Leading Edge Separation Vortex Research Articles

Study of driving force characteristics in the formation of dominant unstable flow events in water jet propulsion pumps

Unstable flow events are pivotal in influencing water jet propulsion pumps' safe and stable operation. However, a notable research gap exists in systematically investigating the underlying driving force characteristics responsible for the formation of predominant unstable flow events in the impeller. To address this gap, we conducted experimental and numerical simulation studies, yielding the following results. As the flow rate decreases, a significant rise in the peak-to-peak value of pressure fluctuations and rigid vorticity occurs on both the blade outlet hub and inlet shroud sides. Two distinct unstable flow events emerge at these locations: the leading-edge separation vortex on the blade inlet shroud side and the horn-like vortex on the blade outlet hub side. To elucidate these phenomena, we conducted an analysis of the driving force characteristics employing the momentum equation of relative motion. Our analysis revealed that, as the flow rate decreases, there is a notable increase in the potential rothalpy gradient (PRG, or the reduced static pressure gradient) at these positions, surpassing the influence of the Coriolis force. Based on these findings, we have constructed the fundamental topological structures of these unstable flow events.

Critical State Calculation of Saddle-Shaped Unstable Region of the Axial-Flow Pump Based on Bifurcation SST k–ω Model

This study aims to investigate the critical state of the saddle-shaped unstable region of the axial-flow pump and propose a suitable criterion for identifying this state. The bifurcation SST k–ω model considered the rotation effect is used in the present work and verified in the numerical calculation of a water jet pump. Then, it is used to simulate the critical state of the axial-flow pump. Results show that the leading-edge separation vortex generates at 0.6Qd, while the head declines only at 0.55Qd. Therefore, using the inflection point of the head-flow curve as the critical state criterion is unsuitable. In addition, the fixed monitoring point is unsuitable for identifying the critical state due to the insensitivity to the amplitude, main frequency, and periodicity changes at the critical state. Finally, to identify the critical state, it is essential to arrange a monitoring point at the leading edge of the blade suction near the shroud, which should rotate with the impeller. The critical state criterion is that the main frequency position of the pressure fluctuation signal is offset at the monitoring point, and the amplitude is increased by 10 times.

Towards Accurate Vortex Separation Simulations with RANS Using Improved k-kL Turbulence Model

In this study, we present our improved RANS results of the missile aerodynamic flow computation involving leading edge vortex separation. We have used our in-house tailored version of the open source finite volume solver FlowPsi. An ongoing study in the NATO STO Applied Vehicle Technologies Panel (AVT-316) has revealed that a highly maneuverable missile configuration (LK6E2) shows unusual rolling moment characteristics due to the vortex–surface interactions occurring during wing leading edge separation of vortices. We show the performance of the recently developed k-kL turbulence model for this test problem. This turbulence model is shown to have superior capabilities compared to other widely used turbulence models, such as Spalart–Allmaras and shear stress transport. With the k-kL turbulence model, it is possible to achieve more realistic computational results that agree better with the physical data. In addition, we propose improvements to this turbulence model to achieve even better predictions of rolling moment behavior. Modifications based on turbulence production terms in the k-kL turbulence model significantly improved the predicted rolling moment coefficient, in terms of accuracy and uncertainty.

Investigation on hydrodynamic characteristics of a hydrofoil based on γ-Reθt transition model

The complex flow field caused by the dynamic stall can affect the operational stability of hydrodynamic machinery. In this paper, the NACA0009 blunt trailing edge hydrofoil is used as the object of study, and the dynamic stall characteristics of the hydrofoil are investigated by using the transition model and the dynamic mesh method. It is found that the hydrofoil deep stall calculated by the transition model is delayed compared to that calculated without the transition model. The hydrofoil dynamic stall can be divided into four stages, initial stage, development stage, stall inception stage and deep stall stage. In the initial stage and the development stage, the lift and drag characteristics are influenced by the shedding vortex. In the stall inception stage and the deep stall stage, the lift and drag characteristics are influenced by the leading edge separation vortex and the trailing edge vortex. The increase of angular velocity and Reynolds number of the dynamic hydrofoil delay the onset of the deep stall while accelerating the boundary layer transition. The research in this paper has a certain guiding effect for the safe and stable operation of hydrodynamic machinery.

