Ø. Andreassen - Academia.edu (original) (raw)

Papers by Ø. Andreassen

Research paper thumbnail of Pseudospectral methods with open boundary conditions for the study of atmospheric wave phenomena

Computer Methods in Applied Mechanics and Engineering, 1990

Abstract We consider Chebyshev pseudospectral methods for the study of atmospheric wave phenomena... more Abstract We consider Chebyshev pseudospectral methods for the study of atmospheric wave phenomena. The governing equations are the two-dimensional Euler equations for gas dynamics with gravity included, where accurate numerical approximation of the nonlinear behaviour is important. The methods are efficiently implemented on a Cray X-MP, and run at nearly optimal speed on one processor. In this case, space derivatives are calculated more efficiently with matrix multiplication than by the Fast Fourier Transform. The boundaries are artificial and we simulate open boundaries by using the characteristic variables of the equations. Different choices of such boundary conditions and their effects on the solutions are discussed. Numerical calculation of an example where the solution may be analytically verified is presented, and the pseudospectral method is seen to be well suited for these computations.

Research paper thumbnail of Simulated correlation tracking on solar granulation

Lecture Notes in Physics, 2000

ABSTRACT

Research paper thumbnail of Breaking gravity waves

The dynamics of the nonlinear wave-wind critical level interaction as explored in 3-D direct nume... more The dynamics of the nonlinear wave-wind critical level interaction as explored in 3-D direct numerical solution are briefly described. The results demonstrate that streamwise oriented vortices dominate the instabilities.

Research paper thumbnail of Some Unusual Types of Noise

71st EAGE Conference and Exhibition incorporating SPE EUROPEC 2009, 2009

Research paper thumbnail of The initial value problem for Kelvin vortex waves

Journal of Fluid Mechanics, 1997

We present a formal solution to the initial value problem for small perturbations of a straight v... more We present a formal solution to the initial value problem for small perturbations of a straight vortex tube with constant vorticity, and show that any initial perturbation to such a tube evolves exclusively as a collection of Kelvin vortex waves. We then study in detail the evolution of the following particular initial states of the vortex tube: (i) an axisymmetric pinch in the radius of the tube, (ii) a deflection in the location of the tube, and (iii) a flattening of the tube's cross-secton. All of these initial states are localized in the direction along the tube by weighting them with a Gaussian function. In each case, the initial perturbation is decomposed into packets of Kelvin vortex waves which then propagate outward along the vortex tube. We discuss the physical mechanisms responsible for the propagation of the wave packets, and study the consequences of wave dispersion for the solution.

Research paper thumbnail of Entrainment-zone restratification and flow structures in stratified shear turbulence

Proceedings of the …, 2002

1. Introduction Stable density stratification poses several significant challenges for turbulence... more 1. Introduction Stable density stratification poses several significant challenges for turbulence model-ing. Unlike the case of unstable stratification (for which numerous potential turbulence nucleation sites may trigger volume-filling motion), turbulence in stable stratification must ...

Research paper thumbnail of In situ measurements of the fine-scale structure and turbulence in the mesosphere and lower thermosphere by means of electrostatic positive ion probes

Journal of Geophysical Research, 1990

... 5538 BLIX ET AL.' TURBULENCE IN THE MIDDLE ATMOSPHERE ... One block then corresponds to ... more ... 5538 BLIX ET AL.' TURBULENCE IN THE MIDDLE ATMOSPHERE ... One block then corresponds to a height interval of about 1 km in the mesosphere. ... This technique first removed the spin frequency and its first harmonic by discrete filtering, and subsequently used a cubic spline ...

Research paper thumbnail of Classification of Critical Points of Turbulent Velocity Field in a Converging-Diverging Channel Flow

6th AIAA Theoretical Fluid Mechanics Conference, 2011

ABSTRACT Based on Direct Numerical Simulations of a channel with a bump,ootnotetextM. Marquillie ... more ABSTRACT Based on Direct Numerical Simulations of a channel with a bump,ootnotetextM. Marquillie et al. (2008), J. Turbulence, vol 9, no 1, pp. 1-23. a critical point generation and classification is performed. Topological features of the flow are found relating critical point type with the distribution of Jacobian Determinants as well as the behavior of skin friction. Eight different types of points are found within the flow and these are visualized using different forms of the Line Integral Convolution visualization technique as described by Aasen and Furuheim (2008).ootnotetextM. Aasen and K. Furuheim (2008), M.Sc. thesis, Department of Informatics, University of Oslo, Oslo, Norway. It is found that the determinant analysis can be a useful method for data reduction of critical points. Local minima and maxima of the skin friction coefficient, Cf, are reliable indicators of development of extreme determinants. Further explanation of the physics is done through the analysis of the velocity and pressure iso-surfaces.

