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At least 1,153 records · Page 64
All sourcesNASA NTRSDOE OSTI
NASA NTRS2014

Model-Invariant Hybrid LES-RANS Computation of Separated Flow Past Periodic Hills

The requirement that physical quantities not vary with a hybrid LESRANS model's blending parameter imposes conditions on the computation that lead to better results across LES-RANS transitions. This promises to allow placement of those transitions so that LES is performed only where required by the physics, improving computational efficiency. The approach is applied to separated flow past periodic hills, where good predictions of separation-bubble size are seen due to the gradual, controlled, LES-RANS transition and the resulting enhanced near-wall eddy viscosity.

Woodruff, Stephen↗
NASA NTRS2013

Transitioning Resolution Responsibility between the Controller and Automation Team in Simulated NextGen Separation Assurance

As part of an ongoing research effort on separation assurance and functional allocation in NextGen, a controller- in-the-loop study with ground-based automation was conducted at NASA Ames' Airspace Operations Laboratory in August 2012 to investigate the potential impact of introducing self-separating aircraft in progressively advanced NextGen timeframes. From this larger study, the current exploratory analysis of controller-automation interaction styles focuses on the last and most far-term time frame. Measurements were recorded that firstly verified the continued operational validity of this iteration of the ground-based functional allocation automation concept in forecast traffic densities up to 2x that of current day high altitude en-route sectors. Additionally, with greater levels of fully automated conflict detection and resolution as well as the introduction of intervention functionality, objective and subjective analyses showed a range of passive to active controller- automation interaction styles between the participants. Not only did the controllers work with the automation to meet their safety and capacity goals in the simulated future NextGen timeframe, they did so in different ways and with different attitudes of trust/use of the automation. Taken as a whole, the results showed that the prototyped controller-automation functional allocation framework was very flexible and successful overall.

NextGen↗
NASA NTRS2014

Hypersonic Separated Flows About "Tick" Configurations With Sensitivity to Model Design

This paper presents computational results obtained by applying the direct simulation Monte Carlo (DSMC) method for hypersonic nonequilibrium flow about "tick-shaped" model configurations. These test models produces a complex flow where the nonequilibrium and rarefied aspects of the flow are initially enhanced as the flow passes over an expansion surface, and then the flow encounters a compression surface that can induce flow separation. The resulting flow is such that meaningful numerical simulations must have the capability to account for a significant range of rarefaction effects; hence the application of the DSMC method in the current study as the flow spans several flow regimes, including transitional, slip, and continuum. The current focus is to examine the sensitivity of both the model surface response (heating, friction and pressure) and flowfield structure to assumptions regarding surface boundary conditions and more extensively the impact of model design as influenced by leading edge configuration as well as the geometrical features of the expansion and compression surfaces. Numerical results indicate a strong sensitivity to both the extent of the leading edge sharpness and the magnitude of the leading edge bevel angle. Also, the length of the expansion surface for a fixed compression surface has a significant impact on the extent of separated flow.

Moss, J. N.↗
NASA NTRS2015

A Reduced-Complexity Investigation of Blunt Leading-Edge Separation Motivated by UCAV Aerodynamics

A reduced complexity investigation for blunt-leading-edge vortical separation has been undertaken. The overall approach is to design the fundamental work in such a way so that it relates to the aerodynamics of a more complex Uninhabited Combat Air Vehicle (UCAV) concept known as SACCON. Some of the challenges associated with both the vehicle-class aerodynamics and the fundamental vortical flows are reviewed, and principles from a hierarchical complexity approach are used to relate flow fundamentals to system-level interests. The work is part of roughly 6-year research program on blunt-leading-edge separation pertinent to UCAVs, and was conducted under the NATO Science and Technology Organization, Applied Vehicle Technology panel.

