Observing magnetospheric waves propagating in the direction of electron drift with Ekaterinburg Decameter Coherent Radar

This paper deals with Pc5 magnetospheric pulsations featuring positive azimuthal wave numbers registered with the mid-latitude coherent decameter radar located near Ekaterinburg (EKB). The azimuthal wave numbers are determined using adjacent high time resolution beams directed toward the magnetic pole. Approximately 13 % of all steady waves registered with the radar propagate eastward. We have examined ten cases of wave observations with both small and high positive wave numbers, which occurred between April 2014 and March 2015. We performed a wavelet analysis of the data sets, estimated wavelength in radial direction for four cases, and determined meridional phase propagation direction. In three cases, the results are consistent with field line resonance behavior. However, in the majority of the studied events wave frequencies are considerably lower than those of field line resonance, which were derived from satellite data on magnetic field and particle density. These waves may be classed with the drift-compressional mode.

Observing magnetospheric waves propagating in the direction of electron drift with Ekaterinburg Decameter Coherent Radar

This paper deals with Pc5 magnetospheric pulsations featuring positive azimuthal wave numbers registered with the mid-latitude coherent decameter radar located near Ekaterinburg (EKB). The azimuthal wave numbers are determined using adjacent high time resolution beams directed toward the magnetic pole. Approximately 13 % of all steady waves registered with the radar propagate eastward. We have examined ten cases of wave observations with both small and high positive wave numbers, which occurred between April 2014 and March 2015. We performed a wavelet analysis of the data sets, estimated wavelength in radial direction for four cases, and determined meridional phase propagation direction. In three cases, the results are consistent with field line resonance behavior. However, in the majority of the studied events wave frequencies are considerably lower than those of field line resonance, which were derived from satellite data on magnetic field and particle density. These waves may be classed with the drift-compressional mode.

Optimal Perturbations of an Oceanic Vortex Lens

The stability properties of a vortex lens are studied in the quasi geostrophic (QG) framework using the generalized stability theory. Optimal perturbations are obtained using a tangent linear QG model and its adjoint. Their fine-scale spatial structures are studied in details. Growth rates of optimal perturbations are shown to be extremely sensitive to the time interval of optimization: The most unstable perturbations are found for time intervals of about 3 days, while the growth rates continuously decrease towards the most unstable normal mode, which is reached after about 170 days. The horizontal structure of the optimal perturbations consists of an intense counter-shear spiralling. It is also extremely sensitive to time interval: for short time intervals, the optimal perturbations are made of a broad spectrum of high azimuthal wave numbers. As the time interval increases, only low azimuthal wave numbers are found. The vertical structures of optimal perturbations exhibit strong layering associated with high vertical wave numbers whatever the time interval. However, the latter parameter plays an important role in the width of the vertical spectrum of the perturbation: short time interval perturbations have a narrow vertical spectrum while long time interval perturbations show a broad range of vertical scales. Optimal perturbations were set as initial perturbations of the vortex lens in a fully non linear QG model. It appears that for short time intervals, the perturbations decay after an initial transient growth, while for longer time intervals, the optimal perturbation keeps on growing, quickly leading to a non-linear regime or exciting lower azimuthal modes, consistent with normal mode instability. Very long time intervals simply behave like the most unstable normal mode. The possible impact of optimal perturbations on layering is also discussed.

The Identification and Prediction of Helical Modes Induced by a Multi-Passage Swirl Stabilised Lean Burn Aero-Engine Fuel Injector Under Steady State and Acoustically Forced Conditions

