Unsteady RANS Research Articles

Alleviating tunnel aerodynamics through hybrid suction & blowing techniques applied to train nose sections

As high-speed railway lines upgrade speeds or develop ultra-high-speed trains, traditional passive measures may struggle to address tunnel aerodynamics and passenger comfort. This study employs numerical calculations to investigate the aerodynamic mitigation of an ultra-high-speed train traveling at U = 600 km/h through a tunnel, utilizing active suction & blowing techniques in its streamlined nose sections. The simulation employs three-dimensional, compressible, unsteady Reynolds-averaged Navier-Stokes (URANS) methods, validated against full-scale experiments. The effects of slot shapes, suction directions, activation periods, and the suction & blowing velocities (SBv) are examined. Results show that the slit design, normal direction, along with continuous activation, outperforms the rectangular design, parallel direction, and partial activation in reducing pressure peaks. Notably, maximum pressure peaks on the train and tunnel surface exhibit an exponential decay pattern as SBv increases. The micro-pressure wave 20 m from the tunnel exit decreases by 28% as SBv increases from 0 to 0.27U. Additionally, maximum slipstream peaks decrease linearly with SBv, with a more pronounced decline on the near side. While drag on the head and middle cars decreases linearly with increasing SBv, the tail car experiences a quadratic increase in drag, leading to an overall reduction in total drag. Furthermore, the reduction in side force and the positive lift of the tail car enhances train safety during tunnel passage. Overall, the hybrid suction & blowing technique offer promising prospects for enhancing the aerodynamic performance of high-speed maglev trains in the future.

Enhancing darrieus wind turbine performance through varied plain flap configurations for the solar and wind tree

Plain flaps (PFs) significantly increase camber, enhancing lift and aerodynamic performance when deployed. In Darrieus Vertical Axis Wind Turbines (VAWTs), which perform efficiently in low-speed, turbulent wind conditions, structural modifications like PFs can improve efficiency. This study explores plain flaps with 10-20-degree deflections at different chord lengths to enhance the NACA 2412 aerofoil’s performance. Using Unsteady Reynolds-Averaged Navier-Stokes (URANS) equations and the Shear Stress Transport (SST) k-ω turbulence model, simulations were conducted across high (Re ≈ 2.71 × 105), medium (Re ≈ 1.35 × 105), and low (Re ≈ 5.4 × 104) Reynolds numbers (Re). The 0.7–10 and 0.8–10 configurations significantly improved torque and Cp. At a Tip Speed Ratio (TSR) of 2.5, the 0.8–10 configuration increased the Cp by 19.51% over the flapless NACA 2412 without PF. The 0.7–10 configuration achieved the highest Cp across all TSRs, while a three-blade setup improved Cp by 43% compared to four- and five-blade configurations. The modified blades demonstrated consistent torque gains across all Re, proving the effectiveness of blade shape modifications in enhancing VAWT efficiency, particularly under fluctuating wind conditions, indicating the potential of PF-modified blades in improving small-scale wind turbine performance in varied urban environments.

An optimized anisotropic pressure fluctuation model for the simulation of turbulence-induced vibrations

A significant aspect of the economic performance and safety of a nuclear reactor involves maintaining the integrity of the fuel rods, which are susceptible to Turbulence-Induced Vibrations (TIV) resulting from the axial flow of the coolant. TIV can instigate severe repercussions, including structural damage such as fatigue and wear. TIV can be studied numerically, using Fluid–Structure Interaction( FSI) simulations. However, high-resolution approaches are computationally too expensive to use for complex FSI simulations, while Unsteady Reynolds-Averaged Navier-Stokes (URANS) simulations severely underpredict the displacement amplitudes of the vibrations as they only resolve the mean flow. Evolving from this shortfall, this paper focuses on a recently developed Anisotropic Pressure Fluctuation Model (AniPFM). This model generates a synthetic velocity fluctuations field, which is used to solve for the pressure fluctuations. Using this model, together with URANS, is a possible way to simulate the excitation mechanisms of TIV of fuel rods in a computationally cheaper way. While previous research has highlighted the potential of this model, there are parameters, definitions, and constants whose impacts on the model are not yet fully understood. Therefore, a comprehensive effort is undertaken to fine-tune the model, optimize its performance, improve understanding of it and further validate it. This is done by applying AniPFM to both pure flow and FSI cases, using high-resolution numerical and experimental data as reference and for comparison. With the optimized model, a substantial decrease in average difference from the experimental data is found for the FSI case under consideration, when compared with the unoptimized version of AniPFM.

