Low Particle Velocities Research Articles

Ultrasound enhanced diffusion in hydrogels: An experimental and non-equilibrium molecular dynamics study.

Focused ultrasound has experimentally been found to enhance the diffusion of nanoparticles; our aim with this work is to study this effect closer using both experiments and non-equilibrium molecular dynamics. Measurements from single particle tracking of 40nm polystyrene nanoparticles in an agarose hydrogel with and without focused ultrasound are presented and compared with a previous experimental study using 100nm polystyrene nanoparticles. In both cases, we observed an increase in the mean square displacement during focused ultrasound treatment. We developed a coarse-grained non-equilibrium molecular dynamics model with an implicit solvent to investigate the increase in the mean square displacement and its frequency and amplitude dependencies. This model consists of polymer fibers and two sizes of nanoparticles, and the effect of the focused ultrasound was modeled as an external oscillating force field. A comparison between the simulation and experimental results shows similar mean square displacement trends, suggesting that the particle velocity is a significant contributor to the observed ultrasound-enhanced mean square displacement. The resulting diffusion coefficients from the model are compared to the diffusion equation for a two-time continuous time random walk. The model is found to have the same frequency dependency. At lower particle velocity amplitude values, the model has a quadratic relation with the particle velocity amplitude as described by the two-time continuous time random walk derived diffusion equation, but at higher amplitudes, the model deviates, and its diffusion coefficient reaches the non-hindered diffusion coefficient. This observation suggests that at higher ultrasound intensities in hydrogels, the non-hindered diffusion coefficient can be used.

Моделирование эрозионного износа титанового сплава высокоскоростным потоком частиц

Introduction. Predicting solid particle erosion (SPE) in gaseous flow and managing its intensity is still a relevant problem in mechanical engineering. It requires the development of a general modeling methodology, which also depends upon many special cases studying various physical processes. Such studies should also include verification analysis, process parameters and model sensitivity studies. Mainly computational fluid dynamics and finite element analysis (and mesh-free methods such as smooth particle hydrodynamics or similar) are used to simulate the erosion process. Papers focused on CFD simulation of solid particle erosion of metal alloys are widely presented, but most of it is associated with relatively low or medium particle velocities (< 100–150 m/s) and is close to uniform diameter distribution. This paper presents a CFD study of Ti6Al4V titanium alloy SPE at relatively high particle velocities and sufficiently non-uniform unimodal particle diameter distribution. The paper also studies the turbulence model influence and particle shape effect which appears as a “shape factor” coefficient in the particle drag model. Methods. The heterogenous flow simulation was based on the Reynolds-averaged Navier-Stokes formulation, where the particles, according to Euler-Lagrange formulation, were simulated as mathematical points with corresponding properties. The influence of turbulence models, such as k-epsilon standard, RNG k-epsilon, and a relatively new Generalized equation k-omega (GEKO) model and its coefficients were also studied. Oka and DNV erosion models were also compared based on the general sample mass loss and more specific erosion intensity profile criterions. The simulation results were compared to the lab-scale experimental results. Results and discussion. It is shown that neither erosion intensity profile or sample mass loss do not depend upon the turbulence model choice or GEKO parameters variation. As expected, erosion is dependent on the erosion model and its coefficients. A notable influence of the shape factor is shown. As the drag coefficient increased due to the particle shape, the erosion intensity decreased and the erosive profile on the surface also changed due to the changing velocity and diameter distribution of the heterogenous flow. It is expected that such results would be useful not only for erosion prediction in all areas of mechanical engineering, but also for wear management in mechanical assemblies and shot peening / shot peen forming management and simulation.

Relativistic dynamical friction in stellar systems

Aims.We extend the classical formulation of the dynamical friction effect on a test star by Chandrasekhar to the case of relativistic velocities and velocity distributions, also accounting for post-Newtonian corrections to the gravitational force.Methods.The original kinetic framework was revised and used to construct a special-relativistic dynamical friction formula where the relative velocity changes in subsequent encounters are added up with Lorentz transformation, and the velocity distribution of the field stars accounts for relativistic velocities. Furthermore, a simple expression is obtained for systems where the post-Newtonian correction on the gravitational forces become relevant even at non-relativistic particle velocities. Finally, using a linearized Lagrangian we derived another expression for the dynamical friction expression in a more compact form than previously used.Results.Comparing our formulation with the classical one, we observe that a given test particle undergoes a slightly stronger drag when moving through a distribution of field stars with relativistic velocity distribution. Vice versa, a purely classical treatment of a system where post-Newtonian (PN) corrections should be included, overestimates the effect of dynamical friction at low test particle velocity, regardless of the form of velocity distribution. Finally, a first-order PN dynamical friction covariant formulation is weaker its classical counterpart at small velocities, but much higher for large velocities over a broad range of mass ratios.

