Peak Value Of Vorticity Research Articles

Experimental study on the air flow around an isolated stepped flat roof building: Influence of snow cover on flow fields

In this study, the flow field distribution around an isolated stepped flat roof building and the influence of snow cover on flow fields are investigated by wind tunnel experiments based on particle image velocimetry (PIV) technology. Firstly, the stable snow cover, which is obtained based on the previous wind tunnel test of snowdrift on stepped flat roofs, is made by using 3D printing technology. Then two groups of models with and without snow cover on roofs are used in a low-speed wind tunnel and the experiments of flow around buildings are carried out. The significant differences of flow characteristic on roofs between without and with snow cover are that when there exists a snow cover, the reverse flow on the lower roof almost disappears and the recirculation vortex only exists at the rear region of lower roof. In addition, when there is with snow cover on roofs, the strength of flow separation and the peak value of vorticity at the higher roof is enhanced. With the existence of snow cover on roofs, turbulent kinetic energy significantly decreases by 51.1% compared with that without snow cover. Finally, the relationship between the distribution coefficient of snow depth and flow patterns is analyzed, and it is found that the linear relationship is fitted well in the front and rear regions on the lower roof.

Thin Shear Layer Structures in High Reynolds Number Turbulence

Three-dimensional tomographic time dependent PIV measurements of high Reynolds number (Re) laboratory turbulence are presented which show the existence of long-lived, highly sheared thin layer eddy structures with thickness of the order of the Taylor microscale and internal fluctuations. Highly sheared layer structures are also observed in direct numerical simulations of homogeneous turbulence at higher values of Re (Ishihara et al., Annu Rev Fluid Mech 41:165–180, 2009). But in the latter simulation, where the fluctuations are more intense, the layer thickness is greater. A rapid distortion model describes the structure and spectra for the velocity fluctuations outside and within ‘significant’ layers; their spectra are similar to the Kolmogorov (C R Acad Sci URSS 30:299–303, 1941) and Obukhov (Dokl Akad Nauk SSSR 32:22–24, 1941) statistical model (KO) for the whole flow. As larger-scale eddy motions are blocked by the shear layers, they distort smaller-scale eddies leading to local zones of down-scale and up-scale transfer of energy. Thence the energy spectrum for high wave number k is \(E_X (k)\sim Bk^{-2p}\). The exponent p depends on the forms of the large eddies. The non-linear interactions between the distorted inhomogeneous eddies produce a steady local structure, which implies that 2p = 5/3 and a flux of energy into the thin-layers balancing the intense dissipation, which is much greater than the mean \(\left \). Thence \(B\sim\left ^{2/3}\) as in KO. Within the thin layers the inward flux energises extended vortices whose thickness and spacing are comparable with the viscous microscale. Although peak values of vorticity and velocity of these vortices greatly exceed those based on the KO scaling, the form of the viscous range spectrum is consistent with their model.

PIV measurement of turbulent mixing layer flow with polymer additives

Turbulent mixing layer flow with polymer additives was experimentally investigated by PIV in present paper. The velocity ratio between high and low speed is 4:1 and the Reynolds number for pure water case based on the velocity differences of two steams and hydraulic diameter of the channel ranges from 14667∼73333. Flow field and turbulent quantities of turbulent mixing layer with 200ppm polymer additives were measured and compared with pure water mixing layer flow. It is shown that the dynamic development of mixing layer is greatly influenced by polymer addictives. The smaller vortices are eliminated and the coherent structure is much clearer. Similar with pure water case, Reynolds stress and vorticity still concentrate in a coniform area of central part of mixing layer and the width will increase with the Reynolds number increasing. However, compared with pure water case, the coniform width of polymer additives case is larger, which means the polymer additives will lead to the diffusion of coherent structure. The peak value of vorticity in different cross section will decrease with the development of mixing layer. Compared with pure water case, the vorticity is larger at the beginning of the mixing layer but decreases faster in the case with polymer additives.