Research paper thumbnail of The vorticity dynamics of instability and turbulence in a breaking internal gravity wave

We perform a three-dimensional simulation of a breaking internal gravity wave in a stratified, co... more We perform a three-dimensional simulation of a breaking internal gravity wave in a stratified, compressible, and sheared fluid to investigate the vorticity dynamics accompanying the transition from laminar to turbulent flow. Baroclinic sources contribute preferentially to eddy vorticity generation during the initial convective instability of the wave field, yielding counter-rotating vortices aligned with the external shear flow. These vortices enhance the spanwise vorticity of the shear flow via stretching and distort the spanwise vorticity via advective tilting. The resulting vortex sheets undergo a dynamical (Kelvin-Helmholtz) instability which rolls the vortex sheets into tubes which link, in turn, with the original streamwise convective rolls to produce a collection of intertwined vortex loops. Following the formation of discrete vortex loops, the most important interactions are the self-interactions of single vortex tubes and the mutual interactions of adjacent vortex tubes in close proximity. The initial formation of vortex tubes from the roll-up of localized vortex sheets imposes axial vorticity variations having both axisymmetric and azimuthal wavenumber two components. Axisymmetric variations excite axisymmetric twist waves, or Kelvin vortex waves, which propagate along the tubes, drive axial flows, and deplete and fragment the tubes. Azimuthal wavenumber two variations excite m = 2 twist waves on the vortex tubes which amplify and unravel single vortex tubes into pairs of intertwined helical tubes. Other interactions, judged less fundamental to the turbulence cascade, include reconnection among tube fragments, mutual stretching of orthogonal tubes in close proximity, excitation of azimuthal wavenumber one twist waves, and the continual roll-up of weaker vortex sheets throughout the evolution. Collectively, these vortex interactions result in a rapid cascade of energy and enstrophy toward smaller scales of motion.

Research paper thumbnail of 7313 On the generation and maintenance of waves and turbulence in simulations of free-surface turbulence

Research paper thumbnail of Studies of Aerodynamically Induced Vibrations on the P-3C Maritime Surveillance Aircraft and Proposed Vibration Reducing Measures

Research paper thumbnail of Integrated Multi-aspect Visualization of 3D Fluid Flows

Research paper thumbnail of Wave Breaking and Transition to Turbulence in Stratified Shear Flows

Journal of the Atmospheric Sciences, 1996

ABSTRACT

Research paper thumbnail of Wave breaking signatures in OH airglow and sodium densities and temperatures: 1. Airglow imaging, Na lidar, and MF radar observations

