Langley Research Center Research Articles

Resurfacing heat stress phenomena in Indian cities during the post‐COVID‐19 lockdown period

This study investigates heat stress in 17 Indian cities during the post‐COVID‐19 lockdown period. The study compares thermal comfort experienced during the COVID‐19 lockdown against that experienced during post‐lockdown, which has not been previously studied. The analysis utilizes daily and monthly climate data from 1991 to 2022 obtained from the Langley Research Centre's official website. The net effective temperature (NET) and thermo‐hygrometric index (THI) were employed to assess heat stress in cities. The findings indicate a sudden increase in heat stress levels during the post‐lockdown period, particularly in cities like Lucknow, Chandigarh, Patna, Kolkata, Ahmadabad, Jodhpur, Guwahati, and Delhi. Moreover, there is a noticeable decline in the number of comfortable days for both THI and NET in certain cities, such as Delhi, Chandigarh, Bhopal, Ahmadabad, and Jodhpur, in 2021 and 2022. This analysis also reveals an overall rise in the number of torrid and very hot days, with significant increases recorded in 2022 compared to 2020. With a few exceptions, most cities show rising trends in THI and NET, causing Indian cities to experience more torrid and very hot months. This study clarifies the effect of the COVID‐19 lockdown on bioclimatic comfort and offers important guidance for future studies in this field.

ORACLES Campaign, September 2016: Inversion of HSRL-2 Observations with Regularization Algorithm into Particle Microphysical Parameters and Comparison to Airborne In-Situ Data

We present microphysical properties of pollution layers observed with NASA Langley Research Center’s airborne high-spectral-Resolution lidar (HSRL-2). The data obtained by HSRL-2 consist of vertical profiles of three backscatter coefficients (β) taken at 355, 532 and 1064 nm and two extinction coefficients (α) measured at 355 and 532 nm. In our study we (1) inverted the 3β + 2α data into particle size distributions with a regularization algorithm, and subsequently computed (2) number concentration and (3) single-scattering albedo for four measurement profiles. We carried out a first comparison to the same particle characteristics measured with airborne in-situ instruments. The in-situ instruments were flown aboard the P-3B aircraft, which followed the flight track of the aircraft ER-2 that carried HSRL-2. We found good agreement of the co-located (space and time) data products, with a degree of reliability reaching 90%. A more detailed study on a larger dataset needs to be carried out in future work to (a) obtain important correction factors, (b) study the influence of different light-scattering models on the inversion results, and (c) identify sources of retrieval and measurement uncertainties.

Deployable vortex generators for low Reynolds numbers applications powered by cephalopods inspired artificial muscles

This paper proposes deployable vortex generators (VGs) powered by twisted spiral artificial muscles (TSAMs). TSAMs take inspiration from cephalopods' papillae and can protrude out of plane upon electro-thermal actuation with an output strain of 2000% and an input voltage of 0.2 V/cm. Unlike passive VGs, designed for specific flow conditions, this technology can adjust to changes in flow conditions by overcoming the limitations of existing active flow control devices in terms of portability and power requirements. Our technology can deploy different VGs configurations on demand, and match a desired target configuration, optimized for a specific flow condition. Experiments were conducted in a wind tunnel using a NASA Langley Research Center LS (1)-0417 GA(W)-1 airfoil. Stall delays and lift increase have been demonstrated for different flow conditions, with Reynolds numbers between 100,000 and 140,000. These findings are promising for enhancing efficiency in small unmanned aerial vehicles operating at low Reynolds numbers.

Application of turbulence filters on PCBoom predictions for two NASA flight test campaigns

In preparation for the NASA X-59 community noise tests, the effects of turbulence on predictions of sonic boom levels are studied. This work builds on previous studies by including the effects of turbulence via finite impulse response filters implemented in the PCBoom prediction program. The resulting turbulized sonic boom predictions are compared to measured boom metric levels from previous NASA flight tests: Quiet Supersonic Flights 2018 and Carpet Determination in Entirety Measurements Phase I. To determine the factors driving differences between predictions and measured levels, least absolute shrinkage and selection operator (Lasso) regression is used to create a reduced-order model for each flight test campaign. [Work supported by NASA Langley Research Center through Analytical Mechanics Associates.]