Helicity in the formation of turbulence

Helicity in vortex structures and spectra is studied in the developmental stages of a numerical simulation of the Navier-Stokes equations using three-dimensional visualizations and spectra. First, time scales are set using the growth and decay of energy dissipation, the peak value of vorticity, and the helicity. Then, two stages between the early time, nearly inviscid Euler dynamics with vortex sheets and a final state of fully developed turbulence with vortex tubes, are described. In the first stage, helicity fluctuations develop in Fourier space during a period still dominated by vortex sheets and rapidly growing peak vorticity. At the end of this period the strongest structure consists of transverse vortex sheets with mixed signs of helicity. During the second stage, a dissipative interaction propagates along one of these vortices as the sheets roll each other into vortex tubes.

Cloud System Families in the Meiyu-Baiu Front Observed during 1-10 July 1991

This paper studies the cloud systems in the stationary Meiyu-Baiu front observed in 1-10 July 1991, using the Geostationary Meteorological Satellite IR data and ECMWF re-analysis data. The Meiyu-Baiu frontal cloud zone consists of a few trains of cloud systems with a length of ∼2000 km, which is recognized as the “cloud system family”. Each family consists of a preceding subsynoptic-scale cloud system, and a few meso-α-scale cloud systems aligned along the trailing portion of the sub-synoptic-scale cloud system. During the 10-day period, 4 sub-synoptic-scale cloud systems, accompanied by Meiyu-Baiu frontal depressions, are formed with an interval of 2-3 days, in association with the short-wave westerly troughs. The sub-synoptic-scale depressions indicate the peak value of vorticity more than 6× 10-5 s-1, at both 850 and 500 hPa. The sub-synoptic-scale cloud system consists of convective clouds in its southern portion, and wide spread clouds in the northeastern portion. A zone of potential vorticity appears in the trailing portion of the sub-synoptic-scale depression. The cold and dry advection occurring to the back of the depression sustain thermal and moisture gradient in the narrow frontal zone after the passage of the depression. A few meso-α-scale cloud systems develop along the trailing portion of the sub-synoptic-scale depression. The main part of the meso-α-scale cloud system consists of the active convective clouds. Some of the cloud systems show features of oval shaped cloud cluster, while others indicate features of leaf shaped cloud system with a cloud band in its trailing portion. These meso-a-scale cloud systems are categorized into three groups based on the horizontal extension of the cloud area, duration and the magnitude of the relative vorticity associated with the cloud system. Type-A meso-α-scale cloud systems are identified for∼2 days, and are accompanied by positive vorticity of ∼4 × 10-5 s-1 in the lower and middle troposphere. Type-B meso-a-scale cloud systems are identified only for∼1 day, and are accompanied by positive vorticity of ∼2 × 10-5 s-1 mainly in the lower troposphere. Type-C meso-α-scale cloud systems are generated during the daytime and decay during the nighttime. The wavelength estimated by Eddy’s baroclinic instability model for the unsaturated frontal zone coincides with the wavelength of the observed sub-synoptic-scale cloud system, while that for the moist frontal zone is consistent with the scale of Type-A meso-a-scale cloud systems. The cloud system families are most clearly observed in 110-140°E, where the Pacific subtropical anticyclone, and the cold low in the northern latitude, provide favorable conditions for the development of cloud systems.

Transition Process of a Two-Dimensional Jet. 1st Report; Vorticity Distribution and Convection Velocity of the Vortex.

The coherent structure dynamics was experimentally investigated in a two-dimensional jet. The fundamental velocity fluctuation modes, i.e., the symmetrical and antisymmetrical modes, were excited acoustically in the jet. Conditional sampling was employed and vorticity contours were calculated from ensemble-averaged velocity components, and the mean velocity at the center of the vortex, the peak value of vorticity, and trajectory of the coherent vortex were educed. The peak position of the coherent vorticity component moved almost parallel to the jet axis in both cases of the excited jets. The convection velocity of the vortex was almost constant throughout the linear interaction and the nonlinear interaction regions independent of the local mean velocity at the center of the vortex . Furthermore, the convection velocity was larger than the local mean velocity throughout the linear interaction regions.