Fluid flow lifting a body from a solid surface.

If a body is at rest on horizontal ground and a sudden horizontal flow of fluid is applied, the body either remains on the ground (rocking, rolling, sliding or spinning) or is lifted off impulsively. This lift-off is followed by a return to the ground or by a fly-away in the sense of continued departure from the ground. Related phenomena arise in the lift-off of an air vehicle from, effectively, moving ground. The present investigation seeks fairly precise mechanistic conditions under which lift-off and subsequent return or fly-away occur for a thin body or more generally for any thin gap of fluid between a body and the ground. Nonlinear fluid-solid interaction takes place in which the motion of the body and the surrounding fluid affect each other. Small-time analysis on lift-off and a numerical study are presented, followed by large-time analysis showing a critical flow speed for fly-away for any shape of the body. The changes in ground effect, from being dominant during lift-off to diminishing in fly-away, are explored together with relevant applications.

Movement of a finite body in channel flow.

A body of finite size is moving freely inside, and interacting with, a channel flow. The description of this unsteady interaction for a comparatively dense thin body moving slowly relative to flow at medium-to-high Reynolds number shows that an inviscid core problem with vorticity determines much, but not all, of the dominant response. It is found that the lift induced on a body of length comparable to the channel width leads to differences in flow direction upstream and downstream on the body scale which are smoothed out axially over a longer viscous length scale; the latter directly affects the change in flow directions. The change is such that in any symmetric incident flow the ratio of slopes is found to be [Formula: see text], i.e. approximately 0.900969, independently of Reynolds number, wall shear stresses and velocity profile. The two axial scales determine the evolution of the body and the flow, always yielding instability. This unusual evolution and linear or nonlinear instability mechanism arise outside the conventional range of flow instability and are influenced substantially by the lateral positioning, length and axial velocity of the body.

Collisions, rebounds and skimming.

Repeated oblique impacts and rebounds of a solid body or bodies on horizontal shallow water are investigated through mathematical modelling. The inclinations from the horizontal are supposed small as the skimming evolves, for a thin typical body shape. The new formulation aimed at improved prediction as well as the background involved is presented together with nonlinear analysis and computation. Comparatively fast or slow collisions and rebounds are found to be of special interest over short time-scales.

Fluid-body interactions: clashing, skimming, bouncing.

Solid-solid and solid-fluid impacts and bouncing are the concern here. A theoretical study is presented on fluid-body interaction in which the motion of the body and the fluid influence each other nonlinearly. There could also be many bodies involved. The clashing refers to solid-solid impacts arising from fluid-body interaction in a channel, while the skimming refers to another area where a thin body impacts obliquely upon a fluid surface. Bouncing usually then follows in both areas. The main new contribution concerns the influences of thickness and camber which lead to a different and more general form of clashing and hence bouncing.

Vortices and flow reversal due to suction slots.

Suction into small two- or three-dimensional surface slots inside an otherwise planar boundary-layer is examined theoretically through a combined analytical and computational approach. Increasing suction strength leads to enhanced nonlinear structures with flow reversals or trailing vortices.