Quantification of the spatial and temporal evolution of stratified shear instabilities at high Reynolds number using quantitative acoustic scattering techniques

Abstract

The spatial and temporal evolution of stratified shear instabilities is quantified in a highly stratified and energetic estuary. The measurements are made using high-resolution acoustic backscatter from an array composed of six calibrated broadband transducers connected to a sixchannel high-frequency (120-600 kHz) broadband acoustic backscatter system. The array was mounted on the bottom of the estuary and looking upward. The spatial and temporal evolution of the waves is described in terms of their wavelength, amplitude and turbulent dissipation as a function of space and time. The observed waves reach an arrested growth stage nearly 10 times faster than laboratory and numerical experiments performed at much lower Reynolds number. High turbulent dissipation rates are observed within the braid regions of the waves, consistent with the rapid transition to arrested growth. Further, it appears that the waves do not undergo periodic doubling and do not collapse once their maximum amplitude is reached. Under some conditions long internal waves may provide the perturbation that decreases the gradient Richardson number so as to initiate shear instability. The initial Richardson number for the observed instabilities is likely between 0.1 and 0.2 based on the slope and growth rate of the shear instabilities.

High frequency acoustic propagation and modeling in stratified estuaries

“…This, together with other remote data from moored stations, could aid in other acoustic monitoring problems and be used in research ranging from climate monitoring and patterns such as those observed in the Arctic by Zeh and others. 2 Additionally, it would be very useful and cost-effective in designing or implementing acoustic communication by being able to model and predict dynamic environments. Covert communications, as explored by Walree and others, may be required for military operations if specific environmental factors might be used or avoided.…”

“…Furthermore, modern code such as BELL-HOP has adopted finite elements approach with geometric beams for the rays, which has shown to be very accurate. 2 Sound will bend as it moves through the water depending on the water properties as given by Snell's Law. As we have a changing sound speed profile for given depths, we can describe this with:…”

“…This is doubly true in the case of military operations where there might be a need for covertness, as studied by Walree and others. 2 The capacity to continue communications and avoid interception can be greatly enhanced if the environment is understood and modelled so that it can be utilized.…”

“…2 shows calculated values for the height of water that must be above our transducer to have different delays in arrival times between a direct path signal and a surface reflection. Figure2.7 shows a minimum time delay of 0.2 msec between the direct path signal and the upper surface reflection.…”

“… Freeman et al, "High-frequency, highly directional short-range underwater acoustic communications" 2. Walree et al, "UUV Covert Acoustic Communications".…”