Impact Pressure Distribution Research Articles

Cavitation in water jet under high ambient pressure conditions

Cavitating jet has been widely used in underground drilling to enhance the rock erosion efficiency. In this circumstance, the high ambient pressure directly influences the bubble traveling process and cavitation impact pressure. To quantitatively assess the erosion intensity of the cavitating jet under the high ambient pressure conditions, a combined numerical and experimental investigation was conducted. A convergent-divergent nozzle was chosen to generate the cavitating jet in a closed test cell. A bubble transportation model was used to simulate the bubble traveling in the water jet through the nozzle and investigate the effects of ambient pressure and flow rate on the transportation efficiency. Pitting analysis with specimen 7075 aluminum alloy was performed to measure the magnitude and distribution of the cavitation impact pressure at different standoff distances. The results reveal the dynamic bubble traveling process and shed light on the cavitation impact field. The bubble transportation capability of the cavitating jet depends on the jet velocity. Given certain ambient pressure and nozzle size, the flow rate must be larger than a certain threshold value for allowing the bubbles to be transported out before collapsing inside the nozzle. The magnitude of cavitation impact pressure is of the order of 1GPa and shows little dependence on standoff distance. However, the overall impact rate and the distribution pattern are significantly influenced by the standoff distance. It is concluded that the variation of the erosion intensity of cavitating jet correlates more closely with the change of the impact rate than with the absolute impact magnitude.

Discrete Elements Model of an Abrasive Water-jet through the Focal Canon to the Work-piece

Abrasive water-jet manufacturing process can shape a lot of materials ranging from metals to glasses. It has a lot of advantages, as its low cutting forces, but remains quite difficult to control. Indeed, the process is leaded by the abrasive particle trajectories which depends on the water static pressure and many other parameters. The impact pressure on the work-piece is commonly modeled by a two Gaussian fit sum which are representative of the particles velocity distribution and the granulometry respectively. Today no studies based on discrete elements take into account the mixing chamber and the focal canon which are the two main steps of the abrasive water-jet tool constitution. In this preliminary work we propose to model the flow through the focal canon until the target impact by an original numeric granular approach. The Non-Smooth Contact Dynamics is an efficient method on a large range of simulation domains. In our case, we consider the water phase and the abrasive phase as two collections of distinct polydisperse elements. The masses are corrected and the contact interaction laws are adjusted to account for an equivalent fluid which similar mechanical properties. These two phases are mixed in a chamber and focalised through the canon, knowing water static pressure and abrasive mass rate. After the canon end the abrasive water-jet evolves in air and thus decelerates by friction. The tool-fluid adapts its geometric configuration from this kinetic energy decrease and impacts a target plane located at a known distance from the canon. Such a model is built on some classic process parameters as the water static pressure, the abrasive mass rate or the work-piece vs. canon distance, but it also naturally takes into account finer mechanical parameters as the abrasive granulometry or friction dissipation. Simulations gives interesting results of impact pressure distribution on the target work-piece with dynamic data of all the collection particles. More generally, this work final aim is to link elemental particle damage studies with a macroscopic wear prediction law.

ショットピーニングにおける衝突面圧分布の計測と関数近似

ショットピーニングにおける衝突面圧分布の計測と関数近似

Impact Pressure Distribution in Flat Fan Nozzles for Descaling Oil Wells

The suitability of High pressure nozzles in terms of impact upon targeted surfaces has indicated its effectiveness for the cleaning of oil production tubing scale, which has recently attract wider industrial applications considering its efficiency, ease of operation and cost benefit[1]. In the Oil and gas production, these nozzles are now used for cleaning the scale deposits along the production tubing resulted mainly from salt crystallization due to pressure and temperature drop. Detailed characterizations of flat-fan nozzle in terms of droplet sizes and mean velocities will benefit momentum computations for the axial and radial distribution along the spray width, with the view of finding the best stand-off distance between the target scale and the spray nozzle. While the droplet sizes and the velocities determine the momentum at impact, although measuring droplet sizes has been known to be difficult especially in the high density spray region [2], still laboratory characterization of nozzles provide a reliable data especially avoiding uncontrollable parameters[3].While several research consider break up insensitive to the cleaning performance, this research investigate the experimental data obtained using Phase Doppler Anemometry (PDA) which led to established variation in momentum across the spray width thus, non-uniformity of impact distribution. Validation model was then developed using Fluent which verify the eroded surfaces of material using the flat-fan atomizer to have shown variability in the extent of impact actions due to kinetic energy difference between the center and edge droplets. The study’s findings could be useful in establishing the effect of droplet kinetic energies based on the spray penetration, and will also add significant understanding to the effect of the ligaments and droplets effects along the spray penetration in order to ascertain their momentum impact distribution along the targeted surface.

