Showing papers on "Secondary circulation published in 2012"

Effects of Vertical Wind Shear

Abstract: The effects of vertical wind shear on convective development have been examined for decades (e.g., Pastushkov 1975; Corbosiero and Molinari 2002; Lang et al. 2007; Ueno 2007). The delayed initiation of convection is associated with a strong vertical wind shear (e.g., Xu et al. 1992). The vertical wind shear affects convective development through the change in vertical transport of horizontal momentum (e.g., Wu and Yanai 1994). The convection becomes increasingly organized in lines because of a strong lower-tropospheric vertical wind shear (e.g., Robe and Emanuel 2001). Vertical wind shears affect convective development in two ways. First, vertical wind shear change intensity of convective systems through the change in dynamic instability. The degree of dynamic instability can be described by a secondary circulation, which is quantified by the magnitude of perturbation kinetic energy. Vertical structures of large-scale horizontal winds may alter perturbation kinetic energy through the change in energy exchange between perturbation kinetic energy and large-scale kinetic energy, which is determined by covariance between perturbation horizontal wind and vertical velocity in the presence of vertical shear of large-scale horizontal winds (e.g., Peixoto and Oort 1992; Li et al. 2002). Second, vertical structures of large-scale horizontal winds may have impacts on horizontal structures of convective systems. Unicell and multicell convections are associated with the unidirectional and reverse wind shears, respectively (e.g., Tao et al. 2003). In this chapter, sensitivity experiments with vertical wind shear excluded (BILISNWVS, PSRNVWS) are compared with the control experiments with vertical wind shear included (BILIS, PSR) to discuss effects of vertical wind shear on rainfall processes during the landfall of sever tropical storm Bilis (2006) and pre-summer heavy rainfall event over southern China in June 2008, based on Wang et al. (2009) and Shen et al. (2011). These sensitivity experiments are identical to the control experiments except in sensitivity experiments vertically varying large-scale zonal winds are replaced with mass-weighted mean large-scale zonal winds from the control experiments.

Topographic forcing of flow partition and flow structures at river bifurcations

Abstract: This paper presents the predicted flow dynamics from the application of a Reynolds-averaged NavierStokes model to a series of bifurcation geometries with morphologies measured during previous flume experiments. The topography of the bifurcations consists of either plane or bedform-dominated beds which may or may not possess discordance between the two bifurcation distributaries. Numerical predictions are compared with experimental results to assess the ability of the numerical model to reproduce the division of flow into the bifurcation distributaries. The hydrodynamic model predicts: (1) diverting fluxes in the upstream channel which direct water into the distributaries; (2) super-elevation of the free surface induced at the bifurcation edge by pressure differences; and (3) counter-rotating secondary circulation cells which develop upstream of the apex of the bifurcation and move into the downstream channels, with water converging at the surface and diverging at the bed. When bedforms are not present, weak transversal fluxes characterize the upstream channel for almost its entire length, associated with clearly distinguishable secondary circulation cells, although these may be under-estimated by the turbulence model used in the solution. In the bedform dominated case, the same hydrodynamic conditions were not observed, with the bifurcation influence restricted and depth scale secondary circulation cells not forming. The results also demonstrate the dominant effect bed discordance has upon flow division between the two distributaries. Finally, results indicate that in bedform dominated rivers. Consequently, we suggest that sand-bed river bifurcations are more likely to have an influence that extends much further upstream and have a greater impact upon water distribution. This may contribute to observed morphological differences between sand-bedded and gravel-bedded braided river networks. Copyright (C) 2012 John Wiley & Sons, Ltd.

