Internal Wave Front Research Articles

Ocular, Corneal and Internal Wavefront Analysis in Refractive Surgery Candidates

Ocular, Corneal and Internal Wavefront Analysis in Refractive Surgery Candidates

Характеристика проявления внутренних волн в приустьевой зоне Дуная по спутниковым данным высокого разрешения

The mechanisms of internal waves manifestation in satellite data of the optical range are considered for the mouth area of the Danube. Three main mechanisms for the manifestation of internal waves are identified - the previously described dynamic (due to a change in the roughness of the sea surface in convergent zones created by a moving internal wave), slick - when surfactants accumulate in convergence zones, and a new one - due to a change in the brightness of the sea surface during modulation internal wave by the scattering layer thickness. Data from the OLI Landsat-8 scanner for 2015-2019 were used for the analysis. It is shown that in different situations, internal waves can manifest themselves either due to various mechanisms, or only due to one of them. Summary maps of manifestations of internal waves in the study area were constructed. Additionally, situations with quasi-synchronous data of MSI Sentinel-2 and C-SAR Sentinel-1, which displayed internal wave packets, are considered. The selection of such pairs made it possible to estimate the phase velocities of internal waves, which ranged from 0.05 m/s (0.19 km/h) to 0.95 m/s (3.43 km/h) in various hydrometeorological situations. Examples of internal wave front transformation on submesoscale eddies are presented

Современные подходы к хирургии катаракт с высокой остротой зрения на основании анализа волнового фронта

Relevance. Until recently, visual acuity was the main criterion for determining the timing of surgery, but already in the early stages of cataract development, patients may suffer from reduced image quality, which affects the quality of life (“night” myopia, light scattering, blurring). Deterioration in visual function in these patients may be explained by refractive errors. Analysis of internal higher-order aberrations resulting from slight opacification of the lens is important for determining the effect of media opacity on wavefront aberrations (WF) and for determining the indications and timing of surgical treatment, especially in patients with high visual acuity. Purpose. Evaluate the optical system of the eye with initial cataracts and high visual acuity. Evaluate the wavefront and its aberrations, contrast sensitivity and Shtrel coefficient. Material and methods. The study included 111 eyes with cataracts and 42 eyes without signs of cataracts (control group). Patients aged on average 57±5.3 years. Wavefront analysis was performed using an iTrace ray tracing aberrometer (Tracey Tech., USA) in a dark room under maximum drug-induced mydriasis. In each group, total, corneal, and internal wavefront aberrations were measured at a pupil diameter of 6.0 mm. Higher order aberrations were analyzed using Zernike polynomials (3rd to 5th order). The modulation transfer function (MTF) at different spatial frequencies (0–30 c/deg) and the Strehl coefficient in patients in the study groups were also assessed. Results. Total and internal higher-order wavefront aberrations were significant in different cataract groups compared with control patients, while higher-order corneal aberrations were not statistically different. We noted a higher level of total and internal aberrations in the group of patients with posterior subcapsular cataract. In this regard, patients with posterior subcapsular cataract require surgical treatment at an earlier stage, despite high visual acuity. Conclusion. We recommend assessing the optical quality of vision and wavefront aberrations when observing early cataracts in order to decide whether to perform surgery at an earlier date. Key words: initial cataract, quality of vision, higher order aberrations, internal wavefront aberrations

Near Vision Tasks and Optical Quality of the Eye.

PurposeTo study the effect of near-vision reading task on optical quality of the eye when performed on a computer monitor and on printed paper, and to identify which of the two results in greater changes.MethodsTwo groups of subjects performed a 30-min reading task in two different conditions: on a computer monitor and on printed paper. Ocular, corneal, and internal wavefront aberrations (Zernike coefficients up to 6 order), root-mean-square of low- and high-order aberrations, spherical equivalent, vectoral components of ocular astigmatism (J45 and J0), and the compensation factor between internal and corneal aberrations were measured before and after the tasks. Their changes were analyzed in each group and between groups.ResultsStatistically significant changes in wavefront aberrations and in root mean square of low- and high-order aberrations were observed in both groups which was significantly greater when the task was performed on printed paper. Partial loss of compensation mechanism and variation in spherical equivalent in a negative direction occurred after both reading tasks; however, it was statistically significant only with printed paper reading task. The vectoral components of ocular astigmatism did not show statistically significant changes in either groups.ConclusionNear-vision reading tasks can change the optical quality of the eye, especially when the task is performed on printed paper.

