Vortex Detachment Research Articles

The Influence of bogie cavity bottom plate on the aerodynamic noise characteristics of high speed trains

The bogie area with a complex structure is an extremely important source of noise for trains. Based on the large-eddy simulation(LES) and Ffowcs Williams-Hawkings(FW-H) analogy, aero-acoustic simulation was conducted on a 3-car high-speed train of a certain type with a bogie cavity bottom plate installed on the front and rear cars. The results show that bottom plate changes the flow field around the bogie, weakens the flow separation of airflow at the leading edge of the bogie cavity, mitigates the flow impact of airflow on various components of the bogie, suppresses the formation and detachment of large-scale vortices in the bogie area, and reduces the formation of pressure pulsations on the geometric surface of the bogie area. The bogie cavity bottom plate scheme reduces the far-field aerodynamic noise of the train by 1.63dBA, and the noise reduction effect is significant. The aerodynamic noise of the measurement point equipped with a bogie cavity bottom plate is lower than that of the conventional bogie cavity in all frequency bands, and the energy reduction is more significant at medium and low frequencies.

Advancements in mathematical modeling and numerical simulation: Immersed boundary method for Navier-Stokes equations applied to stationary and rotating isothermal cylinders

This study investigates the growing scientific and industrial interest in the dynamics of fluids around bodies, with a particular focus on thermal fluctuations. These phenomena are prevalent in various scenarios, such as oil extraction platforms, power transmission lines, and fluid-structure interactions, necessitating a comprehensive understanding of vortex generation, heat transfer, and fluid dynamic forces. We employ the Immersed Boundary Method (IBM) to simulate two-dimensional, incompressible flows around stationary and rotating heated (isothermal) cylinders. Using a computational framework developed in C/C++, we analyze the effects of variations in cylinder rotation rates on flow dynamics and thermal distributions. This study involves multiple simulations to evaluate the stability of the method and extract relevant parameters, including drag, lift coefficients and Nusselt numbers, along with velocity, pressure, vorticity, and temperature fields. Through systematic comparison with existing literature, we aim to validate our findings and contribute to the continuous improvement of numerical accuracy in this domain. By elucidating these complex phenomena, our research aims to provide valuable insights for practical applications in industrial and engineering contexts. The objective of this work is to analyze the combination of heat transfer phenomena with rotation in isothermal cylinders and their thermofluid-structure interaction. To achieve this, a low computational cost C/C++ code (in terms of memory and computational resources) was developed. Furthermore, numerical simulations indicated that with an increase in the Reynolds number, there is an increase in the drag coefficient, highlighting the significant influence of the pressure distribution downstream of the cylinder. This pressure distribution is strongly affected by vortex formation and detachment, thereby validating the methodology.

Enhancing Energy Harvesting Efficiency of Flapping Wings with Leading-Edge Magnus Effect Cylinder.

According to the Magnus principle, a rotating cylinder experiences a lateral force perpendicular to the incoming flow direction. This phenomenon can be harnessed to boost the lift of an airfoil by positioning a rotating cylinder at the leading edge. In this study, we simulate flapping-wing motion using the sliding mesh technique in a heaving coordinate system to investigate the energy harvesting capabilities of Magnus effect flapping wings (MEFWs) featuring a leading-edge rotating cylinder. Through analysis of the flow field vortex structure and pressure distribution, we explore how control parameters such as gap width, rotational speed ratio, and phase difference of the leading-edge rotating cylinder impact the energy harvesting characteristics of the flapping wing. The results demonstrate that MEFWs effectively mitigate the formation of leading-edge vortices during wing motion. Consequently, this enhances both lift generation and energy harvesting capability. MEFWs with smaller gap widths are less prone to induce the detachment of leading-edge vortices during motion, ensuring a higher peak lift force and an increase in the energy harvesting efficiency. Moreover, higher rotational speed ratios and phase differences, synchronized with wing motion, can prevent leading-edge vortex generation during wing motion. All three control parameters contribute to enhancing the energy harvesting capability of MEFWs within a certain range. At the examined Reynolds number, the optimal parameter values are determined to be a∗ = 0.0005, R = 3, and ϕ0 = 0°.

