Vortex-surface Interaction Research Articles

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.

Direct numerical simulations of turbulent jets: vortex–interface–surfactant interactions

We study the effect of insoluble surfactants on the spatio-temporal evolution of turbulent jets. We use three-dimensional numerical simulations and employ an interface-tracking/level-set method that accounts for surfactant-induced Marangoni stresses. The present study builds on our previous work (Constante-Amores et al., J. Fluid Mech., vol. 922, 2021, A6) in which we examined in detail the vortex–surface interaction in the absence of surfactants. Numerical solutions are obtained for a wide range of Weber and elasticity numbers in which vorticity production is generated by surface deformation and surfactant-induced Marangoni stresses. The present work demonstrates, for the first time, the crucial role of Marangoni stresses, brought about by surfactant concentration gradients, in the formation of coherent, hairpin-like vortex structures. These structures have a profound influence on the development of the three-dimensional interfacial dynamics. We also present theoretical expressions for the mechanisms that influence the rate of production of circulation in the presence of surfactants for a general, three-dimensional, two-phase flow, and highlight the dominant contribution of surfactant-induced Marangoni stresses.

Experimental investigation on the streamwise vortex–surface interaction

Interaction of a vortex with a blade, wing or fin may involve both rapid distortion of the incident vorticity field and unsteady flow near the surface. A variety of flow visualization methods were applied in the experiment to observe the change of flow characteristics during a streamwise vortex–surface interaction. Particle image velocimetry measurements were carried out on a 2-D flow field in the near-wall area. The topology structure of the flow field during the vortex–surface interaction was obtained. The near-wall vorticity layer caused by the primary vortex formed secondary vortex. The initial strength of the incident vortex and distance between vortex core and the surface are the main parameters that affect the characteristics of flow field. The relevant results will help further study on the mechanism of interaction.

Flight Dynamics Principles of Canard Aircraft: Implications for UAV Configuration Decision

Marques P, Maligno A, Dierks S and Bachouche A. (2013). Flight dynamics principles of canard aircraft: Implications for UAV configuration decision. International Journal of Unmanned Systems Engineering. 1(2): 12-30. The canard configuration is an appealing alternative to the conventional aft tailplane stabiliser in the design of novel unmanned aerial vehicle (UAV) concepts. A canard foreplane increases the maximum lift coefficient and reduces the trim drag of the aircraft, making the canard set up particularly suitable for high-altitude long-endurance (HALE) UAVs. The inherent flight instability of canard-configured aircraft can be exploited to augment the manoeuvrability of advanced military UAVs at high angles of attack. Canard-wing arrangements result in complex strongly-coupled aerodynamic flow fields that include upwash-downwash effects, vortex-vortex interactions and vortex-surface interactions. This review paper provides a historical overview of the development of the canard aircraft, compares different canard airframes, explores the aerodynamics and aeroelasticity of canard aircraft, assesses longitudinal stability in conventional and canard arrangements, evaluates the aerodynamic efficiency of the tandem wing configuration, and gives examples of canard UAV platform development. The aerodynamic features of the Buraq HALE UAV canard prototype by Tunisia Aero Technologies Industries – UK are outlined. The paper also discusses the advantages and disadvantages of the canard configuration that inform the decision process in the development of next-generation UAV configurations. © Marques Aviation Ltd – Press.

Rayleigh Lecture 2007: Flow-surface interaction noise

A review is given of the theory of vortex sound with emphasis on sound production by vortex–surface interactions involving rigid and deformable, compact and noncompact moving bodies. The analysis is facilitated by use of the ‘compact’ approximation to the acoustic Green's function in cases where the solid surface is either acoustically compact or, for noncompact bodies, where the surface supports locally compact regions of ‘noisy’ flow identified by the presence of singularities in the ‘Kirchhoff vector’ of Green's function. The noncompact problem of the compression wave generated by a high-speed train entering a tunnel is discussed as an example of how accurate predictions of sound generation by identified distributions of vorticity can frequently be made by analytical means when the more conventional numerical approach becomes intractable.

Non-linear oscillations of the vortices in a hollow superconducting YBaCuO cylinder

A response of a hollow YBa 2Cu 3O 7− δ ceramic superconducting cylinder (at a mixed state) to the 100 kHz frequency alternating field was investigated. It was found that the response is non-linear and contains the odd harmonics of the excitation frequency. The amplitudes of the first, third and fifth harmonics are measured depending on an alternating field value h⩽1 Oe, applied to the cylinder external surface, and on a stationary magnetic field H⩽500 Oe driving the sample to the mixed state. For the results analysis a theoretical model of the vortices non-linear oscillations is proposed based on a non-linear vortex–vortex and vortex–surface interaction.

Interaction of trailing vortices in the wake of a wall-mounted rectangular cylinder.

