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1.
In stably stratified media with a Prandtl number Pr ≫ 1, vertical scales of the density (ℓ) and horizontal velocity variation (L) are quite different, ℓ/L = O(Pr−1/2) ≪ 1, and this influences the flow stability. In particular, shear flows without inflection points on the velocity profile are unstable even in an ideal incompressible fluid. The maximum instability growth rate for sufficiently small ℓ/L is of the same order as in homogeneous mixing layers, with mainly three-dimensional rather than two-dimensional oscillations increasing in a wide range of parameters. This paper focuses on the three-dimensional instability of such flows. It is shown that the spectrum of unstable oscillations is essentially anisotropic in the case of a relatively weak stratification when the bulk Richardson number J ≤ O[(ℓ/L)3/2]. The results of the asymptotic analysis are illustrated by calculations for a model flow in a two-layer medium (ℓ = 0) as well as for flows with values of ℓ/L corresponding to a temperature or salinity stratification of the water.  相似文献   

2.
A stability of planetary waves on an infinite beta-plane is investigated in an idealized two-layer fluid system for the large local Rossby numberM. When a primary wave is barotropic, two kinds of barotropic instability modes are found. One of them was previously discussed byGill (1974). When a primary wave is baroclinic, two different kinds of modes that enable barotropic and baroclinic energy transfers are found. The one that has the larger growth rate gains its energy mainly from the mean shear of the primary wave when the internal rotational Froude numberF is smaller than 1/2. WhenF is larger than 1/2, however, the available energy conversion of the primary wave is dominant. This mode has a fairly large part of its energy in the barotropic motion although the primary wave is purely baroclinic.The effect ofO(M –2) corrections is found to have a stabilizing influence on all symmetrical modes. The geophysical applications of the present analysis are suggested in the context.  相似文献   

3.
Studies of mixing were done at the northern flank of Georges Bank in the summer and autumn of 1988. Two time-series of the evolution and intensity of microstructure were examined over a tidal period in the context of tidal forcing and the evolution of the density and velocity field at the site. From the CTD, ADCP and microstructure observations (EPSONDE) on Georges Bank, several interesting features of the mixing processes were found. High dissipation and diffusivity regions appear near the bottom of the Bank. Turbulence near the bottom is highest in intensity and reaches farthest from the bottom at peak tidal flow and diminishes in intensity and vertical extent as the flow decreases. The thickness of the bottom turbulent layer has its maximum value when the flow is strongest and the stratification is weakest. Characterization of the dissipation rate and turbulent diffusivities in respect to buoyancy frequency N, current shear S, Richardson Number Ri and ε/νN2 was done. Dissipation and χT showed little dependence on shear or N2 but decreased at larger Ri. χt was found to be higher in regions of higher N2 and increased as ε/νN2 increased. KT, K and Kν, were all highest near the bottom in excess of 10−2m2s−1 and decreased towards the surface. There was little suggestion of a dependence of mixing efficiency on S2, Ri or ε/νN2, but some indication that Γ decreases with decreasing N2.  相似文献   

4.
At present, the barotropic buoyant stability parameter has been derived from a vertical virtual displacement of a water parcel. The barotropic inertial stability parameter in the eccentrically cyclogeostrophic, basic current field was derived in 2003 from a horizontal cross-stream virtual displacement of a parcel. By expressing acceleration of a parcel due to a virtual displacement, which is arbitrarily sloping within a vertical section across the basic current, in terms of natural coordinates, we derived the vertical component of baroclinic buoyant stability parameter B 2 2, the horizontal component of baroclinic inertial stability parameter I 2 2, the baroclinic joint stability parameter J 2, its buoyant component B 2 and its inertial component I 2. B 2 is far greater than I 2 2, and when neglecting relative vorticity except for vertical shear, a downward convex curve of J 2 plotted against the slope of a virtual displacement follows a trend of B 2 curve. If a parcel displaces along a horizontal surface or an isopycnal surface, however, B 2 vanishes, and J 2 becomes equal to I 2. Actual parcel is apt to displace not only along the bottom slope, but also along the sea surface and an isopycnal interfacial surface, which is approximately equivalent to an isentropic surface, preferred by lateral mixing and exchange of momentum. Such actual displacement makes B 2 vanishing, and grants I 2 an important role. The present analysis of I 2 examining effects due to curvature and horizontal and vertical shear vorticities are useful in deepening our understanding of baroclinic instability in actual oceanic streams.  相似文献   

