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At least 127 records · Page 7

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NASA NTRS2012

CFD Simulations for the Effect of Unsteady Wakes on the Boundary Layer of a Highly Loaded Low-Pressure Turbine Airfoil (L1A)

This report is the third part of a three-part final report of research performed under an NRA cooperative Agreement contract. The first part was published as NASA/CR-2012-217415. The second part was published as NASA/CR-2012-217416. The study of the very high lift low-pressure turbine airfoil L1A in the presence of unsteady wakes was performed computationally and compared against experimental results. The experiments were conducted in a low speed wind tunnel under high (4.9%) and then low (0.6%) freestream turbulence intensity for Reynolds number equal to 25,000 and 50,000. The experimental and computational data have shown that in cases without wakes, the boundary layer separated without reattachment. The CFD was done with LES and URANS utilizing the finite-volume code ANSYS Fluent (ANSYS, Inc.) under the same freestream turbulence and Reynolds number conditions as the experiment but only at a rod to blade spacing of 1. With wakes, separation was largely suppressed, particularly if the wake passing frequency was sufficiently high. This was validated in the 3D CFD efforts by comparing the experimental results for the pressure coefficients and velocity profiles, which were reasonable for all cases examined. The 2D CFD efforts failed to capture the three dimensionality effects of the wake and thus were less consistent with the experimental data. The effect of the freestream turbulence intensity levels also showed a little more consistency with the experimental data at higher intensities when compared with the low intensity cases. Additional cases with higher wake passing frequencies which were not run experimentally were simulated. The results showed that an initial 25% increase from the experimental wake passing greatly reduced the size of the separation bubble, nearly completely suppressing it.

Vinci, Samuel, J.↗

NASA NTRS2014

Wake Vortex Detection: Phased Microphone vs. Linear Infrasonic Array

Sensor technologies can make a significant impact on the detection of aircraft-generated vortices in an air space of interest, typically in the approach or departure corridor. Current state-of-the art sensor technologies do not provide three-dimensional measurements needed for an operational system or even for wake vortex modeling to advance the understanding of vortex behavior. Most wake vortex sensor systems used today have been developed only for research applications and lack the reliability needed for continuous operation. The main challenges for the development of an operational sensor system are reliability, all-weather operation, and spatial coverage. Such a sensor has been sought for a period of last forty years. Acoustic sensors were first proposed and tested by National Oceanic and Atmospheric Administration (NOAA) early in 1970s for tracking wake vortices but these acoustic sensors suffered from high levels of ambient noise. Over a period of the last fifteen years, there has been renewed interest in studying noise generated by aircraft wake vortices, both numerically and experimentally. The German Aerospace Center (DLR) was the first to propose the application of a phased microphone array for the investigation of the noise sources of wake vortices. The concept was first demonstrated at Berlins Airport Schoenefeld in 2000. A second test was conducted in Tarbes, France, in 2002, where phased microphone arrays were applied to study the wake vortex noise of an Airbus 340. Similarly, microphone phased arrays and other opto-acoustic microphones were evaluated in a field test at the Denver International Airport in 2003. For the Tarbes and Denver tests, the wake trajectories of phased microphone arrays and lidar were compared as these were installed side by side. Due to a built-in pressure equalization vent these microphones were not suitable for capturing acoustic noise below 20 Hz. Our group at NASA Langley Research Center developed and installed an infrasonic array at the Newport News-Williamsburg International Airport early in the year 2013. A pattern of pressure burst, high-coherence intervals, and diminishing-coherence intervals was observed for all takeoff and landing events without exception. The results of a phased microphone vs. linear infrasonic array comparison will be presented.

Shams, Qamar A.↗

NASA NTRS2023

Blade-Wake Interaction Noise for Small Hovering Rotors, Part I: Characterization Study

This work illustrates the use of artificial neural network modeling to study and characterize broadband blade-wake interaction noise from hovering small unmanned aerial systems rotors subject to varying airfoil geometries, rotor geometries, and operating conditions. Design of experiments was used to create input feature spaces, and a high-fidelity strategy was implemented at the discrete data points defined by the input feature spaces to design airfoils and rotor blades, predict the unsteady rotor aerodynamics and aeroacoustics, and isolate the blade-wake interaction noise from the acoustic broadband noise. A metric for the blade-wake interaction noise was developed, and the ANOPP2 Artificial Neural Network Tool was used to identify an optimal prediction model for the nonlinear relationship between the input features and the metric for blade-wake interaction noise. This optimal artificial neural network was then validated over training/test data and exhibited prediction accuracy over 91% for data previously unseen by the model. A sensitivity analysis was conducted, which showed that input features that directly modify the thrust coefficient had a dominant effect over blade-wake interaction noise. The optimal prediction model along with aerodynamic simulations were used to further study the effect of varying input features on blade-wake interaction noise, and three types of blade-wake interaction noise were identified