Numerical simulations are performed to investigate three-dimensional unsteady vortex-vortex and vortex-surface interactions in the near field of a wall-mounted rectangular cylinder placed inside a channel. The generation mechanism of the upstream and the trailing vortices from the topologically important critical points and their near-wall evolution pattern have been examined in detail. In the upstream region, a laminar necklace vortex system formed around the junction between the rectangular block (cylinder) and the flat channel floor. A sequence of streamwise vortical rollers dominated the downstream interaction region, and they exhibited strong unsteady vortex-surface interaction. Streamwise vortices which formed upstream of the obstacle exhibited quadrupole structure with the dominant pair being central downwash, whereas those lifting the flow behind the obstacle were of predominantly central upwash. Notably, at some downstream location, the near-wall wake structure was observed to locally disappear due to mutual interaction and annihilation by opposite strength vortices on either side of the wake centerline. During the entire course of unsteady flow evolution, such a disappearance of the wake remained closely associated with local contraction of the limiting streamlines on the channel floor, the development of a pair of topologically important floor critical points (saddles), and the presence of a near-wall node on the vertical symmetry plane. The dominance of inward transverse flow toward these saddles together with flow evolution from the downstream node on the vertical symmetry plane were found to be particularly responsible for facilitating the local interaction of various vortices of opposite strength, leading to significant vorticity cancellation in the region. Moreover, the basic source of the wake vortices and their nature of evolution behind the cylinder were also investigated here, and they were found to be fundamentally different from what one usually observes in the near-wake of a transverse jet. However, the growth of a pair of vertically lifting vortices from the spiraling shear layer nodes just behind the downstream edge of the cylinder base was detected in this flow configuration also.

Solution adaptive grid strategies based on point redistribution

Solution adaptive grid strategies based on the redistribution of a fixed number of points are described in this paper. The redistribution is performed using weight functions that vary based on significant flow features. The weight functions are evaluated using an equidistribution principle. In this paper, emphasis is placed on the development of weight functions applicable to compressible flows exhibiting large scale separated vortical flows, vortex–vortex and vortex–surface interactions, separated shear layers and multiple shocks of different intensities. Algebraic, elliptic and parabolic methods of grid generation have been utilized for structured grid redistribution. Additionally, a point movement scheme is presented for generalized (structured/unstructured/hybrid) grid adaptation. Computer Aided Geometry Design (CAGD) techniques are combined with redistribution schemes to maintain the fidelity of solid boundaries. In particular, solid boundaries are represented using Non-Uniform Rational B-Splines (NURBS). A grid generation software system – Parallel Multiblock Adaptive Grid generation (PMAG) – using an elliptic redistribution scheme is also described with emphasis placed on the parallel implementation for multiblock structured grids with unstructured blocking topologies and on interpolation issues. Computational examples demonstrating the influence of different weight functions and grid redistribution strategies are presented.

Visualization of a mechanism for three-dimensional interaction and near-wall eruption

An experimental configuration has been found which allows detailed observation of three-dimensional vortex–vortex and associated vortex–surface interactions which appear similar to those observed in fully turbulent flow. Hydrogen bubble visualization illustrates a complicated intertwining, or braiding, of two initially co-rotating vortices. It is observed that the three-dimensional interactions of the braided vortices induces a pair of local surface-fluid eruptions reminiscent of the ‘bursting’ behaviour characteristic of the near-wall regeneration process of fully turbulent boundary layers.

Influence of surface barriers on vortex dynamics in Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub 8+δ/ whiskers

Bi/sub 2/Sr/sub 2/CaCa/sub 2/O/sub 8+/spl delta// whiskers are single crystals with highly regular surfaces and few extended defects, making them excellent model systems for studying the role of surfaces in magnetization. Furthermore, they can be elastically bent without losing their superconducting properties, making them potentially important technological materials. We have made a detailed investigation of their magnetic properties using micron-sized Hall probe arrays and show that surface effects play a dominant role in controlling the vortex dynamics, particularly in the moderate to high temperature regime. Local magnetic induction cycles exhibit the asymmetric behaviour characteristic of surface barriers and when the whiskers are slowly field cooled, sharp jumps are observed. The systematic behaviour of these jumps leads us to speculate that they represent sudden configurational rearrangements as a result of the interplay between vortex-surface barrier and vortex-vortex interactions.

A structured grid based solution-adaptive technique for complex separated flows

A structured grid based technique is presented to enhance the predictive capability of widely used CFD codes through the use of solution-adaptive gridding. The procedure redistributes mesh points based upon an existing flowfield. A newly developed weight function employing Boolean sums is utilized to represent the local truncation error. This weight function is then used to construct forcing functions associated with an elliptic system. A NURBS representation is employed to define block surfaces for boundary point redistribution. The technique has been applied to a flowfield about a configuration of practical interest. This flowfield involves supersonic freestream conditions at angle of attack, and exhibits large scale separated vortical flow, vortex-vortex and vortex-surface interactions, separated shear layers and multiple shocks of different intensity. The results demonstrate the capability of the developed weight function to detect shocks of differing strengths, primary and secondary vortices, and shear layers adequately.

