Twin Vortices Research Articles

An integrated approach to the heat and water mass dynamics of a large bay: High-resolution simulations of Funka Bay, Japan

[1] Heat and water mass transports tagged by water type in a bay were investigated using daily outputs from a high-resolution land-sea coupled model. The modeled circulation and water property distribution were similar to those reported by observations. In this paper, the heat angle is introduced to accurately define the roles of the lateral heat flux (LF) into the bay and the net surface heat flux on temperature changes in the bay water. As a result, ocean phenomena in the bay can be categorized by using the heat angle in an intensive LF regime on short-period timescales and a gradual LF regime on intraseasonal timescales. Our close examination revealed that the velocity fields can be classified into three flow patterns: a twin vortex accompanied by positive LF, a clockwise flow with negative LF, and an anticlockwise flow with both LFs. These patterns occur in both intensive and gradual LF regimes. Intensive wind-driven LF forced by atmospheric disturbances was often observed from summer to autumn in 2008, accompanying the intrusion of southern subtropical Tsugaru warm water that was colder than the deep bay water (LF < 0) and subarctic Oyashio water that was warmer than the surface bay water (LF > 0), but both were hardly found in 2009. This thermal contrast affects the interannual difference in the stratification inherent in the bay. Our integrated analysis method is useful for prompt and robust understanding of the thermal and dynamic states in a bay based on ocean simulation data.

Wavy liquid films in interaction with a confined laminar gas flow

AbstractA low-dimensional model capturing the fully coupled dynamics of a wavy liquid film in interaction with a strongly confined laminar gas flow is introduced. It is based on the weighted residual integral boundary layer approach of Ruyer-Quil &amp; Manneville (Eur. Phys. J.B, vol. 15, 2000, pp. 357–369) and accounts for viscous diffusion up to second order in the film parameter. The model is applied to study two scenarios: a horizontal pressure-driven water film/air flow and a gravity-driven liquid film interacting with a co- or counter-current air flow. In the horizontal case, interfacial viscous drag is weak and interfacial waves are primarily driven by pressure variations induced by the velocity difference between the two layers. This produces an extremely thin interfacial shear layer which is pinched at the main and capillary wave humps, creating several elongated vortices in the wave-fixed reference frame. In the capillary wave region preceding a large wave hump, flow separation occurs in the liquid in the form of a vortex transcending the liquid–gas interface. For the gravity-driven film, a twin vortex (in the wave-fixed reference frame) linked to the occurrence of rolling waves has been identified. It consists of the known liquid-side vortex within the wave hump and a previously unknown counter-rotating gas-side vortex, which are connected by the same interfacial stagnation points. At large counter-current gas velocities, interfacial waves on the falling liquid film are amplified and cause flooding of the channel in a noise-driven scenario, while this can be delayed by forcing regular waves at the most amplified linear wave frequency. Our model is shown to exactly capture the long-wave linear stability threshold for the general case of two-phase channel flow. Further, for the two considered scenarios, it predicts growth rates and celerity of linear waves in convincing agreement with Orr–Sommerfeld calculations. Finally, model calculations of nonlinear interfacial waves are in good agreement with film thickness and velocity measurements as well as streamline patterns in both phases obtained from direct numerical simulations.

Relation between Drag Characteristic and Vortex Structure of Accelerating Cylinder from a Rest

The purpose of this study is to understand the relationship between the vortex structure and the drag characteristic of a circular cylinder during accelerated motion from a rest. Wake flow of the accelerated circular cylinder was visualized by fluorescent dye technique. Drag act on the accelerated cylinder was determined by the measurement device using an expansion coil. The development of a twin vortex separated from the cylinder was observed during the accelerated motion. At the beginning of the accelerated motion, drag showed sudden change at the same time that the twin vortex collapsed temporarily. Then the twin vortex was gradually grown by supplying vorticity of shear layer vortices. We confirmed that drag coefficient of accelerated circular cylinder was influenced depending on the accelerations.

