Average Impact Angle Research Articles

The kinetic theory of prompt redeposition in the case of thin Debye sheath

Prompt redeposition is a process in which a neutral atom sputtered from a divertor target or the first wall of a tokamak returns on the surface right after ionization, before experiencing any collisions with the background plasma. This paper presents analytical solutions of kinetic equations for sputtered neutrals and resulting ions. Using obtained distribution functions, the redeposition coefficient and average impact angle are derived under assumptions of thin Debye sheath. Our expression for the prompt redeposition coefficient explicitly depends on the angular and energy distributions of sputtered atoms and, therefore, is more universal than often used Fussmann equation valid only for cosine distribution. The effect of the angular distributions of sputtered particles on the prompt redeposition efficiency and parameters of redeposited ions is analyzed.

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.

Effect of substrate tilt on sputter-deposited AuTa films

Gold-tantalum alloy films are of interest for biomedical and magnetically-assisted inertial confinement fusion applications. However, growth mechanisms of such Au-Ta alloy films are not well understood. Here, we systematically study the effect of substrate tilt on properties of films deposited by direct current magnetron sputtering from a AuTa alloy target. Experimental data is correlated with distributions of landing energies and incident angles of sputtered and backscattered species calculated by Monte Carlo simulations of ballistic collisions and gas phase atomic transport. Results reveal that the deposition rate and film density monotonically decrease with increasing substrate tilt. Properties are similar for films with a tilt of 40° and below: films are amorphous, with an average compressive stress of 0.5 GPa, a density of 16 g/cm3, and electrical resistivity of 200 μΩ cm. The critical substrate tilt angle for the onset of porosity, a transition to a reduced stress state, and an increase in electrical resistivity lies between 40 and 60°. Based on Monte Carlo simulations, these changes in film properties are correlated with an increase in the average impact angle of the ballistic component of atomic flux. Implications of these results to deposition onto non-planar substrates are discussed.

Investigation on particle motions and resultant impact erosion on quartz crystals by the micro-particle laden waterjet and airjet

To investigate particle motions and resultant impact erosion is highly desirable to improve the abrasive waterjet and abrasive airjet micro-machining performance. Numerical study found that particles in waterjet are easier to move following water streamlines than that in airjet due to their larger momentum equilibration numbers, and average impact angle induced by micro-particles in airjet are larger than that driven by waterjet, which in turn affect the resultant impact erosion on quartz crystals. By experimentally analysing the surface morphology of eroded craters generated by the individual micro-particle laden waterjet and airjet, brittle erosion mode is found to significantly dominate the impact erosion of quartz crystal driven by airjet, while both of brittle and ductile erosion modes can be observed by the impacts of the micro-particle laden waterjet. Particularly with relatively low water pressure contributions of ductile and brittle erosions to the material removal process are found to be equally important.

Multiscale Simulation of the Hydroabrasive Erosion of a Pelton Bucket: Bridging Scales to Improve the Accuracy

Erosive wear of hydraulic machines is a common issue, which results in efficiency degradation, the enhancement of cavitation, and the need for expensive maintenance. Although numerical simulations of the erosion process could be very useful, both for understanding and predicting the process, its multiscale nature renders it very difficult to simulate. A previously validated multiscale model of erosion is presented. It consists of two coupled sub-models: On the microscale, the sediment impacts are simulated by means of comprehensive physical models; on the macroscale, the turbulent sediment transport and erosion accumulation are calculated. A multiscale simulation of the erosion of a prototype-scale Pelton bucket impacted by a sediment-laden water jet is presented. The simulation results, namely the erodent flux and the distributions of average impact angle and velocity on the bucket surface, bring insight into the erosion process. Furthermore, the results explain the obtained erosion distribution, which is in very good agreement with the experimental erosion measurements available in the literature for the same test case.

