Formation Of Horseshoe Vortex Research Articles

Numerical simulations of bio-inspired approaches to enhance underwater swimming efficiency

The present study discusses the numerical simulation results of swimming similar to manta rays. The complex three-dimensional kinematics of manta rays were implemented to unravel the intricacies of its propulsion mechanisms by using the discrete vortex method (DVM). The DVM replaces the requirement for a structured grid across the computational domain with a collection of vortex elements. This method simplifies grid generation, especially for intricate geometries, resulting in time and effort savings in meshing complex shapes. By modeling the pectoral fins with discrete panels and utilizing vortex rings to represent circulation and wake, the study accurately computes the pressure distribution, circulation distribution, lift coefficient, and thrust coefficient of the manta ray. This study focuses on the modulation of aerodynamic performance by altering the span length and the length change ratio during the downstroke and upstroke motion (SV). The manta ray's three-dimensional vortex configurations comprise a combination of vortex rings, vortex contrails, and horseshoe vortices. Analysis of the three-dimensional vortex structure indicates the presence of multiple vortex rings and horseshoe vortex rings at higher SV values, while adequate formation of horseshoe vortices is not observed at lower SV values. In terms of propulsive performance, both lift and thrust increase with SV, while the propulsive efficiency demonstrates its peak at SV = 1.75. The analysis reveals that at higher SV values, the net thrust generated primarily originates from the tip of the fins. Moreover, the study illustrates a significant enhancement in propulsive efficiency, particularly in association with optimal Strouhal numbers ranging between 0.3 and 0.4. The key findings of this study may be used in efficient design of agile autonomous underwater vehicles for marine exploration and surveillance applications.

Three-dimensional CFD study on thermo-hydraulic behaviour of finned tubes in a heat exchange system for heat transfer enhancement

Abstract The present study performs a three-dimensional CFD analysis to investigate the hydrodynamic and thermal properties of annular finned tubes in a heat exchange system. All computations are performed in the turbulent flow regime (4330 ≤ Re ≤ 8790), and the Transition SST model is applied for turbulence modelling. The impact of Prandtl number (0.7 ≤ Pr ≤ 50) on the various parameters, such as the heat transfer coefficient, heat transfer rate, and pressure drop, are considered. The results indicate that the thermo-hydraulic behaviour is significantly affected by incrementing both Reynolds and Prandtl numbers. The fin’s surface temperature distribution is examined to get a better insight into its thermal performance, and it is observed that the rear portion of the fin contributes the least to heat transfer. Other important parameters like the fin efficiency and Colburn heat transfer factor are found to significantly impact the performance of the heat exchange system for the above range of settings. The velocity contours show the horseshoe vortex formation near the fin-tube junction, and the channelling effect is observed between consecutive tubes. Different fluids are compared based on the j/f factor for enhanced heat transfer at the minimum possible flow resistance.

Pre-arranged sequences of micropillars for passive mixing control of water and ethanol

Microfluidic reactors have enabled the continuous conduction of chemical reactions with exceptional control of fluids and operating conditions. To enhance reagent mixing in laminar flow conditions, the effective strategy of adding cylindrical obstacles (i.e., pillars) in the mixing channel is appealing. Indeed, they can passively extend the contact surface of the incoming fluid streams, where diffusion and reaction occur. From our previous work, optimized sequences of pillars in a T-microchannel – that differ in diameter and position – can significantly improve the mixing for single-phase flow (i.e., mixing of miscible fluids of similar density and viscosity). However, real-world mixing applications often require the merging of fluids with significantly different fluid properties before, during, and after mixing. Hence, further investigations are required to unveil efficiency and robustness in these more complex practical mixing conditions. We jointly carried out experiments and numerical simulations on miscible fluids in an optimized sequence of pillars called IAF. Our analysis shows how the fluid velocity (i.e., the Reynolds number, Re) and the pillar configuration affect the formation of horseshoe vortices, which we consider crucial for water mixing when 1<Re<100. Then, we analyzed water–water and water–ethanol mixtures at Re=40 to observe how fluid properties alter the flow passively controlled by the pillar sequence. Eventually, the analysis extends within Re=10–100, showing efficiency and pressure drops of optimized designs that depend on the evolution of physical properties. These findings provide insights into the predictions of confined fluid flow around obstacles and its optimization for engineering processes, e.g., nanoparticle precipitation.

