Erosion Theory Research Articles

Erosion and multiphase flow in porous media

ABSTRACT Experimental results from sand production hollow cylinders tests are presented for cavity failure and sand production quantification. The effects of important parameters such as the sandstone class, stress rate, fluid flow rate, and time are presented and show how they influence sand production. A model for hydro-mechanical erosion of porous rocks is presented as it is applied to the sand production problem in hydrocarbon production. The model is based on a continuum theory for erosion, which regards the continuum as a three-phase medium and establishes a set of mass balance equations for the various phases. The solution requires an additional equation in the form of a constitutive or evolution law for the sand rate. Erosion is coupled with the mechanical behavior of a sand-producing cavity, and leads to enlargement of the erosion zone. Finally, multiphase flow and its effect on sand production is presented experimentally and theoretically by analyzing water invasion in the sand production test.

Professor Ioannis Vardoulakis (1949–2009)

Professor Ioannis Vardoulakis (1949–2009)

Determination of gun propellants erosivity: Experimental and theoretical studies

Requirements for higher and higher performances of contemporary guns intensify investigations concerning influence of various factors on barrel erosion. The most important parameter for the life of the gun is the type of the gun propellant. The device based on modification of 37 mm M39 gun for investigating influence of gun propellant on barrel erosion is used. The nozzle (main part of device) mass loss during firing was the measure of gun propellant erosivity. The theory of nozzle erosion includes basic thermal, chemical and metallurgical factors. The main thermal and chemical factors are maximum nozzle inside surface temperature and gun propellant composition. The maximum nozzle surface temperature was determined theoretically by developed interior ballistic model with heat transfer, and experimentally by micro thermocouples measurements and solution of inverse heat conduction problem. The coefficients of gun propellants erosivity are determined in experimental and theoretical way.

Depth of cut models for multipass abrasive waterjet cutting of alumina ceramics

Predictive models for the depth of cut in multipass abrasive waterjet (AWJ) cutting of alumina ceramics are presented. Cutting operations both with and without nozzle oscillation are considered. The modelling process starts with the single pass cutting using a dimensional analysis technique and the particle erosion theories applied to alumina ceramics, before progressing to the development of the models for multipass cutting. The models are finally assessed both qualitatively and quantitatively when cutting an 87% alumina ceramic. It is shown that the model predictions are in good agreement with the experimental data with less than 1% of average errors.

Depth of cut models for multipass abrasive waterjet cutting of alumina ceramics with nozzle oscillation

An experimental study of the depth of cut in multipass abrasive waterjet (AWJ) cutting of alumina ceramics with controlled nozzle oscillation is presented. It is found that this cutting technique can significantly increase the depth of cut by an average of 50.8% as compared to single pass cutting without nozzle oscillation under the corresponding cutting conditions and within the same cutting time. Predictive models for the depth of cut are then developed. The modelling process starts with single pass cutting using a dimensional analysis technique and the particle erosion theories applied to alumina ceramics, before progressing to the development of the models for multipass cutting. The models are finally assessed both qualitatively and quantitatively with experimental data. It is shown that the model predictions are in good agreement with the experimental data with the average deviations of about 1%.

Numerical simulations of Asian dust storms using a coupled climate‐aerosol microphysical model

We have developed a three‐dimensional coupled microphysical/climate model based on the National Center for Atmospheric Research Community Atmospheres Model and the University of Colorado/NASA Community Aerosol and Radiation Model for Atmospheres. We have used the model to investigate the sources, removal processes, transport, and optical properties of Asian dust aerosol and its impact on downwind regions. The model simulations are conducted primarily during the time frame of the Aerosol Characterization Experiment–Asia field experiment (March–May 2001) since considerable in situ data are available at that time. Our dust source function follows Ginoux et al. (2001). We modified the dust source function by using the friction velocity instead of the 10‐m wind based on wind erosion theory, by adding a size‐dependent threshold friction velocity following Marticorena and Bergametti (1995) and by adding a soil moisture correction. A Weibull distribution is implemented to estimate the subgrid‐scale wind speed variability. We use eight size bins for mineral dust ranging from 0.1 to 10 μm radius. Generally, the model reproduced the aerosol optical depth retrieved by the ground‐based Aerosol Robotic Network (AERONET) Sun photometers at six study sites ranging in location from near the Asian dust sources to the Eastern Pacific region. By constraining the dust complex refractive index from AERONET retrievals near the dust source, we also find the single‐scattering albedo to be consistent with AERONET retrievals. However, large regional variations are observed due to local pollution. The timing of dust events is comparable to the National Institute for Environmental Studies (NIES) lidar data in Beijing and Nagasaki. However, the simulated dust aerosols are at higher altitudes than those observed by the NIES lidar.

