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NASA NTRS2001

Market Assessment of Forward-Looking Turbulence Sensing Systems

In recognition of the importance of turbulence mitigation as a tool to improve aviation safety, NASA's Aviation Safety Program developed a Turbulence Detection and Mitigation Sub-element. The objective of this effort is to develop highly reliable turbulence detection technologies for commercial transport aircraft to sense dangerous turbulence with sufficient time warning so that defensive measures can be implemented and prevent passenger and crew injuries. Current research involves three forward sensing products to improve the cockpit awareness of possible turbulence hazards. X-band radar enhancements will improve the capabilities of current weather radar to detect turbulence associated with convective activity. LIDAR (Light Detection and Ranging) is a laser-based technology that is capable of detecting turbulence in clear air. Finally, a possible Radar-LIDAR hybrid sensor is envisioned to detect the full range of convective and clear air turbulence. To support decisions relating to the development of these three forward-looking turbulence sensor technologies, the objective of this study was defined as examination of cost and implementation metrics. Tasks performed included the identification of cost factors and certification issues, the development and application of an implementation model, and the development of cost budget/targets for installing the turbulence sensor and associated software devices into the commercial transport fleet.

Kauffmann, Paul↗

NASA NTRS2006

Progress in Development of an Airborne Turbulence Detection System

Aircraft encounters with turbulence are the leading cause of in-flight injuries (Tyrvanas 2003) and have occasionally resulted in passenger and crew fatalities. Most of these injuries are caused by sudden and unexpected encounters with severe turbulence in and around convective activity (Kaplan et al 2005). To alleviate this problem, the Turbulence Prediction and Warning Systems (TPAWS) element of NASA s Aviation Safety program has investigated technologies to detect and warn of hazardous in-flight turbulence. This effort has required the numerical modeling of atmospheric convection: 1) for characterizing convectively induced turbulence (CIT) environments, 2) for defining turbulence hazard metrics, and 3) as a means of providing realistic three-dimensional data sets that can be used to test and evaluate turbulence detection sensors. The data sets are being made available to industry and the FAA for certification of future airborne turbulence-detection systems (ATDS) with warning capability. Early in the TPAWS project, a radar-based ATDS was installed and flight tested on NASA s research aircraft, a B-757. This ATDS utilized new algorithms and hazard metrics that were developed for use with existing airborne predictive windshear radars, thus avoiding the installation of new hardware. This system was designed to detect and warn of hazardous CIT even in regions with weak radar reflectivity (i.e. 5-15 dBz). Results from an initial flight test of the ATDS were discussed in Hamilton and Proctor (2002a; 2002b). In companion papers (Proctor et al 2002a; 2002b), a numerical simulation of the most significant encounter from that flight test was presented. Since the presentation of these papers a second flight test has been conducted providing additional cases for examination. In this paper, we will present results from NASA s flight test and a numerical model simulation of a turbulence environment encountered on 30 April 2002. Progress leading towards FAA certification of industry built ATDS will also be discussed.

Hamilton, David W.↗

NASA NTRSDate not supplied

Modeling Shock-Separated Flow with the Active Model Split

The predictive accuracy of typical hybrid RANS/LES models is limited by several shortcomings, such as modeled-stress depletion and a dependence on scalar grid measures. The "active model-split" (AMS) hybrid RANS/LES model was developed by Haering, Oliver, and Moser to address these shortcomings. In this model, the mean and fluctuating portions of the stress are modeled separately; this allows for a consistent treatment of the mean flow even in the presence of resolved fluctuations. Part of this hybrid model is an active forcing, which generates turbulent fluctuations in regions that support LES-type fluctuations. In this presentation, some recent results are shown using the active model-split for shock-separated flows. The effects of the active forcing are examined, including the effects on anisotropy and acoustics. Superior predictive accuracy over RANS is shown for some quantities of interest, such as the shock location.

