Time-resolved Velocity Research Articles

Modeling the interplay between the shear layer and leading edge suction during dynamic stall

The dynamic stall development on a pitching airfoil at Re = 106 was investigated by time-resolved surface pressure and velocity field measurements. Two stages were identified in the dynamic stall development based on the shear layer evolution. In the first stage, the flow detaches from the trailing edge and the separation point moves gradually upstream. The second stage is characterized by the roll up of the shear layer into a large scale dynamic stall vortex. The two-stage dynamic stall development was independently confirmed by global velocity field and local surface pressure measurements around the leading edge. The leading edge pressure signals were combined into a single leading edge suction parameter. We developed an improved model of the leading edge suction parameter based on thin airfoil theory that links the evolution of the leading edge suction and the shear layer growth during stall development. The shear layer development leads to a change in the effective camber and the effective angle of attack. By taking into account this twofold influence, the model accurately predicts the value and timing of the maximum leading edge suction on a pitching airfoil. The evolution of the experimentally obtained leading edge suction was further analyzed for various sinusoidal motions revealing an increase in the critical value of the leading edge suction parameter with increasing pitch unsteadiness. The characteristic dynamic stall delay decreases with increasing unsteadiness, and the dynamic stall onset is best assessed by critical values of the circulation and the shear layer height which are motion independent.

Spray Characteristics with High-Speed Tomographic Particle Image Velocimetry Under Non-Flash and Flash Boiling Conditions

Compared with port fuel injection engines, direct injection (DI) gasoline engine is becoming the mainstream of gasoline engines because of its higher fuel economy and excellent transient response. It has been proven that fuel spray characteristics in DI engines are crucial to the performance and emission quality of the engine. Flash boiling spray has great potential to achieve high fuel economy and low emission by dramatically improving the fuel atomization and vaporization and it has different spray-air interaction behavior as compared with non-flash boiling one, while its mechanism is more complex as compared with subcooled spray. We investigate the time-resolved spatial velocity field of the spray using 2-camera high-speed 3D3C (3-dimension 3-component) tomographic particle image velocimetry (PIV) diagnostic technique. A 10mm thick laser sheet is used to illuminate the fuel spray. Characteristics of both non-flash and flash boiling sprays are studied. A single-hole injector is mounted within a heat exchanger so that different fuel temperature can be accessed. In the experiment, n-pentane is used as the fuel. For the non-flash boiling spray, the velocity field of the liquid spray is mostly consistent to the injection direction. With the increase of the degree of superheat (DoS), the overall velocity scale decreases especially at the spray tip. Meanwhile, larger swirls occur at the lower part of the flash boiling spray, which means stronger spray-air interaction occurs at a higher DoS.

Relaxation Dynamics and Genuine Properties of the Solvated Electron in Neutral Water Clusters.

We have investigated the solvation dynamics and the genuine binding energy and photoemission anisotropy of the solvated electron in neutral water clusters with a combination of time-resolved photoelectron velocity map imaging and electron scattering simulations. The dynamics was probed with a UV probe pulse following above-band-gap excitation by an EUV pump pulse. The solvation dynamics is completed within about 2 ps. Only a single band is observed in the spectra, with no indication for isomers with distinct binding energies. Data analysis with an electron scattering model reveals a genuine binding energy in the range of 3.55–3.85 eV and a genuine anisotropy parameter in the range of 0.51–0.66 for the ground-state hydrated electron. All of these observations coincide with those for liquid bulk, which is rather unexpected for an average cluster size of 300 molecules.

