Large eddy simulations (LES) are becoming increasingly popular in engineering to predict complex physical interactions governed by the temporal dynamics of the flow. In the context of propulsion systems with realistic combustors, LES is the ideal choice for modeling complex processes involving multiphase transport, vaporization, and reacting flow physics that span a wide range of scales. However, simulations of a full annular combustor have historically remained out of reach due to the steep cost associated with resolving the energetic scales in systems with such exceedingly complex geometries. Fortunately, with recent advances in supercomputing capabilities, such simulations are now possible and can be used to understand the underpinnings of combustion physics at scales that are often unavailable to experiments. Thus, the objective of this study is to investigate the ability of LES to predict turbulent combustion subject to system-level effects by considering a 360-degree annular combustor from the Energy Efficient Engine (EEE) program. In addition to showcasing LES calculations of the annular combustor, this study investigates the effects of incorporating a complex geometry on predicting self-sustaining oscillatory flows and its impact on combustion. To accomplish this, the massively parallel LES solver, CharLES, is employed, which utilizes a fully compressible Navier-Stokes Equation (NSE) solver and a flamelet progress variable approach on unstructured Voronoi-based cells. Due to the importance of estimating adequate grid size, the effect of mesh resolution is investigated by computing results on three grids and comparing results in terms of mean and fluctuating fields. A non-reacting analysis is first conducted to understand the unsteady fluid dynamics arising from the central recirculation zone, which plays an important role inflame stabilization. Next, the progress variable method is utilized to generate a combustion model coupled with Lagrangian particle tracking models for fuel spray breakup, vaporization, and ignition. The results, when compared to combustor exit temperature measurements from experiments, as well as previous LES simulations in the literature, show good agreement. Further analysis is conducted showing the onset of self-sustained oscillatory flow, owing to the azimuthal coupling, and proper orthogonal decomposition (POD) analysis shows the most energetic modes. Overall, the results are promising and present a novel tool for guiding the development of stable and reliable systems. This work is conducted in support of the DoD High Performance Computing Modernization Program (HPCMP) Frontier program at ARL and NAVAIR in collaboration with the University of Cincinnati, Cadence Design Systems, and Pratt & Whitney.

A fast approach is introduced to aid the evaluation and design of rotating detonation engines (RDEs) using large eddy simulation (LES). The new approach implements an extended flamelet progress variable model in the GPU-accelerated charLES CFD solver from Cascade Technologies. In the extended flamelet approach, tabulated reaction rates from unstretched laminar premixed flames are parameterized and scaled to describe the high-pressure detonation conditions in RDEs. The reaction rate scaling parameters are trained using Zeldovich-von Neumann-Döring (ZND) detonation solutions over a wide range of operating conditions. Sensitivity analysis is performed to assess the impact of the regression algorithm and best-fit scaling parameters on the reconstructed reaction rates. The method is applied to LES of the AFRL 6-inch rotating detonation engine with non-premixed injection of hydrogen in air. The GPU-resident charLES solver demonstrates speedup of almost two orders of magnitude compared to conventional CPU-based solvers and achieves turnaround of high-fidelity RDE simulations in less than one day on modest GPU resources — a critical threshold to enable the use of LES within a design cycle. The combination of reduced-order flamelet modeling and efficient GPU acceleration makes the overall approach a viable candidate for design exploration and optimization of practical RDEs.

Large eddy simulations of supersonic jets from a twin rectangular nozzle are performed with the GPU-accelerated version of Cascade's charLES" compressible flow solver. Comparisons of the numerical noise predictions with near-field and far-field microphone measurements show good agreement for the screech tone frequency, broadband spectra and overall sound pressure levels. For the screech tone amplitude, the agreement is further improved when the reflective surfaces upstream of the nozzle exit are modified in the computational domain to account for the thick acoustic foam covering these surfaces in the wind-tunnel. Such modifications and efficient exploration of the nozzle design space are made possible by the increased computational throughput with the GPU-accelerated solver: for the present O(100) million cell mesh and relatively long noise data collection of 2000 acoustic time units, the simulation results are obtained in about 12 hours on 30 standard GPUs.

A simple algebraic Volume-of-Fluid (VoF) method has been developed based on the finite-volume formulation to solve the transport equation of volume fraction on arbitrary unstructured grids. A blended high-resolution scheme is newly devised in order to preserve the sharpness of material interfaces with minimum numerical dissipation. The high resolution scheme employs a piecewise logistic function in terms of the signed distance to approximate the face fluxes in the VoF transport equation. The face fluxes are blended with those obtained from the central difference scheme based on the directional derivatives of the interface function to reduce the numerical dissipation. Flux limiters are applied to ensure the boundedness of the volume fractions while conserving phase volume. The numerical algorithm is implemented and tested on Voronoi meshes for several canonical problems. The comparative study has demonstrated the enhanced capabilities of the present method in preserving the sharpness and shape of the interfaces over other algebraic VoF methods available in the literature. (C) 2021 Elsevier Inc. All rights reserved.

