Longitudinal Vortex Structures Research Articles

A DES study of the flow around a full-scale train under crosswind condition

A numerical study on the flow around a full-scale train subjected to crosswind has been conducted on the basis of detached-eddy simulation (DES). Finite-volume method is applied to unstructured mesh generated around a three-car-train model whose running speed is fixed to 120 km/h while the crosswind speed is set to 5 m/s,15 m/s, and 25 m/s. A clear longitudinal vortex structure is recognized in the case of the crosswind speed of 25 m/s, where two longitudinal vortices are evolved respectively from the roof and floor of the head car (roof-edge and floor-edge vortices), forming a characteristic vortex pair. This longitudinal vortex structure around the head car is discussed with a focus on its non-trivial role in the side-force mechanism. Our simulation suggests a causal association between these two vortices, which implies an importance of optimum designing of the underfloor equipment in addition to the roof and head shapes of the head car.

Assessment of turbulence and cavitation models in prediction of vortex induced cavitating flow in fuel injector nozzles

Vortex-induced cavitation develops in high pressure fuel injector equipment in areas where despite the very high surrounding pressures (>100 MPa), the rotational motion of the fluid causes large pressure drops in the core of a well-organized longitudinal vortex structure leading to cavitation with normally string type correspondingly also called string cavitation ; typically, the size of the string cavitation is small compared to the overall dimensions while they develop in a transient mode linked with the local flow turbulence. To address these complex interaction processes, some turbulence models have been tested in a diesel injector nozzle for which quantitative experimental data are available and are utilized to assess the predictive capability of relevant models; the flow has been considered compressible, turbulent and isothermal. The closure models considered include the RNG k−ε, SST k−ω, RSM, WALE LES as well as a developed very-large eddy simulation model (VLES). The employed mass-transfer cavitation models are the Schnerr and Sauer (SS), Zwart-Gerber-Belamri (ZGB), HEM and a modified ZGB model with a self-adaptive adjustment of condensation coefficient. The results indicate that the RNG k−ε model and SST k−ω model fail to predict the string cavitation induced by vortex flow in the nozzle due to the drawback of the isotropic vortex-viscosity hypothesis. The RSM model could capture an acceptable string cavitation morphology. The numerical results utilizing the VLES model and LES model are in considerably good agreement with the experimental data, while the VLES model can provide the predictions of cavitating flow as precise as that using the LES model but on a much coarser mesh. The distributions of tangential velocity are similar to the velocity characteristics of Rankin vortex or Taylor vortex, which can help understand the coherent vortex structures in string cavitating flow. It is also shown that the RSM turbulence model with modified ZGB cavitation model improves the accuracy of prediction in the vortex-induced cavitation compared to the other three cavitation models.

Synthesis of multiple longitudinal polarization vortex structures and its application in sorting chiral nanoparticles.

As the essential properties of organisms, detection and characterization of chirality are of supreme importance in physiology and pharmacology. In this work, we propose an optical technique to sort chiral materials by use of longitudinal polarization vortex (LPV) structures, which is generated with tightly focusing Pancharatnam-Berry tailored Laguerre-Gaussian beam. The nonparaxial propagation of the focusing field leads to the creation of multiple pairs of dual LPV structures with arbitrary topological charge and location, which can be independently controlled by the spatial phase modulation applied on the illumination. More importantly, the opposite spin angular momentums carried by each pair of dual foci lead to different energy flow directions, making it suitable to sort nanoparticles by their handedness. In addition, the LPV structures would also bring different dynamic behaviors to the enantiomers, providing a feasible route toward all-optical enantiopure chemical syntheses and enantiomer separations in pharmaceuticals.

Free-Stream Turbulence-Induced Boundary-Layer Transition in Low-Pressure Turbines

Abstract The aerodynamic efficiency of turbomachinery blades is profoundly affected by the occurrence of laminar-turbulent transition in the boundary layer since skin friction and losses rise for the turbulent state. Depending on the free-stream turbulence level, we can identify different paths toward a turbulent state. The present study uses direct numerical simulation as the primary tool to investigate the flow behavior of the low-pressure turbine blade. In the simulations, the flow past only one blade is computed, with periodic boundary conditions in the cross-flow directions to account for the cascade. Isotropic homogeneous free-stream turbulence is prescribed at the inlet. The free-stream turbulence is prescribed as a superposition of Fourier modes with a random phase shift. Two levels of the free-stream turbulence intensity were simulated (Tu=0.19% and 5.2%), with the integral length scale being 0.167c, at the leading edge. We observed that in the case of low free-stream turbulence on the suction side, the Kelvin–Helmholz instability dominated the transition process and full-span vortices were shed from the separation bubble. Transition on the suction side proceeded more rapidly in the high-turbulence case, where streaks broke down into turbulent spots and caused bypass transition. On the pressure side, we have identified the appearance of longitudinal vortical structures, where increasing the turbulence level gives rise to more longitudinal structures. We note that these vortical structures are not produced by Görtler instability.