Probabilistic Prediction of Stability of Ship by Risk Based Approach

Prediction of the stability for ships is very complex in reality. In this paper, risk based approach is applied to predict the probability of capsize for a certified ship, which is effected by the forces of sea especially the wave loading Safety assessment and risk analysis process are also applied for the probabilistic prediction of stability for ships. The probability of shipsencountering different waves at sea is calculated by the existed statistics data and risk based models. Finally, ship capsizing probability is calculated according to single degree of freedom(SDF) rolling differential equation and basin erosion theory of nonlinear dynamics. Calculation results show that the survival probabilities of ship excited by the forces of the seas, especially in the beam seas status, can be predicted by the risk based method.

Sensitivity of surface characteristics on the simulation of wind-blown-dust source in North America

Recently, a wind-blown-dust-emission module has been built based on a state-of-the-art wind erosion theory and evaluated in a regional air-quality model to simulate a North American dust storm episode in April 2001 (see Park, S.H., Gong, S.L., Zhao, T.L., Vet, R.J., Bouchet, V.S., Gong, W., Makar, P.A., Moran, M.D., Stroud, C., Zhang, J. 2007. Simulation of entrainment and transport of dust particles within North America in April 2001 (“Red Dust episode”). J. Geophys. Res. 112, D20209, doi:10.1029/2007JD008443). A satisfactorily detailed assessment of that module, however, was not possible because of a lack of information on some module inputs, especially soil moisture content. In this paper, the wind-blown-dust emission was evaluated for two additional dust storms using improved soil moisture inputs. The surface characteristics of the wind-blown-dust source areas in southwestern North America were also investigated, focusing on their implications for wind-blown-dust emissions. The improved soil moisture inputs enabled the sensitivity of other important surface characteristics, the soil grain size distribution and the land-cover, to dust emission to be investigated with more confidence. Simulations of the two 2003 dust storm episodes suggested that wind-blown-dust emissions from the desert areas in southwestern North America are dominated by emissions from dry playas covered with accumulated alluvial deposits whose particle size is much smaller than usual desert sands. As well, the source areas in the northwestern Texas region were indicated to be not desert but rather agricultural lands that were “activated” as a wind-blown-dust sources after harvest. This finding calls for revisions to the current wind-blown-dust-emission module, in which “desert” is designated to be the only land-cover category that can emit wind-blown dust.

An erosion-based model for abrasive waterjet turning of ductile materials

This paper presents an attempt to model the abrasive waterjet (AWJ) turning process considering material removal from the circumference of a rotating cylindrical specimen. The methodology involves the use of Finnie's theory of erosion to estimate the volume of material removed by the impacting abrasive particles. The proposed model considers the impact of jet at an angle to the workpiece surface to account for the curvature of the workpiece. Unlike earlier works, this model considers the continuous change in local impact angle caused by the change in workpiece diameter. The flow stress of the workpiece material is determined using a novel experiment involving the same abrasive and workpiece materials. The adequacy of the proposed model is examined through AWJ turning tests under various process parameter combinations. The final diameters predicted by the model are found to be in good agreement with the experimental results.

Mechanisms of microhole formation on glasses by an abrasive slurry jet

Abrasive jet micromachining is considered as a promising precision processing technology for brittle materials such as silicon substrates and glasses that are increasingly used in various applications. In this paper, the mechanisms of microhole formation on brittle glasses by an abrasive slurry jet are studied based on the viscous flow and erosion theories. It is shown that the hole cross section is characterized by a “W” shape and can be classified into three zones caused, respectively, by jet direct impact, viscous flow, and turbulent flow induced erosion. An analysis of the surface morphology shows that ductile-mode erosion is dominant. The effect of process parameters on material removal is studied which shows that increasing the pressure and erosion time increases the hole depth, but has little effect on the hole diameter.

Jet-Induced Cratering of a Granular Surface with Application to Lunar Spaceports

The erosion of lunar soil by rocket exhaust plumes is investigated experimentally. This has identified the diffusion-driven flow in the bulk of the sand as an important but previously unrecognized mechanism for erosion dynamics. It has also shown that slow regime cratering is governed by the recirculation of sand in the widening geometry of the crater. Scaling relationships and erosion mechanisms have been characterized in detail for the slow regime. The diffusion-driven flow occurs in both slow and fast regime cratering. Because diffusion-driven flow had been omitted from the lunar erosion theory and from the pressure cratering theory of the Apollo and Viking era, those theories cannot be entirely correct.