Numerical investigation of the benefits of serrated Gurney flaps on an axial flow fan

A novel serrated Gurney flap (SGF) is proposed, and a large eddy simulation is employed to simulate a two-stage variable-pitch axial fan with SGFs. The influence of the novel blades on the aerodynamic performance, sound characteristics and internal dynamics of the fan was examined, and the possible mechanism for noise reduction and vortex structure were investigated. Results indicate that the peak efficiency of the fan shifts toward the large flowrate side, and the high-efficiency region is widened when using SGFs. SGFs with an appropriate serration height reduce high-frequency noise, and the noise reduction gradually improves with decreasing serration height. The use of SGFs decreases the turbulence pulsation intensity, improves the size and distribution of wake vortices, and abates the leading-edge separation vortex and turbulence intensity. Mechanism studies indicate that two counter-rotating vortex pairs in the vertical and horizontal directions interact strongly, which alleviates the aerodynamic acoustics of the fan. A fan with an SGF with a serration height of 0.8% chord achieves the optimal overall performance, including improved efficiency, total pressure rise, and reduced noise.

Low-frequency oscillation characteristics of flow for NACA66 hydrofoil under critical stall condition

Hydrofoil is the core unit of energy conversion for axial-flow pumps and hydro-energy turbines. The dynamic characteristics of hydrofoil significantly affect the operation stability of pumps and turbines, especially under the critical stall condition. In this paper, the unsteady flow field of NACA66 hydrofoil under critical stall condition at Reynolds number 0.75 × 10E6 is investigated by IDDES method. The low-frequency oscillations (LFO) under critical stall condition characterized by two typical frequencies with Strouhal number (St) 0.031 and 0.065 are reported in pressure pulsation and lift coefficient respectively. It is found that the merging phenomenon of separation vortices are the main reason of the wave peak of LFO with St 0.031, while the phenomenon of generation and shedding of separation vortices exist at the moment corresponding to the wave trough. The reason of the LFO of lift coefficient with St 0.065 is found to be the long-short switching of streamwise coverage of the leading edge separation vortex. The change of streamwise coverage is corresponding to the movement of the reattachment point of the separation area. The revelation of LFOs flow mechanism in hydrofoil under critical stall can provide reference for its unsteady flow control and applications.

Aerodynamic performance and wake development of airfoils with wavy leading edges

A comprehensive numerical investigation is performed to study the aerodynamic performance and wake development of a NACA0012 airfoil with wavy leading edges (WLE). The WLEs are in the form of sinusoidal profiles with two design parameters of amplitude and wavelength. Parametric studies of the amplitude and wavelength are conducted to better understand the effect of WLEs. The chord length based Reynolds number is 400 000 and the angle of attack varies from 0° to 20°. For angles of attack less than the stall angle, steady Reynolds-averaged Navier–Stokes (RANS) computations are carried out using the SST (Shear Stress Transportation) k–ω turbulence model. For the post-stall angles of attack, unsteady simulations are carried out using the SST–SAS (Scale-Adaptive Simulation) model to solve the transient separation flow. The numerical results indicate that the aerodynamic performance of the wavy airfoil is sensitive to both the amplitude and wavelength of the WLEs. The WLE with a larger amplitude and smaller wavelength results in a worse aerodynamic performance. The drag coefficient is decreased significantly by the WLEs in the post-stall region. In addition, the wavy airfoils possess a soft stall process without the abrupt loss of the lift when the amplitude is large enough. The wake profiles for the wavy airfoils differ significantly from those for a conventional airfoil, especially for high angles of attack. In the post-stall regime of the baseline airfoil, the wavy airfoils have a higher wake deficit compared to the baseline airfoil, but the turbulence kinetic energy is reduced significantly. The leading edge separation vortex generated by the baseline airfoil is broken down into small vortices by the wavy modification, leading to a reduced lift and drag fluctuations.

Experimental and Numerical Analysis of the Effect of Vortex Generator Installation Angle on Flow Separation Control

In order to explore the effect of the installation angle of vortex generator (VG) on boundary-layer flow control, the vortex characteristics of plate VG and their effect on the aerodynamic characteristics of an airfoil was studied numerically and using wind tunnel experiments. The effects of five VG installation angles (β) of 10°, 15°, 20°, 25°, and 30° on the characteristics of vortices were studied. The results show that the strength of vortices on the leeward side of VG increases with an increased installation angle until, eventually, the vortex core breaks down. During the downstream development of the VG leading-edge separation vortices, these vortices deviate in the radial direction. The larger the installation angle, the larger this deviation distance in the radial direction becomes. The effects of installation angle on the aerodynamic performance of airfoils were studied in a wind tunnel using the same five VG installation angles. The results show that VG can delay flow separation on the airfoil suction surface, thereby increasing lift and reducing drag. The stall angle of the airfoil with VG was increased by 10°. When the installation angle of the VG was 20°, the maximum lift coefficient of airfoil increased by 48.77%. For an airfoil angle of attack (AoA) of 18°, the drag of the airfoil decreased by 88%, and the lift-drag ratio increased by 1146.04%. Considering the best overall distribution of lift-drag ratio, the positive effect of the VG was found to be when β = 20° and the worst VG effectiveness was observed at β = 30°.

