Vorticity Profiles Research Articles

운전점에 따른 3차원 소형축류홴의 와도 특성에 대한 대규모 와 모사

The unsteady-state, incompressible and three-dimensional large-eddy simulation(LES) was carried out to evaluate the vorticity distribution of a small-size axial fan(SSAF). The X-component vorticity profiles developed around blade tips turn from axial to radial, and diminish the density of distribution according to the increase of static pressure. Otherwise, the Z-component vorticity profiles evenly develop at the region larger than the half radial distance of blade at the operating points of A and B, partly at the trailing-edge region of blade and radially over bellmouth according to the increase of static pressure.

Parameterization of Frontal Symmetric Instabilities. I: Theory for Resolved Fronts

A parameterization is proposed for the effects of symmetric instability (SI) on a resolved front. The parameterization is dependent on external forcing by surface buoyancy loss and/or down-front winds, which reduce potential vorticity (PV) and lead to conditions favorable for SI. The parameterization consists of three parts. The first part is a specification for the vertical eddy viscosity, which is derived from a specified ageostrophic circulation resulting from the balance of the Coriolis force and a Reynolds momentum flux (a turbulent Ekman balance), with a previously proposed vertical structure function for the geostrophic shear production. The vertical structure of the eddy viscosity is constructed to extract the mean kinetic energy of the front at a rate consistent with resolved SI. The second part of the parameterization represents a near-surface convective layer whose depth is determined by a previously proposed polynomial equation. The third part of the parameterization represents diffusive tracer mixing through small-scale shear instabilities and SI. The diabatic, vertical component of this diffusivity is set to be proportional to the eddy viscosity using a turbulent Prandtl number, and the along-isopycnal tracer mixing is represented by an anisotropic diffusivity tensor.Preliminary testing of the parameterization using a set of idealized models shows that the extraction of total energy of the front is consistent with that from SI-resolving LES, while yielding mixed layer stratification, momentum, and potential vorticity profiles that compare favorably to those from an extant boundary layer parameterization (Large et al., 1994). The new parameterization is also shown to improve the vertical mixing of a passive tracer in the LES.

Experimental study of heat transfer enhancement with segmented flow in a microchannel by using molecule-based temperature sensors

This paper presents an experimental study of heat transfer enhancement with liquid-gas two-phase segmented microchannel flows by utilizing a novel technique of molecule-based temperature sensors. A serpentine microchannel was constructed and steady liquid and gas segments of ethanol and air were successfully produced at various flow rates. A molecule-based temperature sensor made of Tris(2,2′-bipyridyl) ruthenium was utilized in the liquid segments by dissolving in ethanol as temperature-sensitive fluid and mixing in a dope to produce temperature-sensitive paint in order to retrieve fluid and surface temperature data, respectively. Three cases of different liquid and gas flow rates were tested. The heat transfer enhancements of these three cases were carefully examined under constant heat flux thermal boundary conditions, and the increased heat transfer was evaluated in comparison with the single-phase flow, where only liquid was injected in the microchannel. An improvement of up to 30% in the averaged Nu number of the microchannel flow was determined for Case 3, with a high gas flow rate and short liquid segments. Velocity and vorticity profiles in the liquid segments at different gas and liquid flow rates were also studied using a micro-particle image velocimetry technique. Stronger circulation was observed in Case 3, which confirmed the results from the heat transfer analysis, namely that more efficient heat transfer occurs with strong circulation in short liquid segments when a high gas flow rate is applied. The results of the present study provide detailed temperature profiles in segmented flow as well as the evolution of the Nu number from the entrance to the exit of a serpentine microchannel.

Evolution of a Vortex in a Strain Flow.

Experiments and vortex-in-cell simulations are used to study an initially axisymmetric, spatially distributed vortex subject to an externally imposed strain flow. The experiments use a magnetized pure electron plasma to model an inviscid two-dimensional fluid. The results are compared to a theory assuming an elliptical region of constant vorticity. For relatively flat vorticity profiles, the dynamics and stability threshold are in close quantitative agreement with the theory. Physics beyond the constant-vorticity model, such as vortex stripping, is investigated by studying the behavior of nonflat vorticity profiles.