Implications of two backward blood spatter models based on fluid dynamics for bloodstain pattern analysis

Bloodstain pattern analysis (BPA) is an integral part of crime scene investigation. For violent crimes involving gunshots, standard practice in police departments worldwide have some physical limitations. For instance, the effect of gravity and air drag on trajectories of blood droplets are neglected using current reconstruction methods, which results in a well-known overestimation of the height of the source of blood. As a consequence, more sophisticated models for blood spatter trajectory reconstruction are being developed, two of which are highlighted in the present work. They allow the prediction of bloodstain patterns produced from backward spattered blood droplets from blunt and sharp bullets. Our recent models attribute the splashing of blood to the Rayleigh–Taylor instability which arises when blood is accelerated towards lighter air. This physically-based description comes with the powerful predictive capability to correlate features of bloodstain patterns with the specific bullet and gun that produced them, as well as with the body position. The results of the numerical models were compared with four experiments simulating blood spatter deposition on a vertical wall through the number of stains produced, average stain area, and average impact angle at the surface, and the agreement found is fairly good. Moreover, further insight is obtained by probing and explaining the influence of observable parameters on the resulting spatter pattern, with the goal of aiding BPA experts evaluating a crime scene.

A linear wheel–crossing interaction model

This paper presents the derivation of a linear model for wheel–rail interaction kinematics at railway crossings. The purpose of this model is to demonstrate the fundamental constraints imposed on a crossing geometry if it should be compatible with a given range of wheel profile shapes. In this model, the contact point locations on the wing rail and on the crossing nose are described using linear functions, and the wheel profiles are modelled as conical. Based on these assumptions, a method is developed to adjust the vertical position and longitudinal inclination of the wing rail and the crossing nose in order for the crossing geometry to be compatible with a given range of equivalent wheel profile cone angles. In particular, an expression is derived for the average impact angle. The derived relation highlights the potential of tailoring crossing geometries for a given spread in wheel profile shapes in traffic for minimized loading and damage. Further, the properties of the model are compared to the results of quasi-static multibody simulations for a range of wheel profiles.

Tungsten erosion by impurities and redeposition: focus on the magnetised sheath

The effect of the magnetised sheath on the gross erosion and redeposition of tungsten is examined with an insight on impurity impact energy and angle. A complete treatment of the impact energy is performed leading to a scaling that differs from the usual formula. It is found that even if the energy distribution at the sheath entrance strongly differs from this approximation, the discrepancy remains under 20% for the impact energy. The average impact angle of a set of impurities is calculated. The difference between models with and without sheath electric field depends strongly on the charge state and mass of the impurity considered. This can lead, for example in the case of gross sputtering due to impinging , to an underestimation of 3–4 times at for very grazing angles. Scalings for high and low density cases, which correspond for fusion relevant magnetic fields () to m−3 and m−3, respectively, are deduced and provide a good estimation of the average impact angle of all impurities. Finally the magnetised sheath is shown to have an effect on the redeposition through two contributions: it increases the prompt redeposition contribution to the total redeposition and constitutes a potential barrier for the ions leaving the surface. A discussion is provided about the optimal width of the simulation domain to capture these effects.

Magnetic sheath effect on the gross and net erosion rates due to impurities

Simulations of impurity trajectories in deuterium plasmas in the vicinity of the surface are performed by taking into account the magnetic sheath in conditions relevant for ITER and WEST. We show that the magnetic sheath has a strong effect on the average impact angle of impurities in divertor conditions and that it can lead to an increase of at the gross erosion maximum for neon (Ne+4) compared to the case when only the cyclotron motion is considered. The evaluation of the net erosion has been undertaken by retaining local redeposition of tungsten (W). We investigate how it is affected by the sheath magnetic potential profile. The largest effect is however observed when an energy distribution is considered. In this case the number of particles that manage to exit the sheath is larger as it is dominated by the more energetic particles. The comparison with other work is also discussed. The application to a scenario of the WEST project is finally performed, which exhibits a moderate, however non negligible, erosion of the plasma facing components.

