Maximum Swirl Velocity Research Articles

Parameter Optimization and Experimental Study on Effect of Rounding Off Edge Radius on Heat and Mass Transfer Between Gas Layers and Thermal Performance of Vortex Tube

AbstractSince 1933, the vortex tube (VT) has been used as a device for cooling, heating, and creating separation process (particles, hydrocarbons, energy, etc.). The VT efficiency is very relevant to the characteristics of the inside fluid flow patterns. The most important part in the VT that directs the fluid flow is the “navigator,” which has an angle and a rounded edge. In this study, the effects of change in the radius of the rounded edge (R* = 0, 0.5, 1, 1.5, 2, and 2.5 mm) on flow patterns and thermal performance of the VT are studied experimentally. The results show that with a rounded radius variation from 0 to 2 mm, the VT cooling efficiency increases sharply (34.84%) and with a further increase, there will be a decreasing trend for the cooling efficiency (10.48%). Also, this radius affects the location of the effective cold mass fraction (CMFs related to the highest thermal performance). In addition, variations in the gas velocity (axial/swirl at two positions Z/L = 0.3, 0.7) were studied with complete details and compared with other researchers' valid reports. As the results, the maximum axial velocity near the chamber and the maximum swirl velocity drop along the tube are effective on better cooling/heating performance.

Submesoscale Coherent Vortices Observed in the Northeastern South China Sea

AbstractAlthough submesoscale coherent vortices (SCVs) have been observed in different parts of the world's oceans, most of them were captured by accident by a limited number of hydrographic profiles. Here, using concurrent velocity and temperature/salinity measurements from a submesoscale‐resolving mooring array (2 km), two oppositely rotating SCVs are for the first time reported in detail in the northeastern South China Sea (NESCS). For the anticyclonic (cyclonic) SCV, its core lays at 210 m (180 m) with a mean Rossby number of −0.76 (0.65); its maximum swirl velocity, radius, vertical scale, and Burger number are estimated to be 0.43 m s−1, 31 ± 5 km, 235 m, and 1.5 (0.23 m s−1, 16 ± 3 km, 100 m, and 2.5), respectively. Corresponding to the SCVs' submesoscale nature, their momentum is governed by gradient‐wind rather than geostrophic balance. Consequently, kinetic energy of the anticyclonic (cyclonic) SCV would be on average underestimated (overestimated) by 47% (68%) if the traditional geostrophic relation was used to diagnose the velocity. Further analysis shows that thermohaline properties within the anticyclonic and cyclonic SCVs are close to the Kuroshio water and the local NESCS water, respectively. By combing water mass tracing and high‐resolution simulations, we suggested that the anticyclonic (cyclonic) SCV was generated in the southern Luzon Strait (southwest of Taiwan) through current‐topography interaction. We further proposed that the observed SCVs here should be a common phenomenon in the NESCS, which may provide a novel route for the tracer exchange between the NESCS and the western Pacific.

Non-equilibrium condensation of water vapour in supersonic flows with shock waves

The fluid flow and heat and mass transfer in a supersonic separator are not understood well due to the complicated interaction of the supersonic flow, swirling flow, phase transition and shock waves. In the present study, we develop a wet steam model to investigate the flow structure inside a supersonic separator with the co-existence of non-equilibrium condensation and shock waves. A study of the effect of the inlet subcooling and inlet saturation on the condensation behaviour is conducted to evaluate the performance of the supersonic separation with a focus on the shock wave. The numerical result shows that the degree of supersaturation of the water vapour can reach a maximum value of 4.28 within the designed supersonic separator and generate a peak nucleation rate of approximately 1021 kg m−3 s−1. The occurrence of the shock wave changes the equilibrium thermodynamic state, which leads to the re-evaporation of the condensed droplet. Higher inlet subcooling and inlet saturation not only shift downstream the position of the shock wave, but also induce an earlier condensation and higher liquid fraction. For the present nozzle, when the inlet subcooling and inlet saturation are about 34 K and 0.28 respectively, the shock wave intersects the region of the intense nucleation process, the non-equilibrium condensation process is terminated due to the increase of the pressure and temperature downstream the shock wave. Stronger swirling flow results in non-uniform distribution of the static pressure and decreases the nucleation rate of water vapour. The high swirling flow with a maximum swirl velocity of 150 m/s weakens the liquid fraction by 25% compared to the no swirling flow. This indicates that it is important to balance the swirling flow and condensation process to achieve an efficient performance of the supersonic separator.