VIV and forced convection response of a freely oscillating cylinder to a single impulse perturbation of varying intensity

A numerical investigation into the effects of single gust impulse on the flow physics, dynamic response, and heat transfer from an undamped two degrees of freedom circular cylinder is presented for a constant property, incompressible Newtonian fluid at a low Reynolds number range, 70≤Re≤150 and Prandtl number Pr=7. Structural natural frequency (FN) is set equal to the vortex shedding frequency of a static circular cylinder (fv) at Re=100 and is kept constant in both the degrees of freedom such that FN=FNx=FNy. Mass ratio is m*=10, while structural damping is set to zero (ζ=0). Intensities of gust impulse are varied as a function of inlet velocity while keeping the frequency of gust impulse fixed, in order to isolate the effects of intensity of gust only. Results pertaining to transient effects of gust impulse on the fluidic forces, dynamic response, and conjugate heat transfer are presented. Time histories of lift coefficient show a pattern of quicker recovery from gust in the lock-in regime. Shifting of primary and secondary frequencies due to gust impulse is observed. A very organized and regular pattern of cylinder dislodgement and recovery is observed in the lock-in regimes as opposed to chaotic behaviour out of the lock-in regime. The vortex street is seen to eventually stabilize and restore itself to its original pre-gust oscillatory behaviour. Vorticity contours depict an early vortex detachment due to the shear layers’ stretching followed by a pair of counter rotating vortices shed simultaneously. Consequent evolution of heat transfer behaviour from the cylinder surface is also recorded in the temperature contours. Furthermore, local Nusselt number at the front end of the cylinder depicts remarkable sensitivity to the gust intensity. Increase in gust intensity also causes higher peak values of local skin friction coefficient.

Numerical study on vortex induced vibration of hydrofoils with trailing-edge truncation

Vortex-induced vibration of hydrofoils may cause structural damage and noise hazards to engineering applications under certain unfavorable conditions. Therefore, the vortex-induced vibrations of a hydrofoil under different conditions should be investigated in detail. In this paper, numerical simulations are conducted to study the vortex-induced vibration characteristics of an elastically suspended hydrofoil with trailing-edge truncation. The vibration responses under various structural natural frequencies are calculated. In addition, the effect of six trailing-edge truncation angles from 0° to 40° on the vibration characteristics is explored. Results indicate that the hydrofoil experiences frequency lock-in within a specific frequency range, where structural motion controls vortex shedding patterns and the vortex shedding frequency locks into the structural natural frequency. Moreover, as the trailing-edge truncation angle increases, the upper threshold of frequency lock-in decreases, but the lower threshold shows a first decreasing and then increasing trend, with the minimum value at 30°. Simultaneously, there exists an optimal truncation angle to minimize vibration amplitude at a certain structural natural frequency. The wake flow analysis demonstrates that the vibration amplitude reduction is due to the spatial shift between the vortex detachment locations of upper and lower surfaces, leading to the collision of vortices and energy dissipation.

Wake structure of compound drops oscillating in a viscous fluid

We report an experimental investigation of the structure of the wake and oscillation dynamics of gas-liquid compound drops rising at high Reynolds numbers. Particle image velocimetry (PIV) and high-speed shadowgraph techniques were used to determine the conditions required for the motion to become unstable and to gain insight into the origin of the instability. Three combinations of fluids were tested for a wide range of the diameter ratio db/dcd - i.e., the equivalent diameter of the gas bubble divided by that of the liquid layer -, while the Reynolds number spans roughly from 200 to 700. As db/dcd is increased beyond a critical range, the flow becomes unstable. The compound drops follow a pendular oscillation where the centroid of the liquid acts as a pivot with respect to the top of the compound drop, whose dynamics depends greatly on db/dcd. Measurements of the normal and streamwise vorticities pointed to the instability of the wake as the origin of the oscillation, similarly to a rigid sphere. However, the multiphase structure of the drop also contributed to the destabilization of the system, shifting the motion transition to a lower Reynolds number. The pendular oscillation favors the asymmetry of the flow field behind the compound drop, amplifying the unsteadiness of the wake. The induced path oscillation is therefore shown to result from the coupling of the wake vortex detachment and the pendulum motion induced by the sloshing of the liquid layer inside the compound drop.

