High-fidelity CFD Simulations Research Articles

An efficient parallel multi-fidelity multi-objective Bayesian optimization method and application to 3-stage axial compressor with 144 variables

Parallel infill sampling is a promising approach to improve the efficiency of multi-fidelity multi-objective Bayesian optimization (MOBO). In existing literature, the number of infill samples per iteration is typically limited to 10. Additionally, the application of the multi-fidelity MOBO method in engineering optimization designs with over 100 variables is rare. To that end, a novel generalized expected improvement matrix (GEIM) criterion is proposed by using generalized reference values for the element in expected improvement matrix. Parallel infill sampling strategy based on GEIM is developed and incorporated into the multi-fidelity MOBO framework. The distinct feature of the proposed method is that the number of infill samples at each iteration can be significantly larger than existing methods (i.e. as much as 100), so as to further enhance the optimization efficiency. Empirical experiment results on analytic problems show that the proposed parallel multi-fidelity MOBO method is highly competitive in comparison with existing methods. To address the curse of dimensionality in optimizing multi-stage axial compressor, an efficient modeling method for the Hierarchical Kriging (HK) is incorporated. The HK predictor for the efficiency of the 3-stage compressor with 144 variables is built in 16.71 s with acceptable accuracy. Remarkable improvements over the two objectives of the 3-stage compressor with 144 variables are achieved simultaneously within 870 high-fidelity CFD simulations.

High-fidelity aero-acoustic evaluations of a heavy-lift eVTOL in hover

Electric Vertical Take-off Landing (eVTOL) vehicles can transform aviation, but the unconventional aircraft development currently requires an extensive knowledge base. This paper presents a high-fidelity aerodynamic and acoustic evaluation of a heavy-lift multi-rotor eVTOL design in hover. The design, known as Skybus, has an MTOW of 14,000 kg, and six tiltable rotors attached at the tip of three sets of tandem wings with flaps. High-fidelity scale-adaptive CFD simulations of the full-scale Skybus vehicle in hover, with three design variations, were conducted using the HMB3 framework. The aerodynamic performance and interactions were analysed in detail. The baseline airframe caused a blockage force of about 13% of the MTOW, and the compact hovering rotors induced a sinusoidal single-blade loading variation, blended with small local interactions. The near-field acoustics were then directly extracted from the high-fidelity CFD results, and far-field acoustic propagation was computed using the acoustic analogy, following reference acoustic measurements for eVTOLs proposed by EASA. Different aerodynamic interactions and propeller acoustic characteristics showed strong impacts on the vehicle acoustics. The EPNL metric was also computed and analysed assuming various hover duration, and restrictions on maximum EPNL and hover duration are recommended for future rotorcraft hover noise certification. These novel results provide valuable guidance for eVTOL acoustic design, infrastructure planning, and policy-making.

Numerical Evaluation of Aircraft Aerodynamic Static and Dynamic Stability Derivatives by a Mid-Fidelity Approach

The present study aimed to investigate the capability of mid-fidelity aerodynamic solvers in performing a preliminary evaluation of the static and dynamic stability derivatives of aircraft configurations in their design phase. In this work, the mid-fidelity aerodynamic solver DUST, which is based on the novel vortex particle method (VPM), was used to perform simulations of the static and dynamic motion conditions of the Stability And Control CONfiguration (SACCON): an unmanned combat aerial vehicle geometry developed by NATO’s Research and Technology Organisation (RTO), which is used as a benchmark test case in the literature for the evaluation of aircraft stability derivatives. Two different methods were exploited to extract the dynamic stability derivative values from the aerodynamic coefficient time histories that were calculated with DUST. The results for the mid-fidelity approach were in good agreement with the obtained experimental data, as well as with the results obtained using more demanding high-fidelity CFD simulations. This demonstrates its suitability when implemented in DUST for predicting the static and dynamic behavior of airloads in different conditions, as well as in reliably predicting the values of stability derivatives, with the advantage of requiring limited computational effort with respect to classical high-fidelity numerical approaches and the use of wind tunnel tests.