Journal of Geophysical Research, 1997

The Collaborative Observations Regarding the Nightglow (CORN) campaign took place at the Urbana A... more The Collaborative Observations Regarding the Nightglow (CORN) campaign took place at the Urbana Atmospheric Observatory during September 1992. The instrumentation included, among others, the Aerospace Corporation narrowband nightglow CCD camera, which observes the OH Meinel (6-2) band (hereafter designated OH) and the O2 atmospheric (0-1) band (hereafter designated 02) nightglow emissions; .... ,•,,.•.Ly/L,•..•Lu• lidar; and the University of Illinois MF radar. Here we report on observations of small-scale (below 10-km horizontal wavelength) structures in the OH airglow images obtained with the CCD camera. These small-scale structures were aligned perpendicular to the motion of 30-to 50-km horizontal wavelength waves, which had observed periods of about 10-20 min. The small-scale structures were present for about 20 min and appear to be associated with an overturned or breaking atmospheric gravity wave as observed by the lidar. The breaking wave had a horizontal wavelength of between 500 and 1500 km, a vertical wavelength of about 6 km, and an observed period of between 4 and 6 hours. The motion of this larger-scale wave was in the same direction as the •30-to 50-km waves. While such small-scale structures have been observed before, and have been previously described as ripple-type wave structures [Taylor and Hapgood, 1990], these observations are the first which can associate their occurrence with independent evidence of wave breaking. The characteristics of the observed smallscale structures are similar to the vortices generated during wave breakdown in three dimensions in simulations described in Part 2 of this study [Fritts et al., this issue]. The results of this study support the idea that ripple type wave structures we observe are these vortices generated by convective instabilities rather than structures generated by dynamical instabilities. 1. Illinois at Urbana-Champaign. Copyright 1997 by the American Geophysical Union. Paper number 96JD02619. 0148-0227/97/96JD-02619509.00 b, c]. Much of the work described in the literature up to now has focused on descriptions of the wave characteristics and speculation about the sources of these ripple structures. Until recently, the best explanation for these ripple structures was that they are dynamical instabilities due to short-lived velocity shears generated in situ by the chance combination of wind and wave motions [Taylor and Hapgood, 1990; Taylor et al., 1995a].

Research paper thumbnail of Gravity wave breaking in two and three dimensions: 1. Model description and comparison of two-dimensional evolutions

Journal of Geophysical Research, 1994

A nonlinear, compressible, spectral collocation code is employed to examine gravity wave breaking... more A nonlinear, compressible, spectral collocation code is employed to examine gravity wave breaking in two and three spatial dimensions. Two-dimensional results exhibit a structure consistent with previous efforts and suggest wave instability occurs via convective rolls aligned normal to the gravity wave motion (uniform in the spanwise direction). Three-dimensional results demonstrate, in contrast, that the preferred mode of instability is

Research paper thumbnail of Vorticity dynamics in a breaking internal gravity wave. Part 2. Vortex interactions and transition to turbulence

Journal of Fluid Mechanics, 1998

A companion paper (Part 1) employed a three-dimensional numerical simulation to examine the vorti... more A companion paper (Part 1) employed a three-dimensional numerical simulation to examine the vorticity dynamics of the initial instabilities of a breaking internal gravity wave in a stratified, sheared, compressible fluid. The present paper describes the vorticity dynamics that drive this flow to smaller-scale, increasingly isotropic motions at later times. Following the initial formation of discrete and intertwined vortex loops, the most important interactions are the self-interactions of single vortex tubes and the mutual interactions of multiple vortex tubes in close proximity. The initial formation of vortex tubes from the roll-up of localized vortex sheets gives the vortex tubes axial variations with both axisymmetric and azimuthal-wavenumber-2 components. The axisymmetric variations excite axisymmetric twist waves or Kelvin vortex waves which propagate along the tubes, drive axial flows, deplete the tubes' cores, and fragment the tubes. The azimuthal-wavenumber-2 variations excite m = 2 twist waves on the vortex tubes, which undergo strong amplification and unravel single vortex tubes into pairs of intertwined helical tubes; the vortex tubes then burst or fragment. Reconnection often occurs among the remnants of such vortex fragmentation. A common mutual interaction is that of orthogonal vortex tubes, which causes mutual stretching and leads to long-lived structures. Such an interaction also sometimes creates an m = 1 twist wave having an approximately steady helical form as well as a preferred sense of helicity. Interactions among parallel vortex tubes are less common, but include vortex pairing. Finally, the intensification and roll-up of weaker vortex sheets into new tubes occurs throughout the evolution. All of these vortex interactions result in a rapid cascade of energy and enstrophy toward smaller scales of motion.

Research paper thumbnail of Simultaneous observations of E-region irregularities by ground-based and rocket-borne techniques

Journal of Atmospheric and Terrestrial Physics, 1985

E-region irregularities were observed under various geomagnetic conditions by the SAFARI and STAR... more E-region irregularities were observed under various geomagnetic conditions by the SAFARI and STARE auroral radars and Langmuir probes during the Energy Budget Campaign in November-December 1980. The 10-m and 1-m-wavelength phase irregularities and their phase velocities were measured; associated rocket-borne measurements of electron densities over Kiruna showed vertical/temporal structure, indicating the existence of irregularities in ionization between 90 and 110 km. Furthermore, rocket-borne measurements under moderately disturbed conditions showed the presence of VLF electric fields in the frequency bands of 168-5519 Hz at altitudes between 103 and 145 km at Kiruna. These results, corroborated by some measurements at Andoya, are interpreted in terms of the decay of ion acoustic waves generated by plasma instabilities in the auroral electrojet.

