Foil

Foil may refer to:

Foil (literature)

In fiction, a foil is a character who contrasts with another character (usually the protagonist) in order to highlight particular qualities of the other character. In some cases, a subplot can be used as a foil to the main plot. This is especially true in the case of metafiction and the "story within a story" motif. The word foil comes from the old practice of backing gems with foil in order to make them shine more brightly.

A foil usually either differs dramatically or is extremely similar but with a key difference setting them apart. The concept of a foil is also more widely applied to any comparison that is made to contrast a difference between two things.Thomas F. Gieryn places these uses of literary foils into three categories which Tamara Antoine and Pauline Metze explain as: those that emphasize the heightened contrast (this is different because ...), those that operate by exclusion (this is not X because...), and those that assign blame ("due to the slow decision-making procedures of government...").

Foil (fluid mechanics)

A foil is a solid object with a shape such that when placed in a moving fluid at a suitable angle of attack the lift (force generated perpendicular to the fluid flow) is substantially larger than the drag (force generated parallel the fluid flow). If the fluid is a gas, the foil is called an airfoil or aerofoil, and if the fluid is water the foil is called a hydrofoil.

Physics of foils

A foil generates lift primarily as a result of its shape and angle of attack. When oriented at a suitable angle, the foil deflects the oncoming fluid, resulting in a force on the foil in the direction opposite to the deflection. This force can be resolved into two components: lift and drag. This "turning" of the fluid in the vicinity of the foil creates curved streamlines which results in lower pressure on one side and higher pressure on the other. This pressure difference is accompanied by a velocity difference, via Bernoulli's principle, so the resulting flowfield about the foil has a higher average velocity on the upper surface than on the lower surface.