Shock-induced cavitation and wavefront analysis inside a water droplet

The objective of this study is to develop a basic understanding of the interaction of shock waves with density inhomogeneities. We consider the particular instance of a planar air shock impinging on a spherical water droplet and discuss to what extent this interaction can lead to the inception of cavitation inside the droplet. The effort centers on early phases of the interaction, which are analyzed using both ray theory and a hydrodynamic code. Within the context of ray theory, the occurrence of focusing is examined in detail, and parametric equations are derived for the transmitted wavefront and its multiple internal reflections. It is found that wave patterns predicted by ray calculations compare extremely well with the more accurate numerical solutions from simulations. In particular, it is shown that the internal wavefront assumes a complex time-dependent shape whose dominant feature is the existence of cusp singularities. These singular points are shown to trace out surfaces that are the caustics of the associated system of rays. From the singularities of the energy flux density of the refracted wave, the parametric equations of the caustic surface associated with the kth reflected wavefront are deduced. As a consequence of the focusing process, simulations show the formation of negative-pressure regions in the internal flow field. These low-pressure zones are identified as possible spots at which cavitation may occur, depending on the magnitude of pressure reached. Finally, numerical results provide quantitative information on the dependence of negative-pressure peaks upon incident-shock-wave strength.

Bottom Reverberation in the Presence of Intense Internal Waves

The problem of modeling the broadband coherent field of low-frequency bottom reverberation under conditions of hydrodynamic variability associated with the motion of intense soliton-like internal waves (internal wave solitons) is considered. Expressions are obtained for calculating backscattered signals using mode parabolic equations, as well as approximations of coupled modes. It was assumed that the reverberation signal is received from the direction along or across the internal wave front. In the case of mode coupling, it was shown that the presence of a train of internal wave solitons can increase the intensity of bottom reverberation coming from an area of the bottom behind the front of this inhomogeneity if the sound source is at the center of the waveguide. Comparison of the interference patterns of bottom reverberation in a wide frequency band with and without solitons makes it possible to record this disturbance with greater accuracy and at any depth of the sound source. In detecting a reverberation signal under horizontal refraction conditions, an ordered interference pattern is not observed.

TRANSLATION DE L'AFRIQUE PAR RAPPORT À L´AMÉRIQUE D'APRES L'ETUDE DE LA MIGRATION DES BASSINS SEDIMENTAIRES

During the separation between Africa and South America, the African plate underwent epeirogenic undulation, which was reconstruted from paleogeographic studies of the major sedimentary basins, as well as from the history of the ridges. It exhibits a vertical amplitude of 10 3 to 10 4 meters, a wave lenght of 10 5 to 2 X 10 6 meters, and a period of 10 7 to 10 8 years. The litospheri undulation might be a response to vertical moviments (uplifts or subsidences) affecting the astenosphere or the upper mantle. It also might be due to an internal wave front, whose passage would cause vertical displacements. A third alternative would be a horizontal relative moviment of a flexible Iitosphere over a rigid mantle with elevations and depressions. For the African block, the direction of propagation, and the horizontal speed (0,1 to 10 cm/year), calculated from geologic observations on the migration of sedimentary basins, agree with a possible moviment of the African plate, as derived from the data on sea floor spreading for the South Atlantic. It will be very interesting to investigate whether similar studies on the Brazilian sedimentary basins would allow comparable results.

Wavefront evolution of compression waves propagating in high speed railway tunnels

The amplitude of micro-pressure wave emitted from the exit portal of a high-speed railway tunnel is approximately proportional to the pressure gradient of the internal wavefronts approaching the portal. For long tunnels, the evolution during propagation is an important factor for estimating the pressure gradient in the exit wavefront. Moreover, the relationship between the entrance and exit states depends upon the conflicting influences of inertia and damping along the propagation. In this paper, the investigation is done to study the evolution of wavefronts with different initial waveforms and to assess the influence of friction and train speed in the process. This work is based on numerical simulations of wavefront propagation using a third-order monotonic upwind scheme for conservation laws and backed up by field measurements. Numerical analyses show that the wavefront evolution not only depends on the pressure amplitude and the maximum pressure gradient of the initial wavefront but also depends strongly on the shape and timing of the peak steepness of ∂P/∂t. The predictions also demonstrate the existence of a critical tunnel length. For any particular train and tunnel entrance, the magnitudes of the micro-pressure waves increase with the tunnel length when this length is below the critical one and decrease with the tunnel length otherwise. The critical length decreases with the increase of the tunnel wall friction and the train speed.

