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At least 1,099 records · Page 61
All sourcesNASA NTRSDOE OSTI
NASA NTRSDate not supplied

Understanding the Vertical Slope of Mesoscale Snow Band Reflectivity Signatures

Mesoscale snow bands are frequently responsible for dumping large amounts of snow over a relatively small region and pose a considerable forecasting challenge. The ongoing NASA Investigation of Microphysics and Precipitation for Atlantic Coast-Threatening Snowstorms (IMPACTS) field campaign observed several instances of mesoscale snow bands during January and February of 2020 and 2022. These snow bands were sampled by a combination of airborne (NASA ER-2 and P-3) and ground-based remote sensing and in-situ instruments. Along-band cross sections of these snow bands reveal that many of them are vertically sloped. Our preliminary analysis into one of these snow bands suggests that the vertical slope is not maintained by isentropic lift and does not appear to be tied to horizontal deformation. A combined analysis of the reflectivity and wind retrievals from the ER-2 radars raises the possibility that these features could be produced by vertical wind shear layers deforming fall streaks into bands that extend hundreds of kilometers. The present study will focus on extending our preliminary analysis to additional cases and understanding the mechanisms that support these features using a combination of radar data, in-situ aircraft measurements, and numerical modeling.

Charles Helms↗
NASA NTRSDate not supplied

Design of a Propeller with Global Minimum Torque

Academia, industry, and Government are actively working towards a future where dozens of small to large Unmanned Aerial Systems (UAS) are present within urban environments. In this environment, high noise pollution and inefficiencies have the potential to render these new technologies impractical due to public annoyance and nonacceptance. This study aims to design and analyze a novel propeller design for minimum global torque using an optimization algorithm to find the full three dimensional solution of maximum efficiency. This resulted in a 𝐶 𝑙 distribution defining the global minimum torque solution for propeller design which corresponded with moving as much lift inboard as possible and reducing lift rapidly at the blade tip. The novel propeller (dubbed "Prandtl" propeller) was then compared with the current gold standard in propeller design, a minimum induced loss (MIL) propeller, which had all the same geometric properties except for blade twist and produced the same amount of thrust at the same advance ratio. The results of multiple iterations of Prandtl propellers showed an increase in efficiency of 2.0-3.3% for the Prandtl blade when compared to the equivalent MIL blade. A potential added benefit of this blade design is lower noise generation due to the lower lift loading at the tip reducing the large shear layer intensity which is the point source of noise in propellers. Two major breakthroughs for enabling widespread use of UAS in urban environments are noise reduction and vehicle efficiency, and this new propeller design has the potential to provide both.

Propeller↗
NASA NTRSDate not supplied

Characterization of Reactant Refill and Detonation Wave Dynamics in a GOx/Natural-gas RDRE Using Simultaneous High Repetition-Rate OH-PLIF and Chemiluminescence

The potential application of rotating detonation engines (RDEs) in rocket combustors hinges on a fundamental understanding of detonation wave structure and injector characteristics with fuel and oxidizer compositions relevant in rocket systems. Simultaneous 300 kHz-rate broadband OH* chemiluminescence and OH-PLIF imaging is employed in a fully optically accessible Natural Gas-GOx rotating detonation rocket engine (RDRE) to visualize reactant refill dynamics and detonation wave structure. A custom-built KTP-type optical parametric oscillator (OPO) is coupled with a nanosecond high-repetition-rate burst-mode laser to output284 nm light and target excitation of the Q1(9) transition in the OH radical. Significant deflagrative burning is observed throughout the chamber as a consequence of the oxygen-rich environment. Trailing Azimuthal Reflected Shock Combustion (ARSC) system, similar to those in a H2-air RDE are observed, burning unburned reactants in the region immediately following the primary detonation wave. Contact burning, as indicated in this study, does not seem to be a primary loss mechanism. The simultaneous measurement of OH and OH* show that axial locations exist in the refill process where OH radicals are present, and produced due to shear layer induced deflagration, however, these zones do not produce excited state OH*. While a deeper understanding of the underlying physics in RDRE systems requires further investigation, this work highlights a first-of-its-kind visualization of the turbulent combustion product field and reactant refill characteristics in this highly unsteady environment.

