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At least 469 records · Page 26
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Representation of atmosphere-induced heterogeneity in land–atmosphere interactions in E3SM–MMFv2

In the Energy Exascale Earth System Model (E3SM) Multi-scale Modeling Framework (MMF), where parameterizations of convection and turbulence are replaced by a 2-D cloud-resolving model (CRM), there are multiple options to represent land–atmosphere interactions. Here, we propose three different coupling strategies, namely the (1) coupling of a single land surface model to the global grid (MMF), (2) coupling a single land copy directly to the embedded CRM (SFLX2CRM), and (3) coupling a single copy of land model to each column of the CRM grid (MAML). In the MAML (Multi-Atmosphere Multi-Land) framework, a land model is coupled to CRM at the CRM-grid scale by coupling an individual copy of a land model to each CRM grid. Therefore, we can represent intra-CRM heterogeneity in the land–atmosphere interaction processes. There are 5-year global simulations run using these three coupling strategies, and we find some regional differences but overall small changes with respect to whether a land model is coupled to CRM or a global atmosphere. In MAML, the spatial heterogeneity within CRM induces stronger turbulence, which leads to the changes in soil moisture, surface heat fluxes, and precipitation. However, the differences in the MAML from the other two cases are rather weak, suggesting that the impact of using MAML does not justify the increase in cost.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
NASA NTRS1978

An interaction solution algorithm for viscous aerodynamic performance prediction

A weak-interaction solution algorithm is established for aerodynamic flow field prediction about an isolated airfoil. It requires numerical solution of differential equations governing potential flow, viscous and turbulent boundary layer flow, and the turbulent wake flow downstream of a trailing edge. The algorithm accounts for computed viscous displacement effects on the potential flow. These in turn alter the viscous flow through imposed pressure gradients. Closure for turbulence is accomplished using a second order model. Numerical evaluations assess factors affecting solution accuracy, convergence and stability for the combined potential, boundary layer, and parabolic Navier-Stokes equation systems as solved using a finite element algorithm.

Baker, A. J.↗
NASA NTRS1993

Nonlinear evolution of electromagnetic ion beam instabilities

A comparative study of the ion/ion right-hand resonant instability and the ion/ion nonresonant instability is carried out to investigate and contrast their properties. Linear analysis demonstrates that the nonresonant instability becomes resonant if the density of the ion beam is sufficiently high. Hybrid simulations show that both the resonant and nonresonant instabilities result in the formation of nonlinear pulses, called pulsations, but with distinct features. For example, the pulsations generated by the resonant instability have a positive correlation with the ion density, while those generated by the nonresonant instability are likely to have a relatively weak negative correlation. The waves generated by the nonresonant instability are subject to a parametric decay instability and tend to form a state of condensate where the turbulence becomes nearly monochromatic.

Akimoto, K.↗
NASA NTRS1954

Convection of a pattern of vorticity through a shock wave

An arbitrary weak spatial distribution of vorticity can be represented in terms of plane sinusoidal shear waves of all orientations and wave lengths (Fourier integral). The analysis treats the passage of a single representative weak shear wave through a plane shock and shows refraction and modification of the shear wave with simultaneous generation of an acoustically intense sound wave. Applications to turbulence and to noise in supersonic wind tunnels are indicated.

Ribner, H S↗
NASA NTRS1994

An Experimental Simulation of Flap Flow on Multielement Airfoils at High Reynolds Number

The design of the high lift system has a major impact on the performance of an aircraft yet our understanding of the physics of this flow is still weak. Flow features include interactions between the wakes shed from the upstream elements and the pressure gradients and boundary layers of the downstream elements. Interaction of the turbulent wake of the main element and the flap can cause (1) separation of the flap boundary layer or (2) 'bursting' of the main airfoil wake. Although the first factor is at least partially understood, even the qualitative aspects of (2) remain to be determined. In order to study these phenomena at Reynolds numbers approaching those of flight, a thick high Reynolds number wake is created using a 24 foot flat plate in the long rectangular test section of a 4 ft. by 6 ft subsonic wind tunnel. The design and construction of this test section, plate, and accompanying flap is described. Results obtained in a quarter-scale model were used for design purposes and are also described. Construction of the full scale facility is complete and preliminary results are presented.