5.
The mechanism of the formation of horizontal temperature/density gradients above underwater coastal slopes of natural basins due to heating/cooling from the surface is considered. It is shown that the time required for formation of the gradients is rather small (tens of minutes for a thermocline depth of tens of meters), but the development of the corresponding flows may not be accomplished even in a day long cycle. The time dependence of the horizontal water exchange between the shallow and deep areas is analytically treated. The spatial scale of the problem is the main parameter that defines the resulting quasi-stationary value of the flow rate. The joint analysis of the published field, laboratory, and numerical data of many authors in the range of the above-slope depths of 10−2 m < d < 3 × 102 m (d ≤ D, where D is the thickness of the upper thermally active layer of a basin) indicates that the relation between the value of the horizontal quasi-stationary volumetric flow rate and the local depth looks like Q = 0.00l3 × d 1.37 (R 2 = 0.96). The horizontal convective water exchange is shown to be generally two-layered, ageostrophic, and exhibits its maximum flow rate at the end of the slope. The inferences agree well with the field data and conclusions of other authors.  相似文献   

6.
The dynamic instability of laminated sandwich plates subjected to in-plane partial edge loading is studied for the first time using an efficient finite element plate model. The plate model is based on a refined higher order shear deformation plate theory, where the transverse shear stresses are continuous at the layer interfaces with stress free conditions at plate top and bottom surfaces. Interestingly the plate model having all these refined features requires unknowns at the reference plane only. However, this theory requires C1 continuity of transverse displacement, which is difficult to satisfy arbitrarily in any existing finite element. To deal with this, a new triangular element developed by the authors is used in the present paper.  相似文献   

7.
李博  王鹏皓  卢军炯  王寇 《海洋与湖沼》2019,50(6):1161-1167
本文基于2016年长江口海域(舟山绿华山)的连续观测数据,分析了该海域水文环境要素分布,并估算了溶解氧跨跃层垂向输运。本次观测发现,水体的温度、盐度和溶解氧剖面具有明显的分层结构,三者跃层具有相同的变化趋势且主要受潮流变化影响。通过计算,得到跃层浮性频率的平方(N2)在10–3—10–2/s2之间,剪切的平方(S2)介于10–5—10–2/s2。潮流活动会激发水体剪切不稳定促进水体中溶解氧的垂向交换。最后,采用简化的一维溶解氧垂向输运模型,得到观测周期内跨跃层输运的溶解氧的含量为4.75mmol/(m2·d),佐证层化是长江口海域出现氧亏损现象的主要原因之一。  相似文献   

8.
Using natural coordinates, we have derived a criterion for the inertial instability of arbitrarily meandering currents. Such currents, governed by the eccentrically cyclogeostrophic equation, are adopted as the basic current field for the parcel method. We assume that any virtual displacement which is given to a water parcel moving in the basic field has no influence on this field. From the conservation of mechanical energy for a virtual displacement we derive an inertial instability frequency ω m = [(f + 2u/r)Z]0.5 for the eccentrically cyclogeostrophic current, where f is the Coriolis parameter, u the velocity (always positive), r the radius of curvature of a streamline (negative for an anticyclonic meander), and Z the vertical component of absolute vorticity. If ω m 2 is negative, the eccentrically cyclogeostrophic current becomes unstable. Although the conventional, centrifugal instability criterion, derived from the conservation of angular momentum in a circularly symmetric current field, has a certain meaning for a monopolar vortex, it contains a radial shear vorticity that is difficult to use in arbitrarily meandering currents. The new criterion ω m 2 contains a lateral shear vorticity that is applicable to arbitrarily meandering currents. Examining instabilities of concentric rings with radii of 50–100 km, we consider reasons why the anticyclonic supersolid rotation has been very much less frequently observed than the cyclonic supersolid rotation, despite a prediction of some common stability and a rapid change in radial velocity gradient for the former. Classifying eccentric streamlines into the large and small curvature-gradient types, we point out that the large-gradient curvature in anticyclonic rings is apt to be unstable. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