Propulsion↗
NASA NTRSDate not supplied

Design of a Propeller with Global Minimum Torque

Academia, industry, and Government are actively working towards a future where dozens of small to large Unmanned Aerial Systems (UAS) are present within urban environments. In this environment, high noise pollution and inefficiencies have the potential to render these new technologies impractical due to public annoyance and nonacceptance. This study aims to design and analyze a novel propeller design for minimum global torque using an optimization algorithm to find the full three dimensional solution of maximum efficiency. This resulted in a 𝐶 𝑙 distribution defining the global minimum torque solution for propeller design which corresponded with moving as much lift inboard as possible and reducing lift rapidly at the blade tip. The novel propeller (dubbed "Prandtl" propeller) was then compared with the current gold standard in propeller design, a minimum induced loss (MIL) propeller, which had all the same geometric properties except for blade twist and produced the same amount of thrust at the same advance ratio. The results of multiple iterations of Prandtl propellers showed an increase in efficiency of 2.0-3.3% for the Prandtl blade when compared to the equivalent MIL blade. A potential added benefit of this blade design is lower noise generation due to the lower lift loading at the tip reducing the large shear layer intensity which is the point source of noise in propellers. Two major breakthroughs for enabling widespread use of UAS in urban environments are noise reduction and vehicle efficiency, and this new propeller design has the potential to provide both.

Propeller↗
NASA NTRSDate not supplied

Acoustic Characterization of the NASA Langley 14- by 22-Foot Subsonic Tunnel Using Single-Microphone Analysis Techniques

An experimental campaign was conducted to assess recent acoustic modifications to the NASA Langley Research Center 14- by 22-Foot Subsonic Tunnel. This effort was undertaken in preparation for future rotorcraft, Advanced Air Mobility, and airframe noise acoustic tests. The tunnel is a closed circuit and typically operates in an open-jet configuration for acoustic studies. A vertical linear array of microphones and a phased array were placed on traverses outside of the core flow. Pole-mounted acoustic sources with known waveforms were used to identify reflective surfaces under static conditions with no tunnel flow. A similar process was replicated with more compact sources in an aerodynamic fairing for flow speeds up to Mach 0.16. This enabled investigation of the test section core flow and bounding shear layer impacts on the acoustic measurements. Periodic averaging was employed and was shown to be capable of isolating periodic acoustic signals even for poor signal-to-noise ratio conditions. Benefits and limitations of single-microphone processing methods are identified. A companion paper utilizes a phased array in an effort to address the identified limitations to more traditional single-microphone data collection.

Acoustic Characterization↗
NASA NTRSDate not supplied

Learjet 25D Jet Noise Flyover Measurements: Comparisons with Scale-Model Data

Continued interest in commercial supersonic flight has highlighted the need for accurate jet-noise predictions as jet noise is expected to be a dominant noise source for aircraft takeoff conditions. The current study compares the acoustic spectra obtained from a scale-model investigation to that from a Learjet 25D flyover test. Reasonable agreement was achieved between the scale-model and flight data in both peak level and frequency when the physical scale factor for the scale-model was increased by roughly 6%. The peak perceived noise levels (PNL) for the flight data were between 1.29 dB and 1.65 dB higher than those for the scale model. The roll-off of PNL with time for large emission angles was greater for the flight data than for the scale-model data. Using a distributed source model for shear-layer-refraction corrections to the scale-model data slightly improved comparisons to flight data at low frequencies for emission angles near 70° and between 110° and 130°. Distributed source models had no impact on the scale-model spectra near the peak jet-noise angle. Applying a stretching factor to the distributed source models to capture the effects of a flight stream had a very limited impact on the computed spectra.

Acoustics↗
NASA NTRSDate not supplied

Acoustic Characterization of the NASA Langley 14- by 22-Foot Subsonic Tunnel Using Single-Microphone Analysis Techniques

An experimental campaign was conducted to assess recent acoustic modifications to the NASA Langley Research Center 14- by 22-Foot Subsonic Tunnel. This effort was undertaken in preparation for future rotorcraft, Advanced Air Mobility, and airframe noise acoustic tests. The tunnel is a closed circuit and typically operates in an open-jet configuration for acoustic studies. A vertical linear array of microphones and a phased array were placed on traverses outside of the core flow. Pole-mounted acoustic sources with known waveforms were used to identify reflective surfaces under static conditions with no tunnel flow. A similar process was replicated with more compact sources in an aerodynamic fairing for flow speeds up to Mach 0.16. This enabled investigation of the test section core flow and bounding shear layer impacts on the acoustic measurements. Periodic averaging was employed and was shown to be capable of isolating periodic acoustic signals even for poor signal-to-noise ratio conditions. Benefits and limitations of single-microphone processing methods are identified. A companion paper utilizes a phased array in an effort to address the identified limitations to more traditional single-microphone data collection.