Research paper thumbnail of Synopsis of the D- and E-regions during the Energy Budget Campaign

Journal of Atmospheric and Terrestrial Physics, 1985

... Norway LG SMITH Aeronomy Laboratory, Department of Electrical Engineering, University of Illi... more ... Norway LG SMITH Aeronomy Laboratory, Department of Electrical Engineering, University of Illinois. ... DIX Dixon ROV Rovaniemi GOD Godhavn SOD Sodankyla GOT Godth b STR Sendre Stremfjord HEI Heiss Island THO Thorshavn IVA Ivalo TJ0 Tjornes KEV Kevo TRO Tromse ...

Research paper thumbnail of Three-dimensional evolution of Kelvin-Helmholtz billows in stratified compressible flow

Geophysical Research Letters, 1994

We present results of an initial study of Kelvin-Helmholtz instability in a stratified shear flow... more We present results of an initial study of Kelvin-Helmholtz instability in a stratified shear flow using a three-dimensional, nonlinear, compressible, spectral collocation model. The simulation was performed at an intermediate Reynolds number and significantly extends previous studies of homogeneous and stratified shear flows. Our major findings include a secondary instability oriented along the two dimensional velocity field, with counter-rotating vortices

Research paper thumbnail of Kelvin twist waves in the transition to turbulence

European Journal of Mechanics - B/Fluids, 1998

... Vorticity dynamics in a breaking gravity wave 2: Vortex interactions and transition to turbul... more ... Vorticity dynamics in a breaking gravity wave 2: Vortex interactions and transition to turbulence, J. Fluid Mech., accepted. GERz T., HOWELL J., MAHRT L., 1994, Vortex structures and microfronts, Phys. ... Lord KELVIN, 1880, Vibrations of a columnar vortex, Phil. Mag., 10, 155. ...

Research paper thumbnail of Pseudospectral methods with open boundary conditions for the study of atmospheric wave phenomena

Computer Methods in Applied Mechanics and Engineering, 1990

Abstract We consider Chebyshev pseudospectral methods for the study of atmospheric wave phenomena... more Abstract We consider Chebyshev pseudospectral methods for the study of atmospheric wave phenomena. The governing equations are the two-dimensional Euler equations for gas dynamics with gravity included, where accurate numerical approximation of the nonlinear behaviour is important. The methods are efficiently implemented on a Cray X-MP, and run at nearly optimal speed on one processor. In this case, space derivatives are calculated more efficiently with matrix multiplication than by the Fast Fourier Transform. The boundaries are artificial and we simulate open boundaries by using the characteristic variables of the equations. Different choices of such boundary conditions and their effects on the solutions are discussed. Numerical calculation of an example where the solution may be analytically verified is presented, and the pseudospectral method is seen to be well suited for these computations.

Research paper thumbnail of Simulated correlation tracking on solar granulation

Lecture Notes in Physics, 2000

ABSTRACT

Research paper thumbnail of Breaking gravity waves

The dynamics of the nonlinear wave-wind critical level interaction as explored in 3-D direct nume... more The dynamics of the nonlinear wave-wind critical level interaction as explored in 3-D direct numerical solution are briefly described. The results demonstrate that streamwise oriented vortices dominate the instabilities.

Research paper thumbnail of Some Unusual Types of Noise

71st EAGE Conference and Exhibition incorporating SPE EUROPEC 2009, 2009

Research paper thumbnail of The initial value problem for Kelvin vortex waves

Journal of Fluid Mechanics, 1997

We present a formal solution to the initial value problem for small perturbations of a straight v... more We present a formal solution to the initial value problem for small perturbations of a straight vortex tube with constant vorticity, and show that any initial perturbation to such a tube evolves exclusively as a collection of Kelvin vortex waves. We then study in detail the evolution of the following particular initial states of the vortex tube: (i) an axisymmetric pinch in the radius of the tube, (ii) a deflection in the location of the tube, and (iii) a flattening of the tube's cross-secton. All of these initial states are localized in the direction along the tube by weighting them with a Gaussian function. In each case, the initial perturbation is decomposed into packets of Kelvin vortex waves which then propagate outward along the vortex tube. We discuss the physical mechanisms responsible for the propagation of the wave packets, and study the consequences of wave dispersion for the solution.

