Uniform Acceleration Research Articles

Sea level variability at Colombo, Sri Lanka, inferred from the conflation of satellite altimetry and tide gauge measurements

Accurate long-term measurements of sea level are fundamental to evaluating coastal risks, such as the impact of sea-level rise on near-shore ecosystems, groundwater dynamics, and coastal flooding. This study examines sea-level variability at Colombo, Sri Lanka using satellite altimetry, tide gauge measurements separately and their conflated solution under a single model. Modelling of conflated satellite altimetry and tide gauge measurements shows a geocentric (absolute) local sea-level rise of 3.56 ± 0.32 mm/y without any signature of a uniform acceleration since 1981 at this locality. The measurements disclosed statistically significant periodic changes in sea level of luni-solar origin. The conflated model solution enabled the estimation of a statistically significant in-situ vertical land motion (0.58 ± 0.19 mm/yr) without the aid of global positioning measurements. The conflation model explains 98% of the sea-level variability, which makes it suitable for accurate seal level predictions for coastal risk assessments in Colombo, Sri Lanka.

A Suboptimal Optimizing Strategy for Velocity Vector Estimation in Single-Observer Passive Localization.

In a single-observer passive localization system, the velocity and position of the target are estimated simultaneously. However, this can lead to correlated errors and distortion of the estimated value, making independent estimation of the speed and position necessary. In this study, we introduce a novel optimization strategy, suboptimal estimation, for independently estimating the velocity vector in single-observer passive localization. The suboptimal estimation strategy converts the estimation of the velocity vector into a search for the global optimal solution by dynamically weighting multiple optimization criteria from the starting point in the solution space. Simulation verification is conducted using uniform motion and constant acceleration models. The results demonstrate that the proposed method converges faster with higher accuracy and strong robustness.

Impact dynamics of compound drops of fluids with density contrast

The dynamics of compound drops impacting on a flat substrate is numerically investigated using a ternary-fluid diffuse-interface method, with the aim of assessing the effect of a density difference between the inner and outer droplets (denoted by $\lambda$ ) on the evolution of the interfaces. With the help of numerical simulations, we find that, at the intermediate stage of drop impact, the inner droplet exhibits a self-similar deformation at $\lambda =1$ and relatively high Weber number, and experiences more or less a uniform acceleration for various $\lambda$ . In particular, the acceleration magnitude at $\lambda \ne 1$ can be correlated with the acceleration at $\lambda =1$ and the Atwood number. When the inner droplet is denser than the outer one, a lamella occurs at the spreading front of the inner droplet. We present a scaling analysis of the thickness of the lamella, and the resultant theoretical prediction is in good agreement with numerical results. At the maximal spreading of the compound drop, a bulging structure is formed around the symmetry axis due to the presence of the inner droplet, thereby effectively reducing the liquid supply to the spreading front and leading to a decrease of maximal spreading ratio $\beta _{max}$ as compared with a pure drop. We proposed a corrected Weber number $We^*_\lambda$ by taking account of the combined effects of $\lambda$ , volume fraction of the inner droplet, Weber number and morphology of the compound drop. Integrating $We^*_\lambda$ with the universal model of $\beta _{max}$ for impacting pure drops, we successfully build up a new model for predicting the maximal spreading ratio of impacting compound drops with various $\lambda$ .

Positron acceleration by terahertz wave and electron beam in plasma channel

We present a scheme of positron acceleration by intense terahertz (THz) wave together with the driving large-charge electron beam in a plasma channel. The THz wave rapidly evolves into a transversely uniform acceleration field and a weakly focusing/defocusing lateral field in the channel. The THz wave is partially formed with the scheme of coherent transition radiation when the electron beam goes through a metal foil and partially because of the wakefield in the plasma channel. The electron beam continuously supplies energy to the THz wave. Such a field structure offers the feasibility of long-distance positron acceleration while preserving beam quality. By two-dimensional simulations, we demonstrate the acceleration of positrons from initial 1 GeV to 126.8 GeV with a charge of ∼10 pC over a distance of 1 m. The energy spread of accelerated positrons is 2.2%. This scheme can utilize the electron beam either from laser-driven or conventional accelerators, showing prospects towards high-quality and flexible THz-driven relativistic positron sources of ∼100 GeV.