Natural convection in a trapezoidal cavity filled with a micropolar fluid under the effect of a local heat source

This paper analyzes laminar natural convection of micropolar fluid in a trapezoidal cavity with a local heater. The bottom and top walls of the enclosure are adiabatic while the left vertical wall and part of the right inclined wall are kept at low and high constant tempratures, respectively. The rest part of the inclined wall is adiabatic. Governing equations formulated in dimensionless variables such as stream function, linear vorticity, angular vorticity and temperature have been solved by finite difference method of the second order accuracy. Computations have been carried out to analyze the effects of Rayleigh number, Prandtl number, vortex viscosity parameter and the heater location on streamlines, isotherms and vorticity profiles as well as the variation of the average Nusselt number and fluid flow rate. It has been shown that bottom position of the heater reflects the heat transfer enhancement.

Reconnection of orthogonal cylindrical vortices

This paper reports on a numerical study of the interaction of a pair of orthogonal cylindrical vortices. The vortices have equal radii and vorticity distributions, which are chosen to be either uniform or parabolic, and are initially located at various distances. A vortex-in-cell model simulates the flow evolution in a triply periodic cube; the model contains no viscous terms but it introduces a small numerical viscosity (equivalent to Re>15000). It was found that the time at which the reconnection starts significantly increased with the initial distance between the vortices; in contrast, the reconnection lasted approximately 3/4 of the eddy turnover time for all initial distances and the two vorticity profiles considered in this study. This confirms that the reconnection is a convective process made possible by even the weakest viscosity.

Experimental investigation of the flow structures in the wake of a cross parachute canopy

In the present study, an experimental study was conducted to characterize the flow structures in the turbulent wake of a cross parachute canopy by utilizing Stereoscopic Particle Image Velocimetry (Stereo-PIV) technique. Both flow through gap and flow over arm of the cross parachutes were carefully measured and analyzed. The obtained results showed that the turbulent wake of the flow through gap was larger than that of flow over arm. Besides, the distributions of velocity, vorticity, and Reynolds stress of flow through gap differed significantly with those of flow over arm. In addition, the effect of the arm ratio of the cross canopy on the transverse profiles of velocity, vorticity, and Reynolds stress was also discussed.

Free convection in a trapezoidal cavity filled with a micropolar fluid

The present investigation deals with the study of steady laminar natural convective flow and heat transfer of micropolar fluids in a trapezoidal cavity. The bottom wall of the cavity is kept at high constant temperature, the inclined walls is kept at low constant temperatures while the top horizontal wall is adiabatic. Governing equations formulated in dimensionless stream function and vorticity variables has been solved by finite difference method of the second order accuracy. Comprehensive verification of the utilized numerical method and mathematical model has shown a good agreement with numerical data of other authors. Computations have been carried out to analyze the effects of Rayleigh number, Prandtl number and vortex viscosity parameter both for weak and strong concentration cases. Obtained results have been presented in the form of streamlines, isotherms and vorticity profiles as well as the variation of the average Nusselt number and fluid flow rate. It has been shown that an increase in the vortex viscosity parameter leads to attenuation of the convective flow and heat transfer inside the cavity.

Instability of isolated hollow vortices with zero circulation

Inviscid linear stability analysis and numerical simulations are used to investigate how temporal disturbances evolve in double-annular hollow vortices with an opposite-signed vorticity (the total circulation is zero). Two extrema exist in the vorticity profile and constitute a factor of instability. The dispersion relation is expressed as a simple cubic equation. The results show that the instabilities of vortices are strongly enhanced by the hollow effect of the annular vorticity. In addition, the growth rate of the dominant modes significantly increases with decreasing negative-vorticity thickness. During the initial stage, the dominant unstable modes obtained from simulations are consistent with those obtained from the linear analysis. In nonlinear developments, the flow field stretches out in one direction depending on the motion of the plural vortex pair formed by rolling up the positive and negative vorticities. Once such structures in the vortex are generated, the vortex immediately breaks down and does not become metastable.

Role of stratiform heating on the organization of convection over the monsoon trough

It has been recently demonstrated that stratiform heating plays a critical role in the scale-selection of organized tropical convection, in an aquaplanet version of a coarse-resolution atmospheric general circulation model coupled to a stochastic multicloud cumulus parameterization scheme. It is shown that Madden–Julian oscillation-like organization dominates when the model is tuned to produce strong and long lived stratiform heating while it gives rise to mostly convectively coupled waves in the case of weak and short lived stratiform clouds. The study is extended here to the case of an asymmetric forcing mimicking the migration of the intertropical convergence zone (ITCZ) during summer to understand the impact of changes in stratiform heating on the monsoon dynamics. Consistent with the equatorial ITCZ case, strong and long lived stratiform heating promotes northward and eastward moving intraseasonal disturbances while weak and short lived stratiform heating yields mostly westward propgating synoptic scale low pressure systems. Moreover, the underlying intraseasonal versus low pressure system activity seems to impact the strength and extend of the monsoon trough (MT). In the regime with intraseasonal activity the MT is much stronger and extends northward while in the low pressure system case MT is some what weaker in strength but extends further westward. In the low pressure dominated regime, the background vorticity and zonal wind profiles over the monsoon trough are consistent with the observations.

