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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 667 records · Page 37
All sourcesNASA NTRSDOE OSTI
NASA NTRSDate not supplied

Quantitative Approach for the Accurate CFD Simulation of Hover in Turbulent Flow

Time-dependent Navier-Stokes simulations have been carried out for a V22 rotor in hover using an improved, low-diffusion, HLLE++ upwind algorithm in the OVERFLOW code. Emphasis is placed on lessons learned over the past decade regarding the effects of high-order spatial accuracy, grid resolution, and the use of detached eddy simulation in predicting the rotor performance, i.e., figure-of-merit (FM). A quick-start procedure is described together with a statistical measure of FM convergence that reduces hover computations by 5 times, similar in computational work for forward flight. Furthermore, Cartesian adaptive mesh refinement in the rotor wake revealed a complex turbulent flow with worm-like structures of various scales. These turbulent worms, found computationally more than a decade ago, have recently been observed in a separate German Aerospace Center (DLR) experiment. Moreover, adaptive mesh refinement has been used to resolve the tip-vortex to its correct physical size.

Neal M. Chaderjian↗
NASA NTRS2023

High-Fidelity Simulations of a Tiltwing Vehicle for Urban Air Mobility

NASA is playing an important role in the development of the urban air mobility ecosystem by identifying key research areas and establishing a fleet of conceptual reference vehicles for urban air mobility applications. This paper delves deeper into the study of NASA’s six-passenger tiltwing air taxi aerodynamics using high-fidelity computational fluid dynamics simulations. NASA’s OVERFLOW flow solver has been loosely-coupled with the rotorcraft comprehensive analysis code CAMRAD II. Dual-time stepping, high-order accurate schemes, and the delayed detached-eddy simulation are employed. The tiltwing configuration is analyzed in airplane and helicopter mode, with flight conditions representative of an air taxi mission. Airloads, wake geometry, and performance are studied to identify the unique flow features of this configuration. NASA’s tiltwing vehicle is one of the conceptual designs intended to focus and guide NASA’s research activities in support of aircraft development for vertical take-off and landing air taxi operations.

RVLT↗
NASA NTRSDate not supplied

Enabling Supersonic Over-Land Flight Using Computational Modeling

NASA's QueSST mission is interested in exploring the potential of commercial, over-land supersonic flight, which has been banned since 1973 due to the noise intensity of the sonic booms that supersonic vehicles create. NASA has partnered with Lockheed Martin to design and construct an experimental aircraft, the X-59, that will shape the sonic boom to achieve a perceived loudness on the ground that is lower than 75dB. In this talk, we present new developments in the Launch Ascent and Vehicle Aerodynamics (LAVA) CFD solver framework which are tailored to the analysis of supersonic vehicles, like the X-59, and sonic boom propagation. Utilizing a high-order space marching method, adjoint-based grid adaptation, and robust database capabilities have enabled significant cost savings over traditional CFD methods for the same level of accuracy in perceived boom loudness. Example cases demonstrating the toolset are presented, along with comparisons to experimental wind tunnel data. These developments will enable LAVA to quickly and accurately provide simulation data to the project during X-59's first and subsequent flights in 2024 and beyond.

CST↗
NASA NTRSDate not supplied

Multi-Fidelity Computational Analysis of a Quiet Single-Main Rotor Helicopter for Air Taxi Operations

A multi-fidelity computational study has been conducted on NASA’s quiet single-main rotor helicopter concept for urban air mobility using computational fluid dynamics and rotorcraft comprehensive analysis tools. Two approaches have been followed in order to simulate the rotor flows: (1) a rotor disk model, which models the rotors as source terms, and (2) a high-fidelity approach using high-order accurate schemes and dual-time stepping, which simulates the rotor with its individual rotating blade grids. The delayed detached-eddy simulation model has been employed. A loose-coupling approach between the flow solver and a rotorcraft comprehensive code is utilized to include vehicle trim and motion variables. The quiet single-main rotor helicopter is one of the conceptual designs intended to focus and guide NASA’s research activities in support of aircraft development for vertical take-off and landing air taxi operations.

RVLT↗
NASA NTRSDate not supplied

Towards Gbps Downlinks from Low-Cost Active Phased Arrays

Spacecraft performing science missions use high-directivity antennas to quickly downlink large amounts of collected data. Movement required to keep gimbaled reflector antennas trained on target receivers can impart blockages or vibrations which perturb precision science instruments. Recent manufacturing advances have made Ka-band electronically-steered antenna arrays a cost-effective option to mitigate these challenges, but only if these devices can be proven to deliver comparable quality of service. In this work we present a hardware and software architecture for high-rate links using an active phased array-based terminal in a 1U (10cm cube) form factor. Over-the-air tests in an antenna range characterize two key features of the architecture: high-order (32APSK and above) modulations and dual carriers. We calculate an optimal modulation-dependent input power backoff from array saturation and perform orbital simulations of the link at this operating point. We conclude that reliable links with rates in excess of 1Gbps are possible with current-generation hardware.