Optimizing Duct Geometry for Micro-Scale VAWTs: Exploring Aerodynamics and Aeroacoustics with URANS and DDES Models

Among several advantages of the Vertical Axis Wind Turbines (VAWTs), such as the low cut-in speed, these turbines have lower efficiency compared to Horizontal Axis Wind Turbines (HAWTs). In order to increase the power of VAWTs, a novel approach is employed to address the aforementioned issue. In this study, a convergent-divergent duct (including three components: nozzle, diffuser, and flange) is used, and in the first step of this study, the angles and lengths of all duct components and throat width are optimized (in 3D configuration). In the optimization process, first, the design points are determined by the Design of Experiment (DOE) method, and for each design point, the flow field inside the duct is modeled by the Reynolds-Averaged Navier-Stokes (RANS) approach. Moreover, the objective function of this optimization problem is to maximize the velocity of the flow inside the duct. In the following, the general treatment of the statistical population is forecasted using the Response Surface Method (RSM), and the optimal duct is obtained using the Genetic Algorithm (GA). It is observed that the optimal duct increases the local wind speed up to 1.99 times (from 5 m/s to 9.94 m/s at the throat). Furthermore, a sensitivity analysis was performed on the duct components, which are the most effective parameters in increasing wind speed, nozzle angle, and nozzle length. In the second step, a Darrieus VAWT (with both 2D and 3D configurations) is numerically validated, and then this turbine is scaled-down and located in the optimal duct. The position and tip-clearance of a turbine within the optimal duct are investigated to find the best configuration for higher power generation. Subsequently, both the bare wind turbine and the ducted wind turbine undergo thorough aerodynamic and aeroacoustic analysis, using unsteady RANS and Delayed Detached Eddy Simulations (DDES) approaches, respectively. The results reveal a remarkable improvement in the aerodynamic efficiency (power coefficient increased up to 2.5 times) with the addition of the optimal duct, while the aeroacoustic performance of the system is compromised, leading to an increase in sound pressure level.

Mitigation of train-tunnel aerodynamics through active airflow control at front and rear noses: Impact of slit area

As railway transportation advances toward higher speeds, traditional passive measures may struggle to meet the stringent aerodynamic criteria in tunnels, necessitating the exploration of novel active flow control techniques. This study employs three-dimensional, compressible, unsteady Reynolds-averaged Navier–Stokes simulations to investigate the aerodynamic effects of the suction and blowing slit area (S) positioned on the front and rear noses of the train. The results indicate that suction and blowing activation is particularly effective in alleviating pressure on the narrower side of the tunnel. Specifically, with a 4 m2 slit, the original 4.8% pressure difference between symmetrical points on the train body is fully eliminated. The influence of suction and blowing on the positive pressures is confined to the front and rear noses where the slits are located. Notably, only suction at the front nose mitigates pressure gradients, while blowing at the rear is unrelated. The peak-to-peak pressure (ΔP) on both the train surface and tunnel wall exhibits a linear decline, with reductions of 17.4% and 16.6%, respectively, as S increases from 0 to 4 m2. Similarly, the slipstreams on both sides of the tunnel decrease linearly with increasing slit area: with u/Umax = −0.008S + 0.24 for the near side, and u/Umin = 0.014S − 0.265 for the far side. Additionally, expanding the slit area further boosts the stability and safety of the train during tunnel exit by reducing lateral forces and rolling moments, while also decreasing overall drag, thereby partially compensating for the energy input. Although the maximum lift on the head car increases with slit area, the lift on the tail car initially rises and then decreases, helping to mitigate instability upon tunnel exit. Overall, the hybrid suction and blowing technique offers promising potential for enhancing the tunnel aerodynamics in the future.