236 フローターサープ速度域におけるバレーボール後流のPIV解析(飛翔ボールの空力特性)

Non spinning serve or shot exist as a technique in volleyball and soccer ball. Because the flight paths of these non-spinning balls are not stable. In the volleyball, it is called floater serve. Floater serves in volleyball and erratic path shots of the soccer ball are different in their speed area. In other words, they are the phenomena in a transition region on volleyballs and that in the super critical domain on soccer balls. It is thought that origin of each erratic phenomenon is different. In this study, three dimensional twin vortices were visualized behind volleyball using three-dimensional dynamic PIV and solved the origin of erratic motion on the floater serve. Quick motions of three-dimensional twin vortices were confirmed in the transition region on volleyball.

238 サッカーボールパネルの違いによるボール後流の違い(飛翔ボールの空力特性)

It is known that low rotating or no-rotating balls have the erratic ball path especially in the recemt soccer balls. Aerodyanamic performance and dynamic velocity distributions of the balls wake were measured by 3 component load cell and dynamic PIV system on five types of different panel balls. As a result, the difference of drag coefficient of each ball was found especially in drag crisis regions. Three-dimensional twins vortices were found in the wake behind every balls. The magnitude of the jet stream velocity between each twin vortices was also found to vary according to the difference of the ball panel.

G1102 サッカーボールのパネルの違いによるボール後流の違い(GS11 物体まわり流)

It is known that low rotating or no-rotating balls have the erratic ball path especially in the new different panels of soccer balls. Velocity distributions of the balls wake wake were measured by Dynamic PIV on five balls of the different panels. Three dimensional twin vortices and jets between them were found in each wake of the ball. The velocities of the jet cores were different according to the difference of the ball panel. Especially, large velocity vectors were found in the ball wake which was heard to be big erratic path.

Visualization of the flow field around a flat plate

Vortex dynamics play an important role in the transport of mass, momentum, and energy of a flow field. Thus, vortex manipulation is the key for controlling the efficiency of any device involving flow mixing. For example, the fuel-air mixing in a combustion chamber is the primary process governing the combustion efficiency. In this study we visualize and measure the size of vortices behind a flat plate to address vortex formation, size manipulation, distance of the origin of reverse flow from the plate, and response to the change of the Reynolds (Re) number. This study also investigates the physics behind the generation and size manipulation of twin vortices behind a flat plate and the functional dependence of reverse flow location with the Re number.

A quantitative deconstruction of the morphology of high-latitude ionospheric convection

We present an analysis of ionospheric convection data derived from velocity measurements made by the Super Dual Auroral Radar Network (SuperDARN). Our analysis uses an established technique for combining the network data to produce maps of large-scale convection by fitting a spherical harmonic expansion of the ionospheric electric potential to the radar measurements. We discuss how the basis functions of the spherical harmonic expansion describe different characteristic elements of the ionospheric convection pattern and show how their associated coefficients can be used to quantify the morphology of the convection, much like the total transpolar voltage is used to quantify its strength, in relation to upstream interplanetary magnetic field conditions and associated magnetospheric activity. We find that similar to 2/3 of the voltage associated with the typical convection pattern is described by a simple twin vortex basis function. The magnitude of the twin vortex is strongly dependent on IMF B-Z and the degree of its (typically westward) rotation is weakly dependent on IMF B-Y. The remaining similar to 1/3 of the total voltage is associated with deviations from the basic twin vortex pattern, introduced by the addition of other basis functions, such as IMF B-Y associated dusk-dawn asymmetries, nightside convection features associated with tail activity, and reverse high-latitude convection cells associated with intervals of IMF B-Z > 0.