Three-dimensional evolution of internal waves reflected from a submarine seamount

The interaction between internal waves and submarine seamounts is three-dimensional (3D). In this work, we conduct laboratory experiments to investigate the role of the three-dimensional seamount on the reflection of internal waves. Experimental results show that the energy of internal waves is focused after the reflection and the phase of the internal wave front become curved. The energy of the focusing region was nearly 2 times as that of the scattering region. There is a strong internal wave induced mean flow whose strength is nearly as high as 50% of that of the reflected internal wave. The mean flow was generated by the variation of the velocity amplitude in both the x axis and y axis. These results indicate that the reflection from the 3D seamount could lead to a complex spatial distribution of the internal wave and a different energy conversion process.

Data analysis and modeling of broadband acoustic propagation perpendicular to internal wave fronts and sand dune crests in the South China Sea

Very large subaqueous sand dunes were discovered on the upper continental slope of the northeastern South China Sea (SCS) in the spring of 2007 during an ONR 322OA-funded field experiment which was designed to study the large transbasin internal solitary waves (ISW) that are generated by tidal forcing in the Luzon Strait. These internal waves and sand dunes are important acoustical features, as it is expected that they will cause significant anomalies in the acoustical field. In the spring of 2014, a broadband source was deployed to transmit 850-1150 Hz LFM signals to receivers on a mooring in the center of the sand dune field. The acoustic transect was oriented perpendicular to the dune crests, ISW fronts and isobaths. Data analysis and extended modeling are presented to quantify the degree to which these features impact the propagation of broadband signals in the 100-2000 Hz band as a function of source depth and frequency.

Correlations between Corneal and Internal Wavefront Aberrations through Zernike Coefficients Analysis

Objectives: To examine corneal aberrations are offset by internal aberrations more closely, the Zernike coefficients of corneal aberrations and internal aberrations were compared and analyzed. Methods/Statistical Analysis: Measurements of higher-order aberrations were performed with KR-9000 PW (Topcon Corp., Japan), a Hartmann-Shack Wavefront aberrometer. The coefficients from the 2nd to 4th expressions on Zernike Polynomials were analyzed for cornea and internal optics within 6mm measurement zone. 342 healthy eyes of 171 young adults who have never received eye surgery and do not wear contact lenses were selected as subjects of this study. Findings: There were significant negative correlations among all Zernike coefficients of corneal and internal aberrations. In Z31(r=-0.887, p=0.00), Z4-4(r=-0.698, p=0.00), and Z44(r=-0.688, p=0.00), there were high correlations between corneal and internal aberrations. On the other hand, the correlation coefficients of Z20 and Z22 that belonged to lower-order aberration were very weak at -0.129 and -0.136 respectively, having almost no correlation. Improvements/Applications: Since some signs of their Zernike coefficients are different, it is considered necessary to analyze not the both eyes but a single eye to identify the accurate RMS values of each Zernike coefficients and not only the correlations.

Acoustic propagation in the South China Sea: Internal waves and sand dunes

Very large subaqueous sand dunes were discovered on the upper continental slope of the northeastern South China Sea (SCS) in the spring of 2007 during the ONR 322OA-funded NLIWI Acoustics field experiment. The dunes’ formation mechanism is hypothesized to be the internal solitary waves (ISW) which generate from tidal forcing on the Luzon Ridge on the east side of the SCS, propagate west across the deep basin with amplitudes regularly exceeding 125 m, and dissipate large amounts of energy via turbulent interaction with the continental slope, suspending and redistributing the bottom sediment. These internal waves and sand dunes are important acoustical features, as it is expected that they will cause significant anomalies in the acoustical field. Data analysis and modeling are presented to quantify the degree to which these features impact broadband (850-1200 Hz) signals propagating along an acoustic transect oriented perpendicular to the internal wave fronts and sand dune crests.

