quasi-3D Simulations Research Articles

Enhanced quasi-three-dimensional transient simulation technique incorporating component volume effects for aero engine

Current transient analysis predominantly relies on zero-dimensional/one-dimensional tools, proficient at capturing aerothermodynamic variations across critical engine stations but insufficient for analyzing the internal flow field evolution during transients. Addressing this gap, the study presents an enhanced quasi-three dimensional (quasi-3D) transient simulation technique that integrates component volume effects, offering a significant leap from the preceding quasi-3D transient simulation method based on quasi-steady assumption. By embedding the component volume effects on density, momentum, and energy within the physical temporal dimension of the Navier-Stokes equations, the refined quasi-3D transient model achieves a closer representation of physical phenomena. Validation against a single-shaft turbofan engine’s experimental data confirms the model’s accuracy. Average errors for key performance indicators, including shaft speed, thrust, mass flow rate, and critical component exit temperature and pressure, remain below 0.41%, 5.69%, 2.55%, 3.18% and 0.67%, respectively. Crucially, the model exposes a discernible temporal lag in the compressor outlet pressure and temperature response due to volume effects—previously unquantified in quasi-3D transient simulations. And further exploration of the meridional flow field emphasizes the consequential role of volumes in transient flow field evolution. Incorporating volume effects within quasi-3D transient simulations enhances engine modeling and is pivotal for precise transient analysis in engine design and optimization.

Heuristic inverse design of integrated mode converter by directly reshaping silicon waveguide

Compact photonic devices for mode manipulation in silicon waveguide are of importance as fundamental building blocks in on-chip photonic integrated circuits. In this paper, we present a heuristic inverse design scheme based on directly reshaping the silicon waveguide for TE-polarized mode conversions. No additional complicated internal nanoscale structures are introduced to the waveguide. The boundary deformations of the 6 μm-long functional region are described by Bernstein polynomials and the optimal design is searched and determined by the gradient-based method of moving asymptotes. The TE0-to-TE1 (TE1-to-TE2) mode converter can be efficiently obtained after 19 (52) iterations from a straight waveguide. Quasi-3D simulations show the high mode purity (>0.99) at center wavelength of 1550 nm, while the lowest mode purity keeps higher than 0.97 within the operational bandwidth of 60 nm for all designs. Relatively high transmission efficiency is maintained as well. 3D simulations with SOI configuration validate the functionality and the satisfactory performances. For both 340 nm and 220 nm waveguide thicknesses, the conversion efficiencies respectively keep higher than 0.98 and 0.90 for TE0-to-TE1 and TE1-to-TE2 mode converters at center wavelength. By simply cascading the two proposed mode converters, reciprocal TE0-to-TE2 mode conversion can be achieved. Moreover, we demonstrate the robustness of proposed designs by adding ±10 nm geometric deviations.

Photon Acceleration from Optical to XUV.

The propagating density gradients of a plasma wakefield may frequency upshift a trailing witness laser pulse, a process known as "photon acceleration." In uniform plasma, the witness laser will eventually dephase because of group delay. We find phase-matching conditions for the pulse using a tailored density profile. An analytic solution for a 1D nonlinear plasma wake with an electron beam driver indicates that, even though the plasma density decreases, the frequency shift reaches no asymptotic limit, i.e., is unlimited provided the wake can be sustained. In fully self-consistent 1D particle-in-cell (PIC) simulations, more than 40 times frequency shifts were demonstrated. In quasi-3D PIC simulations, frequency shifts up to 10 times were observed, limited only by simulation resolution and nonoptimized driver evolution. The pulse energy increases in this process, by a factor of 5, and the pulse is guided and temporally compressed by group velocity dispersion, resulting in the resulting extreme ultraviolet laser pulse having near-relativistic (a_{0}∼0.4) intensity.

Prediction of spontaneous imbibition with gravity in porous media micromodels

In this work, theoretical modelling, quasi-three-dimensional (quasi-3D) simulations and micromodel experiments are conducted to study spontaneous imbibition with gravity in porous media micromodels. By establishing the force balance governing the spontaneous imbibition process, we develop a theoretical model for predicting the imbibition length against time in a rectangular capillary. The theoretical model is then extended to the prediction of a compact displacement process in a micromodel by using an equivalent width, which is derived by analogising the micromodel to a rectangular capillary. By simulating spontaneous imbibition in a rectangular capillary with various aspect ratios ( $\varepsilon$ ), we show that the application condition of the quasi-3D method is $\varepsilon \leqslant 1/3$ . Next, we simulate spontaneous imbibition in micromodels with various geometries and flow conditions. Fingering and compact displacement are identified for varying viscosity ratios and gravitational accelerations. At low (high) viscosity ratio of wetting to non-wetting fluids, an upward (downward) gravity can promote the stability of the wetting front, favouring the transition from fingering to compact displacement. In addition, we find that the depth-oriented interface curvature dominates the capillary effect during the imbibition, and such a mechanism is considered by introducing an equivalent contact angle into the theoretical model. With the help of equivalent width and contact angle, the theoretical model is shown to provide satisfactory prediction of the compact displacement process. Finally, a micromodel experiment is presented to further verify the developed theoretical model and the quasi-3D simulation.