Similarity criteria in the erosion theory of brittle materials in abrasive particle flow

Experimental data on the erosion of brittle materials in abrasive particle flow are generalized on the basis of similarity and dimension theory using four similarity criteria. These criteria represent the influence of strength, fracture toughness, dynamic and total pressures resulting from the particle-target impact, and target temperature. The volume erosion rate is represented as power dependence of the similarity criteria. It is shown that strength characteristics and total pressure, which results from wave damping in the particle-target impact, are of major importance in the erosion of brittle materials in addition to the impact velocity. Dynamic pressure and temperature are of minor importance.

Predictive tools for the design of erosion resistant coatings

Aerospace industry is seeking to develop high performance coatings for the protection against erosion by solid particles. However, with many new materials used and tested for different applications and operation under different conditions, conducting experiments for each one of them is becoming increasingly difficult. In the present work, we propose practical semi-empirical and numerical predictive methods to determine erosion resistance of tribological coatings. The paper presents data obtained by finite element (FE) calculations that can be compared with those obtained by classical theories developed for the erosion of materials. The simulation-based approach allows one to express the functional dependence of erosion on the coating properties, and to quantitatively predict the erosion rate. We determined a proportionality coefficient for a wide range of hard coatings. This coefficient was then used, in combination with the semi-empirical expression derived from FE simulations, to determine the erosion rate of different coatings. In addition to the existing erosion theories that tend to emphasize hardness, H, and Young's modulus, E, as the main parameters defining erosion resistance, we focus here on the role of the H/ E and H 3/ E 2 ratios. We demonstrate that the latter characteristics allow one to rank coatings with respect to their erosion performance.

Ar이온 충돌에 의한 Au, Pd(001) 표면에서 재증착 효과의 분자동역학 연구

Atomic behavior during ion beam sputtering was investigated by using classical molecular dynamics simulation. When Ar ion bombards on Au and Pd(001) surface with various incidence energies and angles, some atoms which gained substantial energy by impacting Ar ion were sputtered out and, simultaneously, others were landed on the surface as if surface atoms were redeposited. It was observed that the redeposited atoms are five times for Au and three times for Pd as many as sputtered atoms irrespective of both incidence energy and angle. From sequential ion bombarding calculations, contrary to the conventional concepts which have described the mechanism of surface pattern formation based only on the erosion theory, the redeposition atoms were turned out to play a significant role in forming the surface patterns.

Erosion Processes of the Discharge Cathode Assembly of Ring-Cusp Gridded Ion Thrusters

An ion thruster discharge cathode assembly (DCA) erosion theory is presented based on near-DCA NSTAR plasma measurements and experimental results for propellant flow rate effects on ion number density. The plasma potential structures are utilized in an ion trajectory algorithm to determine the location and angle at the DCA of bombarding ions. These results suggest that the plasma potential structure causes a chamfering of the DCA orifice. Results from tests with an instrumented DCA show that increasing DC propellant flow rate causes a decrease in keeper orifice ion number density, most likely due to charge-exchange and elastic collisions. Combining these two results, the known wear-test and extended life test (ELT) DCA erosion profiles can be qualitatively explained. Specifically, the change in the wear profile from the DCA downstream face to the orifice for the ELT may be a result of the reduction in DCA propellant flow rate when the thruster operating point is changed from the TH 15 to TH 8.

Creeping granular motion under variable gravity levels

In a rotating tumbler that is more than one-half filled with a granular material, a core of material forms that should ideally rotate with the tumbler. However, the core rotates slightly faster than the tumbler (precession) and decreases in size (erosion). The precession and erosion of the core provide a measure of the creeping granular motion that occurs beneath a continuously flowing flat surface layer. Since the effect of gravity on the subsurface flow has not been explored, experiments were performed in a 63% to 83% full granular tumbler mounted in a large centrifuge that can provide very high g-levels. Two colors of 0.5 mm glass beads were filled side by side to mark a vertical line in the 45 mm radius quasi-two-dimensional tumbler. The rotation of the core with respect to the tumbler (precession) and the decrease in the size of the core (erosion) were monitored over 250 tumbler revolutions at accelerations between 1g and 12g. The flowing layer thickness is essentially independent of the g-level for identical Froude numbers, and the shear rate in the flowing layer increases with increasing g-level. The degree of core precession increases with the g-level, while the core erosion is essentially independent of the g-level. Based on a theory for core precession and erosion, the increased precession is likely a consequence of the higher shear rate. Core erosion, on the other hand, is related to the creep region decay constant, which is connected with slow diffusion in the bed and unaffected by gravity.