Measured turbulence statistics from two NASA sonic boom test campaigns

Turbulence in the lower atmosphere can have substantial effects on measured sonic boom waveforms on the ground. These turbulent distortions vary spatially and temporally and are best understood statistically. Simultaneous atmospheric and acoustic data were acquired as part of the NASA Sonic Booms in Atmospheric Turbulence (SonicBAT) and Carpet Determination in Entirety Measurements (CarpetDIEM) tests. Data from these tests, both conducted at NASA Armstrong Flight Research Center, enable a detailed study of atmospheric effects on sonic boom waveforms. The present study expands on previous work [Anderson et al., J. Acoust. Soc. Am. 152, A127 (2022)] by analyzing more booms with additional statistical analyses including metric distributions, standard deviations, and confidence intervals. Results from CarpetDIEM indicate that the confidence interval widths of mean metric levels computed across a seven-microphone array spanning 122 m vary by 1-5 dB, depending on the loudness metric. The effects of peak clipping are also considered, and an analysis is conducted to determine how to best recover from peak clipping for different metrics. [Work supported by NASA Langley Research Center through the National Institute of Aerospace and Analytical Mechanics Associates.]

A reconstructed approach for online prediction of transient heat flux and interior temperature distribution in thermal protect system

Thermal protect system (TPS) is a crucial part of spacecraft to shield them from the aerodynamic heating during the atmospheric entry/re-entry. In this paper, the aerodynamic heating on the surface of a reusable launch vehicle is predicted to achieve rational design of the TPS. The conduction-radiation heat transfer in the TPS that was regard as a kind of index graded medium, was computed using finite volume method and discrete ordinate method. The extended Kalman filtering coupled with Rauch-Tung-Striebel smoother is introduced for calculating the inverse problem to obtain the time-variant heat flux and interior temperature fields in TPS. The impact of the structure parameters and future measurement temperature information on the lag and stability of retrieval results is investigated. The actual heat flux generated at NASA Langley Research Center for the Reusable Launch Vehicle is employed for examining the effectiveness and dependability of the proposed approach. The numerical results demonstrate the present approach can realize precise and on-line prediction of the time-variant heat flux and interior temperature fields in TPS.

Sounding of the Atmosphere Using Broadband Emission Radiometry (SABER): Instrument and Science Measurement Description

AbstractSABER (Sounding of the Atmosphere using Broadband Emission Radiometry) is a 10‐channel infrared radiometer that is one of four instruments on the NASA TIMED (Thermosphere‐Ionosphere‐Mesosphere Energetics and Dynamics) satellite mission to study the structure, energetics, chemistry, and dynamics of the Earth's mesosphere and lower thermosphere. Each of the ten SABER channels has a unique filter over its detector. The filter passes infrared radiation within a defined spectral region that optimizes the ability to derive temperature and constituent concentrations from the infrared radiance measurements. The TIMED spacecraft was launched into a 625 km circular polar orbit (74.1° inclination) via a Boeing Delta II rocket from Vandenberg Air Force Base on 7 December 2001. SABER continues to operate nominally and collect data routinely as it has for over 21 years. Nearly 2,200 peer‐reviewed journal articles have been published worldwide using SABER data. A list of these articles is included in Supporting Information S1 accompanying this paper. This paper presents a detailed technical description of the SABER instrument including major subsystems of the instrument and technical performance parameters. This paper comprehensively describes the instrument and its components and provides final instrument design and performance parameters. The motivation for this paper is to document this information permanently for future reference. The Space Dynamics Laboratory (SDL) of Utah State University designed, fabricated, and calibrated the SABER instrument in close collaboration with NASA Langley Research Center, Hampton University, and Global Atmospheric Technologies and Science (GATS).