Nektar++: Enhancing the capability and application of high-fidelity spectral/[formula omitted] element methods

Nektar++ is an open-source framework that provides a flexible, high-performance and scalable platform for the development of solvers for partial differential equations using the high-order spectral/hp element method. In particular, Nektar++ aims to overcome the complex implementation challenges that are often associated with high-order methods, thereby allowing them to be more readily used in a wide range of application areas. In this paper, we present the algorithmic, implementation and application developments associated with our Nektar++ version 5.0 release. We describe some of the key software and performance developments, including our strategies on parallel I/O, on in situ processing, the use of collective operations for exploiting current and emerging hardware, and interfaces to enable multi-solver coupling. Furthermore, we provide details on a newly developed Python interface that enables a more rapid introduction for new users unfamiliar with spectral/hp element methods, C++ and/or Nektar++. This release also incorporates a number of numerical method developments – in particular: the method of moving frames (MMF), which provides an additional approach for the simulation of equations on embedded curvilinear manifolds and domains; a means of handling spatially variable polynomial order; and a novel technique for quasi-3D simulations (which combine a 2D spectral element and 1D Fourier spectral method) to permit spatially-varying perturbations to the geometry in the homogeneous direction. Finally, we demonstrate the new application-level features provided in this release, namely: a facility for generating high-order curvilinear meshes called NekMesh; a novel new AcousticSolver for aeroacoustic problems; our development of a ‘thick’ strip model for the modelling of fluid–structure interaction (FSI) problems in the context of vortex-induced vibrations (VIV). We conclude by commenting on some lessons learned and by discussing some directions for future code development and expansion. Program summaryProgram Title: Nektar++Program Files doi:http://dx.doi.org/10.17632/9drxd9d8nx.1Code Ocean Capsule:https://doi.org/10.24433/CO.9865757.v1Licensing provisions: MITProgramming language: C++External routines/libraries: Boost, METIS, FFTW, MPI, Scotch, PETSc, TinyXML, HDF5, OpenCASCADE, CWIPINature of problem: The Nektar++ framework is designed to enable the discretisation and solution of time-independent or time-dependent partial differential equations.Solution method: spectral/hp element method

Quasi 3D Nacelle Design to Simulate Crosswind Flows: Merits and Challenges

This paper studies the computational modelling of the flow separation over the engine nacelle lips under the off-design condition of significant crosswind. A numerical framework is set up to reproduce the general flow characteristics under crosswinds with increasing engine mass flow rate, which include: low-speed separation, attached flow and high speed shock-induced separation. A quasi-3D (Q3D) duct extraction method from the full 3D (F3D) simulations has been developed. Results obtained from the Q3D simulations are shown to largely reproduce the trends observed (isentropic Mach number variations and high-speed separation behaviour) in the 3D intake, substantially reducing the simulation time by a factor of 50. The agreement between the F3D and Q3D simulations is encouraging when the flow either fully attached or with modest levels of separation but degrades when the flow fully detaches. Results are shown to deviate beyond this limit since the captured streamtube shape (and hence the corresponding Q3D duct shape) changes with the mass flow rate. Interestingly, the drooped intake investigated in the current study is prone to earlier separation under crosswinds when compared to an axisymmetric intake. Implications of these results on the industrial nacelle lip design are also discussed.

Deterioration effects of coupled blisk blades

Performance degradation due to wear of high pressure compressors (HPC) is a major concern in aero-engine operation and maintenance. Among other effects especially erosion of airfoils leads to changed aerodynamic behavior and therefore to deterioration. These affects engine performance parameter like thrust specific fuel consumption (TSFC) and exhaust gas temperature (EGT). Reaching EGT-limit, the engine typically has to be overhauled during a shop visit to restore safety standards and performance. During state of the art shop visits, engines are repaired based on EGT-specifications. To further enhance the maintenance, tailored repairs for each jet engine based on engine history and operation conditions are necessary to take TSFC into account. To ensure such an effective maintenance, the aerodynamic behavior of deteriorated and repaired airfoils is the key factors. Therefore, geometric properties with high influence on aerodynamic performance have to be known. For blisks (BLade-Integrated-diSK) the approach of tailored maintenance will be even more complicated because the airfoil arrangement cannot be changed or individual airfoils cannot be replaced. Thus, the effects of coupled misshaped airfoils have a high significance. This study will present a Design of Experiments (DoE) for circumferential coupled HPC-airfoils to identify the geometric properties which lead to a reduction of performance. To focus on geometric variations, quasi3D (Q3D) simulations are taken out. Based on a sensitivity analysis, the thickness related parameters, the stagger angle as well as the max. profile camber thickness are identified as the most important parameters which are influencing adjacent airfoils and reduce the aerodynamic performance.