The Effect of Bogie Positions on the Aerodynamic Behavior of a High-Speed Train: An IDDES Study

In this study, an improved delayed detached-eddy simulation method has been used to investigate the aerodynamic behavior of the CRH2 high-speed trains (HST) with different first and last bogie positions. The results of the numerical simulations have been validated against experimental data obtained from a previous wind tunnel test, a full-scale field test and a reduced-scale moving model test. The results of the flow prediction are used to explore the effects of the bogie positions on the slipstream, wake flow, underbody flow and aerodynamic drag. Compared with the original HST model, the downstream movement of the first bogie, has a great effect on decreasing the slipstream velocity and pressure fluctuation aside the HST, especially around the lower part of the HST. Furthermore, the size of the longitudinal vortex structure and slipstream velocity in the near wake region also decrease significantly by moving the last bogie upstream. Additionally, the movement of the first and last bogies toward the HST center, effectively decreases the drag values of the head and tail car, while a lower effect is observed on the intermediate cars.

Деякі особливості втрати стійкості ламінарною течією в трубі

Despite the seeming simplicity of the steady flow in a pipe of constant radius, the question of the cause and process of the transition remains debatable. Especially since the necessary condition for the stability loss of parabolic profile is not satisfied, and the linear theory of hydrodynamic stability for an axisymmetric Poiseuille flow does not give growing axisymmetric eigen solutions for any Reynolds numbers, since the terms characterizing the interaction of disturbances with the initial velocity profile drop out in the linearized equations of momentum conservation. The report presents the results of the study of stages of convective stability loss for the flow at the initial section of the pipe depending on the variable parameters based on the numerical solution of the three-dimensional system of unsteady Navier-Stokes equations and the equation energy transfer. The variable parameters in this study were: Reynolds number, magnitude and gradient sign of the dynamic viscosity coefficient arising in nonisothermal flows. An analogy of the arising secondary axisymmetric large-scale toroidal vortex structures in the near-wall region to Tollmien-Schlichting waves in the region of the transition of the laminar boundary layer on the plate is shown. The subsequent loss of axisymmetry and stability of these torus-like vortex structures is analyzed, which leads to the formation of fairly regular longitudinal vortex structures downstream, the nonlinear interaction of which leads to chaotization of the flow. The lengths of these sections are determined depending on the Reynolds number, the magnitude and sign of the gradient of the dynamic viscosity coefficient.

Supersonic flow in the rectangular duct of an air inlet with the separation-induced interaction of the boundary layer with shock waves

Results of a numerical study of the flow structure in the rectangular duct of a model air inlet with the interaction of the shock wave generated by the inlet cowl with the turbulent boundary layer on the inner duct surfaces are reported. A configuration involving a convergent entrance section and a constant-cross-section throat was analyzed. The numerical simulation of the three-dimensional flow with free-stream Mach number M = 4 was performed on the basis of the Reynolds-averaged Navier-Stokes equations and the k-ω SST model of turbulence. As a result of the calculations, the limit streamlines and the distributions of static pressure on the duct surfaces, as well as the distributions of total pressure and the fields of Mach number in duct cross sections were obtained. The structure of the flow is analyzed and the possibility of occurrence of longitudinal vortex structures in the corner region of the intake-duct throat is shown.

An IDDES investigation of Jacobs bogie effects on the slipstream and wake flow of a high-speed train

This study numerically investigates the effects of Jacobs bogies on the aerodynamic behaviors of a high-speed train using improved delayed detached eddy simulation (IDDES) at Re ​= ​3.3 ​× ​105. The results of the numerical simulations have been validated against the experimental data obtained from a previous reduced-scale moving model test and a wind tunnel test. The slipstream velocity, wake flow, underbody flow and aerodynamic drag of the HST are compared between the conventional bogie case and Jacobs bogie case. The results show that the use of Jacobs bogies can reduce the TSI values of the slipstream velocity at trackside and platform positions by 11.07% and 22.40%, respectively, which thereby shows a positive effect on improving the safety level of trackside workers and passengers standing on the platform. The Jacobs bogies are found to decrease the maximum values of the slipstream velocity and turbulence kinetic energy occurring at the intermediate bogie regions beneath the HST by 30.08% and 41.32%, respectively, which is beneficial for weakening the ballast flight phenomenon. The Jacobs bogies significantly narrow the scale of the longitudinal vortex structure in the wake propagation region. Additionally, the application of Jacobs bogies lowers the aerodynamic drag values of the vehicles and contributes to a 10% total drag reduction.

