Situations where an effectively irrotational freest ream contains regions of concentrated vorticity are common in external aerodynamics, where vortices are known to play an important role in the flight of modern helicopters (Carr 1988) and aircraft (Cunningham 1989, Mabey 1989). Vortices may arise as a consequence of shedding from some upstream surface or near certain three-dimensional boundary geometries that act to promote vortex formation. Examples of the former type of vortex gen­ eration include: 1. vortices that trail from the tips of airfoils (Harvey & Perry 1971) and control surfaces on submarines (Lugt 1983), 2. transverse vortices shed from maneuvering airfoil surfaces and helicopter blades in a process known as dynamic stall (McCroskey 1982, Francis & Keesee 1985, Carr 1988), and 3. shedding from stationary obstacles. Geometry-induced creation can occur in any situation where a flow along a wall approaches a surface-mounted obstacle; examples include: 1. airframe features such as wing/body junctions, 2. conning towers on submarines, and 3 . computer chips mounted on electrical circuit boards. Similar geometries are en-

Vortex Interactions with Walls

The flow induced by a vortex ring approaching a plane wall on a trajectory normal to the wall is investigated for an incompressible fluid which is otherwise stagnant. The detailed characteristics of the interaction of the ring with the flow near the surface have been observed experimentally for a wide variety of laminar rings, using dye in water to visualize the flow in the ring as well as near the plane surface. Numerical solutions are obtained for the trajectory of the ring as well as for the unsteady boundary-layer flow that develops on the wall. The experimental and theoretical results show that an unsteady separation develops in the boundary-layer flow, in the form of a secondary ring attached to the wall. A period of explosive boundary-layer growth then ensues and a strong viscous-inviscid interaction occurs in the form of the ejection of the secondary vortex ring from the boundary layer. The primary ring then interacts with the secondary ring and in some cases was observed to induce the formation of a third, tertiary, ring near the wall. The details of this process are investigated over a wide Reynolds number range. The results clearly show how one vortex ring can produce another, through an unsteady interaction with a viscous flow near the wall.

The impact of a vortex ring on a wall

▪ Abstract Interaction of a vortex, or combinations of them, with a cylinder, blade, or foil may involve both rapid distortion of the incident vorticity field and shedding of vorticity from the surface of the body. This review focuses on the underlying flow physics, with the aim of clarifying the origin of the induced loading. In the case of near or direct encounter of the incident vortex, the relation between three-dimensional features of the flow structure and the local loading poses interesting challenges for further research. With recently developed simulation and laboratory techniques, opportunities now exist to determine the instantaneous quantitative structure of these complex distortions and to interpret them within a theoretical, vorticity-based framework. Effective control of vortex interactions appears to be attainable. It will be necessary, however, to develop creative strategies, distinct from those traditionally employed for control of unstable shear flows.

Vortex-body interactions

Heat transfer and flow structures in an excited circular impinging jet

The structure of the wake behind a kestrel in medium-speed flight down a 36 m length of corridor was analysed qualitatively and quantitatively by stereophotogrammetry of multiple flash photographs of the motion of small soap-covered helium bubbles. The wake consists of a pair of continuous, undulating trailing vortices. The upstroke is therefore aerodynamically active and the circulation appears to remain constant along the wing whose geometry is altered during the course of the wingstroke. It is argued that the flight kinematics, and so the wake structure, of the kestrel may be typical of flapping flight at medium speeds and a flight model based on this wake geometry is presented. Rough estimates of the rate of momentum generated in the wake balance the weight almost exactly and a direct estimate of the induced power requirement from the wake measurements is obtained. The significance of these results for the various alternative aerodynamic descriptions and energetic predictions of models of flapping animal flight is briefly assessed

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The Wake of a Kestrel (Falco Tinnunculus) in Flapping Flight

A vortex-induced unsteady separation was investigated experimentally in the laminar boundary layer produced by an axisymmetric jet impinging normally onto a flat plate. By forcing the air jet, primary ring vortices were periodically generated in the jet shear layer. Phase-locked flow visualization showed that the wall-jet boundary layer separated periodically and evolved into a secondary vortex counter rotating with respect to the primary vortex. The unsteady separation is induced by the primary vortex and moves downstream in the radial mean-flow direction. Phase-averaged hot-wire measurements using a parallel-wire sensor in the vicinity of the unsteady separation provided data for locating the onset of separation in space and time. The data revealed that the unsteady separation originated from a local shear layer which was initiated by the unsteady adverse pressure gradient produced by the primary vortex.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0022112085003469

Norbert Didden

Papers

Unsteady separation in a boundary layer produced by an impinging jet