Variability of nonlinear internal waves in the South China Sea affected by the Kuroshio and mesoscale eddies

AbstractLong‐term observations of nonlinear internal waves in the South China Sea reveal seasonal to interannual variability. During two selected segments of inverted echo sounder observations, tidal forcing in Luzon Strait is almost identical, but the observed amplitudes of nonlinear internal waves in the South China Sea are very different. The effects of the Kuroshio and mesoscale eddies, reproduced by HYbrid Cooridnate Ocean Model (HYCOM) reanalysis simulation, are then investigated. The Kuroshio can enhance the zonal tilt of the thermocline and induce intruding flow in Luzon Strait. During the two selected segments, different thermocline slopes did not significantly change the internal tide generation, but the intruding flow may result in a 11% difference in the amplitude of generated M2 internal tides. During the two selected segments, mesoscale eddies appeared on the path of internal wave propagation, a cold eddy in one case and a warm one in the other. The eddies changed local stratification and induced additional background currents, thus affecting the nonlinear evolution of internal tides. In addition, wave front steering due to the mesoscale eddies dramatically affected the observed amplitude changes of the nonlinear internal waves: the edge, rather than the center, of the nonlinear internal wave front passed through the observational stations, resulting in reduced amplitude in the observations.

The role of water column variability on three-dimensional sound propagation in shallow water

Simultaneous measurements of sound field and the sound speed profile perturbed by passage of internal waves in shallow water and the resulting modal evolution in space-time domain is reported. The waveguide modal behavior for broadband signals centered at 100, 200, and 300 Hz with the bandwidth of 60 Hz during the Shallow Water 2006 (SW06) experiment is studied using an L-shaped hydrophone array. Several cases are considered where the angle of the internal wave front with the acoustic track ranges from −8 to + 83 degrees. Mode filtering obtained from the vertical array and tracking the filtered mode along the horizontal array reveals that individual modes propagating between the source and receiver arrive at different angles. Using the method of vertical modes and horizontal rays the phase of the acoustic field is calculated and the change in the angle of arriving ray at the L-shaped array, that is the horizontal angle of refraction, is obtained. A 3D PE model is used for comparison between the calculations and the experimental data. [Work supported by ONR 322OA.]

Coherence measurements of acoustic normal modes during one month of internal wave events on the New Jersey continental shelf

During the Shallow Water Acoustic Experiment 2006 (SW06) conducted on the New Jersey continental shelf, the three-dimensional (3D) temperature field for one month of internal wave (IW) events has been reconstructed by Badiey et al. [J. Acoust. Soc. Am. El. 134(5), 4035 (2013)]. The aforementioned IW events, with the angle between the acoustic track and the IW front varying from -8° to 83°, were measured while simultaneously acoustic signals were transmitted from fixed sources at an along-shelf distance of about 20 km with frequencies at 87.5–112.5 Hz (m-sequence), 175–225 Hz (m-sequence), 270–330 Hz (chirp), and 470–530 Hz (chirp), respectively. The acoustic signals were recorded by an L-shaped hydrophone array moored inside the area with IW measurements. The main goal of this paper is to analyze the coherence of acoustic normal modes accompany with the simultaneously measured IW events. The coherence of acoustic normal modes decomposed from the measured acoustic field is obtained as a function of frequency and IW parameters, such as the IW propagation direction, amplitude, coherence length, etc. The relationship between the modal coherence and IW parameters is discussed. [Work supported by ONR322OA.]

Time-varying characteristics of the waveguide invariant under internal wave condition in the shallow water area

To explain the change of the waveguide invariant β caused by the internal wave, the waveguide invariant is expressed as a stochastic distribution and the time-varying characteristics of β are analyzed; then the cause and principle of variation of β are discussed in terms of the normal modes theory of the acoustic field. It turns out that the waveguide invariant shows great frequency-dependent characteristics. And in the internal wave environment, when the sound is propagated parallel to the internal wave front, the sound field's composition of normal modes remains unchanged, but the turbulence in the group slowness and phase slowness of the main normal modes in the sound field may also change the distribution of β; when the sound is propagated perpendicular to the internal wave front, the coupling between acoustic normal modes might also cause obvious changes in the values of β.

