Longitudinal Vortices Research Articles

Numerical Study on the Effect of Boat Tail Shape on Aerodynamic Drag of SUV

Longitudinal vortex formed at the rear area of the vehicle is one of the main sources of vehicle aerodynamic drag. The boat tail shape is applied to reduce the longitudinal vortex strength of the SUV so that the aerodynamic drag is improved. For this study, a commercially available SUV produced by domestic automobile company, Hyundai Motor Company, is selected as a baseline model. The boat tail shape is modeled on the baseline model using the morphing technique. The effects of boat tail shape on the aerodynamic drag has been numerically analyzed for a SUV with or without undercover. A commercial flow solver, ANSYS Fluent, is used for this study. The driving speed of SUV is 140 km/h. Even though the same boat tail shape is applied to the same vehicle, the aerodynamic drag improvement depends on the flow pattern behind the vehicle. It can be improved when the strength of the longitudinal vortex is weakened by the airflow generated by the boat tail shape.

Development of a wind turbine driven by longitudinal vortex: Wind tunnel experiment to investigate the basic characteristics of the wind turbine using a single circular cylinder blade

In this study, the basic rotation and output torque characteristics of a longitudinal vortex wind turbine (LV-WT) were investigated by using a wind tunnel experiment. This wind turbine uses a circular cylinder with a diameter d as the driving blade, and a ring-shaped plate with width w ​= ​d is installed behind the cylinder blade with a gap s. A steady lift force to sustain the rotational motion is generated by the longitudinal vortex formed near the intersection of the cylinder blade and the ring plate. The gap ratio s/d is the most dominant parameter for the performance of the LV-WT, and the output torque and efficiency are maximum at s/d ​= ​0.35. A wider ring plate and flanges attached to the blade are promising techniques to improve the performance of the LV-WT. The lift coefficient Cl is approximated by a decreasing linear function of the velocity ratio λ = (rotational velocity V)/(free flow velocity U), as predicted in the preceding numerical study of the authors. Using the drag coefficient determined by the loaded experiment, the performance characteristics of LV-WT are predicted for the widely varying conditions.

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.

Generation of Arbitrary Longitudinal Polarization Vortices by Pupil Function Manipulation

Optical Vortices In article number 2000087, Xiang Hao and co-workers show that by manipulating phase and amplitude at the back pupil of a high numerical-aperture (NA) objective, arbitrary longitudinal polarization vortices within the tightly focused light field are realized.

Effect of separation point on characteristics of cylinder blade wind turbine driven by longitudinal vortex

Effect of separation point on characteristics of cylinder blade wind turbine driven by longitudinal vortex

A Numerical Study on the Effect of a Flow with Turbulence on Aerodynamic Noise Generated from Longitudinal Vortex

A Numerical Study on the Effect of a Flow with Turbulence on Aerodynamic Noise Generated from Longitudinal Vortex

Three-Dimensional Structure of Non-Equilibrium Homogeneous Condensation Flow in a Supersonic Rectangular Nozzle

Numerical simulations have been carried out for a supersonic three-dimensional rectangular arc nozzle, where a secondary flow toward the center of the curvature occurs due to the shape of the nozzle. It is known that secondary flow causes longitudinal vortices to form near the wall of the nozzle corner, making the nozzle outlet flow unstable and induces loss of transport energy. When the working fluid is a condensable gas with relatively large latent heat such as moist air or steam, rapid accelerated expansion in the nozzle causes non-equilibrium condensation due to supersaturation. After the release of latent heat during phase transition, nozzle flow continues expanding at an equilibrium saturation condition. In the absence of foreign particles, e.g. ions or dust particles, condensation nuclei are formed in the gas itself causing non-equilibrium homogeneous condensation. Supersonic nozzle flow properties vary considerably due to the occurrence of condensation phenomenon. The objective of this study is to investigate the effect of non-equilibrium homogeneous condensation on the longitudinal vortices which form in the range close to the corner of rectangular arc nozzle numerically.

Development of circular cylinder blade wind turbine driven by longitudinal vortex

Development of circular cylinder blade wind turbine driven by longitudinal vortex

Numerical investigation of heat exchanger with inserted twisted tape inside and helical fins on outside pipe surface

This research presents numerical investigation of pipe heat exchanger with twisted tape inserted inside and helical fins on outside pipe surface. Various configurations were modelled based on the twist pitch (p) inside the pipe and fin twist pitch (P) over outside of the pipe. The fin twist pitch P (P = P, 2P/3, P/2) on the pipe outside and the inserted tape twist pitch p (p = p, 3p/2, p/2) inside the pipe was varied in the investigation. The hot fluid (water) passes inside the pipe, while the cold fluid (water) passes on the outside of pipe in counter flow direction. The Dittus-Boelter correlation was used for validation. For smooth pipes and the proposed heat exchanger with twisted tapes and helical fins, numerical results were compared. It was found that in inside and outside flows, twisted tape formed longitudinal vortex that helped to increase fluid mixing, thus being higher than that of smooth tube heat transfer rate.

