An Experimental Study of Wing Tip Vortex in the Near Wake of a Rectangular Wing

In this work, the wing tip vortex structure behind a NACA 0015 airfoil with and without small flaps was studied using a Partical Image Velocimetry (PIV) system. The experiment was carried out in a low speed wind tunnel with a test section of 0.5 m × 0.5 m. The Reynolds number (Re), defined by the chord length of the wing (C), was 8.1×104. The angle of attack was fixed at 10°. The PIV measurements were made from 0 to 2C, measured from the trailing edge of the model. The dihedral angle of three flaps was −15°, 0° and 15°, respectively. Compared with the clean airfoil, the one with three flaps significantly changed the wing tip vortex structure, the vorticity and the core of the wing tip vortex. The occurrence of three flaps decreased the gradient of pressure on the two sides of the wing tip, which depressed wing tip vortex formation to some extent. Vortices shed from three flaps influence the evolution of the wingtip vortex generated by the base airfoil. The interaction of those vortices resulted in a weakening of the wing tip vortex.

Models for the rate of decay of rotor tip vortices, based on near-wake data and computations, are in conflict with the observed persistence of rotor tip vortices for many revolutions. A correlation of data from the literature shows that the measured tip vortex strength is only 40% of the strength expected from the peak of the blade bound circulation, for blades with sharply-cut straight-edged tips, over a wide range of tip Mach and Reynolds numbers. Explanations of this drop in the near wake through turbulence models, are seen to be both counter-intuitive and in conflict with measurements of turbulent fluctuations. Using flow visualization and velocimetry in the wake of a 2-bladed rotor in low-speed forward flight, explanations are shown for these mysteries. Shear layer development at the blade tip, and the twist distribution of the blade have a strong influence on the fraction of the blade tip vorticity lost into the inboard counter-rotating wake. The observations of elliptic core shape and of multiple local extrema of axial velocity deficit, are shown to be features of the shear-layer separation and rollup processes. Blade-to-blade differences in vortex evolution from the same rotor are traced to surface texture differences at the blade tip. Results in the literature reporting 90 t0 100% tip circulation recovery for such blades, are seen to be in cases where the induced velocity due to the tip vortex could not be distinguished from that due to the inboard sheet.

Large-eddy simulations of a propeller operating upstream of a hydrofoil at incidence were conducted on a cylindrical grid consisting of almost 4 × 109 points. This manuscript, focused on the interaction between the tip and hub vortices shed by the propeller with the downstream hydrofoil, is part of a broader study, including the analysis of the boundary layer on the hydrofoil [A. Posa and R. Broglia, “Flow over a hydrofoil at incidence immersed within the wake of a propeller,” Phys. Fluids 33, 125108 (2021)] and the downstream development of the wake of the overall system [A. Posa and R. Broglia, “Development of the wake shed by a system composed of a propeller and a rudder at incidence,” Int. J. Heat Fluid Flow 94, 108919 (2022)]. The results demonstrate a substantial influence by the orientation of the hydrofoil on the propeller wake. The pressure and suction side branches of the tip vortices experience outward and inward spanwise shifts across the hydrofoil, respectively. However, these shifts are affected significantly by the angle of incidence. As a result, the propeller wake undergoes an overall contraction on the suction side of the hydrofoil, promoting higher levels of shear and turbulence and a faster growth of the boundary layer thickness. The opposite occurs on the pressure side of the hydrofoil, producing eventually a strong asymmetry of the overall wake. Also the evolution of the hub vortex is substantially affected. It splits into two branches, shifting across the span of the hydrofoil in the direction opposite to the tip vortices. As the incidence angle of the hydrofoil grows, the branch of the hub vortex populating its pressure side becomes larger than the one on its suction side, developing a higher shear with the hydrofoil boundary layer and the suction side branches of the tip vortices.

The aerodynamic performances of aircraft change due to the ground effect for flights near the ground. The air cushion generated near the ground will change the development of the aircraft wake vortex. Researchers have compared several different ground boundary conditions with two-dimensional airfoils, while three-dimensional cases have not been studied specifically. In this study, the effects of stationary and moving wall boundary conditions on the aircraft aerodynamic characteristics and wake development were numerically investigated in three dimensions. The results showed that the stationary wall overestimated the viscous effect of the ground, resulting in a reduction of the wingtip vortex (WTV) vorticity magnitude. The difference appeared at (wingspan of the aircraft) where the secondary vortex was induced by the wingtip and horizontal tail vortex (HTV).

