Dual Vortex Structure Research Articles

Dual vortex structure shedding from low aspect ratio, surface-mounted pyramids

The flow around surface-mounted, square-based pyramids, of apex angles ranging from 5° to 90°, placed in a thin boundary layer with one face normal to the on-coming flow was investigated for Reynolds numbers of 10 000–150 000 using surface pressure measurements and particle image velocimetry as well as oil-film surface-flow patterns. Based on the periodicity of the surface pressure on the pyramid side faces, four distinct shedding regimes were identified: constant local Strouhal number, discrete cellular, low aspect ratio and suppressed. Closer inspection of the low aspect ratio regime indicates that two distinct vortical structures are shed at the same frequency. Vortices are shed alternately from opposing faces in the obstacle base region, while hairpin vortices are shed in-phase and are most easily observed near the tip region of the pyramid. It is further shown that the existence of these structures is consistent with the mean surface-flow topology observed in the wake and side faces of the obstacles.

Eulerian simulation of sedimentation flows in vertical and inclined vessels

Sedimentation of particles in inclined and vertical vessels is numerically simulated using a finite volume method where the Eulerian multiphase model is applied. The particulate phase as well as the fluid phase is regarded as a continuum while the viscosity and solid stress of the particulate phase are modelled by the kinetic theory of granular flows. The numerical results show an interesting phenomenon of the emergence of two circulation vortices of the sedimentation flow in a vertical vessel but only one in the inclined vessel. Several sensitivity tests are simulated to understand the factors that influence the dual-vortex flow structure in vertical sedimentation. Results show that a larger fluid viscosity makes the two vortex centres much closer to each other and the boundary layer effect at lateral walls is the key factor to induce this phenomenon. In the fluid boundary layer particles settle down more rapidly and drag the local carrier fluid to flow downward near the lateral walls and thus form the dual-vortex flow pattern.

Buffeting Flows over a Low-Sweep Delta Wing

An experimental study was conducted with the aim of understanding the unsteady vortex flows and buffeting response of a nonslender delta wing with 50-deg leading-edge sweep angle. Particle image velocimetry and laser Doppler velocimetry measurements, surface flow visualization, force balance measurements, and wing-tip acceleration measurements were used. It was found that there is a profound effect of Reynolds number on the structure of vortical flows. The breakdown of the leading-edge vortices is delayed significantly, and the vortices form more inboard at low Reynolds numbers. The secondary vortex effectively splits the primary vortex into two separate concentrations of vorticity, resulting in a dual vortex structure at small incidences. This dual vortex structure diminishes, and a single primary vortex is observed at higher incidences. At higher Reynolds numbers (on the order of 3 × 10 4) the flow approaches an asymptotic state, with further increases in the Reynolds number resulting in only small variations in the location of vortex core and breakdown. Weak vortex breakdown observed at low incidences is replaced by a conical breakdown with increasing incidences. However, the maximum buffeting occurs prior to the stall, after the vortex breakdown has reached the apex of the wing. The largest velocity fluctuations near the wing surface are observed along the reattachment line. Hence, the shear-layer reattachment, rather than the vortex breakdown phenomenon, is the most important source of increasing buffet in the prestall region as incidence is increased. The velocity fluctuations in the reattachment region have similar dominant frequencies as slender wings in spite of the differences in the physical nature of the flow. With further increase in incidence, the shear-layer reattachment becomes impossible, resulting in very low velocity fluctuations near the wing surface and a precipitous fall in the rms wing-tip acceleration.

On aerodynamic modelling of an insect–like flapping wing in hover for micro air vehicles

