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The Magnus effect is the commonly observed effect in which a spinning ball (or cylinder) curves away from its principal flight path. It is important in many ball sports. It affects spinning missiles, and has some engineering uses, for instance in the design of rotor ships and Flettner aeroplanes. In terms of ball games, topspin is defined as spin about an axis perpendicular to the direction of travel, where the top surface of the ball is moving forward with the spin. Under the Magnus effect, topspin produces a downward swerve of a moving ball, greater than would be produced by gravity alone, and backspin has the opposite effect.〔http://math.ucr.edu/home/baez/physics/General/golf.html〕 Likewise side-spin causes swerve to either side as seen during some baseball pitches, e.g. slider.〔(The Curveball ), The Physics of Baseball.〕 The overall behaviour is similar to that around an aerofoil (see lift force) with a circulation which is generated by the mechanical rotation, rather than by airfoil action.〔Clancy, L.J., ''Aerodynamics'', Section 4.6〕 The Magnus effect is named after Gustav Magnus, the German physicist who investigated it. The force on a rotating cylinder is known as Kutta–Joukowski lift,〔 after Martin Wilhelm Kutta and Nikolai Zhukovsky (or Joukowski), who first analyzed the effect. == Physics == A valid intuitive understanding of the phenomenon is possible, beginning with the fact that, by conservation of momentum, the deflective force on the body is no more or less than a reaction to the deflection that the body imposes on the air-flow. The body "pushes" the air down, and vice versa. As a particular case, a lifting force is accompanied by a downward deflection of the air-flow. It is an angular deflection in the fluid flow, aft of the body. In fact there are several ways in which the rotation might cause such a deflection. By far the best way to know what actually happens in typical cases is by wind tunnel experiments. Lyman Briggs made a definitive wind tunnel study of the Magnus effect on baseballs, and others have produced interesting images of the effect.〔〔 The studies show a turbulent wake behind the spinning ball. The wake is to be expected and is the cause of aerodynamic drag. However there is a noticeable angular deflection in the wake and the deflection is in the direction of the spin. The process by which a turbulent wake develops aft of a body in an air-flow is complex but well-studied in aerodynamics. It is found that the thin boundary layer detaches itself ("flow separation") from the body at some point and this is where the wake begins to develop. The boundary layer itself may be turbulent or not; this has a significant effect on the wake formation. Quite small variations in the surface conditions of the body can influence the onset of wake formation and thereby have a marked effect on the downstream flow pattern. The influence of the body's rotation is of this kind. It is said that Magnus himself wrongly postulated a theoretical effect with laminar flow due to skin friction and viscosity as the cause of the Magnus effect. Such effects are physically possible but slight in comparison to what is produced in the Magnus effect proper.〔 In some circumstances the causes of the Magnus effect can produce a deflection opposite to that of the Magnus effect. The diagram at the head of this article shows lift being produced on a back-spinning ball. The wake and trailing air-flow have been deflected downwards. The boundary layer motion is more violent at the underside of the ball where the spinning movement of the ball's surface is forward and reinforces the effect of the ball's translational movement. The boundary layer generates wake turbulence after a short interval. On a cylinder, the force due to rotation is known as Kutta-Joukowski lift. It can be analysed in terms of the vortex produced by rotation. The lift on the cylinder per unit length, ''F/L'', is the product of the velocity, ''v'' (in metres / second), the density of the fluid, (in kg / m3), and the strength of the vortex that is established by the rotation, ''G'':〔 :, where the vortex strength is given by :, where ''s'' is the rotation of the cylinder (in revolutions / second), ω is the angular velocity of spin of the cylinder (in radians / second) and ''r'' is the radius of the cylinder (in metres). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Magnus effect」の詳細全文を読む スポンサード リンク
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