Experimental and computational investigation of confined laser-induced breakdown spectroscopy

This paper presents an experimental and computational study on laser-induced breakdown spectroscopy (LIBS) for both unconfined flat surface and confined cavity cases. An integrated LIBS system is employed to acquire the shockwave and plasma plume images. The computational model consists of the mass, momentum, and energy conservation equations, which are necessary to describe shockwave behaviors. The numerical predictions are validated against shadowgraphic images in terms of shockwave expansion and reflection. The three-dimensional (3D) shockwave morphology and velocity fields are displayed and discussed

Three Lectures: Nemd, Spam, and Shockwaves

We discuss three related subjects well suited to graduate research. The first, Nonequilibrium molecular dynamics or "NEMD", makes possible the simulation of atomistic systems driven by external fields, subject to dynamic constraints, and thermostated so as to yield stationary nonequilibrium states. The second subject, Smooth Particle Applied Mechanics or "SPAM", provides a particle method, resembling molecular dynamics, but designed to solve continuum problems. The numerical work is simplified because the SPAM particles obey ordinary, rather than partial, differential equations. The interpolation method used with SPAM is a powerful interpretive tool converting point particle variables to twice-differentiable field variables. This interpolation method is vital to the study and understanding of the third research topic we discuss, strong shockwaves in dense fluids. Such shockwaves exhibit stationary far-from-equilibrium states obtained with purely reversible Hamiltonian mechanics. The SPAM interpolation method, applied to this molecular dynamics problem, clearly demonstrates both the tensor character of kinetic temperature and the time-delayed response of stress and heat flux to the strain rate and temperature gradients. The dynamic Lyapunov instability of the shockwave problem can be analyzed in a variety of ways, both with and without symmetry in time. These three subjects suggest many topics suitable for graduate research in nonlinear nonequilibrium problems.Comment: 40 pages, with 21 figures, as presented at the Granada Seminar on the Foundations of Nonequilibrium Statistical Physics, 13-17 September, as three lecture

Generation and characterization of T40/A5754 interfaces with lasersPatrice

Laser-induced reactive wetting and brazing of T40 titanium with A5754 aluminum alloy with 1.5 mm thickness was carried out in lap-joint configuration, with or without the use of Al5Si filler wire. A 2.4 mm diameter laser spot was positioned on the aluminum side to provoke spreading and wetting of the lower titanium sheet, with relatively low scanning speeds (0.1–0.6 m/min). Process conditions did not play a very significant role on mechanical strengths, which were shown to reach 250–300 N/mm on a large range of laser power and scanning speeds. In all cases considered, the fracture during tensile testing occurred next to the TiAl3 interface, but in the aluminum fusion zone. The interfacial resistance was then evaluated with the LASAT bond strength tester, based upon the generation and propagation of laser-induced shock waves. A 0.68 GPa uniaxial bond strength was estimated for the T40/A5754 interface under dynamic loading conditions

Shifting Data Collection from a Fixed to an Adaptive Sampling Paradigm

For domains where data are difficult to obtain due to human or resource limitations, an emphasis is needed to efficiently explore the dimensions of information spaces to acquire any given response of interest. Many disciplines are still making the transition from brute force, dense, full factorial exploration of their information spaces to a more efficient design of experiments approach; the latter being in use successfully for many decades in agricultural and automotive applications. Although this transition is still incomplete, groundwork must be laid for incorporating the next generation of algorithms to adaptively explore the information space in response to data collected, as well as any resulting empirical models (i.e., metamodels). The methodology in the present work was to compare metamodel quality using a fixed sampling technique compared to an adaptive sampling technique based on metamodel variance. In order to quantify metamodeling errors, a delta method was used to provide quantitative model variance estimates. The present methodology was applied to a design space with an air-breathing engine performance response. It was shown that competitive metamodel quality with lower associated error could be achieved for an adaptive sampling technique for the same level of effort as a fixed, a priori sampling technique

Effect Of Shock Tunnel Geometry On Shockwave And Vortex Ring Formation, Propagation, And Head On Collision

Vortex ring research primarily focuses on the formation from circular openings. Consequently, the role of tunnel geometry is less understood, despite there being numerous research studies using noncircular shock tunnels. This experimental study investigated shockwaves and vortex rings from different geometry shock tunnels from formation at the tunnel opening to head on collision with another similarly formed vortex ring using schlieren imaging and statistical analysis. The velocity of the incident shockwave was found to be consistent across all four shock tunnel geometries, which include circle, hexagon, square, and triangle of the same cross-sectional area. The velocity was 1.2 ± 0.007 Mach and was independent of the tunnel geometry. However, the velocities of the resulting vortex rings differed between the shapes, with statistical analysis indicating significant differences between the triangle and hexagon vortex velocities compared to the circle. Vortex rings from the square and circle shock tunnels were found to have statistically similar velocities. All vortex rings slowed as they traveled due to corner inversion and air drag. All shock tunnels with corners produce a wobble in the vortex rings. Vortex rings interact with opposing incident shockwaves prior to colliding with each other. Vortex velocity before and after shock-vortex interaction was measured and evaluated, showing statistically similar results. Shock-vortex interaction slows the shockwave upon interaction, while the shock-shock interaction resulted in no change in shock velocity. Although the vortex rings travel at different velocities, all head-on vortex ring collisions produce a perpendicular shockwave that travels at 1.04 ± 0.005 Mach

Experimental investigation for characterizing and improving inlet designs in rotating detonation engines

Rotating detonation engines (RDEs) present great potential for significant improvement in efficiency for land based power generation systems, in addition to aircraft propulsion devices. They offer the advantage of a net pressure gain across the combustor, as well as high exhaust temperatures and less entropy production due to detonative combustion. These improvements provide direct correlation to improved overall efficiency and thermal efficiency of gas turbine engines. RDEs surpass their conventional combustor counterparts in terms of their geometric size and simpler mechanical design. Among many areas of much needed research to further the technology readiness level (TRL) of RDEs, the inlet design is paramount to the successful operation of a rotating detonation engine. The inlet is one of the central impetuses behind current RDE research.;The existing inlet designs for RDEs in the research community are not optimized for maximum performance, yet are mostly used to operate research combustors. They are shown to induce high pressure drop, anywhere from 50-90%, and provide insufficient mixing for the inlet reactants. They also provide poor interaction between channel pressure fluctuations and detonation propagations. For these reasons, novel inlet design concepts are devised and tested in this work. The primary goal of the work is to design an inlet that is well isolated from the combustion channel, and is conducive to short interruption times of its refueling capability due to shockwave passes. This will precede the loss reduction efforts to the inlet. A combustor from the Air Force Research Lab (AFRL) serves as the baseline geometry for all testing conducted. A linear lab scale testing device, which is a scaled model of the full size cylindrical RDE to allow for lower flow rates and pressures to be used, has been developed for more simplified and rapid experimental testing of inlet concepts. Novel inlet geometries are designed and created using additive manufacturing techniques. Initial experiments are conducted on the baseline inlet and are used as comparison experimental results of new inlet designs. Geometric characteristics are leveraged for their acoustic and resonant properties in order to provide the highest backflow prevention. Experimental results for each design are presented and evaluated. High-speed Schlieren video is used to supplement the quantitative data reported, and is used to analyze the flow structures and interactions with detonation. Novel inlet concepts are presented that show capability to reduce the pressure influence of detonation by 1-2%, and improve the refueling time of the injectors. Improvements from the baseline inlet consist of improvements in backflow length by up to 60%, as well as reduction in recovery times from 20-30%