Luckring, James M.↗
NASA NTRS2015

Centrifugal Size-Separation Sieve for Granular Materials

A centrifugal sieve and method utilizes centrifugal force in rapidly-rotated cylindrical or conical screens as the primary body force contributing to size segregation. Within the centrifugal acceleration field, vibration and/or shearing flows are induced to facilitate size segregation and eventual separation of the fines from the coarse material. Inside a rotating cylindrical or conical screen, a separately-rotated screw auger blade can be used to transport material along the rotating cylinder or conical wall and to induce shearing in the material.

Walton, Otis↗
NASA NTRS2015

Increased Oxygen Recovery from Sabatier Systems Using Plasma Pyrolysis Technology and Metal Hydride Separation

State-of-the-art life support carbon dioxide (CO2) reduction technology is based on the Sabatier reaction where less than 50% of the oxygen required for the crew is recovered from metabolic CO2. The reaction produces water as the primary product and methane as a byproduct. Oxygen recovery is constrained by the limited availability of reactant hydrogen. This is further exacerbated when Sabatier methane (CH4) is vented as a waste product resulting in a continuous loss of reactant hydrogen. Post-processing methane with the Plasma Pyrolysis Assembly (PPA) to recover hydrogen has the potential to dramatically increase oxygen recovery and thus drastically reduce the logistical challenges associated with oxygen resupply. The PPA decomposes methane into predominantly hydrogen and acetylene. Due to the highly unstable nature of acetylene, a separation system is necessary to purify hydrogen before it is recycled back to the Sabatier reactor. Testing and evaluation of a full-scale Third Generation PPA is reported and investigations into metal hydride hydrogen separation technology is discussed.

Zachary W Greenwood↗
NASA NTRS2015

The Effects of Sweeping Jet Actuator Parameters on Flow Separation Control

A parametric experimental study was performed with sweeping jet actuators (fluidic oscillators) to determine their effectiveness in controlling flow separation on an adverse pressure gradient ramp. Actuator parameters that were investigated include blowing coefficients, operation mode, pitch and spreading angles, streamwise location, aspect ratio, and scale. Surface pressure measurements and surface oil flow visualization were used to characterize the effects of these parameters on the actuator performance. 2D Particle Image Velocimetry measurements of the flow field over the ramp and hot-wire measurements of the actuator's jet flow were also obtained for selective cases. In addition, the sweeping jet actuators were compared to other well-known flow control techniques such as micro-vortex generators, steady blowing, and steady vortex-generating jets. The results confirm that the sweeping jet actuators are more effective than steady blowing and steady vortex-generating jets. The results also suggest that an actuator with a larger spreading angle placed closer to the location where the flow separates provides better performance. For the cases tested, an actuator with an aspect ratio, which is the width/depth of the actuator throat, of 2 was found to be optimal. For a fixed momentum coefficient, decreasing the aspect ratio to 1 produced weaker vortices while increasing the aspect ratio to 4 reduced coverage area. Although scaling down the actuator (based on the throat dimensions) from 0.25 inch x 0.125 inch to 0.15 inch x 0.075 inch resulted in similar flow control performance, scaling down the actuator further to 0.075 inch x 0.0375 inch reduced the actuator efficiency by reducing the coverage area and the amount of mixing in the near-wall region. The results of this study provide insight that can be used to design and select the optimal sweeping jet actuator configuration for flow control applications.

Koklu, Mehti↗
NASA NTRS2016

Unmanned Aircraft Systems Human-in-the-Loop Controller and Pilot Acceptability Study: Collision Avoidance, Self-Separation, and Alerting Times (CASSAT)

The Federal Aviation Administration (FAA) has been mandated by the Congressional funding bill of 2012 to open the National Airspace System (NAS) to Unmanned Aircraft Systems (UAS). With the growing use of unmanned systems, NASA has established a multi-center "UAS Integration in the NAS" Project, in collaboration with the FAA and industry, and is guiding its research efforts to look at and examine crucial safety concerns regarding the integration of UAS into the NAS. Key research efforts are addressing requirements for detect-and-avoid (DAA), self-separation (SS), and collision avoidance (CA) technologies. In one of a series of human-in-the-loop experiments, NASA Langley Research Center set up a study known as Collision Avoidance, Self-Separation, and Alerting Times (CASSAT). The first phase assessed active air traffic controller interactions with DAA systems and the second phase examined reactions to the DAA system and displays by UAS Pilots at a simulated ground control station (GCS). Analyses of the test results from Phase I and Phase II are presented in this paper. Results from the CASSAT study and previous human-in-the-loop experiments will play a crucial role in the FAA's establishment of rules, regulations, and procedures to safely, efficiently, and effectively integrate UAS into the NAS.

