 1.38) aligns well with previous studies, while positive downward leaders exhibit lower values (1.08, 1.15, 1.26), denoting a reduced branching behavior; upward leaders demonstrate slightly higher fractal dimensions than downward ones. Additionally, an attempt to correlate the lightning return stroke peak current with the fractal dimension is made. The outcomes of this research may assist in facilitating precise modeling of the lightning attachment phenomenon, thereby contributing to the development of safer lightning protection systems.

The Influence of the Gap Phenomenon on the Occurrence of Consecutive Discharges in WEDM Through High-Speed Video Camera Observation

The Wire Electrical Discharge Machining (WEDM) process is an accurate method for manufacturing high-added-value components for industry. Continuous developments in the process have resulted in specialized machines used in sectors such as aerospace and biomedical engineering. However, some fundamental aspects of the discharge process remain unresolved. This work aims to study the influence of discharge location and bubble expansion on the occurrence of subsequent discharges. A high-speed video camera observation system was constructed to capture images of each discharge. From the acquired images, an algorithm was devised to determine the discharge location based on grayscale analysis. Moreover, the voltage and current waveforms of the discharges and the framing signals of the high-speed video camera were then obtained using an oscilloscope. Synchronizing the observation images and signals allowed for calculating the delay time for each single discharge. The results indicate that most of the discharges occurred near the boundary of the bubble and during bubble expansion. This finding has been observed for a variety of machining conditions and can be explained by the effect of the debris particles concentrated at the bubble boundary. This study provides useful information for better understanding the discharge process in WEDM.

Investigation of high internal phase water-in-oil emulsion on coal gasification fine slag flotation decarburization and its oil saving mechanism

Froth flotation stands as the preferred technique for the separation of charcoal and ash from coal gasification fine slag (CGFS). However, the presence of a well-developed pore structure and an abundance of oxygen-containing functional groups on the surface of residual charcoal necessitates the use of substantial dosage of collectors during the flotation of CGFS. To address this, a high internal phase water-in-oil (HIP W/O) emulsion, with an internal phase water volume fraction of 85 %, was formulated for the flotation decarburization of CGFS, with the objective of reducing the collector dosage. The outcomes of flotation tests demonstrated that at dosages of 50 kg/t for kerosene and 15.04 kg/t for the organic liquid of the HIP W/O emulsion, both were capable of yielding tailings with an ash content exceeding 95 %. The results from Cryo-SEM and viscosity tests suggest that the high viscosity characteristics of the HIP W/O emulsion are primarily attributed to the compact arrangement of emulsion droplets and the active influence of emulsifiers at the oil–water interface. High-speed camera observations revealed that the HIP W/O emulsion droplets exhibited a tendency to disperse into bar-shaped flocs with larger particle sizes in water. This particular dispersion morphology is advantageous for enhancing the oil agglomeration flotation mechanism within the flotation decarburization process of CGFS. LF-NMR tests have further substantiated that the emulsion droplets are not readily absorbed by the microporous structure of the residual charcoal within the CGFS. Wettability tests and FTIR analyses have demonstrated that the hydrophobic modification of the residual charcoal surface, achieved through the HIP W/O emulsion, markedly surpasses that of conventional kerosene. The combination of these results indicates that the use of HIP W/O emulsion in the flotation decarbonization process of CGFS offers multiple benefits. It not only significantly reduces the dosage of organic collectors required, but also enhances the economic efficiency and effectiveness of the flotation process, presenting a novel reagent option for the decarbonization of CGFS.

Dynamic behavior regulation of droplet transfer based on arc-bubble control by ultrasound-induced during underwater wet welding