Time-Volume Estimation of Velocity Fields From Nonsynchronous Planar Measurements Using Linear Stochastic Estimation

The work presented in this paper combines multiple nonsynchronous planar measurements to reconstruct an estimate of a synchronous, instantaneous flow field of the whole measurement set. Temporal information is retained through the linear stochastic estimation (LSE) technique. The technique is described, applied, and validated with a simplified combustor and fuel swirl nozzles (FSN) geometry flow for which three-component, three-dimensional (3C3D) flow information is available. Using the 3C3D dataset, multiple virtual “planes” may be extracted to emulate single planar particle image velocimetry (PIV) measurements and produce the correlations required for LSE. In this example, multiple parallel planes are synchronized with a single perpendicular plane that intersects each of them. As the underlying dataset is known, it therefore can be directly compared to the estimated velocity field for validation purposes. The work shows that when the input time-resolved planar velocity measurements are first proper orthogonal decomposition (POD) filtered, high correlation between the estimations and the validation velocity volumes are possible. This results in estimated full volume velocity distributions, which are available at the same time instance as the input field—i.e., a time-resolved velocity estimation at the frequency of the single input plane. While 3C3D information is used in the presented work, this is necessary only for validation; in true application, planar technique would be used. The study concludes that provided the number of sensors used for input LSE exceeds the number of POD modes used for prefiltering, it is possible to achieve correlation greater than 99%.

Laser-shock compression experiment on magnesium hydride

A dynamic high-pressure experiment was performed on magnesium hydride to measure the Hugoniot equation-of-state point using laser-driven shock waves. By measuring the mean shock velocity of MgH2 and the time-resolved shock velocity of the quartz reference, the Hugoniot state of MgH2 at 150 GPa was determined. The reliability of the data is ensured by using another reference material, polystyrene, which is next to the sample and is compressed by the same. This work validates the experimental methodology to obtain further high-pressure data of MgH2 with high accuracy.

Time-resolved turbulent velocity field reconstruction using a long short-term memory (LSTM)-based artificial intelligence framework

This paper focuses on the time-resolved turbulent flow reconstruction from discrete point measurements and non-time-resolved (non-TR) particle image velocimetry (PIV) measurements using an artificial intelligence framework based on long short-term memory (LSTM). To this end, an LSTM-based proper orthogonal decomposition (POD) model is proposed to establish the relationship between velocity signals and time-varying POD coefficients obtained from non-TR-PIV measurements. An inverted flag flow at Re = 6200 was experimentally measured using TR-PIV at a sampling rate of 2000 Hz for the construction of training and testing datasets and for validation. Two different time-step configurations were employed to investigate the robustness and learning ability of the LSTM-based POD model: a single-time-step structure and a multi-time-step structure. The results demonstrate that the LSTM-based POD model has great potential for time-series reconstruction since it can successfully recover the temporal revolution of POD coefficients with remarkable accuracy, even in high-order POD modes. The time-resolved flow fields can be reconstructed well using coefficients obtained from the proposed model. In addition, a relative error reconstruction analysis was conducted to compare the performance of different time-step configurations further, and the results demonstrated that the POD model with multi-time-step structure provided better reconstruction of the flow fields.

Direct Observations of Particle Dynamics in Magnetized Collisionless Shock Precursors in Laser-Produced Plasmas.

We present the first laboratory observations of time-resolved electron and ion velocity distributions in magnetized collisionless shock precursors. Thomson scattering of a probe laser beam was used to observe the interaction of a laser-driven, supersonic piston plasma expanding through an ambient plasma in an external magnetic field. From the Thomson-scattered spectra we measure time-resolved profiles of electron density, temperature, and ion flow speed, as well as spatially resolved magnetic fields from proton radiography. We observe direct evidence of the coupling between piston and ambient plasmas, including the acceleration of ambient ions driven by magnetic and pressure gradient electric fields, and deformation of the piston ion flow, key steps in the formation of magnetized collisionless shocks. Even before a shock has fully formed, we observe strong density compressions and electron heating associated with the pileup of piston ions. The results demonstrate that laboratory experiments can probe particle velocity distributions relevant to collisionless shocks, and can complement, and in some cases overcome, the limitations of similar measurements undertaken by spacecraft missions.