Heat transfer characteristics of turbulent flow of supercritical carbon dioxide (sCO2) in a short-heated microchannel

A numerical study, validated by experimental results, of convective heat transfer of supercritical carbon dioxide (sCO2) in a short-heated microchannel is reported here. RANS equations were used to predict heat transfer and fluid flow characteristics, such as heat transfer coefficient, Nusselt number, pressure drop, and flow acceleration. This CFD steady state analysis used a microchannel of hydraulic diameter 300 μm, and simulations were performed using Starccm + for five different heat fluxes. Comparisons to available correlations, such as the Gnielinski, Bringer & Smith, Shitsman and Ornatsky, were conducted. It was observed that the Gnielinski correlation predicted the Nusselt number well with CFD simulations at lower heat fluxes but overpredicted the Nusselt number at higher heat fluxes. Overall, Shitsman correlation was able to predict the Nusselt number well compared to other correlations, but it didn't predict the correct trend at high heat fluxes. Key findings include quantification of the effect of flow acceleration on heat transfer using primitive variables and the benefits of operating at higher pressures. A uniform acceleration was observed over most of the length of the heater, and a 75% increase in the flow acceleration resulted in a 22.22% reduction in the Nusselt number. It was also observed that a 36% increase in the operating pressure can result in more than 50% reduction in the required mass flow rate for the same temperature drop of the heater.

Pound’s Falling Light Experiment and Einstein’s Elevator

The experiments with gamma radiation carried out by Pound et al. in the 1960s make use of the Mőssbauer effect. The counting rate of photons emitted from a height of h=22.5 m from a Fe57 source was measured versus the downward speed of the absorber. It was found that the minimum transmission occurred at a speed equal to gh/c, thereby indicating that the excess speed of light was (gh/c2 ) c, i.e. the speed of light was found to be greater than c because of the gravitational effect on the photons. This is in very good agreement with the value predicted in 1907 by Einstein on the basis of his Equivalence Principle (EP) which claims that the effects of a gravitational field are indistinguishable from those of a uniform acceleration of the object. However, it is pointed out that the EP fails to account for the fact that both the speed of light and the associated frequency vary in the same proportion with position of the source in the gravitational field, thereby indicating that the associated wavelength is invariant; the Doppler effect, on the other hand, when applied for the case of upward motion of the laboratory toward the source, leads to the conclusion that the wavelength should decrease in the same proportion as the frequency increases. The fact that the effects of both gravity and the motion of circumnavigating atomic clocks need to be taken into account separately/additively to predict elapsed time delays measured relative to the corresponding clock that remained stationary at the originating airport is proof that the two effects are not equivalent. The Uniform Scaling method is described which accounts for differences in measured values obtained in two different rest frames for all types of physical properties, thereby satisfying the original purpose foreseen for the EP.

Frame dependence of the nonrelativistic limit of quantum fields

We study the nonrelativistic limit of quantum fields for an inertial and a noninertial observer. We show that nonrelativistic particle states appear as a superposition of relativistic and nonrelativistic particles in different frames. Hence, the nonrelativistic limit is frame dependent. We detail this result when the noninertial observer has uniform constant acceleration. Only for low accelerations, the accelerated observer agrees with the inertial frame about the nonrelativistic nature of particles locally. In such a quasi-inertial regime, both observers agree about the number of particles describing quantum field states. The same does not occur when the acceleration is arbitrarily large (e.g., the Unruh effect). We furthermore prove that wave functions of particles in the inertial and the quasi-inertial frame are identical up to the coordinate transformation relating the two frames.

Roles of pre- and post-liquefaction stages in dynamic system response of liquefiable sand retained by a sheet-pile wall

This study examines the dynamic system response of a liquefiable deposit retained by a sheet-pile wall, with emphasis on the roles of pre- and post-liquefaction stages of soil response. A recently developed constitutive model, SANISAND-MSf, is utilized to simulate the pre- and post-liquefaction cyclic response of sands. The model is a stress-ratio controlled, critical state compatible, bounding surface plasticity model, which incorporates the concepts of memory surface and semifluidized state. The model’s performance is validated using a combination of cyclic simple shear tests and dynamic centrifuge tests from the LEAP-2020 project. A sensitivity analysis is then conducted by varying the base input motion intensity and duration. The results reveal that the amplitude of equivalent uniform base acceleration in pre-liquefaction correlates well with the timing of liquefaction triggering, and the cumulative absolute velocity of the base acceleration during the post-liquefaction stage correlates well with the post-liquefaction displacements. The study highlights the importance of accurately simulating response in the pre-liquefaction stage for the extent and timing of occurrence of liquefaction, which regulates the remaining intensity and duration of shaking, and in turn, affects the post-liquefaction permanent deformations at the system level.