Investigating the Exergy of Flow and Three Dimensional Flow Study within the Vortex Tube Device Using Computational Fluid Dynamics (CFD)

In this paper, the effect of inlet pressure on the performance of vortex tube device has been investigated using 3D simulation and CFD technique by Fluent software. The flow inside the device is considered as compressible and turbulent. In order to understand and investigate the effect of inlet pressure, different inlet pressures are entered into the device and the results are extracted and analyzed. The main goal is to achieve the minimum cold exit temperature and maximum swirl velocity in the vortex tube. This paper indicates that inlet pressure of 4.8 bars is an optimal inlet pressure which is justifiable economically and also in terms of the amount of produced cooling. The CFD results show that increase in inlet pressure, increases the entropy production and subsequently the system disorder. Finally, the existing gaps in the previous studies will be filled by examining the inlet and exit exergies in the vortex tube device. Inlet exergy has not considerable changes in terms of α and have a constant value and at α=0.3691 the minimum exergy efficiency is occurred according to the calculations.

Vertical Structure Anomalies of Oceanic Eddies and Eddy-Induced Transports in the South China Sea

Using satellite altimetry sea surface height anomalies (SSHA) and Argo profiles, we investigated eddy’s statistical characteristics, 3-D structures, eddy-induced physical parameter changes, and heat/freshwater transports in the South China Sea (SCS). In total, 31,744 cyclonic eddies (CEs, snapshot) and 29,324 anticyclonic eddies (AEs) were detected in the SCS between 1 January 2005 and 31 December 2016. The composite analysis has uncovered that changes in physical parameters modulated by eddies are mainly confined to the upper 400 m. The maximum change of temperature (T), salinity (S) and potential density (σθ) within the composite CE reaches −1.5 °C at about 70 m, 0.1 psu at about 50 m, and 0.5 kg m−3 at about 60 m, respectively. In contrast, the maximum change of T, S and σθ in the composite AE reaches 1.6 °C (about 110 m), −0.1 psu (about 70 m), and −0.5 kg m−3 (about 90 m), respectively. The maximum swirl velocity within the composite CE and AE reaches 0.3 m s−1. The zonal freshwater transport induced by CEs and AEs is (373.6 ± 9.7)×103 m3 s−1 and (384.2 ± 10.8)×103 m3 s−1, respectively, contributing up to (8.5 ± 0.2)% and (8.7 ± 0.2)% of the annual mean transport through the Luzon Strait.

Dynamical Characterization of a Low Oxygen Submesoscale Coherent Vortex in the Eastern North Atlantic Ocean

AbstractA submesoscale coherent vortex (SCV) with a low oxygen core is characterized from underwater glider and mooring observations from the eastern tropical North Atlantic, north of the Cape Verde Islands. The eddy crossed the mooring with its center and a 1 month time series of the SCV's hydrographic and upper 100 m currents structure was obtained. About 45 days after, and ∼100 km west, the SCV frontal zone was surveyed in high temporal and spatial resolution using an underwater glider. Satellite altimetry showed the SCV was formed about 7 months before at the Mauritanian coast. The SCV was located at 80–100 m depth, its diameter was ∼100 km and its maximum swirl velocity ∼0.4 m s−1. A Burger number of 0.2 and a vortex Rossby number 0.15 indicate a flat lens in geostrophic balance. Mooring and glider data show in general comparable dynamical and thermohaline structures, the glider in high spatial resolution, the mooring in high temporal resolution. Surface maps of chlorophyll concentration suggest high productivity inside and around the SCV. The low potential vorticity (PV) core of the SCV is surrounded by filamentary structures, sloping down at different angles from the mixed layer base and with typical width of 10–20 km and a vertical extent of 50–100 m.