Wing Planform Effect on the Aerodynamics of Insect Wings

Simple SummaryThis study aims to provide an improved understanding of the effect of wing planform shape on the aerodynamic performance of insect flapping wings. We focus our investigation on three planform parameters, namely aspect ratio, radial centroid location, and wing root offset, and their effect on the aerodynamic performance is characterised at a flow Reynolds number most relevant to small insects similar to fruit flies. We show that aspect ratio and root offset mainly influence the flow detachment area near the wingtip, whereas radial centroid location mainly influences the local flow evolution time on the wing surface. Overall, increasing the aspect ratio is beneficial to lift and efficiency up to a limit where flow detachment near the wing tip leads to less-favorable performance. Similarly, increasing the wing root offset leads to an increased flow detachment area near the wing tip, resulting in reduced lift coefficient, but the aerodynamic efficiency remains relatively unaffected by the root offset value for most aspect ratios. Finally, increasing the radial centroid location mainly increases the aerodynamic efficiency.This study investigates the effect of wing planform shape on the aerodynamic performance of insect wings by numerically solving the incompressible Navier-Stokes equations. We define the wing planforms using a beta-function distribution and employ kinematics representative of normal hovering flight. In particular, we use three primary parameters to describe the planform geometry: aspect ratio, radial centroid location, and wing root offset. The force coefficients, flow structures, and aerodynamic efficiency for different wing planforms at a Reynolds number of 100 are evaluated. It is found that the wing with the lowest aspect ratio of 1.5 results in the highest peaks of lift and drag coefficients during stroke reversals, whereas the higher aspect ratio wings produce higher lift and drag coefficients during mid half-stroke translation. For the wings considered, the leading-edge vortex detachment is found to be approximately at a location that is 3.5–5 mean chord lengths from the wing center of rotation for all aspect ratios and root offsets investigated. Consequently, the detachment area increases with the increase of aspect ratio and root offset, resulting in reduced aerodynamic coefficients. The radial centroid location is found to influence the local flow evolution time, and this results in earlier formation/detachment of the leading-edge vortex for wings with a smaller radial centroid location. Overall, the best performance, when considering both average lift coefficient and efficiency, is found at the intermediate aspect ratios of 4.5–6; increasing the centroid location mainly increases efficiency; and increasing the root offset leads to a decreased average lift coefficient whilst leading to relatively small variations in aerodynamic efficiency for most aspect ratios.

How Do Water Filled Traffic Barriers Shake a Suspension Bridge?

The present study stems from the realization that the general problem relating to the analysis of wind-induced vibrations in suspension bridges still requires significant attention. Sidewalk railings, overhaul tracks, and deflectors are known to largely affect such dynamics. Here, the influence of a row of water-filled traffic barriers on the response of a sample suspension bridge is investigated numerically. It is shown that the existence of water barriers causes flow separation and non-negligible vortices with respect to the condition with no water barriers. The vortex shedding frequency at the far end is around 41.30 Hz, relatively close to the real vibration frequency. It is also shown how different incoming angles of attack can change the flow field around the bridge cross-section and the vortex detachment frequency.

Vibrations of Slender Structures Caused by Vortices

Slender cylindrical structures such as overhead transmission lines, skyscrapers, chimneys, risers, and pipelines can experience flow induced vibration (FIV). The vortex vibrations are a type of FIV; they arise because of oscillating forces caused by flow separation and the detachment of vortices. The paper presents a brief overview of experimental research on vortex induced vibration - VIV of short, rigid cylinders elastically supported (with a small aspect ratio). This overview summarizes the basic results of the vortex vibration (VIV) which have been performed in the last five decades. These studies were mainly related to determining the influence of selected parameters - mass, damping and Reynolds number on the cylinder response, either in one direction only or simultaneously in the flow direction and transverse to the flow direction, and with the search for a map of vortex images in the trace (vortex wake pattern map).

