What Is the Coanda Effect?

The Coanda Effect is also known as the Coanda Effect or Coanda Effect. The fluid (water or air flow) tends to deviate from the original flow direction and instead flow with the surface of the protruding object. When there is surface friction between the fluid and the surface of the object through which it flows (also fluid viscosity), as long as the curvature is not large, the fluid will flow along the surface of the object. According to Newton's third law, if an object applies a deflection force to a fluid, the fluid must also apply a reverse deflection force to the object. This force is very obvious on light objects, such as a spoon, but for large aircraft, the proportion is not very large. This role was named after Romanian inventor Henry Conda.

An early description of this phenomenon was

The air flow will introduce air molecules from the surrounding environment, causing a low-pressure "tube" or "sleeve" around the jet. The ambient air from around this low-pressure tube will exert a force on the jet, which when viewed in cross section, is equal in all directions. Therefore, the jet does not move away from a straight line. However, if a solid surface is placed close to the nozzle and is approximately parallel, air entrainment (and therefore removal) between the solid surface and the nozzle will result in a reduction in the side of the jet that cannot be neutralized as quickly as the low pressure area on the "open" side of the jet Air pressure. The pressure difference across the jet causes the jet to deviate from the nearby surface and then adhere to it. Even if it is curved, the jet will adhere to the surface, because each (infinitely small) incremental change in surface direction will bring the effect described by the initial curvature of the jet toward the surface if the surface is not too sharp Bend, where appropriate, even after flowing 180 ° on a cylindrical curved surface, the jet can adhere to the surface and therefore travel in a direction opposite to its original direction. The forces that cause these changes in jet flow direction produce equal and opposite forces on the surface on which the jet flows. These Coand-effect-induced forces can be used to cause lift and other forms of motion, depending on the orientation of the jet and the surface to which the jet is attached. ^[3]

Earlier data provided both theoretical and experimental information, and a detailed explanation of the Coand effect and its limitations needs to be made by comparison. Synergies can occur along curved walls in free jets or wall jets.

The "mechanism of the Kangda effect", the effect of "reducing the lateral pressure of the electric current of the air near the obstacle" described by T. Young's term, means the free nozzle coming out of the nozzle and the surrounding obstacles. It includes the tendency of free jets from the orifice to limit the entrained environment to entrain fluids, without the development of any areas of lower pressure without obstacles around them, such as where turbulent mixing occurs at ambient pressure .

On the correct image, the effect occurs as a wall jet along the curved wall. A two-dimensional wall jet between two parallel plane walls, where the "obstacle" is a quarter-cylindrical section following a flat horizontal rectangular hole, so that no fluid is entrained from the surrounding along the wall at all, but only at the Opposite side mixed with ambient air turbulence.

In order to compare the experience of the theoretical model, we first refer to a two-dimensional plane wall jet with a width h of a circular wall of radius r. The wall jet follows a flat horizontal wall with an infinite radius or its radius is the radius of the earth without separation because the surface pressure and external pressure in the mixed area are equal to atmospheric pressure and the boundary layer does not separate from the wall. The surface pressure was measured along a circular curved wall with a radius r = 12 cm, and the turbulent air (Reynolds number = 106) with a width h was deflected. Due to the local effect at the nozzle outlet, the pressure begins to drop before the starting point of the jet head, thereby generating a jet. If the h / r ratio (ratio of the width of the jet to the radius of curvature of the wall) is less than 0.5, then a true Coand effect is observed, and the wall pressure along the curved wall remains at this low (sub-ambient pressure) level until the jet reaches the wall End (when pressure quickly returns to ambient pressure). If the h / r ratio is greater than 0.5, only local effects occur at the beginning of the jet, after which the jet immediately separates from the wall, and there is no Coand effect.

In 1956, experiments with turbulent air jets with a Reynolds number 106 of various jet widths (h) showed pressure measured along a circular curved wall (radius r) a series of horizontal distances from the origin of the jet.

Below a critical h / r ratio above 0.5, only local effects along the origin of the jet can extend along the curved wall at a small angle of 18 °. The jet immediately separated from the curved wall. Therefore, the Coand effect is not seen here, but only a local attachment: the pressure on the wall is less than the distance corresponding to a small angle of 9 °, followed by an angle equal to 9 °, where the pressure increases until the boundary layer separates The atmospheric pressure is subject to this positive longitudinal gradient. However, if the h / r ratio is less than the critical value of 0.5, the ambient pressure measured on the wall observed on the jet generator is lower than the wall. This is a true Coand effect because the jets adhere to the wall with almost constant pressure, as in traditional wall jets.

Calculations performed by LC Woods on a non-viscous flow for a circular wall in 1954 show that there is a non-viscous solution for any curvature h / r and any given deflection angle up to the separation point on the wall, where singular points have surface pressure curves Infinite slope.

In the calculations, for each value of the relative curvature h / r, the separation angle found in the previous experiment, the image here was recently obtained and shows the inertial effect represented by the non-viscous solution.

However, some scholars today believe that the principle of wing lift is due to the Kangda effect, that is, the wing deflects a large amount of airflow downward and generates a reaction force (lift). This understanding is not entirely correct, the aircraft in the real environment

Turn on the tap and let out a small stream of water. Place the back of the small spoon next to the flow. The water will be attracted to the back of the spoon. This is the result of the Coanda effect and the Venturi Effect. After the water flow is attached to the spoon, the wall effect keeps the water flow on the convex surface of the spoon.

This experiment is a typical example of the reaction force exerted by water on an object. However, many people think that the spoon is adsorbed because of Bernoulli's principle, which causes part of the water to flow faster and the pressure becomes smaller, while the other part has no water and the pressure is greater . Obviously, this statement is ridiculous. The Bernoulli principle cannot be used to compare objects between two different fluids. The front side of the spoon is under atmospheric pressure and the back side is under atmospheric pressure. Although the water flow passes, it does not affect the pressure. A counter-example is that when you face the spoon facing the water flow, that is, let the water flow over the concave side, you can clearly see that the spoon is deflected away from the water flow. ^[6]