Environmental Assessment of Different Flood-Discharge Layout Plans in Baihetan Hydro-Power Project

The optimized design and layout of diverse flood-discharge devices are of great importance for improving the security and reliability of the dams in hydro-power engineering and minimizing their impacts on the regional ecological environment in environmental engineering. In this paper, various kinds of surface and mid-level outlets configurations are compared under multiple operational conditions for the Baihetan hydro-power project. In addition, both the time-averaged and the instantaneous fluctuation impact-pressure distributions in the plunge pool under typical working conditions are measured and investigated in order to study the reasonability of different flood-discharge layout plans. Moreover, the complex hydrodynamic environment and the spatial evolution of time-averaged flow fields along the downstream river are surveyed and further analyzed. The results could serve as the foundation of selecting the optimal flood-discharge and energy-dissipation design, and, more generally, references for the regional ecological environment assessment for Jinsha River after the construction of Baihetan dam.

Influence of the Shape of the Orifice on the Local Heat Transfer Distribution of a Flat Plate Impinged by Under-Expanded Sonic Jets

Experiments are carried out for the circular, square, and equilateral triangular orifices of the same contraction ratio in order to explore the heat transfer characteristics for nozzle pressure ratios of 2.36, 3.04, 3.72, 4.4, and 5.08 at z/d = 2, 4, 6, and 8. The presence of vena contracta and shock cells and its locations are estimated from the impact pressure distribution along the axial direction. The pressure drop during the initial expansion is minimal for the equilateral triangular orifice. However, this pressure drop is comparable for the square and circular orifices. The heat transfer characteristics of the circular orifice are nearly axisymmetric, but that for non-circular jets are asymmetric, and the three dimensional effects are clearly observed. This is inferred through the spatial gradients of the Nusselt number and adiabatic wall temperature distributions. The linear correlations for the average Nusselt number are proposed for the circular, square, and equilateral triangular orifices.

Incubation pit analysis and calculation of the hydrodynamic impact pressure from the implosion of an acoustic cavitation bubble

An experimental study to evaluate cavitation bubble dynamics is conducted. The aim is to predict the magnitude and statistical distribution of hydrodynamic impact pressure generated from the implosion of various individual acoustic cavitation bubbles near to a rigid boundary, considering geometrical features of the pitted area.A steel sample was subjected to cavitation impacts by an ultrasonic transducer with a 5mm diameter probe. The pitted surface was then examined using high-precision 3D optical interferometer techniques. Only the incubation period where surface is plastically deformed without material loss is taken into account. The exposure time was adjusted in the range of 3–60s to avoid pit overlapping and a special procedure for pit analysis and characterisation was then followed. Moreover, a high-speed camera device was deployed to capture the implosion mechanisms of cavitation bubbles near to the surface.The geometrical characteristics of single incubation pits as well as pit clusters were studied and their deformation patterns were compared. Consequently, a reverse engineering approach was applied in order the hydrodynamic impact pressure from the implosion of an individual cavitation bubble to be determined. The characteristic parameters of the cavitation implosion process such as hydrodynamic impact pressure and liquid micro-jet impact velocity as well as the hydrodynamic severity of the cavitation impacts were quantified. It was found that the length of the hypotenuse of the orthographic projections from the center of the pit, which basically represents the deformed area of the pit, increases with the hydrodynamic impact aggressiveness in a linear rate. Majority of the hydrodynamic impacts were in the range of 0.4–1GPa while the corresponding micro-jet velocities were found to be in the range of 200–700m/s. Outcomes of this study, contribute to further understanding the cavitation intensity from the implosion of acoustically generated bubbles and could certainly represent a significant step towards developing more accurate cavitation models.