Christopher S. Thurman↗

NASA NTRSDate not supplied

Perturbations in Brain Functional Connectivity Patterns After Waking From Slow Wave Sleep Under Different Cognitive States

Sleep inertia refers to the state of transition between sleep and wake characterized by impaired alertness, confusion, and reduced cognitive and behavioral performance. While the behavioral symptoms of sleep inertia are well described, the neurological changes that lead to this state remain elusive. Here, to understand the state of sleep inertia and the reorganization that the brain undergoes, we took a graph theoretical approach and compared the EEG derived brain connectivity patterns before sleep and after waking up while participants (n = 10) performed multiple tasks that differed in cognitive complexities. We focused on how the degree and the clustering coefficient of brain regions (EEG sensors) change immediately after participants wake up from slow wave sleep. During a psychomotor vigilance task (PVT), designed to assess vigilant attention, we find that the brain regions with strong network connectivity (degree) before sleep show a reduction in connectivity after waking. In contrast, those with low connectivity before sleep have greater connectivity after waking. The regions that undergo these changes are specific to each participant and these findings are unique to the beta frequency range, which plays a key role in sensorimotor functioning and preserving the current state of the brain. Moreover, in tasks that required inhibitory control and arithmetic reasoning, we found that only regions with weak connectivity before sleep exhibited more connections after waking, but regions with high connectivity prior to sleeping remained unchanged, highlighting task specific effects. Furthermore, we find that during the PVT, the clustering coefficient within low frequency oscillations of the brain is reduced upon waking while it remains unchanged during other tasks. These results suggest that the connections between regions that are lost after abrupt awakening can be reallocated to other regions in order to renormalize the brain. However, this response may only be evident during specific cognitive states and may be more nuanced during complex task performance.

sleep inertia↗

NASA NTRSDate not supplied

Wake Vortex Interaction of Urban Air Mobility Aircraft

This paper presents a study of wake vortex interaction modeling for urban air mobility. Wake vortex modeling approaches for fixed-wing aircraft and rotorcraft are investigated. A wake age model is developed that accounts for the temporal and spatial dissipation of the wake-induced downwash. A wake vortex model is developed for rotorcraft that accounts for blade flapping motion and cyclic pitch control to reduce the lift asymmetry. This in turn results in an equal lift circulation strength on the advancing side and retreating side of the rotor. Two wake vortex interaction simulations are performed to illustrate the modeling approaches.

Wake Vortex↗

NASA NTRS1998

Later endogenous circadian temperature nadir relative to an earlier wake time in older people

The contribution of the circadian timing system to the age-related advance of sleep-wake timing was investigated in two experiments. In a constant routine protocol, we found that the average wake time and endogenous circadian phase of 44 older subjects were earlier than that of 101 young men. However, the earlier circadian phase of the older subjects actually occurred later relative to their habitual wake time than it did in young men. These results indicate that an age-related advance of circadian phase cannot fully account for the high prevalence of early morning awakening in healthy older people. In a second study, 13 older subjects and 10 young men were scheduled to a 28-h day, such that they were scheduled to sleep at many circadian phases. Self-reported awakening from scheduled sleep episodes and cognitive throughput during the second half of the wake episode varied markedly as a function of circadian phase in both groups. The rising phase of both rhythms was advanced in the older subjects, suggesting an age-related change in the circadian regulation of sleep-wake propensity. We hypothesize that under entrained conditions, these age-related changes in the relationship between circadian phase and wake time are likely associated with self-selected light exposure at an earlier circadian phase. This earlier exposure to light could account for the earlier clock hour to which the endogenous circadian pacemaker is entrained in older people and thereby further increase their propensity to awaken at an even earlier time.

NASA Discipline Regulatory Physiology↗

NASA NTRS2017

Impact of Wake Dispersion on Axial Compressor Performance

Detailed development of wakes and their impact on the performance of a low-speed one and half stage axial compressor are investigated with a large eddy simulation (LES). To investigate effects of wake mixing recovery and wake interaction with the boundary layer of the downstream blade, spacing between the rotor blade and the stator is varied. The calculated LES flow fields based on a fine computational grid are compared with related measurements and analyzed in detail at several radial locations. The current LES calculates the effects of wake recovery very well. The effects of wake recovery vary significantly in the radial direction. Loss generation is higher on the pressure side at the stator exit at both near design and near stall condition. The current investigation indicates that better management of wake development can be achieved for improved compressor performance.