Stokes-layer formation under absence of moving parts—A novel oscillatory plasma actuator design for turbulent drag reduction

A novel plasma actuator concept is proposed to mimic the effect of spanwise wall oscillations without mechanically moving parts, where four groups of electrodes and three independently operated high-voltage power supplies maintain a pulsatile dielectric barrier discharge (DBD) array. Time-resolved planar velocity fields are obtained with high-speed particle image velocimetry (PIV) in proximity of the discharge zones for quiescent ambient conditions. Resulting flow topologies and wall-normal velocity profiles indicate the Stokes-layer-like flow formation, which is elevated above the wall due to the no-slip condition. The underlying body forces are derived from the PIV data to provide further insight into cause-effect relations between pulsatile discharge and oscillatory flow. The momentum transfer domain is found to be only interrupted with the width of the exposed electrode, which is an important step toward homogeneous virtual wall oscillations. A comparison with earlier studies by Gatti et al. [“Experimental assessment of spanwise-oscillating dielectric electroactive surfaces for turbulent drag reduction in an air channel flow,” Exp. Fluids 56, 110 (2015)] leads to the hypothesis that DBD-based turbulent drag reduction might be a competing alternative to conventional active and passive shear-layer formation strategies, where the adjustability of both oscillation frequency and velocity amplitude might cover a wide range of Reynolds numbers.

GPU-based, parallel-line, omni-directional integration of measured pressure gradient field to obtain the 3D pressure distribution

The introduction of 3D time-resolved velocity measurement techniques enables calculation of the instantaneous pressure distribution by spatially integrating the material acceleration. This paper introduces an efficient method for 3D integration of the acceleration, which does not require prescribed Dirichlet boundary condition on one of the surfaces, minimizes the propagation of errors in acceleration, and can be easily utilized in flows with complex boundaries. This parallel-line, omni-directional integration procedure (Omni3D) calculates the pressure at every point by integration from all directions, while avoiding regions with large acceleration errors. To reduce the computational costs, the calculations are performed by a GPU-based algorithm, which determines the 3D pressure field from tomographic PIV data in 1 min. The accuracy of Omni3D is compared to that of several techniques, including procedures based on solving the Pressure Poisson Equation (PPE) with different Dirichlet boundary conditions. The error analysis is based on Direct Numerical Simulation (DNS) data for isotropic turbulence, synthetic 3D PIV images for turbulent channel flow generated from DNS data, and experimental data. It examines the effects of spatial resolution, propagation, and avoidance of embedded local errors, boundary conditions, method for calculating the velocity, as well as viscous and sub-grid stresses on the calculated pressures. For acceleration fields with low errors and properly specified boundary conditions, Omni3D and PPE give similar results. However, Omni3D is more effective in suppressing the effects of acceleration errors. Sample experimental results including instantaneous plot of pressure, pressure statistics, and pressure–velocity correlations based on tomographic PIV data are also provided.

Experimental investigation on a high head Francis turbine model during shutdown operation

Increased penetration of intermittent energy resources disturbs the power grid network. The frequency band of the power grid is normally controlled by automatic opening and closing of the guide vanes of hydraulic turbines. This has increased the number of shutdown cycles as compared to the defined ones for the normal operation of turbines. Turbine shutdown induced a significantly higher level of pressure fluctuations and unsteadiness in the flow field, decreasing its expected life. This paper presents experiments performed on a high head model Francis turbine during shutdown. The pressure and 2D Particle Image Velocimetry (PIV) measurements were performed to investigate the pressure fluctuations and flow instabilities in the turbine. The pressure sensors were mounted in the draft tube cone and vaneless space to measure the instantaneous pressure fluctuations. In the present study, the initial high load operating condition was considered to perform the turbine shutdown. The data were logged at the sampling frequency of 40 Hz and 5 kHz for PIV and pressure measurements, respectively. Time-resolved velocity and pressure data are presented in this paper to show the pressure fluctuations and causes of generation of unsteady flow in the draft tube.