Effect of Acute Psychological Stress on Speed Perception: An Event-Related Potential Study

The present study tested the intrinsic ERP features of the effects of acute psychological stress on speed perception. A mental arithmetic task was used to induce acute psychological stress, and the light spot task was used to evaluate speed perception. Compared with judgments in the constant speed and uniform acceleration motion, judgments in the uniform deceleration motion were made more quickly and with higher accuracy; attention control was higher and peaked later; and there was longer N2 peak latency, larger N2 peak amplitude, and lower mean amplitude of the late negative slow wave (SW). Under stress, the reaction time was significantly shorter. The N2 peak amplitude and SW mean amplitude were significantly higher, attention control was higher and appeared earlier, and there was a greater investment of cognitive resources. The type of movement and evoked stress also interacted to predict behavioral and ERP measures. Under acute stress, judgments made in the uniform deceleration motion condition elicited lower N2 peak latency, higher attention control, and later peak attention. The results suggest that judgments of the speed of decelerating motion require a lower investment of cognitive resources than judgments of other kinds of motion, especially under acute stress. These findings are best interpreted in terms of the interaction of arousal and attention.

Instability of bubble expansion at zero temperature

In the context of false vacuum decay at zero temperature, it is well known that bubbles expand with uniform proper acceleration. We show that this uniformly accelerating expansion suffers from an instability related to the bubble size. This can be observed in Minkowski spacetime as a tachyonic mode in the spectrum of fluctuations for the energy functional in the reference frame in which the uniformly accelerating bubble wall appears static. In such a frame, arbitrary small perturbations cause an amplifying departure from the static wall solution. This implies that the nucleated bubble is not a critical point of the energy functional in the rest frame of nucleation but becomes one in the accelerating frame. The aforementioned instability for vacuum bubbles can be related to the well-known instability for the nucleated critical static bubbles during finite-temperature phase transitions in the rest frame of the plasma. It is therefore proposed that zero-temperature vacuum decays as seen from accelerating frames have a dual description in terms of finite-temperature phase transitions.

Epistemic uncertainty in the kinematics of global mean sea-level rise since 1993 and its dire consequences

Recent studies reported an ambiguous global sea level acceleration during the satellite altimetry (SA) era (1993–2017). New SA data created an opportunity to resolve this issue. In this study, two competing kinematic models to represent global mean sea level anomalies are compared. The first model consists of an initial velocity and uniform acceleration. The second model replaces the initial velocity and acceleration with a trend and a representation of a long periodic lunar subharmonic of period 55.8 y, which is determined to be statistically significant at globally distributed tide gauge records. The models also include parameters for the periodic effects of lunisolar origin with periods 18.6 y and 11.1 y annual, and biannual variations in 10-day average of globally SA measurements during 1993–2022. Generalized least squares solutions yielded updated statistically significant estimates for all the model parameters and their statistics for both models. However, the outcome failed to resolve the ambiguity of uniform acceleration in global mean sea level confounded with the long periodic lunar subharmonic of period 55.8 y during this period. This epistemic uncertainty will have a dire impact on climate change risk assessments as demonstrated through the prospective comparison of both kinematic models.

Construction of a System for Dynamic Monitoring of Pollutant Emissions by Transport Flows

The aim of this work is to resolve the urgent problem of building an urban monitoring system which monitors in real time emissions of pollutants from automobile traffic flows. Analysis of the initial data showed a statistically significant division of traffic lanes into homogeneous clusters. A comparative analysis of pollutant emissions was carried out for the three selected clusters with regard to two typical types of traffic at the intersection: free passage without stopping; and uniform acceleration after a complete stop at the stop line. This approach will significantly reduce the load on the computing units and practically implement a dynamic emission monitoring system based on the processing of data from street surveillance cameras.

Profile of Students’ Conceptual Understanding of Physics in Senior High School

This research aimed to identify the profile of physical concept mastery of Senior High School/Aliyah School students in Semarang as a result of the transition from offline to online learning. The research employed the descriptive quantitative approach in which the tests and questionnaires were used to collect data. The school type (Senior High School/Aliyah School), school ownership (Private/State), and student’s interest in learning are all considered in the profile analysis. This study was carried out in Semarang, Indonesia, with a sample of 250 students from MAN 2 Semarang, SMA Negeri 1 Semarang, and SMA Negeri 9 Semarang. The findings revealed: 1) The percentage of students with concept mastery scores with the highest answer being questioned was 12 (88.2%). The results showed: 1) The percentage of students with concept mastery scores that really the highest answer lies in question 12 with a percentage of 88.2% on fluid material (Bernoulli’s Law) with a cognitive level of C2. While the percentage value of the student’s mastery of concepts which is the lowest, lies in questions number 3 (Uniform Speed and Uniform Acceleration subject matter) and 4 (moment of inertia/center of gravity and rigid body balance subject matter) with a percentage of 57% each with successive levels of cognition, C3 and C4, 2) Following tutoring is not effective because it does not affect physical learning achievement, and 3) there is a positive (unidirectional) significant (mean) relationship between independence and learning motivation on physical learning achievement. The results of this study can be used as a reference for researchers, educational policymakers, and stakeholders related to the handling of post-pandemic education.

