Airfoil Nsw

[Deb Cartelli]

Anairfoil (American English) or aerofoil (British English) is a streamlined body that is capable of generating significantly more lift than drag.[1] Wings, sails and propeller blades are examples of airfoils. Foils of similar function designed with water as the working fluid are called hydrofoils.

When oriented at a suitable angle, a solid body moving through a fluid deflects the oncoming fluid (for fixed-wing aircraft, a downward force), resulting in a force on the airfoil in the direction opposite to the deflection.[2][3] This force is known as aerodynamic force and can be resolved into two components: lift (perpendicular to the remote freestream velocity) and drag (parallel to the freestream velocity).

The lift on an airfoil is primarily the result of its angle of attack. Most foil shapes require a positive angle of attack to generate lift, but cambered airfoils can generate lift at zero angle of attack. Airfoils can be designed for use at different speeds by modifying their geometry: those for subsonic flight generally have a rounded leading edge, while those designed for supersonic flight tend to be slimmer with a sharp leading edge. All have a sharp trailing edge.[4]

The wings and stabilizers of fixed-wing aircraft, as well as helicopter rotor blades, are built with airfoil-shaped cross sections. Airfoils are also found in propellers, fans, compressors and turbines. Sails are also airfoils, and the underwater surfaces of sailboats, such as the centerboard, rudder, and keel, are similar in cross-section and operate on the same principles as airfoils. Swimming and flying creatures and even many plants and sessile organisms employ airfoils/hydrofoils: common examples being bird wings, the bodies of fish, and the shape of sand dollars. An airfoil-shaped wing can create downforce on an automobile or other motor vehicle, improving traction.

When the wind is obstructed by an object such as a flat plate, a building, or the deck of a bridge, the object will experience drag and also an aerodynamic force perpendicular to the wind. This does not mean the object qualifies as an airfoil. Airfoils are highly-efficient lifting shapes, able to generate more lift than similarly sized flat plates of the same area, and able to generate lift with significantly less drag. Airfoils are used in the design of aircraft, propellers, rotor blades, wind turbines and other applications of aeronautical engineering.

A lift and drag curve obtained in wind tunnel testing is shown on the right. The curve represents an airfoil with a positive camber so some lift is produced at zero angle of attack. With increased angle of attack, lift increases in a roughly linear relation, called the slope of the lift curve. At about 18 degrees this airfoil stalls, and lift falls off quickly beyond that. The drop in lift can be explained by the action of the upper-surface boundary layer, which separates and greatly thickens over the upper surface at and past the stall angle. The thickened boundary layer's displacement thickness changes the airfoil's effective shape, in particular it reduces its effective camber, which modifies the overall flow field so as to reduce the circulation and the lift. The thicker boundary layer also causes a large increase in pressure drag, so that the overall drag increases sharply near and past the stall point.

Supersonic airfoils are much more angular in shape and can have a very sharp leading edge, which is very sensitive to angle of attack. A supercritical airfoil has its maximum thickness close to the leading edge to have a lot of length to slowly shock the supersonic flow back to subsonic speeds. Generally such transonic airfoils and also the supersonic airfoils have a low camber to reduce drag divergence. Modern aircraft wings may have different airfoil sections along the wing span, each one optimized for the conditions in each section of the wing.

Movable high-lift devices, flaps and sometimes slats, are fitted to airfoils on almost every aircraft. A trailing edge flap acts similarly to an aileron; however, it, as opposed to an aileron, can be retracted partially into the wing if not used.

In two-dimensional flow around a uniform wing of infinite span, the slope of the lift curve is determined primarily by the trailing edge angle. The slope is greatest if the angle is zero; and decreases as the angle increases.[14][15] For a wing of finite span, the aspect ratio of the wing also significantly influences the slope of the curve. As aspect ratio decreases, the slope also decreases.[16]

Thin airfoil theory is a simple theory of airfoils that relates angle of attack to lift for incompressible, inviscid flows. It was devised by German mathematician Max Munk and further refined by British aerodynamicist Hermann Glauert and others[17] in the 1920s. The theory idealizes the flow around an airfoil as two-dimensional flow around a thin airfoil. It can be imagined as addressing an airfoil of zero thickness and infinite wingspan.