Impact characteristics and erosion mechanism of solid particles in a centrifugal pump

To study the impact and erosion mechanism of solid particles in centrifugal pumps, the standard k–ε turbulence model and SIMPLE algorithm are adopted in this paper. Based on the discrete phase model (DPM) of the Lagrange method and McLaury erosion model, the flow impact characteristics and erosion mechanism of solid particles impacting the surface of flow passage components in a single-stage centrifugal pump were numerically simulated, and the test data were compared with the numerical simulation results of the external characteristics of the pump in clean water. The results show that the erosion mechanism of the pressure surface of the blade is mostly the impact erosion caused by high-speed particles with large impact angles, and the impact angle and impact velocity are larger near the tail of the pressure surface. The impact angle of solid particles on the shroud and hub is relatively small, but the erosion mechanism is still impact erosion. The erosion mechanism of the volute wall is mostly the cutting friction erosion caused by the low-velocity particles with small impact angles, and it is only impacted by the particles with large angles near the volute tongue, which is impact erosion. Overall, the average impact angle and impact velocity of the particles on the pressure surface of the blade are higher than those on the volute, so the erosion of the pressure surface is more serious than that of the volute in theory. The research results have certain theoretical reference value for improving the wear resistance of a centrifugal pump.

Performance Evaluation of High Efficiency Particulate Filters for The Removal of Sub-Micrometer Radioactive Aerosols in Nuclear Power Reactors. Simulation Study

The removal of radioactive aerosols has always been one of the key issues of air decontamination in a nuclear power plant to reduce both the exposure of the radiation workers inside the NPP and environmental radiation impact. Radioactive aerosols may be removed using nuclear grade High Efficiency Particulate Air filters. The thickness of the particulate filter is function of many parameters like the particle size, the fiber diameter, the face velocity and the ratio volume of fiber/volume of filter. In this work we evaluated, by using a semi empirical model, the effect of different parameters (fiber diameter, particle size, face velocity) on the performance of the HEPA filters to reach an efficiency removal above 99.99%. The results suggest that thin layers of fiber, can work as an efficient and cost-effective filter. This study revealed that the thickness of the filter showed a positive correlation with the face velocity, but negative correlation with fiber diameter and particle diameter. It is also found that the efficiency removal is higher for lower particle velocities in air stream.

Characterisation of the granular dynamics at the interface between a pipe and a granular flow in a rotating drum

A new mobile bed heat exchanger is presented in this work which is composed of a flowing granular material in a rotating drum and a cylindrical pipe with potential interest in different energy applications as cooling, heating or heat recovery processes. An optimal design of the device requires a characterisation of the phenomena involved at the interface between the granular flow and the pipe. The process is modelled by the discrete element method and a global classification of the flow patterns around the pipe is presented with respect to the three main control parameters of the problem: the Froude number, the diameter ratio and the relative filling height of the drum. The second part is devoted to the characterisation of the structure of the flow at the interface (velocity field, density field) in particular in a so-called Biflow regime where granular motion occurs above as well as below the pipe which is favourable to transfer by convection. A typical behavior at the interface with the pipe consists of a zone I with high velocities of particles at the top of the pipe, a second zone with quasistatic particles or low velocity particles at the front and at the bottom of the pipe and a last zone III of depletion of particles at the back of the pipe. The Froude number has a limited effect on the features of this structure on the first layer in the range of Froude numbers considered whereas the relative height is a more determinant parameter to control the relative magnitude of velocities in zone I and zone II as well as the extent of the depletion zone. This first hydrodynamical characterisation can shed light on the dynamical regimes with improved transfer between the particles and the pipe boundary.