Detail and structural design of a fixed-wing BWB UAV

The current study focuses on the development of a prototype BWB UAV for highway traffic monitoring by supporting a Cooperative Intelligent Transport System (C-ITS). This system allows the monitoring of traffic conditions at large roads and highways. Having determined the mission requirements and concluded the aerodynamic conceptual and preliminary design phases, high fidelity CFD simulations are performed, aiming to calculate the key aerodynamic and stability characteristics of the platform and to optimize its performance throughout the mission. More specifically, regarding the aerodynamic vehicle design, results concerning the calculation of stability derivatives, control surfaces sizing, trim analysis and flight envelope (V-n diagram) are presented, along with the respective methodologies. Considering the structural design of the aircraft, a combination of layout, FE simulations and parameterized design tools were employed, allowing the design and sizing of the skin and the internal structural parts. The parts are mainly made of composite and additively manufactured nylon materials. Coupled interaction loops are conducted among the aerodynamic and structural analyses to optimize the overall performance of the aerial vehicle, maximizing the aerodynamic efficiency, and reducing the structural weight. Finally, the study is concluded by the presentation of the manufactured prototype of the UAV, which satisfies all the structural, aerodynamic, stability and performance requirements for the established highway traffic monitoring mission.

Computational Study on Compressor-Combustor Interactions for Improved Matching of a Micro Gas Turbine

ABSTRACT Understanding complex aerodynamics/combustion interactions across major components of aeroengines, including the compressor, combustor and turbine is demanded for the improved performance of advanced propulsion systems. This study employs high-fidelity CFD simulations to investigate the coupled compressor-combustor interactions for the improved performance of a micro gas turbine. In the prototype configuration, the deficient compressor/combustor matching causes insufficient air/fuel mixing and delayed combustion inside the combustor chamber. By reducing the flow area of the compressor and redistributing the air holes on the combustor chamber, a stable united recirculation structure is formed to promote air/fuel mixing and efficient combustion, achieving a over 50% decrease in OTDF and RTDF at the combustor exit and a total 20% decrease of pressure loss in the entire engine through-flow. Compared with the single-component combustor simulation, the coupled compressor-combustor results exhibit stronger turbulence mixing, increased temperature uniformity inside the combustor chamber. Compared with the single-component compressor simulation, the compressor in the multi-component simulation shows the reduced flow angle and Mach number at the compressor outlet, leading to increased stall critical angles and delayed flow separation at the compressor diffuser and vanes. This demonstrates that the added combustor endues the compressor with an expanded operating range and a lower mass-flow rate instability boundary. This work implies that 3D multi-component CFD simulation is necessary to understand the aerodynamic and combustion interactions and improve the multi-component matching in gas turbine engines.

Computational Rhinology: Unraveling Discrepancies between In Silico and In Vivo Nasal Airflow Assessments for Enhanced Clinical Decision Support.

Computational rhinology is a specialized branch of biomechanics leveraging engineering techniques for mathematical modelling and simulation to complement the medical field of rhinology. Computational rhinology has already contributed significantly to advancing our understanding of the nasal function, including airflow patterns, mucosal cooling, particle deposition, and drug delivery, and is foreseen as a crucial element in, e.g., the development of virtual surgery as a clinical, patient-specific decision support tool. The current paper delves into the field of computational rhinology from a nasal airflow perspective, highlighting the use of computational fluid dynamics to enhance diagnostics and treatment of breathing disorders. This paper consists of three distinct parts-an introduction to and review of the field of computational rhinology, a review of the published literature on in vitro and in silico studies of nasal airflow, and the presentation and analysis of previously unpublished high-fidelity CFD simulation data of in silico rhinomanometry. While the two first parts of this paper summarize the current status and challenges in the application of computational tools in rhinology, the last part addresses the gross disagreement commonly observed when comparing in silico and in vivo rhinomanometry results. It is concluded that this discrepancy cannot readily be explained by CFD model deficiencies caused by poor choice of turbulence model, insufficient spatial or temporal resolution, or neglecting transient effects. Hence, alternative explanations such as nasal cavity compliance or drag effects due to nasal hair should be investigated.