Kinematic analysis of rock flow and deformation temperature of the Sirjan thrust sheet, Zagros Orogen, Iran

AbstractMicrostructural, finite strain and vorticity analyses of quartz-rich mylonites were used in order to investigate kinematics of rock flow and deformation temperature in the Sirjan thrust sheet exposed in a structural window within the Sanandaj–Sirjan High Pressure – Low Temperature (HP–LT) metamorphic belt that forms part of the hinterland of the Zagros orogenic belt of Iran. A dominant top-to-the-SW sense of shear in the study area is indicated by several shear sense indicators such as asymmetric boudins, rotated porphyroclasts, mica fish and S/C fabrics. Quantitative analyses reveal approximately plane strain deformation conditions with Rxz values ranging from 2.5 to 4.3 and increasing towards the Sirjan thrust. Opening angles of quartz c-axis fabrics and recrystallization regimes suggest deformation temperatures vary from 430 to 625 ± 50°C in the hanging wall rocks. Oblique grain shape and quartz c-axis fabrics were used to estimate the degree of non-coaxiality during deformation. The obtained vorticity profile indicates a down-section increase in kinematic vorticity number (Wm) from 0.6 to 0.89. This range of vorticity numbers confirms contributions of both simple (41–68 %) and pure shear (32–59 %) deformation components. The structural characteristics of the study area ultimately were controlled by oblique motion of the Afro-Arabian plate relative to the Iranian plate.

Understanding Instabilities of Tropical Cyclones and Their Evolution with a Moist Convective Rotating Shallow-Water Model

Abstract Instabilities of hurricane-like vortices are studied with the help of a rotating shallow-water model, including the effects of moist convection. Linear stability analysis demonstrates that dominant unstable modes are mixed Rossby–inertia–gravity waves. It is shown that, depending on fine details of the vorticity profile, a wavenumber selection of the instability may operate or not, leading in some cases to an unstable mode with a distinctively maximal growth rate and in other cases to an ensemble of unstable modes with close growth rates. Numerical simulations are performed in order to investigate nonlinear saturation of the instability and to understand the dynamical role of moisture. In agreement with previous studies, the authors confirm axisymmetrization of vorticity in the course of the development of the instability, which induces changes of intensity of the hurricane. In “dry” simulations, winds are intensified only inside the radius of maximum wind, while the maximum value of the wind decreases. “Moist precipitating” simulations (with and without evaporation) exhibit a net increase of winds, also at the radius of maximum wind, as compared to the dry simulations. Dynamical effects of moisture on the reorganization of the vortex and on the efficiency of inertia–gravity wave emission are quantified and shown to be considerable. Periodic bursts in the emission of waves related to the development of the unstable modes inside the vortex are evidenced, as well as the appearance of convectively coupled waves in the moist precipitating simulations with evaporation.

2D-PIV measurement of isothermal air jets from a multi-slot diffuser in aircraft cabin environment

Mixing air ventilation systems are currently used in commercial aircraft environmental control systems (ECS), which are essential for creating a healthy, thermally comfortable and energy-efficient cabin environment. Based on previous studies, the cabin mixing air distribution is mainly driven by the supply air jets. Therefore, understanding the air jet behaviors within the cabin is necessary for evaluating and improving the cabin environmental quality. In this investigation, we performed a detailed and large-scale 2D particle image velocimetry (PIV) measurement to characterize the isothermal jets in the downstream area of one linear slot of a multi-slot diffuser in a ventilated aircraft cabin mockup. The measurements were taken at different inlet Reynolds numbers (Re) ranging from 1480 to 6940 based on the inlet velocities and the slot height. The PIV results clearly revealed the transitional jet structures in the entrainment process downstream of the slot. The free jets discharging from the slot quickly developed into quasi wall jets due to the Coanda effect. The mean velocity profiles, flow patterns, centerline velocity decay and mean vorticity profiles were then analyzed and compared among different inlet Re. Moreover, the cabin jet characteristics were further compared to results from previous studies. The experimental data and analysis in this research can provide references for designing better side ventilation configurations in single-aisle aircraft cabins.