In comparison to welding in air, the periodic formation of unstable arc bubbles is a significant factor contributing to the unstable mass transfer process during underwater wet welding (UWW). Up to now, there is no ideal method to control droplet transfer and improve arc stability. In this study, the ultrasound-induced method (UIM) was used to regulate the droplet transition behavior and improve the process stability. The behavior features and key mechanisms of arc-bubble and droplet transfer induced by ultrasound were revealed with the employment of highspeed camera and X-ray observation, respectively. The incident angle of ultrasound was taken as a variable to reveal its effects and mechanism of action on the behaviors of arc bubble and droplet transfer, weld formation quality, microstructure, and property of weld joint. The results show that ultrasound-induced a new rising mode of arc bubble, which can significantly reduce the repulsive resistance that come from the bubble rising on the droplet transfer process. Compared with underwater wet welding without ultrasound, the average diameter of molten droplets reduces from 3.2 mm to 1.7 mm, and the droplet transfer frequency increases from 7.8 Hz to 13 Hz as the incident angle of ultrasound increases to 60°. In addition, the weld seam without obvious defects could be produced by UWW with UIM, of which the Variation coefficient of weld reinforcement is reduced from 21.3 to 11.6 and 17.4 at an ultrasonic incident angle of 30° and 45°. With increasing of ultrasound incident angle, it showed that the microhardness of the weld joint has a slight increase. In the heat-affected zone, the microhardness for the joint increased from 290 HV to 320 HV while the incident angle increased to 60°. This research shows that an appropriate incident angle of ultrasound can obtain good-quality joints which has a stable metal transfer process in underwater wet welding.

Real-time mapping of photo-sono therapy induced cavitation using Doppler optical coherence tomography.

Photo-sono therapy (PST) is an innovative anti-vascular approach based on cavitation-induced spallation. Currently, passive cavitation detection (PCD) is the prevalent technique for cavitation monitoring during treatment. However, the limitations of PCD are the lack of spatial information of bubbles and the difficulty of integration with the PST system. To address this, we proposed a new, to the best of our knowledge, cavitation mapping method that integrates Doppler optical coherence tomography (OCT) with PST to visualize bubble dynamics in real time. The feasibility of the proposed system has been confirmed through experiments on vascular-mimicking phantoms and in vivo rabbit ear vessels, and the results are compared to high-speed camera observations and PCD data. The findings demonstrate that Doppler OCT effectively maps cavitation in real time and holds promise for guiding PST treatments and other cavitation-related clinical applications.

Insights into ground strike point properties in Europe through the EUCLID lightning location system

Abstract. Evaluating the risk of lightning strikes to a particular structure typically involves adhering to the guidance outlined in IEC 62305-2. Among the multitude of factors influencing the overall risk, flash density emerges as a crucial parameter. According to its definition, each flash is assigned only one contact point to ground. Nevertheless, it is well known that, on average, flashes exhibit multiple ground termination points as shown by high-speed camera observations. In this research, lightning data collected by the European Cooperation for Lightning Detection (EUCLID) network are utilized in combination with a ground strike point (GSP) algorithm that aggregates individual strokes within a flash into ground strike points. This approach enables the examination of spatio-temporal patterns of GSPs across Europe throughout a decade, spanning from 2013 to 2022. Average GSP densities exhibit variations across the European continent, mirroring the observed patterns in flash densities. The highest densities are concentrated along the Adriatic Sea and the western Balkan region, reaching peak values of up to 8.5 GSPs km−2 yr−1. The spatial distribution of the mean number of ground strike points per flash reveals a noticeable increase in the Mediterranean, Adriatic, and Baltic Sea regions compared to inland areas. Moreover, it has been determined that the average number of GSPs per flash reaches its peak between September and November. Additionally, a daily pattern is discernible, with the lowest number of GSPs per flash occurring between 12:00 and 18:00 UTC (universal time coordinated). It is found that 95 % of the separation distances between distinct GSPs are less than 6.7 km. Lastly, it is worth noting that the presence of the Alps has an impact on GSP behaviour, resulting in lower GSP counts in comparison to the surrounding areas, along with the shortest average distances between different GSPs.