LES↗

NASA NTRS1971

Experiment S026: Ion wake measurement

Measurements of the ion-wake and electron-wake structure and other perturbations of the ambient medium produced by the Gemini spacecraft in orbit are reported. Thruster firings in the TDA-south configuration produced a decrease in the observed ion flux to the outboard ion sensor, an apparent increase in the ion flux to the inboard ion sensor, and an enhanced electron concentration to the outboard electron sensor. Visual inspection of strip-chart data was indicative that definitive wake-cone angles could be determined. It also was apparent for many cases that the electron distribution followed the ion depletion effects; this was indicative that the wake was a plasma, not an ion wake. Analysis of the data from the GATV orbits was indicative that electron and ion temperatures were higher than was estimated. The high electron temperature accounted for the sensor saturation observed during the mission. This temperature measurement was 0.3 electron volt at a 200-n. mi. altitude during daytime conditions. Spacecraft wake structure has been observed as far as 100 feet from the vehicle.

Medved, D. B.↗

NASA NTRS1974

Studies of aircraft wake chemistry and dispersion

Use of aerospace technology to study aircraft wakes is reviewed. It is shown how aerospace vehicles can be used to provide data for increased understanding of the atmosphere and of aircraft exhaust trails where knowledge is inadequate to evaluate fully the potential impact of the engine emissions. Models of aircraft near-field exhaust wakes are characterized by jet, vortex, and dispersion regimes. Wake growth in the jet regime is self-determined and rapid, whereas further spreading is inhibited in the vortex regime because of circulating vortex motion. Wake diffusion in the dispersion regime is initially influenced by aircraft induced turbulence but is dominated later by small-scale atmospheric turbulence. Computed fluid mechanical results show the importance of effects such as wake buoyancy, wind shear, turbulence, and traffic corridor exhaust buildup on dispersion of the wake. In the jet regime the exhaust characteristics and thermochemistry serve to illustrate initial chemical changes involving potential pollutant species.

Poppoff, I. G.↗

NASA NTRS1974

Measurements of supersonic jet aircraft wakes in the stratosphere

Progress is reported in an experimental program consisting of making supersonic aircraft wakes visible in the stratosphere so that photographs can be taken which yield wake dimensions vs time, and so that aircraft equipped with instruments which measure key exhaust species can find and penetrate the wakes. The object is to provide verification of fluid dynamic and chemical models. Three available models are summarized, and the experimental methods used are described. The Lockheed model predictions correspond in general most closely to experimental wake cross-sectional aerea. NO data are in reasonable agreement with NO predictions, but chemical models are relatively undeveloped, and additional measurements of exhaust products are needed before verification of chemical models becomes possible. However, the overall feasibility of wake visualization for model verification is substantiated. It is emphasized that significant model differences exist that must be resolved before reliable predictions can be made in the wake regimes studied.

Farlow, N. H.↗

NASA NTRS1976

Vortex interactions in multiple vortex wakes behind aircraft

A flow visualization technique has been developed which allows the nature of lift-generated wakes behind aircraft models to be investigated. Several different configurations of a 0.61-m span model of a Boeing 747-type transport aircraft were tested to allow observation of typical vortex interactions and merging in multiple vortex wakes. The vortices were identified by emitting tracer dyes from selected locations on the model. Wing span loading and model attitude were found to effect both vortex motions within the wake and resulting far-field wake velocity. Landing gear deployment caused a far-field reformation of vorticity behind a model configuration which dissipated concentrated vorticity in the near-field wake. A modified landing configuration was developed which appeared to significantly alleviate the concentrated wake vorticity.

Ciffone, D. L.↗

NASA NTRS1978

Some features of a wake tone flow field

An experimental investigation of a two-dimensional model of bluff body wake-surface interaction noise examines the changes in the flow field associated with changes in sound generated. Results show three distinct flow regimes separated by large differences in wake structure. In the first, with the surface in the near wake, discrete frequency sound is totally suppressed. At greater separation distances an amplified discrete frequency tone, a wake tone, is generated with the frequency a function of the separation distance. The final regime, with the surface in the far wake, is similar to a wake with no surface interaction.

Smith, C. A.↗

NASA NTRS1978

Wake flow measurements with a mobile laser Doppler velocimeter system

Wake flow measurements were conducted with a mobile laser Doppler velocimeter (LDV). The measurements included surveys of aircraft wake vortices behind a B-747 aircraft, aircraft carrier wake measurements from aboard the U.S.S. Nimitz, and tower wake measurements for a 100 kW wind turbine. Results of these tests demonstrated that a mobile ground-based LDV is a versatile and useful tool for the measurement of full-scale three-dimensional wake flows. The potential is demonstrated for utilization of this system to study complex wakes for a variety of applications.