On the Effect of Beating during Nonlinear Frequency Chirping

Spectral analyses of energetic particle (EP) driven bursts of MHD fluctuations in magnetically confined plasmas often exhibit multiple simultaneous chirps. While the superposition of oscillations at multiple frequencies necessarily causes beating in the signal acquired by a localized external probe, self-consistent hybrid simulations of chirping EP modes in a JT-60U tokamak plasma have demonstrated the possibility of global beating, where the electromagnetic field vanishes globally between beats and reappears with opposite phase. This implies that there can be a single field mode that oscillates at multiple frequencies simultaneously when resonantly driven by multiple density waves in EP phase space. Conversely, this means that the EP density waves are mutually coupled and interfere with each other via the jointly driven field, a mechanism ignored in some theories. In this treatise, we study the role of field pulsations in general and beating in particular using the Hamiltonian guiding center orbit-following code ORBIT with a reduced wave-particle interaction model in realistic geometry. Through amplitude pulsations and phase jumps, beating is found to drive the evolution of EP phase space structures. Observations: (1) Beating causes density wave fronts to advance radially in pulses. The resulting chirps become staircase-like. (2) The beats facilitate convective transfer of material between neighboring layers of phase space density waves. On the one hand, this may delay detachment of solitary vortices. On the other hand, it facilitates the accumulation of hole and clump fragments into larger structures. (3) Long-range chirping occurs when massive holes or clumps detach and drift away from the turbulent belt around the seed resonance. The detached vortices can remain robust and, on average, maintain their concentric nested layers while being perturbed by the field's continued beating.

A transition point for the blood flow wall shear stress environment in the human fetal left ventricle during early gestation

Development of the fetal heart is a fascinating process that involves a tremendous amount of growth. Here, we performed image-based flow simulations of 3 human fetal left ventricles (LV), and investigated the hypothetical scenario where the sizes of the hearts are scaled down, leading to reduced Reynolds number, to emulate earlier fetal stages. The shape and motion of the LV were retained over the scaling to isolate and understand the effects of length scaling on its fluid dynamics. We observed an interesting cut-off point in Reynolds number (Re), across which the dependency of LV wall shear stress (WSS) on Re changed. This was in line with classical fluid mechanic theory where skin friction coefficient exhibited first a decreasing trend and then a plateauing trend with increasing Re. Below this cut-off point, viscous effects dominated, stifling the formation of LV diastolic vorticity structures, and WSS was roughly independent of Reynolds number. However, above this cut-off, inertial effects dominated to cause diastolic vortex ring formation and detachment, and to cause WSS to scale linearly with Reynolds number. Results suggested that this transition point is found at approximately 11 weeks of gestation. Since WSS is thought to be a biomechanical stimuli for growth, this may have implications on normal fetal heart growth and malformation diseases like Hypoplastic Left Heart Syndrome.

Delaying leading edge vortex detachment by plasma flow control at topologically critical locations

Flapping wing propulsion offers unrivalled manoeuvrability and efficiency at low flight speeds and in hover. These advantages are attributed to the leading edge vortex developing on an unsteady wing, which induces additional lift. We propose and validate a manipulation hypothesis that allows prolongation of the leading edge vortex growth phase, by delaying its detachment with the aid of flow control. This approach targets an overall lift increase on unsteady airfoils. A dielectric barrier discharge plasma actuator is successfully used to compress secondary structures upstream of the main vortex on a pitching and plunging flat plate. To determine flow control timing and location, the tangential velocity on the airfoil surface is used, which is also used to quantify topological effects of flow control. This flow control is then tested for different motion kinematics and on a NACA 0012 airfoil. Significant increase of the peak circulation of the leading edge vortex of about 20% for all cases with flow control indicates that this approach is applicable for various kinematics, dynamics and airfoil types.

Predicting the Behaviour of a Vortex Shedding-Based Passive Mechanical Micro Heat Exchanger

In the recent years, microscale applications are gaining increasing importance. Despite their advances, the technology and resources needed to develop new designs may be a drawback for reduced scale engineering testing. To overcome this, computational methods are an efficient tool to predict how a real-life system may behave prior physical construction. The present work aims to investigate numerically effective models to predict the conditions at which a micro heat exchanger (MHE) is able to promote mixing by vortex shedding mechanics. In spite of vortex-shedding is a well-known mechanism in flow physics, it is not possible to know a priori whether a configuration (for a given geometry and flow velocity) may or may not lead to this desired vortex detachment to enhace mixing. Thus, Machine Learning methods are used for prediction, trained with finite-volume numerical simulations of different MHE devices selected based on their performance. A classification model is used to predict which configurations lead to vortex shedding. Also, a correlation regression model is developed to predict the critical Reynolds number. When the critical Reynolds number is surpassed for a given geometry, vortex shedding appears and its intensity controls the thermal mixing efficiency of the microdevice. These predictors could be useful in the search of optimal configurations by optimisation algorithms, since in the sampling process could be used to define constrains.