A quantitative deconstruction of the morphology of high‐latitude ionospheric convection

We present an analysis of ionospheric convection data derived from velocity measurements made by the Super Dual Auroral Radar Network (SuperDARN). Our analysis uses an established technique for combining the network data to produce maps of large‐scale convection by fitting a spherical harmonic expansion of the ionospheric electric potential to the radar measurements. We discuss how the basis functions of the spherical harmonic expansion describe different characteristic elements of the ionospheric convection pattern and show how their associated coefficients can be used to quantify the morphology of the convection, much like the total transpolar voltage is used to quantify its strength, in relation to upstream interplanetary magnetic field conditions and associated magnetospheric activity. We find that ∼2/3 of the voltage associated with the typical convection pattern is described by a simple twin vortex basis function. The magnitude of the twin vortex is strongly dependent on IMF BZ and the degree of its (typically westward) rotation is weakly dependent on IMF BY. The remaining ∼1/3 of the total voltage is associated with deviations from the basic twin vortex pattern, introduced by the addition of other basis functions, such as IMF BY associated dusk‐dawn asymmetries, nightside convection features associated with tail activity, and “reverse” high‐latitude convection cells associated with intervals of IMF BZ &gt; 0.

Flow Analysis around an In-Line Forced Oscillating Circular Cylinder Using IB-Method

Flow around an in-line forced oscillating circular cylinder was simulated numerically by using OpenFOAM in order to clarify the mechanism of flow induced vibration. Immersed boundary Method is used to solve the moving boundary. Reynolds number is set to 1000 and the reduced velocity is set to the range from 0 to 10. In the first excitation region, it is shown that negative drag force which is a factor for an in-line oscillation of a circular cylinder comes from contacting between high pressure region and a circular cylinder. The present simulation shows that twin vortex has an important role on the contact phenomena. In the second excitation region, it is shown that time averaged lift drag doesn't become zero on some oscillating conditions. It is considered that a cross-flow oscillation comes from the phenomena.

G060032 水平加熱正方形柱まわりの強制-自然並行流共存対流の流動と伝熱

Experimental investigations have been carried out on the aiding flows of forced and natural convection around heated horizontal square cylinder. The experiments covered the ranges of Reynolds and modified Rayleigh numbers as; 50<Red <1300, 10^4<Ra_d^* <7.0x10^5. The flow fields around the cylinder were visualized with fine particles. The result showed that the wake region around the cylinder is reduced with increasing the buoyancy force, and steady, counter-rotating twin vortex appears behind the cylinder. The local heat transfer coefficients were measured. The result showed that the local and average Nusselt numbers from the cylinders are markedly increased with the buoyancy force. It is also found that the forced, natural and mixed convective regions can be classified with the non-dimensional parameter [Gr_d^*/(Nu_dRe_d^2)].

S053043 静止状態から加速運動を行う円柱の抗力特性と渦構造の関係

Vortex structure and drag characteristic of a circular cylinder accelerated from a rest were investigated by drag measurement and flow visualization techniques. The circular cylinder installed in a linear-motor actuator system moves with a constant acceleration in a water tank. The drag measurement of the circular cylinder was conducted by the measurement equipment using the expansion of coil. The wake flow of a circular cylinder is visualized by using the dye. The formation and the collapse process of the twin vortex and shedding vortices were observed during accelerated motion of the circular cylinder. From the results of flow visualization, the drag was increased after the formation of twin vortex. The drag gradually increased after the collapse of twin vortex. Then the drag increased rapidly after the formation of shedding vortices.

Fluid dynamics modeling of a stratified disk burner in swirl co-flow

The work describes Large Eddy Simulations of an unconfined partially premixed propane-air burner with swirl. A premixing section consisting of two axisymmetric cavities, formed along three concentric disks, supplies the afterbody disk recirculation with a radial equivalence ratio gradient. Flame stabilization is established there supported by a swirling co-flow of air. Lean and ultra-lean conditions are examined by regulating the fuel-air stratification and the swirl. An implicit finite-volume LES method is applied, employing the dynamic Smagorinsky model and the Eddy Dissipation Concept. Propane combustion and NOx production are modelled with an eleven-step scheme. The LES reveals intrinsic features of the flame front anchoring close to the burner rim under the influence of inlet mixture stratification and reproduces large and small scale features of the primary vortex flame stabilization and its interaction with the downstream vortex breakdown zone. Comparisons with measurements indicate a satisfactory agreement in the mean and turbulent velocity fields, while the impact of combustion and heat release on the wake development is reproduced well by the model. Deviations in the mean and fluctuating fields increase in the vicinity of the interaction of the twin vortex system. Preliminary comparisons between simulations and global species measurements suggest that the turbulence-chemistry model represents satisfactorily the experimental trends. ▸ Large Eddy Simulations of stratified disk burner are performed. ▸ Combustion is stabilized on a cavity premixer/disk burner under the action of swirl. ▸ Turbulent reactions were treated with the EDC model with eleven-step chemistry. ▸ Simulations represented satisfactorily both large and small scale burner characteristics. ▸ Intrinsic features of stratified flame stabilization in the primary vortex are exposed.

