Search Results (137)

Reliable prediction of tunnel thruster performance under reverse, or off-design, reverse operating direction (ROD) conditions, is crucial for modern vessels that require bidirectional thrust from a single unit—such as yachts and offshore support vessels. Despite the increasing demand for such a capability, there remains limited understanding of the unsteady hydrodynamic behavior and performance implications of ROD operation. This study addresses this gap through a scale-resolving computational fluid dynamics (CFD) investigation of a full-scale, fixed-pitch propeller with a diameter of 0.62, installed in a tunnel geometry representative of yacht-class side thrusters. Using advanced turbulence modeling, we compare the thruster’s performance under both the normal operating direction (NOD) and ROD. The results reveal notable differences: in ROD, the upstream separation zone was more compact and elongated, average thrust increases by approximately 3–4%, and torque and pressure fluctuations rise by 15–30%. These findings demonstrate that a single tunnel thruster can meet bidirectional manoeuvring requirements. However, the significantly elevated unsteady loads during ROD operation offer a plausible explanation for the increased noise and vibration frequently observed in practice. Full article

Three-dimensional (3D) computational fluid dynamic (CFD) simulations have been performed to investigate the complex flow features and stripping of fluid materials from a cylindrical water drop at the late-stage in a Shock Liquid Drop Interaction (SLDI) process when the drop’s downstream end experiences compression after it is impacted by a supersonic shock wave ($Ma$ = 1.47). The drop trajectory/breakup has been simulated using a Lagrangian model and the unsteady Reynolds-averaged Navier–Stokes (URANS) approach has been employed for simulating the ambient airflow. The Kelvin–Helmholtz Rayleigh–Taylor (KHRT) breakup model has been used to capture the liquid drop fragmentation process and a coupled level-set volume of fluid (CLSVOF) method has been applied to investigate the topological transformations at the air/water interface. The predicted changes of the drop length/width/area with time have been compared against experimental measurements, and a very good agreement has been obtained. The complex flow features and the qualitative characteristics of the material stripping process in the compression phase, as well as disintegration and flattening of the drop are analyzed via comprehensive flow visualization. Characteristics of the drop distortion and fragmentation in the stripping breakup mode, and the development of turbulence at the later stage of the shock drop interaction process are also examined. Finally, this study investigated the effect of increasing $Ma$ on the breakup of a water drop by shear stripping. The results show that the shed fluid materials and micro-drops are spread over a narrower distribution as $Ma$ increases. It illustrates that the flattened area bounded by the downstream separation points experienced less compression, and the liquid sheet suffered a slower growth. Full article

The present work aims to propose a new calibration strategy of the Hall–Thollet Body Force (BF) model to simulate the flow in multi-stage compressors and to capture inlet distortion effects within the machine. Both global (0D) and radial (1D) correction terms are introduced and calibrated to improve predictions in multi-stage compressors, accounting for highly interacting, highly loaded blades, falling outside the validity range of the model’s original coefficients. The modified model has been tested on the 3.5-stage high-pressure compressor CREATE, for which experimental data are available. The modified model is then employed to study different patterns of inlet distortion. The results show a very good agreement between Unsteady Reynolds-Averaged Navier–Stokes (URANS) simulations and Body Force calculations in terms of performance, key quantities along the radial and circumferential directions and distortion transfer across the compressor. Full article

Ocean energy represents a promising resource for renewable energy generation. Hydrokinetic turbines (HKTs) provide a sustainable method to extract energy from ocean currents. However, turbine efficiency remains limited, particularly in marine environments with low flow velocities. A parametric evaluation of blade configurations is conducted in this study to assess their effect on the power and torque performance of a double-stage drag-based Savonius HKT. Numerical simulations are conducted using the Unsteady Reynolds-Averaged Navier-Stokes (URANS) equations with the k-ω SST turbulence model. The numerical model is validated against published data, and analyses on mesh density, domain size, and time step are performed to ensure accuracy. Three blade configurations—(0°, 0°), (0°, 45°), and (0°, 90°) are evaluated under flow velocities of 0.6 m/s, 0.8 m/s, and 1.0 m/s. Results indicate that blade configuration significantly affects turbine performance. The (0°, 0°) configuration performs best at high flow velocity (1.0 m/s), while the (0°, 45°) setup achieves the highest efficiency at 0.6 m/s. The (0°, 90°) configuration performs the least effectively across all conditions. A similar performance trend is observed for the torque coefficient. This study recommends selecting blade configurations based on flow velocity, providing design guidance for double-stage HKTs operating in varying marine conditions. Full article