Leading edge vortex formation and detachment on a flat plate undergoing simultaneous pitching and plunging motion: Experimental and computational study

This study focuses on the formation and detachment of a leading edge vortex (LEV) appearing on an airfoil when its effective angle of attack is dynamically changed, inducing additional forces and moments on the airfoil. Experimental measurements of the time-resolved velocity field using Particle Image Velocimetry (PIV) are complemented by a computational study using an URANS (Unsteady Reynolds-Averaged Navier–Stokes) framework. In this framework a transition-sensitive Reynolds-stress model of turbulence, proposed by Maduta et al. (2018), which combines the near-wall Reynolds-Stress model by Jakirlic and Maduta (2015) and a phenomenological transition model governing the pre-turbulent kinetic energy by Walters and Cokljat (2008), is employed. Combined pitching and plunging kinematics of the investigated flat plate airfoil enable the effective inflow angle to be arbitrarily prescribed. A qualitative assessment of flow fields and a quantitative comparison of LEV characteristics in terms of its center position and circulation as well as an investigation of the mechanism causing the vortex to stop accumulating circulation revealed close agreement between the experimental and simulation results. Further considerations of the lift contribution from the pressure and suction side of the airfoil to the overall lift indicates that the qualitative lift evolution is reproduced even if the pressure side contribution is neglected. This reveals important characteristics of such airfoil dynamics, which can be exploited in future experimental studies, where direct aerodynamic force and moment measurements are greatly inhibited by dominating inertial forces.

Dynamics of periodically-excited vortices in swirling flames

This work investigates the dynamics of periodically-excited vortices in swirling cold flow and hot flame using high-speed particle image velocimetry, for three different Strouhal numbers, 0.68, 1.02, and 1.70. The periodic upstream perturbation induces coherent vortices in the shear layers both inside and outside of the flame surface. We find that the outer vortex rings (OVRs) play a dominant role in tuning the flame dynamics and heat release, whereas the inner vortex rings (IVRs) are related to the formation of the center recirculation zone (CRZ) and suffer from much stronger dissipation. The evolutions of core vorticity, circulation, trajectory, and convective velocity are quantitatively analyzed for the OVRs to understand two key mechanisms: vortex formation and vortex detachment. The results show that the growing and shedding of the OVRs are governed by the inlet shear layer as well as the variations in inlet velocity and acceleration. Furthermore, both the circulation increment during vortex formation and the vortex convection after detachment are qualitatively captured by existing vortex models, demonstrating the prospect of vortex dynamics in understanding general flame-vortex interaction problems.

Insights into leading edge vortex formation and detachment on a pitching and plunging flat plate

The present study is a prelude to applying different flow control devices on pitching and plunging airfoils with the intention of controlling the growth of the leading edge vortex (LEV); hence, the lift under unsteady stall conditions. As a pre-requisite the parameters influencing the development of the LEV topology must be fully understood and this constitutes the main motivation of the present experimental investigation. The aims of this study are twofold. First, an approach is introduced to validate the comparability between flow fields and LEV characteristics of two different facilities using water and air as working media by making use of a common baseline case. The motivation behind this comparison is that with two facilities the overall parameter range can be significantly expanded. This comparison includes an overview of the respective parameter ranges, control of the airfoil kinematics and careful scrutiny of how post-processing procedures of velocity data from time-resolved particle image velocimetry (PIV) influence the integral properties and topological features used to characterise the LEV development. Second, and based on results coming from both facilities, the appearance of secondary structures and their effect on LEV detachment over an extended parameter range is studied. A Lagrangian flow field analysis based on finite-time Lyapunov Exponent (FTLE) ridges allows precise identification of secondary structures and reveals that their emergence is closely correlated to a vortex Reynolds number threshold computed from the LEV circulation. This threshold is used to model the temporal onset of secondary structures. Further analysis indicates that the emergence of secondary structures causes the LEV to stop accumulating circulation if the shear layer angle at the leading edge of the flat plate has ceased to increase. This information is of particular importance for advanced flow control applications, since efforts to strengthen and/or prolong LEV growth rely on precise knowledge about where and when to apply flow control measures.Graphical abstract