Research paper thumbnail of Entrainment-zone restratification and flow structures in stratified shear turbulence

Proceedings of the …, 2002

1. Introduction Stable density stratification poses several significant challenges for turbulence... more 1. Introduction Stable density stratification poses several significant challenges for turbulence model-ing. Unlike the case of unstable stratification (for which numerous potential turbulence nucleation sites may trigger volume-filling motion), turbulence in stable stratification must ...

Research paper thumbnail of In situ measurements of the fine-scale structure and turbulence in the mesosphere and lower thermosphere by means of electrostatic positive ion probes

Journal of Geophysical Research, 1990

... 5538 BLIX ET AL.' TURBULENCE IN THE MIDDLE ATMOSPHERE ... One block then corresponds to ... more ... 5538 BLIX ET AL.' TURBULENCE IN THE MIDDLE ATMOSPHERE ... One block then corresponds to a height interval of about 1 km in the mesosphere. ... This technique first removed the spin frequency and its first harmonic by discrete filtering, and subsequently used a cubic spline ...

Research paper thumbnail of Classification of Critical Points of Turbulent Velocity Field in a Converging-Diverging Channel Flow

6th AIAA Theoretical Fluid Mechanics Conference, 2011

ABSTRACT Based on Direct Numerical Simulations of a channel with a bump,ootnotetextM. Marquillie ... more ABSTRACT Based on Direct Numerical Simulations of a channel with a bump,ootnotetextM. Marquillie et al. (2008), J. Turbulence, vol 9, no 1, pp. 1-23. a critical point generation and classification is performed. Topological features of the flow are found relating critical point type with the distribution of Jacobian Determinants as well as the behavior of skin friction. Eight different types of points are found within the flow and these are visualized using different forms of the Line Integral Convolution visualization technique as described by Aasen and Furuheim (2008).ootnotetextM. Aasen and K. Furuheim (2008), M.Sc. thesis, Department of Informatics, University of Oslo, Oslo, Norway. It is found that the determinant analysis can be a useful method for data reduction of critical points. Local minima and maxima of the skin friction coefficient, Cf, are reliable indicators of development of extreme determinants. Further explanation of the physics is done through the analysis of the velocity and pressure iso-surfaces.

Research paper thumbnail of The vorticity dynamics of instability and turbulence in a breaking internal gravity wave

We perform a three-dimensional simulation of a breaking internal gravity wave in a stratified, co... more We perform a three-dimensional simulation of a breaking internal gravity wave in a stratified, compressible, and sheared fluid to investigate the vorticity dynamics accompanying the transition from laminar to turbulent flow. Baroclinic sources contribute preferentially to eddy vorticity generation during the initial convective instability of the wave field, yielding counter-rotating vortices aligned with the external shear flow. These vortices enhance the spanwise vorticity of the shear flow via stretching and distort the spanwise vorticity via advective tilting. The resulting vortex sheets undergo a dynamical (Kelvin-Helmholtz) instability which rolls the vortex sheets into tubes which link, in turn, with the original streamwise convective rolls to produce a collection of intertwined vortex loops. Following the formation of discrete vortex loops, the most important interactions are the self-interactions of single vortex tubes and the mutual interactions of adjacent vortex tubes in close proximity. The initial formation of vortex tubes from the roll-up of localized vortex sheets imposes axial vorticity variations having both axisymmetric and azimuthal wavenumber two components. Axisymmetric variations excite axisymmetric twist waves, or Kelvin vortex waves, which propagate along the tubes, drive axial flows, and deplete and fragment the tubes. Azimuthal wavenumber two variations excite m = 2 twist waves on the vortex tubes which amplify and unravel single vortex tubes into pairs of intertwined helical tubes. Other interactions, judged less fundamental to the turbulence cascade, include reconnection among tube fragments, mutual stretching of orthogonal tubes in close proximity, excitation of azimuthal wavenumber one twist waves, and the continual roll-up of weaker vortex sheets throughout the evolution. Collectively, these vortex interactions result in a rapid cascade of energy and enstrophy toward smaller scales of motion.

