An Experimental/Computational Investigation of the Response of a Compliant Panel to Turbulent and Transitional Shock-wave/Boundary-layer Interactions in Hypersonic Flow
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Simultaneous fine structure observation of wind and temperature profiles by the Arecibo 430-MHz radar and in situ measurements
A simultaneous campaign of balloon and radar measurements took place on March 14 to 16, 1984, above the Arecibo 430-MHz radar. This radar was operating with a vertical resolution of 150 m following two antenna beam directions: 15 deg. from the zenith, respectively, in the N-S and E-W directions. The main results concerning the comparison between the flight and simultaneous radar measurements obtained on March 15, 1984 are analyzed. The radar return power profile (S/N ratio in dB) exhibits maxima which are generally well correlated with step-like structures in the potential temperature profile. These structures are generally considered as the consequence of the mixing processes induced by the turbulence. A good correlation appears in the altitude range 12.5 to 19 km between wind shears induced by a wave structure observed in the meridional wind and the radar echo power maxima. This wave structure is characterized by a vertical wavelength of about 2.5 km, and a period in the range 30 to 40 hours. These characteristics are deduced from the twice daily rawinsonde data launched from the San Juan Airport by the National Weather Service. These results pointed out an example of the interaction between wave and turbulence in the upper troposphere and lower stratosphere. Turbulent layers are observed at locations where wind shears related to an internal inertia-gravity wave are maxima.
Thomas, D.↗
Wave patterns and clear air turbulence.
Two cases of CAT in which the turbulence was apparently associated with Kelvin-Helmholtz billows are discussed. In the first case, long waves embedded in the mean flow appeared to modify a deep shear layer and produce local regions of enhanced shear, which subsequently gave rise to billow development. In the second case, the billows formed near the lower boundary of a strong baroclinic zone. In both cases, moderate CAT was encountered by T-33 aircraft, and the clear air structures were discernible with a sensitive 10.7-cm wavelength radar.
Hardy, K. R.↗
On the transport and acceleration of solar flare particles in a coronal loop
The turbulent environment of a flaring solar coronal loop directly affects the population of particles to be accelerated or already accelerated. Under the assumption of a uniform turbulent MHD wave field within the loop, the behavior of a particle distribution as it interacts with the turbulence is discussed, including particle precipitation to the footpoints of the loop and the evolution of the energy distribution as the particles undergo second-order stochastic acceleration. Two cases are discussed in detail: (1) particles spatially diffusing within the loop and precipitating with minimal acceleration in the short time scale of an impulsive event and (2) particles diffusing in both real and momentum space in a long duration event. Collisional losses due to ambient electrons are included. The gamma-ray flare of June 3, 1982 is modeled, and good agreement is obtained between predicted and observed time profiles if the loop length is 100,000 km with an intrinsic spatial diffusion time of 100-450 s. It follows that the production of high-energy neutrons and pi mesons extends over a time scale of 1000 s as observed.
Ryan, James M.↗
Production of Loop-Top Hard X-Ray Emission
The main goal of this work has been to understand the particle acceleration mechanism (or mechanisms) in impulsive solar flares, using hard X-ray observations of high spatial and spectral resolution. Several new observations, including the observations by YOHKOH that reveal emission from both footpoints and loop tops, have suggested to us that a model employing stochastic acceleration is the most likely candidate for explaining energetic particles in a majority (if not all) of impulsive flares. In this model, most of the flare energy is initially converted into plasma waves and turbulence, which in turn can accelerate particles from thermal to relativistic energies on sub-second timescales. This research has provided important tools for the interpretation of the high spectral and spatial resolution observations obtained from the Ramaty High Energy Solar Spectroscopic Imager (RHESSI) . In the course of our research, we have focused on two different (but possibly related) stochastic acceleration models: (1) a model that employs high-frequency whistler turbulence and concentrates on the energetic electrons, and (2) a model that employs low frequency MHD waves, and is able to accelerate both the ambient electrons and ions.