Three-Dimensional Visualization of Flow Characteristics Using a Magnetic Resonance Imaging in a Lattice Cooling Channel

Flow structures in lattice cooling channels are investigated experimentally by measuring three-dimensional (3D) velocity components over entire duct. The lattice cooling structure is formed by crossing two sets of parallel inclined ribs. Heat transfer is enhanced when coolant flows through the narrow subchannels between the ribs. According to the past literature, longitudinal vortex structures are formed inside the subchannels due to interactions between crossing flows. In this study, 3D velocity field measurement is performed using magnetic resonance imaging (MRI) scanner to clarify the flow mechanism. The rib inclination angle is varied from 30 to 60 deg. Reynolds number is set at approximately 8000 based on the whole duct inlet hydraulic diameter and bulk velocity. Working fluid is 0.015 mol/L copper sulfate aqueous solution. Measured results show that coolants in the upper and lower subchannels interact not only at the both ends of the duct, but also at diamond-shaped openings formed by opposite subchannels. The exchange of momentum between the upper and lower subchannels occurs at the openings, leading to sustained longitudinal vortex in each subchannel as mentioned in the literature. When the ribs are arranged with obtuse angle, a large vortex spreads across the contact surface, while the vortex structure independently stays in each subchannel for acute rib angle. The measured velocity fields are compared with numerically-simulated ones using a Reynolds-averaged Navier-Stokes (RANS) solver. Overall flow pattern is captured, but flow interaction between the upper and lower subchannels is underestimated.

Assessment of two fluidic actuators for active flow control on a one-sided diffuser

The manipulated flow along a one-sided diffuser is investigated experimentally by means of particle image velocimetry and wall pressure measurements. Two fluidic actuators are considered: a fluidic diverter emitting two alternately pulsed jets and a fluidic oscillator generating one spatially oscillating jet. Active separation control is performed with these devices to suppress the pressure-induced separation bubble present in the uncontrolled flow field. The method of proper orthogonal decomposition is employed to identify inherent physical mechanisms subject to varied jet emission angles as well as actuation intensities and construct a reduced order model by utilizing modal coefficients to obtain phase information regarding the periodic actuation process. As a main result, we show that longitudinal vortex structures determining the separation control capability, are more distinct for the fluidic oscillator. Furthermore, the flow fields manipulated by both actuators at a low momentum input are dominated by a spatial mode resembling the base flow. However, this feature is suppressed by the fluidic diverter at greater velocity ratios, whereas the variation of actuation intensity does not yield significant alterations to the spatial modes observed for the fluidic oscillator.

Flow over a flat surface behind delta winglets of varying aspect ratios

The effect of winglet aspect ratio on the generated vortical flow over a flat surface was experimentally scrutinized in a wind tunnel using a triple sensor hot-wire. Aspect ratios of 1, 2 and 4 were considered at a Reynolds number based on winglet height of 6000 and an angle of attack of 30 degrees. A large longitudinal vortex structure was observed in the cross-stream plane. The streamwise velocity deficit at the main vortex core lessened, while that at the Upwash Region remained unaltered, with increasing aspect ratio. Moreover, the vortex moved downward and inward and its intensity decreased. The turbulence level decreased with a corresponding increase in Taylor microscale. The integral length was found to be independent of the aspect ratio but scaled with the winglet height.

3D structure of flow in the near field of the quasi-two-dimensional turbulent jet

The work explores the three-dimensional structure of the near field of a quasi-two-dimensional turbulent jet. The jet stream forms at an outflow from the rectangular channel with aspect ratio B/h equal to 1:2.5 into the channel, formed by two plane-parallel plates. The experiments were carried out for a wide range of Re number from 2 000 to 20 000. The measurements were performed using Tomographic PIV method with high time resolution. It is shown that for the whole investigated range of Re number in the near-field of the quasi-two-dimensional jet, there are formed secondary flows, caused by the presence of longitudinal vortex structures in the flow.

