Direct Numerical Simulations of Localised Forced Ignition in Turbulent Mixing Layers: The Effects of Mixture Fraction and Its Gradient

The effects of mixture fraction value ξ and the magnitude of its gradient |∇ξ| at the ignitor location on the localised forced ignition of turbulent mixing layers under decaying turbulence is studied based on three-dimensional compressible Direct Numerical Simulations (DNS) with simplified chemistry. The localised ignition is accounted for by a spatial Gaussian power distribution in the energy transport equation, which deposits energy over a prescribed period of time. In successful ignitions, it is observed that the flame shows a tribrachial structure. The reaction rate is found to be greater in the fuel rich side than in stoichiometric and fuel-lean mixtures. Placing the ignitor at a fuel-lean region may initiate ignition, but extinction may eventually occur if the diffusion of heat from the hot gas kernel overcomes the heat release due to combustion. It is demonstrated that ignition in the fuel lean region may fail for an energy input for which self-sustained combustion has been achieved in the cases of igniting at stoichiometric and fuel-rich locations. It is also found that the fuel reaction rate magnitude is negatively correlated with density-weighted scalar dissipation rate in the most reactive region. An increase in the initial mixture fraction gradient at the ignition centre for the ignitor placed at stoichiometric mixture decreases the spreading of the burned region along the stoichiometric mixture fraction isosurface. By contrast, the mass of the burned region increases with an increase in the initial mixture fraction gradient at the ignition location, as for a given ignition kernel size the thinner mixing layer includes more fuel-rich mixture, which eventually makes the overall burning rate greater than that compared to a thicker mixing layer where relatively a smaller amount of fuel-rich mixture is engulfed within the hot gas kernel. Submitted as a full-length article to Flow Turbulence and Combustion.     

Numerical Study of Turbulent Jet Ignition in a Lean Premixed Configuration

Direct numerical simulations (DNS) of a hot combustion product jet interacting with a lean premixed hydrogen-air coflow are conducted to fundamentally investigate turbulent jet ignition (TJI) in a three-dimensional configuration. TJI is an efficient method for initiating and controlling combustion in ultra-lean combustion systems. Fully compressible gas dynamics and species equations are solved with high order finite difference methods. The hydrogen-air reaction is simulated with a reliable detailed chemical kinetics mechanism. The physical processes involved in the TJI-assisted combustion are investigated by considering the flame heat release, temperature, species concentrations, vorticity, and Baroclinc torque. The complex turbulent flame and flow structures are delineated in three main: i) hot product jet, ii) burned-mixed, and iii) flame zones. In the TJI-assisted combustion, the flow structures and the flame features such as flame speed, temperature, and species distribution are found to be quite different than those in “standard” turbulent premixed combustion due to the existence of a high energy turbulent hot product jet. The flow structures and statistics are also found to be different than those normally seen in non-isothermal non-reacting jets.     

Three-Dimensional Simulation of Turbulent Hot-Jet Ignition for Air-CH<Subscript>4</Subscript>-H<Subscript>2</Subscript> Deflagration in a Confined Volume

This work describes essential aspects of the ignition and deflagration process initiated by the injection of a hot transient gas jet into a narrowly confined volume containing air-CH₄-H₂ mixture. Driven by the pressure difference between a prechamber and a long narrow constant-volume-combustion (CVC) chamber, the developing jet or puff involves complex processes of turbulent jet penetration and evolution of multi-scale vortices in the shear layer, jet tip, and adjacent confined spaces. The CVC chamber contains stoichiometric mixtures of air with gaseous fuel initially at atmospheric conditions. Fuel reactivity is varied using two different CH₄/H₂ blends. Jet momentum is varied using different pre-chamber pressures at jet initiation. The jet initiation and the subsequent ignition events generate pressure waves that interact with the mixing region and the propagating flame, depositing baroclinic vorticity. Transient three-dimensional flow simulations with detailed chemical kinetics are used to model CVC mixture ignition. Pre-ignition gas properties are then examined to develop and verify criteria to predict ignition delay time using lower-cost non-reacting flow simulations for this particular case of study.     

Residual Stress Determination Using Hole Drilling and 3D Image Correlation

In recent years, the hole drilling method for determining residual stresses has been implemented with optical methods such as holographic interferometry and ESPI to overcome certain limitations of the strain rosette version of hole drilling. Although offering advantages, the interferometric methods require vibration isolation, a significant drawback to their use outside of the laboratory. In this study, a 3D image correlation approach was used to measure micron-sized surface displacements caused by the localized stress relief associated with hole drilling. Residual stresses were then found from the displacements using non-dimensional relations previously derived by finite element analysis. A major advantage of image correlation is that it does not require interferometric vibration isolation. Experiments were performed to check the ability of this new approach for uniaxial and equi-biaxial states of stress. Stresses determined by the approach were in good agreement with computed values and those determined by hole drilling using holographic interferometry.     