Three-dimensional numerical simulation of wave-induced scour around a pile on a sloping beach

A fully-coupled (hydrodynamic and morphologic) numerical model based on the open-source CFD package OpenFOAM is presented and utilized to simulate breaking wave induced scour around a pile on a sloping beach. The hydrodynamic model is based on RANS equations together with k-ω turbulence closure and volume of fluid method for capturing the free surface. These are then coupled with both bed load and suspended load transport descriptions, which drive resultant morphology of the bed. The model is validated against large-scale laboratory experiments involving interaction of breaking solitary waves with a pile and induced flow and scour. The flow features and hydrodynamic characteristics around the pile are described in detail, and the scouring mechanism at different locations of the pile is thoroughly analyzed. The simulated flow field confirms the formation of horseshoe vortex in front of the pile induced by plunging breaking wave. The temporal variation of scour depth at the side of the pile is satisfactorily simulated while those at the offshore and onshore are under-predicted. The scour depth at the side of the pile increases twice during run-up and draw-down. The scour at the onshore location of the pile starts to increase rapidly at the end of wave drawdown.

Scour protection around bridge pier and two-piers-in-tandem arrangement

ABSTRACT In the present study, experiments were carried out on circular model bridge piers individually as well as with two-piers-in-tandem arrangement under clear water scour conditions predominantly caused by formation of horseshoe vortex. Independent collar plates around bridge piers and a continuous collar plate covering both the piers were installed as countermeasures against scouring. In case of two piers, efficacy of scour-protection devices for front and rear piers was checked for the overall stability. Performance potential was noted to be about 67% and 100% for continuous collar plates of sizes 2.5D and 3.0D around two piers at average bed level with spacing of 2.0D; where D is the diameter of the pier. During degradation of general bed level, a group of three continuous collar plates was employed around both the piers to ensure considerable reduction of scour. A group of three collar plates of sizes 2.5D and 3.0D having an inter-plate spacing Cs equal to D/4 with the middle plate placed at the average bed level reduced scour by 79 and 82%, respectively, in comparison to the scour depth for an unprotected pier. A mathematical model was developed by multivariate regression analysis to prediction the scour depth for both the pier arrangements.

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.

Asymmetric nose-fairing for wing–body interference flow

In the context of the wing–fuselage junction flow the purpose of a nose-fairing is to prevent the oncoming boundary-layer flow separation, which in turn eliminates the horseshoe vortex formation at the junction. A simple procedure is proposed here to arrive at an asymmetric fairing shape for a non-zero wing incidence angle. Even though it is a simple method to decide on fairing shape and size, the fairing designed by this method for a wing at 4° incidence is shown to satisfy the purpose of the nose-fairing. The wind tunnel experimental results clearly show that the designed fairing improves the flow quality at the wing–body intersection by preventing flow separation and vortex formation. Relatively lower momentum deficit in the boundary-layer (behind the wing trailing edge) hints at drag-reduction for the faired configuration.

Numerical Study of Seabed Boundary Layer Flow Around Monopile and Gravity-Based Wind Turbine Foundations

Computational fluid dynamics (CFD) has been used to study the seabed boundary layer flow around monopile and gravity-based offshore wind turbine foundations. The gravity-based foundation has a hexagonal bottom slab (bottom part). The objective of the present study is to investigate the formation of horseshoe vortex and flow structures around two different bottom-fixed offshore wind turbine foundations in order to provide an assessment of potential scour for engineering design. Three-dimensional CFD simulations have been performed using Spalart–Allmaras delayed detached eddy simulation (SADDES) at a Reynolds number 4 × 106 based on the freestream velocity and the diameter of the monopile foundation, D. A seabed boundary layer flow with a boundary layer thickness D is assumed for all the simulations. Vortical structures, time-averaged results of velocity distributions and bed shear stresses are computed. The numerical results are discussed by studying the difference in flows around the monopile and the gravity-based foundations. A distinct horseshoe vortex is found in front of the monopile foundation. Two small horseshoe vortices are found in front of the hexagonal gravity-based foundation, i.e., one is on the top of the bottom slab and one is near the seabed in front of the bottom slab. The horseshoe vortex size for the hexagonal gravity-based foundation is found to be smaller than that for the monopile foundation. The effects of different foundation geometries on destroying the formation of horseshoe vortices (which is the main cause of scour problems) are discussed.