Aerodatabase Development and Integration and Mission Analysis of a Mach 2 Supersonic Civil Aircraft

The request for faster and greener civil aviation is urging the worldwide scientific community and aerospace industry to develop a new generation of supersonic aircraft, which are expected to be environmentally sustainable and to guarantee a high-level protection of citizens. A key aspect to monitor the potential environmental impact of new configurations is the aerodynamic efficiency and its impact onto the real mission. To pursue this goal, this paper discloses increasing-fidelity aerodynamic modeling approaches to improve the conceptual design of high-speed vehicles. The disclosed methodology foresees the development of aerodynamic aerodatabases by means of incremental steps starting from simplified methods (panels methods and/or low-fidelity CFD simulations) up to very reliable data based on high-fidelity CFD simulations and experimental measurements with associated confidence levels. This multifidelity approach enables the possibility of supporting the aircraft design process at different stages of its design cycle, from the estimation of preliminary aerodynamic coefficients at the beginning of the conceptual design, up to the development of tailored aerodatabases at advanced design phases. For each design stage, a build-up approach is adopted, starting from the investigation of the clean external configuration up to the complete one, including control surfaces’ effects and, if any, the effects of the integration of the propulsive effects. In addition, the applicability of the approach is guaranteed for a wide range of supersonic and hypersonic aircraft, and the developed methodology is here applied to the characterization of Mach 2 aircraft configuration, a relevant case study of the H2020 MORE&LESS project.

A wall-modeled hybrid RANS/LES model for flow around circular cylinder with coherent structures in subcritical Reynolds number regions

In the present paper, a hybrid RANS/LES model with the wall-modelled LES capability (called WM-HRL model) is developed to perform the high-fidelity CFD simulation investigation for complex flow phenomena around a bluff body with coherent structure in subcritical Reynolds number region. The proposed method can achieve a fast and seamless transition from RANS to LES through a filter parameter <i>r</i><sub>k</sub> which is only related to the space resolution capability of the local grid system for various turbulent scales. Furthermore, the boundary positions of the transition region from RANS to LES, as well as the resolving capabilities for the turbulent kinetic energy in the three regions, i.e. RANS, LES and transition region, can be preset by two guide index parameters <i>nr</i><sub>k1-Q</sub> and <i>nr</i><sub>k2-Q</sub>. Through a series of numerical simulations of the flow around a circular cylinder at Reynolds number <i>Re</i> = 3900, the combination conditions are obtained for such two guide index parameters <i>nr</i><sub>k1-Q</sub> and <i>nr</i><sub>k2-Q</sub> that have the capability of high-fidelity resolving and capturing temporally- and spatially-developing coherent structures for such complex three-dimensional flows around such a circular cylinder. The results demonstrate that the new WM-HRL model is capable of accurately resolving and capturing the fine spectral structures of the small-scale Kelvin-Helmholtz instability in the shear layer for flow around such a circular cylinder. Furthermore, under a consistent grid system, through different combinations of these two guide index parameters <i>r</i><sub>k1</sub> and <i>r</i><sub>k2</sub>, the fine structuresof the recirculation zones with two different lengths and the U-shaped and V-shaped distribution of the average stream-wise velocity in the cylinder near the wake can also be obtained.

On the aerodynamic performance of redundant propellers for multi-rotor eVTOL in cruise

As multi-rotor and convertible configurations gain compelling popularity in Advanced Air Mobility designs, the handling of redundant propellers during cruise flight regimes becomes a new challenge. This paper presents a systematic study of the aerodynamic performance of feathered and windmilling redundant propellers on a simplified multi-rotor, multi-wing configuration. High-fidelity CFD simulations were performed with a systematic test matrix including the blade feathering angle, azimuth shift, coning angle, and windmilling pitch angle and RPM. For the feathered blades, the feathering angle had a strong impact on the aerodynamic performance. The feathered blades could substantially reduce the overall lift and increase the overall drag at high feathering angles (up to 25% lift reduction and 70% drag increase). This is correlated with changes in the sectional wing inflow angles induced by the upstream feathered blades. Windmilling propellers, in ideal cases, were found capable of considerable wind energy extraction (50% of required cruise power) and could create slowed inflows for downstream thrusting propellers, at the cost of excessive drag (up to 90% more). Comparisons of vehicle performance with feathered and windmilling blades, showed similar aerodynamic forces and energy consumption. This shows that windmilling could be a feasible option for redundant propellers, given careful balancing of the drag and energy conversions. The presented results provide valuable guidance for the handling of redundant blades, for future multi-rotor aircraft designs for sustainable aviation.