Wingtip vortices are an important phenomenon in fluid dynamics due to their complex and negative impacts. Despite numerous studies, the current understanding of the inner vortex is very limited; thus, a basis for the design of effective wingtip geometry and vortex manipulation is narrow. This work examines the structure of the trailing vortex shed from a swept-tapered wing, analogous to a commercial aircraft topology. Stereoscopic particle imaging velocimetry has been utilized to compare the vortex structure and development through several angles of attack at various downstream stations for a fixed Reynolds number (Re = 1.5 × 106). After correcting for vortex meander through helicity-based spatial localization of the vortex core, relationships between the vortex core velocity/vorticity fields, core shape, and turbulent properties have been examined. Subsequently, the vortex is found to exhibit a layered structure with slow linear rates of dissipation indicative of laminar diffusion mechanisms, despite being a turbulent vortex. The turbulent kinetic energy distribution in the vortex signals that relaminarization of the inner core occurs. Consideration of the streamline curvature around the core, via examination of the local Richardson number, indicated that a laminar core structure had formed within which large-scale turbulent eddies could not contribute to the turbulent diffusion of vorticity away from the core. The normalized circulation within the vortex core has been shown to exhibit self-similar behavior typical of the fully developed axisymmetric vortices.

The impact of Gurney flaps, of different heights and perforations, on the growth and development of a tip vortex, both along the tip and in the near field of a finite NACA 0012 wing, at Re = 1.05 × 105 was investigated by using particle image velocimetry (PIV). Wind-tunnel force balance measurements were also made to supplement the PIV results. This study is a continuation of the work of Lee and Ko (Exp Fluids 46(6):1005–1019, 2009) on the near-wake measurements behind perforated Gurney flaps. The present results show that along the tip, the overall behavior of the secondary vortices and their interaction with the primary, or tip, vortex remained basically unchanged, regardless of flap height and perforation. The peak vorticity of the tip vortex, however, increased with flap height and always exhibited a local maximum at x/c = 0.8 (from the leading edge). In the near field, the strength and structure of the near-field tip vortex were found to vary greatly with the flap height and perforation. The small flaps produced a more concentrated tip vortex with an increased circulation, while the large Gurney flaps caused a disruption of the tip vortex. The disrupted vortex can, however, be re-established by the addition of flap perforation. The larger the flap perforation the more organized the tip vortex. The Gurney flaps have the potential to serve as an alternative off-design wake vortex control device.

The current experimental study investigates the details of the formation, structure, and development of wing tip vortices that are generated by an aerofoil of NACA0012 profile having a rectangular planform of 151 mm chord using the particle image velocimetry technique. Experiments have been carried out at different angles of attack, which affect the vortex strength, vortex size, tangential velocity, and vorticity distribution along the downstream stations at chord lengths ranging from x/ c = 0.1 to 1.6, at a fixed free-stream velocity of 212 mm/s corresponding to the Reynolds number, Rec = 32 000 based on the chord length. The radius of the vortex core is found to vary approximately between 5 and 7.5 per cent of the chord length of the wing. The results show that the flow around the tip is dominated by the structures of multiple vortices. The maximum normalized tangential velocity increases with increasing angle of attack. The peak magnitude of the tangential velocity is ∼15 per cent of the free-stream velocity for an angle of attack of α = 4°, whereas it is ∼40 per cent of the free-stream velocity for α = 12°.

The relationship between rotor-blade aeroelasticity and tip-vortex stability is investigated numerically. An aeroelastic framework based on the free-vortex wake and finite element methods is employed to model a subscaled helicopter rotor in hover and forward-tilted conditions. A linear eigenvalue stability analysis is performed on tip vortices to associate the coupled impact of aeroelastic effects and vortex evolution. Prior numerical investigations have shown that highly flexible wind turbine rotor-blades have the potential to decrease levels of the instability of tip vortices. The present work focuses on testing these findings against a subscaled rotor within the range of helicopter operational rotation frequencies. The presented work aims to develop further insight into rotor-wake interactions and blade-vortex interaction to explore the mitigation of adverse rotorcraft operational conditions, such as their effect on aerodynamic-induced airframe vibrations and the associated life-cycle fatigue performance.