Acoustic Characterization↗
NASA NTRSDate not supplied

Zemax Modeling of a Self-aligned Focusing Schlieren System with Simulated Compressible Flow

A self-aligned focusing schlieren system is simulated in the ANSYS Zemax OpticStudio software environment. The model uses a custom scatter-defining dynamic link library on the retroreflective background in order to scatter rays in the direction of the incident ray rather than the specular ray. Simulated compressible flow refractive index fields are imported into Zemax by use of the grid gradient object and a workaround to the maximum point limit is introduced. The Ronchi ruling is built as a high resolution bitmap image in MATLAB and set in Zemax as a slide object allowing the bitmap image to either transmit or terminate rays at each line pair. The Zemax simulations are compared to computational numerical schlieren and experimental conventional, path-integrated schlieren results from a hypersonic stagnation point injection flow configuration and an experimental image of a shock wave from a laser-induced breakdown. In the stagnation point injection simulations, the Zemax schlieren images capture complex flow features such as the Mach disk, shear layers, contact surface, and bow shock well, but contains long, curved features (striations) between the model and the bow shock that are not present in the synthetic schlieren image from the CFD or the experimental image. The image from Zemax of the spark-induced shock wave captures the shape and curvature of the shock but appears blurry due to grid discretization in the computational data.

Nicholas A Mejia↗
NASA NTRSDate not supplied

Aeroheating Predictions for a Hypersonic, Turbulent Near-Wake

The accuracy of heating predictions using various turbulence models is examined for an axisymmetric near-wake at Mach 6. The CFD predictions are compared with experimental data collected under AGARD Working Group 18 on the wake of a 70-degree sphere-cone. The impact of grid resolution and discretization error is estimated, which allows a comparison of stacked-block and conventional structured meshes. The accuracy of steady Reynolds-averaged Navier-Stokes (RANS) models is contrasted with that of a hybrid RANS/Large-Eddy Simulation model. The predictions are made with three different CFD codes (LAURA, FUN3D, and HyperSolve), to demonstrate the code-to-code variation in the results. Steady SST models capture the qualitative nature of the heating in the wake, including the increasing peak heating with increasing Reynolds number. Spalart-Allmaras models, including SA-Catris, under-predicted the peak heating in the wake. Hybrid RANS/LES models improve upon the SA results but have their own modeling difficulties near the shear layer impingement. These results are generally consistent across solvers and grid topologies.

RANS↗
NASA NTRSDate not supplied

Aeroheating Predictions for a Hypersonic, Turbulent Near-Wake

The accuracy of heating predictions using various turbulence models is examined for an axisymmetric near-wake at Mach 6. The CFD predictions are compared with experimental data collected under AGARD Working Group 18 on the wake of a 70-degree sphere-cone. The impact of grid resolution and discretization error is estimated, which allows a comparison of stacked-block and conventional structured meshes. The accuracy of steady Reynolds-averaged Navier-Stokes (RANS) models is contrasted with that of a hybrid RANS/Large-Eddy Simulation model. The predictions are made with three different CFD codes (LAURA, FUN3D, and HyperSolve), to demonstrate the code-to-code variation in the results. Steady SST models capture the qualitative nature of the heating in the wake, including the increasing peak heating with increasing Reynolds number. Spalart-Allmaras models, including SA-Catris, under-predicted the peak heating in the wake. Hybrid RANS/LES models improve upon the SA results but have their own modeling difficulties near the shear layer impingement. These results are generally consistent across solvers and grid topologies.

RANS↗
NASA NTRSDate not supplied

Wake Velocimetry of a Sphere-Cone Model in a Mach 10 Air Freestream using FLEET

Velocity measurements in the wake behind a sphere-cone-shaped vehicle wind tunnel model in a Mach 10 hypersonic air flow using femtosecond laser electronic excitation tagging (FLEET) are reported in this work. The FLEET measurements were performed at 1 kHz using a femtosecond laser centered at 800 nm and an ultrahigh-speed intensified camera system. The results reported here are for a single freestream condition corresponding to approximately Re ∞ /L = 3.6∙10 6 m -1 . The performance of the velocimetry instrument was evaluated in the quiescent test section at conditions relevant to the wake measurements. For velocity measurements in the wake, the FLEET measurement volume was translated to several radial locations from the model centerline in the vertical direction and at a streamwise location corresponding to approximately 12 mm from the payload. Reverse flow with mean velocities ranging from -12 m/s to -48 m/s were observed in the recirculating region of the wake, whereas hypersonic velocities in the range of 1079 m/s to 1183 m/s were observed in the free shear layer. A velocity uncertainty methodology is also outlined and applied for the reported measurements. The velocity data reported in this work is expected to play a significant role in the validation of computational codes modeling the hard-to-predict separated hypersonic wake.