Understanding Tornadic Wind Effects on Manufactured or Mobile Homes through High-Fidelity CFD Simulations

Understanding Tornadic Wind Effects on Manufactured or Mobile Homes through High-Fidelity CFD Simulations

A geometric sensitivity study for the aerodynamics of a strut-braced airframe

A global sensitivity analysis method, the adaptive-cut high-dimensional model representation method (HDMR) is considered to evaluate the impact of geometrical changes of a ultra-high aspect ratio strut-braced wing configuration on the aerodynamic performance. A data-driven reduced order model based on Proper Orthogonal Decomposition is used to keep the computational cost of the analysis at a manageable level. The airframe configuration is described using 7 geometrical parameters, which comprise the design space. The geometrical parameters are decomposed and analysed across their whole range of values by the HDMR to assess their influence on the Drag coefficient, Lift coefficient and Lift-to-Drag ratio of the aircraft. These results form the basis for a qualitative exploration of the aerodynamics of such airframes further augmented by high-fidelity CFD simulations. Results show that the sweep angle of the wing is the dominant parameter in terms of Drag due to changes in shock wave intensity. Changes in the wing root chord also implicitly affect the local geometry at the wing-strut junction, which influences the blockage effect and thus the intensity of the shock waves at the junction. Lift is affected primarily by the twist of the wing. Overall, the strut is shown to have significant effects on the performance of the aircraft design due to the presence of strong shock waves at the wing-strut junction and interference effects.

Neural-physics multi-fidelity model with active learning and uncertainty quantification for GPU-enabled microfluidic concentration gradient generator design

The microfluidic concentration gradient generator (μCGG) is an important biomedical device to generate concentration gradients (CGs) of biomolecules at the microscale. Nonetheless, determining their operational parameter values to generate complex, user-specific, biologically desired CGs is not trivial. This paper presents a neural-physics multi-fidelity model (NP-MFM) to predict CGs with equivalent accuracy as high-fidelity CFD simulation at ultra-fast computational speed through a novel uncertainty- and distance-based active learning process. The verified NP-MFM, along with the genetic algorithm, is implemented on a GPU platform to search optimal values of operational parameters that generate CGs closely matching user-prescribed profiles. Results show that the NP-MFM is a feasible multi-fidelity modeling approach for rapid and accurate prediction of CGs (with 0.019s/simulation) and can be used for GPU-enabled μCGG design optimization and automation. Furthermore, design CGs generated by the proposed method match user-prescribed CGs very well with an averaged discrepancy less than 0.34.