Miller, James A.↗
Analysis of wave mode content in fully turbulent, moderately collisional plasma laboratory experiment and kinetic simulation
Our major goal for the project was to make magnetic field and plasma density fluctuation measurements using identical diagnostic and acquisition setups on two different laboratory-based turbulent plasma devices: SSX at Swarthmore College and a new plasma source in development at Bryn Mawr College (BMX). We used single-point correlation and k-filtering metrics using these experimental measurements. These metrics were compared to kinetic simulations of the experimental environments in order to determine wave mode content of the laboratory turbulence.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Analysis of wave mode content in fully turbulent, moderately collisional plasma laboratory experiment and kinetic simulation. Final Report
A final report of the activities of Bryn Mawr College for award DE-SC0018258. The major goal for the project is to make magnetic and Langmuir-probe-based density fluctuations measurements using identical diagnostic and acquisition setups on two different laboratory-based turbulent plasma devices: SSX at Swarthmore College and a new plasma source in development at Bryn Mawr College (BMX). We propose to generate single-point correlation and k-filtering metrics using these experimental measurements. These metrics will be compared directly to kinetic simulations of the experimental environments in order to determine wave mode content of the laboratory turbulence.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Collaborative Research: Analysis of wave mode content in fully turbulent, moderately collisional plasma laboratory experiment and kinetic simulation
Our major goal for the project is to make magnetic and Langmuir-probe-based density fluctuations measurements using identical diagnostic and acquisition setups on two different laboratory-based turbulent plasma devices: SSX at Swarthmore College and a new plasma source in development at Bryn Mawr College (BMX). We propose to generate single-point correlation and k-filtering metrics using these experimental measurements. These metrics will be compared directly to kinetic simulations of the experimental environments in order to determine wave mode content of the laboratory turbulence.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Theory and simulation of the beam cyclotron instability.
A theory of plasma beam cyclotron instability is developed on the basis of computer simulation experiments. The theory holds that at certain turbulence levels, electron cross-field diffusion which supresses the electron gyroresonances is created by turbulent wave-particle interactions in a plasma beam after a period of quasi-linear exponential development of turbulence. The stabilizing effect of Landau ion damping is noted. The behavior of cold and hot ions is discussed.
Lampe, M.↗
Field structure of collapsing wave packets in 3D strong Langmuir turbulence
A simple model is constructed for the electric fields in the collapsing wave packets found in 3D simulations of driven and damped isotropic strong Langmuir turbulence. This model, based on a spherical-harmonic decomposition of the electrostatic potential, accounts for the distribution of wave-packet shapes observed in the simulations, particularly the predominance of oblate wave packets. In contrast with predictions for undamped and undriven subsonic collapse of scalar fields, oblate vector-field wave packets do not flatten during collapse but, instead, remain approximately self-similar and rigid.
Newman, D. L.↗
An investigation of the relationships between mountain-wave conditions and clear air turbulence encountered by the XB-70 airplane in the stratosphere
XB-70 aircraft for investigating and predictory xB-70 aircraft for investigating and predicting mountain waves
Incrocci, T. P.↗
On the generation of sound by turbulent convection. I - A numerical experiment
Motivated by the problem of the origin of the solar p-modes, we study the generation of acoustic waves by turbulent convection. Our approach uses the results of high-resolution 3D simulations as the experimental basis for our investigation. The numerical experiment describes the evolution of a horizontally periodic layer of vigorously convecting fluid. The sound is measured by a procedure, based on a suitable linearization of the equations of compressible convection that allows the amplitude of the acoustic field to be determined. Through this procedure we identify unambiguously some 400 acoustic modes. The total energy of the acoustic field is found to be a fraction of a percent of the kinetic energy of the convection. The amplitudes of the observed modes depend weakly on (horizontal) wavenumber but strongly on frequency. The line widths of the observed modes typically exceed the natural linewidths of the modes as inferred from linear theory. This broadening appears to be related to the (stochastic) interaction between the modes and the underlying turbulence which causes abrupt, episodic events during which the phase coherence of the modes is lost.
