Critical Standoff Distance Research Articles

Water-augmented vaporizing foil actuator welding: Process performance and mechanisms clarifications

This paper proposes a novel impact welding method named water-augmented vaporizing foil actuator welding, in which liquid water is introduced to alter the foil’s vaporization process, thus altering the induced driving pressure and the final welding performance. A series of comparative experiments were performed to confirm the effectiveness of the process and to understand the underlying mechanism. The proposed process decreased the lower-bound required discharge energy by 58–69%, increased the peel strength by 30–60%, and significantly reduced the variability in the critical standoff distance at the lower bound of the weld window. In addition, the proposed process resolved the robustness issue by producing a much more regular and repeatable shape of the weld area compared to conventional VFAW. Furthermore, three potential mechanisms have been proposed to explain the process improvement: the added water 1) increased the electrical energy deposition, 2) introduced additional energy from the aluminum-water chemical reaction, and 3) provided a more efficient force-transfer medium. In summary, this paper introduces a novel impact welding method with much improved process capability and robustness compared to conventional vaporizing foil actuator welding. In addition, due to its simplicity, this method may be easily extended for other forms of impulse metalworking, which may stimulate a wide interest for the community of high velocity metalworking.

Numerical modeling for assessing progressive collapse risk of RC buildings exposed to blast loads

The safety of strategic structures against accidental loading (such as blast-generated waves) has recently received much attention from practicing structural engineers. Terrorist attacks in the proximity of urban environments may bring blast threats close to the perimeter of the buildings, thereby making them exposed to progressive collapse. This paper aims at investigating the progressive collapse possibility of an existing RC (reinforced concrete) building against blast-generated waves. Data was collected for a typical RC building constructed in a congested area in Riyadh city. A simple NLD (nonlinear dynamic) analysis approach was carried out to examine the progressive collapse possibility of the building when exposed to blast threat events. In this regard, 3D nonlinear two-stage FEA (finite element analysis) was conducted. In the first stage, a local model was created for a typical individual RC column. The local model was validated utilizing the published results of an RC column subjected to blast load. The goal of the local analysis is to evaluate the individual column response against blast load and to come up with the critical stand-off distance at which the column gets severely damaged. In the second stage, a global model (comprising shell and beam elements) was created for the whole building. The goal of the global analysis is to examine the global behavior of the building owing to the removal of the columns identified from the local model stage. The progress of damage caused to different structural elements and the state of stress in these elements was studied. The results enhanced the knowledge about the progressive collapse potential in RC buildings and will be helpful in the development of mitigation strategies.

A Study of Swirling Jet Drilling and Its Specific Energy for Hydrate-Bearing Sediments

SummaryNatural gas hydrate (NGH) is a form of clean energy. The marine NGH solid fluidization exploitation method and the integrated radial jet drilling and completion method are considered as two ways to improve gas production, and water jet is the key technology of the two mining methods. This paper first compared the flow field distribution of conical jet and swirling jet at different outlet velocities through numerical simulation. The volume change of conical jet and swirling jet erosion hydrate-bearing sediment (HBS) was analyzed when the jet outlet velocity and standoff distance were changed. Then, based on the experimental apparatus for NGH generation and jet erosion, we carried out the swirling jet erosion HBS experiments. The depth and diameter of the erosion pits were about 10% larger than the numerical simulation. Finally, we evaluated the jet erosion performance with specific energy calculations. The results show that the reaming ability of the swirling jet is 2.4 times than that of the conical jet at the jet velocity of 100 m/s and the standoff distance of 5 mm. When the jet velocity ranges from 30 to 100 m/s, the erosion efficiency of the swirling jet is higher than that of the conical jet. The critical standoff distance is calculated as 12.5 mm (12.5 times nozzle throat) in current study, larger than which the advantages of swirling jet erosion HBS become less obvious. The excellent performance of swirling jet erosion HBS under certain conditions is described by specific energy calculation. This study can help for better understanding the performance of water jet drilling and provides an idea for improving the production efficiency of NGH.