Heat transport from a bluff body near a moving wall at Re = 100

Numerical simulations are performed for the flow and heat transfer from a stationary 2D square cylinder placed near a moving wall at Reynolds number 100 and cylinder-to-wall gap ratios in the range 0.1 ⩽ G/ D ⩽ 4. The governing equations are solved using a finite volume method. Flow and thermal field resemble that of isolated case when the cylinder is far away from the moving wall. When G/ D < 1, the twin vortex shedding pattern is transformed into single row of negative vortices and eventually the flow becomes steady when G/ D < 0.3. Lift ( C ¯ L ) and drag coefficient ( C ¯ D ) of the cylinder are higher compared to that of an isolated cylinder. Strouhal number ( St) of the cylinder increases with decrease in gap ratio from 4 to 1. With a further decrease in gap ratio, St reduces drastically before vanishing at G/ D = 0.3 as a consequence of vortex shedding suppression. In general, the heat transfer rates are higher than the case of an isolated cylinder. The mean Nusselt number (Nu M) increases with decrement in gap ratio. However, the trend is not the same. Nu M increases gradually when G/ D is varied from 4 to 0.5, while it suddenly decreases with decrement in G/ D from 0.5 to 0.3 and again rises sharply for G/ D ⩽ 0.2. Results indicate that an enhancement of 27.45% in mean Nusselt number can be achieved, compared to the isolated case, by placing the cylinder at G/ D = 0.1.

Convection surrounding mesoscale ionospheric flow channels

[1] We evaluate data from the European Incoherent Scatter (EISCAT) Svalbard radar (ESR) and Defense Meteorological Satellite Program (DMSP) spacecraft coupled with data from the Super Dual Auroral Radar Network (SuperDARN) polar cap convection patterns in order to study how the ionospheric convection evolves around a sequence of transient, mesoscale flow channel events in the duskside of the cusp inflow region. On a northwestward convection background for the interplanetary magnetic field (IMF) BY positive and BZ negative, a sequence of three eastward flow channels formed over the course of 1 hour in response to three sharp IMF rotations to IMF BY negative and IMF BZ positive. The first and third channels, due to IMF BY negative periods of ∼13 min and >30 min, respectively, develop in a similar manner: they span the entire ESR field of view and widen poleward with increasing time elapsed since their first appearance until the IMF rotates back. The convection patterns are consistent with the line-of-sight data from the ESR and DMSP within a 10 min adaption time. The flow lines form a twin-vortex flow, with the observed channel being the twin vortices' center flow. The fitting algorithm was pushed to its limits in terms of spatial resolution in this study. During portions of the channel events, the suggested twin-cell flow is not in agreement with our physical interpretation of the flow channels being reconnection events because cell closure is suggested across an anticipated nonreconnecting open-closed boundary. For these segments, we present simulated patterns which have been arrived at by a combination of looking at the raw data and examining the fitted convection patterns.

Matched refractive-index PIV visualization of complex flow structure in a three-dimentionally connected dual elbow

