Lift-off Speed Research Articles

The Hydrodynamics of Rheotaxis in the Plaice (Pleuronectes Platessa L.)

ABSTRACT Plaice resting on the bottom of a flume respond to the current with a clearly defined pattern of behaviour. A simple hydrodynamic model which relates the slip-speed (Us) and lift-off speed (UL) to the physical forces acting on the fish has been verified experimentally using freshly killed fish and a rigid model. With an asymmetric semi-ellipsoid shape and a fineness ratio of 14 the plaice appears to be morphologically adapted to have minimum drag when heading into the current. As a result of this streamlined shape the lift force experienced by the fish is 10–20 times greater than the drag force, and the rheotactic behaviour therefore appears to be principally adapted to counteract hydrodynamic lift. Values of Us and UL, above which live fish must expend energy against the current, are exceeded for much of each tidal cycle in the southern North Sea. The model is of general application to benthic organisms in flowing water for Reynolds numbers between 104 and 106; outside these limits some modification of the constants is required.

A Theoretical Investigation of the Multileaf Journal Bearing

The finite-element method is used to solve the compressible Reynolds equation which governs the fluid flow between the journal surface and the leaves in a multileaf journal bearing. The solution obtained is then coupled with the load-deflection equations of the leaves to obtain such information as liftoff speed and minimum film thickness. In addition, the load-deflection equations yield the initial preload on the journal, the startup torque, the stiffness coefficients, and the equivalent damping coefficients. Such details as leaf curvature, friction between leaves, and between leaves and journal surface are considered in the load-deflection equations. Results are obtained for a wide range of operating parameters.

Hydrofoils: Optimum Lift-Off Speed for Sailboats

For a hydrofoil sailboat there is a unique optimum lift-off speed. Before this speed is reached, if there are no parasitic vertical hydrofoil appendages, the submerged or partially submerged hydrofoils increase drag and degrade performance. As soon as this speed is reached and the hydrofoils are fully and promptly deployed, the performance of a hydrofoil-borne craft is significantly improved. At speeds exceeding optimum lift-off speed, partially submerged hydrofoils impair performance if there is no significant effect of loading on the hydrofoil lift-to-drag ratio.