A stationary 3-D free tip hydrofoil was constructed and tested in a water tunnel at the Marine Hydrodynamics Laboratory at the Massachusetts Institute of Technology. The foil had moderate skew and was designed to produce a tip vortex like that typically seen on a modem free tip propeller blade. At the fixed loading condition, studies on the foil and its tip vortex were collected at ca vita ting and non-cavitating conditions. Laser Doppler Velocimetry (LDV) was used to collect the tip vortex velocity profiles at various axial positions at both non-cavitating and cavitating conditions. Also using the LDV system, closed contour velocity profiles were taken at various spanwise positions and used to measure the span loading distribution. Tip vortex circulation growth and core radius growth as a function of streamwise position were extracted from the tip vortex velocity profiles. growth and core radius growth as a function of streamwise position were extracted from the tip vortex velocity profiles. Pictures were taken of the cavitating vortex as well as foil sheet cavities at inception to document cavitation observations.

To investigate the feature of turbulence developing behind the filter device in a current flow, the flow fields at intermediate downstream distance of an immersed grid in an open water channel are recorded using a two-dimensional (2D) Particle Image Velocimetry (PIV) system. The measurements on a series of vertical and horizontal sections are conducted to reveal the stream-wise evolution and depth diversity of grid turbulence in the free surface flow. Unlike the previous experiments by Laser Doppler Velocimetry (LDV) and Hot-Wire Anemometry (HWA), the integral scales and space-time correlations are estimated without using the Taylor hypothesis in this paper. The distributions of mean velocity, turbulence intensity and integral scale show the transition behavior of grid-generated flow from perturbations to fully merged homogenous turbulence. The distributions of velocity and turbulence intensity become more uniform with increasing distance. While the spatial divergence of integral scale becomes more pronounced as the flow structures develop downstream. The vertical distributions of flow parameters reveal the diversity of flow characteristics in the water depth direction influenced by free surface and the outer part of turbulence boundary layer (TBL) from the channel bottom. The applicability of the newly proposed two-order elliptic approximation model for the space-time correlations of the decaying grid turbulence in channel flow is verified at different positions. The calculated convection velocity for large-scale motion and sweep velocity for small-scale motion based on this model bring a new insight into the dynamic pattern of this type of flow.

[Abstract] An experimental study has been conducted to examine the effects that Gurney flap span has on the lift and drag of the wing-flap configuration and the near-field trailing vortex strength. The configurations tested included a rectangular wing, a full-span Gurney flap and two Gurney flaps having spans equal to one half the wing span, one positioned outboard, and the other inboard. All three flaps had a height equaling six percent of the wing chord. As expected, all three flaps significantly changed the wing lift and drag characteristics. While the flapped configurations developed much higher both lift and drag coefficients, the overall effect on the wing lift-to-drag ratio has been found to be detrimental for all three flap configurations as compared to the baseline wing, the negative effect increasing with the flap span. Also, significant flattening of the lift-to-drag vs. lift coefficient curve has been observed. As expected, the addition of the flaps led to additional trailing vortices which affect the overall trailing vortex rollup and strength. The effect of the flaps on the trailing vortex strength in the near field has been quantified using a rollup tightness factor. This measure has indicated that while the configurations employing the full-span Gurney flap and the 50-percent-span flap located over the outboard wing sections exhibited trailing vortex strength reductions over most of the range of angles of attack tested, the configuration involving the inboard 50-percent flap developed stronger trailing vortices in the near field. These results have indicated that an effective means for trailing vortex reduction via Gurney flap use should be sought within the design space incorporating flaps located outboard. Future studies should include outboard Gurney flaps of varying relative span, other wing planforms and higher Reynolds numbers. Also, far field effects need to be examined.

The dynamics of a pair of counter-rotating wingtip vortices formed in the wake of a slotted wing are investigated numerically at Reynolds numbers 3000 to 10 000, considering a NACA12 airfoil with various angles of attack ranging from 5 to 10 degrees. Two parts of the wake are investigated. Firstly, we focus on the von Kármán wake of the wing and explore its properties in reference to inifinite wing results. The finiteness of the wing introduces notable differences. Secondly, we examine the dynamics of the wingtip vortices and the instabilities that are obtained. The influence of the von Kármán wake on this vortex is emphasized. Two behaviors are identified, depending on the angle of attack and Reynolds number. An orthogonal proper orthogonal decomposition (POD) is applied to identify the coherent structures that superimpose upon the mean flow. A Fourier analysis of the vortex centerline is conducted to highlight the dominant wavelengths.

Instability characteristics of a co-rotating wingtip vortex pair based on bi-global linear stability analysis

Tip vortex prediction for contra-rotating propeller using large eddy simulation

Particle image velocimetry (PIV) measurements of tip vortex wake structure of wind turbine