Numerical analysis on the ducted propeller aerodynamics in sidewall-ground effect

Owing to their compact structure and robust protective features, unmanned aerial vehicles (UAVs) equipped with ducted propellers are particularly suited for search and detection missions in confined environments. However, in such spaces, proximity effects can lead to pronounced instability in the aerodynamic performance of the UAV, particularly under the influence of multiple wall interactions. This study employs a sliding mesh technique and the Unsteady Reynolds-Averaged Navier-Stokes method to perform computational fluid dynamics simulations, analyzing the ground-sidewall effect's impact on ducted propeller aerodynamic performance across various hovering positions. Research shows that sidewall effects remain largely unaffected by ground effects. However, when the ground height is less than 2r and the sidewall distance is less than r, the ground effect noticeably alters the strength of the sidewall effect. In this region, sidewall suction effects increase sharply as ground height decreases; however, once the ground height falls below 1r, the mean side force diminishes rapidly. Based on the simulation results, this study proposes an empirical formula for side force under coupled sidewall-ground effects, with a mean absolute percentage error of approximately 10% compared to simulation results. Through an analysis of the unstable motion of vortex structures, this study further explains the causes of substantial transient force fluctuations observed near the walls. The findings of this study provide theoretical guidance for the design of flight controllers and the planning of safe flight paths in confined environments.

Unsteady RANS simulations of under-expanded hydrogen jets for internal combustion engines

Hydrogen can be considered a suitable fuel for heavy-duty reciprocating internal combustion engines (ICEs) in order to limit carbon dioxide emissions. The low volumetric power density of hydrogen and the backfire problem suggest to employing the direct injection technology with relatively high nozzle pressure ratios (NPRs). This paper provides the analysis of under-expanded hydrogen jet dynamics using an open-source high-fidelity simulation tool based on the OpenFOAM framework. The unsteady Reynolds-averaged Navier–Stokes (URANS) equations are solved by an efficient pressure-based solver for compressible flow. URANS equations are attractive for fast engineering analysis of 3D engine cycle and optimization, where large eddy simulation (LES) is too computationally expensive. The accuracy of the simulations is enhanced by employing the weighted essentially non-oscillatory (WENO) approach for the spatial discretization, considering schemes from second-order to fourth-order accuracy. Those schemes are embedded in a pressure-implicit with splitting of operators (PISO) algorithm, obtaining a very robust and accurate numerical method for compressible multi-species flows, which can be shared in an open access framework. Hydrogen injection in air is simulated, with several values of the NPR typical of direct injection ICE in the low-medium range, 8.5≤NPR≤30. The main features of the developing jet are analyzed, such as barrel shock dimensions, cone angle and hydrogen–air mixing. The results are validated with respect to experimental and LES data available in the recent literature, demonstrating the efficiency and the accuracy of the employed URANS approach and evaluating its limits.

Investigation of transonic flows through an idealized ORC turbine vane using Delayed Detached Eddy simulations