RANS↗

NASA NTRS1997

Finite Element Aircraft Simulation of Turbulence

A turbulence model has been developed for realtime aircraft simulation that accommodates stochastic turbulence and distributed discrete gusts as a function of the terrain. This model is applicable to conventional aircraft, V/STOL aircraft, and disc rotor model helicopter simulations. Vehicle angular activity in response to turbulence is computed from geometrical and temporal relationships rather than by using the conventional continuum approximations that assume uniform gust immersion and low frequency responses. By using techniques similar to those recently developed for blade-element rotor models, the angular-rate filters of conventional turbulence models are not required. The model produces rotational rates as well as air mass translational velocities in response to both stochastic and deterministic disturbances, where the discrete gusts and turbulence magnitudes may be correlated with significant terrain features or ship models. Assuming isotropy, a two-dimensional vertical turbulence field is created. A novel Gaussian interpolation technique is used to distribute vertical turbulence on the wing span or lateral rotor disc, and this distribution is used to compute roll responses. Air mass velocities are applied at significant centers of pressure in the computation of the aircraft's pitch and roll responses.

McFarland, R. E.↗

NASA NTRS1999

Juneau Airport Doppler Lidar Deployment: Extraction of Accurate Turbulent Wind Statistics

A 2 micrometer pulsed Doppler lidar was deployed to the Juneau Airport in 1998 to measure turbulence and wind shear in and around the departure and arrival corridors. The primary objective of the measurement program was to demonstrate and evaluate the capability of a pulsed coherent lidar to remotely and unambiguously measure wind turbulence. Lidar measurements were coordinated with flights of an instrumented research aircraft operated by representatives of the University of North Dakota (UND) under the direction of the National Center for Atmospheric Research (NCAR). The data collected is expected to aid both turbulence characterization as well as airborne turbulence detection algorithm development activities within NASA and the FAA. This paper presents a summary of the deployment and results of analysis and simulation which address important issues regarding the measurement requirements for accurate turbulent wind statistics extraction.

Hannon, Stephen M.↗

NASA NTRS1982

Evaluation of turbulence induced noise in coherent anti-Stokes Raman scattering

The effect of turbulence in a transonic wind tunnel on coherent anti-Stokes Raman scattering is considered. The driving pump and Stokes waves are taken to be coaxially propagating Gaussian beam waves which are focused on the Raman active medium through the turbulent boundary layer of the flow tube. The random index of refraction variations in the layer are modeled as phase perturbations of the driving waves which cause a reduction of the mean on-axis field and an increase in the mean diameter of the beams. Effective Gaussian beam parameters are developed and the radiated anti-Stokes power calculated as a function of the phase screen parameters. A significant reduction in signal strength occurs for realistic estimates of the phase screen parameter appropriate to a confined transonic flow. A method for estimating the signal degradation which could be applied to other experimental situations is presented.

Elliott, R. A.↗

NASA NTRS2011

Low Frequency Turbulence as the Source of High Frequency Waves in Multi-Component Space Plasmas

Space plasmas support a wide variety of waves, and wave-particle interactions as well as wavewave interactions are of crucial importance to magnetospheric and ionospheric plasma behavior. High frequency wave turbulence generation by the low frequency (LF) turbulence is restricted by two interconnected requirements: the turbulence should be strong enough and/or the coherent wave trains should have the appropriate length. These requirements are strongly relaxed in the multi-component plasmas, due to the heavy ions large drift velocity in the field of LF wave. The excitation of lower hybrid waves (LHWs), in particular, is a widely discussed mechanism of interaction between plasma species in space and is one of the unresolved questions of magnetospheric multi-ion plasmas. It is demonstrated that large-amplitude Alfven waves, in particular those associated with LF turbulence, may generate LHW s in the auroral zone and ring current region and in some cases (particularly in the inner magnetosphere) this serves as the Alfven wave saturation mechanism. We also argue that the described scenario can playa vital role in various parts of the outer magnetosphere featuring strong LF turbulence accompanied by LHW activity. Using the data from THEMIS spacecraft, we validate the conditions for such cross-scale coupling in the near-Earth "flow-braking" magnetotail region during the passage of sharp injection/dipolarization fronts, as well as in the turbulent outflow region of the midtail reconnection site.