9.
The saturated moist air with temperature T, pressure P and specific humidity q may be considered as an equivalent dry model air with temperature T* and pressure P under the condition of constant enthalpy. The features of the equivalent dry model air are described and the diagnostic analyses for the summer monsoon over Eastern Asia are examined. Results show that there are obvious baroclinicity features of equivalent temperature, i. e., wet baroclinicity, with a strong gradient belt of equivalent temperature for a time scale mean about 30 days in summer monsoon region and it is in accord with the confluence belt of the warmer air and colder air expressed by equivalent temperature; the mean updraft was observed at its warmer part of that region and downdraft at its colder part; the vertical shear of horizontal wind vector between 150 hPa and 850 hPa is related to the mass field expressed by mean equivalent temperature in that layer.These seem to indicate that the wet baroclinicity in summer monsoon region  相似文献   

10.
The strength of the vertical mixing in the bottom mixed layer near the continental shelf break in the East China Sea was directly measured with the Micro-Scale Profiler (MSP). It has been shown that there is no significant statistical relation between the turbulent energy dissipation and the degree of the stratificationN 2. It seems that the vigorous turbulence occurs not constantly but intermittently in the bottom mixed layer so that a large variation of is found depending on the time. In contrast to , the coefficient of the vertical eddy diffusivityK z is mostly determined byN such thatK z is large in the bottom mixed layer and small in the thermocline. Large value ofK z in the bottom mixed layer is also found in the time series ofK z estimated in terms of Richardson number calculated from the data obtained with electromagnetic current meters. The value ofK z more than 10 cm2s–1 frequently occur in the layer of 20–25 m thick just above the bottom.  相似文献   

11.
The vertical mixing process induced by internal tides was investigated by repeated conductivity, temperature, and depth (CTD) measurements and bottom-mounted acoustic Doppler current profiler (ADCP) in Uchiura Bay from July 24 to 25, 2001. Internal tides were observed with a wave height of 40 m and a horizontal current of 0.3 ms−1. Density inversions were found in the CTD data, and the method of Galbraith and Kelley (1996) was applied to the data to identify overturns and to calculate Thorpe scale. Most of the overturns distributed in the region of low Richardson number, so that they were considered to be caused by shear instability associated with the internal tides. Thorpe scale was calculated to be 0.48 m. From the Thorpe scale, the vertical eddy diffusivity due to internal tides in Uchiura Bay was estimated as K ρ ∼ 10−4 m2s−1. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

12.
A simple conceptual formulation to compute seabed shear stress due to asymmetric and skewed waves is presented. This formulation generalizes the sinusoidal wave case and uses a variable friction factor to describe the physics of the boundary layer and to parameterize the effects of wave shape. Predictions of bed shear stresses agree with numerical computations using a standard boundary layer model with a k–ε turbulence closure. The bed shear stress formulation is combined with a Meyer-Peter and Müller-type formula to predict sheet flow bedload transport under asymmetric and skewed waves for a horizontal or sloping bed. The predictions agree with oscillatory water tunnel measurements from the literature.  相似文献   