An idealized transonic blade configuration is studied using Delayed Detached Eddy Simulations (DDES), with focus on the loss mechanisms associated with the base pressure and turbulent wake development. The working fluid is Novec649, which is commonly used in low temperature Organic Rankine Cycle (ORC) power plants. Due to the molecular complexity of the fluid, a lower pressure ratio and exit Mach number are achieved compared to an air flow through the same geometry, a phenomenon that becomes more pronounced when considering thermodynamic operating conditions that lead to stronger non-ideal gas behavior, and which is well captured by models of standard use in ORC design and analysis, such as Reynolds-Averaged Navier–Stokes (RANS) models. Overall, as the fluid isentropic exponent and the exit Mach number decrease, the losses tend to increase. When performing a breakdown of the losses, the wake region provides the dominant contribution. Unlike RANS models, the DDES resolves the largest turbulent scales in the wake region, allowing fine-detail analysis of the unsteady wake dynamics, namely vortex shedding, and its influence on the loss coefficients. In the low supersonic Mach number range considered (M2∼1–2), the vortex shedding exhibits a flapping motion in the reattachment region behind the trailing edge. The Strouhal number, classically formed with the diameter of the trailing edge, is found to increase with increasing outlet Mach number. However, when introducing a new scaling based on the neck width of the reattachment region, directly related to the shape of the recirculation bubble and the associated reattachment shocks, the Strouhal number collapses to a relatively constant value of about 0.3 in all cases. DDES also shows that, due to the large heat capacity of Novec649, temperature fluctuations in the wake are greatly reduced, leading to the suppression of the so-called energy separation phenomenon occurring in air flows. While RANS simulations capture satisfactorily the main features and global quantities of the flow, they fail in describing the flow around the trailing edge. This results in inaccurate estimates of the back pressure, which ultimately leads to an underestimation of the losses by about 20%. Unsteady RANS simulations can predict the wake unsteadiness, but do not capture the couplings between large coherent structures, the recirculation, and the shocks, also resulting in inaccurate performance predictions. Thus, the present result showcase the importance of using advanced scale-resolving simulations for improved analysis and design of ORC turbines.

Advanced heave control of a hydrofoil-based autonomous surface vehicle: CFD modeling with integrated variable propeller revolution control

An innovative control strategy employing the Proportional-Integral-Derivative (PID) algorithm is proposed to address the heave stability challenges in the in-house developed hydrofoil-based Autonomous Surface Vehicle (HASV). This method regulates the HASV's heave motion by adjusting the propeller's revolution speed. After confirming that the aerodynamic load on the superstructure contributes less than 2% compared to the hydrodynamic load, a detailed numerical simulation study and recirculating water tunnel test were conducted to validate the reliability of the established Unsteady Reynolds-Averaged Navier-Stokes (URANS) CFD model for the HASV substructure. The integration of the PD controller for propeller revolutions into the CFD free-running model, which incorporates fully nonlinear and viscous effects, was achieved using the ‘Region-Moving’ method for the background domain and the Body Force Method (BFM) for the propeller domain. By analyzing the impact of various PD coefficients on heave control, optimal settings for the HASV were determined. Subsequent studies examined the vehicle's capability in draft correction and draft maintenance under wave conditions. The results of direct CFD-PD simulations demonstrate that the proposed heave control method effectively stabilizes the HASV under complex nonlinear environmental conditions, offering valuable insights for addressing similar heave control challenges in hydrofoil-based vehicles.

A study of gust wind speed using a novel unsteady Reynolds-Averaged Navier-Stokes model

In this study, a novel unsteady Reynolds-Averaged Navier-Stokes (URANS) model is proposed in conjunction with a prespecified averaging time and turbulent inflow, and a new peak factor that considers the effect of averaging time used in URANS is derived to calculate gust wind speed. Firstly, the URANS incorporated with a prespecified averaging time and turbulent inflow is proposed to predict the time series of wind speed over flat terrain and investigate the variation of higher-order moments and zero-crossing rate at which the fluctuating wind speed changes algebraic sign with the averaging time. The predicted higher-order moments are less sensitive to the averaging time, but the predicted zero-crossing rate slightly decrease with increasing the averaging time due to the moving average effect. Secondly, a new peak factor is proposed to consider the averaging time. Finally, the gust wind speeds over flat terrain and around a single building predicted using the proposed URANS and the new peak factor based on the Hermite model are found to be in good agreement with the LES results, while those predicted by the conventional models deviate more from the LES results.