Enabled by national commercialization of massive shale resources, Gas Turbines continue to be the backbone of power generation in the US. With the ever-increasing demand on efficiency, GT combustion sections have evolved to include shorter combustion lengths and multiple axial staging of the fuel, while at the same time operating at ever increasing temperatures. This paper presents the results of very detailed Large Eddy Simulations of one (or two) combustor can(s) for a 7HA GE Gas Turbine Engine over a range of operating parameters. The model of the simulated combustor can(s) includes (include) all the details of the combustor from compressor diffuser to the end of the stationary part of the first stage of the turbine. It includes the geometries of multiple pre-mixers within the combustion can(s) and the complete design features for axial fuel staging. All simulations in this work are performed using the CharLES flow solver developed by Cascade Technologies. CharLES is a suite of massively parallel CFD tools designed specifically for multiphysics LES in high-fidelity engineering applications. Thermo acoustic results from LES were validated first in the physical GE lab and then in full-engine testing. Both the trend as well as the predicted amplitudes for the excited axial dominant combustion mode matched the data produced in the lab and in the engine. The simulations also revealed insight into the ingestion of hot gases by different hardware pieces that may occur when machine operates under medium to high combustion dynamics amplitudes. This insight then informed the subsequent design changes which were made to the existing hardware to mitigate the problems encountered.

Gas turbine engines are required to operate at both design and off-design conditions that can lead to strongly unsteady flow-fields and aerodynamic losses severely impacting performance. Addressing this problem requires effective use of computational fluid dynamics tools and emerging models that resolve the large scale fields in detail while accurately modeling the under-resolved scale dynamics. The objective of the current study is to conduct massively parallel large eddy simulations (LES) of rotating turbomachinery that handle the near-wall dynamics using accurate wall models at relevant operating conditions. The finite volume compressible CharLES solver was employed to conduct the simulations over moving grids generated through Voronoi-based unstructured cells. A grid sensitivity analysis was carried out first to establish reliable parameters and assess the quality of the results. LES simulations were then conducted to understand the impact of blade tip clearance and operating conditions on the stage performance. Variations in tip clearance of 3% and 16% chord were considered in the analysis. Other design points included operation at 100% rotor speed and off-design conditions at 75% and 50% of the rotor speed. The simulation results showed that the adiabatic efficiency improves dramatically with reduction in tip gap due to the decrease in tip leakage flow and the resulting flow structures. The analysis also showed that the internal flow becomes highly unsteady, undergoing massive separation, as the rotor speed deviates from the design point. This study demonstrates the capability of the framework to simulate highly turbulent unsteady flows in a rotating turbomachinery environment. The results provide much needed insight and massive data to investigate novel design concepts for the US Army Future Vertical Lift program.

Large eddy simulations are performed for a cold ideally-expanded dual-stream jet issued from cylindrical co-axial nozzles, with supersonic primary stream (Mach number M 1 = 1 . 55 ) and subsonic secondary stream ( M 2 = 0 . 9 ). The geometry includes the internal screw holes used to fasten the two nozzles together and to the plenum chamber. These slanted cylindrical holes over which the secondary stream flows were not covered in the experiment and were seamlessly captured in the computational mesh thanks to a novel grid generation paradigm based on the computation of Voronoi diagrams. A simulation with the screw holes covered is also performed and the preliminary results tends to indicate that these features have minimal impact on the flow and acoustic fields for the present operating conditions. As expected, the present dual-stream configuration with subsonic annular stream surrounding the primary supersonic stream features a reduced shear-layer growth, a longer potential core and a lack of strong Mach wave radiation. A long LES database is currently being collected for analysis and modeling of wavepackets and noise sources in such complex turbulent jets