A numerical investigation of the roll motion characteristics of the Basic Finner Model was performed. The study of roll motion is essential in the design and performance evaluation of aerospace vehicles, particularly for stability and maneuverability purposes. The numerical investigation was conducted employing the Unsteady Reynolds-Averaged Navier-Stokes (URANS) solver coupled with k-ε realizable turbulence model. The simulations were performed for a range of Mach numbers and angles of attack to assess the influence of these parameters on the model’s roll motion characteristics. The CFD procedure was validated based on an experimental database from previous work and the literature. The influence of roll motion on aerodynamic forces and moments at different flow conditions were analyzed to obtain a better understanding of the physics. The variation of forces and moments with roll angle, Mach number, and angle of attack, as well as the pressure distribution at different flow conditions, are discussed, also covering aerodynamic interactions between the fins and body. This numerical investigation contributes to understanding the aerodynamic behavior of the Basic Finner Model during roll motion. The findings are valuable for the design and optimization of aerospace vehicles, aiding in the development of more efficient and stable configurations. Future research can be based upon these results to explore additional factors that may impact roll motion characteristics and can further refine the design and performance evaluation processes for aerospace vehicles. Full article

To investigate the size effect on the energy characteristics of axial flow pumps, this study scaled the original model size based on the head similarity principle, resulting in four size schemes (Schemes 2–4 correspond to 3, 5, and 10 times the size of Scheme 1, respectively). By solving the unsteady Reynolds-averaged Navier–Stokes (URANS) equations with the Shear Stress Transport (SST) k-omega turbulence model, the external characteristic parameters and internal flow field structures were predicted. Additionally, the spatial distribution of internal hydraulic losses was analyzed using entropy generation theory. The results revealed three key findings: (1) the efficiency of axial flow pumps significantly improves with increasing size ratio, with Scheme 4 exhibiting a 6.1% efficiency increase compared to Scheme 1; (2) as the size ratio increases, the entropy production coefficients of all hydraulic components decrease, with the impeller and guide vanes in Scheme 4 showing reductions of 55.1% and 56.5%, respectively, compared to Scheme 1; (3) the high entropy generation coefficient regions in the impeller and guide vanes are primarily concentrated near the rim, with their area decreasing as the size ratio increases. Specifically, the entropy production coefficients at the rim of impeller and guide vanes in Scheme 4 decreased by 84.85% and 58.2%, respectively, compared to Scheme 1. These findings provide valuable insights for the selection and optimization of axial flow pumps in applications such as cross-regional water transfer, agricultural irrigation, and urban drainage systems. Full article

For a spinning projectile, coning motion induced by disturbances during flight can have a unique impact on the lateral force and yawing moment, which may further affect flight stability and maneuverability. The flow over a coupled spinning–coning projectile and a spinning projectile was numerically simulated by solving the unsteady Reynolds-averaged Navier–Stokes (URANS) equation with an implicit dual-time stepping method and a spinning–coning coupled motion model established through a dynamic mesh technique. The variation in transient and time-averaged aerodynamic characteristics with the angle of attack (AoA), dimensionless spin rate, and dimensionless cone rate was analyzed, and the specific effect of coning motion on the lateral force and yawing moment was revealed. Based on these findings, the yawing moment term in traditional angular motion theory was modified, and the flight response to the initial disturbance was discussed. The results indicate that the time-averaged lateral force and yawing moment of the spinning–coning coupled projectile are multiplied compared with those of the spinning projectile and vary linearly with the dimensionless spin rate and cone rate. The main factors affecting the lateral force are the coning motion-induced effective angle of sideslip (AoS), asymmetric expansion waves, and asymmetric vortices. The much larger yawing moment induced by spinning–coning coupled motion can more easily cause AoA divergence and flight instability. Full article

This study evaluated the performance of a ship propeller numerically using the Coanda effect. The simulations applied a model based on a 6.5K DWT tanker and conducted self-propulsion assessments for three types of propellers: the original propeller, a normal propeller, and a Coanda propeller. The numerical simulations used the unsteady Reynolds-averaged Navier–Stokes (URANS) equations, incorporating the SST k–ω turbulence model. The influence of the additional thrust generated by the Coanda effect on the hull resistance and self-propulsion factors was analyzed. The key findings showed that the Coanda-based propeller achieved efficient propulsion performance by generating additional lift even at low rotational speeds. A self-propulsion analysis showed that the Coanda propeller required approximately 7.8% less delivered power than the original propeller. These results suggest that propulsion systems utilizing the Coanda effect offer superior efficiency and economic advantages over traditional technologies. This study provides critical baseline data for assessing the feasibility of a Coanda propeller, with further validation planned through full-scale ship simulations. Full article