Numerical simulations of the effects of KC number and entrapped water on the subsea cover hydrodynamic characteristics

ABSTRACT The entrapped water in subsea structures with openings significantly influences hydrodynamic forces and structural stability. This study utilises Unsteady Reynolds-Averaged Navier-Stokes (URANS) equations with the k − ω Shear Stress Transport (SST) turbulence model to investigate oscillatory flows around a subsea cover under varying conditions, including different levels of entrapped water (obtained by different opening dimensions) and KC numbers. Numerical simulations are validated through experimental data from oscillatory flows over a subsea cover on a plate. The study examines lift and in-line force coefficients to assess the impact of opening dimensions and KC numbers on hydrodynamic forces. Findings show that opening size primarily affects the lift coefficient due to changes in the behaviour of entrapped water. The influence of entrapped water decreases as both the KC number and opening size increase. A 30% opening yields the smallest lift-to-in-line ratio ( C Lrms / C Drms ) and reduced C Lrms and C Lstd values. Additionally, the flow fields around the cover are also discussed. Smaller openings reduce the amplitude of velocity, vorticity, and pressure inside the cover. Openings mitigate vortex formation and shedding caused by jets between the structure and the wall. The results also reveal a phase shift in lift coefficients for different opening sizes. These findings provide critical insights for the hydrodynamic behaviour and design of similar subsea structures.

The Development of Machine Learning Models for Radial Compressor Monitoring With Instability Detection

Abstract Modern radial compressors are designed to operate with high performance and a wide range of applications while minimizing their environmental impact. It is necessary to study the machine near the stability limit and gain a comprehensive understanding of the fluid dynamic mechanisms that trigger the instability in the system to extend the operating range at high efficiency. Machine learning aids in developing pattern identification models for detecting compressor instability. In a prior study, a two-stage radial compressor for refrigerant gas underwent extensive simulation, capturing unsteady RANS conditions and generating a substantial dataset of pressure signals from multiple probes. Selected signals, coupled with detailed computational fluid dynamics (CFD) post-processing, revealed fluid dynamic structures near surge conditions. This study utilizes all pressure signals to assess the stage's operational state (stable, transient, or unstable/surge). An additional goal is to develop an algorithm predicting flow patterns in different stage sections with minimal pressure signals at the rotor inlet. This is a preliminary step toward the development of a smart monitoring and diagnostic model for the compressor installed in a plant. The developed model consists of three submodels, trained with CFD results obtained from unsteady Reynolds averaged Navier–Stokes (URANS) simulations. The three submodels are a regression submodule, a classification submodule, and a forecasting algorithm. The regression submodule predicts diffuser static pressure fields, the classification submodule categorizes whether the compressor is in a critical condition or not, and the forecasting algorithm predicts the inducer pressure signals in the future impeller rotations according to the actual operation history. These sub-modules work together to forecast whether the compressor, according to the operation strategy, is going into instability conditions.

Assessment of Cavitation Erosion Using Combined Numerical and Experimental Approach

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.

Quantifying the Tip Leakage Vortex Wandering in a Low-Speed Axial Flow Fan via URANS Simulation

The tip leakage vortex is a dominant noise and aerodynamic loss source of low-speed axial flow fans. The tip leakage vortex often has an unsteady behavior, like the periodic motion of the vortex core: the so-called vortex wandering. This periodic vortex wandering results in additional noise, aerodynamic loss, and an oscillation in the moment acting on the blading, which causes an unfavorable vibration of the rotor. In the research campaigns focusing on vortex wandering, complicated measurement techniques (PIV) or high-fidelity but computationally costly simulations are commonly used. The present paper aims to introduce a relatively cost-effective unsteady Reynolds-averaged Navier-Stokes simulation-based investigation method of vortex wandering. The capabilities of the proposed technique are presented through a low-speed axial flow fan case study.

