Search Results (194)

The transition toward carbon-neutral energy systems has accelerated interest in hydrogen-fueled combustion technologies, where efficient fuel–air mixing is essential for stable and clean combustion. In the present study, the mixing characteristics of under-expanded supersonic jets injected into a pressurized environment are numerically investigated using validated computational fluid dynamics simulations. Two nozzle configurations are examined: a straight nozzle and sudden-expansion nozzles with different expansion ratios and expansion locations. The governing compressible flow equations are solved using the rhoCentralFoam solver with the SST k–ω turbulence model. The numerical framework is validated against Sod’s shock tube solution and experimental data for under-expanded supersonic free jets. The results show that sudden-expansion nozzles significantly modify the shock-wave structure, jet penetration, and lateral spreading compared with the straight nozzle. Among the investigated configurations, nozzles with intermediate expansion-section lengths exhibited pronounced Mach-disk oscillations with a dominant frequency of approximately 10 kHz. The normalized supersonic core length decreased from 17.79 for the straight nozzle to 5.50 for the best-performing sudden-expansion configuration, while the normalized jet half-width increased from 0.82 to 1.70, indicating substantially enhanced mixing performance. The findings demonstrate that nozzle geometry strongly governs the trade-off between flow stability and mixing enhancement in high-pressure supersonic jets. Full article

A numerical study of shock wave propagation through multiple raindrops is presented using a density-based compressible two-phase flow solver coupled with a sharp-interface volume-of-fluid (VoF) method. The piecewise linear interface calculation (PLIC) approach is employed to reconstruct gas–liquid interfaces and capture droplet deformation during shock interaction. The numerical framework is first validated using a one-dimensional gas–liquid shock tube problem and a shock–helium bubble interaction benchmark. The method is then applied to investigate shock interactions with single, double, and multiple raindrops under compressible flow conditions. Numerical results show that complex wave structures, including shock reflection, diffraction, and wave interference, develop during shock propagation through raindrop fields. Interactions between neighboring droplets lead to local pressure amplification and non-uniform flow structures. Full article

Shock-induced gas/water interfacial instability is important in multiphase flow processes involving rapid deformation, mixing, and breakup. In this study, the evolution of shock-impacted gas/water interfaces was investigated using high-speed images from previously conducted shock-tube experiments and two-phase numerical simulations. Interface contours were extracted through digital image processing, and spatial Fourier analysis was used to describe the modal evolution of interfacial perturbations. A numerical model based on the VOSET interface-capturing method and the SST $k-\omega$ turbulence model was established, with the compressibility of both phases considered. A mode number–amplitude–time (K-L-t) diagnostic framework was proposed. The results show that this framework can distinguish the dominant stages associated with Richtmyer–Meshkov (RM), Rayleigh–Taylor (RT), and Kelvin–Helmholtz (KH) instabilities. In the double-liquid-column case, the downstream interface exhibits a delayed transition, which may be associated with shielding and wake interference. Increasing the shock Mach number accelerates modal growth and advances the transition times, whereas increasing the liquid-column diameter delays the instability evolution because of larger inertia. A modified RM dispersion equation incorporating compressibility and finite-thickness effects was further proposed, showing improved agreement with the CFD-extracted initial growth rates. Full article

Stiffened plate structures widely used in military aircraft are frequently subjected to severe shock environments, such as those generated by gunfire or explosive blasts, which can significantly compromise the integrity and reliability of onboard equipment and devices. Accurate characterization and prediction of the shock response, especially the damping behavior of such structures, remains a critical yet challenging problem in aeronautical engineering. This study presents an integrated experimental–numerical framework for analyzing the shock response and damping characteristics of representative stiffened plates under shock wave excitation. Controlled shock loading is applied using a shock tube, with real-time acceleration responses measured at multiple locations on both plain and rib-reinforced plates. A high-fidelity finite element model is developed, and three commonly used damping models—Rayleigh Damping, wave attenuation Model, and Maximum Loss Factor Model—are systematically evaluated. Damping parameters are identified through a Particle Swarm Optimization (PSO) algorithm, using the shock response spectrum (SRS) as the performance metric. Experimental results reveal that the incorporation of reinforcing ribs can reduce peak acceleration responses and significantly enhance the damping performance, particularly in the mid-to-high frequency range. The identified damping parameters further show that the maximum loss factor model provides superior agreement with experimental SRS data compared to traditional approaches. The proposed methodology offers a robust method for modeling damping behavior in stiffened plates, providing practical insights for the design of aircraft structures exposed to shock environments. Full article