Adam Gannon↗
NASA NTRSDate not supplied

A New Integral Field Spectrograph for Broadband Coronagraph Demonstrations at the High Contrast Imaging Testbed Facility

An integral field spectrograph (IFS) camera may help fulfill the exoplanet characterization goals of a future Habitable Worlds Observatory. During the Nancy Grace Roman Space Telescope mission formulation phase, the Coronagraph Instrument Project established the laboratory performance baseline of a combined coronagraph and IFS system with the Prototype Imaging Spectrograph for Coronagraphic Exoplanet Studies (PISCES; 1E-8 contrast over an 18% bandpass; 3--9 lambda/D bowtie-shaped control region). New laboratory demonstrations are needed to expand on this milestone in terms of contrast, bandpass, and field of view towards the requirements of a future mission capable of characterizing the atmospheres of Earth-like exoplanets. Here we present the design of a successor to PISCES that can observe a 20 lambda/D-diameter field of view with an instantaneous bandpass up to 30%, at a resolving power R > 70 at visible wavelengths. This instrument will interface with the existing coronagraph layout in the High Contrast Imaging Facility DST-2 vacuum chamber. The addition of a spectroscopic imaging camera at HCIT will benefit the wider coronagraph technology community by enabling other NASA-supported investigators to automatically obtain multi-wavelength measurements of speckles both inside and outside of their control region, and the instrument will support community-led demonstrations of high-order wavefront sensing and control techniques such as dark hole maintenance and linear dark field control.

coronagraph↗
NASA NTRSDate not supplied

Constrained Field Construction Using Bernstein Polynomial Derived Operators

This method recasts the mass-conservation equation as a Sylvester equation employing high-order derivative operators derived from modified Bernstein polynomial expansions. Given prescribed velocity fields, the algorithm yields a constrained, least-squared solution for the associated density field. To demonstrate the practical utility of this methodology, it is applied to a computationally-derived two-dimensional isolator dataset. This reconstruction method is envisioned to be used in conjunction with diagnostic techniques to aid in the quantification of isolator flow fields, since obtaining highly characterized datasets within this engine component is exceedingly difficult.

Isolator↗
NASA NTRSDate not supplied

The Parabolic Deformable Mirror Testbed at NASA Goddard Space Flight Center

One of the primary goals of future coronagraph space missions is to spectrally characterize Earth-like exoplanets. Observing efficiency is critical and directly related to how large of a field of view we can correct and the bandwidth of that correction. In addition to the coronagraph itself, the bandwidth and depth of contrast is related to the wavefront control system, which classically uses two deformable mirrors (DMs) in series. The minimum distance between these DMs is determined by the Talbot distance of the beam, meaning it scales as the square of the beam size. NASA Goddard is exploring an alternative design architecture to the two-DM approach in which the reimaging optics for the coronagraph are also deformable and we only retain a high-order pupil DM. With these off-axis parabolic elements being made deformable, the long propagation distance between the two DMs is eliminated. Goddard has procured an off-axis parabolic DM (PDM or OAP DM) from vendor ALPAO, France. A testbed is currently being built to test the PDM and demonstrate its utility for achieving high contrast in a coronagraph. The testbed is also being built with an integrated low-order wavefront sensing capability, allowing for integrated testing of a dynamic environment with multiple estimation and control loops.

Hari Subedi↗
NASA NTRSDate not supplied

Extending an Economical Third-Order Inviscid Nodal-Gradient Cell-Centered Finite-Volume Method to Mixed-Element Grids

In this paper, we extend an economical third-order nodal-gradient cell-centered finite-volume method, originally developed for tetrahedral grids, to mixed-element grids. It is shown that the efficient quadratic interpolation formula essential to eliminating second derivatives from a third-order accurate discretization can be easily extended to an arbitrary cell type. For the surface flux integration, we consider a split-face approach, where a quadrilateral face is split into two triangles, and the third-order method for tetrahedra is directly applied. Also, we derive a second-derivative-free, high-order, volume quadrature formula for an arbitrary cell. Numerical results are presented for accuracy verification and applications with three-dimensional nontetrahedral grids.

Computational Fluid Dynamics↗
NASA NTRSDate not supplied

Enhanced Simulation Techniques in Predicting Sonic Boom Loudness Using CFD

This paper outlines advancements in predicting sonic boom loudness within the Launch, Ascent, and Vehicle Aerodynamics (LAVA) computational framework. Traditionally, a two step process consisting of a steady state computational fluid dynamics problem for near-field analysis and a far-field propagation solver for calculation of loudness metrics has been used. Improvements to this process made in this work include utilizing a high-order space marching method for mid-field computations, developing a novel output-based mesh adaptation method targeting error in near-field pressure sig-natures, and developing a robust scripting system using curvilinear grids to increase robustness and simplify the process of running large databases of simulation cases. These advancements are detailed and applied to the simulation of the X-59, presenting comparative cost and timing analyses between the prior two step workflow and the current three step procedure. We achieve increased accuracy and robustness for loudness predictions with at least a50%computational cost reduction.