Dynamics of the Vorticity Distribution in Endwall Junctions

The dynamics of the vorticity distribution for an unsteady, laminar necklace vortex system are established using quantitative particle image velocimetry techniques. The vortex system is composed of necklace vortices, which form periodically in front of a bluff body junction, translate toward the body, and subsequently amalgamate with a sustained corner vortex. It is shown that cross-diffusion of the necklace vortex vorticity with opposite-sign vorticity generated by a necklace vortex-surface interaction is the dominant mechanism creating a vorticity balance in the system. The instability of the impinging shear layer, under the influence of the adverse pressure gradient generated by the presence of the bluff body, is proposed as the mechanism by which the periodic vortex system frequency is selected.

Spatial-temporal turbulent flow-field and heat transfer behavior in end-wall junctions

Abstract The spatial-temporal velocity and surface heat transfer behavior in a rectangular end-wall junction are examined experimentally for an impinging turbulent boundary layer. The study is done in a water channel on a fast response, constant heat-flux surface using flow visualization, scanned-laser particle image velociemtry (PIV), and thermochromic liquid crystals (LC). Particle image visualizations and PIV-derived vorticity isocontours in the junction region of a rectangular block illustrate that a dominant unsteady horseshoe vortex resides in the junction region, as suggested by previous studies 2 (Devenport and Simpson 1990; Pierce and Shin 1992). The horseshoe vortex moves sporadically within a spatial envelope, due to the interaction with both (1) vortices from the impinging turbulent boundary layer; and (2) intermittent, opposite rotation vortices spawned by eruptive vortex-surface interactions (Walker 1978; Peridier et al. 1991). Companion surface heat transfer studies using thermochromic liquid crystals of both a canonical turbulent boundary layer and the rectangular block junction illustrate the presence of initially discrete spanwise variations in heat transfer on the approach surface caused by the presence of the streamwise, near-wall “streaks” in the impinging turbulent boundary layer; in the junction region, these streaks undergo a rapid metamorphosis into discrete, transverse regions of high heat transfer. These localized regions display significant transient, three-dimensional (3-D) behavior, appearing to be directly associated with vortex-surface interactions by the resident horseshoe vortex. This milieu of turbulent vortex development/interaction results in increases of 62% in spatially averaged Stanton number and increases of over 200% in local Stanton number compared to the canonical turbulent boundary layer.

Fine-grained YBaCuO: the formation of the initial vortex lattice and the magnetization curves

Drastic changes in the experimental magnetization curve behaviour of fine-grained YBaCuO were observed when the grain sizes D became commensurable with penetration depth lambda . In particular, the magnetic field Hmax (when M(H) reached its maximum) increased significantly when the grain size decreased. A theoretical study of initial flux line structure formation in the case of D approximately lambda was carried out, drawing an analogy between HTSC samples at high temperatures and an isotropic type II superconductor on the basis of a model taking into account both vortex-vortex and vortex-surface interactions. The formation fields of the first seven vortices as well as the calculated magnetization curves M(H) were obtained for a wide range of D/ lambda ratio. It was shown that a significant increase in the fields of penetration, not only for the first but also for the subsequent vortices, took place. This effect allowed us to explain peculiarities of experimental M(H) curves for fine-grained HTSC samples.

The generation and regeneration of single hairpin vortices

The generation and growth of single hairpin vortices created by controlled surface fluid injection were examined experimentally within a laminar boundary layer over a range of Reynolds numbers. Flow visualization, using both dye and hydrogen bubbles, was employed in conjunction with hot-film anemometry to investigate the growth characteristics and evolution of these single hairpin vortices. Hydrogen-bubble visualization results reveal that the passage of a hairpin vortex can give rise to a low-speed streak pattern adjacent to the surface, and a turbulent pocket-like pattern farther removed from the surface. When the displacement and injection Reynolds numbers exceed critical levels, regeneration processes occur, which result in the development of new hairpin-like vortices by both (i) lateral deformation of the vortex lines comprising the initial hairpin vortex and (ii) a process of vortex-surface interaction, which causes the ejection of surface fluid and subsequent hairpin formation by viscous-inviscid interactions. The combination of these processes results in both lateral and streamwise growth of the initial hairpin-vortex structure, yielding a symmetric turbulent-spot type of behaviour.

Vortex distortion during vortex-surface interaction in a Mach 3 stream

An experimental study has been conducted in Mach 3 wind tunnel to investigate the behavior of supersonic vortices as they interact with a wedge surface placed in their passage. The experimental setup was arranged so that interactions resulted in a close encounter of the vortex core and the wedge leading edge. Spark shadow photographs of the flowfield along the pressure measurements on the wedge surface were used to study the interaction problem. In their most organized form, distortion of streamwise vortices upon interacting with the wedge was found to result in formation of symmetric detached shock fronts far upstream of the wedge leading edge followed by an apparent slip surface separating a subsonic region from a supersonic zone. Interaction experiments leading to substantial changes in the structure of vortices revealed that the supersonic vortex distortion has strong resemblances to the incompressible 'B-breakdown' reported in the literature. Experimental results also indicate that the interaction strongly depends on the vortex strength and vortex proximity to the wedge leading edge, and the generated flowfield was found to be highly unsteady. Interaction of concentrated streamwise vortices with the oblique shock formed over the wedge surface resulted in formation of a locally three-dimensional shock wave with a limited subsonic region downstream of the shock.