The stability of oceanic fronts in a shallow water model

We investigate mechanisms through which energy cascades from the mesoscale, $O(100~\text{km})$, to the submesoscale, $O(10~\text{km})$, for oceanic fronts in a reduced gravity shallow water model using two different profiles. The first idealization of an ocean front has an interfacial depth that is a smooth hyperbolic tangent profile and is an extension of the piecewise constant potential vorticity profile studied in Boss et al. (J. Fluid Mech., vol. 315, 1996, pp. 65–84). By considering a range of minimum depths, all of which have the same velocity profile, we are better able to isolate the effect of vanishing layer depths. We find that the most unstable mode exists in a one-layer model and does not need two layers, as previously speculated. Moreover, we find that even without a vanishing layer depth there are other modes that appear at both larger and smaller length scales that have a gravity wave structure. The second profile is the parabolic double front from Scherer & Zeitlin (J. Fluid Mech., vol. 613, 2008, pp. 309–327). We find more unstable modes than previously presented and also categorize them based on the mode number. In particular, we find there are pairs of unstable modes that have equal growth rates. We also study the nonlinear evolution of these oceanic fronts. It is determined that vanishing layer depths have significant effects on the unstable dynamics that arise. First, stronger gravity wave fields are generated. Second, cyclonic fluid that moves into the deeper waters is stretched preferentially more in comparison to the deep water scenario and destabilizes more easily. This results in smaller scale vortices, both cyclones and anticyclones, that have length scales in the submesoscale regime. Our results suggest that the nonlinear dynamics of a front can be very efficient at generating submesoscale motions.

Lagrangian investigations of vortex dynamics in time-dependent cloud cavitating flows

Lagrangian investigations of vortex dynamics, including Lagrangian Coherent Structures (LCS) and particle trajectory, are conducted to highlight the mechanisms of cloud cavitating flows around a Clark-Y hydrofoil. Numerical simulations are performed using a transport equation-based cavitation model and the large eddy simulation (LES) approach. Good agreements are observed between numerical predictions and experimental measurements, including time-averaged turbulence statistics, velocity, vorticity profiles and the periods of unsteady shedding process of the vortex structures near the trailing edge. Besides, present numerical predictions are capable of capturing the unsteadiness of cloud cavitation, including the initiation, growth toward the trailing edge and subsequent shedding of cavities. Based on the Lagrangian analysis of vortex dynamics in non-cavitating flows, two LCSs, namely LE-LCS and TE-LCS, are defined. In cloud cavitating flows, distributions of the two LCSs in different cavitation developing stages illustrate different behaviors of vortex structures. (a) In the attached sheet cavity growing stage, the LE-LCS extends to the trailing edge, which implies the expansion of the attached sheet cavity, and the TE-LCS rolls up and extends downstream, which implies the detachment of cloud cavity. In addition, particle tracers indicate that the Leading edge vortex (LEV) is enhanced by the attached sheet cavity, and there is no direct interaction between attached cavity’s expansion and cloud cavity’s shedding. (b) In the re-entrant jet developing stage, the LE-LCS and TE-LCS connect together near the middle of the hydrofoil, which implies that two vortex structures mix together inside of the stable attached cavity. Particle tracers clearly show the re-entrant jet flow and the unsteadiness of the vortex structures inside of the stable attached cavity. Furthermore, trapped particles tracers indicate the semi-Vortex Street in the wake, which is induced by the resistance effect from the stable cavity on the partial shedding of the LEV. (c) In the cloud cavity shedding stage, no connection between the LE-LCS and TE-LCS can be observed near the middle part of the hydrofoil, which implies the break-up of vortex structures inside the attached cavity. Meanwhile, particle tracers show the breakup of vortex structure inside the attached cavity, as well as the shedding process of the rear part, which is enhanced by the detached cloud cavity.

Sensitivity of tropical cyclone intensification to inner-core structure

In this study, the dependence of tropical cyclone (TC) development on the inner-core structure of the parent vortex is examined using a pair of idealized numerical simulations. It is found that the radial profile of inner-core relative vorticity may have a great impact on its subsequent development. For a system with a larger inner-core relative vorticity/inertial stability, the conversion ratio of the diabatic heating to kinetic energy is greater. Furthermore, the behavior of the convective vorticity eddies is likely modulated by the system-scale circulation. For a parent vortex with a relatively higher inner-core vorticity and larger negative radial vorticity gradient, convective eddy formation and radially inward propagation is promoted through vorticity segregation. This provides a greater potential for these small-scale convective cells to self-organize into a mesoscale inner-core structure in the TC. In turn, convectively induced diabatic heating that is close to the center, along with higher inertial stability, efficiently enhances system-scale secondary circulation. This study provides a solid basis for further research into how the initial structure of a TC influences storm dynamics and thermodynamics.