Steady-state entanglement for rotating Unruh-DeWitt detectors

The Unruh-DeWitt model of particle detector is widely used to probe quantum effects in noninertial frames and curved spacetimes. In this paper, we investigate the entanglement dynamics of a quantum system composed of two rotating Unruh-DeWitt detectors in interaction with a fluctuating massless scalar field in vacuum. We obtain the necessary and sufficient condition for the detectors to achieve steady-state entanglement, and systematically investigate the steady-state entanglement of two Unruh-DeWitt detectors rotating in coaxial orbits with the same orbital radius and angular velocity. When the separation between the detectors is vanishing, the detectors can obtain steady-state entanglement dependent on the initial state, as the detectors in uniform acceleration and static detectors in a thermal bath do. Remarkably, however, when the separation between the detectors is nonvanishing but small compared with the transition wavelength of detectors, the detectors can obtain steady-state entanglement irrespectively of what the initial state is. This is the unique phenomenon caused by the centripetal acceleration and is not present in the uniform acceleration case and the case of static detectors in a thermal bath.

Smooth Trajectory Planning at the Handling Limits for Oval Racing

In motion planning for autonomous racing, the challenge arises in planning smooth trajectories close to the handling limits of the vehicle with a sufficient planning horizon. Graph-based trajectory planning methods can find the global discrete-optimal solution, but they suffer from the curse of dimensionality. Therefore, to achieve low computation times despite a long planning horizon, coarse discretization and simple edges that are efficient to generate must be used. However, the resulting rough trajectories cannot reach the handling limits of the vehicle and are also difficult to track by the controller, which can lead to unstable driving behavior. In this paper, we show that the initial edges connecting the vehicle’s estimated state with the actual graph are crucial for vehicle stability and race performance. We therefore propose a sampling-based approach that relies on jerk-optimal curves to generate these initial edges. The concept is introduced using a layer-based graph, but it can be applied to other graph structures as well. We describe the integration of the curves within the graph and the required adaptation to racing scenarios. Our approach enables stable driving at the handling limits and fully autonomous operation on the race track. While simulations show the comparison of our concept with an alternative approach based on uniform acceleration, we also present experimental results of a dynamic overtake with speeds up to 74 m/s on a full-size vehicle.

Numerical Study on Improved Geometry of Outlet Pressure Ripple in Parallel 2D Piston Pumps

Because the axial piston pump is often used in the aerospace and aviation fields, it is necessary to pay attention to its outlet pressure and flow characteristics. The parallel 2D piston pump proposed, based on the axial piston pump, has no structural flow ripple because it has a rail with a uniform acceleration and deceleration. Now, the pump is used in the special working conditions of the aerospace field, and it is required to meet the rated flow of 50 L/min, the rated load of 8 MPa, and an extremely low-pressure ripple. Based on CFD technology, this paper studies the pump’s outlet flow and pressure ripples through numerical simulation. According to the causes of the outlet pressure ripple, an improved geometry is determined to further reduce the outlet pressure ripple. Using a high-frequency pressure sensor to measure the outlet pressure ripple of the optimized pump prototype, it was found that the outlet pressure ripple rate of the prototype was only 6%. The parallel 2D piston pump has been proved by the simulation and test that its outlet pressure ripple is extremely low. However, it is not effective to reduce the outlet flow ripple by increasing the pre-pressure and reducing the backflow. In parallel 2D piston pumps, it is still necessary to find a new method to further reduce outlet pressure and flow ripples.