Dynamic stall control of an offshore wind turbine airfoil using a leading-edge microcylinder

This study conducted a computational investigation to examine the impact of passive flow control using a microcylinder positioned near the leading edge of a dynamically stalled NACA0012 offshore wind turbine airfoil at a Reynolds number of 1 × 106. Unsteady Reynolds-Averaged Navier-Stokes (URANS) simulations were used for parametric analyses to determine optimal control parameters, while Delayed Detached Eddy Simulation (DDES) provided insights into the transient vortex structures on the airfoil's suction surface, elucidating the control mechanisms of the microcylinder. The results reveal a strong correlation between the microcylinder's effectiveness and its geometric configuration, particularly its diameter and distance from the airfoil surface. The microcylinder effectively modifies the flow field by generating vortices that interact with the boundary layer, thereby enhancing its resistance to adverse pressure gradients and delaying flow separation. The optimal configuration—characterized by a microcylinder with a diameter of 1% of the chord length (D/c = 0.01) positioned 1.5% of the chord length (L/c = 0.015) away from the airfoil surface—resulted in a 62% reduction in peak drag coefficient and a 50% decrease in aerodynamic hysteresis loop area. These improvements significantly enhance the dynamic aerodynamic performance and stability of the oscillating NACA0012 airfoil, effectively mitigating the detrimental effects of dynamic stall.

The impact of roughness on the resistance of full-scale ships

Abstract This study aims to explore the impact of hull roughness on ship resistance, which provides a reference for enhancing the operational efficiency and energy-saving levels of ships. Initially, the development and accuracy of Computational Fluid Dynamics (CFD) numerical simulations are elucidated through literature, and the current state of research on hull roughness by scholars both domestically and internationally is introduced. Subsequently, taking the 2000 TEU container ship SITCCAGAYAN as the object of study, a CFD simulation is performed based on the unsteady Reynolds-averaged Navier-Stokes equations in Star CCM+ software. An improved wall function method and roughness function are used to design the simulation experiments, conducting the resistance prediction under various conditions, including the fully smooth hull, fully rough hull, and different levels of roughness at various parts of the hull. The experiment, based on results such as the increase in resistance, roughness impact factor, and roughness Reynolds number, demonstrates that the surface roughness of the hull significantly affects the magnitude of ship resistance. An increase in local and overall roughness leads to an increase in total resistance, but the roughness at the bow part of the ship has the greatest impact on resistance.

The use of Approximate Entropy analysis for flow pattern identification in radial compressors to detect instable operating conditions

Abstract This study uses a statistical analysis on given pressure signals to identify the surge condition in radial compressors, providing insight into their dynamics and performance. Specifically, the investigation focuses on a two-stage centrifugal compressor used in refrigeration applications. Through Computational Fluid Dynamics (CFD) Unsteady Reynolds-Averaged Navier-Stokes (URANS) simulations, static pressure time-series are collected to cover four distinct operational points of the machine, ranging from a stable condition to an unstable one. To assess the stability of the compressor operation, the analysis introduces the Approximate Entropy (ApEn) as a statistical parameter to measure the regularity and predictability of the time-series signals. The results show that the proposed methodology is effective in pattern recognition, able to identify sources of instabilities, and to distinguish between different operating conditions. This approach enhances the comprehension of the operational characteristics of centrifugal compressors and sets the stage for improved monitoring and diagnostic methodologies. ApEn can therefore be a useful index to analyse time-series from sensors easy to install in real compressor configurations. The proposed methodology is included in a set of models, based on machine learning techniques, previously developed by the authors, to identify compressor instability during operation.

Conjugate heat transfer simulations of a radially cooled gas turbine blade leading edge using a vortex-based fluidic oscillator for sweeping jet impingement

