Progressive short waves with a narrow frequency band are known to be accompanied by long set-down... more Progressive short waves with a narrow frequency band are known to be accompanied by long set-down waves travelling with the groups. In finite depth, scattering of short waves by a large structure or a varying coastline can radiate free long waves which propagate faster than the incident set-down. I n a partially enclosed harbour attacked by short waves through the entrance, such free long waves can further resonate the natural modes of the harbour basin. In this paper an asymptotic theory is presented for a harbour whose horizontal dimensions are much greater than the entrance width, which is in turn much wider than the short wavelength.
Fractal solutions of the inviscid water-wave problem are presented. For gravity waves (neglecting... more Fractal solutions of the inviscid water-wave problem are presented. For gravity waves (neglecting surface tension) free surfaces with fractal dimensions 2¼ and 2 ! 3 are obtained. For capillary waves (neglecting gravity), subfractal free surfaces with dimension 2 are shown to exist. However, the situation is reversed if one considers time series of the surface elevation taken at a fixed point. In this case the capillary wave solution produces graphs with dimension 1~, whereas the graph for gravity waves has dimension 1.
The probability of freak waves in an inhomogeneous ocean is studied by integration of Alber's equ... more The probability of freak waves in an inhomogeneous ocean is studied by integration of Alber's equation. The special phase structure of the inhomogeneous disturbance, required for instability, is provided by bound waves, generated by the quadratic interaction of the stochastic sea with a deterministic, long swell. The probability of freak waves higher than twice the significant wave height increases by a factor of up to 20 compared to the classical value given by Rayleigh's distribution. The probability of exceptionally high freak waves, with height larger than three times the significant wave height, is shown to increase some 30 000-fold compared to that given by the Rayleigh distribution, which renders their encounter feasible.
In Zakharov's equation, the spectral function represents the entire horizontal plane. In practica... more In Zakharov's equation, the spectral function represents the entire horizontal plane. In practical applications, one often has to use a finite number of Fourier modes that are determined for limited regions of the horizontal plane, but vary from region to region.
Abstract—The feasibility for speed of sound dispersion (SOSD) imaging was investigated here. A th... more Abstract—The feasibility for speed of sound dispersion (SOSD) imaging was investigated here. A through transmission new method for measuring the SOSD was utilized. With this method a long pulse comprising of two frequencies one being the double of the other is transmitted through the object and detected on its other side. SOSD projection images were obtained by scanning objects immersed
Mass transfer in an oscillatory electro-osmotic flow (EOF) is theoretically studied, for the case... more Mass transfer in an oscillatory electro-osmotic flow (EOF) is theoretically studied, for the case of a cylindrical tube with a reactive wall. An expression for the dispersion coefficient, reflecting the time-averaged mass flux of an electrically neutral solute, is derived analytically. Under the influence of a reversible solute-wall mass exchange, the dispersion coefficient exhibits a complex dependence on the various parameters representing the effects of the electric double-layer thickness, oscillation frequency, solution transport properties, solute partitioning, and reaction kinetics. Our results suggest that, in the presence of a reversible mass exchange at the wall, an oscillatory EOF may be used for separation of species. It is found that optimal conditions for separation are achieved for a thin double-layer, where an inert solute, or one with slow exchange kinetics, experiences virtually no dispersion while the dispersion is maximized for the reactive solute exhibiting fast kinetics.