Abstract WMP33: Towards Cerebral Aneurysm Rupture Risk Prediction Using Quantitative Analysis by 4D Flow MRI: Intra-Aneurysmal Vortical Blood Flow and Association to Wall Shear Stress

Introduction: While previous studies have shown a link between changes in aneurysmal wall shear stress (WSS) and risk of rupture, the association between the development of WSS variations and intra-aneurysmal hemodynamic flow patterns remain unknown. Understanding such link could potentially lead to novel early biomarkers for risk stratification of aneurysmal rupture. Aim: To assess the association between intra-aneurysmal flow patterns behavior and WSS in cerebral aneurysm patients. Methods: Time-resolved 3D blood flow velocities were acquired by dual-venc 4D flow MRI (kt-GRAPPA R=5, voxel size (1.0x 1.0 x 1.0 mm 3 ), TE/TR 3.5/6.2 ms, temporal resolution 57ms, venc low /venc high 50/100 cm/s) in 12 patients (66 ± 9 years, 8 F), who presented with a total of 14 cerebral aneurysms (average 10-time points per aneurysm synchronized with the cardiac cycle). Time-resolved median WSS was quantified over the 3D aneurysm surface. To quantify 3D blood flow pattern behavior from 4D Flow MRI, we used the fluid dynamics-based helicity metric. Helicity is a signed quantity that measures the strength and alignment between vortical pattern and blood flow directions. Helicity magnitudes take positive values at regions where both vortical pattern and blood flow align in the same direction while regions with opposing directions have negative values. Time-resolved positive and negative helicity were quantified separately by instantaneous volumetric median and correlated with WSS. Correlation was tested using pooled paired data of all acquired time points from all aneurysms (N=147 samples). Results: (Fig.1) Strong correlations were found between WSS and flow patterns behavior with the strongest correlation (R=0.91, p<0.001) found with negative helicity. Conclusions: We revealed that intra-aneurismal blood flow pattern behavior strongly correlates with wall shear stress. These results may help in the future to identify early hemodynamic markers for risk of aneurysmal rupture.

Abstract TMP108: Evaluation of Cerebral Arteriovenous Malformations using Flow Distribution Network Graphs and Dual-VENC 4D Flow MRI

Introduction: Cerebral arteriovenous malformation (AVM) is the most common cause of hemorrhagic stroke in young adults. AVM affects the total blood flow to the brain as well as the extranidal flow distribution, yet quantitative data relating AVM hemodynamic parameters to hemorrhage risk are lacking. 4D Flow MRI provides time-resolved 3D blood flow velocity with full volumetric coverage of principal arteries and veins. This study aims to investigate the impact of anatomical differences in vessel connectivity on blood flow, and to identify potential quantitative biomarkers for AVM stroke risk stratification. Methods: 4D flow MRI was obtained for healthy controls (n = 26) and patients with unruptured AVM (n = 5, Fig. 1A). A semiautomated in-house tool was used to compute blood flow and peak velocity at multiple points per vessel from 4D flow data (Fig. 1B). Results and vessel connectivity are schematically summarized as a Flow Distribution Network Graph (FDNG) for group comparison and secondary metrics. Results: In the healthy cohort, we identified 3 main COW variants: A (complete COW, n = 13), B (no RPCOM, n = 5) and C (both PCOMs missing, n = 4). Group comparison FDNGs (Fig. 1C) show significant differences between groups A and B in RICA flow (p = 0.026) and right A/P flow ratio (p = 0.036). AVMs were characterized by total cerebral blood flow and flow diversion to the nidus. AVM draining veins had significantly higher peak velocity (p = 0.032) relative to feeding arteries than non-draining veins relative to non-feeding arteries and control venous/arterial ratios (p = 0.002, Fig. 1D). Conclusions: Network analysis of 4D flow MRI data can identify significant differences in flow distribution between normal variant groups and provide standardized metrics of AVM hemodynamics, including peak velocity, which specifically distinguishes AVM nidal flow. These metrics account for individual vascular network variation and may provide a path forward for risk stratification.