Study of air-backed and water-backed carbon fiber composite plates subjected to underwater shock loading

A collection of experimental and numerical studies was conducted for laminated carbon fiber composite plates of different layup orientations to investigate their response and failure when subjected to underwater shock loading. The composite plates were clamped within a rigid fixture and submerged within an anechoic water tank while having either an air-back or water-back supporting condition. Plastic bottles partially filled with liquid nitrogen were used to generate shock pressures whose characteristics were closer to that resulting from the pulsation of a gas bubble rather than the detonation of an explosive. A multiphysics model based on fluid–structure interaction was considered for the numerical study. A multiscale approach was also used for the numerical study to predict failure based on the constituent material level (i.e., fiber and matrix material level). Both experimental and numerical results agreed well, indicating that the water-backed carbon fiber plate had a very localized failure while the air-backed composite plate showed global failure. The critical stand-off distance for the failure of the water-backed carbon fiber composite was approximately half of that for the air-backed composite plate of the same layup orientation. In other words, the water-backed carbon fiber composite plate had greater damage tolerance than the air-backed plates.

Response of an underwater cylindrical composite shell to a proximal implosion

An experimental investigation is conducted to study the dynamic underwater response of a cylindrical composite shell under near critical hydrostatic pressure, to the implosion of another shell in proximity. A primary cylindrical composite shell is imploded in proximity to a secondary shell which is similar in all respects except for the secondary shell having a smaller length. Length differences of 10% and 20% are chosen to simulate variations in collapse pressures occurring in shells from real life manufacturing defects and/or degradation during operational use. The response of the secondary shell is investigated to understand if and how its collapse occurs in addition to studying the Fluid-Structure Interaction (FSI) phenomenon. The pair of shells are subjected to underwater hydrostatic loading using a large pressure vessel suitable for high-speed photography in conjunction with 3D Digital Image Correlation (DIC). 3D-DIC is employed to obtain full-field displacement measurements of both the shells, and local dynamic pressure histories are also simultaneously recorded. The primary shell always imploded first, causing a dynamic loading on the secondary shell. In cases of implosion of the secondary shell, although the transient radial deformations occurred in mode 2, the failure itself occurred with a localized failure of the shell walls. It is observed that there exists an inner critical stand-off distance for the secondary shell to fail catastrophically upon the implosion of the primary shell, and an outer critical stand-off distance beyond which the secondary shell does not implode. A critical stand-off distance is found to exist only in the case of the 10% smaller secondary shell length. If the secondary shell stand-off distance is more than the outer critical distance or when length of the secondary shell is 20% smaller, the secondary shell responds with bending and breathing modes and no visible damage is recorded. When the stand-off distance is in between the inner and the outer critical distances, the relative orientation of the incipient modal shapes of the two shells is the factor governing the collapse of the secondary shell. A method is also developed to decouple full-field 3D-DIC measurements into bending and breathing deformation measurements.

Anomalous blow-off limit of methane-air premixed flame in a micro preheated combustor with a flame holder

The present work first reports the combustion characteristics of methane/air premixed fuel mixture (Lewis number Le ≈ 1.0) at different Reynolds numbers (Re) in a micro preheated combustor with a flame holder which can be used as heat source for the micro power generators. Unexpectedly, the previously reported anomalous blow-off of lean H2/CH4/air (Le < 1.0) premixed flame in the mesoscale combustor appears in the present micro combustor, which is observed for the first time. The flame dynamics in the micro combustor also presents other new features comparing with that in the similar mesoscale combustor. Three neighboring cases (Re = 40, 80, and 120) are adopted to reveal the underlying mechanisms of the anomalous blow-off in terms of the flow field, stretch effect, conjugate heat exchange, and preferential transport effect. A better heat recirculation effect results in the increase of blow-off limit from Re = 120 to 80. A worse flow recirculation effect, a larger heat loss ratio to the environment, and a worse preferential transport effect lead to the decrease of blow-off limit from Re = 80 to 40. In the end, the critical standoff distance between the flame root and flame holder presents a second-order function relation with the change of Re.