Khazanov, George V.↗

NASA NTRS1990

Modern developments in shear flow control with swirl

Passive and active control of swirling turbulent jets is experimentally investigated. Initial swirl distribution is shown to dominate the free jet evolution in the passive mode. Vortex breakdown, a manifestation of high intensity swirl, was achieved at below critical swirl number (S = 0.48) by reducing the vortex core diameter. The response of a swirling turbulent jet to single frequency, plane wave acoustic excitation was shown to depend strongly on the swirl number, excitation Strouhal number, amplitude of the excitation wave, and core turbulence in a low speed cold jet. A 10 percent reduction of the mean centerline velocity at x/D = 9.0 (and a corresponding increase in the shear layer momentum thickness) was achieved by large amplitude internal plane wave acoustic excitation. Helical instability waves of negative azimuthal wave numbers exhibit larger amplification rates than the plane waves in swirling free jets, according to hydrodynamic stability theory. Consequently, an active swirling shear layer control is proposed to include the generation of helical instability waves of arbitrary helicity and the promotion of modal interaction, through multifrequency forcing.

Farokhi, Saeed↗

NASA NTRS1990

Modern developments in shear flow control with swirl

Farokhi, S.↗

NASA NTRS1993

Annual Research Briefs, 1992

This report contains the 1992 annual progress reports of the Research Fellows and students of the Center for Turbulence Research. Considerable effort was focused on the large eddy simulation technique for computing turbulent flows. This increased activity has been inspired by the recent predictive successes of the dynamic subgrid scale modeling procedure which was introduced during the 1990 Summer Program. Several Research Fellows and students are presently engaged in both the development of subgrid scale models and their applications to complex flows. The first group of papers in this report contain the findings of these studies. They are followed by reports grouped in the general areas of modeling, turbulence physics, and turbulent reacting flows. The last contribution in this report outlines the progress made on the development of the CTR post-processing facility.

Spinks, Debra↗

NASA NTRS1994

Dynamics of the magnetic shearing instability and magnetohydrodynamic turbulence in accretion disks. 1: Vertical magnetic field

Recently, the magnetic shearing instability (MSI) has been proposed as a dynamical mechanism for angular momentum transport in accretion disks (Balbus & Hawley 1991; Hawley & Balbus 1991). In this paper, the nonlinear dynamics of MSI modes in the presence of a vertical magnetic field B(sub 0) is discussed. In particular, the saturation levels of the fluctuating fields, the angular momentum flux, and the energy dissipation mechanism, are examined in detail. It is shown that MSI induces strong magnetohydrodynamic (MHD) turbulence in a range of wavenumbers 1/H is less than K is less than or equal to Omega/V(sub A)(sub 0)), where H is the thickness, Omega is the rotation frequency of the disk, and V(sub A)(sub 0) is the Alfven velocity. Despite the fact that the linear growth rate of MSI is maximal at small-scale (i.e., k is approximately Omega/V(sub A(sub 0)), angular momentum transport due to MSI turbulence is dominated by the magnetic Reynolds stress driven by large-scale modes (k is approximately 1/H). It is shown that the amplitude of low k(sub r) MSI eddies is limited primarily by subscale shear flow instability. Thus, dominant MSI cells are quasi-isotropic. In a stationary state, the effective Shakura-Sunyaev 'alpha' value is predicted to be of order V(sub A)(sub 0)0/C(sub s). In addition, the veritcal magnetic-field-induced MSI cells convert vertical magnetic field B(sub 0) into azimuthal magnetic field B(sub theta) in the disk. The generation of azimuthal magnetic field in turn introduces new physical processes, such as dynamo activity and azimuthal MSI turbulence. We conclude that it is not possible to decouple vertical MSI saturation from azimuthal MSI evolution. Low-frequency MSI cells are shown to co-exist with high-frequency radial buoyancy or internal waves. We show that modulational interaction between waves on these two frequency ranges is usually weak in the case when mean magnetic field is vertical. Thus, MSI and internal wave dynamics must be treated on an equal footing.