Structural Improvement of Differential Motion Assembly in In Situ Pressure-Preserved Coring System Using CFD Simulation

In situ pressure-preserved coring (IPP-Coring) is one of the most efficient methods for identifying the scale of the oil and gas content. However, the differential motion assembly of the IPP-Coring system often undergoes ball and ball seat seal failure and sticking due to surface erosion, and a greater pressure drop may unexpectedly trigger the assembly. This paper addresses these issues by improving the hydraulic structure of an assembly based on a deep understanding of the flow characteristics in the assembly, thus increasing the success rate of the IPP-Coring. Computational fluid dynamics (CFD) was employed to investigate flows in a differential motion assembly. The effects of the diameter and outlet structure of the ball seat on the fluid status, velocity, and pressure distribution were thoroughly analyzed. When the ball seat diameter increased from 30 to 40 mm, the maximum velocity and pressure drop decreased to 0.55 and 0.2 times their original values, respectively. There was a severe vortex area in the differential motion assembly due to the presence of the ball seat, but changing the outlet structure in the ball seat to an arc structure decreased the length of the vortex area and the fluid velocity near the wall to 0.7 and 0.4 times, respectively, compared with those with the original right-angled structure. In addition, the pressure drop decreased to 0.33 times the original value. Thus, the hydraulic structure of the assembly was improved, and a 40 mm diameter ball seat and an arc-shaped ball seat outlet were selected. Particle trajectory and erosion calculation results showed that the improved structure has a lower particle velocity and less impact on the wall, and the average erosion rate is only 0.42 times the value of the original structure. Due to the better erosion resistance and smaller pressure drop, the improved structure shows promise for field performance.

Can the Nucleation Phase be Generated on a Sub-fault Linked to the Main Fault of an Earthquake?

We study the effects of seismic coupling, friction, viscous, and inertia on earthquake nucleation based on a two-body spring-slider model in the presence of thermal- pressurized slip-dependent friction and viscosity. The stiffness ratio of the system to represent seismic coupling is the ratio of coil spring K between two sliders and the leaf spring L between a slider and the background plate and denoted by s=K/L. The s is not a significant factor in generating the nucleation phase. The masses of the two sliders are m1 and m2, respectively. The frictional and viscous effects are specified by the static friction force, fo, the characteristic displacement, Uc, and viscosity coefficient, h, respectively. Numerical simulations show that friction and viscosity can both lengthen the natural period of the system and viscosity increases the duration time of motion of the slider. Higher viscosity causes lower particle velocities than lower viscosity. The ratios g=h2/h1, f=fo2/fo1, y=Uc2/Ucl, and m=m2/m1 are four important factors in influencing the generation of a nucleation phase. When s>0.17, g>1, 1.15>f>1, y<1, and m<30, simulation results exhibit the generation of nucleation phase on slider 1 and the formation of P wave on slider 2. The results are consistent with the observations and suggest the possibility of generation of nucleation phase on a sub-fault. Results exhibit independence of P wave at slider 2 on the shape and duration time of nucleation phase at slider 1.

Atomistic characterization of impact bonding in cold spray deposition of copper

We investigate the cold-spray additive manufacturing process of nanometric copper particles impacting a copper substrate using molecular dynamics (MD) and focus on the interplay between particle size (2–32 nm), particle velocity (100–2800 m/s), ensuing particle penetration depth and the impact bonding energy (IBE) binding the impinging particle with the substrate. It is observed, based on trends in the IBE, that even at low particle velocities (∼100 m/s) smaller sized particles (<8 nm) bind to the substrate, while there is a size-dependent critical velocity (≥200 m/s) for binding of larger particles (≥8 nm). At velocities near and above 500 m/s, material jetting and significant increase in particle penetration into the substrate are observed for larger particles. Rapid temperature increase, accompanied by adiabatic shear banding and instability (ASI), is observed, especially at the particle-substrate interface, resulting in particle-cratering at higher velocities in the range studied. Further, while jetting and ASI are observed together at high impact velocities, simulations show that ASI is not essential for jetting, consistent with recent investigations. In addition, both ASI and jetting are not prerequisites for particle binding to the substrate. A significant observation involves self-similar variations in particle penetration depth and IBE with particle kinetic energy across all particle sizes, which is captured by a quantitative constitutive relationship governing the particle's kinetic energy, penetration depth, and IBE.

Regularity of weak solutions for the relativistic Vlasov–Klein–Gordon system

This article is devoted to the study of the relativistic Vlasov–Klein–Gordon system in space dimension three. The dynamical model describes a collisionless ensemble of classical particles coupled with a Klein–Gordon field. By combining the Fourier method and smoothing effect of low velocity particles, we prove a higher regularity of the field, which is widely used for the study of Vlasov equation.