Comstock, James R., Jr.↗
NASA NTRS2016

Human-In-The-Loop Investigation of Interoperability Between Terminal Sequencing and Spacing, Automated Terminal Proximity Alert, and Wake-Separation Recategorization

A human-in-the-loop simulation study addressed terminal-area controller-workstation interface variations for interoperability between three new capabilities being introduced by the FAA. The capabilities are Terminal Sequencing and Spacing (TSAS), Automated Terminal Proximity Alert (ATPA), and wake-separation recategorization, or 'RECAT.' TSAS provides controllers with Controller-Managed Spacing (CMS) tools, including slot markers, speed advisories, and early/late indications, together with runway assignments and sequence numbers. ATPA provides automatic monitor, warning, and alert cones to inform controllers about spacing between aircraft on approach. ATPA cones are sized according to RECAT, an improved method of specifying wake-separation standards. The objective of the study was to identify potential issues and provide recommendations for integrating TSAS with ATPA and RECAT. Participants controlled arrival traffic under seven different display configurations, then tested an 'exploratory' configuration developed with participant input. All the display conditions were workable and acceptable, but controllers strongly preferred having the CMS tools available on Feeder positions, and both CMS tools and ATPA available on Final positions. Controllers found the integrated systems favorable and liked being able to tailor configurations to individual preferences.

ATD-1↗
NASA NTRS2016

Model-Invariant Hybrid Computations of Separated Flows for RCA Standard Test Cases

NASA's Revolutionary Computational Aerosciences (RCA) subproject has identified several smooth-body separated flows as standard test cases to emphasize the challenge these flows present for computational methods and their importance to the aerospace community. Results of computations of two of these test cases, the NASA hump and the FAITH experiment, are presented. The computations were performed with the model-invariant hybrid LES-RANS formulation, implemented in the NASA code VULCAN-CFD. The model- invariant formulation employs gradual LES-RANS transitions and compensation for model variation to provide more accurate and efficient hybrid computations. Comparisons revealed that the LES-RANS transitions employed in these computations were sufficiently gradual that the compensating terms were unnecessary. Agreement with experiment was achieved only after reducing the turbulent viscosity to mitigate the effect of numerical dissipation. The stream-wise evolution of peak Reynolds shear stress was employed as a measure of turbulence dynamics in separated flows useful for evaluating computations.

Woodruff, Stephen↗
NASA NTRS2016

Full-Scale Turbofan Engine Noise-Source Separation Using a Four-Signal Method

Contributions from the combustor to the overall propulsion noise of civilian transport aircraft are starting to become important due to turbofan design trends and expected advances in mitigation of other noise sources. During on-ground, static-engine acoustic tests, combustor noise is generally sub-dominant to other engine noise sources because of the absence of in-flight effects. Consequently, noise-source separation techniques are needed to extract combustor-noise information from the total noise signature in order to further progress. A novel four-signal source-separation method is applied to data from a static, full-scale engine test and compared to previous methods. The new method is, in a sense, a combination of two- and three-signal techniques and represents an attempt to alleviate some of the weaknesses of each of those approaches. This work is supported by the NASA Advanced Air Vehicles Program, Advanced Air Transport Technology Project, Aircraft Noise Reduction Subproject and the NASA Glenn Faculty Fellowship Program.