13.
The onset of a three-dimensional jet flow in a stratified fluid is studied with the aid of a direct numerical simulation. An initially cylindrical jet with a Gaussian velocity profile is considered in a fluid with stable linear density stratification. The results indicate that, if an initial small perturbation of the velocity field has a wide spectrum, an exponential growth of the isolated quasi-two-dimensional mode occurs and its spectral maximum is shifted toward smaller wave numbers in comparison with the maximum of the helical mode of the instability of a nonstratified jet. The growth rate is proportional to Ri0.5, where Ri is the global Richardson number. The onset of the instability leads to the formation of the flow’s vortex structure, which consists of a collection of different-polarity quasi-two-dimensional vortices located in a horizontal plane near the longitudinal axis of the jet. At sufficiently long times (Nt > 100, where N is the buoyancy frequency and t is time), the growth of instability reaches the saturation stage and further fluctuations in velocity and density decay under the effect of viscous diffusion. At this stage, the flow becomes self-similar and the time dependences of the transverse and vertical widths of the jet are consistent with the asymptotic behaviors of integral parameters of the flow that are observed experimentally in the far stratified wake. The results suggest that the onset of the instability of a quasitwo-dimensional mode can play the determining role in the dynamics of flow in the far stratified wake.  相似文献   

14.
A circular storm moves with a constant speedc along a geostrophic flow similar to a western boundary current in the upper layer of a two-layer ocean with the motionless lower layer. The linear inertia terms are retained. Effects of the current becomes more conspicuous for smallerc and insignificant forc above 10 m s–1. The inertia effects are manifested in cellular patterns of the interface perturbations with cell lengths of(c–vf –1 in a wake of the storm with a radius of an order of 100 km, wherev is the current velocity. On the left hand edge where the flow has a strong shear, the interface displacements have large amplitudes which increase with a distance along the path in a wake of the storm. These disturbances propagate to the left of the edge within an angle of cot–1 (c 2/gH0–1), whereg is the reduced gravity andH 0 is the depth of the interface at the edge of the current. Comparison with the observations during Typhoon Trix in 1971 south of Japan suggests that fluctuations of the daily mean sea level with several days' periods observed along the southern coast of Japan may be due to the stationary oscillations of the Kuroshio caused by the inertia undulations along its left edge or due to the propagating perturbations to the left.  相似文献   

15.
In order to simulate the response of the primary production dynamics to the decay of the Warm-Core Ring (WCR) 86-B off the east coast of Japan, we made a numerical model which consists of a three dimensional physical model (modified GCM) and we used the same biological model that Frankset al. (1986) did. According to the three dimensional model, the well known sub-surface chlorophyll maximum was reproduced but horizontal distributions of Chl.a and NO3 –-N showed different patterns corresponding to different initial conditions of nutrient. This is because the weak vertical velocity in the WCR does not play an important role on the ecosystem but only the light intensity and the balance between uptake and vertical diffusion of dissolved nutrient is important. This result differs from that of Frankset al. (1986). The two WCRs interaction model suggests that a weak upwelling could exist between two WCRs accompanied by baroclinic instability.  相似文献   

16.
High spatial resolution measurements of current velocity performed by the shipboard mounted Acoustic Doppler Current Profiler (ADCP) in the lateral boundary layer of the southern Gulf of Finland during two 5-day periods are described and analysed with a focus on the dominant dynamics. The measurement site represents a small (15×20 km), relatively deep (up to 100 m) bay opened to large-scale estuarine circulation. The measurement period was characterized by calm winds and a strong seasonal pycnocline (Brunt-Väisälä frequencyN=6–9*10−2 s−1). The quasi-steady velocity field revealed polarization of currents along the shore whereas an intensive baroclinic coastal jet was observed over a cross-shore scale of 1–2 km. The level of vertical separation of the alongshore flow coincided with the pycnocline at the coast, but was shifted below it in the offshore region. The cross-shore flow was considerably weaker and showed a three-layer structure with an opposite phase between the first and second surveys. It is suggested that the observed jet resembles a non-locally forced eastward propagating coastally trapped wave. In the offshore area the alongshore flow field satisfies local geostrophic balance quite well, except in the pycnocline where strong vertical stratification exerts considerable vertical stress. As vertical velocity shear is well correlated with vertical stratification, the horizontal advection prevails over vertical mixing. Horizontal inhomogeneities of density distribution are partly explained by vertical velocities with an estimated magnitude of less than 0·6 mm/s and the spatial pattern following bottom topography.  相似文献   