Thermoacoustic instabilities in gas turbine combustion systems, caused by a feedback loop between acoustic fluctuations and the flame, can be a major factor in determining the durability of the combustor. Of interest here are helical modes caused by a Kelvin-Helmholtz instability emanating from a region of high shear close to the outlet of the fuel injector. A liquid fuelled lean burn fuel injector, containing three air flow passages is studied in the present work using non-reacting compressible unsteady RANS CFD simulations. An acoustic wave is injected at the downstream boundary with excitation frequencies of 300Hz and 450Hz to compare to an unforced case. Analysis of the flow response is carried out using linear stability analysis, Proper Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD). The linear stability analysis required interpolation of the solution from the unstructured CFD grid onto a uniform cylindrical polar mesh. The analysis found an absolute instability in the shear region between two passages. This m = −2 mode is unstable over frequencies from 400Hz to 1000Hz with wavelengths of 1.08 to 1.41 of the injector outer diameter. For the unforced case the POD identifies the first two modes with azimuthal wave number m = 1 and these are seen to spiral from the splitter plate inwards to disturb the pilot and outwards to the main. The dominant frequency is around 450Hz which is consistent with measurements and close to the linear stability analysis value. For the 300Hz forced case POD identifies the first four modes as being helical but has difficulty determining the dominant azimuthal wave number. There is shown to be a significant interaction between the acoustic and helical modes and double the total resolved kinetic energy as compared to the unforced case. The 450Hz forced case shows the asymmetric m = 1 mode to be damped and the m = 2 helical mode is relatively unchanged. The resolved kinetic energy was only marginally higher than the unforced case and significantly lower than the 300Hz forced case. The DMD analysis showed how, as the forcing increased the flow through the injector, the flow is simultaneously pushed radially inwards and accelerated azimuthally. It also identified the region downstream of the splitter plate with significant fluctuations and is likely to be the wavemaker region responsible for the generation of helical instabilities. This work improves understanding of how helical modes of different azimuthal wave numbers react to acoustic forcing. The ability to manipulate the strength of these modes through alteration of the fuel injector geometry gives designers an additional tool to control thermo-acoustic instabilities.

Large-scale coordinated observations of Pc5 pulsation events

HF (high-frequency) radars belonging to SuperDARN (Super Dual Auroral Radar Network) receive backscatter over substantial fields of view which, when combined, allow for simultaneous returns over extensive regions of the polar caps and midlatitudes. This makes them ideal instruments for the observation of pulsations in the Pc5 (1–5 mHz) frequency band. Relatively few pulsation events observed by multiple radars have been reported in the literature. Here we describe observations of three such events which extend over more than 120° of magnetic longitude in the Northern Hemisphere and one of which is also detected in the Southern Hemisphere. All three events show characteristics of field line resonances. In one case the pulsation has also been observed by magnetometers under or near the radar fields of view. The extensive longitudinal coverage allows accurate determination of azimuthal wave numbers. These are at the upper end of the lower values associated with external sources such as those in the solar wind. Such sources imply antisunward flow. However, the azimuthal wave number is negative, implying westward propagation at magnetic local times on both sides of noon, as would be expected from drift–bounce resonance with positive particles. Quiet conditions and a very low ring current during the events argue against this. The identification of the source of pulsations from a number of different mechanisms remains a problem of interest.

Three-Dimensional Viscous Potential Electrohydrodynamic Kelvin–Helmholtz Instability Through Vertical Cylindrical Porous Inclusions With Permeable Boundaries

In this paper, the electrohydrodynamic three-dimensional Kelvin–Helmholtz instability of a cylindrical interface with heat and mass transfer between liquid and vapor phases is studied. The liquid and the vapor are saturated, two coaxial cylindrical porous layers, and the suction/injection velocities for the fluids at the permeable boundaries are also taken into account. The dispersion relation is derived and the stability analysis is discussed for various parameters. It is found that the streaming velocity has a destabilizing effect, while the axial electric field has a stabilizing one. The suction for both the liquid and the steam has a destabilizing effect in contrast with the injection at both boundaries. The flow through porous structure is more stable than the pure flow. The case of the axisymmetric (for zero value of the azimuthal wave number m) and asymmetric (for nonzero value of the azimuthal wave number m) disturbances at large wavelength (at the wave number k→0) are always stable. Meanwhile, it is the same dispersion relation for the plane geometry at large wave number. Finally, our results are corroborated by comparing them with the previous published results.