Nowadays, a lot of efforts are being made to increase turbine inlet temperatures (TIT), with the aim of increasing efficiency in aircraft and power generation turbines. Due to the higher temperature level, advanced cooling solutions to preserve material durability are necessary. It is essential to avoid contact between hot gases and the temperature-sensitive components, such as the stator and rotor cavity disks. Modern gas turbine performance optimization centers on reducing leakage and refining sealing systems. The interaction between the main flow and cavity flow in stator/rotor systems has a significant role in loss generation. This study employs Unsteady Reynolds-Averaged Navier–Stokes (URANS) simulations to investigate the unsteady interactions within the stator/rotor cavity of a low-pressure turbine. Numerical results are compared and validated against experimental data obtained in the cavity rig of the University of Genova. The research focuses on the effects of stator/rotor interactions, including wake ingestion from upstream rotor bars and the blocking influence of downstream potential effects on cavity sealing effectiveness. In this paper, a comparison between the zero cooling air flow rate and cavity sealing condition is shown. Special attention is given to unsteady loss mechanisms occurring downstream of the vane row and in areas where the cavity flow re-enters the main channel, showing how cooling flow rates affect these losses. From this study, it can be seen that by increasing the cooling flow rate injected into the cavity, there is an increase in the hub’s passage vortex effect and there is a more intense interaction between the main flow and the cavity flow. These results offer valuable insights into the mechanisms of interaction between the main flow and cavity flow. Full article

The Darrieus vertical axis wind turbine (VAWT) is categorized as a lift-based turbomachine. It faces challenges in the low tip speed ratio (TSR) range and requires initial torque for the starting operation. Ongoing efforts are being made to enhance the turbine’s self-starting capability. In this study, Computational Fluid Dynamics (CFD) simulations were utilized to tackle the identified challenge. The Unsteady Reynolds-Averaged Navier–Stokes (URANS) approach was employed, combined with the shear–stress transport (SST) $k-\omega$ turbulence model, to resolve fluid flow equations. The investigation focused on optimizing the placement of auxiliary blades by considering design parameters such as the pitch angle and horizontal and vertical distances. The goal was to increase the turbine efficiency and initial torque in the low-TSR range while minimizing efficiency loss at high-TSR ranges, which is the primary challenge of auxiliary blade installation. Implementing the auxiliary blade successfully extended the rotor’s operational range, shifting the rotor operation’s onset from TSR 1.4 to 0.7. The optimal configuration for installing the auxiliary blade involves a pitch angle of 0°, a horizontal ratio of 0.52, and a vertical ratio of 0.41. To address the ineffectiveness of auxiliary blades at high-TSRs, installing deflectors in various configurations was explored. Introducing a double deflector can significantly enhance the overall efficiency of the conventional Darrieus VAWT and the optimum rotor with the auxiliary blade by 47% and 73% at TSR = 2.5, respectively. Full article

This paper aims to numerically assess the cavitation damage of hydrodynamic machines and hydraulic components and its development in time, based on cavitation erosion tests with samples of used materials. The theoretical part of this paper is devoted to the numerical simulation of unsteady multiphase flow by means of computational fluid dynamics (CFD) and to the prediction of the erosive effects of the collapses of cavitation bubbles in the vicinity of solid surfaces. Compressible unsteady Reynolds-averaged Navier–Stokes equations (URANS) are solved together with the Zwart cavitation model. To describe the destructive collapses of vapor bubbles, the modeling of cavitation bubble dynamics along selected streamlines or trajectories is applied. The hybrid Euler–Lagrange approach with one-way coupling and the full Rayleigh–Plesset equation (R–P) are therefore utilized. This paper also describes the experimental apparatus with a rotating disc used to reach genuine hydrodynamic cavitation and conditions similar to those of hydrodynamic machines. The simulations are compared with the obtained experimental data, with good agreement. The proposed methodology enables the application of the results of erosion tests to the real geometry of hydraulic machines and to reliably predict the locations and magnitude of cavitation erosion, so as to select appropriate materials or material treatments for endangered parts. Full article