Research paper thumbnail of 7313 On the generation and maintenance of waves and turbulence in simulations of free-surface turbulence

Research paper thumbnail of Studies of Aerodynamically Induced Vibrations on the P-3C Maritime Surveillance Aircraft and Proposed Vibration Reducing Measures

Research paper thumbnail of Integrated Multi-aspect Visualization of 3D Fluid Flows

Research paper thumbnail of Wave Breaking and Transition to Turbulence in Stratified Shear Flows

Journal of the Atmospheric Sciences, 1996

ABSTRACT

Research paper thumbnail of Wave breaking signatures in OH airglow and sodium densities and temperatures: 1. Airglow imaging, Na lidar, and MF radar observations

Journal of Geophysical Research, 1997

The Collaborative Observations Regarding the Nightglow (CORN) campaign took place at the Urbana A... more The Collaborative Observations Regarding the Nightglow (CORN) campaign took place at the Urbana Atmospheric Observatory during September 1992. The instrumentation included, among others, the Aerospace Corporation narrowband nightglow CCD camera, which observes the OH Meinel (6-2) band (hereafter designated OH) and the O2 atmospheric (0-1) band (hereafter designated 02) nightglow emissions; .... ,•,,.•.Ly/L,•..•Lu• lidar; and the University of Illinois MF radar. Here we report on observations of small-scale (below 10-km horizontal wavelength) structures in the OH airglow images obtained with the CCD camera. These small-scale structures were aligned perpendicular to the motion of 30-to 50-km horizontal wavelength waves, which had observed periods of about 10-20 min. The small-scale structures were present for about 20 min and appear to be associated with an overturned or breaking atmospheric gravity wave as observed by the lidar. The breaking wave had a horizontal wavelength of between 500 and 1500 km, a vertical wavelength of about 6 km, and an observed period of between 4 and 6 hours. The motion of this larger-scale wave was in the same direction as the •30-to 50-km waves. While such small-scale structures have been observed before, and have been previously described as ripple-type wave structures [Taylor and Hapgood, 1990], these observations are the first which can associate their occurrence with independent evidence of wave breaking. The characteristics of the observed smallscale structures are similar to the vortices generated during wave breakdown in three dimensions in simulations described in Part 2 of this study [Fritts et al., this issue]. The results of this study support the idea that ripple type wave structures we observe are these vortices generated by convective instabilities rather than structures generated by dynamical instabilities. 1. Illinois at Urbana-Champaign. Copyright 1997 by the American Geophysical Union. Paper number 96JD02619. 0148-0227/97/96JD-02619509.00 b, c]. Much of the work described in the literature up to now has focused on descriptions of the wave characteristics and speculation about the sources of these ripple structures. Until recently, the best explanation for these ripple structures was that they are dynamical instabilities due to short-lived velocity shears generated in situ by the chance combination of wind and wave motions [Taylor and Hapgood, 1990; Taylor et al., 1995a].

Research paper thumbnail of Gravity wave breaking in two and three dimensions: 1. Model description and comparison of two-dimensional evolutions

Journal of Geophysical Research, 1994

A nonlinear, compressible, spectral collocation code is employed to examine gravity wave breaking... more A nonlinear, compressible, spectral collocation code is employed to examine gravity wave breaking in two and three spatial dimensions. Two-dimensional results exhibit a structure consistent with previous efforts and suggest wave instability occurs via convective rolls aligned normal to the gravity wave motion (uniform in the spanwise direction). Three-dimensional results demonstrate, in contrast, that the preferred mode of instability is

Research paper thumbnail of Vorticity dynamics in a breaking internal gravity wave. Part 2. Vortex interactions and transition to turbulence