FLEET↗
NASA NTRSDate not supplied

Wake Velocimetry of a Sphere-Cone Model in a Mach 10 Air Freestream using FLEET

Velocity measurements in the wake behind a sphere-cone-shaped vehicle wind tunnel model in a Mach 10 hypersonic air flow using femtosecond laser electronic excitation tagging (FLEET) are reported in this work. The FLEET measurements were performed at 1 kHz using a femtosecond laser centered at 800 nm and an ultrahigh-speed intensified camera system. The results reported here are for a single freestream condition corresponding to approximately Re ∞ /L = 3.6∙10 6 m -1 . The performance of the velocimetry instrument was evaluated in the quiescent test section at conditions relevant to the wake measurements. For velocity measurements in the wake, the FLEET measurement volume was translated to several radial locations from the model centerline in the vertical direction and at a streamwise location corresponding to approximately 12 mm from the payload. Reverse flow with mean velocities ranging from -12 m/s to -48 m/s were observed in the recirculating region of the wake, whereas hypersonic velocities in the range of 1079 m/s to 1183 m/s were observed in the free shear layer. A velocity uncertainty methodology is also outlined and applied for the reported measurements. The velocity data reported in this work is expected to play a significant role in the validation of computational codes modeling the hard-to-predict separated hypersonic wake.

FLEET↗
NASA NTRS1976

Turbulence measurements in a transonic boundary layer and free-shear flow using laser velocimetry and hot-wire anemometry techniques

Quantitative measurements of the turbulence fluctuations in velocity and mass flux have been obtained in Mach 0.6 and 0.8 turbulent boundary layer and free-shear layer flows by laser velocimetry and hot-wire anemometry techniques. To evaluate the effects of compressibility, these transonic data are compared to available incompressible and supersonic results. Based on some simplifying assumptions, estimates of the rms density fluctuations are made for which error bounds are given. In addition to these fluctuation data, the compressible mean velocity data obtained with the laser velocimeter are presented and compared to pitot tube results. The investigation was conducted in the Ames 6- by 6-Foot Supersonic Wind Tunnel at free-stream Mach numbers of 0.6 and 0.8 for a unit Reynolds number of about 10,000,000 per meter.

Johnson, D. A.↗
NASA NTRS1975

The measurement of shear stress and total heat flux in a nonadiabatic turbulent hypersonic boundary layer

Turbulent shear stress and direct turbulent total heat-flux measurements have been made across a nonadiabatic, zero pressure gradient, hypersonic boundary layer by using specially designed hot-wire probes free of strain-gauging and wire oscillation. Heat-flux measurements were in reasonably good agreement with values obtained by integrating the energy equation using measured profiles of velocity and temperature. The shear-stress values deduced from the measurements, by assuming zero correlation of velocity and pressure fluctuations, were lower than the values obtained by integrating the momentum equation. Statistical properties of the cross-correlations are similar to corresponding incompressible measurements at approximately the same momentum-thickness Reynolds number.

Mikulla, V.↗
NASA NTRS1981

Climatological characteristics of high altitude wind shear and lapse rate layers

Indications of the climatological distribution of wind shear and temperature lapse and inversion rates as observed by rawinsonde measurements over the western United States are recorded. Frequencies of the strongest shear, lapse rates, and inversion layer strengths were observed for a 1 year period of record and were tabulated for the lower troposphere, the upper troposphere, and five altitude intervals in the lower stratosphere. Selected bivariate frequencies were also tabulated. Strong wind shears, lapse rates, and inversion are observed less frequently as altitude increases from 175 millibars to 20 millibars. On a seasonal basis the frequencies were higher in winter than in summer except for minor influences due to increased tropopause altitude in summer and the stratospheric wind reversal in the spring and fall.

Ehernberger, L. J.↗
NASA NTRS1980

Sporadic-E layers and unstable wind shears

Electron density profiles of sporadic-E layers have been observed with good height resolution using rocket-borne probes. These generally show a simple shape consistent with the effect of a linear wind shear acting on metallic ions. Occasionally more complex shapes have been recorded, including double peaks and, on one occasion, a nearly rectangular profile. A direct method of obtaining the wind profile from the concentration profile of metallic ions has been developed. The metallic ion concentration profile itself is obtained from the electron density profile. Both procedures derive from the steady-state continuity equation. For linear wind shears it is found that the maximum value of the shear is about 50 m/s/km which corresponds to a Richardson number of 1/4. Layers of complex shape are associated with non-linear wind shears in which the maximum shear considerably exceeds this value. It is concluded that the complex profiles of sporadic-E layers can be interpreted as an effect of unstable wind shears.

Smith, L. G.↗