Bogdan, Thomas J.↗
On turbulent spots in plane Poiseuille flow
Turbulence characteristics inside a turbulent spot in plane Poiseuille flow are investigated by analyzing a database obtained from a direct numerical simulation. The spot is found to consist of two distinct regions - a turbulent area and a wave area. The flow inside the turbulent area has a strong resemblance to that found in the fully developed turbulent channel. Suitably defined mean and r.m.s. fluctuations as well as the internal shear-layer structures are found to be similar to the turbulent counterpart. In the wave area the inflexional mean spanwise profiles cause a rapid growth of oblique waves, which break down to turbulence. The breakdown process of the oblique waves is reminiscent of the secondary instability observed during transition to turbulence in channel and boundary-layer flows. Other detailed characteristics associated with the Poiseuille spot are presented and are compared with experimental results.
Henningson, Dan S.↗
On turbulent spots in plane Poiseuille flow
Turbulence characteristics inside a turbulent spot in plume Poiseuille flow are investigated by analyzing a data base obtained from a direct numerical simulation. The spot is found to consist of two distinct regions - a turbulent area and a wave area. The flow inside the turbulent area has a strong resemblance to that found in the fully developed turbulent channel. Suitably defined mean and rms fluctuations as well as the internal shear layer structures are found to be similar to the turbulent counterpart. In the wave area, the inflexional mean spanwise profiles cause a rapid growth of oblique waves, which break down to turbulence. The breakdown process of the oblique waves is reminiscent of the secondary instability observed during transition to turbulence in channel and boundary layer flows. Other detailed characteristics associated with the Poiseuille spot are presented and are compared with experimental results.
Henningson, Dan S.↗
Numerical simulation of nonlinear development of instability waves
The nonlinear interactions of high amplitude instability waves in turbulent jets are described. In plane shear layers Riley and Metcalf (1980) and Monkewitz (1987) have shown that these interactions are dependent, among other parameters, on the phase-difference between the two instability waves. Therefore, here researchers consider the nonlinear development of both the amplitudes and the phase of the instability waves. The development of these waves are also coupled with the development of the mean flow and the background turbulence. In formulating this model it is assumed that each of the flow components can be characterized by conservation equations supplemented by closure models. Results for the interactions between the two instability waves under high-amplitude forcing at fundamental and subharmonic frequencies are presented here. Qualitative agreements are found between the present predictions and available experimental data.
Mankbadi, Reda R.↗
Small scale turbulence in the Crab Nebula: Evidence of lower hybrid parametric instabilities driven by the pulsar wave
Strong small scale turbulence is observed in the Crab Nebula from the temporal pulse broadening data. It is shown that the strong 30 Hz pulsar wave can parametrically excite instabilities near the lower hybrid frequency in the thermal plasma of the Crab Nebula with a characteristic wavelength of the order of the scale size a of the turbulence observed. These instabilities provided a coupling mechanism between the pulsar wave and the Nebula plasma.
Lee, L. C.↗
Solar Wind Five
Topics of discussion were: solar corona, MHD waves and turbulence, acceleration of the solar wind, stellar coronae and winds, long term variations, energetic particles, plasma distribution functions and waves, spatial dependences, and minor ions.
Neugebauer, M.↗
Waves associated with quasi-parallel shocks: Generation, mode conversion and implications
Ions that are energized at quasi-parallel collisionless shocks and move back upstream generate low-frequency waves, largely on the fast/magnetosonic branch. At sufficient Mach number, the waves are convected back into the shock, lead to shock re-formation, and are mode-converted into downstream (magnetosheath) Alfvenic turbulence. Other waves are generated more locally at the interface of the incoming solar wind and the partially thermalized plasma. This paper reviews how recent simulation studies of collisionless shocks in conjunction with linear kinetic theory and proper wave diagnostics have aided in our understanding of the upstream and magnetosheath waves.
Krauss-Varban, D.↗