Semi-analytical and CFD formulations of a spherical floater

Today, humanity is facing the great pressure of fossil fuels exhaustion and environmental pollution. This obliges governments and industries to make accelerated efforts on producing green energy. The focus is spotted on marine environment which is a vast source of renewable energy. Among several classes of designs proposed for wave energy conversion, spherical Wave EnergyConverters (WECs) have received considerable attention. The problems of water wave diffraction and radiation by a sphere has been examined by a substantial amount of literature, i.e., [1]–[4], whereas in [5]–[8] linear hydrodynamic effects on a spherical WEC have been examined. All these research works are based on potential flow methodologies. Nevertheless, overthe last decade there has been a significant interest on Computational Fluid Dynamics CFD modelling due to its detailed results, focusing also to spherical WECs [9]–[10].In the present work a semi-analytical model is applied to solve the wave radiation problem around a spherical WEC (Figure 1), in the context of linear potential theory. The outcomes of the theoretical analysis are supplemented and compared with high fidelity CFD simulations (Figure 2 for a semi-submerged sphere). Furthermore, the two methodologies are compared with a theoretical approach for the hydrodynamic analysis of floating bodies with vertical axis as being presented in [11]. The method is based on the discretization of the flow field around the body using coaxial ring elements, which are generated from the approximation of the sphere’s meridian line by a stepped curve.Numerical results are given from the comparison of the three formulations, and some interesting phenomena are discussed concerning the viscous effects on the floater.
 [1] Havelock, T. H. 1955. Wave due to a floating sphere making periodic heaving oscillations. R. Soc. London,A231, 1-7.[2] Hulme, A. 1982. The wave forces acting on a floating hemisphere undergoing force periodic oscillation. J. FluidMech., 121, 443-463.[3] Wang, S. 1986. Motions of a spherical submarine in waves. Ocean Engng., 13, 249-271.[4] Wu, G.X. 1995. The interaction of water waves with a group of submerged spheres. Appl. Ocean. Res., 17, 165-184.[5] Srokosz, M.A. 1979. The submerged sphere as an absorber of wave power. J. Fluid Mech., 95, 717-741.[6] Thomas, G.P., Evans, D.V. 1981. Arrays of three-dimensional wave energy absorbers. J. Fluid Mech., 108, 67-88.[7] Linton, C.M. 1991. Radiation and diffraction of water waves by a submerged sphere in finite depth. Ocean Engng.,18, 61-74.[8] Meng, F., et al. Modal analysis of a submerged spherical point absorber with asymmetric mass distribution.Renew. Energy 130, 223-237.[9] Shami, E.A., et al. 2021. Non-linear dynamic simulations of two-body wave energy converters via identificationof viscous drag coefficients of different shapes of the submerged body based on numerical wave tank CFD simulation.Renew. Energy, 179, 983-997.[10] Katsidoniotaki, E., et al. 2023. Validation of a CFD model for wave energy system dynamics in extreme waves.Ocean Engng., 268, 113320.[11] Kokkinowrachos, K., et al. 1986. Behaviour of vertical bodies of revolution in waves. Ocean Engng., 13, 505-538

A study on tidal rotors under the combined effects of currents and waves using actuator-line CFD simulations

Determining loads and power on a tidal rotor under realistic flow conditions is challenging. Methods used by offshore industries for the design and deployment of marine devices rely on simulations of a significant number of cases that combine different environmental and operational conditions. Design studies of tidal rotors rely on engineering models of low to medium computational cost that often omit important hydrodynamic effects such as blockage, multi-rotor interactions, wave diffraction, etc., and have a questionable accuracy under conditions such as high-thrust regimes or highly unsteady flows. Alternatively, high-fidelity CFD simulations can implicitly capture most of the relevant physics, but their computational cost often limits their application in engineering practice.
 This study will present a numerical study of a tidal rotor operating near the free surface and affected by combined currents and surface waves. The simulations will be performed with an intermediate fidelity actuator-line model implemented within OpenFOAM. The AL model is embedded within a simulated tank discretised with the finite volume method, that employs a RANS turbulence model with a Volume of Fluid (VoF) approach for the free surface. Relaxation zones, implemented with the Waves2Foam library, are used for the generation and absorption of surface waves and reflections. A preliminary render of the assembled model simulating a test condition can be seen in fig. 1.
 The paper manuscript will present the results of an initial validation of the modelling strategy that will compare simulated cases with towing-tank experiments. The validation will be followed by a quantification of the interactions between the rotor and the free surface, as well as an analysis of the transient loads and power fluctuations for different operating conditions. 