Zhang, W.↗

NASA NTRS2019

Solar Activity Modeling: From Subgranular Dynamical Scales to the Solar Cycles

The dynamical effects of solar magnetoconvection span a wide range spatial and temporal scales that extend from the interior to the corona and from fast turbulent motions to global magnetic activity. To study the solar activity on short temporal scales (from minutes to hours), we use 3D radiative MHD simulations that allow us to investigate complex turbulent interactions that drive various phenomena, such as plasma eruptions, spontaneous formation of magnetic structures, funnel-like structures and magnetic loops in the corona, and others. In particular, we focus on multi-scale processes of energy exchange across layers of the solar interior and atmosphere, which contribute to coronal heating and eruptive dynamics. For modeling global-scale activity, we use a data assimilation approach that has demonstrated great potential for building reliable long-term forecasts of solar activity. In particular, it has been shown that the Ensemble Kalman Filter (EnKF) method applied to the Parker-Kleeorin-Ruzmakin dynamo model is capable of predicting solar activity up to one sunspot cycle ahead in time, as well as estimating the properties of the next cycle a few years before it begins. In this presentation, using the available magnetogram data, we discuss development of the methodology and forecast quality criteria (including forecast uncertainties and sources of errors). We demonstrate the influence of observational limitations on prediction accuracy, and we present the EnKF predictions of the upcoming Solar Cycle (25) based on both the sunspot number series and observed magnetic fields and discuss the uncertainties and potential of the data assimilation approach for modeling and forecasting solar activity.

Kitiashvili, I. N.↗

NASA NTRS1987

Turbulence control on an airborne laser platform

An active flow control device to generate large-scale, periodic structures in a turbulent shear flow is developed. Together with adaptive optics, the device may be used on airborne laser platforms to reduce or eliminate optical distortion caused by the turbulence in the aircraft's boundary layer. A cyclic jet issuing from a spanwise slot is used to collect the turbulent boundary layer for a finite time and then release all of the flow instantaneously in one large eddy that convects downstream. Flow visualization and hot-film probe measurements are used together with pattern recognition algorithms to demonstrate the viability of the flow control method. A flat plate towed in a water channel is used as a test bed. The instantaneous velocity signal is used to compute important statistical quantities of the random velocity field, such as the mean, the root-mean-square, the spectral distribution, and the probability density function. When optimized for a given boundary layer, it is shown that the cyclic jet will produce periodic structures that are similar to the random, naturally occurring ones. These structures seem to trigger the onset of bursting events near the wall of the plate. Thus, the present device generates periodic structures in both the outer and inner regions of a turbulent boundary layer.

Gad-El-hak, Mohamed↗

NASA NTRS1990

Hydrodynamic instability and coherent structures in turbulence

The evolution of a coherent structure in the near wall region is examined in the context of linear theory. In accordance with this theory, a three-dimensional inviscid disturbance is shown to grow into a streamwise elongated streaky structures in a manner similar to what has been observed in experiments and numerical simulations. It is found that spanwise asymmetry is necessary for the structures to evolve into long streaks. The implications of the results for turbulent drag reduction through active or passive means are discussed.

Landahl, Marten T.↗

NASA NTRS1993

Nonthermal ions and associated magnetic field behavior at a quasi-parallel earth's bow shock

Attention is given to ion and magnetic field measurements at the earth's bow shock from the AMPTE-UKS and -IRM spacecraft, which were examined in high time resolution during a 45-min interval when the field remained closely aligned with the model bow shock normal. Dense ion beams were detected almost exclusively in the midst of short-duration periods of turbulent magnetic field wave activity. Many examples of propagation at large elevation angles relative to the ecliptic plane, which is inconsistent with reflection in the standard model shock configuration, were discovered. The associated waves are elliptically polarized and are preferentially left-handed in the observer's frame of reference, but are less confined to the maximum variance plane than other previously studied foreshock waves. The association of the wave activity with the ion beams suggests that the former may be triggered by an ion-driven instability, and possible candidates are discussed.