In this study, a third-order meshless method is presented through adopting WENO-Z reconstruction as a substitute for traditional linear reconstruction. In order to achieve a third-order reconstruction of WENO-Z, the required three-point stencils are created by introducing ghost points on the lines through each pair of the central and satellite points of the meshless cloud. The flow variables of the ghost point are evaluated by a proposed interpolation technique, in which only available information associated with the cloud is utilized. Based on each resultant stencil of the ghost-central-satellite points, the WENO-Z is then implemented for computing the variables at the midpoints between the central and satellite points of the cloud. In this way, the resulting meshless method could be expected to be of third-order accuracy while obtaining an oscillation-free property. A series of typical model cases, including linear advection of sinusoid wave, convection of an isentropic vortex, and two well-known shock-tube problems, are selected to be simulated for validation. The expected third-order of accuracy and inherit ability of shock capturing are achieved regardless of whether the meshless points distributed are regular or irregular. In addition, a set of subsonic, transonic, and supersonic flows over aerodynamic bodies like single-and multi-element airfoils are also demonstrated for the compressible Euler equations, and obtained numerical results compare well with the reference data in the literature. Full article

To accurately describe the combustion process of ammonia/diesel dual-fuel, this paper develops a simplified kinetic mechanism for ammonia/diesel dual-fuel based on the decoupling method and a modular approach, comprising 212 components and 620 elementary reactions. The diesel component is represented by four components: n-heptane, n-hexadecane, isohexadecane, and α-methylnaphthalene. The mechanism was validated using shock tube experimental data. The results indicate that the developed mechanism can accurately predict key parameters such as ignition delay time and laminar flame speed under different ammonia-blending ratios, showing good agreement with experimental values. Single-component ignition delay prediction error ≤ 6%; laminar flame speed deviation error ≤ 2%; CFD validation metrics (e.g., peak cylinder pressure error within 1.35%) Furthermore, the mechanism was coupled with 3D CFD software to validate the cylinder pressure and heat release rates, using a six-cylinder, heavy-duty diesel engine with a bore of 114 mm, a stroke of 145 mm, a displacement of 8.9 L, and a compression ratio of 16.6 as the study subject. Based on the validation of the model and the feasibility of the mechanism, further studies were conducted on combustion and emission patterns under different load conditions and ammonia substitution rates. The results indicate that at low ammonia substitution rates, as the load decreases, the combustion rate slows down and thermal efficiency declines, while the indicated thermal efficiency first decreases and then increases; load primarily influences ignition and combustion processes by altering the thermodynamic state within the cylinder. At ammonia substitution rates of 20–60%, the heat release rate exhibits a “bimodal” pattern under different load conditions. NO, NO₂, and N₂O emissions first increase and then decrease with increasing ammonia substitution rate, peaking in the 40–60% range; CO₂ emissions gradually decrease as the ammonia substitution rate increases. Full article

Organic Rankine Cycles (ORCs) are widely recognized as an effective solution for waste heat recovery (WHR). However, the design and optimization of these systems must address the tradeoff between computational efficiency and the need to capture complex component behavior. This requires moving beyond purely energetic 0D modeling approaches to account for constructional, spatial, and operational constraints. This work presents a novel modeling framework with a specific focus on the expansion device. Radial inflow turbine stages are selected for their capability to achieve high pressure ratios while maintaining compactness and high efficiency. Heat exchangers follow a generic one-dimensional counterflow configuration, with a shell-and-tube geometry adopted for sizing purposes. The turbine stages are modeled by resolving several internal sections in order to capture local thermofluid dynamic conditions. The framework predicts turbine efficiency and incorporates a newly developed formulation for shock-induced losses, improving performance prediction under trans-sonic flow conditions. After validation against experimental data, the model is applied to a WHR system integrated with an internal combustion engine fueled by biofuels. The results highlight the existence of optimal operating conditions arising from competing physical mechanisms. The analysis also shows the transition from single-stage to two-stage turbine configurations at high pressure ratios and emphasizes the role of real gas effects in determining stage performance and optimal expansion distribution. The results of simulations carried out for three different working fluids (ethanol, toluene, and R1234ze(E)) highlight that the available mechanical power ranges from 10 to 22 kW for single-stage turbine configurations and from 24 to 36 kW for two-stage configurations, with total system volumes varying between approximately 600 and 9000 L. Among the working fluids considered here, ethanol provides the best overall performance for the present case study. Overall, the proposed approach provides a reliable and computationally efficient tool for the preliminary design and optimization of ORC-based WHR systems. Full article

