Showing papers by "David C. Fritts published in 1996"

Evolution and breakdown of Kelvin-Helmholtz billows in stratified compressible flows. Part I: Comparison of two- and three-dimensional flows

Abstract: The authors use a nonlinear, compressible, spectral collocation code to examine the evolution and secondary instability of Kelvin–Helmholtz billows in stratified shear flows at intermediate Reynolds numbers. Two-dimensional results exhibit structure consistent with previous numerical studies and suggest dissipation via diffusive transports within the billow cores. Results obtained permitting three-dimensional structures show, in contrast, that secondary instability results in a series of counter-rotating vortices that occupy the outer portions of the billow structures, are oriented in the plane of two-dimensional motion, largely along the two-dimensional velocity field, and contribute substantially to mixing and homogenization of the billow cores at later times. Examination of the flow structure leading to secondary instability also suggests an alternative explanation of the nature of this instability in stratified flows to that offered previously. Comparison of the two-dimensional and spanwise-a...

Wave Breaking and Transition to Turbulence in Stratified Shear Flows.

Abstract: In a previous study the authors used a nonlinear, compressible, spectral collocation numerical model to examine the evolution of a breaking gravity wave in two and three dimensions. The present paper extends that effort to examine the implications of higher resolution and smaller dissipation for wave and instability evolutions, transports, and energetics in shear flows aligned with and having a component transverse to the direction of wave propagation. A component of mean shear transverse to the direction of wave propagation (denoted as a skew shear) results in the alignment of instability structures with the background shear flow rather than in the direction of wave propagation. This alignment leads to asymmetric instability structures and less rapid instability growth relative to the parallel shear flow. Slower instability evolution due to a skew shear has several implications for wave breakdown, including a delayed state of maximum instability, a larger wave amplitude prior to and throughout w...

Evolution and Breakdown of Kelvin–Helmholtz Billows in Stratified Compressible Flows. Part II: Instability Structure, Evolution, and Energetics

Abstract: A companion paper by Fritts et al. employed a nonlinear, compressible, spectral collocation code to examine the effects of secondary instability on the evolution of Kelvin–Helmholtz billows in stratified shear flows at intermediate Reynolds numbers. The purpose of this paper is to examine the structure, sources, evolution, and energetics of the secondary instability itself. It is found that this instability comprises counterrotating vortices aligned largely along the two-dimensional velocity field (with spanwise wavenumber), with initial instability occurring in the stably stratified braids between adjacent billows and thereafter in the billow cores as maximum KH amplitudes are achieved. The more energetic secondary instabilities are confined to the billows, where the major sources of instability energy are buoyancy and shear due to negative stratification and the solenoidal generation of negative spanwise vorticity within the billow cores. Strain also represents a significant source of streamwis...

Gravity wave variability and interaction with lower-frequency motions in the mesosphere and lower thermosphere over Hawaii

Abstract: Radar observations of the mesosphere and lower thermosphere over Kauai, Hawaii, have provided measurements of the horizontal wind field since September 1990. Horizontal velocity variances in the gravity wave band of frequencies were computed for several averaging intervals over the entire dataset. The results, correlated with tidal and lower-frequency winds, show evidence of filtering of gravity waves by tidal and other lower-frequency motions. Spectra of gravity wave variances for two years of data display peaks at planetary wave as well as tidal periods, primarily near 12-, 24-, 48-h, and 16-day periods. The modulation of gravity wave variance during four distinct background regimes is examined: one in which the diurnal tide predominates; another in which the 2-day wave is large, a third in which the zonal mean wind and tidal amplitudes are large; and a fourth in which the zonal mean, tidal, and 2-day amplitudes are all small. Intervals in which the diurnal tide or the 2-day wave predominates s...