This theoretical paper discusses recent advances in the fluid dynamics of insect and micro air vehicle (MAV) flight and considers theoretical analyses necessary for their future development. The main purpose is to propose a new conceptual framework and, within this framework, two analytic approaches to aerodynamic modelling of an insect-like flapping wing in hover in the context of MAVs. The motion involved is periodic and is composed of two half-cycles (downstroke and upstroke) which, in hover, are mirror images of each other. The downstroke begins with the wing in the uppermost and rearmost position and then sweeps forward while pitching up and plunging down. At the end of the half-cycle, the wing flips, so that the leading edge points backwards and the wing's lower surface becomes its upper side. The upstroke then follows by mirroring the downstroke kinematics and executing them in the opposite direction. Phenomenologically, the interpretation of the flow dynamics involved, and adopted here, is based on recent experimental evidence obtained by biologists from insect flight and related mechanical models. It is assumed that the flow is incompressible, has low Reynolds number and is laminar, and that two factors dominate: (i) forces generated by the bound leading-edge vortex, which models flow separation; and (ii) forces due to the attached part of the flow generated by the periodic pitching, plunging and sweeping. The first of these resembles the analogous phenomenon observed on sharp-edged delta wings and is treated as such. The second contribution is similar to the unsteady aerodynamics of attached flow on helicopter rotor blades and is interpreted accordingly. Theoretically, the fluid dynamic description is based on: (i) the superposition of the unsteady contributions of wing pitching, plunging and sweeping; and (ii) adding corrections due to the bound leading-edge vortex and wake distortion. Viscosity is accounted for indirectly by imposing the Kutta condition on the trailing edge and including the influence of the vortical structure on the leading edge. Mathematically, two analytic approaches are proposed. The first derives all the quantities of interest from the notion of circulation and leads to tractable integral equations. This is an application of the von Kármán-Sears unsteady wing theory and its nonlinear extensions due to McCune and Tavares; the latter can account for the bound leading-edge vortex and wake distortion. The second approach uses the velocity potential as the central concept and leads to relatively simple ordinary differential equations. It is a combination of two techniques: (i) unsteady aerodynamic modelling of attached flow on helicopter rotor blades; and (ii) Polhamus's leading-edge suction analogy. The first of these involves both frequency-domain (Theodorsen style) and time-domain (indicial) methods, including the effects of wing sweeping and returning wake. The second is a nonlinear correction accounting for the bound leading-edge vortex. Connections of the proposed framework with control engineering and aeroelasticity are pointed out.

Superposed epoch analysis applied to large-amplitude travelling convection vortices

Abstract. For the six months from 1 October 1993 to 1 April 1994 the recordings of the IMAGE magnetometer network have been surveyed in a search for large-amplitude travelling convection vortices (TCVs). The restriction to large amplitudes (>100 nT) was chosen to ensure a proper detection of evens also during times of high activity. Readings of all stations of the northern half of the IMAGE network were employed to check the consistency of the ground signature with the notation of a dual-vortex structure moving in an azimuthal direction. Applying these stringent selection criteria we detected a total of 19 clear TCV events. The statistical properties of our selection resemble the expected characteristics of large-amplitude TCVs. New and unexpected results emerged from the superposed epoch analysis. TCVs tend to form during quiet intervals embedded in moderately active periods. The occurrence of events is not randomly distributed but rather shows a clustering around a few days. These clusters recur once or twice every 27 days. Within a storm cycle they show up five to seven days after the commencement. With regard to solar wind conditions, we see the events occurring in the middle of the IMF sector structure. Large-amplitude TCVs seem to require certain conditions to make solar wind transients 'geoeffective', which have the tendency to recur with the solar rotation period.Key words. Ionosphere (Aural ionosphere; Ionosphere- magnetosphere interactions) · Magnetospheric Physics (current system)

Superposed epoch analysis applied to large-amplitude travelling convection vortices

For the six months from 1 October 1993 to 1 April 1994 the recordings of the IMAGE magnetometer network have been surveyed in a search for large-amplitude travelling convection vortices (TCVs). The restriction to large amplitudes (>100 nT) was chosen to ensure a proper detection of evens also during times of high activity. Readings of all stations of the northern half of the IMAGE network were employed to check the consistency of the ground signature with the notation of a dual-vortex structure moving in an azimuthal direction. Applying these stringent selection criteria we detected a total of 19 clear TCV events. The statistical properties of our selection resemble the expected characteristics of large-amplitude TCVs. New and unexpected results emerged from the superposed epoch analysis. TCVs tend to form during quiet intervals embedded in moderately active periods. The occurrence of events is not randomly distributed but rather shows a clustering around a few days. These clusters recur once or twice every 27 days. Within a storm cycle they show up five to seven days after the commencement. With regard to solar wind conditions, we see the events occurring in the middle of the IMF sector structure. Large-amplitude TCVs seem to require certain conditions to make solar wind transients 'geoeffective', which have the tendency to recur with the solar rotation period.Key words. Ionosphere (Aural ionosphere; Ionosphere- magnetosphere interactions) · Magnetospheric Physics (current system)