Review of self-induced roll oscillations and its attenuation for low-aspect-ratio wings

Micro aerial vehicles are currently receiving growing interest because of their broad applications in many fields. In their flight tests, the onset of unwanted large amplitude roll oscillations was reported, which resulted in difficulties with flight control, and this has become one of the major challenges of current micro aerial vehicles design. In this review type of article, the low Reynolds number flow characteristics of a low-aspect-ratio wing are reviewed, and the self-induced roll oscillations are discussed with special attention being payed to the interaction between the three-dimensional flow structure and wing in reciprocatively rolling motion. The roll attenuation methods are introduced via flow control approaches, which can suppress the roll oscillations effectively by manipulating the leading-edge flow separation and tip vortices of the low-aspect-ratio wings.

Computational Investigation of Vertical Stabilizer with Vortex Generators and Dorsal Fin

This paper describes methods to improve the performance of a vertical stabilizer in terms of yawing moment with corotating blade-type vortex generators and a dorsal fin over a wide range of sideslip angles. A vertical stabilizer designed by ONERA was mounted on the NASA Common Research Model to investigate the performance of sets of vortex generators and dorsal fins at several rudder deflection angles. Computational fluid dynamics simulations were performed to reveal basic flow characteristics of the vertical stabilizer, including its rudder, and to understand how vortices generated around the vortex generators and dorsal fins interact with the leading-edge separation vortex and boundary layer on the vertical stabilizer and how the vortices improve the lateral performance of the vertical stabilizer. Wind-tunnel tests were also conducted to validate the computational results. Vortex generators improved the performance of the vertical stabilizer slightly at low sideslip angles by reducing flow separation on the rudder, and a dorsal fin assisted greatly at high sideslip angles due to two vortices it induced: the primary dorsal fin vortex and the leading-edge separation vortex.

Low Reynolds number effect on energy extraction performance of semi-passive flapping foil

In this paper, 2-D numerical solution scheme is used to study the performance of semi-passive flapping foil flow energy harvester at Reynolds numbers ranging from 5000 to 50,000. The energy harvester comprises of NACA0015 airfoil which is supported on a translational spring and damper. An external sinusoidal pitch excitation is provided to the airfoil. Energy is extracted from the flow induced vibration of airfoil in translational mode. Movement of airfoil is accommodated in fluid domain by using a hybrid meshfree Cartesian fluid grid. A body conformal meshfree nodal cloud forms the near field domain, encompassing the airfoil. During the simulation, the solid boundary causes the motion of the meshfree nodal cloud, without necessitating re-meshing. In the far field, the static Cartesian grid encloses and partly overlaps the meshfree nodal cloud. A coupled mesh based and meshfree solution scheme is utilized to solve laminar flow, viscous, incompressible equations, in Arbitrary-Lagrangian-Eulerian (ALE) formulation, over a hybrid grid. Spatial discretization of flow equations is carried out using radial basis function in finite difference mode (RBF-FD) over meshfree nodes and conventional finite differencing over Cartesian grid. Stabilized flow momentum equations are used to avoid spurious fluctuations at high Reynolds numbers. A closely coupled, partitioned, sub-iteration method is used for fluid structure interaction. The study is focused to analyse the behaviour of flow energy harvesters at various Reynolds numbers. Effects of changing the translational spring stiffness and pitch activation frequency are also investigated. Instantaneous flow structures around the airfoil have been compared at different Reynolds numbers and pitch amplitudes. It is found that net power extracted by the system increases at high Reynolds numbers. Moreover, re-attachment of leading edge separation vortex plays an important role in their overall system performance.

Experimental Validation of Fluid–Structure Interaction Computations of Flexible Composite Propellers in Open Water Conditions Using BEM-FEM and RANS-FEM Methods