Measured three-dimensional effects of mode-1 and mode-2 nonlinear internal waves on broadband acoustic wave propagation in shallow water

Horizontal shadowing effects and frequency shifts of acoustic intensity level curves were measured with a bottomed horizontal array in the East China Sea during the summer of 2008. Low-frequency acoustic pulses were transmitted by two fixed sources at 33 km (270–330 Hz LFM) and 20km (450–550 Hz LFM) range. Strong shadowing effects were observed when mode-1 nonlinear internal wave fronts were nearly parallel to the acoustic propagation path. Numerical studies indicated that shadowing effects are more complex for mode-2 nonlinear internal waves due to acoustic-mode dependent focusing and defocusing. These effects were further analyzed using 3-D PE simulations for more dynamic mode-2 nonlinear internal waves observed on the US New Jersey Shelf. The shadowing was less pronounced for internal waves with curved wave-fronts and small amplitudes. However, regular and irregular frequency shifts were still present. The experimental observations coupled with 3-D PE simulations suggest that nonlinear internal waves may be sensed and characterized via low-frequency acoustic signals. [Work supported by the Office of Naval Research.]

Time reversal of modal arrivals for broadband signals in horizontally stratified shallow water due to internal waves

Shallow water waveguides in the presence of internal waves can make a time-varying, horizontally stratified medium that can significantly affect broadband pulse propagation. In this paper, we analyze a unique data set obtained during the passage of an internal wave event in a region where a broadband sound source (270 to 330 Hz) was received by an L-shaped hydrophone array about 20 km away in 80 meters on the New Jersey continental shelf. During the time 20:30 and 22:07 GMT, on 17 August 2006, an approaching IW affected the dispersion characteristic of the broadband LFM chirp signal while passing an acoustic track in shallow water waveguide. Modal behavior is examined before and after the internal wave front crossed the source-receiver track. While dispersion characteristics of the signals changed, modal arrival time reversal occurred. The corresponding group and phase velocities that signals experienced during this phenomenon are analyzed using the theory of horizontal rays and vertical modes. These results have motivated theoretical and modeling studies of the waveguide behavior since it was first reported in 2010. [Work supported by ONR322OA.]

Multifocal IOL Implant with or without Capsular Tension Ring: Study of Wavefront Error and Visual Performance

To evaluate visual performance and wavefront error after multifocal intraocular lens (MIOL) implant with or without capsular tension ring (CTR). Sixty eyes of 60 candidates for phacoemulsification and MIOL implantation were included in the study and were randomly divided into 2 groups. Thirty eyes (group 1) were implanted with the AcrySof ReSTOR® SN60D3 MIOL and Corneal® ACPI 11 CTR and 30 eyes (group 2) received the same MIOL with no CTR. Twenty days and at 360 days after surgery, the uncorrected and corrected distance visual acuity at high and low contrast were not significantly different between the 2 groups. The root mean square (RMS) of ocular and internal wavefront error was significantly different in the 2 groups early after surgery and did not modify significantly over the 1-year follow-up period. The RMS of total and internal high-order aberrations and ocular and internal trefoil and coma aberrations were significantly lower in group 1 compared to group 2 (p<0.05). Ocular, corneal, internal modulation transfer function at high and low spatial frequencies and ocular, corneal, internal Strehl ratio were not significantly different between the groups. The IOL decentration and tilt were higher in group 1 compared to group 2. ReSTOR MIOL was effective in improving visual performance and provided a good quality of vision due to a significant reduction of high-order spherical aberration. The implant of CTR additionally reduced the ocular wavefront error related to a reduction of third-order aberration related to better IOL position.

Acoustic frequency shifts due to internal tides and nonlinear internal waves

Significant frequency shifts of acoustic intensity level curves in broadband signal spectrograms were measured in the East China Sea during the summer of 2008. Broadband pulses at 270-330 Hz were transmitted from a fixed source and received at a bottomed horizontal array, located at 33 km range. The acoustic intensity level curves of the received signals indicate regular frequency shifts that are well correlated with the measured internal tides and nonlinear internal waves. Regular frequency shifts due to nonlinear internal waves are observed only when their wave-fronts are nearly parallel to the acoustic propagation path, causing an effective change in the sound speed profile. Similar effects were observed in 3-D numerical simulation results when curved nonlinear internal wave fronts are used. These observations and simulations indicate the potential of monitoring internal tides and nonlinear internal waves using low-frequency acoustic signals when the acoustic source and receiver are strategically placed. [Work supported by the Office of Naval Research.]