Simulation of Spray Cooling on Hot Steel Slabs in Continuous Casting

Abstract Continuous casting is an important process to solidify molten steel. Efficient and uniform heat removal by water spray without cracking or deforming steel slabs is a major challenge during continuous casting. With the aid of computational fluid dynamics (CFD), the current study presents a numerical model of spray cooling for process understanding and improvement. The Eulerian–Lagrangian approach is utilized to track the movement of water droplets that are injected from a hydraulic nozzle. The initial droplet size is predicted by the linear instability sheet atomization (LISA) model and the droplet distribution satisfies the Rosin-Rammler distribution. Droplet-slab impingement is modeled by the wall jet model and the associated heat transfer accounts for both heat conduction and radiation. Heat transfer coefficient (HTC) on slab surface is validated against a hot-plate benchmark experiment and shows good agreement. The significance of spray standoff distance and spray direction on heat transfer is also investigated. The results indicate the existence of the critical standoff distance beyond which the cooling effect becomes negligible. Spray direction plays an important role in determining the heat transfer rate as well, and insufficient cooling is observed when droplets are sprayed against gravity. Additional 10% to 15% of water will compensate for such low heat transfer. The current study should benefit the steel industry by providing fundamental insights into the process. The current model can also be applied to other types of spray nozzles and can further improve the efficiency of future simulations.

Dual-processing by abrasive waterjet machining—A method for machining and surface modification of nickel-based superalloy

Abstract Abrasive waterjet (AWJ) is widely used for machining of advanced (e.g. nickel-based) superalloys as it offers high material removal rates and low cutting temperatures. However, the inadequate surface integrity, e.g. large number of scratches and embedded particles in the machined surface, which would induce severe deteriorations of the materials functional performance, has been one of the greatest issues of the AWJ machining technique. To solve this problem, this research proposed a dual-process abrasive waterjet machining method, whereby two different functions of abrasive waterjet were employed: materials removal (first process) and surface modification (secondary process), hence, to improve the workpiece surface integrity. Two types of entrained particles, i.e. with sharp cutting edges (e.g. garnet) and smooth surfaces (e.g. stainless steel ball), that depending on their kinetic energy density can either cut or modify the workpiece surface respectively, are employed for these the two constitutive processes of the proposed dual-waterjet machining method. A critical standoff distance and inclination angle of the waterjet nozzle has been defined for the surface modification process thus, to eliminate the embedded particles and scratches left by the first cutting process while also introducing a surface strengthening effect. To support this approach, a mathematical model has been proposed for determining the surface modification parameters (e.g. jet feed speed and abrasive flow rate). In-depth analysis of the microstructural and metallurgical alternations of the machined workpiece surface and superficial layer have also been conducted to reveal the mechanisms responsible for the surface damage elimination and surface strengthening. Moreover, a four point bending fatigue test has been conducted to validate the mechanical performance, whereby a significant improvement of the fatigue life on the machined workpiece was achieved when compared with the case that single AWJ cutting method (91 %) and conventional machining (34 %) are employed. This proves that the proposed dual-processing AWJ machining method is of high efficiency to improve the functional performance of components on a single machine tool platform.

Resistance of museum artefacts against blast loading

Abstract The response of museum artefacts and statues subjected to deliberate explosions of moderate intensity is investigated and their vulnerability assessed. The study focuses on the most predominant failure mechanisms, namely overturning and fracture due to the tensile stresses developed by the impact of shock waves. The rocking response is investigated relying on the existing knowledge and theory of inverted pendulum structures subjected to earthquake loading. An analytical, established approach for determining the overturning domain, developed in a previous study, is used to investigate the critical stand-off distance between the target and the explosive in order to avoid toppling. The proposed analytical model is adopted by defining appropriate correction parameters to consider the real geometry of the museum objects. We assess the overturning domain of some emblematic statues of high aesthetic and cultural value, and namely: Michelangelo's David, Farnese Hercules, Aphrodite of Milos, Athena Giustiniani, Laocoon and His Sons, and Belvedere Torso. Finally, direct damage due to the high tensile stresses is investigated and computed for different target geometries. For the dimensions and explosive quantities herein considered, overturning is found to prevail over direct material damage for targets with regular geometry.