Schneider, S.↗
DOE OSTI2022

Anomalous spin polarization from turbulent color fields

Here we study the important, yet widely overlooked, role of gluons for spin transport with a connection to local parity violation in quark gluon plasmas. We employ the formalism of quantum kinetic theory to quarks in weakly coupled quantum chromodynamics to derive the source terms for quark spin polarization. These source terms involve parity-odd correlators of dynamically generated color fields in near-equilibrium quark gluon plasmas and give rise to locally fluctuating axial charge currents. Our results provide a possible explanation for the spin alignment of vector mesons measured in high-energy nuclear collisions.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2022

Pressure–Strain Interaction as the Energy Dissipation Estimate in Collisionless Plasma

The dissipative mechanism in weakly collisional plasma is a topic that pervades decades of studies without a consensus solution. We compare several energy dissipation estimates based on energy transfer processes in plasma turbulence and provide justification for the pressure–strain interaction as a direct estimate of the energy dissipation rate. The global and scale-by-scale energy balances are examined in 2.5D and 3D kinetic simulations. We show that the global internal energy increase and the temperature enhancement of each species are directly tracked by the pressure–strain interaction. The incompressive part of the pressure–strain interaction dominates over its compressive part in all simulations considered. The scale-by-scale energy balance is quantified by scale filtered Vlasov–Maxwell equations, a kinetic plasma approach, and the lag dependent von Kármán–Howarth equation, an approach based on fluid models. We find that the energy balance is exactly satisfied across all scales, but the lack of a well-defined inertial range influences the distribution of the energy budget among different terms in the inertial range. Therefore, the widespread use of the Yaglom relation in estimating the dissipation rate is questionable in some cases, especially when the scale separation in the system is not clearly defined. In contrast, the pressure–strain interaction balances exactly the dissipation rate at kinetic scales regardless of the scale separation.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1990

Upstream waves at Mars - Phobos observations

The region upstream from the Mars subsolar bow shock is surveyed for the presence of MHD wave phenomena using the high temporal resolution data from the Magma magnetometer. Strong turbulence is observed when the magnetic field is connected to the Mars bow shock in such a way as to allow diffuse ions to reach the spacecraft. Also weak waves are observed at the proton gyro frequency. These waves are left-hand elliptically polarized and may be associated with the pick-up of protons from the Mars hydrogen exosphere.

Russell, C. T.↗
NASA NTRS1992

Mass loss in main-sequence A-type stars?

We present new observations of the H-alpha line profile for five main-sequence A-type stars, where very high SNRs were achieved. A search for weak asymmetries was carried out to detect a stellar wind, but the many telluric absorption lines in this range prevent us from taking the full benefit of the high SNRs. This situation was improved by modeling the telluric absorptions to remove them from the observed spectra, but the H-alpha profiles were nevertheless found to be quite symmetric. We calculated the profile of the H-alpha line for a grid of model atmospheres of an A-type star including a weak wind, in order to assess an upper limit on the mass loss rate. The asymmetry is found to be sensitive to the velocity law and to the turbulent velocity of the wind, but its first moment depends much less on them. The upper limits deduced on the mass loss rate are between 1 and 2 x 10 exp -10 solar mass/yr. An improvement up to a factor 10 could be anticipated if new observations could be secured from a much drier location, achieving similar SNRs.

Lanz, T.↗
NASA NTRS1994

Characteristics of civil aviation atmospheric hazards

Clear air turbulence, wake vortices, dry hail, and volcanic ash are hazards to civil aviation that have not been brought to the forefront of public attention by a catastrophic accident. However, these four hazards are responsible for major and minor injuries, emotional trauma, significant aircraft damage, and in route and terminal area inefficiency. Most injuries occur during clear air turbulence. There is significant aircraft damage for any volcanic ash encounter. Rolls induced by wake vortices occur near the ground. Dry hail often appears as an area of weak echo on the weather radar. This paper will present the meteorological, electromagnetic, and spatiotemporal characteristics of each hazard. A description of a typical aircraft encounter with each hazard will be given. Analyzed microwave and millimeter wave sensor systems to detect each hazard will be presented.