Wilson, D. J.↗

NASA NTRS1980

Wake flowfields for Jovian probe

The wake flow field developed by the Galileo probe as it enters the Jovian atmosphere was modeled. The wake produced by the probe is highly energetic, yielding both convective and radiative heat inputs to the base of the probe. A component mathematical model for the inviscid near and far wake, the viscous near and far wake, and near wake recirculation zone was developed. Equilibrium thermodynamics were used for both the ablation and atmospheric species. Flow fields for three entry conditions were calculated. The near viscous wave was found to exhibit a variable axial pressure distribution with the neck pressure approximately three times the base pressure. Peak wake flow field temperatures were found to be in proportion to forebody post shock temperatures.

Engel, C. D.↗

NASA NTRS1980

Structure and decay characteristics of turbulence in the near- and far-wake of a moderately loaded compressor rotor-blade

The wake of a turbomachinery rotor-blade is turbulent, highly three-dimensional, and nonisotropic with appreciable curvature in the trailing-edge and near-wake regions. The characteristics of the turbulence vary considerably with radius, blade loading, free-stream turbulence, Reynolds number, and the rotor-blade geometry. This paper is concerned with the turbulence properties of a moderately loaded compressor blade, particularly near the blade trailing-edge. The tangential variation of the axial, tangential and radial intensities and stresses across the wake, as well as their decay characteristics were measured with a tri-axial hot-wire probe in the rotor frame of reference. The decay of intensities and stresses were found to be very rapid in the trailing-edge and near-wake regions and slow in the far-wake region. The effects of inlet-guide-vane and the hub-wall boundary layers on the rotor wake turbulence spectra are also discussed. Similarity rules for the three components of intensity are also derived and presented in this paper.

Ravindranath, A.↗

NASA NTRS1982

A prescribed wake rotor inflow and flow field prediction analysis, user's manual and technical approach

A user's manual is provided which includes the technical approach for the Prescribed Wake Rotor Inflow and Flow Field Prediction Analysis. The analysis is used to provide the rotor wake induced velocities at the rotor blades for use in blade airloads and response analyses and to provide induced velocities at arbitrary field points such as at a tail surface. This analysis calculates the distribution of rotor wake induced velocities based on a prescribed wake model. Section operating conditions are prescribed from blade motion and controls determined by a separate blade response analysis. The analysis represents each blade by a segmented lifting line, and the rotor wake by discrete segmented trailing vortex filaments. Blade loading and circulation distributions are calculated based on blade element strip theory including the local induced velocity predicted by the numerical integration of the Biot-Savart Law applied to the vortex wake model.

Egolf, T. A.↗

NASA NTRS1981

Interaction of compressor rotor blade wake with wall boundary layer/vortex in the end-wall region

This paper reports the experimental study of the three-dimensional characteristics of the mean velocity of the rotor wake inside the annulus- and hub-wall boundary layers. The measurements were taken with a rotating three-sensor hot wire behind the rotor. This set of measurements probably represents the first set of comprehensive measurements taken inside the annulus- and hub-wall boundary layers. The wake was surveyed at several radial locations inside the boundary layer region and at several axial locations. Interaction of the wake with the annulus-wall boundary layer, secondary flow, tip-leakage flow, and the trailing vortex system results in slower decay and larger width of the wake. The presence of a strong vortex and its merger with the wake is also observed. The end-wall boundary layers and the secondary flow were found to have a substantial effect on both the decay characteristics and the profile of the wake. These and other measurements are reported and interpreted in this paper.

Lakshminarayana, B.↗

NASA NTRS1983

Flow visualization of the wake of a transport aircraft model with lateral-control oscillations

An exploratory flow visualization study conducted in the Langley Vortex Research Facility to investigate the effectiveness of lateral control surface oscillations as a potential method for wake vortex attenuation on a 0.03 scale model of a wide body jet transport aircraft is described. Effects of both asymmetric surface oscillation (control surfaces move as with normal lateral control inputs) and symmetric surface oscillation (control surfaces move in phase) are presented. The asymmetric case simulated a flight maneuver which was previously investigated on the transport aircraft during NASA/FAA flight tests and which resulted in substantial wake vortex attenuation. Effects on the model wake vortex systems were observed by propelling the model through a two dimensional smoke screen perpendicular to the model flight path. Results are presented as photographic time histories of the wake characteristics recorded with high speed still cameras. Effects of oscillation on the wake roll up are described in some detail, and the amount of vortex attenuation observed is discussed in comparative terms. Findings were consistent with flight test results in that only a small amount of rotation was observed in the wake for the asymmetric case. A possible aerodynamic mechanism contributing to this attenuation is suggested.

Jordan, F. L., Jr.↗