Simple Models of Helical Baroclinic Vortices

Two distinct asymptotic solutions of the inviscid Boussinesq equations for a steady helical baroclinic Rankine-like vortex with pre- scribed buoyancy forcing are considered and critically compared. In both cases the relative distribution of the velocity components is the same across the vortex at all altitudes (the similarity assumption). The first vortex solution demonstrates monotonic growth with height of the vortex core radius, which becomes infinite at a certain critical altitude, and the corresponding attenuation of the vertical vorticity. The second vortex solution schematises the vortex core as an inverted cone of small angular aperture. These idealised vortices are then embedded in a convectively unstable boundary layer; the resulting approximate vortex solutions have been applied to determine the maximum rotational velocity in vortices. Both models predict essentially the same dependence of the model-inferred peak rotational velocity on the swirl number (the ratio of the maximum swirl velocity to the average vertical velocity in the main vortex updraft). The helicity budget of the vortex flow is analysed in detail, where applicable.

Sea surface structure of North Brazil Current rings derived from shipboard and moored acoustic Doppler current profiler observations

[1] In the western tropical Atlantic, the North Brazil Current retroflection periodically sheds large anticyclonic rings, which then propagate northwestward. Between 1998 and 2000, the North Brazil Current Rings Experiment sampled a large number of these rings by shipboard and moored acoustic Doppler current profiler. Ten of the sampled rings are analyzed in this study, focusing on their sea surface dynamic properties. The rings exhibit a radial structure consisting of two regimes, an “inner” core region in near solid body rotation and an “outer” ring regime with an approximately exponentially decaying structure. The observations show a sharp change in vorticity at the regime transition and the presence of a strong opposite vorticity shield bounding the inner solid body core. We show that Gaussian models, commonly used to represent the surface expression of these and other rings, are adequate for determining the sea surface height anomaly but tend to poorly estimate other properties such as the maximum swirl velocity. Therefore, we propose a new two-part model as a better approximation of the rings' radial structure. According to the cyclogeostrophic balance approximation, the sea surface height distribution across the inner ring has a parabolic shape, while the outer ring has an exponential structure similar to the velocity field. Interestingly, many of the observed rings have an intensity very close to the theoretical limit for anticyclones at these latitudes, which is believed to be due to inertial instability.

A subsurface warm‐eddy off northern Baja California in July 2004

Upper‐ocean eddies are commonly observed from remote sensing, but submerged eddies are more difficult to detect. During July 2004, a 21‐day hydrographic survey in the southern region of the California Current was carried out to investigate the mesoscale variability. We observed for the first time a subsurface anticyclonic eddy off northern Baja California with the same water mass characteristics as the California Undercurrent. The core of the eddy was quasi‐circular with radii of 35 km and thickness of 250 m. The maximum swirl velocity was ∼3 cms−1. The water mass of the core of the eddy was characterized by potential temperature of 11°C, salinity of 34.5, and dissolved oxygen of 1.4 mll−1. The eddy propagated westward. The subsurface warm‐eddy could transport relatively saline water into the North Pacific subtropical gyre.

Steady‐state properties and statistical distribution of atmospheric dust devils

A steady Rankine‐like vortex model of a dust devil is reported, which takes into account the sunlight absorption by airborne dust particles and the diabatic heating of a rotating dust column. For the maximum swirl velocity being a few times larger than the mean updraft velocity, the model admits significant simplifications allowing for its complete analytical treatment. The competitive effects of (i) the surface heat fluxes and (ii) the suspended‐dust‐caused diabatic heating on the vortex constitution (strength, height and shape) are examined. The second factor is found to be more influential for the strongest vortices. A reference exponential distribution of dust devils over their radius (visible diameter), which was introduced based on general arguments of mathematical information theory, is shown to fit satisfactory well the observational data on dust devils in Arizona and southern California and, in a preliminary way, on Mars.

Unsteady nature of leading edge vortices

Flow visualization and velocity measurements were carried out in leading edge vortices over a delta wing. Very large velocity fluctuations were observed well upstream of breakdown and also in the absence of breakdown. The maximum rms swirl velocity can exceed the free stream velocity depending on the angle of attack. Examination of the probability density functions and spectra suggest that the core of the vortex is characterized by large amplitude, broad band random velocity fluctuations. It was suggested that these large velocity fluctuations are due to apparently random displacements of the vortex core. A simple model of the flow that produced the essential features of the experimental results was presented.

