Jin Wu

Bio: Jin Wu is an academic researcher from University of Delaware. The author has contributed to research in topics: Wind wave & Wind speed. The author has an hindex of 19, co-authored 41 publications receiving 2160 citations. Previous affiliations of Jin Wu include National Cheng Kung University.

Wind-Stress coefficients over Sea surface near Neutral Conditions—A Revisit

Abstract: A scaling law of wind-stress coefficients is presented to illustrate explicitly that the coefficient increases with wind velocity and decreases with fetch; physical reasonings of both trends are discussed. Besides being shown previously to be related to a criterion determining airflow separation from waves, the Charnock relation is further associated with the critical roughness Reynolds number identifying regimes of the atmospheric surface layer. Intrinsic errors and limitations of the Charnock relation, which provides an overall correlation between stress coefficient and wind velocity, are illustrated. A probable nondimensional expression, a refinement of the Charnock relation, is proposed between the roughness length and the wind-friction velocity involving not only gravity but also surface tension and viscosity. Previous compilation of wind-stress data obtained with eddy-correlation and wind-profile methods is found to be consistent with recent results obtained with similar techniques. A singl...

Wind-induced drift currents

Abstract: Systematic measurements of drift currents below and of airflows above an air-water interface have been made under various wind conditions. The current near but not immediately below the water surface is found to follow a Karman-Prandtl (logarithmic) velocity distribution. The current immediately below the water surface varies linearly with depth. The transitions of the current boundary layer to various regimes appear to lag behind, or to occur a t a higher wind velocity than, those of the airflow. The fraction of the wind stress supported by the wave drag seems to vary with the wind and wave conditions: a large fraction is obtained at low wind velocities with shorter waves and a small fraction is obtained a t high wind velocities with longer waves. At the air-water interface, the wind-induced current is found to be proportional to the friction velocity of the wind. The Stokes mass transport, related to wave characteristics, is only a small component of the surface drift in laboratory tanks. However, in terms of the fraction of the wind velocity, the mass transport increases, while the wind drift decreases, as the fetch increases. The ratio between the total surface drift and the wind velocity decreases gradually as the fetch increases and approaches a constant value of about 3·5% at very long fetches.

Wind stress and surface roughness at air‐sea interface

Abstract: Based on the compiled data of thirty independent oceanic observations, this article systematically presents the wind-stress coefficient, the surface roughness, and the boundary layer flow regime at the air-sea interface under various wind conditions. The air flow near the water surface is shown to be aerodynamically rough or in the transition region except at a very low wind velocity (U10 < 3 m/sec). Both the wind-stress coefficient and the surface roughness are found to increase with the wind velocity when U10 is less than 15 m/sec and to reach a saturated value for U10 greater than 15 m/sec. From the oceanic wave observations, the presence of this discontinuity at U10 = 15 m/sec is found to be due to an increase in the wind velocity (measured at the significant wave amplitude above the mean water level) beyond the average wave phase velocity. This finding provides a well-defined separation for the often quoted terms ‘light’ and ‘strong’ winds. The compiled data also show that the surface roughness is governed by the amplitude of the short gravity waves. Charnock's relationship is shown to be applicable to most of the oceanic data and Charnock's proportionality constant is determined, η/(u*2/g) = 0.0156. Finally, two approximate formulas for the windstress coefficient C10 = 0.5 × U101/2 × 10−3 for light wind and C10 = 2.6 × 10−3 for strong wind are suggested for oceanic applications.

Oceanic Whitecaps and Sea State

Abstract: Results of white-cap coverages of the ocean surface obtained by previous investigators in both the Atlantic Ocean and the Pacific Ocean are reanalyzed. The variation of coverage with wind velocity appears to be related to the rate of energy supplied by the wind. The coverage is also found to vary with stability conditions of the atmospheric surface layer. Empirical formulas are deduced for various sea states and stability conditions, and application of these formulas to remote sensing of marine wind velocity is discussed.

Variations of whitecap coverage with wind stress and water temperature

Abstract: Results of whitecap coverages from five previously reported oceanic experiments by Monahan and coinvestigators have been analyzed; the fractional coverage of the sea surface(W) was shown to follow a power law, W = 2U3.7510, where U10 is the wind velocity at 10 m above the mean sea surface. Attempts were then made to associate the coverage with the wind-friction velocity (u*) deduced from the wind velocity and air–sea temperature differences as W = 0.2u3*. Finally, we discuss effects of the water temperature on the coverage.