Spatially coordinated airborne data and complementary products for aerosol, gas, cloud, and meteorological studies: the NASA ACTIVATE dataset

Abstract. The NASA Aerosol Cloud meTeorology Interactions oVer the western ATlantic Experiment (ACTIVATE) produced a unique dataset for research into aerosol–cloud–meteorology interactions, with applications extending from process-based studies to multi-scale model intercomparison and improvement as well as to remote-sensing algorithm assessments and advancements. ACTIVATE used two NASA Langley Research Center aircraft, a HU-25 Falcon and King Air, to conduct systematic and spatially coordinated flights over the northwest Atlantic Ocean, resulting in 162 joint flights and 17 other single-aircraft flights between 2020 and 2022 across all seasons. Data cover 574 and 592 cumulative flights hours for the HU-25 Falcon and King Air, respectively. The HU-25 Falcon conducted profiling at different level legs below, in, and just above boundary layer clouds (< 3 km) and obtained in situ measurements of trace gases, aerosol particles, clouds, and atmospheric state parameters. Under cloud-free conditions, the HU-25 Falcon similarly conducted profiling at different level legs within and immediately above the boundary layer. The King Air (the high-flying aircraft) flew at approximately ∼ 9 km and conducted remote sensing with a lidar and polarimeter while also launching dropsondes (785 in total). Collectively, simultaneous data from both aircraft help to characterize the same vertical column of the atmosphere. In addition to individual instrument files, data from the HU-25 Falcon aircraft are combined into “merge files” on the publicly available data archive that are created at different time resolutions of interest (e.g., 1, 5, 10, 15, 30, 60 s, or matching an individual data product's start and stop times). This paper describes the ACTIVATE flight strategy, instrument and complementary dataset products, data access and usage details, and data application notes. The data are publicly accessible through https://doi.org/10.5067/SUBORBITAL/ACTIVATE/DATA001 (ACTIVATE Science Team, 2020).

A hybrid soft material robotic end-effector for reversible in-space assembly of strut components.

Based on the NASA in-Space Assembled Telescope (iSAT) study (Bulletin of the American Astronomical Society, 2019, 51, 50) which details the design and requirements for a 20-m parabolic in-space telescope, NASA Langley Research Center (LaRC) has been developing structural and robotic solutions to address the needs of building larger in-space assets. One of the structural methods studied involves stackable and collapsible modular solutions to address launch vehicle volume constraints. This solution uses a packing method that stacks struts in a dixie-cup like manner and a chemical composite bonding technique that reduces weight of the structure, adds strength, and offers the ability to de-bond the components for structural modifications. We present in this paper work towards a soft material robot end-effector, capable of suppling the manipulability, pressure, and temperature requirements for the bonding/de-bonding of these conical structural components. This work is done to investigate the feasibility of a hybrid soft robotic end-effector actuated by Twisted and Coiled Artificial Muscles (TCAMs) for in-space assembly tasks. TCAMs are a class of actuator which have garnered significant recent research interest due to their allowance for high force to weight ratio when compared to other popular methods of actuation within the field of soft robotics, and a muscle-tendon actuation design using TCAMs leads to a compact and lightweight system with controllable and tunable behavior. In addition to the muscle-tendon design, this paper also details the early investigation of an induction system for adhesive bonding/de-bonding and the sensors used for benchtop design and testing. Additionally, we discuss the viability of Robotic Operating System 2 (ROS2) and Gazebo modeling environments for soft robotics as they pertain to larger simulation efforts at LaRC. We show real world test results against simulation results for a method which divides the soft, continuous material of the end-effector into discrete links connected by spring-like joints.

Probability of Trajectory Deviation of Unmanned Aerial Vehicle in Presence of Wind

Incorporating unmanned aerial vehicles (UAVs) into the United States National Airspace System would demand enhanced airspace safety technologies for the safety of the UAVs, people, and property on the ground. One of the safety-critical factors to consider is the risk of a UAV deviating from its planned trajectory, which may result in loss of separation between other vehicles or obstacles or may cause early depletion of battery power. In this paper, we studied the effect of wind on UAV trajectory deviation by incorporating wind velocity as a drag component in a six-degrees-of-freedom trajectory simulation comprising a rotorcraft lumped-mass model. Both steady-state wind and wind turbulence effects were investigated. We validated our approach using real flight data from UAV experiments conducted at NASA Langley Research Center. The proposed approach would enable risk-informed decision making by timely mitigation of current and future collision events in an uncertain and dynamic environment.