Large-Eddy Simulation of a Tornado’s Interaction with the Surface

High-resolution, fully three-dimensional, unsteady simulations of the interaction of a tornado vortex with the surface were performed in an attempt to answer questions about the character of turbulent transport in this unique flow. The authors demonstrate that sufficient resolution was achieved for the particular physical conditions of their example that the time-averaged velocity and pressure distributions showed little sensitivity in the region of maximum velocities to either finer resolution or modified subgrid turbulent model. The time-averaged velocity distributions show the maximum velocity values occurring within 50 m of the surface. The instantaneous velocity distributions show the turbulence dominated by a relatively small number of strong secondary vortices spiralling around the main vortex with the maximum instantaneous velocities typically one-third larger than the maximum time-averaged velocity. These eddies are centered a little inside of the cone of maximum mean swirl velocity and spiral around the mean vortex at velocities less than the average maximum velocity. Statistical analysis of the velocity fluctuations induced by the secondary vortices shows that the turbulent transport of angular momentum is predominantly inward at low levels, allowing the inner recirculating flow to acquire values of angular momentum of up to 30% of that provided by the inflow boundary conditions, thus enhancing the surface intensification of the velocities.

Rings of the North Brazil Current: Their structure and behavior inferred from observations and a numerical simulation

Large anticyclonic rings are shed from the retroflecting North Brazil Current (NBC) near 8°N in the tropical western Atlantic. New subsurface velocity and temperature measurements within three such rings are presented here and are found to be consistent with previous in situ and remotely sensed NBC ring measurements. A high‐resolution numerical model of the Atlantic Ocean forced by monthly wind stress and an imposed meridional overturning cell is found to shed NBC rings that approximate those observed. The model rings are more surface‐intensified than those observed and somewhat smaller in diameter. Both observed and modeled NBC rings move northwestward along the coast of South America with a speed of 8–16 cm/s, considerably slower than predicted by analytical theories describing westward ring propagation. At least 2–3 rings per year separate from the NBC retroflection. Annually, 1–3 rings translate intact from their formation region near 50°W to the islands of the southeastern Caribbean, where they disintegrate after a lifetime of about 100 days. The volume of fluid trapped within the core of an NBC ring and isolated from external mixing is estimated using potential vorticity as a tracer. The horizontal limits of the trapped core volume closely coincide with the radius of maximum swirl velocity, while the vertical limit of the core is typically less than the subsurface extent of significant swirl velocity. The core volume of a typical observed ring is 3.2±1.0×1013 m3. This corresponds to an annualized per‐ring mass transport near 1 Sv (106 m3/s), similar to previous estimates. This study is the first to make use of subsurface temperature and velocity data to compute the volume of the anomalous ring core. NBC rings may be responsible for 3–4 Sv of direct mass transport across the equatorial‐tropical gyre boundary or 20–25% of the total upper ocean cross‐gyre transport required by the Atlantic meridional overturning cell. Translating NBC rings may contribute 20% of the total meridional heat transport by the ocean at this latitude.

Unsteady, Turbulent Convection into a Homogeneous, Rotating Fluid,with Oceanographic Applications

Abstract Turbulent convection into a homogeneous, rotating fluid has been generated in laboratory tanks, for both laterally confined and unconfined domains. When a given experiment was in a solid-body rotation, a source located at the top surface of the water column was activated to release denser saltwater into the underlying, less-dense fluid of total depth H. As a result, a downward propagating 3D turbulent front was formed. Eventually, at a transition depth zc, rotational effects dominated the turbulence and many quasi-2D vortices were generated, which then penetrated downward beneath the upper 3D turbulent layer. Measurements in the confined experiments gave zc ≈ (12.7 ± 1.5) (B0/f3)1/2; the mean diameter (Dv) of the quasi-2D vortices as Dv≈(15.0±1.5) (B0/f3)1/2, their downward speed of propagation (uc) as uc ≈ (1.0 ± 0.1) (B0/f)1/2, and the maximum swirl velocity (uv) of an individual vortex as uv ≈ (4.0 ± 0.4)(B0/f)1/2 (where B0 is the surface buoyancy flux and f the Coriolis parameter). All are in...