Theoretical and Experimental Analysis of Wave Impact Pressures on Curved Seawalls

Experimental model tests were performed in a wave flume with regular waves to measure the magnitude and distribution of impact pressures caused by breaking waves on a curved seawall model having different radii of curvatures. The base structure of the wall has a foreshore slope of 1/10. Theoretical studies based on pressure impulse theory were carried out to obtain the numerical results of breaking wave impact pressures on curved seawalls. The boundary element method was used for the numerical solution of the governing equation. The novel aspect of this study was to investigate the applicability of pressure impulse theory to curved seawalls. The results showed that the pressure impulse model can be used to model the wave impact pressures and their distribution on curved seawall models with good accuracy. A slight decrease has been observed in pressures for increasing radii of curvatures, especially for the case which the water depth at wall was 14 cm. The location of the maximum impact pressure was found to occur above the still water level for all cases tested in this study.

Numerical and Experimental Studies of Cavitation Behavior in Water-Jet Cavitation Peening Processing

Water-jet cavitation peening (WCP) is a new technology for the surface modification of metallic materials. The cavitation behavior in this process involves complex and changeable physics phenomena, such as high speed, high pressure, multiple phases, phase transition, turbulence, and unstable features. Thus, the cavitation behavior and impact-pressure distribution in WCP have always been key problems in this field. Numerous factors affect the occurrence of cavitation. These factors include flow-boundary conditions, absolute pressure, flow velocity, flow viscosity, surface tension, and so on. Among these factors, pressure and vapor fraction are the most significant. Numerical simulations are performed to determine the flow-field characteristics of both inside and outside the cavitating nozzle of a submerged water jet. The factors that influence the cavitation intensity of pressure are simulated. Fujifilm pressure-sensitive paper is used to measure the distribution of impact pressure along the jet direction during the WCP process. The results show that submerged cavitation jets can induce cavitation both inside and outside a conical nozzle and a convergent-divergent nozzle when the inlet pressure is 32 MPa. Moreover, the shock wave pressure induced by the collapse of the bubble group reaches up to 300 MPa.

Numerical and Experimental Studies of Cavitation Behavior in Water-Jet Cavitation Peening Processing

Water-jet cavitation peening (WCP) is a new technology for the surface modification of metallic materials. The cavitation behavior in this process involves complex and changeable physics phenomena, such as high speed, high pressure, multiple phases, phase transition, turbulence, and unstable features. Thus, the cavitation behavior and impact-pressure distribution in WCP have always been key problems in this field. Numerous factors affect the occurrence of cavitation. These factors include flow-boundary conditions, absolute pressure, flow velocity, flow viscosity, surface tension, and so on. Among these factors, pressure and vapor fraction are the most significant. Numerical simulations are performed to determine the flow-field characteristics of both inside and outside the cavitating nozzle of a submerged water jet. The factors that influence the cavitation intensity of pressure are simulated. Fujifilm pressure-sensitive paper is used to measure the distribution of impact pressure along the jet direction during the WCP process. The results show that submerged cavitation jets can induce cavitation both inside and outside a conical nozzle and a convergent-divergent nozzle when the inlet pressure is 32 MPa. Moreover, the shock wave pressure induced by the collapse of the bubble group reaches up to 300 MPa.

DISTRIBUTION OF IMPACT INDUCED PRESSURES AT THE FACE OF UNIFORMLY SLOPED SEA DIKES: PRELIMINARY 2D EXPERIMENTAL RESULTS

Although existing knowledge on the vertical distribution of impact pressures on sea-dikes is well established only very little is known with respect to their horizontal distribution. A collaboration developed between the University of Southampton, Uk and FZK, Hannover looks in more detail at the distribution of pressures induced by waves breaking on the face of a sea-dike. For this, 2D large scale experiments with waves breaking on a 1:3 sea dike were conducted but instead of pressure transducers a tactile pressure sensor was used to map the impact pressures. Such sensors were initially used with breaking waves in the University of Southampton and their use for large scale experiments was attempted here for the first time. In the current paper the calibration and application of the tactile sensor for experiments involving up to 1m high and 8sec long waves are initially described. Preliminary results illustrating the simultaneous distribution of impact induced pressures over an area of 426.7x487.7mm are then presented. Based on these pressure maps the vertical and horizontal location of maximum breaking wave induced pressures is also deduced.