Three-dimensional flow characteristics near a circular cylinder mounted on horizontal plate at low Reynolds number

Flow field characteristics in the neighbourhood of a finite height circular cylinder mounted on a horizontal plate have been investigated numerically. Three-dimensional simulations are carried out for low Reynolds number (Re) incompressible flow using commercial software ANSYS Fluent 14.0. Topological characteristics of the flow have been investigated near the cylinder of aspect ratio (AR) ranging from 0.5 to 3.0 with Re varied in the range from 40 to 300. Time-averaged streamline plots are presented in various planes for different values of AR and Re to understand the flow field characteristics. For a developing flow impinging on the cylinder, flow separation in upstream of cylinder close to the cylinder-bottom wall junction leads to horseshoe vortex formation. Various topological characteristics like saddle, node have been identified in upstream and downstream of the cylinder. Positions of saddles are shown to depend on the values of AR and Re. Development of recirculation region and formations of vortex bubble have been studied in form of their projections on different planes. The variation of mean drag coefficient on the cylinder with Re for different values of AR has also been presented.

Effects of Boundary-Layer Ingestion on the Aero-Acoustics of Transonic Fan Rotors

The use of boundary-layer-ingesting, embedded propulsion systems can result in inlet flow distortions where the interaction of the boundary-layer vorticity and the inlet lip causes horseshoe vortex formation and the ingestion of streamwise vortices into the inlet. A previously-developed body-force-based fan modeling approach was used to assess the change in fan rotor shock noise generation and propagation in a boundary-layer-ingesting, serpentine inlet. This approach is employed here in a parametric study to assess the effects of inlet geometry parameters (offset-to-diameter ratio and downstream-to-upstream area ratio) on flow distortion and rotor shock noise. Mechanisms related to the vortical inlet structures were found to govern changes in the rotor shock noise generation and propagation. The vortex whose circulation is in the opposite direction to the fan rotation (counter-swirling vortex) increases incidence angles on the fan blades near the tip, enhancing noise generation. The vortex with circulation in the direction of fan rotation (co-swirling vortex) creates a region of subsonic relative flow near the blade tip radius that decreases the sound power propagated to the far-field. The parametric study revealed that the overall sound power level at the fan leading edge is set by the ingested streamwise circulation, and that for inlet designs in which the streamwise vortices are displaced away from the duct wall, the sound power at the upstream inlet plane increased by as much as 9 dB. By comparing the far-field noise results obtained to those for a conventional inlet, it is deduced that the changes in rotor shock noise are predominantly due to the ingestion of streamwise vorticity.

Numerical simulation of coherent turbulent structures and of passive scalar dispersion in a canopy sub-layer

This study deals with the problem of turbulent atmospheric boundary-layer flow over a forest canopy. Numerous previous works showed that this flow presents more similarities with a mixing-layer flow than with the standard boundary-layer flow. In this paper, this problem was studied for homogeneous canopies, using large eddy simulation (LES). The numerical results reproduced correctly the various steps of development of this flow: the appearance of a first generation of coherent structures resulting from the development of a primary Kelvin–Helmholtz instability, the reorganization of these structures, by vortex pairing and kinking, the development of a secondary instability and the formation of horseshoe vortices. Then, the process of transport of a passive scalar from a forest canopy into a clear atmosphere was studied in two cases, i.e., when the passive scalar concentration at the surface foliage is either constant or time-varying. Even though this small difference has little influence on the concentration patterns, the results showed that it can significantly affect the concentration magnitude as well as the dynamics of the total concentration in the atmosphere.