Three-dimensional instabilities in tornado-like vortices with secondary circulations

Abstract: Tornadoes and other intense atmospheric vortices are known to occasionally transition to a flow structure with multiple vortices within their larger circulations. This phenomenon has long been ascribed to fluid dynamical instability of the inner-core circulation, and many previous studies have diagnosed low-wavenumber unstable modes in tornado-like vortices that resemble the observed structures. However, relatively few of these studies have incorporated the strong vertical motions of the inner-core circulation into the stability analysis, and no stability analyses have been performed using a complete, frictionally driven secondary circulation with strong radial inflow near the surface. Stability analyses are presented using the complete circulations generated from idealized simulations of tornado-like vortices. Fast-growing unstable modes are found that are consistent with the asymmetric structures present in these simulations. Attempts to correlate the structures and locations of these modes with instability conditions for vortices with axial jets derived by Howard & Gupta and by Leibovich & Stewartson produce only mixed results. Analyses of perturbation energy growth show that interactions between eddy fluxes and the radial shear of the azimuthal wind contribute very little to the growth of the dominant modes. Rather, the radial shear of the vertical wind and the vertical shear of the vertical wind (corresponding to deformation in the axial direction) are the primary energy sources for perturbation growth. Relatively weak axisymmetric instabilities are also identified that have some similarity to symmetric oscillations that have been observed in tornadoes.

Strain-vorticity induced secondary motion in shallow flows

Abstract: Deviations from two-dimensionality of a shallow flow that is dominated by bottom friction are quantified in terms of the spatial distribution of strain and vorticity as described by the Okubo-Weiss function. This result is based on a Poisson equation for the pressure in a quasi-horizontal (primary) flow. It is shown that the Okubo-Weiss function specifies vertical pressure gradients, which for their part drive vertical (secondary) motion. An asymptotic expansion of these gradients based on the smallness of the vertical to horizontal scale ratio demonstrates that the sign and magnitude of secondary circulation inside the fluid layer is dictated by the signs and magnitude of the Okubo-Weiss function. As a consequence of this, secondary motion as well as nonzero horizontal divergence do also depend on the strength, i.e., the Reynolds number of the primary flow. The theory is exemplified by two generic vortical structures (monopolar and dipolar structures). Most importantly, the theory can be applied to more ...

Shear-flow induced secondary circulation in parallel underwater topographic corrugation and its application to satellite image interpretation

Abstract: This study aims to figure out satellite imaging mechanisms for submerged sand ridges in the shallow water region in the case of the flow parallel to the topography corrugation. Solving the disturbance governing equations of the shear-flow yields the analytical solutions of the secondary circulation. The solutions indicate that a flow with a parabolic horizontal velocity shear and a sinusoidal vertical velocity shear will induce a pair of vortexes with opposite signs distributed symmetrically on the two sides of central line of a rectangular canal. In the case of the presence of surface Ekman layer with the direction of Ekman current opposite to (coincident with) the mean flow, the two vortexes converge (diverge) at the central line of canal in the upper layer and form a surface current convergent (divergent) zone along the central line of the canal. In the case of the absence of surface Ekman layer, there is no convergent (divergent) zone formed over the sea surface. The theoretical results are applied to interpretations of three convergent cases, one divergent case and statistics of 27 cases of satellite observations in the submerged sand ridge region of the Liaodong Shoal in the Bohai Sea. We found that the long, finger-like, bright patterns on SAR images are corresponding to the locations of the canals (or tidal channels) formed by two adjacent sand ridges rather than the sand ridges themselves.

Observations and large-eddy simulations of the thermally driven cross-basin circulation in a small, closed basin

Abstract: Differential solar irradiation on opposing mountain sidewalls produces local temperaturegradients. Flows across the valley or basin develop due to the ensuinghorizontal pressure gradients, which are directed from the less irradiated and coldersidewall toward the more irradiated and warmer sidewall. These thermal flows areinvestigated for the small and almost circular basin of Arizona’s Meteor Crater usingobservations and numerical simulations. Observations from the Meteor Crater showa pronounced cross-basin flow in the center of the crater basin under undisturbedconditions, which develops as an easterly flow in the morning when the sun is tothe east and the west sidewall is more strongly irradiated, and which then shiftsto a southerly direction around noon and eventually to a westerly direction in theevening. The direction of the cross-basin flow agrees with the direction of the crossbasintemperature and pressure gradients as the sun moves across the sky during theday. Large-eddy simulations for an idealized, rotationally symmetric basin producea cross-basin circulation with a three-layer structure in the morning, that is, a nearsurfacesoutheasterly cross-basin flow topped by an opposing, northwesterly returnflow and a secondary southeasterly flow near or above the top of the basin. Based onan analysis of the horizontal momentum and the thermodynamic balance equations,a different formation mechanism is identified for each layer, with each of the formationmechanisms being related to asymmetric irradiation. Additional simulations arerun with a prescribed surface heat flux, which produces a spatially constant heat-fluxgradient, and with varying background wind speeds and directions for different basinsizes. Results indicate that persistent cross-basin flows develop only in basins that aresmaller than 5 km. Background winds induce a secondary circulation near the top ofthe basin, which interacts with the thermally driven circulation. The resulting windfield depends on the direction of the background winds with respect to the prescribedheat-flux gradient and on the stratification of the basin atmosphere.