A comparative study of atmospheric and laboratory‐analogue numerical tornado‐vortex models

AbstractA detailed sensitivity study of a tornado‐vortex model developed by Howells and Smith is presented in order to draw a comparison between the results of various axisymmetric models that purport to simulate laboratory and tornadic vortices. The model incorporates a stretched finite‐difference mesh in both radial and vertical directions and this provides for an increase in resolution in the vicinity of the vortex core and lower‐boundary layer. The development of the vortex is studied for different values of two key parameters. These are the ratio of the applied tangential velocity to the mean vertical velocity (the swirl ratio) and the Reynolds' number based on the eddy diffusivity coefficient, the mean radial velocity, and the radius of the domain. Most of the simulations are for a no‐slip lower boundary, but a few free‐slip experiments are presented for comparison. These serve to highlight the importance of the radial inflow jet for the no‐slip case.Emphasis is placed in the no‐slip experiment on the relative importance of the primary meridional circulation associated directly with the applied forcing, and the secondary circulation induced by the lower‐boundary layer. For weak applied swirl (tangential) velocity (1ms−1), the secondary circulation is relatively weak and the maximum tangential velocity is associated with the convergence produced by the primary circulation. Accordingly, this maximum occurs at a substantial height above the lower boundary. For moderate applied swirl velocity (4ms−1), the secondary circulation is manifest as a strong radial inflow jet near the lower boundary. This jet advects rotating air close to the axis. The flow that evolves is dependent on the magnitude of the eddy turbulent diffusivity coefficient. For a relatively low diffusivity coefficient (10m2s−1), a large amplitude centrifugal wave forms and breaks violently near the corner region. At higher values (20m2s−1), a quasi‐steady toroidal vortex develops in the region where the stream surfaces expand, and the maximum swirl velocity is found to occur at low levels near the corner region. This breakdown feature is advected out of the domain when the diffusivity coefficient is increased to 30m2s−1. When the simulations are performed using high imposed swirl velocities (10ms−1), a very intense vortex forms with maximum tangential velocity exceeding 55ms−1, despite the frictional losses near the ground. This is considerably stronger than the equivalent free‐slip vortex which attains a maximum tangential velocity of 44ms−1. Although many of these results have been obtained in past numerical studies, a clear demonstration of the importance of the secondary circulation as compared with the primary circulation has not been forthcoming due to a wide variety of experimental configurations and external parameters.

The Effects of Swirl on the Performance of Supercritical Convergent-Divergent Nozzles

SummaryThe quasi-one-dimensional theory due to Carpenter and Johannesen is applied to supercritical swirling flows in convergent-divergent nozzles. Only flows with uniformly constant stagnation pressure and entropy are considered. Values of exit impulse function and area ratio are given for various types of swirling flow with a range of back-pressure ratios. Thrust coefficients are calculated using these values and specific thrust coefficients plotted against maximum swirl velocity for various cases. In some cases the specific thrust for swirling nozzle flows very slightly exceeds the no-swirl value. The effects of nozzle wall curvature and flight speed on swirling nozzle flows are discussed. The contribution of post-exit thrust to the total thrust is estimated for swirling flow. Finally, the implications are considered of using swirl at the cruise conditions of a typical SST.

The stability of a trailing line vortex. Part 1. Inviscid theory

The inviscid stability of swirling flows with mean velocity profiles similar to that obtained by Batchelor (1964) for a trailing vortex from an aircraft is studied with respect to infinitesimal non-axisymmetric disturbances. The flow is characterized by a swirl parameterqinvolving the ratio of the magnitude of the maximum swirl velocity to that of the maximum axial velocity. It is found that, as the swirl is continuously increased from zero, the disturbances die out quickly for a small value ofqifn= 1 (nis the azimuthal wavenumber of the Fourier disturbance of type exp{i(αx+nϕ − αct)}); but for negative values ofn, the amplification rate increases and then decreases, falling to negative values atqslightly greater than 1·5 forn= −1. The maximum amplification rate increases for increasingly negativenup ton= −6 (the highest mode investigated), and corresponds toq≃ 0·85. The applicability of these results to attempts at destabilizing vortices is briefly discussed.