Erik Mollo-Christensen

Experiments on Jet Flows and Jet Noise Far- Field Spectra and Directivity Patterns

The results of measurements of far-field sound emitted from jets are reported. The narrow-band power spectral density of the sound in the far field was measured for three jet diameters, three Mach numbers, and five angular positions. The intensity distribution of mean-square pressure fluctuation in the far field in several wide frequency ranges were also measured. The similarity relations found from the experiments are reported.

Sound emission from jets at high subsonic velocities

Measurements of spectra of sound emission to the far field from jets at high subsonic velocities are presented. The similarity relations found in the experiments suggest a mechanism of sound generation and scattering where the latter is of dominant importance. A possible mechanism is described.

Oscillatory behavior in Arctic sea ice concentrations

We have done a frequency analysis of the 9-year record of sea ice concentrations obtained with the NASA Nimbus 7 scanning multichannel microwave radiometer (SMMR). Because of the periodic revisit time of the SMMR, we are able to examine recurrence of shorter-period oscillations, as aliased by the revisit frequency, and longer-period oscillations per se. A number of findings result from a sequence of two analysis schemes, as follows. We have first determined, then removed, the mean, linear trend, and seasonal cycle (the latter composed of five harmonics of the annual cycle) from the time series for each pixel of gridded SMMR ice concentration data in the Arctic, using multiple linear regression. We find that the 9-year trends are most positive in the Beaufort and East Greenland Seas, whereas the most negative areas are in the Barents Sea and the sea ice surrounding Svalbard ; this is similar to trend patterns in the lengths of the sea ice season reported earlier by Parkinson [1992]. In the residual data the standard deviation ranges from 12% to 16% in the central Arctic basin, considerably above the estimated wintertime ice concentration accuracies of 5% in that region ; this is in part due to fluctuations in the ice concentration rather than noise. Secondly, we have used Fourier analysis and multiple-taper filtering techniques to identify tentatively some of these fluctuations as tidal components or multiyear components similar to those found in the El Nino-Southern Oscillation Index, and we show their spatial distribution. However, there are some high-amplitude, periodic components that we have not yet been able to identify ; we show the spatial distribution of one of these components.

Modulational development of nonlinear gravity-wave groups

Observations of the development of nonlinear surface gravity-wave groups are presented, and the amplitude and phase modulations are calculated using Hilbert-transform techniques. With increasing propagation distance and wave steepness, the phase modulation develops local phase reversals whose locations correspond to amplitude minima or nodes. The concomitant frequency modulation develops jumps or discontinuities. The observations are compared with recent similar results for wavetrains. The observations are modelled numerically using the cubic nonlinear Schroedinger equation. The motivation is twofold: to examine quantitatively the evolution of phase as well as amplitude modulation, and to test the inviscid predictions for the asymptotic behavior of groups versus long-time observations. Although dissipation rules out the recurrence, there is a long-time coherence of the groups. The phase modulation is found to distinguish between dispersive and soliton behavior.

Allowable discontinuities in a Gerstner wave field

Velocity discontinuity surfaces can be inserted at will in a Gerstner edge wave on an inclined boundary in a rotating fluid.

Wave response of a spar buoy with and without a damping plate

Abstract Report on a computer analysis of wave response of a spar buoy that has been operated successfully in the Mediterranean over the past ten years, to find out how the presence of a large horizontal plate at the bottom affects its wave response. The calculations show that the addition of a damping plate decreases have response for short waves but increases the response for very long waves.

Heat Storage in the Oceanic Upper Mixed Layer Inferred from Landsat Data

From the spacing of internal wave packets generated by tidal flow over topography, one can determine their propagation speed. The propagation speed depends upon the density anomaly and depth of the upper mixed layer. Attributing the density anomaly to temperature only, one can calculate the heat storage in the upper oceanic layer. On the basis of Landsat images of the New England continental shelf, the heat storage calculated from satellite data has been compared with available in situ observations. The data show that the method may have merit and is deserving of further refinement.

Effects of rotation and shear on doubly diffusive instability

A linear stability analysis of a doubly diffusive system, with rotation and shear, shows that overstable oscillations can occur in stratifications typical of the equatorial ocean, that internal waves encountering an equatorial current can exchange energy with the current, and that the wave-induced fluxes of salt and heat can lead to layer formation in the salinity, temperature and velocity fields.

Experimental and theoretical investigation of the stability of air flow over a water surface

An experimental investigation of the instability of a laminar air flow over water shows two distinct modes of unstable oscillations as predicted by theory. The Tollmien–Schlichting waves instability could be excited by a ribbon vibrating in the air, and the neutral stability curve determined. The water wave instability mode could be excited by a ribbon vibrating in the water. The growth rates of these waves show only fair agreement with theoretical predictions.

Edge waves as a cause of ice rideup onshore

Edge waves on ice-covered water, generated elsewhere by wind, current, or wave impingement at the ice edge, can reach large amplitudes where bottom slope and current combine to give low group velocity. The amplitude of water velocity normal to shore can become large, and the resulting acceleration of the ice cover onto shore may be sufficient to cause ice rideup. Edge waves on an ice covered ocean with a mean current are analyzed to a linear approximation, and estimates are given of velocities and amplitudes.