We introduce a method for measuring the speed of sound dispersion. It combines a short pulse tran... more We introduce a method for measuring the speed of sound dispersion. It combines a short pulse transmission followed by a long burst comprised of two frequencies, one being double that of the other. The method allows the determination of the speed of sound dispersion using a single transmission. To validate the method, the dispersion was first measured in plastic samples
Marine aerosols produced by sea-waves and associated winds are a potentially important climate fo... more Marine aerosols produced by sea-waves and associated winds are a potentially important climate forcing factor. Acting as efficient cloud condensation nuclei, marine sea-salt aerosols (SSA) could affect cloud formation and precipitation processes. In spite of the importance of impact of SSA variations on climate change, there are no regular sea-salt aerosol measurements in the open sea, where SSA are mainly produced and where their effects on climate are maximal. In order to partly fill the gap in our understanding of the SSA processes, model-based daily forecasts of three-dimensional distribution of SSA could be helpful, providing valuable information about space and time distribution of these types of aerosol. Model performance over the open sea has been verified in this study by comparing modeled SSA concentrations with observed wave height. Two sea-wave monitoring buoys, located in the Eastern Mediterranean near Haifa and Ashdod, provided us with information about wave height dur...
Fractal solutions of the inviscid water-wave problem are presented. For gravity waves (neglecting... more Fractal solutions of the inviscid water-wave problem are presented. For gravity waves (neglecting surface tension) free surfaces with fractal dimensions 2¼ and 2 ! 3 are obtained. For capillary waves (neglecting gravity), subfractal free surfaces with dimension 2 are shown to exist. However, the situation is reversed if one considers time series of the surface elevation taken at a fixed point. In this case the capillary wave solution produces graphs with dimension 1~, whereas the graph for gravity waves has dimension 1.
In this paper we modify and investigate a nonparametric, nearest-neighbor forecasting method. We ... more In this paper we modify and investigate a nonparametric, nearest-neighbor forecasting method. We compare a minimal ‘volume’ simplex method, constructed out of the E+1 E-dimensional points, with the E+1 nearest points method. We then show nonlinearities in some precious metals commodities.
A new compatibility condition, applied to the velocity of the free surface particles, enables the... more A new compatibility condition, applied to the velocity of the free surface particles, enables the simulation of plunging breakers, up to the creation of a thin jet, without numerical smoothing. An improved formulation of a boundary element method for the simulation of surface waves problems is derived and tested through some numerical examples. A numerical wave channel with a wavemaker and an energy absorber is developed and used for a comparison of the non-linear solution with first and second order steady solutions in the frequency domain. Finally, the shoaling of with published experimental results.
The probability of freak waves in an inhomogeneous ocean is studied by integration of Alber's equ... more The probability of freak waves in an inhomogeneous ocean is studied by integration of Alber's equation. The special phase structure of the inhomogeneous disturbance, required for instability, is provided by bound waves, generated by the quadratic interaction of the stochastic sea with a deterministic, long swell. The probability of freak waves higher than twice the significant wave height increases by a factor of up to 20 compared to the classical value given by Rayleigh's distribution. The probability of exceptionally high freak waves, with height larger than three times the significant wave height, is shown to increase some 30 000-fold compared to that given by the Rayleigh distribution, which renders their encounter feasible.
ABSTRACT A solution of Rayleigh's instability equation, which circumvents the apparent cr... more ABSTRACT A solution of Rayleigh's instability equation, which circumvents the apparent critical-layer singularity, is provided. The temporal and spatial growth rates of water waves exposed to a logarithmic wind profile are calculated and discussed. The findings are similar to previously published results, except for shear velocity-to-wave celerity ratios larger than 2, where the newly calculated growth rates start to decrease after having reached a distinct maximum. The ratio of the spatial to temporal growth rates is examined. It is shown to deviate by up to 20% from the leading-order value of 2. The implications of the growth rate to the modal distributions of energy input from wind to waves, for young and mature seas, and in temporal/spatial growth scenarios, are analyzed.