Characterization of Slat Noise Radiation from a High-Lift Common Research Model

A test campaign was conducted in the open-jet test section of the NASA Langley Research Center’s 14- by 22-Foot Subsonic Tunnel on a 10%-scale semispan version of the High-Lift Common Research Model incorporating a leading-edge slat, a trailing-edge flap, and removable high-fidelity main landing gear. Computational simulations were used to support the model development, provide important information such as load distributions along the high-lift elements, and aid in the design of noise reduction devices. Aerodynamic and far-field acoustic measurements were obtained on baseline and acoustically treated model configurations using an ensemble of time-averaged and unsteady surface pressure sensors paired with a traversing 97-microphone phased array viewing the pressure side of the airframe. A deconvolution method incorporating corrections for shear layer acoustic wave decorrelation was employed to determine the locations and strengths of relevant noise sources along the span of the slat and in the vicinity of the flap edges and landing gear. The main goal of the test campaign was to use the surface pressure and array data to evaluate the effectiveness of various slat noise reduction concepts. It was found that slat-gap fillers produced significant broadband noise reduction with minimal aerodynamic performance penalties for all Mach numbers and airfoil angles of attack that were investigated. The test campaign clearly demonstrated the noise reduction benefits that can be obtained by applying appropriate treatments to leading-edge slats on commercial transport-class aircraft.

Conceptual design of a new experimental setup to simulate aircraft tyre spin-up dynamics

PurposeThe first touchdown moment of aircraft tyres on a runway is the critical phase where maximum of the vertical and horizontal ground loads is produced. Some valuable drop tests have been performed at Langley research centre to simulate the touchdown and the spin-up dynamics. However, a long impact basin and a huge power source to accelerate and decelerate the landing gear mechanism have been used. Based on a centrifugal mechanism, the purpose of this paper is to propose the conceptual design of a new experimental setup to simulate the spin-up dynamics.Design/methodology/approachA schematic view of the proposed mechanism is presented, and its components are introduced. Operating condition of the system and the test procedure are discussed in detail. Finally, tyre spin-up dynamics of Boeing 747 is considered as a case study, and operating condition of the system and the related test parameters are extracted.FindingsIt is shown that the aircraft tyre spin-up dynamics can be simulated in a limited laboratory space with low energy consumption. The proposed setup enables the approach velocity, sink rate and vertical ground load to be adjusted by low power actuators. Hence, the proposed mechanism can be used to simulate the tyre spin-up dynamics of different types of aircraft.Research limitations/implicationsIt is important to note that more details of the setup, including the braking and actuating mechanisms together with their control procedures, should be clarified in practice. In addition, the curved path introduced as the runway will cause errors in the results. Hence, a compromise should be made between the tyre pressure, path curvature, the induced error and the cost of the experimental setup.Practical implicationsThe proposed experimental setup could be constructed in a limited space and at a relatively low cost. Low power actuators are used in the proposed system. Hence, in addition to the performance tests, fatigue tests of the landing gear mechanism will also be possible.Originality/valueBased on a centrifugal mechanism, the conceptual design of a new experimental setup is presented for simulating the tyre spin-up dynamics of aircraft. Considering that the drag load developed during tyre spin-up following initial touchdown is an important factor governing the design of the landing gear mechanism and aircraft structure, the authors hope this paper encourages engineers to continuously make efforts to increase the transparency of the touchdown process, enabling optimisation of landing gear design.