Combustor Noise↗
NASA NTRS2017

Extraction-Separation Performance and Dynamic Modeling of Orion Test Vehicles with Adams Simulation: 3rd Edition

NASA's Orion Capsule Parachute Assembly System (CPAS) Project is now in the qualification phase of testing, and the Adams simulation has continued to evolve to model the complex dynamics experienced during the test article extraction and separation phases of flight. The ability to initiate tests near the upper altitude limit of the Orion parachute deployment envelope requires extractions from the aircraft at 35,000 ft-MSL. Engineering development phase testing of the Parachute Test Vehicle (PTV) carried by the Carriage Platform Separation System (CPSS) at altitude resulted in test support equipment hardware failures due to increased energy caused by higher true airspeeds. As a result, hardware modifications became a necessity requiring ground static testing of the textile components to be conducted and a new ground dynamic test of the extraction system to be devised. Force-displacement curves from static tests were incorporated into the Adams simulations, allowing prediction of loads, velocities and margins encountered during both flight and ground dynamic tests. The Adams simulation was then further refined by fine tuning the damping terms to match the peak loads recorded in the ground dynamic tests. The failure observed in flight testing was successfully replicated in ground testing and true safety margins of the textile components were revealed. A multi-loop energy modulator was then incorporated into the system level Adams simulation model and the effect on improving test margins be properly evaluated leading to high confidence ground verification testing of the final design solution.

Varela, Jose G.↗
NASA NTRS2017

Material Characterization for the Analysis of Skin/Stiffener Separation

Test results show that separation failure in co-cured skin/stiffener interfaces is characterized by dense networks of interacting cracks and crack path migrations that are not present in standard characterization tests for delamination. These crack networks result in measurable large-scale and sub-ply-scale R curve toughening mechanisms, such as fiber bridging, crack migration, and crack delving. Consequently, a number of unknown issues exist regarding the level of analysis detail that is required for sufficient predictive fidelity. The objective of the present paper is to examine some of the difficulties associated with modeling separation failure in stiffened composite structures. A procedure to characterize the interfacial material properties is proposed and the use of simplified models based on empirical interface properties is evaluated.

Davila, Carlos G.↗
NASA NTRS2017

A Fuzzy Logic Approach for Separation Assurance and Collision Avoidance for Unmanned Aerial Systems

In the coming years, operations in low altitude airspace will vastly increase as the capabilities and applications of small Unmanned Aerial Systems (sUAS) continue to multiply. Therefore, solutions to managing sUAS in highly congested airspace must be explored. In this study, a Fuzzy Logic based approach was used to help mitigate the risk of collisions between aircraft using separation assurance and collision avoidance techniques. The system was evaluated for its effectiveness to mitigate the risk of mid-air collisions between aircraft. This system utilizes only current state information and can resolve potential conflicts without knowledge of intruder intent. The avoidance logic was verified using formal methods and shown to select the correct action in all instances. Additionally, the Fuzzy Logic Controllers were shown to always turn the vehicles in the correct direction. Numerical testing demonstrated that the avoidance system was able to prevent a mid-air collision between two sUAS in all tested cases. Simulations were also performed in a three-dimensional environment with a heterogenous fleet of sUAS performing a variety of realistic missions. Simulations showed that the system was 99.98 effective at preventing mid-air collisions when separation assurance was disabled (unmitigated case) and 100 effective when enabled (mitigated case).

Verification & Validation of a Fuzzy Logic Control↗
NASA NTRS2018

Wall-Modeled Lattice Boltzmann and Navier-Stokes Approaches for Separated Flows

Lattice Boltzmann (LB) and hybrid Reynolds-averaged Navier-Stokes/large eddy simulation (RANS/LES) methods within the Launch Ascent and Vehicle Aerodynamics (LAVA) solver framework are applied to NASA's Revolutionary Computational Aerosciences (RCA) standard test cases for separated flows. A detailed comparison between the performance and accuracy of the two emerging numerical methodologies for turbulence resolving simulations, i.e. the LB and hybrid RANS/LES methods will be presented. This contribution addresses the RCA technical challenge to identify and down-select critical turbulence, transition, and numerical method technologies for 40% reduction in predictive error for standard turbulence separated flow test cases. Results for the 2D NASA wall-mounted hump and the axisymmetric transonic bump including time-averaged pressure coefficient, skin friction, and velocity pro les, as well as resolved and modeled Reynolds stresses for both numerical approaches will be presented and differences between LB and hybrid RANS/LES will be discussed.