The K-type and H-type transitions of natural convection boundary layers

The K-type and H-type transitions of a natural convection boundary layer of a fluid of Prandtl number 7 adjacent to an isothermally heated vertical surface are investigated by means of three-dimensional direct numerical simulation (DNS). These two types of transitions refer to different flow features at the transitional stage from laminar to turbulence caused by two different types of perturbations. To excite the K-type transition, superimposed Tollmien–Schlichting (TS) and oblique waves of the same frequency are introduced into the boundary layer. It is found that a three-layer longitudinal vortex structure is present in the boundary layer undergoing the K-type transition. The typical aligned $\wedge$-shaped vortices characterizing the K-type transition are observed for the first time in pure natural convection boundary layers. For exciting the H-type transition, superimposed TS and oblique waves of different frequencies, with the frequency of the oblique waves being half of the frequency of the TS waves, are introduced into the boundary layer. Unlike the three-layer longitudinal vortex structure observed in the K-type transition, a double-layer longitudinal vortex structure is observed in the boundary layer undergoing the H-type transition. The successively staggered $\wedge$-shaped vortices characterizing the H-type transition are also observed in the downstream boundary layer. The staggered pattern of $\wedge$-shaped vortices is considered to be caused by temporal modulation of the TS and oblique waves. Interestingly the flow structures of both the K-type and H-type transitions observed in the natural convection boundary layer are qualitatively similar to those observed in Blasius boundary layers. However, an analysis of turbulence energy production suggests that the turbulence energy production by buoyancy rather than Reynolds stresses dominates the K-type and H-type transitions. In contrast, the turbulence energy production by Reynolds stresses is the only factor contributing to the transition in Blasius boundary layers.

Three-dimensional structure of longitudinal vortices shedding from cruciform two-cylinder systems with different geometries

In previous research the present authors have shown that cross flow vibrations can be induced on a cylinder by setting another cylindrical body downstream in cruciform arrangement with a certain gap s between them. These vibrations are excited by two longitudinal vortices (LVs), i.e. the trailing vortex (TV) and the necklace vortex (NV), shedding periodically near the cross of the system. In this study, LVs shedding from three cruciform systems, i.e. circular-cylinder/circular-cylinder (CC/CC), circular-cylinder/strip-plate (CC/SP) and square-cylinder/strip-plate (SC/SP) system, are observed from three directions using water tunnel and surface-die-streak technique to clarify their three-dimensional structure. The gap-to-diameter ratio s/d is set at 0.08 or 0.28 as the representative values of TV and NV, respectively. Formation of TV or NV depends on the system geometry s/d and Reynolds number Re. In the case of CC/CC, TV is formed at s/d = 0.08 and NV at s/d = 0.28 steadily when 1400 ≤ Re ≤ 2000, and they shed periodically when 3300 ≤ Re ≤ 5700. In the case of CC/SP, steady shedding of TV is observed at the both values of s/d in the lower Re range and periodic shedding is observed only for TV at s/d = 0.08 in the higher Re range, but with poorer periodicity. In the case of SC/SP, NV is formed at the both values of s/d and periodic shedding is not observed up to the highest value of Re = 5000 in this experiment.

Numerical study on characteristics of heat transfer and friction factor in a circular tube with central slant rods

In the present work, a numerical study on the characteristics of heat transfer and friction factor of laminar water flow in a circular tube fitted with a new tube insert (central slant rod inserts) is presented. Results reveal that the new inserts can fully disturb the core flow in the tube and create multiple longitudinal vortex structures, which are confirmed to be excellent flow structures for heat transfer enhancement. In addition, we carried out stereoscopic particle image velocimetry (SPIV) measurements in exactly the same facility as that of the numerical study to validate the accuracy of our numerical study. The results show good agreement between the experiment and numerical simulation. Following this, the effects of three parameters of central slant rods, including the slant angle, pitch and length in the radial direction, are discussed numerically. The computational results indicate that a moderate slant angle ranging from 20° to 30° can obtain the largest Nusselt number; however, the effect of slant angle on the friction factor is quite limited. The Nusselt number and friction factor both increase with a decrease in rod pitch and an increase in rod length in the radial direction. The simulation results show that the Nusselt number and friction factor are enhanced by 1.81–5.05 times and 2.49–6.92 times, respectively, to that of a plain tube. The performance ratio R3 values are in the range of 1.74–4.60. Empirical formulas for Nu and f are obtained based on calculation results. All results indicate that the central slant rod is a promising high-performance tube insert for heat transfer enhancement in practical applications.