ABSTRACT Recent studies have provided preliminary indication that an osmotic-induced backwash of ... more ABSTRACT Recent studies have provided preliminary indication that an osmotic-induced backwash of RO membranes may offer an innovative, effective and potentially chemical-free cleaning method. In this paper, we present numerical simulations of the two-dimensional, transient concentration field during an osmotic backwash event. Presented results illustrate the dynamics of the de-polarization process, permeation rate and characteristic time-scales, as influenced by various transport mechanisms present. Different possible configurations for inducing the osmotic backwash are considered, illustrating possible advantages and shortcomings. For a backwash cycle initiated by reduction of the trans-membrane pressure, it is shown that during short times, the backwash process is only weakly affected by the presence of a crossflow velocity, whereas it is this axial advection mechanism which strongly influences the permeation rate at longer times. For an osmotic backwash induced by injection of a high concentration ‘draw’ solution, it is shown that the pulse duration should be longer than the residence time for a maximum achievable cycle-averaged permeation rate. A shorter pulse is significantly diluted, particularly on the membrane surface, to the point where its concentration may drop below that required for inducing osmotic flow. Consequently, the pulse concentration and duration must be carefully optimized if efficient osmotic cleaning is to be achieved throughout the full length of a membrane train.
A two-dimensional, boundary layer model is presented, for describing the heat transfer in the fee... more A two-dimensional, boundary layer model is presented, for describing the heat transfer in the feed channel of a vacuum membrane distillation (VMD) module. The model formulation allows for variations of viscosity with temperature, and introduces an 'effective' slip coefficient so as to account for the possible deviation of the flow and heat transfer characteristics over a liquid-gas interface, from those at a solid surface. The model solution provides the temperature field in the feed channel and its dependence on the bulk velocity and temperature, as well as the vapor mass flux across the membrane. While the effect of a temperature variable viscosity becomes increasingly apparent at higher temperatures, its effect on the evaporation mass flux is not substantial. The relative contribution of transverse convection to the evaporation mass flux is shown to be insignificant, within the range of parameters examined. In the presence of an 'effective' velocity slip, the degree of temperature polarization is reduced and a corresponding increase in the evaporation mass flux is observed. Calculated results are in good agreement with experimental data. The presented results suggest that the effect of alternating solid and gas interfaces, encountered by the fluid, on the flow field and resulting heat transfer, should be accounted for if accurate predictions are to be made.
Progressive short waves with a narrow frequency band are known to be accompanied by long set-down... more Progressive short waves with a narrow frequency band are known to be accompanied by long set-down waves travelling with the groups. In finite depth, scattering of short waves by a large structure or a varying coastline can radiate free long waves which propagate faster than the incident set-down. I n a partially enclosed harbour attacked by short waves through the entrance, such free long waves can further resonate the natural modes of the harbour basin. In this paper an asymptotic theory is presented for a harbour whose horizontal dimensions are much greater than the entrance width, which is in turn much wider than the short wavelength.
Fractal solutions of the inviscid water-wave problem are presented. For gravity waves (neglecting... more Fractal solutions of the inviscid water-wave problem are presented. For gravity waves (neglecting surface tension) free surfaces with fractal dimensions 2¼ and 2 ! 3 are obtained. For capillary waves (neglecting gravity), subfractal free surfaces with dimension 2 are shown to exist. However, the situation is reversed if one considers time series of the surface elevation taken at a fixed point. In this case the capillary wave solution produces graphs with dimension 1~, whereas the graph for gravity waves has dimension 1.
The probability of freak waves in an inhomogeneous ocean is studied by integration of Alber's equ... more The probability of freak waves in an inhomogeneous ocean is studied by integration of Alber's equation. The special phase structure of the inhomogeneous disturbance, required for instability, is provided by bound waves, generated by the quadratic interaction of the stochastic sea with a deterministic, long swell. The probability of freak waves higher than twice the significant wave height increases by a factor of up to 20 compared to the classical value given by Rayleigh's distribution. The probability of exceptionally high freak waves, with height larger than three times the significant wave height, is shown to increase some 30 000-fold compared to that given by the Rayleigh distribution, which renders their encounter feasible.
In Zakharov's equation, the spectral function represents the entire horizontal plane. In practica... more In Zakharov's equation, the spectral function represents the entire horizontal plane. In practical applications, one often has to use a finite number of Fourier modes that are determined for limited regions of the horizontal plane, but vary from region to region.