Flow structure in a three-dimensionally connected dual elbow is visualized using a 1/15-scale experimental apparatus simulating the 1st and 2nd elbows of JSFR cold-leg piping. A matched refractive-index PIV measurement clarifies that a low-velocity region formed on the inner wall side of the 1st elbow develops toward the 2nd elbow. This low-velocity region consists of the following two ones: a flow separation region accompanied mainly with the generation and disappearance of transverse vortices, and a velocity recovery region that has longitudinal vortices with strong unsteadiness. These longitudinal vortices exist as twin vortices in the time-averaged flow field, and their dynamic characteristics highly depend on high-velocity creeping flows generated in the 1st elbow that flow into the velocity recovery region through the side walls. Since the velocity recovery region reaches the 2nd elbow, the geometry of the 2nd elbow has a significant impact on the characteristics of the vortex shedding in the velocity recovery region. On the other hand, obvious flow separation is not observed in the 2nd elbow, whereas high-velocity flow with intense velocity fluctuation is confirmed on the inner wall side. Furthermore, the unsteady vortices shed from the velocity recovery region are transferred to the central area of the 2nd elbow while growing significantly. The visualization of the secondary elbow shortly after the 2nd elbow clarifies that a strong swirling flow is formed in the 2nd elbow. These flow structures are due to the distorted flow formed in the 1st elbow and the shape effect of the 2nd elbow.

Effects of Diffuser Leading-Edge Geometries on Performance and Noise Characteristics of a Centrifugal Compressor

The noise reduction radiated from a centrifugal compressor with vaned diffuser using hooked and tapered diffuser vane are discussed by experiments and CFD. Most dominant noise of the centrifugal compressor is interaction tone noise between impeller-discharged flow and diffuser leading-edge. Thus, the shape of diffuser leading-edge is important to reduce the noise level of interaction tone noise. In the experiments, the compressor performance by using hooked diffuser vane is deteriorated because of increase of the leakage-flow passing through the hooked portion while the hooked diffusers are able to reduce the noise level effectively. On the other hand, tapered diffuser vanes are able to reduce the noise and to maintain high compressor performance simultaneously. In the CFD, the twin vortices are generated by the leakage-flow passing through the tapered portion and the vortices induce secondary flow near the pressure surface at diffuser leading-edge and suppress the flow separation from the diffuser leading-edge.

Volume of Sedimentary Particles Lifted up by a Swirl Motion of Bubbling Jet

In our previous research, we mentioned about mixing of small solid sedimentary particles using a swirl motion of a bubbling jet. The lifting behaviors of the particles were classified into two types depending on the aspect ratio (LN/D) of the bath, where LN the nozzle depth and D the bath diameter. The two types of lifting behaviors were closely associated with the isolated vortex appearing for LN/D=0.2–0.4 and twin vortices appearing for LN/D=0.5–0.8. These vortices were similar to the tornado. In this study, empirical equations are proposed for the volume of sedimentary particles lifted up by a swirl motion of a bubbling jet as functions of the gas flow rate, vessel diameter, bath depth, and the physical properties of fluids.

0118 流れ方向強制振動直列二円柱からの渦放出(OS1-4 噴流,後流および剥離流れ現象の解明と制御,オーガナイズドセッション)

In this study, the flow features of vortex shedding from tandem arranged circular cylinders oscillating along the direction of the flow and its characteristics of fluid force were simulated using a vortex method at the ranges of interval ratio L/d=1.5〜5.5, the amplitude ratios 2a/d=0 and 0.25, the frequency ratios f/f_k= 1.6 and f/f_k=2.8 and Reynolds number Re=620. As a result of calculations, it was confirmed that the calculated flow patterns of stationary case were in reasonable agreement with previous experimental results. The variation of fluid force and vortex shedding frequency was shown for every interval ratio L/d of tandem cylinders, and relationship between the 1st cylinder and the 2nd cylinder was clarified. In the flow pattern of oscillating tandem cylinders, although the vortex street of mushroom section form was formed, the twin vortex street was not formed. The influence of vibration appears notably in the 2nd cylinder.

이동로봇의 장애물 회피를 위한 복소 포텐셜 항법의 개선

This paper deals with the enhancement of the complex potential navigation for wheeled mobile robots. The circle theorem from complex function theory is used to avoid an obstacle, and the enhancement to avoid multiple obstacles is proposed. The limit cycle navigation can be combined for robot to kick the ball to the intentioned direction. Avoiding step and superposing twin vortices can be applied to adjust the direction of robot's trajectory. The proposed method is verified through a set of simulation works, and the feasibilities for the enhancement of complex potential theory are successful.