The fundamental physical process of an atomizing spray produced by a pulsed injector plays a crucial role in analyzing the combustion dynamics in many propulsion-related applications. A full understanding of the primary atomization process has not been achieved for several reasons, including difficulties in visualizing the optically dense region. Due to the recent advances in numerical methods and computing resources, high-resolution simulations of atomizing flows are becoming available to provide new insights into the complex process. In the present study, an unstructured, unsplit volume-of-fluid (VoF) method is employed to model the liquid/gas interface and droplet formation dynamics for a single pulsed-injection event issued from a complex diesel injector. Two single-component reference fuels, n-paraffin (n-dodecane) and isoparaffin (isooctane), were selected to study the influence of hydrocarbon fuel properties on the spray formation mechanisms. The spray is released into a quiescent environment filled with nitrogen gas at 20 bar and 300 K. The fuel transport properties at peak conditions for the cases considered are in the range of 6.9 x 10(3) < Re < 2.5 x 10(4), 5.4 x 10(4) < We < 1.25 x 10(5), and 0.01 < Oh < 0.03 and set the spray in a transitional atomization regime. The simulations provide microscopic-level detail of the liquid/gas interface dynamics and droplet formation process with quantified statistics from first principles. Comparisons with experimental measurements and theory demonstrate the validity of the interface-capturing approach and provide a novel analysis tool to explore the underlying breakup physics.

Unstructured large eddy simulations are performed with the compressible flow solver ”Charles” developed at Cascade Technologies, for heated supersonic over-expanded jets issued from a twin nozzle. The study focuses on the modeling of the internal flow and its effects on the flow-field in the jet plume and ultimately on the radiated noise. In this work, near-wall adaptive mesh refinement, synthetic inflow turbulence and wall modeling are used inside the nozzle. In addition to the converging-diverging nozzle geometry, the computational domain includes the Y-duct, S-ducts, and angle adapter upstream of the nozzles, to realistically reproduce the elements of the experimental configuration where the internal flow conditions and wall modeling can be expected to affect the external flow-field and sound-field predictions. Comparisons between the numerical predictions and experimental PIV and far-field noise measurements from NASA Glenn Research Center will be presented and discussed.

In direct injection engines, the jet primary and secondary breakup processes have a significant influence on the fuel/air mixture formation and drop-size distribution directly affecting the fuel conversion efficiency and combustion characteristics. In this work the disintegration process of turbulent liquid jets from a realistic diesel injector issuing into a still environment is investigated numerically using a coupled liquid/gas interface capturing technique and a high-fidelity DNS/LES approach. This study is aimed at assessing the influence of NJFCP aviation jet fuel mixtures on the disintegration and droplet-size spray characteristics at simulated diesel operating conditions. For this purpose, an unstructured unsplit Volume-of-Fluid method is employed in conjunction with a realistic diesel injector geometry to simulate the pulsed jet disintegration and breakup process. Flow and droplet PDF statistics are extracted to demonstrate the impact of physical properties (A2, C3 fuel) on the mixing behavior and droplet distribution. The simulations are compared against X-ray radiography volume fraction measurements from Argonne National Laboratory and also serve as numerical benchmarks for calibration of lower fidelity models.

: A high-resolution numerical simulation of jet breakup and spray formation from a complex diesel fuel injector at diesel engine type conditions has been performed. A full understanding of the primary atomization process in diesel fuel injection has not been achieved for several reasons including the difficulties accessing the optically dense region. Due to the recent advances in numerical methods and computing resources, high resolution simulations of atomizing flows are becoming available to provide new insights of the process. In the present study, an unstructured un-split Volume-of-Fluid (VoF) method is employed to simulate the injection event with prescribed bulk inflow conditions. An axial single-hole ARL fuel injector was X-ray scanned at The Advanced Photon Source Facility from Argonne National Laboratory for this work to define the internal geometry. The working conditions correspond to orifice dimensions of 90 m fueled with n-paraffin (n-dodecane) and iso-paraffin (iso-octane) reference fuels for a detailed investigation of fuel specific mixing mechanisms.

The details of liquid atomization and the resulting spray formation processes in realistic complex fuel injectors are not well understood, because experimental access to the atomization region is typically severely limited, if not outright impossible. A significant portion of the atomization process occurs in spatial regions adjacent to solid walls that block experimental access into the injector so that experimental studies are limited to either far field measurements of complex injectors, after most of the atomization has occurred, or to simple injector geometries such as a circular cross-section pipes injecting into crossflow channels. Due to recent advances in numerical methods and available computational resources, high-fidelity simulations of the atomization process are starting to emerge as an alternative to experimental studies for understanding and predicting the spray formation process. In the present study, a novel unstructured Volume-of-Fluid (VoF) method coupled to a Lagrangian spray approach is employed to simulate the atomization process in a realistic high shear nozzle typical of gas turbine injectors, consisting of six liquid jets injecting into a crossflow swirling gaseous nozzle flow. Simulations are performed at ambient conditions and compared to experimental data of the resulting spray at di↵erent far field measurement planes downstream of the nozzle.