Effects of Orientation and Position of the Combustor-Turbine Interface on the Cooling of a Vane Endwall

First stage, nozzle guide vanes and accompanying endwalls are extensively cooled by the use of film cooling through discrete holes and leakage flow from the combustor-turbine interface gap. While there are cooling benefits from the interface gap, it is generally not considered as part of the cooling scheme. This paper reports on the effects of the position and orientation of a two-dimensional slot on the cooling performance of a nozzle guide vane endwall. In addition to surface thermal measurements, time-resolved, digital particle image velocimetry (TRDPIV) measurements were performed at the vane stagnation plane. Two slot orientations, 90 deg and 45 deg, and three streamwise positions were studied. Effectiveness results indicate a significant increase in area averaged effectiveness for the 45 deg slot relative to the 90 deg orientation. Flowfield measurements show dramatic differences in the horseshoe vortex formation.

0318 エントロピー生成に着目したフィルム冷却流れのLES解析(OS3 流体機械の研究開発とそれに関連した複雑流動現象,オーガナイズドセッション)

Recent advanced film-cooling techniques tend to be multipurpose, e.g. cooling and flow control, and therefore the total performance is difficult to be evaluated. This paper has proposed a numerical method to estimate local rate of mean entropy generation based on the flow statistics obtained by large eddy simulation. The method is applied to evaluate the effectiveness of a novel film cooling concept, which not only cools the endwall region upstream a leading-edge of turbine vane but also suppresses the formation of horseshoe vortex, which is known to be responsible for overheating and aerodynamic losses by the legs of the vortex downstream. As a result, it is clearly suggested that the film coolant injection upstream the leading edge has a potential both to cool the enwall and to reduce aerodynamic losses simultaneously.

Endwall convective heat transfer for bluff bodies

The endwall heat transfer characteristics of forced flow past bluff bodies have been investigated using liquid crystal thermography (LCT). The bluff body is placed in a rectangular channel with both its ends attached to the endwalls. The Reynolds number varies from 50,000 to 100,000. In this study, a single bluff body and two bluff bodies arranged in tandem are considered. Due to the formation of horseshoe vortices, the heat transfer is enhanced appreciably for both cases. However, for the case of two bluff bodies in tandem, it is found that the presence of the second bluff body decreases the heat transfer as compared to the case of a single bluff body. In addition, the results show that the heat transfer exhibits Reynolds number similarity. For a single bluff body, the Nusselt number profiles collapse well when the data are scaled by Re0.55; for two bluff bodies arranged in tandem, the heat transfer scaling is changed to Re0.51, indicating that the power index of Reynolds number is flow dependent.

Supersonic Fin-Body Interactions in an Internal Scramjet Flow

This paper reports on the study of a fin-body junction flow in the context of an internal scramjet flowpath. Threedimensional Reynolds-averaged Navier-Stokes numerical simulations of the flowfield inside a particular scramjet configuration have been performed using the commercial computational fluid dynamics code CFD++. The junction is formed by a central strut fuel injector spanning the entrance to a direct-connect combustor. The local flow Mach number is 2.5. The bow shock generated by the blunt strut injector causes flow separation on the side wall, which leads to the formation of horseshoe vortices. The computed results in the symmetry plane and on the injector surface near the leading edge of the strut are in good agreement with the solutions obtained by other investigators for isolated fin-body junctions. Further downstream, reflections of the strut bow shock at the internal flow top wall and then the strut surface have significant effect on the propagation of the fin-body interaction. The shock-vortex interaction was found to despin the primary horseshoe vortex; i.e., oppositely orientated vorticity is induced, and hence dissipate vorticity. Further reduction of the vorticity, associated with the observed vortices, occurs due to viscous dissipation of the horseshoe vortices in the internal flow side wall boundary layer.

Qualitative and quantitative measurements of horseshoe vortex formation in the junction of horizontal and vertical plates

Horseshoe vortex formation around a vertical flat plate mounted in the water channel is experimentally investigated. In order to demonstrate the development of the horseshoe vortex system in the junction of the horizontal and vertical plates a Particle Image Velocimetry (PIV) and dye visualization techniques were used. Instantaneous and time-averaged velocity vector fields, corresponding streamline topology, vorticity values were analyzed. Boundary layer separation, developing vortices, secondary or counter-rotating vortices, merging of developing and counter-rotating vortices which results in a primary horseshoe vortex system and corner vortex were demonstrated qualitatively and quantitatively.