Modeling and simulation of a micro gas-cooled nuclear reactor using Modelica

Highly compact micro nuclear reactors, which offer extensive energy benefits across ocean, land, space, and sky applications, have recently emerged as a popular research topic within the international nuclear industry. Due to its excellent inherent safety characteristics, the gas-cooled graphite-moderated reactor with TRISO fuel has attained extensive attention. Nonetheless, micro-reactors exhibit a high degree of system integration, characterized by the tight coupling and mutual constraints among various system functions. Conventional discipline-specific decoupled design patterns find it challenging to tackle the complexity arising from multi-disciplinary couplings. In response, this paper investigates the application of Modelica, a multi-domain unified modeling language, to construct models for several subsystems, encompassing the reactor, energy conversion system, and control system. This approach aims to enhance support for cross-disciplinary design. The accuracy of the reactor core model was verified by high-fidelity CFD simulation results, demonstrating a good agreement. Further investigations were then conducted on the safety and operational characteristics of the whole system. Typically, two simulations were conducted on the Gas cooled micro nuclear reactor (GCMR) design: one focused on an anticipated transients without scram accident scenario and the other on load-following operation. The simulation results demonstrated that the reactor possesses excellent inherent safety, even during extreme accidents. In such scenarios, the reactor is able to achieve shutdown solely through the negative reactivity resulting from increased core temperature. Furthermore, considering the heat accommodated in the reactor system and the constantly generated decay heat, a passive air-cooling mechanism has been investigated and successfully demonstrated with the model. The reactor also exhibits good load-following performance, which can be achieved by simply adjusting helium inventory (or pressure) and control drum position, while maintaining constant core temperature and power generation efficiency. These results can be leveraged to provide guidelines for further detailed designs of the GCMR.

Intelligent mesh refinement based on U-NET for high-fidelity CFD simulation in numerical reactor

Thermal-hydraulic analysis of reactors is fundamental for studying many service behaviors inside the reactor core, and high-fidelity thermal-hydraulic simulation based on advanced computing techniques and supercomputing resources has attracted widespread attention. The construction of high-quality fluid domain mesh models for reactors with complex and sophisticated core structures is fundamental for high-fidelity thermal-hydraulic simulations. However, mesh models built entirely with commercial tools have limitations in terms of model quality and construction performance. An intelligent mesh refinement method based on the U-Net neural network model (Unet-IMR) is designed, which attempts to use AI technology to assist in generating high-quality mesh models. Specifically, an intelligent prediction model is constructed and used to predict the poor mesh in a given mesh model, and the designed region-sensitive refinement strategy is further applied to iteratively optimize and refine the mesh to construct a high-quality mesh model. Experimental results show that the mesh distribution and the transition between sparse and dense meshes are more reasonable in the Unet-IMR based mesh model, and meshes located at sophisticated locations have relatively high goodness-of-fit. All the meshes in the model can pass the correctness checks of the CVR-PACA software, and the simulation results are consistent with physical expectations. Furthermore, the Unet-IMR based mesh model can achieve similar simulation results with fewer meshes compared to the mesh model built with commercial tools.

Data-driven aerodynamic models for aeroelastic simulations

Multiple approaches are available for calculating the time-dependent aerodynamic loads of thin, flexible structures subjected to airflow: analytical, semi-empirical, CFD-based, and various reduced-order models. Analytical and semi-empirical models are usually limited for certain structures (e.g., wing profiles, bridge sections, cylinders and prismatic bodies) and small deformations. In the case of large deformations, reduced-order models and CFD simulations are used to calculate aerodynamic loads, but these approaches are computationally costly. We applied a data-based identification method to calculate the aerodynamic loads acting on a flat plate performing heaving and pitching motions. The data-based model was based on high fidelity CFD simulations. The SINDy (Sparse Identification of Nonlinear Dynamics) algorithm was used for model construction. The LASSO (Least Absolute Shrinkage and Selection Operator) and STLSQ (Sequentially Thresholded Least Squares) optimization routines were used to fit the coefficients of the aerodynamic model. The individual models obtained using this procedure have a limited validity range in the reduced frequency of the system. To extend the applicability of the model, linear interpolation was used. The resulting combined model was applied to a two-degree-of-freedom (2-DOF) aeroelastic system. Stability and numerical bifurcation analysis were carried out to investigate the dynamics of the system. The most significant advantage of this method is that the aeroelastic simulations run very quickly after the initial model identification process for a wide range of system parameters and are accurate for large deformation angles.