A study of particle splash on developing ripple forms for two bed materials

A custom-designed Particle Tracking Velocimetry (PTV) system was developed to measure particle splash dynamics in two materials: mixed quartz sand and ground acrylic particles approximating the density of snow. The strengths and limitations of the PTV system are described in the context of five experiments carried out at friction velocities slightly greater than the threshold friction velocity for particle entrainment and saltation. Analysis of particle collisions on the bed surface (splash) demonstrates that the simulated snow particles impact the surface at a smaller angle than quartz sand. They also eject a greater number of particles, though at lower velocities and angles than observed for the sand particles. These relations appear to be highly sensitive to the porosity of the bed surface, while particle shape and elasticity play a secondary role. With regard to PTV measurements carried out along the wind-aligned axis of migrating aeolian ripples, particles ejected from the lee slope tend to be slightly larger than those on the stoss slope, and in general are ejected with a lower speed and greater angle to the surface. Irrespective of the type of granular material, when the average impact angle is close to or greater than the negative slope of the evolving bedform, the particle ejection rate from the stoss slope was measured to be approximately twice that for the lee slope. This empirical observation confirms the assumption made in models of ripple development that the ejection flux governs the bedform migration rate. However, systematic under-prediction of this rate by an early model suggests that additional processes, inclusive of surface creep, must also be accounted for.

Porosity in Spray-Formed Materials

Porosity in spray-formed materials is an important issue, but the formation of porosity is not completely understood. Many experimental results and some theoretical models have been presented in the past. Nevertheless, the prediction of porosity in a deposit is still not possible today. The paper will give some examples picked from literature, which show some general correlations between process parameters and porosity. These correlations can be helpful to form a basic understanding of the process. Finally it is necessary to know more about the conditions of the droplets and the deposit at the point of impingement. These impacting conditions have to be correlated to the porosity to improve the understanding of the process and to make a prediction possible. Determining the impact conditions is a challenge because usually they are not constant with time and some values are difficult to measure. Our experiments show a strong correlation between the surface temperature of the deposit and the porosity. For IN718 and U720 as-sprayed porosities below 1 vol.% were achieved if the deposit surface temperature is app. 1250 °C. The average impact angle weighted by the local particle mass flux is also an important parameter. The probability of low as-sprayed porosity is high if the average weighted impact angle is below 25° but decreases dramatically for higher impact angles.

Angular ion and neutral energy distribution in a collisional rf sheath

A numerical study on the ion and neutral angular impact energy distribution at the rf-driven electrode of a reactive ion etcher is presented. The calculations for the ions are performed using a Monte Carlo method that includes charge exchange and elastic scattering. The contribution of both collision processes to the angular ion impact energy distribution is studied. For the case that charge exchange is the only collision process, the Monte Carlo results can be checked against those of a method based on a spatially uniform and time independent collision rate. In that case, both methods yield the same ion impact energy distribution. The position, velocity, and propagation angle of the energetic neutrals created in collisions of ions with the background gas are stored. These are used as input data for a separate code that follows the evolution of the angular neutral energy distribution, taking into account (multiple) neutral elastic scattering. From the ion and neutral distributions, the number of neutrals per ion, the average impact energy, and the energy-weighted average impact angle have been derived. It is shown that these parameters are well described by simple expressions. Finally, the sputter yield is calculated. The results show that the contribution of the angular distributions of both ions and neutrals to the yield can be neglected.

The lateral surface drag coefficient of cylindrical spacecraft in a rarefied finite temperature atmosphere

In the present determination of the free molecule flow drag coefficient for a cylindrical spacecraft flying parallel to its principal axis, the lateral surface effects of thermal motion are explicitly included in terms of the average impact angle of the incident gas momentum vector. Kinetic theory is used to characterize self-shadowing, as well as to obtain an expression for the lateral surface coefficient in terms of the average impact angle of the incident momentum vector and the fractional momentum transfer along the line of impact. It is found that, for a length/diameter ratio of about 5, the lateral surface contribution to the drag coefficient is comparable to that of the front face.