This study focuses on analyzing the thermohydraulic characteristics and entropy generation (EG) in a streamlined pin–fin heat sink. Three different streamlined profiles were investigated to evaluate the effect of pin–fin shape. The profiles were compared based on both heat transfer and flow resistance. The tear-ellipse configuration exhibited the highest Nusselt number while maintaining a low friction factor by reducing the wake zone and enhancing flow mixing. Therefore, this configuration was selected for further detailed analysis of the thermohydraulic characteristics and EG. The effects of Reynolds number (Re = 8,558 − 17,115), pin–fin height (H = 42 − 48 mm), and pin–fin aspect ratio (e = 0.55 − 0.75) were analyzed. The results show that the Nusselt number increases and total EG decreases with increasing Re, decreasing e, or increasing H. Thermal EG predominates over frictional EG throughout the entire domain under investigation. Finally, RSM-based multi-objective optimization using NSGA-II is performed to maximize thermohydraulic performance (THP) and minimize entropy generation ratio (EGR), followed by LINMAP to identify the most suitable compromise solution from the Pareto front. The optimal result was identified at Re = 14,749.5, e = 0.55, and H = 48 mm, yielding predicted THP and EGR values of 1.4374 and 0.8297, respectively. Compared with the reference configuration, the numerical results show that Nusselt number increased by 34.8% and total EG (Stotal) decreased by 17.1%. These results provide useful insights for designing heat sinks with streamlined pin-fins that achieve high thermohydraulic performance and low irreversible characteristics.
The mixing layer growth characteristics are crucial for exploring the working mechanism of ejectors. This work carries out numerical simulations to investigate the effects of back pressure and nozzle position. The relationship between the mixing layer growth characteristics and the entrainment performance is investigated under the critical mode, and the mechanism behind the performance variation is analyzed. The entrainment performance and the mixing layer growth rate share the same trend. The back pressure does not alter the growth trajectory of the mixing layer. Shifting the nozzle exit position changes the structure of the mixing channel and directly affects the mixing layer growth rate. The continued mixing after flow choking is an important component that accounts for the total pressure loss. A higher mixing layer growth rate corresponds to more intense energy transfer, which matches to a higher mixed-flow sound speed and thereby increases the entrainment ratio. This study demonstrates that the mixing layer growth characteristics are closely linked to entrainment performance. The performance optimization is an external manifestation of mixing enhancement. Deepening the research on mixing layer characteristics can be an effective approach to designing high-efficiency ejectors.
This numerical study investigates the transition process in a Falkner–Skan–Cooke boundary layer on a homogeneously-roughened swept flat plate, with varying roughness wavelengths and heights. Using direct numerical simulations, we determine the onset and development of stationary crossflow vortices (CFVs) and their eventual breakdown via secondary instabilities (SIs). The results highlight the wavelength-selective nature of roughness-induced instability: roughness matching the most unstable mode for the stationary CFV triggers the earliest CFV development and collapse. In contrast, both longer and shorter wavelengths alter the growing mode, ultimately delaying the collapse of CFVs and the transition to turbulence. Through spectral and statistical analyses, we identify clear milestones—occurrences of CFVs and SIs, and collapse of CFVs—which correspond well with flow visualizations. While canonical SI types (Type-1, 2, 3) are observed on short-wavelength roughness, mid- and long-wavelength cases reveal atypical unsteady behavior lacking clear spectral peaks. The observed delay in transition due to wavelength mismatches would have practical relevance to turbulent transition control.