Autoignition of n-decane Droplets in the Low-, Intermediate-, and High-temperature Regimes from a Mixture Fraction Viewpoint

Detailed numerical simulations of isolated n-decane droplets autoignition are presented for different values of the ambient pressure and temperature. The ignition modes considered included single-stage ignition, two-stage ignition and cool-flame ignition. The analysis was conducted from a mixture fraction perspective. Two characteristic chemical time scales were identified for two-stage ignition: one for cool-flame ignition, and another for hot-flame ignition. The appearance and subsequent spatial propagation of a cool flame at lean compositions was found to play an important role in the ignition process, since it created the conditions for activating the high-temperature reactions pathway in regions with locally rich composition. Single-stage ignition was characterized by a single chemical time scale, corresponding to hot-flame ignition. Low-temperature reactions were negligible for this case, and spatial diffusion of heat and chemical species mainly affected the duration of the ignition transient, but not the location in mixture fraction space at which ignition first occurs. Finally, ignition of several cool flames of decreasing strength was observed in the cool-flame ignition case, which eventually lead to a plateau in the maximum gas-phase temperature. The first cool flame ignited in a region where the fuel / air mixture was locally lean, whereas ignition of the remaining cool flames occurred at rich mixture compositions.     

Operational Modal Analysis of a Helicopter Rotor Blade Using Digital Image Correlation

A novel procedure to perform operational modal analysis on a reduced-scale, 2 m diameter helicopter rotor blade is described. Images of the rotor blade rotating at 900 RPM are captured by a pair of high-speed digital cameras at a sampling rate of 1000 frames per second. From these images, the out-of-plane bending deformation of the rotor blade is measured using Digital Image Correlation, with a spatial resolution of 7.2 mm and an accuracy of 60 μm, or 0.006 % of the rotor radius. Modal parameters including natural frequencies and mode shapes are determined from the bending deformation through application of the Ibrahim Time Domain method. The first three out-of-plane bending modes were identified at each rotational speed and compared to an analytical finite element model of the rotor blade. The experimental and analytical natural frequencies agreed to within 0.2 % in the best case and 10.0 % in the worst case. The experimental mode shapes were also found to closely match the analytical predictions. The results of this study demonstrate the ability of this procedure to accurately determine the modal parameters of rotating helicopter rotor blades.     

Accessing Preferential Foam Flow Paths in 2D Micromodel Using a Graph-Based 2-Parameter Model

Transport in Porous Media - Foam is to be used as a blocking agent for confining a pollutant source zone and avoid spreading in an aquifer. To this end, it is necessary to determine where injected...     

Studies of Subcritical Crack Growth in Viscoelastic Anisotropic Bodies Using the Continued Fraction Operator Method: Synthesis and Summary*

International Applied Mechanics - A technique of solving problems of linear viscoelasticity is presented. Some basics are given on the application of continued fractions to solve some problems for...     

Simulation of Three-Phase Displacement Mechanisms Using a 2D Lattice-Boltzmann Model

Using a numerical technique, known as the lattice-Boltzmann method, we study immiscible three-phase flow at the pore scale. An important phenomenon at this scale is the spreading of oil onto the gas–water interface. In this paper, we recognize from first principles how injected gas remobilizes initially trapped oil blobs. The two main flow mechanisms which account for this type of remobilization are simulated. These are the double-drainage mechanism and (countercurrent) film flow of oil. The simulations agree qualitatively with experimental findings in the literature. We also simulate steady-state three-phase flow (fixed and equal saturations) in a small segment of a waterwet porous medium under both spreading and nonspreading conditions. The difference between the two conditions with respect to the coefficients in the generalized law of Darcy (which also includes viscous coupling) is investigated.     