Wilkinson, W. P.↗

NASA NTRS2001

Formation of Planetary Systems

Stars are observed to be forming within cold regions of the galaxy called molecular clouds. A clump of gas and dust within a molecular cloud can collapse into a rotationally- supported disk orbiting the pre s sure -supported star. Such a disk has the same initial elemental composition as the growing star. At sufficient distances from the central star, it is cool enough for approx. 1 - 2% of this material to be in solid form, either remnant interstellar grains or condensates formed within the disk. During the infall stage, the disk is very active and probably highly turbulent. When the infall slows substantially or stops, the disk becomes more quiescent. The dust grows from micron-sized dust to kilometer-sized planetesimals by physical collisions, possibly aided by collective gravity. The dynamics of larger solid bodies within protoplanetary disks are better characterized. The primary perturbations on the orbits of kilometer-sized and larger planetesimals in protoplanetary disks are mutual gravitational interactions and physical collisions. These interactions lead to accretion (and in some cases erosion and fragmentation) of planetesimals. The most massive planets have the largest gravitationally-enhanced collision cross-sections, and accrete almost everything with which they collide. The size distribution of solid bodies becomes quite skewed, with a few large bodies growing much faster than the rest of the swarm, until they have accumulated most of the small bodies within their gravitational reach. Slower growth continues (at least for solid, earth-like planets) as the eccentricities of planetary embryos are pumped up by long-range mutual gravitational perturbations. As planetary masses increase, they become more efficient at stirring random velocities of neighboring bodies. If sufficiently massive and dense planets exist far enough from the star, they can eject material into interstellar space. In most models, giant planets begin their growth like terrestrial planets, but if they become massive enough before the protoplanetary disk dissipates, then they are able to accumulate substantial amounts of gas. Giant planet growth times predicted by current models are similar to estimates of the lifetime of the gaseous protoplanetary disk, so we don't know whether or not giant planets form in most protoplanetary disks. However, planets that become massive while a substantial amount of gas remains in the disk may migrate into the star as a consequence of their gravitational interactions with the disk.

Lissauer, Jack J.↗

NASA NTRS2023

Structured Type III Radio Bursts Observed in Interplanetary Space

Context . The last few decades have seen numerous studies dedicated to fine structures of type III radio bursts observed in the meter–decameter wavelengths. Most of the explanations of the structured radio emission involve the propagation of electron beams through the strongly inhomogeneous plasma in the low corona. To date, only a few type III bursts with fine structures, observed at hecto-kilometric wavelengths, have been reported. Aims . We report here the existence of numerous structured type III radio bursts observed during the STEREO era by all three WAVES instruments on board STEREO A, B, and Wind. The aim of the study is to report and classify structured type III bursts, and to present the characteristics of their fine structures. The final goal is to try to understand the physical mechanism responsible for the generation of structured radio emission. Methods . In this study we used data from all available spacecraft, specifically STEREO and Wind. We employed 1D density models to obtain the speed of the source of type III radio emission, the electron beam. We also performed a spectral analysis of the fine structures in order to compare their characteristics with the metric–decametric fine structures. Results . The presented similarities of the type III fine structures in the metric to decametric and interplanetary wavelengths indicate that the physical processes responsible for the generation of structured type III radio bursts could be the same, at heights from the low corona to the interplanetary range. We show that the observed structuring and intermittent nature of the type III bursts can be explained by the variation in the level of density fluctuations, at different distances from the Sun.

Non-thermal radiation mechanisms↗

NASA NTRS1992

Modeling of turbulent shear flows

The current progress is documented in the research and development of modeling techniques for turbulent shear flows. These include a two-scale model for compressible turbulent flows and a new energy transfer model. The former represents the status of the efforts to identify compressibility effects in turbulence. The energy transfer model refines a weakly nonlinear wave model developed earlier, which models directly the turbulent large structures. The objective of these activities is to develop second-order closures for compressible turbulent flows.

Liou, William W.↗