CST↗
NASA NTRSDate not supplied

Implicit Preconditioning for Explicit Multigrid Solvers on Cut-Cell Cartesian Meshes

This work assesses the effectiveness of linearized implicit Euler preconditioning for multigrid solvers using an unpreconditioned, Jacobian-free Newton Krylov method to converge the linear system of equations. Multigrid convergence rates improve to approximately 0.75 across the cases tested including a Mach 2 supersonic wedge, transonic NACA 0012 airfoil, and ONERA M6 wing. While larger Krylov subspaces increase the convergence rate, they also increase the computational cost, such that 4-8 Krylov vectors often offers the fastest turnaround. Further reductions in computational cost are achieved with a sequential hybrid preconditioner that begins with the explicit multigrid solver before transitioning to the preconditioned algorithm later on. In addition, a novel implementation of dual time stepping is extended to include both common BDF methods as well as high-order implicit Runge-Kutta schemes. This particular formulation, which uses A −1 preconditioning, is amenable to matrix-free solvers, and the L-stable methods are especially suited for meshes with arbitrarily small cut-cells. Asymptotic order of convergence is demonstrated for BDF1, BDF2, SDIRK2, and 3rd-order Radau IIA time integration with unsteady 2D vortex simulations.

ARMD↗
NASA NTRSDate not supplied

High-Fidelity Computational Analysis Of Nasa’s Air Taxi Concepts

High-fidelity Computational Fluid Dynamics (CFD) simulations have been carried out for NASA’s air taxi concepts. The high-fidelity approach uses high-order accurate schemes and dual-time stepping, which simulates the rotor with its individual rotating blade grids. The delayed detached-eddy simulation model has been employed. A loose-coupling approach between the flow solver and a rotorcraft comprehensive code is utilized to include vehicle trim and motion variables.NASA’s air taxi conceptual designs are intended to focus and guide NASA’s research activities in support of aircraft development for vertical take-off and landing air taxi operations

ARMD↗
DOE OSTI2025

Coupling Approaches with Non-matching Grids for Classical Linear Elasticity and Bond-based Peridynamic Models in 1D

Local-nonlocal coupling approaches provide a means to combine the computational efficiency of local models and the accuracy of nonlocal models. To facilitate the coupling of the two models, non-matching grids are often desirable as nonlocal grids usually require a finer resolution than local grids. In that case, it is often convenient to resort to interpolation operators so that models can exchange information in the overlap regions when nodes from the two grids do not coincide. This paper studies three existing coupling approaches, namely 1) a method that enforces matching displacements in an overlap region, 2) a variant that enforces a constraint on the stresses instead, and 3) a method that considers a variable horizon in the vicinity of the interfaces. Further, the effect of the interpolation order and of the grid ratio on the performance of the three coupling methods with non-matching grids is carefully studied on one-dimensional examples using polynomial manufactured solutions. The numerical results show that the degree of the interpolants should be chosen with care to avoid introducing additional modeling errors, or simply minimize these errors, in the coupling approach.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2025

An extension of the localized artificial diffusivity method for immiscible and high density ratio flows

The localized artificial diffusivity (LAD) method is widely regarded as the preferred multi-material regularization scheme for the compact finite difference method, because it is conservative, easy to implement, and generally robust for a wide range of multi-material problems. However, traditional LAD methods face significant challenges when applied to flows with large density ratios and when maintaining thermodynamic equilibrium across material interfaces. These limitations arise from the formulation of the artificial diffusivity flux and the reliance on enthalpy diffusion for interface regularization. Additionally, traditional LAD methods struggle to ensure stability under large density ratio conditions, fail to maintain a finite interface thickness, and are therefore unsuitable for modeling immiscible interfaces. Here, in this work, we discuss the origins of these issues in traditional LAD methods and propose modifications which enable the simulation of large density ratio and immiscible flows. The proposed method targets the artificial diffusion fluxes at gradients and ringing in the volume fraction, rather than the mass fraction in traditional methods, to consistently regularize large density ratio interfaces. Furthermore, the proposed method introduces an artificial bulk density diffusion term to enforce equilibrium conditions across interfaces. To address the challenge of modeling immiscible flows, a conservative diffuse interface term is incorporated into the formulation to ensure a finite interface thickness. Specific consideration is taken in the design of the method to ensure that these crucial properties are maintained for N -material flows. The effectiveness of the proposed method is demonstrated through a series of canonical test cases, and its accuracy is validated by comparison with experimental data on micro-bubble collapse in water. These results highlight the method’s robustness and its ability to overcome the limitations of traditional LAD approaches.

Artificial diffusivity↗
NASA NTRS2004

Nonlinear optics and crystalline whispering gallery mode resonators

We report on our recent results concerning fabrication of high-Q whispering gallery mode (WGM) crystalline resonators, and discuss some possible applications of lithium niobate WGM resonators in nonlinear optics and photonics. In particular, we demonstrate experimentally a tunable third-order optical filter fabricated from the three metalized resonators; and report observation of parametric frequency dobuling in a WGM resonator made of periodically poled lithium niobate (PPLN).

whispering gallery mode↗