Experiments on buoyant-parcel motion and the generation of internal gravity waves

Abstract

Laboratory experiments were conducted to study the motion of miscible buoyant fluid parcels in both homogeneous and stratified media, and were complemented with numerical experiments using the model of Orlanski & Ross (1973). Parcel motion in the homogeneous case was found to be shape-preserving, in agreement with the large body of data for such motion. The stratified medium experiments were such that the parcel attained equilibrium positions. It was found that the time for the parcel to reach maximum depth was approximately 0·7 times the buoyancy period, independent of that depth. The subsequent collapse of the parcel and generation of internal gravity waves was observed. Ray-like patterns of waves propagated out from the collapse region, and wave frequencies near 0·7 times the Brunt-Väisälä frequency were predominant. The internal wave energy radiated away from the collapsed parcel was estimated and found to be 20 to 25% of the change in the system potential energy.

The internal wavefield generated by a towed sphere at low Froude number

“…The estimated measurement error is ∼10 % for these data, as well as for the wavelength values shown in figure 6(b). These values of θ are somewhat larger than the range measured by Dohan & Sutherland (2005), θ = 46 ± 5 • , for IWs generated by a continuous turbulent source, by Cerasoli (1978) for IWs resulting from the collapse of a buoyant plume where the angle ranged between θ = 35 • and 50 • , and by Wu (1969) for the collapse of a mixed region where θ = 36 • . In numerical simulations of wake turbulence, steeper angles were found by Diamessis & Abdilghanie (2011), with θ = 31 • at low Re = 5 × 10 3 and θ = 45 • at the higher Re = 10 5 .…”

“…Nevertheless, the experimental cross-plane cuts show an apparent dominant wave pattern, which is used to qualitatively characterize the wavefield, as has been employed in other experimental investigations (e.g. Cerasoli 1978;Dohan & Sutherland 2005). This wavefield characterization approach also pertains to the cuts in the horizontal plane discussed in § 3.2.4.…”

“…This is expected since the lee wave forcing scale is considerably larger than the characteristic dimension of the wake eddies. The increase in the spreading angle and increased wavelengths further from the source were explained by Cerasoli (1978) based on an argument of Wu (1969). In effect, as the IWs are generated by a broadband source, the waves with longer wavelengths will have larger phase and group speeds than those with shorter wavelengths, with the result that they will propagate farther from the source and more rapidly propagate off track, resulting in an increasing angle of the rays connecting the crests and troughs.…”

Internal gravity waves generated by convective plumes

“…An experimental study of the energy lost to waves from a thermal was undertaken by Cerasoli (1978). The energy of the internal gravity waves propagating away from the collapse region was estimated by comparing the difference between the initial and final potential energy of the mean state.…”

Generation of internal gravity waves by a katabatic wind in an idealized alpine valley

“…These results have been verified experimentally (Ermanyuk & Gavrilov (2003)). Interestingly, when a mixed region is released at its equilibrium level (Wu (1969)) or above it (Cerasoli (1978)), in a constant N stratified fluid, the power spectrum of the radiated waves is peaked at a frequency Ω such that Ω/N ≃ 0.8 as well. From these results, it follows that two systems of oscillations are expected in the stably-stratified atmospheric boundary layer of a valley: (i) a system of temporal oscillations located near the slope at frequency ω such that ω/N =sinα and (ii) a system of propagating motions, consisting of the internal gravity wave field, propagating from the slope and away from it at frequency Ω/N ≃ 0.8.…”