Abstract—The feasibility for speed of sound dispersion (SOSD) imaging was investigated here. A th... more Abstract—The feasibility for speed of sound dispersion (SOSD) imaging was investigated here. A through transmission new method for measuring the SOSD was utilized. With this method a long pulse comprising of two frequencies one being the double of the other is transmitted through the object and detected on its other side. SOSD projection images were obtained by scanning objects immersed
Mass transfer in an oscillatory electro-osmotic flow (EOF) is theoretically studied, for the case... more Mass transfer in an oscillatory electro-osmotic flow (EOF) is theoretically studied, for the case of a cylindrical tube with a reactive wall. An expression for the dispersion coefficient, reflecting the time-averaged mass flux of an electrically neutral solute, is derived analytically. Under the influence of a reversible solute-wall mass exchange, the dispersion coefficient exhibits a complex dependence on the various parameters representing the effects of the electric double-layer thickness, oscillation frequency, solution transport properties, solute partitioning, and reaction kinetics. Our results suggest that, in the presence of a reversible mass exchange at the wall, an oscillatory EOF may be used for separation of species. It is found that optimal conditions for separation are achieved for a thin double-layer, where an inert solute, or one with slow exchange kinetics, experiences virtually no dispersion while the dispersion is maximized for the reactive solute exhibiting fast kinetics.
We introduce a method for measuring the speed of sound dispersion. It combines a short pulse tran... more We introduce a method for measuring the speed of sound dispersion. It combines a short pulse transmission followed by a long burst comprised of two frequencies, one being double that of the other. The method allows the determination of the speed of sound dispersion using a single transmission. To validate the method, the dispersion was first measured in plastic samples
Marine aerosols produced by sea-waves and associated winds are a potentially important climate fo... more Marine aerosols produced by sea-waves and associated winds are a potentially important climate forcing factor. Acting as efficient cloud condensation nuclei, marine sea-salt aerosols (SSA) could affect cloud formation and precipitation processes. In spite of the importance of impact of SSA variations on climate change, there are no regular sea-salt aerosol measurements in the open sea, where SSA are mainly produced and where their effects on climate are maximal. In order to partly fill the gap in our understanding of the SSA processes, model-based daily forecasts of three-dimensional distribution of SSA could be helpful, providing valuable information about space and time distribution of these types of aerosol. Model performance over the open sea has been verified in this study by comparing modeled SSA concentrations with observed wave height. Two sea-wave monitoring buoys, located in the Eastern Mediterranean near Haifa and Ashdod, provided us with information about wave height dur...
Fractal solutions of the inviscid water-wave problem are presented. For gravity waves (neglecting... more Fractal solutions of the inviscid water-wave problem are presented. For gravity waves (neglecting surface tension) free surfaces with fractal dimensions 2¼ and 2 ! 3 are obtained. For capillary waves (neglecting gravity), subfractal free surfaces with dimension 2 are shown to exist. However, the situation is reversed if one considers time series of the surface elevation taken at a fixed point. In this case the capillary wave solution produces graphs with dimension 1~, whereas the graph for gravity waves has dimension 1.
In this paper we modify and investigate a nonparametric, nearest-neighbor forecasting method. We ... more In this paper we modify and investigate a nonparametric, nearest-neighbor forecasting method. We compare a minimal ‘volume’ simplex method, constructed out of the E+1 E-dimensional points, with the E+1 nearest points method. We then show nonlinearities in some precious metals commodities.
A new compatibility condition, applied to the velocity of the free surface particles, enables the... more A new compatibility condition, applied to the velocity of the free surface particles, enables the simulation of plunging breakers, up to the creation of a thin jet, without numerical smoothing. An improved formulation of a boundary element method for the simulation of surface waves problems is derived and tested through some numerical examples. A numerical wave channel with a wavemaker and an energy absorber is developed and used for a comparison of the non-linear solution with first and second order steady solutions in the frequency domain. Finally, the shoaling of with published experimental results.