Flow Visualization on the Characteristics and Actions of Longitudinal Vortices Appearing during the Formation of Karman Vortex Street

Flow Visualization on the Characteristics and Actions of Longitudinal Vortices Appearing during the Formation of Karman Vortex Street

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.

Drag force of circular cylinder blade wind turbines driven by steady lift force of longitudinal vortex

Drag force of circular cylinder blade wind turbines driven by steady lift force of longitudinal vortex

A Simple Passive Device for the Drag Reduction of an Ahmed Body

In this paper, a simple passive device is proposed for drag reduction on the 35° Ahmed body. The device is a simple rectangular flap installed at the slant surface of the model to investigate the effect of slant volume, formed between the device and the slant surface, on the flow behaviour. The slant volume can be varied by changing the flap angle. This investigation is performed using the FLUENT software at a Reynolds number of 7.8 ×〖10〗^5 based on the height of the model. The SST k-omega model is used to solve the Navier-stokes equations. It is found that this passive device influences the separation bubbles created inside the slant volume and provides a maximum drag reduction of approximately 14% at the flap angle of 10°. Moreover, the device delays the main separation point, which changes the flow conditions at the back of the model. The drag reduction was found to mainly dependent on the suppression of the separation bubbles formed inside the slant volume, which leads to faster pressure recovery. The cause of this pressure recovery is found to be the reduction in recirculation length and width. Also, the addition of a flap reduces the turbulent kinetic energy, which lessened the wake entrainment in the recirculation region, leading to a drag reduction. Also, it hinders the formation of horseshoe vortex that provides a pressure recovery and influence the wake width. However, the investigation also reveals that this device does not reduce the induced drag due to longitudinal vortex from the side edges.

Experimental study of heat transfer and flow of delta winglets inline arrays in a tube heat exchanger for enhanced heat transfer

AbstractThis experiment was carried out using delta winglet arrays of vortex generators (VG) with inline arrangement in a tube heat exchanger to study enhanced heat transfer and flow behaviour. The experiment was conducted for the turbulent flow (Re = 6000 to 27000). In this experiment, different parameters, pitch ratios (PR = 1.6, 2.4, and 4.8), lengths (L = 10, 15, and 20 mm), and attack angles (B = 0°, 10°, 20°, 30°, and 45°) were studied and then their effect on thermal performance was observed. Results indicate that the PR affected f and Nu significantly. For PR = 1.6, VGs showed the highest f and Nu for all of the cases. Vortex generators with L10 B45 PR4.8 achieved the best TPE with 1.23 at Re = 6000. Attack angle B indicated a significant impact on thermal performance and 45 degree showed the TPE of 1.23 at lower Re. Oil film flow and smoke flow visualization were employed to identify the flow vortices and understand flow mechanism. The oil film flow and smoke flow visualization clearly traced longitudinal vortex, and induced vortex, which induced impingement flow and recirculation zone that lead to significant heat transfer enhancement.

Generation of Arbitrary Longitudinal Polarization Vortices by Pupil Function Manipulation

The properties of optical vortex provide new insights into a wide range of optical and physical phenomena, giving rise to numerous applications in multiple disciplines. Notably, as much as the associated technologies have become increasingly flexible, it is still challenging to simultaneously generate multiple longitudinal polarization optical vortices with random topological charges (l) and tunable intensities at arbitrary positions. Herein, a novel approach that, by manipulating phase and amplitude at the back pupil of a high numerical‐aperture (NA) objective, realizes arbitrary longitudinal polarization vortices within the tightly focused light field is developed. Building upon the same model, the impacts of polarization on the optical vortex field and its potential of tuning the ratio of the longitudinal polarization component are further investigated.

Local heat transfer enhancement induced by a piezoelectric fan in a channel with axial flow

The present work experimentally and numerically investigates the local heat transfer enhancement induced by a piezoelectric fan interacting with a cross flow in a local heated channel. The piezoelectric fan is placed along the flow direction and tested under different amplitudes and flow rates. In the simulations, a spring-based smoothing method and a local remeshing technique are used to handle the moving boundary problems. Hybrid mesh is used to reduce the size of dynamic mesh domain and to improve computational efficiency. The experimental and numerical values of the time-averaged mean Nusselt number are found to be in good agreement, with deviations of less than 10%. The experimental result shows that the heat transfer performance of the heated surfaces is substantially enhanced with a vibrating piezoelectric fan. The numerical result shows that the heat transfer enhancement comes from the strong longitudinal vortex pairs generated by the piezoelectric fan, which significantly promote heat exchange between the main flow and the near-wall flow. In the case of a=0.66 (a is the dimensionless amplitude) and Re=1820, the enhancement ratio of the time-averaged mean Nusselt number reaches 119.9%.