Thin airfoil theory assumes the air is an inviscid fluid so does not account for the stall of the airfoil, which usually occurs at an angle of attack between 10 and 15 for typical airfoils.[20] In the mid-late 2000s, however, a theory predicting the onset of leading-edge stall was proposed by Wallace J. Morris II in his doctoral thesis.[21] Morris's subsequent refinements contain the details on the current state of theoretical knowledge on the leading-edge stall phenomenon.[22][23] Morris's theory predicts the critical angle of attack for leading-edge stall onset as the condition at which a global separation zone is predicted in the solution for the inner flow.[24] Morris's theory demonstrates that a subsonic flow about a thin airfoil can be described in terms of an outer region, around most of the airfoil chord, and an inner region, around the nose, that asymptotically match each other. As the flow in the outer region is dominated by classical thin airfoil theory, Morris's equations exhibit many components of thin airfoil theory.

The dream of soaring in the sky like a bird has captivated the human mind for ages. Although many failed, some eventually succeeded in achieving that goal. These days we take air transportation for granted, but the physics of flight can still be puzzling.

Notice that as the wind hits a blade of grass, that blade naturally bends in the direction of the blowing gust, and the faster that gust, the stronger the bending. A single blade indicates the direction and speed of the flow of air in that area.

The arrows are convenient, but the grassy scene also has another aid for visualizing flows. Many light objects like leaves, flower petals, dust, or smoke are very easily influenced by the motion of the surrounding air. They quickly change their velocity to match the flow of the wind. We can replicate the behavior of these light objects with little markers that are pushed around by that flow. You can see them on the right side:

On the other hand, the little markers are actively following the flow, letting us see how the air is actually moving through space, with the ghosty trails giving us some historical overview of where this parcel of air has come from.

At room temperature the average speed of a particle in air is an astonishing 1030 mph1650 km/h, which is many times higher than even the most severe hurricanes. Given the size of the cube, this means that even at the fastest speed of simulation everything happens 11 billions time slower than in real life.

Recall that the scale of the large central arrow is much larger than the scale of individual tiny arrows attached to each particle. Despite that increase in size, the arrow practically disappears when we average out a larger number of particles and we can clearly see that the average velocity of particles is more or less zero even in this extremely small volume.

An imperfect, but convenient analogy is to imagine a swarm of bees flying in the air. While all the individual insects are actively roaming around at different speeds, the group as a whole may steadily stay in one place.

However, when we use the procedure of averaging the velocity of all the particles, we can reveal the motion of their group in the box of a given size, at a specific speed of the flow:

Because the motion of each individual particle is so disordered, we have to look at many of them at once to discern any universal characteristics. And when we do just that, from all the chaos emerges order.

Naturally, the averaging box needs to be large enough to avoid the jitteriness related to aggregation of too few particles, but at any scale that we could care about the noisy readout completely disappears.

The average motion of particles is very different than the motion of each individual molecule. Even in very fast flows, many of the molecules move in the opposite direction than what the arrow indicates, but if we tally up all the particle motion, the air as a whole does make forward progress in the direction of velocity.

In my previous article I presented a more elaborate description of the interplay between forces and objects, but to briefly recap here, if forces acting on an object are balanced, then that object will maintain its current velocity.

The markers show that the flow splits ahead of the airfoil, then it gently changes direction to glide above and below the shape. Moreover, the markers right in front of the airfoil gradually slow down and lag behind their neighbors. The air somehow senses the presence of the body.

In the demonstration below, you can see air particles bombarding a small box. Every time a collision happens I briefly mark it with a dark spot on the surface of that cube:

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