In the past several decades, many papers have been published on fluid–structure coupled calculations to analyse the hydro-elastic response of flexible (composite) propellers. The flow is usually modelled either by the Navier–Stokes equations or as a potential flow, by assuming an irrotational flow. Phenomena as separation of the flow, flow transition, boundary layer build-up and vorticity dynamics are not captured in a non-viscous potential flow. Nevertheless, potential flow based methods have been shown to be powerful methods to resolve the hydrodynamics of propellers. With the upcoming interest in flexible (composite) propellers, a valid question is what the consequences of the potential flow simplifications are with regard to the coupled fluid–structure analyses of these types of propellers. This question has been addressed in the following way: calculations and experiments were conducted for uniform flows only, with a propeller geometry that challenges the potential flow model due to its sensitivity to leading edge vortex separation. Calculations were performed on the undeformed propeller geometry with a Reynolds-averaged-Navier–Stokes (RANS) solver and a boundary element method (BEM). These calculations show some typical differences between the RANS and BEM results. The flexible propeller responses were predicted by coupled calculations between BEM and finite element method (FEM) and RANS and FEM. The applied methodologies are briefly described. Results obtained from both calculation methods have been compared to experimental results obtained from blade deformation measurements in a cavitation tunnel. The results show that, even for the extreme cases, promising results have been obtained with the BEM-FEM coupling. The BEM-FEM calculated responses are consistent with the RANS-FEM results.

Research of the leading edge separation vortex characteristics due to the inlet velocity shape

The zero adverse pressure gradient plate is taken as the study object, in order to study the leading edge separation vortex characteristics due to the inlet velocity shape. With the CFD method, we classify the study cases into 6 groups (h/δ = 0.1,0.2,0.5,1.0,1.5,2.0(h for vortex generator(VG) height, δ of inlet boundary layer thickness)) to study the inflow boundary layer’s influence law to the vortex generator characteristics. Calculation results show that, vortex generator leading separation vortex strength is proportional to fluid average kinetic energy within vortex generator height range. Vortex strength shows essentially inverse relationship with the flow boundary layer thickness, while the vortex generator shape resistance also shows the inverse proportion to the flow boundary layer thickness. Namely, heavier flaw boundary layer thickness leads to lower vortex strength and lower vortex generator shape resistance simultaneously.

Numerical study of a variable camber plunge airfoil under wind gust condition

Variable camber deformation is observed during the flight of some insects and bird species; however, the effect of this special airfoil shape motion on the aerodynamic characteristics of the airfoil is not well understood, especially for the airfoil under gust wind. We did a numerical study to investigate the aerodynamic characteristics of a variable camber plunge airfoil under wind gusts. A weak coupling program was developed to simulate the interaction between the variable camber plunge airfoil and fluid, and the flow field, aerodynamic force and energy efficiency of different camber airfoils under different wind gust conditions are investigated. It was found that camber deformation influences the aerodynamic characteristics of the airfoil greatly. If the airfoil has an appropriate camber deformation, the deformation can increase the mean thrust and the propulsive efficiency of the airfoil. Moreover, the aerodynamic characteristics of the appropriate camber airfoils are not significantly affected by the gust frequency, and there exists a range of gust amplitude where the aerodynamic characteristics of the airfoils are also not significantly affected by the gust amplitude, which may be beneficial for aerodynamic stability of the airfoil. The results also show that appropriate camber deformation can suppress leading edge vortex separation, which improves the aerodynamic characteristics of the airfoil.

デルタ形状ボルテックスジェネレーターによる後縁フラップ上の離流れ制御について

Low-speed wind tunnel tests were performed to investigate aerodynamic effects induced by delta-shaped vortex generators installed on the upper surface of a trailing-edge flap. The vortex generator was applied to control the flow field around the trailing-edge flap where the flow was separated from the hinge line of the trailing-edge flap. The test results showed that the vortex generators increase the lift coefficient and the lift-to-drag ratio when the trailing-edge flap deflects to 30degrees. The leading-edge separation vortices formed from the delta-shaped vortex generators suppressed the flow separation on the trailing-edge flap. The height of the vortex generator geometrically changes depending on the trailing-edge flap deflection angles. Therefore, when the trailing-edge flap is not deflected such as cruise flight condition where the flow field is dominated by the attached flow, the vortex generator does not impose a penalty on the aerodynamic performance.

Aerodynamic Performance Prediction of Straight-Bladed Vertical Axis Wind Turbine Based on CFD

Numerical simulation had become an attractive method to carry out researches on structure design and aerodynamic performance prediction of straight-bladed vertical axis wind turbine, while the prediction accuracy was the major concern of CFD. Based on the present two-dimensional CFD model, a series of systematic investigations were conducted to analyze the effects of computational domain, grid number, near-wall grid, and time step on prediction accuracy. And then efforts were devoted into prediction and analysis of the overall flow field, dynamic performance of blades, and its aerodynamic forces. The calculated results agree well with experimental data, and it demonstrates that RNG k-ε turbulent model is great to predict the tendency of aerodynamic forces but with a high estimate value of turbulence viscosity coefficient. Furthermore, the calculated tangential force is more dependent on near-wall grid and prediction accuracy is poor within the region with serious dynamic stall. In addition, blades experience mild and deep stalls at low tip speed ratio, and thus the leading edge separation vortex and its movement on the airfoil surface have a significant impact on the aerodynamic performance.