Simulation of Wave Slamming on Open-Piled Structures Based on FLUENT

In order to study the specialties of wave slamming on open-piled structures, a two-dimensional regular wave tank was established based on commercial CFD software FLUENT. Three typical cases of regular wave slamming on the open-piled structures were reproduced by using the numerical wave tank and compared with the experimental data available. Good agreements were obtained between the numerical and experimental results and the average of peak impact pressure was chosen as the characteristic impact pressure. Then regular wave impact pressure on the open-piled structures under various wave height, period and over height were simulated. The influences of the three parameters on the distribution of impact pressure were analyzed.

Evaluation of the hydrodynamic pressure of cavitation impacts from stress–strain analysis and geometry of individual pits

The present paper focuses on the development of an inverse procedure able to infer the hydrodynamic pressure distribution acting on the material surface and responsible for the creation of the cavitation pit based on the stress in a cavitation pit and its geometrical features. To achieve this goal, experimental pitting and nanoindentation measurement techniques were used together with a model of pit formation based on a Gaussian distribution of the hydrodynamic impact pressure pulse.The pitting tests were performed at four different operating pressures on aluminum alloy samples and the geometrical characteristics of the pits were measured. Then the strain, stress, and load in a cavitation pit were quantified by coupling the pitting test analysis with the material information obtained via the indentation tests. Finally, a Gaussian distribution of the hydrodynamic impact pressure on the material surface has been hypothesized and the peak of this distribution as well as its width have been inferred.This procedure, allowing the evaluation of the hydrodynamic impact pressure and load responsible for the material erosion and the deduction of insightful information on the flow aggressiveness at different operating conditions, could certainly represent a significant step in developing a technique able to evaluate the cavitation intensity from pitting tests.

流体構造連成解析の水面衝撃問題への適用性に関する一考察

In this paper, a series of water impact simulations is carried out by using fluid-structure interaction (FSI) function of the nonlinear dynamic structural analysis code LS-DYNA. Water impact phenomena of wedge shaped body and circular cylinder body are simulated in the present study. For the purpose of the validation of numerical simulations, the water impact pressure's peak values, pressure distributions and structural responses are compared with conventional theories and experimental results. A series of rigid wedges' water impact simulations is performed and water pressure values are compared with conventional Wagner's theory and experimental values. Then, a series of elastic cylinders' water impact simulations is performed and structural responses are compared with experimental values. It is found from the present study that water pressures and structural responses calculated by FSI analyses using some models are in good agreement with Wagner's theory and experimental results.

Numerical Simulation of High Velocity Waterjet Characteristics and Impact Pressure

This paper presents a numerical simulation approach to analyze high velocity waterjet characteristics and impact pressure. For the complexity of waterjet formation in air, multiphase mixture flow model is used, and the simulation is performed in FLUNET software. The simulation includes the hydrodynamic characteristics and pressure distribution of high velocity waterjet in air. The decay of pressure at different distance along centerline under different pump pressure is analyzed and the length of the initial region of waterjet is determined. In addition, the impact pressure of waterjet at different stand-off distance is also simulated, and the impact pressure distribution and its changing tendency with the stand-off distance are obtained. This paper provides theoretical parameters for waterjet incremental sheet metal forming.

Numerical Simulation on the Pressure Field of Gas Knives

In the continuous galvanizing line, gas knives are used to control the liquid coating thickness. They are based on the principle of turbulent planar impinging jet. The CFD (Computational Fluid Dynamics) method is used to research the turbulent planar impinging jet of gas knives. With the implement of FLUENT software, the realizable k-ε model is adopted to simulate the pressure field. The transverse impact pressure distributions on the steel strip are demonstrated. For gas knives with even nozzle slot, the plot of impact pressure is analogous to parabola, reducing from middle to two ends. This numerical result shows agreement with experimental one. It indicates that the realizable k-ε model can accurately predict the impinging jet. Besides, for gas knives with uneven nozzle slot, the impact pressure is basically equivalent. Furthermore, several main factors having effect on the coating thickness are studied. The numerical results reveal that, the higher the inlet pressure, the thinner the coating; and the longer the distance between jet and strip, the thicker the coating.

Can we learn more from the data underlying the statistical α–β model with respect to the dynamical behavior of avalanches?