Multiscale Vortex Characteristics of Dynamic Stall from Empirical Mode Decomposition

The multiscale development of dynamic stall was studied using a scale-based modal analysis technique. Time-resolved velocity field data around an airfoil during dynamic stall were used for this analysis, with experimental conditions corresponding to and . Intrinsic mode functions were determined by a multivariate, multidimensional empirical mode decomposition method, which ensures mode alignment between velocity components and modal dependency on the full spatiotemporal flowfield. Individual intrinsic mode functions were linked to physically significant phenomena in the flowfield, including the formation of vortical structures due to the Kelvin–Helmholtz instability, interactions between shear-layer vortices leading to vortex pairing, and ultimately shear-layer roll-up and the formation of the dynamic stall vortex. In addition to qualitatively describing these flow structures and interactions, their characteristic length and time scales within the intrinsic mode functions were quantified. These quantitative measures were found to be consistent with time and length scales reported in the literature on experimental investigations of canonical fluid dynamic processes.

Pressure from 2D snapshot PIV

In this study, we quantify the accuracy of a simple pressure estimation method from 2D snapshot PIV in attached and separated flows. Particle image velocimetry (PIV) offers the possibility to acquire a field of pressure instead of point measurements. Multiple methods may be used to obtain pressure from PIV measurements, however, the current state-of-the-art requires expensive equipment and data processing. As an alternative, we aim to quantify the efficacy of estimating instantaneous pressure from snapshot (non-time resolved) two-dimensional planar PIV (the simplest type of PIV available). To make up for the loss of temporal information, we rely on Taylor’s hypothesis (TH) to replace temporal information with spatial gradients. Application of our approach to high-resolution 2D velocity data of a turbulent boundary layer flow over ribs shows moderate to good agreement with reference pressure measurements in average and fluctuations. To assess the performance of the 2D TH method beyond average and fluctuation statistics, we acquired a time-resolved measurement of the same flow and determined temporal correlation values of the pressure from our method with reference measurements. Overall, the correlation attains good values for all measured locations. For comparison, we also applied two time-resolved approaches, which attained values of correlation similar to our approach. The performance of the 2D TH method is further assessed on 3D time-resolved velocity data for a turbulent boundary layer and compared with 3D methods. The root-mean-square (RMS) pressure fluctuations of the 2D TH, 3D TH and 3D pseudo-Lagrangian methods closely follow the pressure fluctuation distribution from DNS. These observations on the RMS pressure estimates are further supported by similar analysis on synthetic PIV data (based on DNS) of a turbulent channel flow. The values of spatial correlation between the 2D TH method and the DNS pressure fields in this case, are similar to the temporal correlations achieved in the turbulent flow over the ribs. Finally, we discuss the accuracy of instantaneous pressure estimates and provide a rule of thumb to determine regions where the pressure fluctuation estimate from the 2D TH methods is likely to fail.Graphical abstract

Time-resolved droplet size and velocity distributions in a dilute region of a high-pressure pulsed diesel spray

The principal objective of the present work is to experimentally investigate the time-resolved spray characteristics. Droplets data in a dilute region of a high-pressure diesel spray were obtained by employing the PDIA (particle/droplet image analysis) method with double pulse laser illumination. Eulerian specification was adopted to describe the transient spray microscopic behavior within a fixed test window at ten representative instants after triggering the injection. The experimental results indicate that the transient spray evolution can be characterized in three distinct stages. During stage I (the latency stage), the spray tip develops and reaches the test location, but no discrete droplets were clearly observed. During stage II, the spray passes through the test window, from which discrete droplets are observed to disperse in the dilute region and the SMD of the droplets increases rapidly. During stage III, bulk spray disappears in the test window and only discrete droplets were observed. The SMD of the droplets is decreased to a “steady-state” value. Based on these sampled and processed data, the droplet size distribution functions were obtained and compared with the Rosin-Rammler and the Nukiyama-Tanasawa function. Finally, the Stokes number and the Weber number of each droplet were determined, and the movement of the droplets were analyzed. The measured time dependent droplet behaviors are believed to provide important data for spray modeling and simulation.

Femtosecond predissociation dynamics of ethyl iodide in the B-band.