Compression–torsion lattice structures (CTLS) exhibit coupled compressive–torsional deformation, yet their response under underwater shock loading remains to be further investigated. In this study, sandwich structures with CTLS cores were investigated through a combination of shock tube experiments, digital image correlation (DIC), and nonlinear finite element analysis. The underwater shock response and protective performance were evaluated based on rear-plate kinetic energy, central deflection, and plastic deformation. The results indicate that, at the same relative density, CTLS sandwich structures reduce the rear-plate kinetic energy by more than 42% and the peak deflection by 12.4%, compared with sandwich structures employing traditional straight lattice structures (TSLS). Under identical compressive stiffness, CTLS provide superior protective performance to TSLS, and this advantage becomes more pronounced with increasing ligament diameter. Furthermore, CTLS sandwich structures extend the tunable range of the core energy absorption ratio from 33–35% to 24–38%, reflecting enhanced flexibility in energy distribution within the structure. Full article

This article presents the design and detailed characterization of a new supersonic wind tunnel at the Aerospace Laboratory for Lasers, ElectroMagnetics, and Optics of Texas A&M University, tailored for optical diagnostic development and sub-scale fundamental compressible fluid dynamics research. A Ludwieg tube tunnel architecture was selected due to its robustness, versatility, and low operational costs. The tunnel consists of a 50-foot-long driver tube constructed from modular Tri-Clamp spools, a Mach 4 nozzle with 3 in. exit diameter configured as a free jet, and a fast-acting valve with 14 ms opening time for high-duty-cycle operation. Such construction proved to be a robust, compact, and affordable solution for academic applications. Characterization methods consisted of simultaneous high-speed dot-schlieren, total and static pressure measurements, and femtosecond laser electronic excitation tagging. Average flow velocity for the first steady-state test time was measured via FLEET at (668.0 ± 5.7) m/s. The Mach number was calculated based on the angles of the attached oblique shocks formed near the 30° cone model. Calculated Mach number was repeatable from run to run and had small oscillations near the average value of 3.96 ± 0.03. Based on the simultaneously measured velocity and Mach number, the static temperature was calculated to be between (68.6 ± 0.3) K and (66.3 ± 0.3) K throughout the 400 ms test time, completely defining the thermodynamic state of the generated freestream flow. Full article

A transient numerical framework incorporating dynamic mesh techniques was developed to simulate the launch process. On this basis, a thermal–fluid–structural multi-physics coupling paradigm was proposed to interpret the evolution of the flow field and the associated load response throughout the entire firing sequence. The results show that flow development follows a multi-stage dynamic pattern, comprising gas-impact filling, gap-jet formation, and subsequent free-jet expansion. A pronounced spatially heterogeneous phase lag was observed in the pressure–Mach number response. This phenomenon arises from a mismatch among the characteristic time scales of pressure-wave propagation, flow inertia, and shock–boundary-layer interaction. Quantitative analysis further indicates that the spatial superposition of high-temperature zones, high-Mach regions, and elevated-pressure areas activates a thermal–fluid–structural positive-feedback loop that drives the local peak temperature to approximately 2.5 × 10³ K. The temperature response lags behind the pressure maximum by approximately 30–50 ms, reflecting the governing time scale of thermal inertia. In addition, vortical structures near the tube base account for nearly 15% of the total thrust. These findings provide a theoretical foundation for predicting transient peak loads in concentric cylindrical systems and for optimizing instantaneous thermal protection strategies. Full article

Ammonia is considered as a promising hydrogen carrier and a carbon-free fuel. Methods for improving ammonia combustion characteristics often involve its co-firing with more reactive fuels (natural gas, biofuels, etc.). Among the natural gas components, ethane is second most abundant. Therefore, the development of detailed chemical–kinetic mechanisms that accurately consider the interactions between ammonia and each component of natural gas is very important. Such mechanisms must be based on experimental data obtained under a wide range of conditions. In this work, NH₃/C₂H₆/O₂/Ar blends were studied in JSR (φ = 0.5–2.0, p = 1 atm, τ = 1 s, T = 800–1300 K) and in a shock tube (p = 7.3–8.6 atm, T = 1260–1590 K). Additionally, the structure of premixed flames was investigated (φ = 0.8–1.2, p = 1–5 atm). Eleven recently published detailed chemical–kinetic mechanisms were tested. The model Shrestha-2025 was updated to achieve better agreement with the entire set of experimental data. The effect of p and φ on intermediate species concentration was analyzed. Ammonia and ethane consumption pathways were also examined. Full article