The turbine blades of aero engines are subjected to extremely high temperatures, particularly at the leading edge, where temperatures can reach approximately 1800–2000 K. Therefore, effective heat load management is crucial. A vortex-based fluidic oscillator for sweeping jet impingement was proposed as an innovative cooling method to enhance heat transfer at leading edge of high-pressure gas turbine blades. This numerical investigation evaluates the cooling performance of a vortex-based sweeping jet compared to steady and conventional sweeping jets in a radially cooled high-pressure turbine blade. In this study, a conjugate heat transfer model based on three-dimensional unsteady Reynolds-averaged Navier–Stokes (URANS) equations is employed. The shear stress transport (SST k–ω) model is specifically selected to predict the flow field and heat transfer characteristics of a vortex-based fluidic oscillator applied to the leading edge. To verify the accuracy of numerical calculations, two sets of experimental data were used as benchmark. The results demonstrated strong qualitative and quantitative agreement with experimental data. Various parameters, including coolant mass flow rates (0.171, 0.514, and 0.857 g/s), aspect ratios (0.5, 0.65, and 1), jet-to-wall spacings (H/D = 2, 4, and 6), and pressure drop, were examined to assess overall cooling effectiveness and heat transfer performance. Time-averaged and time-resolved flow field measurements revealed that vortex-based fluidic oscillator significantly enhanced cooling effects and covered a larger impinging area compared to a steady jet. Notably, the vortex-based fluidic oscillator achieved a 24.3% higher heat transfer performance than the steady jet at H/D = 2, with an average temperature decrease in approximately 21 K at leading edge.

Insights into flame flashback phenomenon utilizing a Strut-Cavity flame holder inside scramjet combustor

Understanding upstream flame propagation in scramjets is challenging, particularly concerning flame flashback in a combustor with a novel strut-cavity flame holder. Two-dimensional unsteady Reynolds-averaged Navier–Stokes (URANS) simulations were performed to investigate how Mach number and wall divergence affect flame behavior. The utility of the strut-cavity flame holder was highlighted through a study of its non-reacting flow characteristics. Flow dynamics are significantly altered as the shear layer above the cavity interacts with the downstream hydrogen jet. Shear layer dynamics and fuel-air mixing are improved through key factors such as shock-train behavior, cavity oscillations, and transverse fuel injection. The submerged fuel jet is less exposed to supersonic flow and demonstrates reduced entropy rise, achieving a 16% increase in mixing efficiency compared to standalone struts and a 46% improvement over transverse injection without a flame holder. Thermal choking shifts the shock train upstream, facilitating interactions with the shear layer and enhancing vortex formation, which decreases flow speed and promotes upstream flame propagation. The presence of OH radicals indicates that flame flashback follows a periodic pattern with an initial gradual slope, suggesting effective anchoring. Stability and flashback likelihood are affected by low-speed zones, vortex merging, and wall divergence. At Mach 3, combustion efficiency improves without wall divergence due to increased heat release, while wall divergence prevents flame flashback by sustaining supersonic core flow and managing flow-flame interactions. At higher core flow velocities, flame stabilization occurs at the cavity's separation corner, despite a tendency for upstream propagation, with validation of the URANS results achieved through two-dimensional large eddy simulations.

Rotor Airfoil Dynamic Stall Numerical Calculation in High Advance Ratio

Abstract The characteristics of rotor airfoil dynamic stall in high advance ratio were investigated based on the CFD (Computational Fluid Dynamics)method. The unsteady RANS(Reynolds-averaged Navier-Stokes) equation and overset mesh method were used in CFD calculation, and the variation of Mach and attack Angle were simulated by transitional and pitching motion of the airfoil. The airfoil dynamic lift coefficient, moment coefficient, and flow field of airfoils at different rotor blade positions were analyzed, and the influence of flight parameters and airfoil aerodynamic shape was investigated by comparison with baseline conditions. The results show that the dynamic stall of the rotor airfoil at the retreating blade in a high advance ratio is mainly caused by the extension of trailing edge separation; reducing rotor lift in a high advance ratio by a suitable flight strategy is an effect way to reduce dynamic stall; rotor airfoil leading edge droop can reduce dynamic moment, but will lead to dragging increase and drag divergence Mach decrease, the negative peak of the dynamic moment is reduced by about 60%, the drag coefficient is increased by about 0.01, and the drag divergence Mach is decreased by about 0.01 in present computation.