Femtosecond time-resolved velocity map ion imaging experiments are reported on the second absorption band (B-band) of ethyl iodide at 201.19 and 200.08 nm, corresponding to the 000 and 1810 transitions, i.e., the origin of the band and the first most intense vibronic state assigned to one quantum of excitation in the methyl torsion mode. Electronic predissociation lifetimes and the temporal evolution of the anisotropy have been determined by time-resolved resonance-enhanced multiphoton ionization of iodine and ethyl fragment images. A shorter lifetime measured at the origin of the band in comparison with methyl iodide indicates that predissociation in ethyl iodide is more favorable due to a stronger coupling between the initial Rydberg state and the valence repulsive state correlating with the dissociation fragments. Moreover, vibrational activity in the methyl torsion in the Rydberg state seems to enhance the probability of transfer of population to the valence repulsive state leading to a faster dissociation. The perpendicular character of the transition at early times and the loss of anisotropy as a function of time have been determined from the time-resolved angular distributions of the iodine and ethyl ion images. The initial anisotropy value is consistent with a purely perpendicular transition compatible with the excitation of the [6A'', 7A'] states with a minor parallel component to the C-I bond. The loss of initial anisotropy over time highlights the parent molecular rotation during predissociation and is compatible with a rotational temperature of the parent molecule of 100 K.

Higher Pulsatility in Cerebral Perforating Arteries in Patients With Small Vessel Disease Related Stroke, a 7T MRI Study.

Background and Purpose- Cerebral small vessel disease (SVD) is a major cause of stroke and dementia, but underlying disease mechanisms are still largely unknown, partly because of the difficulty in assessing small vessel function in vivo. We developed a method to measure blood flow velocity pulsatility in perforating arteries in the basal ganglia and semioval center. We aimed to determine whether this novel method could detect functional abnormalities at the level of the small vessels in patients with stroke attributable to SVD. Methods- We investigated 10 patients with lacunar infarction (mean age 61 years, 80% men), 11 patients with deep intracerebral hemorrhage (ICH) considered to be caused by SVD (ICH, mean age 58 years, 82% men) and 18 healthy controls that were age- and sex-matched. We performed 2-dimensional phase contrast magnetic resonance imaging at 7 T to measure time-resolved blood flow velocity in cerebral perforating arteries of the semioval center and the basal ganglia. We compared the number of detected arteries, pulsatility index and mean velocity between the patient groups and controls. Results- In the basal ganglia, the number of detected perforators was lower in lacunar infarction (26±9, P=0.01) and deep ICH patients (28±6, P=0.02) than in controls (35±7). The pulsatility index in the basal ganglia was higher in lacunar infarction (1.07±0.13, P=0.03), and deep ICH patients (1.02±0.11, P=0.11), than in controls (0.94±0.10). Observations in the semioval center were similar. Number of detected perforators was lower in lacunar infarction (32±18, P=0.06), and deep ICH patients (28±18, P=0.02), than in controls (45±16). The pulsatility index was higher in lacunar infarction (1.18±0.15, P=0.02), and deep ICH patients (1.17±0.14, P=0.045) than in controls (1.08±0.07). No velocity differences were detected. Conclusions- This exploratory study shows that SVD can be expressed in terms of functional measures, such as pulsatility index, which are derived directly from the small vessels themselves. Future studies may use this technique to further unravel the mechanisms underlying SVD.

Flow MR fingerprinting.