Two‐dimensional lattice Boltzmann simulation of natural convection in differentially heated square cavity: Effect of Prandtl and Rayleigh numbers

A thermal lattice Boltzmann method is utilized for the numerical simulation of the natural convection heat transfer phenomena inside a differentially heated square cavity. Numerical simulations are performed to elucidate the combined effects of Prandtl number (0.71 ≤ Pr ≤ 100) and Rayleigh number (104 ≤ Ra ≤ 106) on heat transfer and fluid flow characteristics inside the cavity. Detailed insights are gained by the evaluation of isotherms, stream functions and vorticity profiles. For higher Prandtl and Rayleigh numbers, streamlines become more confined to hot wall with the loss of symmetry. Similarly, isotherms patterns become more stratified towards the hot wall. The centre‐line velocity and temperature profiles are also analyzed. Further, the heat transfer rate is estimated in terms of average Nusselt number. It suggests a proportional increase in the average Nusselt number with the increasing values of Prandtl number for considered range of Rayleigh numbers. Finally, the numerical data for the average Nusselt number are presented in the form of a correlation for their best utilization in design and engineering practices.

Reynolds number effects in the near-field of a turbulent square jet

High-resolution particle image velocimetry (PIV) was used to study the flow characteristics in the near-field of a turbulent square water jet issuing from a smooth contraction nozzle. Mean velocity and turbulence statistics are investigated over a range of jet Reynolds number ReD (based on jet exit velocity and equivalent nozzle diameter) varying from 10,000 to 41,400. The influence of ReD on the shear layer formation in terms of momentum thickness, Taylor length scale and the evolution of the turbulent/non-turbulent interface are also studied. The velocity measurements reveal that the jet in the near-field is dependent on Reynolds number at the lower end of the range (ReD=10,000) despite the fact that all exit jet profiles closely approximate a “top-hat” shape. It is shown that the jet shear layer grows faster at lower Reynolds number which, combined with an increase of the spanwise turbulence component (vrms) along the jet centerline, suggest rapid axis switching. Much weaker Reynolds number dependence of the mean velocity, turbulence intensities, momentum thickness and jet centerline anisotropy (urms/vrms) was found for square jets at ReD>104. The Taylor length scale calculated along the jet centerline decreases with increasing Reynolds number and asymptotes to a constant value for X/D>4. The turbulent/non-turbulent (T/NT) regions of the jet have been identified using the velocity criteria proposed in the literature. Evolution of the T/NT interface and the effect of ReD on the conditionally averaged streamwise velocity and vorticity are also investigated in the near-field of the jet. No jump of the conditionally averaged streamwise velocity and vorticity profiles was noted in the near-field of the square jet.

Spatio-temporal development of the pairing instability in an infinite array of vortex rings

In this paper, we study the linear stability of an infinite vortex ring array with respect to the pairing instability, using a spectral code. The base flow solution, obtained after a short relaxation process, is composed of rings with a Gaussian azimuthal vorticity profile. The temporal stability properties are first obtained and compared to the theoretical predictions obtained by Levy and Forsedyke (1927 Proc. R. Soc. Lond. A 114 594–604). The spatio-temporal evolution of a localized perturbation is then computed. The growth rate of the perturbation in the frame moving at the speed v is obtained for all v. The variation of with respect to the parameters of the flow is provided.

The influence of nonlinear bottom friction on the properties of decaying cyclonic and anticyclonic vortex structures in a shallow rotated fluid

AbstractThe problem of the decay of intense vortices in a shallow rotated neutrally stratified fluid is considered using simulations with a modified model of von Kármán type and laboratory experiments. The numerical model describes a forced axisymmetric vortex, vertically confined, but infinite in the horizontal plane. It may be used for comparisons with laboratory experiments, in which a quasi-turbulent eddy flow is generated, using magnetohydrodynamic forcing. A detailed analysis of simulations of the free decay of the flow from an initial state, given either by an arbitrary Poiseuille or by a forced stationary profile of vorticity, is provided. Based on this analysis, three different regimes of decay of intense anticyclones in the parameter space of the Ekman and initial Rossby numbers are found. It is shown that anticyclones with large enough Rossby and small enough Ekman numbers may decay to a non-trivial stationary state, or at least they decay much slower than cyclones of the same intensity. The laboratory experiments show much slower decay of intense anticyclones than weak anticyclones or cyclones, and also a dominance of anticyclones over cyclones during the initial stage of decay. These observations qualitatively agree with theoretical predictions.