Rocking response of inverted pendulum structures under blast loading

We investigate the dynamics of inverted pendulum structures under fast-dynamic excitations arising from an explosion. We model blast actions using established empirical models and best-fit interpolations of existing experimental tests. We focus attention on pure rocking response mechanisms. We present moment balance equations and overturning conditions. Inspired by previous works in the frame of earthquake engineering, we derive new analytical, closed-form solutions for the rocking response and the overturning domain of slender blocks due to explosions.The analytical findings and assumptions are validated through existing experimental tests and detailed three-dimensional numerical simulations, which consider the full interaction between the blast waves and the structure. We show that unilateral rocking response and overturning are predominant mechanisms compared to sliding and up-lifting.Finally, we develop design charts to be used as a straightforward decision making tool for determining the critical stand-off distance between the explosive source and the target in order to prevent overturning. Our model can be easily applied for the design of protective devices to preserve artefacts, secure buildings and humans, as well as for devising energy absorbing systems based on rocking.

Designing of Blast Resistant Lightweight Elevation System - Numerical Study

In recent times, the designing of the blast resistant lightweight elevation systems has become an important engineering issue. It is clear that such structures do not fit any commonly known standards, thus designing process induces the application of a sophisticated numerical procedures, complex material constitutive laws and finally detailed experimental test. In this paper some details of a numerical study of an aluminum blast resistant multilayered elevations are discussed. For the assumed critical stand-off distance, values of a TNT equivalence and velocities of debris, common for improvised charges, the series of a lightweight elevation system was designed. The system is modular and its installation is possible without use of a heavy equipment.

Analysis of the Response of a One-Storey One-Bay Steel Frame to Blast

In recent years, there has been a considerable increase in perceived risks of blast loading attacks or similar incidents on structures. Blast design is therefore a necessary aspect of the design for building structures globally and as such building design must adapt accordingly. Presented herein is an attempt to determine the numerical response of a seismically designed single-degree-of-freedom (SDOF) structure to blast loading. The SDOF model in the form of a portal frame was designed to withstand a typical seismic occurrence in Northern Trinidad. Blast loads caused by applying a 500 kg charge weight of TNT at standoff distances of 45 m, 33 m, and 20 m were then applied to the model. The blast loading on the frame was determined using empirical methods. The analytical study showed that the seismically designed SDOF plane frame model entered the plastic region during the application of the blast load occurring up to the critical standoff distance.

Hydrodynamics of fluid around a collapsing bubble in the spark bubble droplet generation process

Summary This article aims to study in detail the bubble oscillation near confined free surfaces and the ensuing droplet formation using a combined boundary element-finite difference method. Three configurations are considered: (i) the bubble oscillation near a circular aperture made in a flat plate, (ii) the bubble oscillation inside and near the top opening of a vertical cylinder and (iii) the bubble oscillation between a perforated flat plate and a horizontal solid boundary. The effects of standoff distance on the bubble dynamics and on the surrounding fluid dynamics are examined. Completely different bubble shapes, free surface motions, jetting patterns and pressure distributions under different standoff distances could be observed in the present work. In addition, it was found that for the configurations (i) and (iii), the bubble reentrant jet is always directed away from the free surface. For the cylinder case, however, a critical standoff distance was found for which the pressure distribution in the fluid above and below the bubble are the same and the bubble takes the shape of an hourglass at the time of jet impact. For subcritical standoff distances the bubble reentrant jet is always away from the free surface. However, for supercritical standoff distances the direction of the reentrant jet is interestingly directed towards the free surface. Finally, the effect of plate and cylinder configurations on the resulting droplet dynamics was investigated. It was found that the driving force for the droplet formation is the initial velocity induced by the bubble reentrant jet rather than the pressure difference inside the liquid. Besides, in the cylinder case the droplet size is smaller and its pinch-off happens earlier than the plate case.