Analysis of Cracks and Deformations in a Full Scale Reinforced Concrete Beam Using a Digital Image Correlation Technique

This paper deals with the use of a digital image correlation technique for the determination of the actual mechanical behaviour of a full scale reinforced concrete beam after 25 years of service in a severe industrial environment. The objective is to investigate the influence of the service conditions on the cracking process and the flexural behaviour of the beam. For this purpose, one beam is removed from the existing structure before being tested in four point bending in laboratory. Displacement fields derived from digital images captured during five loading cycles are analysed in terms of crack detection and measurement, beam deflection and curvature. Owing to its good resolution, the method proves suitable for early crack detection and measurement. A comparison between experimental results and theoretical values derived from Eurocode 2 design code in the serviceability state suggests the existence of a longitudinal compressive force in the beam. A complementary analysis confirms the validity of this hypothesis. It is concluded that the cracking and the flexural behaviour of the tested beam are significantly affected by the existence of an initial compressive stress, which is possibly resulting from a swelling of the concrete due to long term exposure to wet atmosphere and elevated temperature.     

Full-field Thermal Deformation Measurements in a Scanning Electron Microscope by 2D Digital Image Correlation

Using recently developed methods for application of a nano-scale random pattern having high contrast during SEM imaging, baseline full-field thermal deformation experiments have been performed successfully in an FEI Quanta SEM using 2D-DIC methods. Employing a specially redesigned commercial heating plate and control system, with modified specimen attachment procedures to minimize unwanted image motions, recently developed distortion correction procedures were shown to be effective in removing both drift and spatial distortion fields under thermal heating. 2D-DIC results from heating experiments up to 125°C on an aluminum specimen indicate that (a) the fully corrected displacement components have nearly random variability and a standard deviation of 0.02 pixels (≈25 nm at 200× and ≈0.5 nm at 10,000×) in each displacement component and (b) the unbiased measured strain fields have a standard deviation ≈150 × 10⁻⁶ and a mean value that is in good agreement with independent measurements, confirming that the SEM-DIC based method can be used for both micro-scale and nano-scale thermal strain measurements.

Dynamic Subsurface Deformation and Strain of Soft Hydrogel Interfaces Using an Embedded Speckle Pattern With 2D Digital Image Correlation

Background

Subsurface mechanisms can greatly affect the mechanical behavior of biological materials, but observation of these mechanisms has remained elusive primarily due to unfavorable optical characteristics. Researchers attempt to overcome these limitations by performing experiments in biological mimics like hydrogels, but measurements are generally restricted due to the spatio-temporal limitations of current methods.

Objective

Utilization of contemporary 3D printing techniques into soft, transparent, aqueous yield-stress materials have opened new avenues of approach to overcoming these roadblocks. By incorporating digital image correlation with such 3D printing techniques, a method is shown here that can acquire full-field deformation of a hydrogel subsurface in real-time.

Methods

Briefly, the method replaces the solvent of a transparent and low polymer concentration yield-stress material with an aqueous hydrogel precursor solution, then a DIC speckle plane is 3D printed into it. This complex is then polymerized using photoinitiation thereby locking the speckle plane in place.

Results

Full-field deformation measurements are made in real-time as the embedded speckle plane (ESP) responds with the bulk to the applied load. Example results of deformation and strain fields associated with indentation, relaxation, and sliding contact experiments are shown.

Conclusions

This method has successfully observed the subsurface mechanical response in the bulk of a hydrogel and has the potential to answer fundamental questions regarding biological material mechanical behaviors.

     

High-Accuracy 2D Digital Image Correlation Measurements with Bilateral Telecentric Lenses: Error Analysis and Experimental Verification

By comparing two digital images of a test planar specimen surface recorded in different configurations, two-dimensional digital image correlation (2D-DIC) provides full-field displacements to sub-pixel accuracy and full-field strains in the recorded images. For the 2D-DIC systems using an optical lens, a simple pinhole imaging model is commonly used to describe the linear relationship between the measured sensor plane displacements and the actual displacements in the object surface. However, in a practical measurement, various unavoidable disadvantageous factors, such as small out-of-plane motion of the test object surface occurred after loading, small out-of-plane motion of the sensor target due to the self-heating or temperature variation of a camera, and geometric distortion of the imaging lens, may seriously impair or slightly change the originally assumed linear correspondence. In certain cases, these disadvantages may lead to significant errors in displacements and strains measured by 2D-DIC. In this work, the measurement errors of 2D-DIC due to the above three disadvantageous factors are first described in detail. Then, to minimize the errors associated with these disadvantages, a high-accuracy 2D-DIC system using a bilateral telecentric lens is established. The performance of the established 2D-DIC system and other two 2D-DIC systems using a conventional lens and an object-side telecentric lens are investigated experimentally using easy-to-implement stationary, out-of-plane and in-plane rigid body translation tests. A detailed examination reveals that a high-quality bilateral telecentric lens is not only insensitive to out-of-plane motion of the test object and the self-heating of a camera, but also demonstrates negligible lens distortion. Uniaxial tensile tests of an aluminum specimen were also performed to quantitatively compare the axial and transversal strains measured by the proposed 2D-DIC system and those measured by strain gage rosettes. The perfect agreement between the two measurements further verifies the accuracy of the established 2D-DIC system.     