To investigate the feasibility to quantify blood velocities within the magnetic resonance fingerprinting framework, while providing relaxometric maps of static tissue. Bipolar gradients are inserted into an SSFP-based MRF sequence to achieve velocity-dependent signal phases, allowing tri-directional time-resolved velocity component quantification. The accuracy of both relaxometric mapping and velocity quantification was validated in vivo and in phantom studies. Simulations determined that even for strong cardiac cycle length variations (700-1400 ms) Flow-MRF determines accurate velocity maps deviating <0.1% from the ground truth on average. The cardiac cycle length variability only results in reduced velocity-to-noise ratios. Good agreement in the velocity quantification between a standard phase-contrast cine and the Flow-MRF sequence was reached in phantom experiments. Relaxometric phantom experiments determined mean deviations between Flow-MRF and spin-echo-based reference measurements of 89 ± 25 ms / 0.8 ± 2.5 ms over the range of 630-2630 ms / 49-145 ms for T1 / T2 , respectively. The in vivo study of a human knee determined mean T1 / T2 values of 1383 ± 75 ms / 26 ± 4 ms for the gastrocnemius muscle that agree with literature values. Flow-MRF presents a novel way of quantifying velocities while simultaneously providing relaxometric maps of static tissue and it can potentially be a viable method to accelerate the inherently long acquisition times of time-resolved velocity quantification.

Validated simulations of dynamic crack propagation in single crystals using EFEM and XFEM

Brittle and quasibrittle materials such as ceramics and geomaterials fail through dynamic crack propagation during impact events. Simulations of such events are important in a number of applications. This paper compares the effectiveness of the embedded finite element method (EFEM) and the extended finite element method (XFEM) in modeling dynamic crack propagation by validating each approach against an impact experiment performed on single crystal quartz together with in-situ imaging of the dynamic fracture using X-ray phase contrast imaging (XPCI). The experiment is conducted in a Kolsky bar (generating a strain rate on the order of $$10^3\,\text {s}^{-1}$$ ) that is operated at the synchrotron facilities at the advanced photon source (APS). The in situ XPCI technique can record the dynamic crack propagation with micron-scale spatial resolution and sub-microsecond temporal resolution, and the corresponding images are used to extract the time-resolved crack propagation path and velocity. A unified framework is first presented for the dynamic discretization formulations of EFEM and XFEM. This framework clarifies the differences between the two methods in enrichment techniques and numerical solution schemes. In both cases, a cohesive law is used to describe the fracture process after crack initiation. The simulations of the dynamic fracture experiment using the two simulation approaches are compared with the in situ experimental observations and measurements. The performance of each method is discussed with respect to capturing the early crack propagation process.

The effects of cross-flow fuel injection on the reacting jet in vitiated cross-flow

The effects of cross-flow fuel injection on a slotted jet flame consisting of an ethylene-air premixture are investigated experimentally. Cross-flow conditions of 900 K and 100 m/s were chosen to closely simulate the environment of a secondary combustor in a staged combustion system. It was found that increasing the cross-flow equivalence ratio (Φ∞) requires a consequent reduction in the jet equivalence ratio (Φj) for jet flame stabilization in order to avoid the formation of locally rich mixtures beyond the flammability limits of the flame. Stable flames were achieved for a low cross-flow equivalence ratio of Φ∞ = 0.4 across a range of jet equivalence ratio values and momentum flux ratios, demonstrating the ability of the transverse jet to extend the flammability limits of the cross-flow mixture. Significantly, as Φ∞ is increased beyond a certain point, no ethylene is required to be present in the jet mixture, and a jet consisting only of air is able to stabilize the flame. Due to the fluidic nature of the flame stabilization mechanism of these flames, they are dubbed fluidically stabilized flames (FSF). OH* chemiluminescence and high-speed particle image velocimetry were utilized to gain deeper understanding of the flame behavior and flow field features of the FSF. In contrast to bluff-body stabilized flames, it was found that the FSF provides increased control of the flame shape, with increasing flame width and penetration for higher jet momentum flux ratios (J). The FSF was also demonstrated to be a highly dynamical phenomenon, characterized by a dominant peak frequency that is dependent on both Φj and Φ∞. Proper orthogonal decomposition of the time-resolved velocity fields shows that heat release affects the dynamics of the FSF in a similar manner to the reacting jet in cross-flow (RJICF), as demonstrated in a previous study. Finally, the flame behavior was found to be highly dependent on the cross-flow fueling mechanism, with coherent flame oscillations present when the fuel injection point is closely-coupled to the flame stabilization location.