Low-frequency fluctuations in the yearly misclosures of the global mean sea level budget during 1900–2018

Abstract Sub- and super-harmonics of luni-solar forcing are proxies for the natural variations in sea levels observed at tide gauge stations with long records as demonstrated in earlier studies. This study also identified their signatures in the noisy yearly misclosures of the global mean sea level budget for the period 1900–2018. The analyses of the yearly misclosures revealed a temporal linear systematic error of 0.08 ± 0.02 mm/year, which is not explained by the budget components. The estimate is statistically significant (α = 0.05) but small in magnitude and accounts for only 11% (adjusted R 2) of the variations in the yearly misclosures. Meanwhile, the yearly misclosures have also a statistically significant constant bias as large as −12.2 ± 0.9 mm, which can be attributed to the lack of a common datum definition for the global mean sea level budget components. Modeling the low-frequency changes of luni-solar origin together with a trend and constant bias parameters reduces variability in the misclosures. Accounting for their effects explains 50% (adjusted R 2) of the fluctuations in the yearly misclosures compared to the 11% if they are not. In addition, unmodeled low-frequency variations in the yearly global budget closure assessments have the propensity of confounding the detection of a statistically significant recent uniform global sea level acceleration triggered by anthropogenic contributors.

Carbon emission model of vehicles driving at fluctuating speed on highway.

Carbon emission changes significantly when the vehicle is driving at a variable speed. Given that the rule of vehicle carbon emissions under different driving behaviors is underexplored, this study filled this research gap by establishing a theoretical carbon emission model when vehicles are driving at fluctuating speed. The model was established based on the law of conservation of mechanical energy, the first law of thermodynamics, the theory of vehicle dynamics. Two passenger cars and three trucks were selected as typical vehicles. A field test was carried out under different driving behaviors with uniform acceleration. The proposed model can forecast the carbon emissions of vehicles that fluctuate in speed during travel and showed strong accuracy in model validation. As the speed fluctuation increased, the influence of acceleration on carbon emissions became more apparent. Gradient has a more significant influence on vehicle carbon emissions, followed by acceleration. By controlling the driving behavior of the vehicle on the round trip, the same carbon emissions as driving on a flat route at a constant speed can be achieved. The findings of this study will provide a theoretical basis for low-carbon transportation.

Transient Characteristic Analysis of Variable Frequency Speed Regulation of Axial Flow Pump

In order to explore the influence of different rotational acceleration on the transient internal and external flow characteristics of the axial flow pump and improve the performance of the pump, numerical simulations and experiments were used to analyze the variable frequency speed regulation characteristics of the axial flow pump. Taking three-dimensional turbulent numerical simulation as the main research method and CFX as the calculation platform, three variable frequency speed regulation methods were used to conduct transient numerical simulation; keep the acceleration constant, increased and decreased; and obtain the real-time pump performance curve and pressure characteristic curve. Uniform acceleration and deceleration with constant acceleration maintained the stable change and good stability of head and shaft power, and the pressure change was the most stable in the process. The acceleration and deceleration with decreasing acceleration ran most smoothly at high speed, and the frequency conversion effect was the best. At the same time, the transition to steady-state operation was also the most stable. The research in this paper can provide reference for the stable operation of variable frequency speed regulation of the axial flow pump.

Numerical Simulation of Axial-Flow Pump Cavitation Based on Variable Frequency Speed Regulation

In order to investigate the influence of variable voltage and variable frequency (VVVF) regulation on the cavitation performance of the axial-flow pump, numerical simulation and experiments were used to analyze the cavitation performance of an axial-flow pump under different VVVF modes. The VVVF modes were uniform acceleration with constant acceleration, variable acceleration with increasing acceleration, variable acceleration with decreasing acceleration, and its corresponding deceleration scheme. Furthermore, a comprehensive performance test rig was built for the pump to carry out cavitation visualization tests which verified the accuracy of numerical simulation. For the uniform acceleration scheme with constant acceleration, the change of flow field inside the impeller was stable, the expansion rate of cavitation was slow, and the growth rate of the cavitation volume was the slowest. For the variable acceleration scheme with decreasing acceleration, the cavitation extended rapidly due to the large initial velocity. For the variable acceleration scheme with increasing acceleration, cavitation extension was the slowest. The growth rate of the cavity volume of the two variable acceleration schemes was faster than that of the uniform acceleration scheme, and the changing trend was consistent. This feature indicates that the impeller rotation speed has a significant impact on cavitation, and excessive rotation speed will rapidly extend the cavitation. By monitoring the influence of cavitation on pressure distribution under VVVF, it was shown that the three acceleration schemes all produce large pressure fluctuation. For the uniform acceleration scheme with constant acceleration, the fluctuation range of pressure was more balanced, and the pressure dropped slowly. For the acceleration scheme with higher acceleration, the pressure fluctuation amplitude increased in the late stage of acceleration and the pressure decline speed accelerated. For the acceleration scheme with decreasing acceleration, the pressure showed a downward trend with violent fluctuations in the early stage and gradually tended to be flat in the late stage.