Daniela Di Iorio

Path‐averaged turbulent dissipation measurements using high‐frequency acoustical scintillation analysis

The path‐averaged turbulent kinetic energy dissipation (per unit mass) is estimated in a shallow tidal channel using acoustical scintillation analysis. The tidal current ensures that fully developed turbulence prevails. In order to make measurements of the turbulence parameters, a high‐frequency (67‐kHz) acoustic propagation experiment was conducted. Our acoustic data is compared to the weak scattering theory of Tatarski assuming a Kolmogorov turbulence model and compared with available oceanographic data. Analysis of log‐amplitude, phase, and phase‐difference spectra shows close agreement with the theory. Comparison of the acoustic data with direct measurement of temperature and salinity fluctuations using in situ sensors allows evaluation of the contribution of turbulent velocity fluctuations to the scintillation signal. The results show that turbulent velocity fluctuations are the dominant (∼95%) component of the observed acoustic scintillation, leading to estimates of the path‐averaged turbulent energ...

Separation of current and sound speed in the effective refractive index for a turbulent environment using reciprocal acoustic transmission

Here a high-frequency acoustic experiment through a turbulent flow in Cordova Channel is described which is specifically designed to measure the contribution of scalar and vector fluctuations to the total scattered signal over a 48-h period. The effective refractive index fluctuations are determined by both the random changes in sound speed (scalars) and the random motion of the medium (vectors), and so in this paper it is demonstrated that reciprocal acoustic transmission is a technique which unambiguously separates their effects. The effective refractive index structure parameter, Cηeff2 is defined as the sum of scalar Cηs2 and vector Cηv2 contributions through the equation Cηs2+11/6Cηv2. The effective refractive index structure parameter is also measured from the forward propagation acoustical scintillation variances, thus providing an independent check on the interpretation.

Two‐dimensional angle of arrival fluctuations

Angle of arrival fluctuations are one manifestation of acoustic propagation through a turbulent flow. Here the two‐dimensional angle of arrival distribution for a 670‐m acoustic path through a high Reynolds number flow in a tidal channel is examined and its origin and relationship to the flow field is determined. Over scales greater than the variability due to turbulence, a solution of the ray propagation equation explains the effect of advection on the horizontal arrival angle. The rapidly fluctuating two‐dimensional angle of arrival distribution shows a degree of scatter consistent with the level of turbulent intensity. After removal of effects due to the finite aperture, orthogonal components are correlated during strongly sheared flow implying that the turbulence is weakly anisotropic over the measured scales. This anisotropy is discussed in terms of the cross stream velocity gradients ∂v/∂x′ and ∂v/∂z′, where (x′,z′) are perpendicular diagonal coordinates.

Thirty years of scintillating acoustic data in diverse ocean environments, thanks to David Farmer

The acoustic scintillation method was first applied in a coastal tidal channel in the early 1980s by David Farmer and this laid the foundation for studies of estuarine channel flows, bottom boundary layer dynamics, deep-sea hydrothermal plumes, and now more recently hydrocarbon seeps. Over short distances, using high frequencies and high transmission rates, amplitude and phase fluctuations measured over transmitter and receiver arrays have been used to infer horizontal (or vertical) flows and turbulent motions, all averaged along the acoustic path over the range separating transmitters and receivers. Autonomous and cabled instrumentation have provided measurements of temporally and spatially averaged quantities, continuously in time. This ability to make long-term continuous measurements has shown major advances in our understanding of acoustic forward scatter from velocity and temperature fluctuations in moving random media and for identifying strong turbulence levels in a variety of ocean settings. Much...

Acoustic measurements of vertical velocity and temperature fluctuations of a hydrothermal plume with comparisons to large eddy simulations

The acoustic scintillation method is used to study the hydrothermal plume of Dante within the Main Endeavour vent field (MEF) at the Endeavour segment of the Juan de Fuca Ridge. Forty days of vertical velocity and temperature fluctuations were obtained from the rising plume above the Dante edifice in an environment where the flow is dominated by strong (5cm/s) semi-diurnal tidal currents and a northerly mean residual current (3cm/s). These acoustic measurements provide a window on deep-sea hydrothermal plume dynamics in strong oscillatory cross flows. A large eddy simulation, parameterized with anisotropic mixing coefficients, taking into account ambient stratification and time-dependent background flows and calibrated by the acoustic measurements, yields insight into turbulent processes, entrainment, plume bending, rise height, and, inferentially, mound heat flux. The turbulent dissipation rates for kinetic energy (e) and thermal variance (eθ) is approximated by computing the Reynolds averaged sub-grid s...