Towards high-quality X-59 sonic thump measurements

Brigham Young University has been studying questions related to NASA X-59 community noise testing. A summary of the key findings and recommendations to date are presented. Among these are weather-resistant ground microphone setups that reduce wind noise, methods for treating the problem of ambient noise contamination on sonic boom metrics and spectra, and the turbulence-induced variability seen over relatively small-aperture arrays. Also discussed are recommendations for data post-processing, e.g., employing a digital pole-shifting filter and zero-padding to improve the fidelity and smoothness of low-frequency spectral data. Finally, the relative merits of various sonic boom metrics are considered. Although Perceived Level has been the most widely-used metric, it is relatively sensitive to ambient noise contamination and turbulence-induced variability. [Work supported by NASA Langley Research Center through the National Institute of Aerospace.]

A Psychoacoustic Test for Urban Air Mobility Vehicle Sound Quality

This paper describes a psychoacoustic test in the Exterior Effects Room (EER) at the NASA Langley Research Center. The test investigated the degree to which sound quality metrics (sharpness, tonality, etc.) are predictive of annoyance to notional sounds of Urban Air Mobility (UAM) vehicles (e.g., air taxis). A suite of 136 unique (4.6 second duration) UAM rotor noise stimuli was generated. These stimuli were based on aeroacoustic predictions of a NASA reference UAM quadrotor aircraft under two flight conditions. The synthesizer changed rotor noise parameters such as the blade passage frequency, the relative level of broadband self-noise, and the relative level of tonal motor noise. With loudness constant, the synthesis parameters impacted sound quality in a way that created a spread of predictors both in synthesizer parameters and in sound quality metrics. Forty subjects listened to the suite of UAM noise stimuli in the EER and judged each sound individually on a standard scale of annoyance. Additionally, a subset of the UAM noise stimuli were compared to a reference sound that varied in loudness. From these responses, the relative effect of changes in loudness or changes in other sound quality metrics on annoyance was evaluated. This paper covers background and motivation for the test, details of how the sound stimuli were generated, and details of the test design and execution. Test results investigate how sound quality may affect perceived annoyance to UAM vehicle noise, indicating the importance of sharpness, tonality, impulsiveness, and roughness on annoyance to UAM noise.

Finite Element Simulation of Three Full-Scale Crash Tests for Cessna 172 Aircraft

The NASA Emergency Locator Transmitter Survivability and Reliability project was initiated in 2013 to assess the crash performance standards for the next generation of emergency locator transmitter (ELT) systems. Three Cessna 172 aircraft were acquired to perform crash testing at the NASA Langley Research Center Landing and Impact Research Facility. Full-scale crash tests were conducted in the summer of 2015, and each test article was subjected to severe, but survivable, impact conditions including a flare-to-stall during emergency landing and two controlled-flight-into-terrain scenarios. Finite element analyses were performed to numerically simulate the aircraft response to the crash tests. The first test simulated impacting a concrete surface represented analytically by a rigid plane. Tests 2 and 3 simulated impacting a dirt surface represented analytically by an Eulerian grid of brick elements using a Mohr–Coulomb material model. The objective of this paper is to summarize the test and analysis results for the three full-scale crash tests. Simulation models of the airframe that correlate well with the tests are needed for future studies of alternate ELT mounting configurations.

Velocity discontinuity propagation model validation using an approximate point source in motion.

The classic geometric acoustic solution for propagation through a mean flow velocity discontinuity is evaluated experimentally using an approximate point source in motion. The geometric approximation of the pressure field in the frequency and time domains is, first, revisited for arbitrary subsonic flow and source motion along the flow axis. The derivation, computed via the method of stationary phase, shows the expected Doppler behavior of the radiated acoustic field due to the source motion acting in conjunction with the convective amplification effect for a stationary source in flow. The model is validated with a minimally intrusive, approximate point source of heat in the Quiet Flow Facility at the NASA Langley Research Center. Within the limitations of the experiment in this open-jet wind tunnel, data generally show agreement with the model across a range of flow speeds and microphone positions for a broad range of frequencies. Where disagreement between measurement and theory is noted, possible causes are discussed.