Marshall, Robert E.↗
DOE OSTI2024

Small-amplitude Compressible Magnetohydrodynamic Turbulence Modulated by Collisionless Damping in Earth’s Magnetosheath: Observation Matches Theory

Plasma turbulence is a ubiquitous dynamical process that transfers energy across many spatial and temporal scales and affects energetic particle transport. Recent advances in the understanding of compressible magnetohydrodynamic (MHD) turbulence demonstrate the important role of damping in shaping energy distributions on small scales, yet its observational evidence is still lacking. This study provides the first observational evidence of substantial collisionless damping (CD) modulation on the small-amplitude compressible MHD turbulence cascade in Earth’s magnetosheath using four Cluster spacecraft. Based on an improved compressible MHD decomposition algorithm, turbulence is decomposed into three eigenmodes: incompressible Alfvén modes and compressible slow and fast (magnetosonic) modes. Our observations demonstrate that CD enhances the anisotropy of compressible MHD modes because CD has a strong dependence on wave propagation angle. The wavenumber distributions of slow modes are mainly stretched perpendicular to the background magnetic field (B 0 ) and weakly modulated by CD. In contrast, fast modes are subjected to a more significant CD modulation. Fast modes exhibit a weak, scale-independent anisotropy above the CD truncation scale. Below the CD truncation scale, the anisotropy of fast modes enhances as wavenumbers increase. As a result, fast-mode fractions in the total energy of compressible modes decrease with the increase of perpendicular wavenumber (to B 0 ) or wave propagation angle. Our findings reveal how the turbulence cascade is shaped by CD and its consequences for anisotropies in the space environment.

97 MATHEMATICS AND COMPUTING↗
NASA NTRS1992

Turbulence modeling and experiments

The best way of verifying turbulence is to do a direct comparison between the various terms and their models. The success of this approach depends upon the availability of the data for the exact correlations (both experimental and DNS). The other approach involves numerically solving the differential equations and then comparing the results with the data. The results of such a computation will depend upon the accuracy of all the modeled terms and constants. Because of this it is sometimes difficult to find the cause of a poor performance by a model. However, such a calculation is still meaningful in other ways as it shows how a complete Reynolds stress model performs. Thirteen homogeneous flows are numerically computed using the second order closure models. We concentrate only on those models which use a linear (or quasi-linear) model for the rapid term. This, therefore, includes the Launder, Reece and Rodi (LRR) model; the isotropization of production (IP) model; and the Speziale, Sarkar, and Gatski (SSG) model. Which of the three models performs better is examined along with what are their weaknesses, if any. The other work reported deal with the experimental balances of the second moment equations for a buoyant plume. Despite the tremendous amount of activity toward the second order closure modeling of turbulence, very little experimental information is available about the budgets of the second moment equations. Part of the problem stems from our inability to measure the pressure correlations. However, if everything else appearing in these equations is known from the experiment, pressure correlations can be obtained as the closing terms. This is the closest we can come to in obtaining these terms from experiment, and despite the measurement errors which might be present in such balances, the resulting information will be extremely useful for the turbulence modelers. The purpose of this part of the work was to provide such balances of the Reynolds stress and heat flux equations for the buoyant plume.

Shabbir, Aamir↗
NASA NTRS1997

Flow Quality and Boundary Layer Transition

The widely held view is that transition to turbulence in the Blasius boundary layer occurs via amplification and eventual nonlinear breakdown of initially small amplitude instabilities i.e. Tollmien-Schlichting (TS) waves. However this scenario is only observed for low amplitude free-stream turbulence levels, i.e. u/U < 0.1%. Bypass of linear TS instability mechanism occurs for higher EST levels, yet considerable differences exist between the few experiments carefully designed to assess the effect of EST on transition. The consensus is that EST leads to longitudinal streaks that form near the leading edge in the boundary layer . These streaks appeal to be regions of concentrated streamwise vorticity and they are often referred to as Klebanoff modes. The importance of mean flow free-stream nonuniformity (FSN) is not as widely appreciated as EST for characterizing wind tunnel flow quality. Here it is shown that, although the v like generated by a d=50micron wire located upstream of the contraction (Re(sub d)=6.6, x/d=45,000) is immeasurably small by the time it interacts with the leading edge in the test section, it is responsible for generation of a pair of weak streamwise vortices in the boundary layer downstream. The characteristics of these wake-induced vortices and their effect on TS waves are demonstrated. Small remnant FSN variations are also shown to exist downstream of a turbulence grid. The question arises Are the adverse effects introduced by the turbulence grid caused by FST or by small remnant FSN variations?

Watmuff, Jonathan H↗
NASA NTRS1994

Heat-transfer measurements and computations of swept-shock-wave boundary-layer interactions

An experimental and computational research program providing new knowledge of the heat transfer in swept-shock-wave/boundary-layer interactions is described. An equilibrium turbulent boundary layer on a flat plate is subjected to impingement by a swept planar shock wave generated by a sharp fin. Five different interactions with fin angles ranging from 10 to 20 deg at freestream Mach numbers of 3 and 4 produce a variety of interaction strengths ranging from weak to very strong. A foil heater generates a uniform heat flux over the flat plate surface, and miniature thin-film-resistance sensors are used to measure the local surface temperature. The heat convection equation is then solved for the heat transfer distribution within an interaction, yielding an uncertainty of about +/- 10%. These data are compared with numerical Navier-Stokes solutions that employ a k-epsilon turbulence model. A simple peak heat transfer correlation for fin interactions is suggested.