Radio Frequency Quadrupole for Bunching Electron Beam: Electromagnetic Field, Particle Velocity Range, and Accuracy at 10 GHz

The Radio Frequency Quadrupole (RFQ) accelerator invented by Kapchinskii and Tepliakov can focus, bunch, and accelerate charged-particle beams simultaneously. Typically, it operates at frequencies up to 500 MHz, for low particle velocities ( β ). The first attempt to design cylindrical RFQ for electrons in the GHz region was done using 3 GHz at Frascati in 1990. In this paper, an analytical approximation of the electromagnetic field is given, and linearized in the beam region for a rectangular Electron Radio Frequency Quadrupole (ERFQ). The differences between the proton-RFQ and the electron-RFQ are discussed. Then, it will be shown that contrary to the quadrupoles for protons or heavy-ions, the ERFQ is suited for electron velocities in the range 0.5 - 0.7 c, and possible applications are given. Finally, it is illustrated, with numerical field computations that this approach gives sufficient accuracy at 10 GHz.

The influence of different preparation methods on the erosion behavior of NiP-ZrO2 nanocomposite coating

Three different deposition techniques were utilized to synthesize NiP-ZrO2 nanocomposite coatings. The structural and mechanical merits for the as-prepared coatings were compared through XRD, XRF, SEM, AFM, microhardness, and nanoindentation analysis. The pulse-electrodeposition was the most suitable technique with a hardness value of 653HV25. The erosion behavior for the different NiP-ZrO2 coatings was investigated based on erosion duration, particles velocity, and impact angle. The results shows that the erosion rate decreases linearly with increasing angle of incidence and increases with increasing particle velocity. The erosion mechanism is mainly fracturing of splats and ploughing action at low particle velocity due to the ductility merits of the different coatings. In contrast, larger craters, radial cracks, and micro cuttings were observed at higher speeds.

Helioscope for gravitationally bound millicharged particles

Particles may be emitted efficiently from the solar interior if they are sufficiently light and weakly coupled to the solar plasma. In a narrow region of phase space, they are emitted with velocities smaller than the escape velocity of the Solar System, thereby populating a gravitationally bound density that can accumulate over the solar lifetime, referred to as a ``solar basin.'' Detection strategies that can succeed in spite of (or even be enhanced by) the low particle velocities are therefore poised to explore new regions of parameter space when taking this solar population into account. Here we identify ``direct deflection'' as a powerful method to detect such a population of millicharged particles. This approach involves distorting the local flow of gravitationally bound millicharges with an oscillating electromagnetic field and measuring these distortions with a resonant $LC$ circuit. Since it is easier to distort the flow of slowly moving particles, the signal is parametrically enhanced by the small solar escape velocity near Earth. The proposed setup can probe couplings an order of magnitude smaller than other methods for millicharge masses ranging from 100 to 100 eV and can operate concurrently as a search for sub-GeV millicharged dark matter. The signal power scales as the millicharge coupling to the eighth power, meaning that even with conservative assumptions, direct deflection could begin to explore new regions of parameter space. We also highlight novel features of millicharge solar basins, including those associated with the phase-space distribution and the possibility for the occupation number to vastly exceed that of a thermal distribution.

Orthogonal Collision of Particles Produces New Physical State

Collider is a machine or device that usually causes two beams of high-speed particles moving to collide in a straight line. The fundamental purpose of a collision is to obtain an abnormal mass-energy density and attempt to discover new physics and new substances namely new physical states. However, linear collisions are not easy to achieve the above purpose. Through the comparable experiment of rear-end collision, head-on collision and orthogonal collision of two low-velocity particles, this paper theoretically proposes a new idea that the orthogonal collision between two-beam high-velocity particles can really produce an abnormal mass-energy density. This machine based on the new idea of orthogonal collision can not only greatly reduce the construction cost of colliders, but also is the most effective way to achieve the purpose of collision.