High-fidelity aerodynamic and acoustic design and analysis of a heavy-lift eVTOL

This work presents the high-fidelity aerodynamic and acoustic modelling processes supporting the preliminary design of a large eVTOL vehicle at the University of Glasgow in collaboration with GKN Aerospace. To support the GKN heavy-lift eVTOL design, known as Skybus, a range of tools of various fidelity levels were adopted and integrated. This paper first presents the conceptual design and initial sizing of the Skybus vehicle. The airframe design was then analysed using high-fidelity methods with parametric variations of wing an/di-hedral designs. By adjusting the interactions between the wings and the airframe, 5∘ front wing anhedral combined with 10∘ aft wing dihedral offered the maximum lift increase. High-fidelity CFD simulations of the complete Skybus vehicle in forward flight with two and four operating propellers were later carried out. The aerodynamic interactions between the propellers and the wings were analysed in detail. In cruise, the four-rotor configuration showed stronger aerodynamic interference effects and hence required more power than the two-rotor counterpart. Near-field acoustics of the vehicle was then extracted directly from the flow solutions and analysed. Far-field noise features were also computed using the FW-H equations and the CFD solutions. The peak noise levels of the heavy-lift vehicle in forward flight at 90 m/s were about 75 dB SPL perceived on the ground 1000 m below. Acoustic features and differences of the two configurations along the flight path and in the lateral directions were presented and discussed.

Multi-fidelity aerodynamic design and analysis of propellers for a heavy-lift eVTOL

This paper presents a multi-fidelity propeller design and analysis process, and demonstrates it for the preliminary design of a heavy-lift eVTOL vehicle proposed by GKN aerospace, known as Skybus. The multi-fidelity framework integrates tools and results of variable fidelity levels. A global scan of the design space was first carried out at the low-fidelity stage. The low-fidelity output was converted to 3D shapes and grids through an automatic meshing tool, and high-fidelity CFD simulations and gradient-based optimisation were launched to deliver further improved designs. The method was first verified using a benchmark multi-modal test function. The framework was then demonstrated for the propeller design of the Skybus vehicle. Rectangular blade designs with linear twist were first derived through the Blade Element Momentum Theory and used as the input to the low-fidelity stage for simplicity. Upon the baseline design, the low-fidelity stage managed to find an improved shape with 3% reduced power, while the high-fidelity optimisation further reduced the power by 2.2% under the equality thrust constraint. In addition, this work also reports the propeller pitch-RPM performance map tool that has effectively supported the Skybus development through fast performance predictions and operating condition determination. For propellers with both pitch and RPM regulation, the performance map explicitly correlates various performance scopes and to aid optimisation. The multi-fidelity construction of the performance map and demonstrations of its usage are then presented.

Assessment of an E10 gasoline surrogate: Qualitative and quantitative comparisons of in-cylinder spray morphology

A minimum-component gasoline fuel surrogate that captures both chemical and physical behaviors of a full-distillate fuel is needed for high-fidelity CFD simulations. This study evaluates gasoline spray characteristics in a direct-injection spark-ignition engine under motored operation. Two fuels are compared; PACE-20, which is a 9-component surrogate formulation of RD5-87, is compared with its target fuel RD5-87, which is a full-boiling range research grade E10 gasoline. The spray morphologies of both fuels are recorded for a centrally-located direct-injection 8-hole spray subject to intake air cross-flow during the early part of the intake stroke. High-speed imaging recorded scattered light of the side and axial projections of the liquid spray. Quantitative metrics were developed and employed to facilitate comparison of spray morphologies as well as to identify the transition in spray morphology due to flash boiling. This paper builds on a previous study of RD5-87 where coolant temperature (20°C–100°C), in-cylinder pressure (40–110 kPa), engine speed (650–1950 rpm), and injection pressure (60–180 bar) were systematically changed to span operating conditions with and without flash boiling. Images of the PACE-20 morphology are selected for a sub-set of operating conditions from the previous study where distinctive morphology changes occurred. Visual inspection of the images and quantitative metrics demonstrate that the PACE-20 spray morphology is equivalent to that of the RD5-87 in most cases. The exception was for changes in the ambient-gas pressure where the flash-boiling transition occurred at ∼5 kPa higher in-cylinder pressure for PACE-20. Three empirical metrics, Merging Index, Asymmetry, and Flash Index are proposed here and they were found to be useful both as quantitative comparisons of the fuel morphologies, and for identifying the transition in spray morphology due to flash boiling.