Abstract Hazard and risk assessment in avalanche-prone areas involves estimation of runout distances of potential avalanches. Methods for determination of the runout may be divided into two categories: 1) methods based on statistical approaches such as the well known α – β model or 2) methods based on numerical avalanche models such as the PCM-model or VS-type models (just to name the more traditional ones). Methods in the second group have the advantage that besides the runout distance, velocity and impact pressure distributions along the avalanche track can also be obtained, this being a requisite for meaningful risk assessments. However, the predictive power of dynamical models depends on the use of appropriate rheological models and their parameters. In the statistical α – β model, the maximum runout distance is solely a function of topography. The runout distance equations were found by regression analysis, correlating the longest registered runout distance of several hundred avalanche paths with a selection of topographic parameters. In this paper, we re-evaluate Norwegian and Austrian avalanche data, which served as basis for the α – β model in the respective countries, and additional avalanche data with respect to dynamical measures. As most of those avalanche data originate more or less from extreme events (i.e. avalanches with return periods of the order of 100 years), the dynamical measures may give hints about an appropriate rheology for dynamical models suitable for extreme avalanche events. The analysis raises reasonable doubt whether the classical ansatz for the retarding acceleration of snow avalanches with additive terms involving Coulomb-friction and a velocity-squared dependency, which is used in many avalanche models, is adequate for a physically-based model. Back-calculations of runout distances using a simple block model show a discrepancy between commonly proposed parameter values (and of the underlying rheological models) and the observations.

0406 壁面近傍での非球状気泡崩壊挙動に関する一考察(OS4-2 キャビテーションおよびマイクロバブル効果とその応用技術,オーガナイズドセッション)

Collapsing bubble near wall boundary is numerically analyzed to clarify the mechanism of cavitation bubble collapse and develop numerical prediction method of cavitation erosion. 2D axisymmetric and 3D plane-symmetric calculations using a locally homogeneous model of gas-liquid two-phase medium are performed until the second bubble collapse. It is confirmed that microjet generation, propagation of induced pressure wave and collapse of torus bubble can be realized and that the second collapse is responsible for the intensity of cavitation erosion. It is also found that the collapse of slightly asymmetric bubble from spherical bubble causes the different distribution of maximum impact pressure.

Long-term avalanche hazard assessment with a Bayesian depth-averaged propagation model

AbstractWhile performing statistical–dynamical simulations for avalanche predetermination, a propagation model must reach a compromise between precise description of the avalanche flow and computation times. Crucial problems are the choice of appropriate distributions describing the variability of the different inputs/outputs and model identifiability. In this study, a depth-averaged propagation model is used within a hierarchical Bayesian framework. First, the joint posterior distribution is estimated using a sequential Metropolis–Hastings algorithm. Details for tuning the estimation algorithm are provided, as well as tests to check convergence. Of particular interest is the calibration of the two coefficients of a Voellmy friction law, with model identifiability ensured by prior information. Second, the point estimates are used to predict the joint distribution of different variables of interest for hazard mapping. Recent developments are employed to compute pressure distributions taking into account the rheology of snow. The different steps of the method are illustrated with a real case study, for which all possible decennial scenarios are simulated. It appears that the marginal distribution of impact pressures is strongly skewed, with possible high values for avalanches characterized by low Froude numbers. Model assumptions and results are discussed.

Multidroplet Impact Model for Prediction of Residual Stresses in Water Jet Peening of Materials

In this article, a multidroplet impact model, proposed for predicting residual stresses induced on materials subjected to water jet peening, is presented. This approach considers the impact pressure distribution due to high-velocity droplets impinging on the material surface instead of stationary pressure distribution for prediction of residual stresses on water jet-peened surfaces. It makes use of Reichardt's theory for predicting the velocity distribution of droplets and liquid impact theory for predicting the impact pressure and duration of impact of high-velocity droplets. For predicting residual stresses on the surface and subsurface of material subjected to water jet peening, finite element modeling approach was adopted by using transient elastoplastic finite element analysis by considering an impingement of a set of droplets in succession to one another over a certain time period after which this pressure is released. The effectiveness of the proposed approach was demonstrated bv comparing the predicted residual stresses with those predicted by using the single set of droplets approach proposed by Rajesh et al. [6]. Finally, the practical relevance of the proposed approach was shown by comparing the predicted results with the experimental results obtained by water peening of 6063-T6 aluminium alloy.