Physics

# Bernoullis Principle of Lift

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Behind everything that happens around us lie principles that were discovered centuries ago. As human knowledge reaches its peek, almost every action has a scientific explanation just to satisfy man's curiosity. These principles are then applied in everyday life to lighten workloads and as well as to explain every single phenomenon. One of the principles that had contributed in modern technology is the Bernoulli's Principle.

It was in the 18th century that a scientist named Daniel Bernoulli discovered that an increase in fluid (referring to both gas and liquid) velocity would cause a decrease in pressure as well as a decrease in air velocity would increase in pressure. The principle has contributed much in fluid dynamics in explaining liquid movement (incompressible flow) as well as gas movement (compressible flow).

This principle can be well represented if a venture tube. The horizontal tube has wide and narrow sections to which a U-shaped tube is connected on each section. The increase in air velocity in the narrow part of the tube would cause a decrease in pressure on the part of the U-tube connected to it that would then cause the water to rise higher in that area than the water in the tube connected to the wider horizontal tube. This is because the increase in air velocity in the narrow part of the tube causes a decrease in pressure in the part of the U-tube connected to it that would cause the water level in that area to rise. The lower air velocity in the wider tube causes an increase in pressure in the U-tube connected to it that causes the lower level of water in the area.

BERNOULLI'S PRINCIPLE AND THE LAW OF CONSERVATION OF ENERGY

The decrease in pressure due to the increase in air velocity could be correlated with the principle of conservation of energy. This principle states that energy cannot be created nor destroyed such that energy would always remain constant. In Bernoulli's principle, the increase in kinetic energy as air passes through the narrow tube would be balanced by a decrease in pressure.

The amount of both the potential and kinetic energy remains constant as always. In a horizontal tube with constrictions, the energy is conserved since what would just dictate on which type of mechanical energy was greater is the velocity of fluid. A decrease in fluid velocity means that it has moved from an area of lower pressure to an area with higher pressure. The opposite applies in the fluid movement with high velocity fluids. Thus, in a horizontal tube, the highest speed of fluid will be in areas with decreased pressure while the lowest fluid speed would take place in areas where the pressure is highest.

BERNOULLI'S PRINCIPLE AND AERODYNAMICS

Bernoulli's principle has been used widely in explaining the mechanisms in the lift of the airplane's wings during flight. Imagine a plane's wing in a cross-sectional picture with the bottom part more flat while the upper part of the wings is slightly sloping. There would be a difference in the flow of air molecules as they hit a barrier, which in this case is the airplane's wing.

Air molecules hitting the wings would be forced to either flow on top or at the bottom of the wing. The air tend to travel faster as it passes through the top sloping part while the air in the bottom part moves slower relative to it. The air traveling on top requires a longer distance to travel due to the curvature to keep pace in meeting the air passing at the bottom part for them to meet at the rear side of the wing. The greater speed of air molecules on top of the wing is because there is smaller space due to the interference of the airfoil as air molecules are pushed between it and the outer layer of air. This causes an increase in pressure underneath the wing compared to the upper part thus the wing will be lifted upwards during flights.

The same mechanism happens with birds in flight. The structure of the birds' wings also have curvatures on the top part and the bottom part is more likely flat. This would require the air traveling on top to move at a higher velocity to meet at other end side of the wings the air molecules that passed through the bottom just like the principle of lift on airplanes.

The flow of fluids in the scenario of an airplane is the turbulent flow. Compared to laminar flow, where fluid flows smoothly at same speed, the turbulent flow would require the air to flow at different speed or direction such that the air that travels on top as well as the bottom of the wing would both reach the rear end at the same time. The difference in pressure is what makes a plane fly due to lift.

To determine the lift of an aircraft, the formula is:

Lift=(1/2)(air density)(velocity expressed in feet squared)(aircraft's wing area)(coefficient of lift)

Some factors in this formula, however, would not be constant since it would be dependent on some other external factors. For instance, air density is dependent on the altitude and the coefficient of lift uses the pitch angle velocity relationship curve since it is dependent on the shape of the aircraft's wings as well as its pitch angle.

OTHER USES OF BERNOULLI'S PRINCIPLE OF LIFT

The principle also applies in day-to-day life. Take for example a chimney on a windy day. The increased air velocity would cause the air pressure on top of the chimney to be low, thus, creating greater pressure at the bottom. In atomizers, the high pressure inside the bottle is due to low air circulation inside. Therefore, pressing the pump would cause the bottle content to flow from the inside (higher pressure) to the top (lower pressure) where it would exit in the nozzle.

The principle had been able to explain the principles behind some of the phenomena that happen in our everyday lives. Even if there are many speculations today on the relevance of Bernoulli's principles due to recent explanations from physicists regarding new discoveries, it is still the basic principle that could clearly explain man's quest for knowledge to understand things better.

References:

http://library.thinkquest.org/2819/bernoull.htm
http://uk.encarta.msn.com/encyclopedia_761557396/aerodynamics.html
http://en.wikipedia.org/wiki/Venturi_effect
http://www.aviation-history.com/theory/airfoil.htm
http://en.wikipedia.org/wiki/Aerodynamics
http://www.scienceclarified.com/everyday/Real-Life-Chemistry-Vol-3-Physics-Vol-1/Bernoulli-s-Principle.html