Onset of cellular flame instability in adiabatic CH4/O2/CO2 and CH4/air laminar premixed flames stabilized on a flat-flame burner

This paper presents numerical and experimental studies on the onset and evolution of laminar cellular flames of CH4/O2/CO2 (oxy-fuel) and CH4/air mixtures under adiabatic conditions, stabilized in the proximity of a flat-flame burner at atmospheric pressure. In the numerical simulations, a two-dimensional domain with periodicity at the transverse far field boundaries is resolved using a high accuracy finite difference method and employing a detailed chemical kinetic mechanism and detailed transport properties. In the experiments a specially designed adiabatic flat-flame burner, a so-called heat flux burner, is employed. A key parameter, the standoff distance between the flame front and the burner exit plate, is identified. A critical standoff distance is found, above which cellular flame instability is observed. It is shown that the critical standoff distance is closely related to the density ratio and the laminar flame thickness for each flame studied. The observed onset of cellular flames is governed by the hydrodynamic instability mechanism, which is generally suppressed by the burner when the flame is very close to the burner plate. Diffusive-thermal effects play an important role in the flame instability when the flame is far from the burner. The critical standoff distance has no clear correlation with the Lewis number, indicating a less significant effect of diffusive-thermal instability on the flames near the burner.

De-painting with high-speed water jets: Paint removal process and substrate surface roughness

Although the use of water jets for paint removal processes is an accepted procedure, there are just a few studies known which discuss parameter optimization and surface topography in some detail. The paper investigates the effects of water jet kinetic energy and stand-off distance on the mass loss of an organic paint system applied to a steel substrate. It was shown that the material removal process was characterized by a combination of loading intensity and loading frequency. Water drops, formed in the water jet at long stand-off distances, played a notable role. For rather high water jet energies, mass loss exhibited high values at high stand-off distances. For lower water jet energies, however, maximum material loss values appeared at a critical stand-off distance. The transition water jet energy was 600kJ. It could be shown that the steel substrate topography was not compromised due to secondary surface preparation by water jets. Secondary blast cleaning, however, reduced the profile of the substrate.

Experimental research on waterjet street surface cleaning

Using pure waterjets for cleaning street surfaces is very important for road maintenance and vehicle protection. It is also relevant to environmental protection. Commonly, street surfaces are cleaned by fan-shaped waterjets and many parameters can influence the effectiveness of the cleaning. To date, there has been a dearth of research into this subject. The study of the relationship between the parameters and the results is the basis for optimising these parameters to obtain the best cleaning effectiveness, productivity and energy consumption. This paper reports on a series of experiments conducted to study the correlation between cleaning rate and cleaning parameters of pressure p, traverse rate ν, standoff distance and nozzle installation angle. It is found that the cleaning rate is proportional to p 3 and ν−0·5. At the same time, there was a critical standoff distance where the cleaning rate and average clean width could reach the maximum value; the cleaning rate could reach the maximum value when the installation angle was near 40°.

Mathematical Modeling and Experimental Verification of Stationary Waterjet Cleaning Process

The use of stationary waterjet for the removal of coating material from the substrate is investigated analytically and experimentally. In the analysis, the cleaning width as a function of standoff distance, water pressure, and nozzle radius is derived by considering the structure of waterjet and the cleaning mechanism. Also derived are the relations of the optimal cleaning standoff distance and maximum cleaning width to the critical cleaning standoff distance, and how the water pressure and nozzle radius affect this critical standoff distance. These derived analytical relations are verified with experimental results.