Urban air mobility (UAM) vehicles are gaining considerable attention because of their potential to transform future transportation systems. When a propeller operates near the ground, its wake impinges on the ground, creating complex flow structures. These interactions influence aerodynamic performance, acoustic emission, and unsteady blade loading. Therefore, this study investigated the aerodynamics and aeroacoustics of propeller–ground interactions through experimental measurements and numerical simulations. A twisted two-bladed rotor was used as the reference configuration for both approaches. Large eddy simulation (LES) was conducted to investigate the effects of rotor-ground distance on wake development and turbulent flow characteristics at four clearance ratios, defined as the ratio of rotor height above the ground to the rotor radius: 0.3, 1, 2, and 3. The computed pressure coefficients obtained from LES were validated against experimental data across all clearance ratios. The results revealed that propeller–ground distance significantly affects wake evolution and flow dynamics. At various clearance ratios, the tip vortex impingement location shifts and the tip jet flow becomes stronger, producing pronounced upwash and downwash regions beneath the rotor. These flow features substantially alter the near-field pressure distribution and enhance unsteady flow fluctuations. Additionally, far-field acoustic measurements indicated that noise level increases as the propeller approaches the ground, although the magnitude depends on the observer angle. Spectral analysis confirmed that the elevated noise is predominantly broadband in nature, with noticeable amplification at the blade passing frequencies.
Drop-in sustainable aviation fuels coupled with high overall pressure ratio gas turbine engines constitute a promising pathway toward near-term carbon neutrality in the aviation sector. Under typical high-power operating conditions, however, the pressure in the combustor exceeds the fuel’s critical pressure, so injection occurs in a transcritical regime. In this regime, the thermophysical and transport properties of the working fluid exhibit pronounced non-linear variations, which significantly influence the mixing processes. While the effects of pressure and temperature on transcritical injection are well established for cryogenic rocket propellants, substantial uncertainty persists regarding how surrogate fuel composition affects numerical predictions for sustainable aviation fuels. In this work, we first validate a numerical framework for representing transcritical, multicomponent aviation fuels by coupling OpenFOAM with Cantera. We then investigate the effect of the number of chemical species in SAF surrogates on turbulent transcritical mixing under aero-engine relevant conditions through the diffuse-interface large-eddy simulation approach. The study focuses on the alcohol-to-jet fuel POSF-11498, classified as a category C fuel within the National Jet Fuels Combustion Program. The results demonstrate that a two-component surrogate, designed to capture the dominant C12 and C16 highly branched iso-paraffins of POSF-11498, reproduces the behavior of a more detailed seven-component surrogate with no appreciable loss of accuracy, at less than half its computational cost. In contrast, a single-component representation departs appreciably from the multicomponent surrogates in the prediction of density, heat capacity and thermal conductivity. In all cases the mixing field itself is nearly surrogate-independent, so the composition sensitivity stems from the real-fluid thermophysical properties rather than from the mixing dynamics. The proposed numerical framework provides a versatile foundation for high-fidelity LES investigations of transcritical injection of sustainable aviation fuels.
Triply Periodic Minimal Surfaces (TPMS) are increasingly adopted as heat-transfer structures thanks to their smooth curvature, tunable porosity and high surface-to-volume ratio. In the present work, we focus on the Gyroid topology in laminar flow (20≤Re≤100) and develop a modified Reynolds analogy linking thermal and hydraulic behaviour through compact engineering correlations. Using CFD datasets obtained under both prescribed wall temperature and imposed wall heat-flux boundary conditions, we analyse the friction factor, Nusselt number and Stanton number over the porosity range 0.3≤φ≤0.7. A Darcy–Forchheimer interpretation of the hydraulic behaviour shows that the investigated operating window spans the transition from a predominantly Darcian regime to a mixed viscous–inertial regime while remaining entirely laminar. Three alternative forms of modified Reynolds analogy are calibrated, yielding correlations that reproduce the CFD data with residual deviations of 10%–15%. The resulting correlations are then employed for reduced-order thermo–hydraulic optimisation based on heat-transfer enhancement and pumping-power minimisation. Among the three formulations, only the correlation including a porosity-dependent Reynolds-number correction generates an internal Pareto front, highlighting the role of inertial transport in determining the optimal thermo–hydraulic trade-off. The proposed framework enables the prediction of thermal performance directly from hydraulic information and provides a reduced-order design tool for Gyroid-based heat-transfer applications.