Experiments on hypersonic boundary-layer instability of a fin–cone configuration were conducted in a Φ 0.5 m Mach 6 Ludwieg tube tunnel. Infrared thermography and high-frequency pressure sensors were used to measure the transition front and instability waves under four different nose bluntness conditions. On the leeward surface, transition is delayed near the centerline due to expansion waves generated by the double-cone structure. The region close to the corner is strongly influenced by the horseshoe vortex, whereas instability waves around 110 kHz manifest as the flow moves away from it. In contrast, transition on the windward surface occurs earlier and broadband high-frequency instability waves of 160–300 kHz are present near the corner. Increasing nose bluntness strongly suppresses transition away from the fin root, especially near the centerline and on the fin-off cone side, but has a relatively limited impact on the shock-interaction regions near the fin–cone corner. Transition on the fin surface remains insensitive to nose bluntness variations. This work elucidates the distinct transition behaviors across different regions of a complex fin–cone configuration and their differential responses to nose bluntness, providing valuable insights for the aerodynamic design and transition prediction of hypersonic vehicles. Full article

To improve the seismic performance of buildings and reduce earthquake-related disaster risks, this study employs the MIDAS finite element analysis platform to establish a numerical model of a 15-story concrete-filled steel tube frame-shear wall structure. Recorded natural ground motion data are used as the primary input, and a main shock-aftershock sequence is constructed using an attenuation-based method. On this basis, a seismic fragility analysis framework is adopted to derive structural fragility curves, which are subsequently assembled into a comprehensive seismic fragility matrix. The results indicate that, under identical main shock-aftershock sequences, aftershock effects increase the collapse probability of the unretrofitted structure by approximately 17–37%. Furthermore, when buckling-restrained braces are introduced, the structural strength at the same damage state increases by about 8% under the action of the main shock alone and by nearly 24% when both the main shock and aftershocks are considered. Full article

This work investigates how the vehicle-to-tube suspension gap governs compressible flow physics and operating margins in Hyperloop-class transport at 10 kPa. To our knowledge, this is the first study to apply adjoint aerodynamic optimization to mitigate gap-induced choking and shock formation in a full pod–tube configuration. Using a steady, pressure-based Reynolds-averaged Navier-Stokes (RANS) framework with the GEnerlaized K-Omega (GEKO) turbulence model, a simulation for the cruise conditions was performed at M = 0.5–0.7 with a mesh-verified analysis (medium grid within 0.59% of fine) to quantify gap effects on forces and wave propagation. For small gaps, the baseline pod triggers oblique shocks and a near-Kantrowitz condition with elevated drag and lift. An adjoint shape update—primarily refining the aft geometry under a thrust-equilibrium constraint—achieves 27.5% drag reduction, delays the onset of choking by ~70%, and reduces the critical gap from d/D ≈ 0.025 to ≈0.008 at M = 0.7. The optimized configuration restores a largely subcritical passage, suppressing normal-shock formation and improving gap tolerance. Because propulsive power at fixed cruise scales with drag, these aerodynamic gains directly translate into operating-power reductions while enabling smaller gaps that can relax tube-diameter and suspension mass requirements. The results provide a gap-aware optimization pathway for Hyperloop pods and a compact design rule-of-thumb to avoid choking while minimizing power. Full article

With the rapid expansion of electric vehicles, the risk of battery fires has become a critical safety concern. Conventional suppression methods, such as submerging battery packs in large water tanks, are inefficient due to long response times and potential secondary hazards. This study introduces a polymer-based fire suppression tube system that automatically activates under specific conditions. The system utilizes energy from a C4 explosion to rupture the tube, rapidly releasing the extinguishing agents stored inside. Explicit dynamics simulations in ANSYS Workbench 2024 R2 were conducted by varying tube thickness from 0.5 mm to 2.0 mm to evaluate the structural response of adjacent components. Three indices were examined: total deformation, deformation of the adjacent plate, and deformation of the tube itself. The results showed that thinner tubes (0.5 mm) allowed for greater propagation of blast energy, increasing the risk of damage, whereas thicker tubes (≥1.5 mm) effectively confined the explosive energy and reduced shock transmission. These findings confirm that tube thickness is a key parameter governing blast-induced deformation, with 1.5 mm identified as the threshold for minimizing structural damage. This study provides practical design guidelines for polymer-based automatic suppression systems, contributing to safer fire protection solutions for electric vehicles and related industrial applications. Full article