Flow and Heat Transfer Characteristic of Inclined Oval Trench Dimples With Numerical Simulation

In this work, flow and heat transfer in a channel having oval trench dimples were investigated numerically. 3-D channel flow with a cross-section of 300-mm width and 32-mm height were created using Computational Fluid Dynamic (CFD) with ANSYS, Fluent (V.15.0). Five oval trench dimples with 3-mm depth, 10.0-mm width and 45-mm length arranging with a single row and in-line configuration were located on the bottom surface of the channel. Reynolds number based on hydraulic diameter of the channel were fixed at Re=20,000 whereas a dimple inclined angle defined as the angle of dimple centreline to the mainstream was varied at 0, 15, 30 and 45 degrees. SST turbulent model was used to solve governing Equations. The result show that longitudinal vortex flow occurred at ?=15° to 45° which would be enhance heat transfer on the surface. When inclined angle became larger, the areas of Nusselt number and high total pressure coefficient took place at dimple edge in +Z direction. The peak of average spanwise Nusselt numbers took place for the case of ?=45°. Moreover, the area of high spanwise average Nusselt numbers (>100) for the case ?=30° was the largest.

Thermal performance analysis of fin-tube heat exchanger with staggered tube arrangement in presence of rectangular winglet pairs

Longitudinal vortices generated by winglet type vortex generators mounted on fin surface improve fluid mixing with disruption of boundary layer growth, which results in convective heat transfer enhancement. The present study reports the three-dimensional numerical investigation of airflow through heated fin-tube heat exchangers in presence of common flow down configured rectangular winglet pairs (RWPs) for staggered arrangements of circular tubes. It is well known that the thermal performance of a fin-tube heat exchanger is majorly influenced by various locations of winglets corresponding to tube centre. Henceforth, fluid flow and heat transfer characteristics of different possible RWP locations concerning each tube are examined. Further, for higher performance, a search for an effective angle of attack ranging from 15° to 60° is also performed for optimized locations. The effect of different inlet flow conditions is also investigated for Reynolds number ranges from 2000 to 10,000. Thermohydraulic performance is studied by considering Nusselt number, friction factor, secondary flow intensity and thermal performance factor. Irreversibilities due to RWPs are analysed by calculating the entropy generation rate due to viscous and heat transfer effects. Obtained results illustrate that the selection of promising RWP locations plays a vital role in improving thermal performance. The relationship between heat transfer, secondary flow and irreversibility parameters are also reported.

Heat transfer enhancement through longitudinal vortex generators in compact heat exchangers with flat tubes

The thermal performance of a compact heat exchanger with flat tubes was numerically investigated in a laminar regime by introducing longitudinal vortex generators (LVG) in arrays comprising: 9 alternating, 18 alternating, 18 aligned, 27 alternating and 39 alternating LVG. Vortex generators, in the form of delta winglets with an angle of attack of 35o, were used. For each geometry, the local heat flux, local and global Nusselt number and the friction factor were analysed. The Reynolds number varied between 400 and 2000.

Impact of the bogies and cavities on the aerodynamic behaviour of a high-speed train. An IDDES study

This study investigates the effects of bogie cavities and bogies on the aerodynamic behaviour of a high-speed train (HST) using improved delayed detached eddy simulation (IDDES) at Re ​= ​3.3 ​× ​105. The main aim of this work is to identify the individual influence of bogies and cavities on the surrounding flow, thereby revealing aspects to further improve the HST aerodynamic performance. The accuracy of the numerical method has been validated against the experimental data obtained from a previous reduced-scale moving-model test, a wind tunnel test and a full-scale field test. The underbody flow, wake flow, slipstream velocity, aerodynamic drag and the computational costs are compared for three cases. The results show that installing the bogies around the cavities in Case 1 and sealing the cavities in Case 3 can effectively reduce the turbulence kinetic energy (TKE) and slipstream velocity around the HST. The cavities in Case 2 produce the highest level of TKE and slipstream velocity distribution in both underbody flow region and the wake region, compared to other two cases. This presents the largest scales of the shear vortices in the cavities and longitudinal vortices in the wake. Compared to Case 2, the TSI values of the slipstream velocity at the trackside position decreases by about 18.9% in Case 1 and 56.9% in Case 3, respectively.The cavities account for approximately 65% of the aerodynamic drag of the HST. Installing bogies in Case 1 and sealing cavities in Case 3 gives a 24% and 56% drag reduction for the overall HST. It is recommended to invest 10% higher resources to achieve an accurate surrounding flow prediction of a HST in presence of bogies.