Film-Cooling Flowfields With Trenched Holes on an Endwall

The leading edge region along the endwall of a stator vane experiences high heat transfer rates resulting from the formation of horseshoe vortices. Typical gas turbine endwall designs include a leakage slot at the combustor-turbine interface as well as film-cooling holes. Past studies have documented the formation of a horseshoe vortex at the leading edge of a vane, but few studies have documented the flowfield in the presence of an interface slot and film-cooling jets. In this paper, a series of flowfield measurements is evaluated at the leading edge with configurations including a baseline with neither film-cooling holes nor an upstream slot, a row of film-cooling holes and an interface slot, and a row of film-cooling holes in a trench and an interface slot. The results indicated the formation of a second vortex present for the case with film-cooling holes and a slot relative to the baseline study. In addition, turbulence intensity levels as high as 50% were measured at the leading edge with film-cooling holes and a slot compared with the 30% measured for the baseline study. A trench was shown to provide improved overall cooling relative to the no trench configuration as more of the coolant stayed attached to the endwall surface with the trench.

Horseshoe vortex studies in the passage of a model plate-fin-and-tube heat exchanger

An experimental investigation is reported using particle image velocimetry of the flow structure in the flow through a rectangular duct containing a circular cylinder with axis normal to the flow. A duct with a narrow gap was designed to simulate a fin-tube heat exchanger containing a single circular cylinder. The time-averaged velocity vectors map, 〈 V〉, patterns of streamlines, 〈 ψ〉 and corresponding vorticity contours, 〈 ω〉 were obtained using 490 instantaneous images. Development of the entrainment process between the main flow and wake-flow regions is magnified by a helical horseshoe vortex system emerging upstream of the cylinder. The occurrence and development of boundary layer separation and the formation of horseshoe vortices upstream of the cylinder close to both upper and lower plate surfaces and the merging of these developing horseshoe vortices into a primary horseshoe vortex system were also observed in the side-view plane for the Reynolds number range of 1500 ⩽ Re d ⩽ 6150.

Secondary instability in forced wavelength Görtler vortices

From an experimental study on the onset of secondary instability in forced wavelength Görtler vortices, it is found that the breakdown process of the Görtler vortices is due to the development of varicose and sinuous mode instabilities. The development of the varicose mode is characterized by the formation of horseshoe vortices that evolve downstream to form mushroom-like structures. This phenomenon is then followed by meandering of the vortices as an indication of the onset of sinuous mode instability, prior to turbulence. The spectrum analysis applied to the fluctuating velocity component shows the occurrence of peak frequency of about 150Hz, which is attributed to the fundamental secondary instability mode with its wavelength comparable to the spanwise wavelength of the primary Görtler vortices, at the location where the mushroom-like structures are clearly depicted in the mean velocity contours on the y-z plane. This confirms that the secondary instability is of the varicose type at the onset that is followed by the sinuous type downstream, prior to the breakdown of the vortices that will lead the boundary layer flow to turbulence. It is also found that the growth rate of the secondary instability observed is about 6.5 times higher than that of the Görtler instability, which leads to a breakdown of the streamwise structures and hence to turbulence.

Fluid-flow and endwall heat-transfer characteristics of an ultralight lattice-frame material

This paper describes an experimental study of the pressure loss and endwall heat transfer in a highly porous, ultralightweight, multifunctional lattice-frame material (LFM) subjected to forced air convection. The influence of LFM morphology on the endwall flow and heat transfer patterns in a unit cell is examined in detail. Depending on the orientation of the LFM, the pressure loss across a unit cell was found to be between about 30% and 60% of the dynamic pressure over a wide range of Reynolds numbers. For the two selected orientations, the LFM has almost the same overall heat transfer performance. Detailed endwall surface heat transfer distributions were obtained using thermochromic liquid crystals (TLCs). The results reveal that the local endwall heat transfer was sensitive to details such as the location of vertices and the inclination angle of struts. High heat transfer regions were clearly observed around the vertices due to the formation of horseshoe vortices and behind the inclined and/or yawed struts. It was also found that the use of high conductivity materials such as aluminum alloy for the struts enhances the rate of heat transfer by a factor of approximately 2.5 when compared to using a low conductivity material such as polycarbonate.