Shedding of Cavitation Clouds in an Orifice Nozzle

Focused on the unsteady property of a cavitating water jet issuing from an orifice nozzle in a submerged condition, this paper presents a fundamental investigation of the periodicity of cloud shedding and the mechanism of cavitation cloud formation and release by combining the use of high-speed camera observation and flow simulation methods. The pattern of cavitation cloud shedding is evaluated by analyzing sequence images from a high-speed camera, and the mechanism of cloud formation and release is further examined by comparing the results of flow visualization and numerical simulation. It is revealed that one pair of ring-like clouds consisting of a leading cloud and a subsequent cloud is successively shed downstream, and this process is periodically repeated. The leading cloud is principally split by a shear vortex flow along the nozzle exit wall, and the subsequent cloud is detached by a re-entrant jet generated while a fully extended cavity breaks off. The subsequent cavitation cloud catches the leading one, and they coalesce over the range of x/d≈1.8~2.5. Cavitation clouds shed downstream from the nozzle at two dominant frequencies. The Strouhal number of the leading cavitation cloud shedding varies from 0.21 to 0.29, corresponding to the injection pressure. The mass flow rate coefficient fluctuates within the range of 0.59~0.66 at the same frequency as the leading cloud shedding under the effect of cavitation.

Experimental Study on Energy Evolution and Acoustic Emission Characteristics of Fractured Sandstone under Cyclic Loading and Unloading

To investigate the dynamic response of fractured rock under cyclic loading and unloading, a WHY-300/10 microcomputer-controlled electro-hydraulic servo universal testing machine was used to conduct uniaxial cyclic loading and unloading tests. Simultaneously, acoustic emission (AE) and a CCD high-speed camera were employed to monitor the fracturing characteristics of sandstone. The mechanical properties, energy evolution, AE characteristics, and deformation of 45° sandstone were analyzed. The results indicate that as the load cycle level increases, both the elastic modulus and deformation modulus exhibit a “parabolic” increase, with a rapid rise initially and a slower rate of increase later. The damping ratio generally shows a decreasing trend but tends to rise near the peak load. The total energy, elastic energy, dissipated energy, damping energy, and damage energy all follow exponential function increases with the load level. The b-value fluctuates significantly during the stable crack propagation phase, unstable crack propagation phase, and peak phase. When the FR (Felicity ratio > 1), the rock is relatively stable; when the FR (Felicity ratio < 1), the rock gradually extends towards an unstable state. The Felicity ratio can be used as a predictive tool for the precursors of rock failure. Shear fractures dominate during the compaction and peak phases, while tensile fractures dominate during the crack propagation phase, ultimately leading to a failure characterized by tensile fracture. High-speed camera observations revealed that deformation first occurs at the tips of the prefabricated cracks and gradually spreads and deflects toward the ends of the sandstone. This study provides theoretical support for exploring the mechanical behavior and mechanisms of fractured rock under cyclic loading and unloading, and it has significant practical implications.

Plasma pretreatment to enhance the sedimentation of ultrafine kaolinite particles: Experiments and mechanisms

Coal-slime water treatment not only has a direct impact on the reuse of water as well as the recovery of coal-slime, but also has an essential significance for the sustainable development of a green ecology and carbon neutrality. In this study, the sedimentation behavior of ultrafine kaolinite particles (the main component of high-ash fine slime) using plasma pretreatment was investigated via sedimentation tests, turbidity detections and high-speed camera observations. The results showed that with plasma pretreatment, the arrival time of the maximum (final) thickness of the thickening layer considerably shortens, the final thickness of the thickening layer slightly decreases, and the turbidity of the supernatant significantly reduces. The mechanism of the plasma pretreatment method for the enhanced sedimentation of ultrafine kaolinite particles was elaborated via laser particle size analysis, microscopic observations, zeta potential determinations, and particle interaction energy calculations. The results showed that with plasma pretreatment, the zeta potential of kaolinite particles lowers, and the decrease in repulsive electrostatic force strengthens the coagulation of kaolinite particles, therefore making kaolinite flocs more compact and enhancing the sedimentation process of ultrafine kaolinite particles. And the enhanced sedimentation mechanism of ultrafine kaolinite particles via plasma pretreatment was drawn.