Frictional convergence, atmospheric convection, and causality

Abstract: Frictional convergence in an atmospheric boundary layer topped by a stable free troposphere is considered. In agreement with extensive previous work, we find that atmospheric stability reduces the vertical scale of the free tropospheric secondary circulation associated with frictional convergence. Associated with this reduction in vertical scale is a proportional reduction in the time scale for the frictional spindown of an atmospheric circulation. This reduction in time scale alters the balance between terms in the component of the momentum equation along the isobars. In particular, for disturbance scales less than a few hundred kilometers in typical tropical conditions, the momentum tendency term comes into approximate balance with the friction term, with the Coriolis term becoming less important. This reduces the magnitude of the cross-isobaric flow and the strength of the ascent in regions where this flow converges. If some other mechanism such as moist convection produces enough boundary layer convergence to nullify the spindown of the disturbance in question, then the magnitude of the convergence equals that predicted by the steady-state frictional convergence formulation. However, in this case the arrow of causality is reversed from that assumed in a naive treatment of frictional convergence. Frictional convergence is not “causing” the convection; the convection is actually causing the convergence, and the mechanism forcing the convection must be sought elsewhere. This distinction is crucial in understanding what drives deep convection. The present analysis is linearized and the picture may change when nonlinear effects become important. It is also limited to situations in which the boundary layer winds are relatively weak. Tropical cyclones, with their strong winds and nonlinear behavior, thus deserve an independent analysis.

Secondary circulation antibacterial system with dialysis water supply pipeline

Abstract: The invention discloses a secondary circulation antibacterial system with a dialysis water supply pipeline. The secondary circulation antibacterial system comprises a main dialysis water supply pipeline, a main dialysis water supply pipeline flow adjusting valve, a secondary circulation system inlet, a secondary circulation system inlet side pipeline, a secondary circulation system tee joint, a secondary circulation system return side pipeline and a secondary circulation system outlet, and is characterized in that the flow adjusting valve is mounted on the main dialysis water supply pipeline; the secondary circulation system inlet and the secondary circulation system outlet are respectively formed on the opposite sides of the two ends of the flow adjusting valve; the secondary circulation system inlet side pipeline is connected in the position of the secondary circulation system inlet and is a hose pipe with a length of about 300cm; the other end of the hose pipe is connected with the secondary circulation system tee joint; the secondary circulation system return side pipeline is connected with the other port of the secondary circulation system tee joint and is the same as the secondary circulation system inlet side pipeline in length and material; the rest port of the secondary circulation system tee joint is connected with a dialysis machine connecting pipe of 3-5cm; and the other end of the connecting pipe is connected with a dialysis machine. According to the secondary circulation antibacterial system, the problems that the dialysis water supply pipeline is provided by a disinfection dead cavity and can not be completely disinfected are solved and the bacterial growth and proliferation can be effectively suppressed.

Diagnosis and Numerical Simulation of a Convective Rainstorm Process of Block Type Occurred in North China

Abstract: Using the routine and intensive observational data,the NCEP reanalysis data,the satellite images and the high resolution simulation result by MM5,a diagnosis and numerical simulation of a block type convective rainstorm process occurred in North China on July 23,2006 were conducted.The result showed that the process happened in the low pressure circulation,in which the East Siberia Block was the main feature.The broad low pressure area and the vertical secondary circulation induced by the jet stream provided the valuable environmental condition and dynamic forcing for the rainstorm.The water vapor transportation of synoptic scale on 700 hPa and 850 hPa,which was westerly direction to the south of 40°N and easterly direction on the north side of the low,improved the environmental moisture conditions around Beijing.Just before the occurrence of rainstorm,the mesoscale moisture transportation with south-westerly direction on 850 hPa is a key of forming rainstorm.MM5 model successfully reproduced the formation and evolution processes of the mesoscale convective systems(MCS),which was the directly weather system causing the rainstorm.The horizontally scale of MCS crossed 0.5 longitudes and from surface to 300 hPa vertically with a typical warm heart construction.The strong vertical secondary circulation,which was consistent with strong divergence on 400 hPa and convergence on 900 hPa,produced a favorable condition for the exploding of strong convection.Meanwhile,the instable energy transportation on the south side of the MCS,in which the intensive potential equivalent temperature gradient appeared,contributed to the rainstorm evidently.