The influence of oblique currents in narrow and shallow channels causes the fluid flow around ships to become complex. To analyze the hydrodynamic characteristics of a ship in such channels, it is essential to examine the influence of oblique currents on the ship’s hydrodynamic characteristics. In this study, current direction, ship speed, current speed, and water depth were identified as determinants affecting the hydrodynamic characteristics of a ship. Numerical simulations were conducted on a large oil tanker to investigate the effects of these factors on the ship’s hydrodynamic characteristics. The viscous fluid flow was modeled using the unsteady Reynolds-averaged Navier–Stokes (URANS) equations in conjunction with the k-ε turbulence model. The URANS equations were discretized using the finite volume method. The numerical results indicate substantial differences in the hydrodynamic characteristics of ships under oblique current conditions compared to still-water conditions. At a current direction of β = −45°, the direction of the sway force is consistent with that of still water’s sway force, which is an attractive force. The yaw moment at β = −45° changes from a bow-out moment under still-water conditions to a bow-in moment. Conversely, at a current direction of β = 45°, the sway force shifts from an attractive force under still-water conditions to a repulsive force. The yaw moment acts as a bow-out moment, which is consistent with that observed in still-water conditions. Furthermore, the influence of hydrodynamic characteristics on a ship varies significantly with changes in ship speed, current speed, and water depth. To ensure the safe navigation of ships, it is essential to develop and apply comprehensive strategies and countermeasures that account for practical conditions. Full article

Transonic buffet flow is a classical complex and unstable flow that has a negative effect on aircraft fly safety. Therefore, it is crucial to study the unsteady characteristics of buffet flow. The numerical analysis method is very useful in achieving the aforementioned goal. In this paper, focused on the typical supercritical airfoil OAT15A in fixed and pitching conditions, unsteady Reynolds averaged Navier–Stokes (URANS) closed with the sst-kω turbulence mode, coupled with the structure dynamical equation, is utilized to investigate the transonic buffet flow. Firstly, from the perspective of coherent flow structure, flow velocity divergence snapshots constructed from unsteady flow solutions are used to analyze the feature of transonic buffets in the two cases mentioned. Then, DMD modes are extracted by the dynamic mode decomposition technique from the velocity snapshots and adopted to analyze the flow modes of the two distinct flow fields. The numerical simulation results show that, in the fixed case, the regular motion feature of the buffet is present, the shock oscillation is closely related to the vortex structure, and the durations of rearward and forward movements of the shock are both equal to half of the buffet period. In the pitching case, the duration of the rearward motion of the primary shock is approximately five eighths of one buffet period, and the secondary shock appears with the primary one moving downstream, and they interact with each other. The region of the shock movement is larger than that of the fixed case, and there is chaotic flow rather than periodic flow in its wake. Structural elastic oscillation changes the characteristics of the aerodynamic response, which is solely affected by the frequency of the pitching oscillation. Full article

Blind tees, as important junctions, are widely used in offshore oil and gas transportation systems to improve mixing flow conditions and measurement accuracies in curved pipes. Despite the significance of blind tees, their unsteady flow characteristics and mixing mechanisms in turbulent flow regimes are not clearly established. Therefore, in this study, Unsteady Reynolds-Averaged Navier–Stokes (URANS) simulations, coupled with Explicit Algebraic Reynolds Stress Model (EARSM), are employed to explore the complex turbulent flow characteristics within blind-tee pipes. Firstly, the statistical flow features are investigated based on the time-averaged results, and the swirl dissipation analysis reveals an intense dissipative process occurring within blind tees, surpassing conventional elbows in swirling intensity. Then, the instantaneous flow characteristics are investigated through time and frequency domain analysis, uncovering the oscillatory patterns and elucidating the mechanisms behind unsteady secondary flow motions. In a 2D-length blind tee, a nondimensional dominant frequency of oscillation (St_bt = 0.0361) is identified, highlighting the significant correlation between dominant frequencies inside and downstream of the plugged section, which emphasizes the critical role of the plugged structure in these unsteady motions. Finally, a power spectra analysis is conducted to explore the influence of blind-tee structures, indicating that the blind-tee length of l_bt = 2D enhances the flow-mixing conditions by amplifying the oscillation intensities of secondary flow motions. Full article

With the impact of size on low-pressure turbines (LPTs) increasing, the gap between the blades has shrunk, inevitably influencing the unsteady effects inside the turbine. In this study, the aerodynamic effect of the blade gap size is investigated using a compressible unsteady Reynolds-averaged Navier–Stokes (URANS) model on the basis of a four-stage LPT. Simulations are conducted in which the gap between the third-stage stator (S3) and rotor (R3) varies from 0.2 to 0.8 times the axial chord length of the R3 blade. The multi-stage environment reflects the complexity of real low-Reynolds flow fields. Computational fluid dynamics is used to analyze the flow field in detail. The results demonstrate that in the small-gap (AG-0.2) case, the turbulence kinetic energy (TKE) level of the S3 wake close to the R3 leading edge is four-thirds of that in the large-gap (AG-0.8) case. The higher intensity of the wake impacting on the blade results in a higher inverse pressure gradient in the rear part of the R3 suction surface, which increases the profile loss. However, the AG-0.2 case leads to fewer losses caused by the passage vortex in the hub area under the influence of the higher intensity of the wake. Full article