17.
In the context of the problem of global warming, the question of how much the observed increase in the mean air temperature over the last decades is likely due only to natural variability is studied. It is assumed that an observed temperature-data series of length N is a segment of a stationary random sequence with a spectrum known on an interval that does not include the lowest frequencies. The variance σ2(N) of the sample mean m* and the standard deviation σ(N), which determines the width of the confidence interval of m*, are calculated for different continuations of the spectrum to the low-frequency region. For a continuation of the spectrum that boundlessly increases, as does ω−2α (0 < α < 1/2) when the frequency ω tends to zero (red-noise spectrum), it is shown that, the closer the parameter α is to 1/2, the more slowly σ(N) tends to zero at N → ∞. Using an empirical spectrum of a global mean annual temperature series as an example, it is shown that the standard deviation significantly depends on the form of the spectrum in the lowest frequency region. An attempt has been made to assess the measure of standard-deviation uncertainty due to a lack of exact information on the spectrum in the low-frequency region. Important series characteristics—the equivalent number of independent observations N eq(N) and the time scale of correlation T 1(N) which are determined through the variance σ2(N) of m*—also depend on the form of the spectrum in the low-frequency region. For the red-noise spectrum, N eq(N) increases with an increase in N proportionally to N 1–2α (but not proportionally to N as in the case of bounded spectrum); the correlation scale T 1(N) is no longer constant (as in the case of bounded spectrum) and increases with an increase in N proportionally to N 2α.  相似文献   

18.
Abstract

Under seismic loading, the soil layer is subjected to multidirectional cyclic shear stress with different amplitudes and frequencies because of the coupling of multiple shear waves and the soil element within a slope or behind a retaining wall is subjected to initial static shear stress before subjected to cyclic loading. Due to the complexity of seismic loading propagation, a phase difference exists between the initial static shear stress and cyclic shear stress. To investigate the influence of the phase difference and initial static shear stress on cyclic shear strain, cyclic modulus, and cyclic strength, a series of laboratory tests are performed on Wenzhou marine soft clay by multi-directional simple shear system, which can simulate the actual state better by controlling the horizontal cyclic stress in the x and y directions simultaneously. As the phase difference varies from 0° to 90°, the dynamic shear modulus increases and cyclic strain accumulation decreases with an increasing number of cycles. The shear strain increases with the initial shear stress.  相似文献   

19.
To understand the transport process of lithogenic particles in the ocean, we measured the grain size distributions of lithogenic particles and measured the opal, La, Yb, Th, and Sc concentrations of the settling particles collected from time-series sediment traps at Sta. KNOT (44°N, 155°E, water depth 5320 m) from June 2002 to May 2004. The annual mean lithogenic particle flux observed at the lower sediment trap (5100 m) was twice as high as that at the upper sediment trap (770 m). The contribution of Asian loess estimated by the La/Yb and the Th/Sc ratios in the lower layer was greater than that in the upper layer. The fluxes of small lithogenic particles with sizes of 3–4 μm at the lower layer (5 to 65 mg/m2/day) were approximately four times larger than that at the upper layer (0.6 to 27 mg/m2/day). These results indicate that the horizontal addition of small particle sizes of Asian loess is a main factor in the increase of lithogenic particles at the lower layer. The temporal variations in the small lithogenic particle flux at the lower layer had a positive correlation with those at the upper layer (r = 0.71). The small lithogenic particle fluxes showed a strong positive correlation with the opal fluxes (r = 0.9). We therefore conclude that the small lithogenic particles were laterally transported and scavenged by the formation of aggregates with opal.  相似文献   

20.
Visualization of a collapsing vertical packet of patches in a laboratory experiment and the results of acoustical sounding under field conditions allowed us to discriminate the stage of its evolution which was previously unknown. It is defined by the variations of the horizontal dimensions of patches and their effective scattering surface with a period ofT=150–250/N, whereN is the Brunt-Väisälä frequency. The horizontal dimensions of patches can reach 40% of their final dimensions at the evolution stage discriminated.Translated by Mikhail M. Trufanov.  相似文献   

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