Long‐term variability in hydrothermal vent flow and temperature fluctuations by acoustic scintillation

An acoustical scintillation instrument is described that has been used to measure flow and temperature fluctuations at a hydrothermal vent plume. The vertical buoyancy driven flow together with the root‐mean‐square temperature fluctuations are obtained for vent structure 8F of the Main Endeavour vent field on the Endeavour segment of the Juan de Fuca Ridge. Long term (71 days) measurements are obtained and an estimate of the heat flux density which is of the order 0.06 MW m−2 is calculated. Measurements also show oscillations in the log‐amplitude variance that result from either plume advection or increased entrainment of ambient fluids by the ambient tidal currents, thus demonstrating the need for a long time series measurement of simultaneous horizontal and vertical flow. Future measurements in the Main Endeavour vent field, with possible integration into the NEPTUNE seafloor observatory, will be over extended periods of time so that comparisons of these processes can be made with independent seismic...

On the multifrequency method of the range dependent transversal current monitoring

Transverse flow of inhomogeneous current produces fluctuations of the acoustic signal passing through it. These fluctuations vary with the signal frequency due to the variation of the Fresnel zone size. Respectively, fluctuations of signals at two different frequencies are coherent in a low‐frequency range of the spectrum and noncoherent in the high‐frequency band when ocean inhomogeneities are smaller than the transverse size of the Fresnel zone. The frequency cutoff of the coherence function of signals at different frequencies depends on current flow characteristics. Di Iorio and Farmer [J. Acoust. Soc. Am. 100, 814–834 (1996)] describe high‐frequency acoustic scintillation measurements in a tidally forced flow through a Cordova Channel specifically designed to characterize the two‐dimensional angle of arrival fluctuations in terms of the mean and turbulent properties of sound speed and current. We present new results from this data set which was collected in an alternating dual frequency mode. The multifrequency technique gives results which are consistent with previous results, but eventually will allow the measurement of current profiles across the channel.Transverse flow of inhomogeneous current produces fluctuations of the acoustic signal passing through it. These fluctuations vary with the signal frequency due to the variation of the Fresnel zone size. Respectively, fluctuations of signals at two different frequencies are coherent in a low‐frequency range of the spectrum and noncoherent in the high‐frequency band when ocean inhomogeneities are smaller than the transverse size of the Fresnel zone. The frequency cutoff of the coherence function of signals at different frequencies depends on current flow characteristics. Di Iorio and Farmer [J. Acoust. Soc. Am. 100, 814–834 (1996)] describe high‐frequency acoustic scintillation measurements in a tidally forced flow through a Cordova Channel specifically designed to characterize the two‐dimensional angle of arrival fluctuations in terms of the mean and turbulent properties of sound speed and current. We present new results from this data set which was collected in an alternating dual frequency mode. The mult...

The measurement of flow and turbulence levels using acoustical scintillation analysis

For acoustical propagation in the coastal environment, small scale fluctuations in the temperature/salinity and flow velocity distorts the spherically spreading phase fronts which result in amplitude and phase fluctuations (scintillation). As the fine structure variability in the medium has a natural coherence time, it will be transported by the current. Detection of pattern motion is an essential concept exploited in acoustical scintillation and has been demonstrated in a variety of coastal channels. Important features of this approach are that the measurement is path averaged, is of the flow component perpendicular to the acoustic path and is sensitive to the Fresnel scale size which is chosen to lie within the inertial subrange of the turbulent random medium. Analysis of the signals goes beyond measurement of flow speed, and has included detection of turbulent velocity and scalar levels and turbulent anisotropy. For one‐way propagation the turbulent kinetic energy dissipation rate is derived from the v...

A maximum likelihood estimation algorithm for tracking two multipaths

A high‐frequency (67 kHz) acoustical scintillation experiment was carried out in the northern entrance to Hood Canal, Puget Sound, WA. This experiment made use of a 4‐transmitter and 4‐receiver array configured as a T‐shape. The two‐dimensional feature of this array was designed to measure both along‐channel small‐scale properties as a result of advection and vertical properties as a result of refraction from temperature/salinity stratification. Sampling was carried out at 5 Hz for all transmitters so as to understand the turbulent effects of the medium on acoustic scattering. This experiment tested if acoustical scintillation techniques can be extended to long distances (2380 m) and in deep (100 m) coastal channels having strong tidal currents and temperature/salinity stratification. With the longer path lengths and stable stratification, acoustic propagation resulted in multipath arrivals which were separable for most of the measurement period. In order to study the oceanographic effects on the direct p...

Acoustic observations of Mediterranean flow into the Black Sea

The physical behavior of the Mediterranean flow entering the Black Sea through the Bosphorus Strait is described using a variety of high‐frequency acoustic systems. Because of the density difference between salty Mediterranean and fresh Black Sea water, a two‐layer exchange is formed which is confined within a canyon in the Black Sea exit region of the Bosphorus Strait. A 307‐kHz acoustical scintillation system placed 6 m from the seafloor and covering a 300‐m propagation path is used to describe the mean Mediterranean current speed and the turbulent velocity fluctuations within the bottom boundary layer of the Mediterranean flow during a 4‐day period when the exchange was maximal. In the idealized case of isotropic and homogenous turbulence, estimates of the turbulent kinetic energy dissipation rate leads to values ranging from 1×10−6 to 5×10−5 W/kg−1. A 600‐kHz broadband acoustic Doppler current profiler placed within the canyon shows that the two‐layer exchange displays temporal variability over scales...