Microstructure and properties of Al–Cu–Mg alloy welded by 14 μm single-mode laser small oscillation welding

In this paper, the 14 μm single-mode laser oscillation welding technique was adopted to achieve high-quality welding of 2A12 aluminum alloy. Specifically, its influence on the microstructure and mechanical properties of the joint and the formation mechanism were investigated. The results showed that the ultra-small spot laser (28 μm) was able to achieve sequential oscillation of the molten pool with small amplitude (0.1 mm). The increased frequency caused the average grain size to decrease and the grain boundary length increased in the fusion zone. Meanwhile, the average grain orientation difference increased, and the growth orientation was more dispersed and homogeneous. While reducing weld morphology defects and refining grain microstructure, there was little influence on the penetration and width morphology of the weld. Finally, successfully prepared welds with tensile strength up to 410.4 MPa and elongation simultaneously increased to 4.065 % at 200 Hz. Furthermore, the influence of keyhole oscillation on the molten pool flow at small amplitude was explained by high-speed camera observation, which explored the mechanism of grain growth in the oscillating joints and provided new ideas for the welding of 2xx aluminum alloys.

Wave-converging pressure increase in curved cylindrical rotating detonation combustors

The internal combustion structure of a curved cylindrical rotating detonation combustor (RDC) was experimentally investigated through optical observation and pressure measurements. Experiments were conducted under low back pressure conditions using two combustion chambers having, in both cases, an internal diameter of 17.5 mm, a curvature of radius of 38.1 mm, and an outlet angle of 90 deg, with gaseous C2H4 and O2 as the propellants. One was made of SUS and the other of resin. In addition to vibrations and bottom center pressure, pressure distribution on the sidewalls was measured for the SUS combustor, and high-speed camera observations of self-luminescence from the radial direction were conducted for the resin combustor. These measurements were compared by varying the equivalence ratio. The frequency analysis results obtained from vibrations and self-luminescence indicated that the strongly-coupled detonation mode exhibited a higher peak frequency, suggesting that the detonation waves may have different propagation speeds or rotational positions. In terms of pressure distribution and self-luminescence, only the strongly-coupled detonation mode inside near the bottom surface exhibited high pressure and brightness values. This suggested the potential for converging pressure by employing rotating detonation waves and a curved tube.

Discharge Characteristics and Mechanisms of Electrolytic Discharge Processing by Jet Mask

As a novel microfabrication method, electrochemical discharge machining has remarkable effects on the forming and processing of brittle and hard materials and non-conductive materials, but little research has been done on the electrochemical discharge mode in the jet state. To fulfil the potential of this technology, innovative research on the discharge characteristics and mechanism of electrochemical discharge machining in the jet mask is proposed. A high-speed camera observation experiment was set up to record the process of the jet flow column discharge formation and penetration. Changes in the electric field of the electrolytic jet channel were analysed by simulation software, and the morphology of the machined micro-pits was observed using a microscope. A mathematical derivation of the dielectric electric field in the gas–liquid two-phase jet column reveals the mechanism of discharge channel formation in the jet state. The experiments show that when the processing voltage is 400 V, a stable continuous spark appears, realizing the unique characteristics of a large-gap long-distance discharge and a flat small circle-shaped discharge mark produced at the bottom of the crater. The actual field strength within the bubble of this model obtained by mathematical derivation is approximately 61.5 kV/cm greater than the critical field strength for air bubble breakdown in the standard state, where bubble breakdown occurs in the discharge.

Improved formation accuracy and mechanical properties of laser-arc hybrid additive manufactured aluminum alloy through beam oscillation

High-frequency circular beam oscillation was employed for the laser-arc hybrid additive manufacturing of AA4047 aluminum alloy. Under the optimized oscillating frequency of 300 Hz, the effects of oscillating diameter (D) on the forming accuracy, microstructure and mechanical properties of deposited thin-walls were studied. It was found that the effective forming width reached a maximum of 88.9 %, and the porosity decreased from 0.1 % to 0.01 % when the D increased from 0 to 1.2 mm. Furthermore, the machining allowance was reduced from 1.51 to 1.25 mm, which indicated that the formation accuracy was improved by 17 %. The increasing D reduced the average size of grains from 55.9 to 44.8 μm, showing a refinement degree of 19.9 %. These improvements reduced the microhardness distribution variance of thin-wall from 31.82 to 13.37. The ultimate tensile strength increased from 219.1 to 227.3 MPa, and the elongation increased from 12.9 % to 14.6 %. Based on the high-speed camera observation, the study discussed the influence of laser beam oscillation on the droplet transfer and melt flow behavior, as well as the improvement mechanism of deposition quality.