Hyperthermal Collisions of O+(4S3/2) with Methane at 5 Electron Volts

Preliminary studies were carried out for the O + ( 4 S3� 2) + methane reaction, which serves as a benchmark for developing the theory of polymer erosion by O + under low Earth orbit conditions. Ab initio electronic structure calculations show that the interaction of O + with CH4 can lead to a large number of reaction products such as charge transfer, hydride abstraction, and H elimination. Based on the information obtained from these quantum chemistry calculations, a direct dynamics classical trajectory simulation was carried out at 5-eV relative translation energy and the chemical reaction channels predicted by the ab initio calculations are confirmed.

Artificial Neural Network Prediction of Material Removal Rate in Electro Discharge Machining

ABSTRACT Thermal erosion theory is widely accepted as an explanation of the erosion process in electro-discharge machining (EDM). Theoretical models are based on the solution of the transient heat conduction equation, which is modeled considering suitable assumptions with appropriate initial and boundary conditions. The closed form solutions result only after considering too many assumptions, which are far from actual machining conditions. The growth of the plasma channel, energy sharing between electrodes, process of vaporization, formation of recast layer, plasma-flushing efficiency, and temperature sensitivity of thermal properties of the work material are a few physical phenomena that render the machining process highly complex and stochastic. The mathematical consideration of all these complex phenomena is very difficult. Therefore, mathematical prediction of material removal rate when compared with the experimental results shows wide variation. In such circumstances, an attempt has been made to develop an artificial feed forward neural network based on the Levenberg-Marquardt back propagation technique of appropriate architecture of the logistic sigmoid activation function to predict the material removal rate. Such a neural network model is expected to perform well under the stochastic environment of actual machining conditions without understanding the complex physical phenomena exhibited in electro-discharge machining. The validity of the neural network model is checked with the experimental data, and we conclude that the artificial neural network model for EDM provides faster and more accurate results.

Investigation of Soil Erosion from Bare Steep Slopes of the Humid Tropic Philippines

Abstract At the Visayas State College of Agriculture (ViSCA) on the island of Leyte in the Philippines, hydrologic and soil-loss measurements were recorded for 32 erosion events over 3 yr on three 12-m-long bare soil plots with slopes of approximately 50%, 60%, and 70%. Measurements included rainfall and runoff rates at 1-min intervals, total soil lost per event from the plot, rill details when observed after an erosion event, and soil settling-velocity characteristics. Storm events are characterized by high rainfall rates but quite low rates of runoff, because of the consistently high infiltration rate of the stable clay soil (an Oxic Dystropept). Both observation and modeling indicated that overland flow is commonly so shallow that much of the soil surface is likely to be unsubmerged. For the 70% slope plot, half the events recorded mean sediment concentrations from 100 to 570 kg m−3. A somewhat constant hydrologic lag between rainfall and runoff is used to estimate a Manning’s roughness coefficient n of about 0.1 m−1/3 s, a value used to estimate velocity of overland flow. Possible effects of shallow flows and high sediment concentrations on existing erosion theory are investigated theoretically but are found to have only minor effects for the ViSCA dataset. A soil erodibility parameter β was evaluated for the data whenever rilling was recorded following an erosion event. The values of β indicate that, except for events with higher stream powers, other erosion processes in addition to overland flow could have contributed to soil loss from erosion plots in a significant number of events.

Ion beam erosion of amorphous materials: evolution of surface morphology

In this work we present our results concerning the formation of self-organized nanoscale structures during the bombardment with a low-energy defocused Ar ion beam. We studied glass surfaces because of their physical properties, technological interest and cheapness. The evolution of sample surface was studied ex situ by atomic force microscopy. We found, in agreement with Bradley and Harper, a morphology characterized by a regular ripple structure with the wave vector perpendicular or parallel to the ion beam direction. This structure periodicity was found to vary in the range 90–350nm with a linear time evolution. In order to gain further information about the sputtering process and for comparison with the existing continuum theories of surface erosion, we studied the scaling behaviour of surface roughness.