A coupled CFD–DEM approach to model the in-trough mixing in a multi-stage solar particle receiver

Concentrated solar power (CSP) is expected to reach economic competitiveness with conventional power plants in the foreseeable future. The use of fine-grained ceramic particles as a heat transfer fluid (HTF) for CSP is a promising alternative to other HTFs such as molten salts and nanofluids. However, research on ceramic particle based CSP is stymied due to the inherent difficulty in obtaining in-depth particle mixing processes. Acquiring this highly sought information would allow engineers and scientists to improve the design and optimization of next-generation CSP receivers thereby potentially leading to improved thermal efficiencies. To this end, a high-fidelity numerical model, namely a coupled computational fluid dynamics and discrete element method (CFD–DEM) is developed and implemented on a novel multi-stage solar particle receiver to provide an in-depth analysis of the dynamical and temporal evolution of the in-trough particle mixing processes. The results obtained from this numerical model will assist engineers and scientists to better optimize particle based CSP systems. The validated numerical model accounts for particle–particle, particle–wall, particle–fluid interactions of multiphase solid-gas flows in a novel multi-stage solar particle receiver. This study is the first attempt to conceptualize a novel multi-stage solar and to develop and implement a high-fidelity numerical model to provide an in-depth analysis of the in-trough particle mixing processes. For the first time, important flow characteristics, namely particle mixing, of the particle curtain are investigated. The results showed that the ‘catch-and-release’ troughs have a profound implication on the in-trough mixing process depending on the location of the particle in the particle curtain. The numerical model revealed that specific regions of the multi-stage receiver exhibit high particle residence times and low particle velocities. The rear particle of a curtain takes about 15% longer to reach the 2nd bin compared to the front and centre particle, and the rear particle remains immersed in the 2nd bin for about 14% longer compared to the front and centre particle. As such, the large particle residence time due to the in-trough mixing could potentially lead to better heat transfer performance of a solar receiver. The high-fidelity numerical model could potentially serve as an advanced tool for engineers and scientists to carefully design, optimize, and guide-the scale up and commercialization of novel solar particle receivers, particle heat exchangers, and various other industrial applications such as solar gasification (syngas) and green hydrogen technologies.

Investigation of the hydrodynamics in the regenerator of fluid catalytic cracking unit integrated by chemical looping combustion

Oil refineries are responsible for 4–6% of global CO2 emissions, and 20–35% of these emissions released from the regenerator of Fluid Catalytic Cracking (FCC) units, which are the essential units for the conversion of heavier petroleum residues (vacuum gas oil) into more valuable products. Chemical looping combustion (CLC) has been recently proposed to mitigate the CO2 emissions released from the regenerator of FCC units with a lower energy penalty. However, a detailed experimental and modelling investigation is still necessary in order to identify the hydrodynamics in the regenerator of chemical looping combustion integrated with fluidised catalytic cracking (CLC-FCC). A computational fluid dynamic (CFD) study was conducted to understand the hydrodynamic behaviours of gas-solid two-phase flow in the regenerator of the CLC-FCC unit, based on a three-dimensional multiphase model (Eulerian-Eulerian) with the kinetic theory of granular flow.The results provide a useful insight into regenerator hydrodynamics, in terms of oxygen carrier modified FCC catalysts and FCC coke distribution profiles, in the regenerator of CLC-FCC. The conventional drag models (Syamlal-O'Brien and Gidaspow) predict a bed density profiles of a dense phase (250–300 kg/m3) at the dense phase (0–0.25 of h/H), and a dilution phase from h/H = 0.25 to 0.50 of regenerator. The bed density profile is indistinguishable from the industrial data provided for conventional FCC regenerators. The fluidisation gas (CO2) passes through the centre of the regenerator where the fluidisation gas splits the catalyst particles from the centre to the walls, to create a dilute particle phase in the centre and a dense particle-phase near the wall, which is one of the characteristic flow regimes in circulating fluidised bed reactors. The particles in the centre demonstrate an upward flow trend with a particle velocity above 3.0 m/s while the dense particles near the wall tend to go down with relatively low particle velocity of <0.5 m/s, which creates vortexes and a non-uniform particle distribution in the regenerator. The distribution of the fluidising gas provides better mixing of solid particles in the entrance and the optimisation of the superficial gas velocities (1.0 m/s) to create a distributed flow regime with developed vortexes through the dense and dilute phases. Furthermore, the laminar and turbulent flow models demonstrated no significant differences in terms of axial bed density profile in the regenerator of the CLC-FCC concept. These findings demonstrated that the hydrodynamics of catalysts in the CLC-FCC regenerator successfully predicted with CFD modelling and the prediction results aligned well with the conventional FCC regenerator.