Physics of liquid jets

Abstract: Jets, ie collimated streams of matter, occur from the microscale up to the large-scale structure of the universe Our focus will be mostly on surface tension effects, which result from the cohesive properties of liquids Paradoxically, cohesive forces promote the breakup of jets, widely encountered in nature, technology and basic science, for example in nuclear fission, DNA sampling, medical diagnostics, sprays, agricultural irrigation and jet engine technology Liquid jets thus serve as a paradigm for free-surface motion, hydrodynamic instability and singularity formation leading to drop breakup In addition to their practical usefulness, jets are an ideal probe for liquid properties, such as surface tension, viscosity or non-Newtonian rheology They also arise from the last but one topology change of liquid masses bursting into sprays Jet dynamics are sensitive to the turbulent or thermal excitation of the fluid, as well as to the surrounding gas or fluid medium The aim of this review is to provide a unified description of the fundamental and the technological aspects of these subjects

Coefficients for sea surface wind stress, heat flux, and wind profiles as a function of wind speed and temperature

Abstract: Surface layer coefficients for wind profiles, wind stress, and heat flux in typical open sea conditions are briefly reviewed. Businger-Dyer flux-gradient relationships and a Charnock wind stress formula fit the empirical data and are dimensionally consistent. These have been solved by an iterative method, and the results are presented in a tabular form suitable for climatological calculations from marine wind and temperature data.

Directional spectra of wind-generated waves

Abstract: From observations of wind and of water surface elevation at 14 wave staffs in an array in Lake Ontario and in a large laboratory tank, the directional spectrum of wind-generated waves on deep water is determined by using a modification of Barber's (1963) method. Systematic investigations reveal the following: (a) the frequency spectrum in the rear face is inversely proportional to the fourth power of the frequency $\omega $, with the equilibrium range parameter and the peak enhancement factor clearly dependent on the ratio of wind speed to peak wave speed; (b) the angular spreading $\theta $ of the wave energy is of the form sech$^{2}$ ($\beta \theta $), where $\beta $ is a function of frequency relative to the peak; (c) depending on the gradient of the fetch, the direction of the waves at the spectral peak may differ from the mean wind direction by up to 50 degrees, but this observed difference is predictable by a similarity analysis; (d) under conditions of strong wind forcing, significant effects on the phase velocity caused by amplitude dispersion and the presence of bound harmonics are clearly observed and are in accordance with the Stokes theory, whereas (e) the waves under natural wind conditions show amplitude dispersion, but bound harmonics are too weak to be detected among the background of free waves.

Spectral and statistical properties of the equilibrium range in wind-generated gravity waves

Abstract: Recent measurements of wave spectra and observations by remote sensing of the sea surface indicate that the author's (1958) conception of an upper-limit asymptote to the spectrum, independent of wind stress, is no longer tenable. The nature of the equilibrium range is reexamined, using the dynamical insights into wave–wave interactions, energy input from the wind and wave-breaking that have been developed since 1960. With the assumption that all three of these processes are important in the equilibrium range, the wavenumber spectrum is found to be of the form , where p ∼ ½ and the frequency spectrum is proportional to u*gσ−4. These forms have been found by Kitaigorodskii (1983) on a quite different dynamical basis; the latter is consistent with the form found empirically by Toba (1973) and later workers. Various derived spectra, such as those of the sea-surface slope and of an instantaneous line traverse of the surface, are also given, as well as directional frequency spectra and frequency spectra of slope.The theory also provides expressions for the spectral rates of action, energy and momentum loss from the equilibrium range by wave-breaking and for the spectrally integrated rates across the whole range. These indicate that, as a wave field develops with increasing fetch or duration, the momentum flux to the underlying water by wave-breaking increases asymptotically to a large fraction of the total wind stress and that the energy flux to turbulence in the water, occurring over a wide range of scales, increases logarithmically as the extent of the equilibrium range increases. Interrelationships are pointed out among different sets of measurements such as the various spectral levels, the directional distributions, the total mean-square slope and the ratio of downwind to crosswind mean-square slopes.Finally, some statistical characteristics of the breaking events are deduced, including the expected length of breaking fronts (per unit surface area) with speeds of advance between c and c+dc and the number of such breaking events passing a given point per unit time. These then lead to simple expressions for the density of whitecapping, those breaking events that produce bubbles and trails of foam, the total number of whitecaps passing a given point per unit time and, more tenuously, the whitecap coverage.

A new phylum of Archaea represented by a nanosized hyperthermophilic symbiont

Abstract: According to small subunit ribosomal RNA (ss rRNA) sequence comparisons all known Archaea belong to the phyla Crenarchaeota, Euryarchaeota, and--indicated only by environmental DNA sequences--to the 'Korarchaeota'. Here we report the cultivation of a new nanosized hyperthermophilic archaeon from a submarine hot vent. This archaeon cannot be attached to one of these groups and therefore must represent an unknown phylum which we name 'Nanoarchaeota' and species, which we name 'Nanoarchaeum equitans'. Cells of 'N. equitans' are spherical, and only about 400 nm in diameter. They grow attached to the surface of a specific archaeal host, a new member of the genus Ignicoccus. The distribution of the 'Nanoarchaeota' is so far unknown. Owing to their unusual ss rRNA sequence, members remained undetectable by commonly used ecological studies based on the polymerase chain reaction. 'N. equitans' harbours the smallest archaeal genome; it is only 0.5 megabases in size. This organism will provide insight into the evolution of thermophily, of tiny genomes and of interspecies communication.