Lee, Y.↗
NASA NTRS1972

Flow field measurements for space-shuttle-related cylindrical configurations in hypersonic streams.

Use of surface pressures, heat-transfer rates, and flow-visualization photographs to define the flowfields for simulated infinite cylinders. Four cylindrical cross sections were studied in the experimental program, which was conducted at free-stream Mach numbers of 5 to 15, free-stream unit Reynolds numbers from 2 to 20 million per foot, and angles of attack from 30 to 90 deg. Although swept-cylinder theory provides a reasonable correlation of the measured laminar heat-transfer rates in the plane of symmetry, improved correlation with the measurements is obtained if the three-dimensional nature of the flow is taken into account. In cases where the boundary layer on the windward forebody was turbulent, the large velocity gradients near the chines produced relaminarization of the boundary layer prior to separation. The average heat-transfer-rate in the separated region, which is weakly dependent on the free-stream conditions, was found to be a linear function of the angle of attack. For a given flow condition the effect of the leeward geometry on heat transfer can be correlated by a single geometric parameter which is dependent on the location of separation.

Bertin, J. J.↗
NASA NTRS1989

Inception length to a fully-developed fin-generated shock wave boundary-layer interaction

An experimental study of fin-generated shock wave turbulent boundary-layer interactions confirmed previous observations that, sufficiently far from the fin apex, such interactions become conical. The inception length to conical symmetry was found to increase weakly with Mach number for Mach numbers from 2.5 to 4 and fin angles from 4 to 22 deg. For the range of interactions examined, the inception length was found to depend primarily upon the inviscid shock angle, this angle ranging from 21 to 40 deg. The behavior of the inception length with shock angle can be broadly divided into two categories. For 'weak' interactions with shock angles less than about 35 deg, the inception length decreased as the shock angle increased. For 'strong' interactions with shock angles greater than about 35 deg, the inception region was small and was approximately constant at three boundary-layer thicknesses in length. In the latter, strong interaction case, the inception length was an order of magnitude smaller than that found in the weakest interactions examined, to the extent that strong interactions were practically fully-developed from the apex.

Lu, Frank K.↗
NASA NTRS1992

The spectrum of random magnetic fields in the mean field dynamo theory of the Galactic magnetic field

The fluctuation spectrum that must arise in a mean field dynamo generation of galactic fields if the initial field is weak is considered. A kinetic equation for its evolution is derived and solved. The spectrum evolves by transfer of energy from one magnetic mode to another by interaction with turbulent velocity modes. This kinetic equation is valid in the limit that the rate of evolution of the magnetic modes is slower than the reciprocal decorrelation time of the turbulent modes. This turns out to be the case by a factor greater than 3. Most of the fluctuation energy concentrates on small scales, shorter than the hydrodynamic turbulent scales. The fluctuation energy builds up to equipartition with the turbulent energy in times that are short compared to the e-folding time of the mean field. The turbulence becomes strongly modified before the dynamo amplification starts. Thus, the kinematic assumption of the mean dynamo theory is invalid. Thus, the galactic field must have a primordial origin, although it may subsequently be modified by dynamo action.

Kulsrud, Russell M.↗
NASA NTRS1998

Fast Reconnection of Weak Magnetic Fields

Fast magnetic reconnection refers to annihilation or topological rearrangement of magnetic fields on a timescale that is independent (or nearly independent) of the plasma resistivity. The resistivity of astrophysical plasmas is so low that reconnection is of little practical interest unless it is fast. Yet, the theory of fast magnetic reconnection is on uncertain ground, as models must avoid the tendency of magnetic fields to pile up at the reconnection layer, slowing down the flow. In this paper it is shown that these problems can be avoided to some extent if the flow is three dimensional. On the other hand, it is shown that in the limited but important case of incompressible stagnation point flows, every flow will amplify most magnetic fields. Although examples of fast magnetic reconnection abound, a weak, disordered magnetic field embedded in stagnation point flow will in general be amplified, and should eventually modify the flow. These results support recent arguments against the operation of turbulent resistivity in highly conducting fluids.

Zweibel, Ellen G.↗