Analysis and Artifact Correction for Volume Correlation Measurements Using Tomographic Images from a Laboratory X-ray Source

The effects of three artifacts (reconstruction, beam hardening and temperature of the X-ray tube) associated with the use of a lab tomograph are analyzed in terms of their induced biases for Digital Volume Correlation (DVC) from a series of reconstructed volumes acquired successively. The most detrimental effect is due to spurious dilatational strains induced by temperature variations in the tomograph. If they are not accounted for, any quantitative kinematic measurement is impossible for strain levels below 0.5%.     

Full Field Strain Measurement of Resistant Spot Welds Using 3D Image Correlation Systems

This paper describes local strain measurements of electrical resistance spot welds for three sheet stack-ups of Dual Phase (DP600) and Mild steel under quasi-static tensile loading. The experiments were designed to measure local strain distributions at the vicinity of the spot welds using a modified tensile-shear specimen geometry that allows a strain measurement system to access such area. The electrode tip type used was B-nose, and surface indentation levels were 30% and 50% of the sheet thickness. Local strains at critical locations in resistance spot welds were measured using non-contact 3D image correlation systems (ARAMIS) during quasi-static tension tests. The measured local strains were also compared with results offered by a finite element analysis.     

Dynamic Systems Analysis and Control Based on a Configuration Manifold Model

This paper presents a configuration manifold model for the analysis of dynamic systems and the development of control algorithms from both geometrical and topological points of view. The fundamental theory of surfaces and differential manifolds endowed with Riemannian metrics is overviewed. The concepts of configuration manifolds (C-manifolds) and their immersions and embeddings are then introduced and applied to dynamic systems modeling. An explicit form of the smooth embedding for a given dynamic system with its C-manifold is derived. In an open serial-chain robotic system, a topological equivalence, i.e. a homeomorphism, is found and shown to be useful for dynamic model reduction. With topology being viewed as the structure of geometry, we discover and prove that the kinematics of a dynamic system determines its topology so that the kinematics is virtually a structure of the system's dynamics. This key point of view is further extended to the development of an adaptive control strategy. A computer simulation study is finally performed to verify the proposed model and adaptive control scheme.     

Continuum-Based Shape Sensitivity Analysis for 2D Coupled Atomistic/Continuum Simulations Using Bridging Scale Decomposition

In this paper, we propose the first attempt to perform shape sensitivity analysis for two-dimensional coupled atomistic and continuum problems using bridging scale decomposition. Based on a continuum variational formulation of the bridging scale, the sensitivity expressions are derived in a continuum setting using both direct differentiation method and adjoint variable method. To overcome the issue of discontinuity in shape design due to the discrete nature of the molecular dynamics (MD) simulation, we define design velocity fields in a way that the shape of the MD region does not change. Another major challenge is that the discrete finite element (FE) mass matrix in bridging scale is not continuous with respect to shape design variables. To address this issue, we assume an evenly distributed mass density when evaluating the material derivative of the FE mass matrix. In order to support accuracy verification of sensitivity results using overall finite difference method, we use regular-shaped finite elements and only allow shape change in one direction in our example problems, so that design perturbations can be made to the discrete FE mass matrix. However, the sensitivity formulation is sufficiently general to support irregular-shaped finite elements and arbitrary design velocity fields. The sensitivity analysis results, verified using overall finite difference method, reveal the impact of macroscopic shape design changes on microscopic atomistic responses.     

Asymmetrical effects in a 2D stenosis

An integral Interacting Boundary Layer theory is presented for the steady laminar flow in a asymmetrical bidimensional channel at high Reynolds number. The effect of asymmetry of the geometry is taken into account into the ideal fluid pressure expression. The effect is small but noticeable. Comparison with a Navier–Stokes solution shows the trend of asymmetry: increasing the pressure drop on the more curved wall, decreasing it on the other. Separation and reattachment of the boundary layer are obtained and compare well.     

Numerical Investigation of the Temperature Macroparameters in a Shock Wave in a Binary Gas Mixture Using the Kinetic Boltzmann Equation

The effect of the Mach number and the concentration and mass ratios on the behavior of the parallel, radial, and total temperatures of the components in a shock wave in a binary gas mixture is studied. The results obtained are compared with the theoretical and experimental results of other investigators.