Initial experiments and analysis of blunt-edge vortex flows for VFE-2 configurations at NASA Langley, USA

A review is presented of the initial experimental results and analysis that formed the basis the Vortex Flow Experiment 2 (VFE-2). The focus of this work was to distinguish the basic effects of Reynolds number, Mach number, angle of attack, and leading-edge bluntness on separation-induced leading-edge vortex flows that are common to slender wings. Primary analysis is focused on detailed static surface pressure distributions, and the results demonstrate significant effects regarding the onset and progression of leading-edge vortex separation.

Aerodynamic Investigations of Low-Aspect-Ratio Thin Plate Wings at Low Reynolds Numbers

ABSTRACTTo realize the relationship between flow structures of wingtip vortices and post stall characteristics of low aspect-ratio wings, this paper experimentally studies the aerodynamic characteristics and the corresponding flow structures of the rectangular thin-plate wings at Reynolds numbers between 104 and 105. The aerodynamic properties to be studied include lift, drag, slopes at linear and nonlinear range of the lift curves and lift-to-drag ratios of the tested wings with the aspect ratio varying from 1.0 to 3.0. The flow structures regarding the leading-edge separation vortices and wingtip vortices at upper surface and near-wake regions of the wings are also investigated by smoke-wire visualization. Results indicate that the high stall angle of attack and vortex lift are clearly manifested to induce the nonlinear increase in the lift curves as the aspect ratio reaches less than 1.6. This phenomenon is specifically observed to augment the aerodynamic properties with the decrease of the aspect ratio. Additionally, the corresponding flow visualization also indicates that the wingtip vortices and the areas of highly affected regions are duly increased with the increase of the angle of attack up to 40°, which makes certain that the extra increase of the nonlinear lift results from these vortices. This result can be practically applied to the planform design for unmanned aerial vehicles.

Separation control on NACA23012 using synthetic jet

A numerical study of separation control has been made to investigate aerodynamic characteristics of NACA23012 airfoil with synthetic jets. Computed results demonstrated that stall characteristics and control surface performance could be substantially improved by resizing separation vortices. The maximum lift was obtained when the separation point coincides with the synthetic jet location and the non-dimensional frequency is about 1. In addition, separation control effect was proportional to the peak velocity of the synthetic jet. It was observed that the actual flow control mechanism and flow structure is fundamentally different depending on the range of synthetic jet frequency. For low frequency range, small vortices due to synthetic jet penetrated to the large leading edge separation vortex, and as a result, the size of the leading edge vortex was remarkably reduced. For high frequency range, however, small vortex did not grow up enough to penetrate into the leading edge separation vortex. Instead, synthetic jet firmly attached the local flow and influenced the circulation of the virtual airfoil shape which is the combined shape of the main airfoil with the separation vortex. As a way to reduce the jet peak velocity, performance of a multi-array synthetic jet was investigated. Moreover, a high frequency multi-location synthetic jet was exploited to efficiently eliminate the unstable flow structure which was observed in low frequency range. Finally, by changing the phase angle in multi-location synthetic jets, highly controlled flow characteristics could be obtained with multi-array/multi-location synthetic jets. This shows efficiency of the current approach in separation control using synthetic jet.

Visualization by PIV of dynamic stall on a vertical axis wind turbine

The aerodynamic behavior of a vertical axis wind turbine (VAWT) is analyzed by means of 2D particle image velocimetry (PIV), focusing on the development of dynamic stall at different tip speed ratios. The VAWT has an unsteady aerodynamic behavior due to the variation with the azimuth angle θ of the blade’s sections’ angle of attack, perceived velocity and Reynolds number. The phenomenon of dynamic stall is then an inherent effect of the operation of a VAWT at low tip speed ratios, impacting both loads and power. The present work is driven by the need to understand this phenomenon, by visualizing and quantifying it, and to create a database for model validation. The experimental method uses PIV to visualize the development of the flow over the suction side of the airfoil for two different reference Reynolds numbers and three tip speed ratios in the operational regime of a small urban wind turbine. The field-of-view of the experiment covers the entire rotation of the blade and almost the entire rotor area. The analysis describes the evolution of the flow around the airfoil and in the rotor area, with special focus on the leading edge separation vortex and trailing edge shed vorticity development. The method also allows the quantification of the flow, both the velocity field and the vorticity/circulation (only the results of the vorticity/circulation distribution are presented), in terms of the phase locked average and the random component.