Advances in Aircraft System Identification at NASA Langley Research Center

Advances in aircraft system identification at NASA Langley Research Center are discussed. The relevant time period includes the years since the last summary paper of this kind, which was published in the Journal of Aircraft in 2005. Research advances were achieved in flight test experiment design, frequency-domain modeling, real-time autonomous global modeling, rapid simulation development and updating, dynamic modeling in turbulence, flight data corrections, model uncertainty characterization, and aeroelastic modeling using distributed sensing. Possible future developments in the field are identified.

Airborne HSRL-2 measurements of elevated aerosol depolarization associated with non-spherical sea salt

Airborne NASA Langley Research Center (LaRC) High Spectral Resolution Lidar-2 (HSRL-2) measurements acquired during the recent NASA Earth Venture Suborbital-3 (EVS-3) Aerosol Cloud Meteorology Interactions over the Western Atlantic Experiment (ACTIVATE) revealed elevated particulate linear depolarization associated with aerosols within the marine boundary layer. These observations were acquired off the east coast of the United States during both winter and summer 2020 and 2021 when the HSRL-2 was deployed on the NASA LaRC King Air aircraft. During 20 of 63 total flight days, particularly on days with cold air outbreaks, linear particulate depolarization at 532 nm exceeded 0.15–0.20 within the lowest several hundred meters of the atmosphere, indicating that these particles were non-spherical. Higher values of linear depolarization typically were measured at 355 nm and lower values were measured at 1,064 nm. Several lines of evidence suggest that these non-spherical particles were sea salt including aerosol extinction/backscatter ratio (“lidar ratio”) values of 20–25 sr measured at both 355 and 532 nm by the HSRL-2, higher values of particulate depolarization measured at low (< 60%) relative humidity, coincident airbornein situsize and composition measurements, and aerosol transport simulations. The elevated aerosol depolarization values were not correlated with wind speed but were correlated with salt mass fraction and effective radius of the aerosol when the relative humidity was below 60%. HSRL-2 measured median particulate extinction values of about 20 Mm−1 at 532 nm associated with these non-spherical sea salt particles and found that the aerosol optical depth (AOD) contributed by these particles remained small (0.03–0.04) but represented on average about 30%–40% of the total column AOD. Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) spaceborne lidar aerosol measurements during several cold air outbreaks and CALIOP retrievals of column aerosol lidar ratio using column AOD constraints suggest that CALIOP operational aerosol algorithms can misclassify these aerosols as dusty marine rather than marine aerosols. Such misclassification leads to ∼40–50% overestimates in the assumed lidar ratio and in subsequent retrievals of aerosol optical depth and aerosol extinction.

Reconstruction of aerothermal heating for the thermal protection system of a reusable launch vehicle

The aim of this paper is to reconstruct the aerothermal heating profile for a three-layered thermal protection system of a reusable launch vehicle (RLV). Computational methods enable the simulation of such systems and contribute to understanding of their function, which is essential for safety. The mathematical model considered in this paper uses a one-dimensional heat conduction equation. The reconstructed heat flux on the outer surface is time-dependent. The implicit scheme of the finite difference method and the Levenberg–Marquardt methods are used to solve the direct and inverse problems, respectively. The developed algorithm is first tested on a benchmark example with a known analytical solution. Next, the algorithm is used for reconstructing the aerothermal heating profile for the thermal protection system of a RLV from known temperature values at a selected point of its thermal protection system. Input data for the inverse problem are obtained by solving the direct problem for the atmospheric entry heating profile on the surface of the RLV as measured by the NASA Langley Research Center. For the inverse problem of the main example, the aerothermal heat flux is reconstructed in the form of first- or third-order splines for various numbers of interpolating points. The model and algorithm produced a very good match between the reconstructed heat flux and the measured one. The best match is obtained for third-order splines and twenty interpolation nodes.

Design of Hypervelocity Flow Generator (HFG) and Its Diagnostics

Ground facility for hypersonic research is a key element for successful development of aerodynamically proven hypervelocity vehicles. Design concept and diagnostics of hypervelocity flow generator (HFG) were made as a test platform for hypersonic/hypervelocity spacecraft models at NASA Langley Research Center.