The Coandă effect is a physical phenomenon in fluid mechanics that refers to the tendency of fluids, such as air or water, to adhere to a curved surface instead of following a straight path.
This fluid dynamics phenomenon can be observed in various situations from the flight of an airplane to the operation of a steam turbine.
Physical explanation
To better understand the Coandă effect, it is useful to remember some basic concepts about the nature of fluids. Fluids, whether liquids or gases, tend to move from areas of high pressure to areas of low pressure.
When air flows around a curved surface, the air pressure on the outside of the curve is greater than on the inside. This creates a force that pushes the air toward the curved surface, causing it to stick to it instead of continuing in a straight line.
The fluid particles adhere to the surface due to a combination of forces acting on them. When a fluid, whether a gas or a liquid, flows over a curved surface, differences in the pressure of the fluid along the surface are generated that generate a resultant force that acts on the particles of the fluid, pushing them towards the curved surface. .
Boundary layer
One of the key factors contributing to this adhesion is the fluid velocity gradient. In the boundary layer, which is the region of the fluid that is in direct contact with the surface, the fluid velocity gradually decreases from zero at the surface to the free flow value.
This decrease in velocity creates a velocity gradient that induces a drag force on the fluid particles, attracting them toward the surface.
Fluid Viscosity
In addition, the viscosity of the fluid also plays an important role. Viscosity is the fluid's resistance to flow and affects how layers of fluid adjacent to the surface behave.
In the presence of a curved surface, the viscosity of the fluid causes the particles in contact with the surface to adhere to it, following its contour instead of separating and following a straight path.
Molecular explanation
From a molecular perspective, the adhesion of fluid particles to a surface can be understood in terms of intermolecular forces and the movement of individual molecules within the fluid.
In a fluid the molecules are constantly in motion and colliding with each other. When fluid flows over a solid surface, molecules in the boundary layer that is in direct contact with the surface experience attractive forces toward the surface due to intermolecular interactions.
For example, in the case of air, gas molecules interact primarily through Van der Waals forces and dipole-dipole attractions. These forces cause air molecules near the solid surface to be attracted to it.
As the fluid molecules approach the solid surface, their speed decreases due to these attractive forces and collisions with other molecules. This results in a gradual decrease in fluid velocity as we approach the solid surface, creating the velocity gradient characteristic of the boundary layer mentioned above.
From the point of view of viscosity, in a viscous fluid the molecules are more tightly bound to each other, which increases the resistance to flow and makes the molecules in contact with the solid surface adhere to it more easily.
Discovery of the phenomenon: Henri Coandă
The discovery of the Coandă effect came about thanks to Romanian engineer Henri Coandă in the 1930s. Coandă was experimenting with a jet engine he had designed when he noticed an unexpected phenomenon: the airflow did not behave as he expected.
Coandă observed that when a jet of air came out of a tube and passed over a curved surface, such as the edge of a plate, instead of maintaining a straight path, the air adhered to the curved surface and followed it. This discovery contradicted conventional expectations about the behavior of moving fluids.
He later investigated this phenomenon further and discovered that the effect was due to differences in air pressure along the curved surface, which generated a suction force that pulled the air flow toward the surface.
Examples and applications
This principle has important implications in various areas of physics, everyday life and engineering.
Aviation
For example, in aviation, this effect is exploited in the design of aircraft wings and control surfaces.
On an airplane wing, the Coandă effect occurs when air flowing over the upper surface of the wing adheres to its curved contour, creating a low-pressure zone. This results in a pressure difference that generates lift, allowing the plane to stay in the air.
On the other hand, in a helicopter propeller, the Coandă effect drives the airflow downward, providing the thrust necessary to lift the helicopter. Both phenomena illustrate how aerodynamic design takes advantage of airflow to achieve flight and propulsion.
Meteorology and wind currents
The Coandă effect also has implications in meteorology, especially in the formation and behavior of clouds and atmospheric winds. The Coandă effect can influence the direction and speed of atmospheric flow around natural obstacles, such as mountains, buildings, and bodies of water.
When wind flows around a mountain, for example, something similar to the Coandă effect happens: the wind tends to stick to the surface of the mountain and follow its contour instead of flowing directly over it.
This can lead to interesting weather phenomena, such as the formation of lenticular clouds on top of mountains, where moist air cools and condenses as it rises over the mountain and then moves down the opposite side.
Additionally, it can also influence wind direction in urban areas where there are buildings and structures. Wind flowing around buildings can follow their contours and create areas of accelerated or swirling wind.
Wind power
Wind turbines are devices used to convert the kinetic energy of the wind into electrical energy. In these systems, the airflow around the wind turbine blades is influenced by the Coandă effect, which affects its performance and power generation capacity.
When wind flows over the blades of the wind turbine, the air tends to adhere to the curved surface of the blades and follow their contour rather than flowing directly through them.
By following the contour of the blades, the airflow creates pressure differences that generate aerodynamic forces, thus driving the movement of the blades and generating mechanical energy. This mechanical energy is then converted into electrical energy through a generator.
Steam turbines
The Coandă effect can also influence the design and operation of steam turbines in a nuclear power plant. These turbines convert the thermal energy of the steam generated by the nuclear reactor into mechanical energy to generate electricity.
In a steam turbine, high-pressure water vapor is directed through a series of fixed and moving blades in the turbine. When steam flows over these blades, it expands and undergoes a decrease in pressure, generating thrust forces that rotate the turbine.
As steam flows over the blades, it tends to adhere to the curved surfaces of the blades and follow their contours. This adhesion of water vapor to the blades affects the flow of steam and the distribution of thrust forces on the surface of the blades depending on their shape and arrangement.