The probability of freak waves in an inhomogeneous ocean is studied by integration of Alber's equ... more The probability of freak waves in an inhomogeneous ocean is studied by integration of Alber's equation. The special phase structure of the inhomogeneous disturbance, required for instability, is provided by bound waves, generated by the quadratic interaction of the stochastic sea with a deterministic, long swell. The probability of freak waves higher than twice the significant wave height increases by a factor of up to 20 compared to the classical value given by Rayleigh's distribution. The probability of exceptionally high freak waves, with height larger than three times the significant wave height, is shown to increase some 30 000-fold compared to that given by the Rayleigh distribution, which renders their encounter feasible.
ABSTRACT A solution of Rayleigh's instability equation, which circumvents the apparent cr... more ABSTRACT A solution of Rayleigh's instability equation, which circumvents the apparent critical-layer singularity, is provided. The temporal and spatial growth rates of water waves exposed to a logarithmic wind profile are calculated and discussed. The findings are similar to previously published results, except for shear velocity-to-wave celerity ratios larger than 2, where the newly calculated growth rates start to decrease after having reached a distinct maximum. The ratio of the spatial to temporal growth rates is examined. It is shown to deviate by up to 20% from the leading-order value of 2. The implications of the growth rate to the modal distributions of energy input from wind to waves, for young and mature seas, and in temporal/spatial growth scenarios, are analyzed.
ABSTRACT Recent studies have provided preliminary indication that an osmotic-induced backwash of ... more ABSTRACT Recent studies have provided preliminary indication that an osmotic-induced backwash of RO membranes may offer an innovative, effective and potentially chemical-free cleaning method. In this paper, we present numerical simulations of the two-dimensional, transient concentration field during an osmotic backwash event. Presented results illustrate the dynamics of the de-polarization process, permeation rate and characteristic time-scales, as influenced by various transport mechanisms present. Different possible configurations for inducing the osmotic backwash are considered, illustrating possible advantages and shortcomings. For a backwash cycle initiated by reduction of the trans-membrane pressure, it is shown that during short times, the backwash process is only weakly affected by the presence of a crossflow velocity, whereas it is this axial advection mechanism which strongly influences the permeation rate at longer times. For an osmotic backwash induced by injection of a high concentration ‘draw’ solution, it is shown that the pulse duration should be longer than the residence time for a maximum achievable cycle-averaged permeation rate. A shorter pulse is significantly diluted, particularly on the membrane surface, to the point where its concentration may drop below that required for inducing osmotic flow. Consequently, the pulse concentration and duration must be carefully optimized if efficient osmotic cleaning is to be achieved throughout the full length of a membrane train.
A two-dimensional, boundary layer model is presented, for describing the heat transfer in the fee... more A two-dimensional, boundary layer model is presented, for describing the heat transfer in the feed channel of a vacuum membrane distillation (VMD) module. The model formulation allows for variations of viscosity with temperature, and introduces an 'effective' slip coefficient so as to account for the possible deviation of the flow and heat transfer characteristics over a liquid-gas interface, from those at a solid surface. The model solution provides the temperature field in the feed channel and its dependence on the bulk velocity and temperature, as well as the vapor mass flux across the membrane. While the effect of a temperature variable viscosity becomes increasingly apparent at higher temperatures, its effect on the evaporation mass flux is not substantial. The relative contribution of transverse convection to the evaporation mass flux is shown to be insignificant, within the range of parameters examined. In the presence of an 'effective' velocity slip, the degree of temperature polarization is reduced and a corresponding increase in the evaporation mass flux is observed. Calculated results are in good agreement with experimental data. The presented results suggest that the effect of alternating solid and gas interfaces, encountered by the fluid, on the flow field and resulting heat transfer, should be accounted for if accurate predictions are to be made.
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Papers by Yehuda Agnon