Optical Investigation of a Spark Gap for DC Protection around Current Zero

The applied experimental setup consists of a pair of fixed electrodes forming a short gap in air. The electrodes are connected with a thyristor forming a parallel current path. Once the thought flashover ignited arc has burnt a given time in the range of several hundreds of microseconds, the thyristor will be fired causing a commutation of the current in the semiconductor path. Hence, the current through the electrode gap will be reduced. The extinguishment of the arc permits the creation of a current zero in the gap. The focus of the experimental work is set to the analysis of the arc plasma during the DC phase and its distinction in the current extinction phase (current zero). High speed camera observation and optical emission spectroscopy are carried out and combined with the electrical measurements.

In Situ Observation of Bubbles and the Effect of Ultrasonic Vibration on Bubble Behavior in EDM.

Accumulation and concentration of debris in deep hole electrical discharge machining (EDM) significantly hinder its machining efficiency and accuracy. It is believed that the movement of bubbles associated with the discharge gap flow field play a pivotal role in debris removal and influence the discharge conditions. Ultrasonic vibration (USV) of the electrode is thought to be an effective method for improving EDM-generated bubbles and debris exclusion. In this study, we first elucidated the behavior of bubbles during EDM of holes with varying aspect ratios. Subsequently, USV was introduced to EDM. The behavior of dielectric fluid flow under the influence of ultrasonic vibration was analyzed using computational fluid dynamics (CFD), which revealed time-varying changes in discharge gap flow pressure and velocity. The velocity of the dielectric flow field near the electrode's side face was found to reach a maximum of approximately 15.2 m/s, greatly facilitating debris removal. High-speed camera observations revealed that bubbles were dispersed within the side gap, with most of them adhering to the electrode's wall. Furthermore, the bubbles exhibited a tendency to continuously break up and coalesce near the hole's outlet before escaping in the USV-assisted EDM. These observed characteristics of bubble behavior under the influence of USV are expected to significantly enhance debris removal and promote efficient dielectric exchange.

Numerical analysis of the effect of an oscillating metal vapor plume on the keyhole and molten pool behavior during deep penetration laser beam welding

The effect of the oscillating metal vapor plume on the keyhole and molten pool behavior during the laser beam welding of AlMg3 aluminum alloys is investigated by experimental and numerical methods. The real-time height of the metal vapor plume is measured by high-speed camera observation. The obtained experimental results are used to evaluate the additional heating source and laser beam attenuation caused by the scattering and absorption based on the Beer–Lambert theory. Furthermore, the dynamic behavior of the metal vapor plume is incorporated into a 3D transient heat transfer and fluid flow model, coupled with the ray tracing method, for the laser beam welding of the AlMg3 alloy. It is found that additional heating resulting from the scattered and absorbed laser beam energy by the metal vapor plume significantly expands the shape of the molten pool on the top region. Moreover, the oscillating metal vapor plume caused the fluctuation of the high-temperature region in the molten pool. The probability of keyhole collapse at the bottom increases 17% due to the oscillating laser power induced by the laser beam attenuation. The internal interplay between the metal vapor plume, molten pool shape, and keyhole collapse is obtained. The developed model has been validated by experiments, which shows a good agreement.

Experimental study on the drop test on wet deck slamming for a SWATH segment model

Wet deck slamming is a serious issue for high-speed catamarans if the relative vertical motions are higher. In this study, a drop test on wet deck slamming is conducted for a small waterplane area twin hull segment model, and the vertical water-entry model test is performed under different drop heights and speeds. The slamming pressure under the wet deck and the acceleration are measured. The flow field is obtained through particle image velocimetry measurement. A high-speed camera observation during wet deck slamming shows that air is pressed into water, and a water–gas mixture is formed due to the air compressibility effect. The slamming pressure shows an oscillating property with multiple frequency components, which is due to the air cushion effect caused by the volume variation of the water–gas mixture and the acoustic wave oscillation with the air pocket. The effects of the drop height variation and the constant-drop speed on the slamming load are analyzed. The results show that the pressure peak appears during the demihull's bottom slamming and wet deck slamming. The acceleration time record produces two peaks, one for the demihull's bottom slamming and another for the wet deck slamming. In conclusion, the air cushion effect can reduce the wet deck slamming pressure.