Study on Lateral Flow Distribution and Momentum Analysis at Flood season and Neap tide of the Seokmo Channel in the Han River estuary

Abstract: This research observed the cross section current of 7 survey lines in Seokmo Channel of Gyeonggi bay with a lot of freshwater inflow and S-shaped for 13 hours during flood season and neap tide. We indicated the distribution of the current velocity by comprehending the speed and direction of the current velocity of each line during maximum flood, ebb tide and observed the distribution of salinity. Moreover, in order to understand what lateral momentum causes the lateral flow in each survey line, we practiced the momentum analysis through the observation data. As a result, the lateral baroclinic pressure gradient force and vertical friction of the Seokmo channel during neap tide were the strongest, and this is why the flow by the distribution of salinity and stratification most often occurs. In north of the Seokmo channel, where have wide intertidal and a lot of freshwater inflow, the secondary circulation is caused by balance of lateral baroclinic pressure gradient force and other forces, and the vertical friction was strong in the lines with small depth. On the other hand, in the southern part of the Seokmo channel where the water is deep and the waterway is curved, the advective acceleration and centrifugal force become stronger by the geographical causes during ebb and the influence of fresh water. Therefore, the lateral flow in the Seokmo channel was caused by the distribution of the momentum that differs by location, depth, curve, etc.

Establishment of a multiple-Doppler radar wind retrieval system and its application to the analysis of mesoscale convective systems

Abstract: A multiple-Doppler radar wind retrieval system is established by using a three dimensional variation method. The system consists of two parts. One is to interpolate reflectivities and radial velocities of nine Doppler radars of the Korea Meteorological Administration (KMA) into one analysis domain. The other is to retrieve three dimensional winds by minimizing a cost function that includes the following costs and constraints: the observed radial velocity cost, background wind cost, continuity constraint and smoothness constraint. In order to verify the performance of the system, retrieved winds are compared with observed winds obtained from five wind profilers of KMA. The performance of the system depends on the relative position to the baselines between Doppler radars. However, the performance of the system is enhanced when the number of overlaps among the radial velocities increases. The system is applied to the analysis of the evolution of a mesoscale convective system (MCS) on the Changma front on 1 July 2005. The analysis result shows that a new convective cell is developed by the convergence of the low troposphere winds at the organizing stage. The analysis of the vertical vorticity reveals that, among the two vorticity generation terms to be calculated utilizing the retrieved winds, tilting or twisting source dominates the divergence source in most convective regions. The strong downdrafts associated with the storm are produced on the downdraft branch of a meridional direction secondary circulation across the Changma front.

Pressure compensation device in aircraft

Abstract: Pressure compensation device ( 100 ) deployed in an element that contains a flammable substance that compensates pressures between interior ( 101 ) of the element and exterior atmosphere ( 102 ) in which said element is deployed, by means of a continuous primary circulation ( 300 ) of air through primary channel ( 103 ) in said device ( 110 ) that communicates interior ( 100 ) and exterior ( 102 ) of the element, further comprising a secondary channel ( 104 ) integrated in the device ( 100 ) itself, communicating interior ( 102 ) and exterior ( 102 ) of the element, comprising said secondary channel ( 104 ) in the interior zone of the element a sheet ( 105 ) such that, in the event primary continuous circulation ( 300 ) of air through primary channel ( 103 ) is interrupted, said continuous circulation continues through secondary circulation ( 400 ) of air to interior ( 101 ) of the element, this secondary circulation ( 400 ) of air being capable of breaking sheet ( 105 ) of secondary channel ( 104 ).