GS0524 時間変動するサイドジェットの数値解析と可視化

In the present study, we investigated the mechanism of side jet generation by numerical calculation. The side jet occurrence condition is that inflow gaseous density smaller enough than ambient gas. The side jet is the jet phenomenon at right angles with mainstream flow direction. The side jet is expected that used for mixture control. The analysis of vortical structure by ∇^2P has shown the side jet occur between two longitudinal vortical structures that difference direction of rotation. Also, the analysis of these longitudinal vortical structures and baroclinic torque by (∇ρ×∇p) have shown longitudinal vortical structures reach out from the rip of baroclinic ring. Consequently, the vortical structure breakdown by the instability of the shear layer and baroclinic torque play an important role in the side jet generation.

Wall shear stress characterization of a 3D bluff-body separated flow

Efficient flow control strategies aimed at reducing the aerodynamic drag of road vehicles require a detailed knowledge of the reference flow. In this work, the flow around the rear slanted window of a generic car model was experimentally studied through wall shear stress measurements using an electrochemical method. The mean and fluctuating wall shear stress within the wall impact regions of the recirculation bubble and the main longitudinal vortex structures which develop above the rear window are presented. Correlations allow a more detailed characterization of the recirculation phenomenon within the separation bubble. In the model symmetry plane the recirculation structure compares well with simpler 2D configurations; specific lengths, flapping motion and shedding of large-scale vortices are observed, these similarities diminish when leaving the middle plane due to the strong three-dimensionality of the flow. A specific attention is paid to the convection processes occurring within the recirculation: a downstream convection velocity is observed, in accordance with 2D recirculations from the literature, and an upstream convection is highlighted along the entire bubble length which has not been underlined in some previous canonical configurations.

Comparative efficiency of shear, elongation and turbulent droplet breakup mechanisms: Review and application

The study of phase dispersion of two immiscible fluids in different flows requires identifying the relevant breakup mechanisms. We propose here a detailed investigation of droplet breakup in a multifunctional exchanger-reactor of the vortex generator type in which transfer intensification is due to longitudinal vortical structures. We compare the efficiency of the mean gradients and turbulent mechanisms in droplet breakup in this industrial reactor. This efficiency is essentially characterized by the resulting distribution of droplet diameters. Then, the roles of the mean flow and the turbulent field, intensity, energy spectrum, and turbulence scales are examined in relation to the liquid/liquid dispersion in order to explore the governing mechanisms of drop breakup. In the complex flow considered here – nonhomogeneous and anisotropic turbulence at moderate Reynolds numbers (<15,000) – with weak turbulence intensity (about 10%), it can be demonstrated that turbulent breakup mechanisms largely dominate mean flow effects; elongation and shear effects are shown to have minor effects on the breakup mechanisms. Moreover, the global characteristic scales of the flow are not the relevant parameters in predicting the final size of the emulsion, but instead the Kolmogorov microscale, implying that the residence time in the reactor is not a limiting factor. Hence, the local dissipation rate governs the performance of the actual multifunctional reactor. This study provides some insight in the design and scaling-up of multiphase reactors.

Aerodynamic drag control by pulsed jets on simplified car geometry

Aerodynamic drag control by pulsed jets is tested in a wind tunnel around a simplified car geometry named Ahmed body with a rear slant angle of 35°. Pulsed jet actuators are located 5 × 10−3 m from the top of the rear window. These actuators are produced by a pressure difference ranging from 1.5 to 6.5 × 105 Pa. Their excitation frequency can vary between 10 and 550 Hz. The analysis of the control effects is based on wall visualizations, aerodynamic drag coefficient measurements, and the velocity fields obtained by 2D PIV measurements. The maximum drag reduction is 20 % and is obtained for the excitation frequency Fj = 500 Hz and for the pressure difference ∆P = 1.5 × 105 Pa. This result is linked with a substantial reduction in the transverse development of the longitudinal vortex structures coming from the left and right lateral sides of the rear window, with a displacement of the vortex centers downstream and with a decrease in the transverse rotational absolute values of these structures.

An Experimental Study of the Flow Around a Formula One Racing Car Tire

The wake of the front tires affects the airflow over the remainder of a fenderless race car. The tires can also be responsible for up to 40% of the vehicle’s drag. Prior experiments have used compromised models with solid, symmetric hubs and nondeformable tires. The present experiment acquires particle image velocimetry measurements around a 60% scale model of a deformable pneumatic tire fitted to a spoked Formula 1 wheel with complete brake geometry and supplementary brake cooling ducts. The results show reversed flow regions in the tire wake, asymmetric longitudinal vortex structures behind the tire, and a tire wake profile that is unlike previous experimental results and postulations. The flow through the hub of the wheel causes a shift of the wake inboard (toward the car) so that the outboard side of the wake does not extend past the outline of the tire.