Turn-off instabilities in large area IGBTs

In this paper, an experimental study is proposed to investigate the failure effects in IGBT large-area devices due to hard gate driving strategies. Thanks to quasi-3D simulations, a large overcurrent is predicted in few cells and corresponds to a large spike in the gate voltage during collector voltage transient, which is able to trigger the device instability.An interpretation of the phenomenon is given that attributes such spike to a strong current imbalance on the large area device, so that a latch-up failure mechanism is proposed.

QUASI-3D MODELLING OF SURF ZONE DYNAMICS

A quasi-three-dimensional model (quasi-3D) has been developed through the implementation of an analytical 1DV flow model in existing depth-averaged shallow water equations. The model includes the effects of waves and wind on the vertical distribution of the horizontal velocities. Comparisons with data from both physical and field cases show that the quasi-3D approach is able to combine the effect of vertical structures with the efficiency of depth-averaged simulations.
 Inter-comparisons with three-dimensional simulations show that the quasi-3D approach can represent similar velocity profiles in the surf zone. Quasi-3D morphodynamic simulations show that the bed dynamics in the surf zone represent the
 relevant 3D effects in the surf zone much more than the depth-averaged computations. It was shown that the quasi-3D approach is computationally efficient as it only adds about 15-20% to the runtimes of a 2DH simulation which is minor compared to a run time increase of 250-800% when switching to a 3D simulation.

A PIC-MCC code for simulation of streamer propagation in air

A particle code has been developed to study the distribution and acceleration of electrons in electric discharges in air. The code can follow the evolution of a discharge from the initial stage of a single free electron in a background electric field to the formation of an electron avalanche and its transition into a streamer. The code is in 2D axi-symmetric coordinates, allowing quasi 3D simulations during the initial stages of streamer formation. This is important for realistic simulations of problems where space charge fields are essential such as in streamer formation. The charged particles are followed in a Cartesian mesh and the electric field is updated with Poisson’s equation from the charged particle densities. Collisional processes between electrons and air molecules are simulated with a Monte Carlo technique, according to cross section probabilities. The code also includes photoionisation processes of air molecules by photons emitted by excited constituents. The paper describes the code and presents some results of streamer development at 70km altitude in the mesosphere where electrical discharges (sprites) are generated above severe thunderstorms and at ∼10km relevant for lightning and thundercloud electrification. The code is used to study acceleration of thermal seed electrons in streamers and to understand the conditions under which electrons may reach energies in the runaway regime. This is the first study in air, with a particle model with realistic spatial dependencies of the electrostatic field. It is shown that at 1atm pressure the electric field must exceed ∼7.5 times the breakdown field to observe runaway electrons in a constant electric field. This value is close to the field where the electric force on an electron equals the maximum frictional force on an electron – found at ∼100eV. It is also found that this value is reached in a negative streamer tip at 10km altitude when the background electric field equals ∼3 times the breakdown field. At higher altitudes, the background electric field must be relatively larger to create a similar field in a streamer tip because of increased influence of photoionisation. It is shown that the role of photoionization increases with altitude and the effect is to decrease the space charge fields and increase the streamer propagation velocity. Finally, effects of electrons in the runaway regime on negative streamer dynamics are presented. It is shown the energetic electrons create enhanced ionization in front of negative streamers. The simulations suggest that the thermal runaway mechanism may operate at lower altitudes and be associated with lightning and thundercloud electrification while the mechanism is unlikely to be important in sprite generation at higher altitudes in the mesosphere.

Applicability of Quasi-3D and 3D MOSFET Simulations in the ‘Atomistic’ Regime

The importance of intrinsic fluctuations to the next generation of CMOS circuits has renewed interest in simplified, computationally inexpensive routes to the analysis of circuit parameters such as threshold voltage and subthreshold slope in ensembles of devices. A quasi-3D analytic approach to the statistical analysis of these parameters in 100 × 100 nm, 70 × 70 nm and 35 × 35 nm devices has been compared to the more computationally expensive full 3D simulation. The quasi-3D approach is useful in predicting variations in subthreshold slope, although its predictions become inaccurate for devices of approximately 35 × 35 nm or smaller. It is less effective in considering variations in threshold voltage, erroneously predicting a rise of the ensemble average threshold voltage and significantly exaggerating the threshold voltage variations over the ensemble.

An accurate on-resistance model for low voltage VDMOS devices

An accurate on-resistance model of VDMOS devices in the low voltage regimes is proposed and verified by numerical simulation results. The model considers the lateral Gaussian doping profiles in the channel region and exact current spreading angles in the epitaxial layer for both linear and cellular geometries by employing the conformal mapping. Results of the proposed model are in good agreement with those of 2D and quasi-3D simulations obtained by PISCES-IIB for a VDMOS device with 100 V rating. The on-resistance of a low voltage VDMOS may be considerably overestimated if it is analyzed by the conventional method.