Lattice Boltzmann↗
NASA NTRS2018

Study of Skin-Stringer Separation in Postbuckled Composite Aeronautical Structures

Aeronautical composite stiffened structures have the capability to carry loads deep into postbuckling, yet they are typically designed to operate below the buckling load to avoid potential issues with durability and structural integrity. Large out-of-plane postbuckling deformation of the skin can result in the opening of the skin-stringer interfaces, especially in the presence of defects, such as impact damage. To ensure that skin-stringer separation does not propagate in an unstable mode that can cause a complete collapse of the structure, a deeper understanding of the interaction between the postbuckling deformation and the development of damage is required. The present study represents a first step towards a methodology based on analysis and experiments to assess and improve the strength, life, and damage tolerance of stiffened composite structures subjected to postbuckling deformations. Two regions were identified in a four-stringer panel in which skin-stringer separation can occur, namely the region of maximum deflection and the region of maximum twisting. Both regions have been studied using a finite element model of a representative single-stringer specimen. For the region of maximum deflection, a seven-point bending configuration was used, in which five supports and two loading points induce buckling waves to the specimen. The region of maximum twisting was studied using an edge crack torsion configuration, with two supports and two loading points. These two configurations were studied by changing the positions of the supports and the loading points. An optimization procedure was carried out to minimize the error between the out-of-plane deformation of the representative single-stringer specimen and the corresponding region of the four-stringer panel.

Kootte, Lucas J.↗
NASA NTRS2019

Building Booster Separation Aerodynamic Databases for Artemis II

NASA’s Artemis II mission will mark the return of humans to near-lunar space for the first time since Apollo. Shortly after launch on the Space Launch System (SLS), a critical phase of ascent occurs when 16 small rockets fire to push the boosters away from the core. Minimizing the risk of failure during separation requires the construction of multiple 13-dimensional databases, including perturbations in position, flight conditions, and engine thrust. The SLS Computational Fluid Dynamics team used NASA’s FUN3D flow solver on the Pleiades and Electra supercomputers to run 5,780 simulations at nominal conditions and over 8,000 simulations with a core stage engine failure to generate the databases needed to verify the booster separation system for Artemis II.

Meeroff, Jamie G.↗
NASA NTRS2020

Adaptive Airborne Separation to Enable UAM Autonomy in Mixed Airspace

The excitement and promise generated by Urban Air Mobility (UAM) concepts have inspired both new entrants and large aerospace companies throughout the world to invest hundreds of millions in research and development of air vehicles, both piloted and unpiloted, to fulfill these dreams. The management and separation of all these new aircraft have received much less attention, however, and even though NASA’s lead is advancing some promising concepts for Unmanned Aircraft Systems (UAS) Traffic Management (UTM), most operations today are limited to line of sight with the vehicle, airspace reservation and geofencing of individual flights. Various schemes have been proposed to control this new traffic, some modeled after conventional air traffic control and some proposing fully automatic management, either from a ground-based entity or carried out on board among the vehicles themselves. Previous work has examined vehicle-based traffic management in the very low altitude airspace within a metroplex called UTM airspace in which piloted traffic is rare. A management scheme was proposed in that work that takes advantage of the homogeneous nature of the traffic operating in UTM airspace. This paper expands that concept to include a traffic management plan usable at all altitudes desired for electric Vertical Takeoff and Landing urban and short-distance, inter-city transportation. The interactions with piloted aircraft operating under both visual and instrument flight rules are analyzed, and the role of Air Traffic Control services in the postulated mixed traffic environment is covered. Separation values that adapt to each type of traffic encounter are proposed, and the relationship between required airborne surveillance range and closure speed is given. Finally, realistic scenarios are presented illustrating how this concept can reliably handle the density and traffic mix that fully implemented and successful UAM operations would entail.

Cotton, William B.↗