Gas-liquid mass transfer in a gas–liquid-solid three-phase moving bed

The gas–liquid mass transfer was investigated in a gas–liquid-solid three-phase moving bed based on the dissolved oxygen method, and a semi-quantitative gas–liquid boundary identification method was developed to measure the gas–liquid interfacial area. Results showed that the effects of gas velocity and liquid velocity on gas–liquid mass transfer in the three-phase moving bed were the same as those in the trickle bed. Since the gas–liquid interfacial area and its renewal speed were promoted at the low particle velocity and inhibited at the high particle velocity, the liquid-phase volumetric mass-transfer coefficient first increased and then decreased with the increasing particle velocity. Moreover, an empirical correlation for the liquid-phase volumetric mass-transfer coefficient was established based on 296 data, and the average relative deviation was 8.11%. The sensitivity analysis showed that the liquid velocity was more important to gas–liquid mass transfer than gas velocity and particle velocity.

Numerical and experimental research on the erosion of solid-liquid two-phase flow in transport butterfly valve based on DEM method

PurposeThe purpose of this paper is to analyze the mechanism of particle erosion in butterfly valve pipelines under hydraulic transportation conditions. The results will affect the sealing and safety of butterfly valve pipelines and hopefully serve as reference for the anti-erosion design of butterfly valve pipelines.Design/methodology/approachThrough the discrete element method (DEM) simulation that considers the force between particles, the detached eddy simulation (DES) turbulence model based on realizable k-epsilon is used to simulate the solid-liquid two-phase flow-induced erosion condition when the butterfly valve is fully opened. The simulation is verified by building an experimental system correctness. The solid-liquid two-phase flow characteristics, particle distribution and erosion characteristics of the butterfly valve pipeline under transportation conditions are studied.FindingsThe addition of particles may enhance the high-speed area behind the valve. It first increases and then decreases with increasing particle size. With increasing particle size, the low-velocity particles change from being uniformly distributed in flow channel to first gathering in the front of the valve and, then, to gathering in lower part of it. Fluid stagnation at the left arc-shaped flange leads to the appearance of two high-speed belts in the channel. With increasing fluid velocity, high-speed belts gradually cover the entire valve surface by focusing on the upper and lower ends, resulting in the overall aggravation of erosion.Originality/valueConsidering the complexity of solid-liquid two-phase flow, this is the first time that the DEM method with added inter-particle forces and the DES turbulence model based on realizable k-epsilon has been used to study the flow characteristics and erosion mechanism of butterfly valves under fully open transportation conditions.

Atmospheric vertical velocity - a crucial component in understanding proximal deposition of volcanic ash

The simulation of volcanic ash transport and deposition (VATD) for distances within a few kilometres from the vent (proximal region) is challenging owing to a combination of unresolved volcanogenic effects and the impact of the volcano's orography. Due to the urgency of calculations or sometimes lack of access to computational resources or expertise, atmospheric vertical velocity (w) is often underestimated in VATD modelling. The error associated with this underestimation has, however, never been properly quantified. Here, we use a weak vulcanian eruption that occurred at Sakurajima volcano on 1/10/2017 as a first step to addressing this limitation. We combine deposit characteristics observed by disdrometer measurements with high-resolution atmospheric and VATD modelling to validate and illustrate the differences in modelled trajectories when w is either ignored or accounted for. The Weather Research and Forecasting (WRF) model is used to model the orogenic effects down to a resolution of 50 m. Eulerian and Lagrangian VATD models (FALL3D and LagTrack, a newly-developed Matlab code, respectively) are used to describe the particle trajectories. Results confirm the importance of w in the case of low-altitude eruptions for capturing the complex, near-vent trajectory of ash particles: when neglected fall velocities were seen to differ up to 2–5 m s−1 depending on the particle size. Although the impact of w is most notable within 10 km from the vent, the forced sedimentation of low terminal velocity particles can have a significant secondary effect at larger distances.

Photonic and plasmonic transition radiation from graphene

We theoretically study the transition radiation in the framework of full Maxwell equations, when a swift electron crosses a monolayer graphene. Based on the Sommerfeld integration, we demonstrate in the frequency domain the spatial distribution of this free-electron radiation, which clearly shows the broadband excitation of both photons and graphene plasmons. Moreover, the radiation spectra for photons and graphene plasmons are analytically derived. We find that the excitation of photons and graphene plasmons favors different particle velocities. To be specific, a higher particle velocity gives rise to the excitation of photons with better directivity and higher intensity, while a lower particle velocity enables the efficient excitation of graphene plasmons in a broader frequency range. Our work indicates that the interaction between swift charged particles and various 2D materials or van der Waals heterostructures is promising for the design of terahertz on-chip radiation sources.