Journal of Fluid Mechanics, 1998

A companion paper (Part 1) employed a three-dimensional numerical simulation to examine the vorti... more A companion paper (Part 1) employed a three-dimensional numerical simulation to examine the vorticity dynamics of the initial instabilities of a breaking internal gravity wave in a stratified, sheared, compressible fluid. The present paper describes the vorticity dynamics that drive this flow to smaller-scale, increasingly isotropic motions at later times. Following the initial formation of discrete and intertwined vortex loops, the most important interactions are the self-interactions of single vortex tubes and the mutual interactions of multiple vortex tubes in close proximity. The initial formation of vortex tubes from the roll-up of localized vortex sheets gives the vortex tubes axial variations with both axisymmetric and azimuthal-wavenumber-2 components. The axisymmetric variations excite axisymmetric twist waves or Kelvin vortex waves which propagate along the tubes, drive axial flows, deplete the tubes' cores, and fragment the tubes. The azimuthal-wavenumber-2 variations excite m = 2 twist waves on the vortex tubes, which undergo strong amplification and unravel single vortex tubes into pairs of intertwined helical tubes; the vortex tubes then burst or fragment. Reconnection often occurs among the remnants of such vortex fragmentation. A common mutual interaction is that of orthogonal vortex tubes, which causes mutual stretching and leads to long-lived structures. Such an interaction also sometimes creates an m = 1 twist wave having an approximately steady helical form as well as a preferred sense of helicity. Interactions among parallel vortex tubes are less common, but include vortex pairing. Finally, the intensification and roll-up of weaker vortex sheets into new tubes occurs throughout the evolution. All of these vortex interactions result in a rapid cascade of energy and enstrophy toward smaller scales of motion.

Research paper thumbnail of Simultaneous observations of E-region irregularities by ground-based and rocket-borne techniques

Journal of Atmospheric and Terrestrial Physics, 1985

E-region irregularities were observed under various geomagnetic conditions by the SAFARI and STAR... more E-region irregularities were observed under various geomagnetic conditions by the SAFARI and STARE auroral radars and Langmuir probes during the Energy Budget Campaign in November-December 1980. The 10-m and 1-m-wavelength phase irregularities and their phase velocities were measured; associated rocket-borne measurements of electron densities over Kiruna showed vertical/temporal structure, indicating the existence of irregularities in ionization between 90 and 110 km. Furthermore, rocket-borne measurements under moderately disturbed conditions showed the presence of VLF electric fields in the frequency bands of 168-5519 Hz at altitudes between 103 and 145 km at Kiruna. These results, corroborated by some measurements at Andoya, are interpreted in terms of the decay of ion acoustic waves generated by plasma instabilities in the auroral electrojet.

Research paper thumbnail of Synopsis of the D- and E-regions during the Energy Budget Campaign

Journal of Atmospheric and Terrestrial Physics, 1985

... Norway LG SMITH Aeronomy Laboratory, Department of Electrical Engineering, University of Illi... more ... Norway LG SMITH Aeronomy Laboratory, Department of Electrical Engineering, University of Illinois. ... DIX Dixon ROV Rovaniemi GOD Godhavn SOD Sodankyla GOT Godth b STR Sendre Stremfjord HEI Heiss Island THO Thorshavn IVA Ivalo TJ0 Tjornes KEV Kevo TRO Tromse ...

Research paper thumbnail of Three-dimensional evolution of Kelvin-Helmholtz billows in stratified compressible flow

Geophysical Research Letters, 1994

We present results of an initial study of Kelvin-Helmholtz instability in a stratified shear flow... more We present results of an initial study of Kelvin-Helmholtz instability in a stratified shear flow using a three-dimensional, nonlinear, compressible, spectral collocation model. The simulation was performed at an intermediate Reynolds number and significantly extends previous studies of homogeneous and stratified shear flows. Our major findings include a secondary instability oriented along the two dimensional velocity field, with counter-rotating vortices

Research paper thumbnail of Kelvin twist waves in the transition to turbulence

European Journal of Mechanics - B/Fluids, 1998

... Vorticity dynamics in a breaking gravity wave 2: Vortex interactions and transition to turbul... more ... Vorticity dynamics in a breaking gravity wave 2: Vortex interactions and transition to turbulence, J. Fluid Mech., accepted. GERz T., HOWELL J., MAHRT L., 1994, Vortex structures and microfronts, Phys. ... Lord KELVIN, 1880, Vibrations of a columnar vortex, Phil. Mag., 10, 155. ...