Experimental investigation on the leading-edge vortex formation and detachment mechanism of a pitching and plunging plate

Abstract

Dynamics of bubbles rising in pseudo-2D bubble column: Effect of confinement and inertia

2D Rectangular columns, formed within a thin gap between two enclosed parallel plates, are often used in visualisation and CFD studies of multiphase flows. In this study, a numerical analysis is performed on the dynamics of a bubble rising in such setup (D = 5–7 mm, Re ≈ 1000–2000), with the aim of determining the effect of the size of the gap (h) on the velocity field around it. For most cases, the bubble adopts an oblate spheroidal shape, with chaotic rise behaviour and continual fluctuations in both aspect ratio, orientation, and velocity. The bubble aspect ratio generally decreases with increasing level of confinement, because of increased level of resistance provided by the walls. With increasing Reynolds number, however, the bubble (D = 7 mm, h/D = 1) is found to display a more elongated oblate spheroidal shape and a rise behaviour that is largely rectilinear. This change in behaviour has been attributed towards severe disruptions in the development of toroidal vortex formation at the walls, confining the formation of hairpin vortices towards the side interface of the bubble at the centreline of the column. This prevents the development of large-scale fluctuations and confines the path oscillation of the bubble towards high-frequency fluctuations. Further, the formation of a region of recirculating flow is also evident, as a result of rapid detachment of vortices at the bubble equator. Intense fluctuation in velocity in the horizontal direction parallel to the wall is developed, and the dissipation in this fluctuation, as well as in the time-averaged velocity below the bubble, is found to occur more gradually than in the case of columns with larger gaps.

Experiment and modeling of lift force acting on single high Reynolds number bubbles rising in linear shear flow

The lift force acting on single bubbles rising in a downward linear shear flow is measured experimentally. The flow is generated by a 90° sharply bent rectangular water channel, with uniformly distributed vanes installed directly after the corner, and the linear velocity profile is validated by particle image velocimetry. In the flow, single contaminated bubbles with a volume equivalent diameter d ranging from 2 mm to 20 mm are tested, corresponding to a Reynolds number Re based on d and relative velocity of the bubble in the quasi-steady state from 440 to 7200. The experimental results of the lift coefficient CL at d < 8 mm (Re < 2100) are consistent with both the data and correlation in the literature. In the case of d > 8 mm, the results of CL exhibit similar tendencies and magnitudes to those of Li et al. (2016a). Based on the ideal modeling of the vortex detachment, the wake-induced contribution of CL is estimated. Good agreement is achieved between the newly derived model and experimental results.

Análise dinâmica elástica em torres tubulares de aço para aerogeradores de eixo horizontal

This article aims to obtain the dynamic elastic analysis of a steel tube tower for horizontal axis wind turbine using finite elements of bar (own code) and finite elements of bark and solids (modeling in ANSYS). For this, a theoretical foundation is presented that consists in the development of the movement equation from the Lagrange equation. Next, the modal overlay method is presented for solving the matrix motion equation and for obtaining the dynamic response of the tower. In the second part of this article, the development is presented for the assembly of the mass and damping matrices of the tower modeled with its own code in finite elements of the bar. In the part of results, initially, the rigidity, mass and damping matrices are exposed for a certain level of discretization of the modeled tower with finite elements of bar. Then, the results of the modal analysis were done for both the finite element shell model, with and without flexible base, and the finite element model of the bar, in which the modes and the frequencies of vibration of the tower in all the models are presented. Finally, the responses of the tower were analyzed: in the direction of the wind flow, when submitted by a vector of resonant forces to its 1st mode of vibration, representing the floating part of the wind; and, in the transverse direction to the wind flow due to the von K&aacute;rman cadenced vortex detachment. The results of the modal analysis made for both the finite element shell model, with and without flexible base, and the finite element model of the bar are similar, especially in the first mode of vibration of the tower. Therefore, the model represented by finite elements (FE) of bar can be used as representative of the dynamic behavior of the tower when it is subjected to the resonant excitations at its fundamental frequency.