>> Saturday, October 2, 2010
BERNOULLI'S THEOREM :
In a streamline flow of an idle fluid (which is not viscous) the sum of potential energy ( energy due to its position) , pressure energy and kinetic energy ( energy due to its speed ) will remain constant.
As per Bernoulli's theorem ,
P.E + Pr.E + K.E = Constant
When considering the flow of air the potential energy can be ignored; the statement can therefore be modified ; for all practical aerodynamic purposes, by saying that "the sum of kinetic energy and pressure energy of smooth flow of air is always constant."
Therefore, Pr.E + K.E = Constant
It means that , the pressure is low where the velocity is high and vice versa to keep the energy constant during flow.
One of the most simplest and burning example of Bernoulli's theorem is venturi tube.
"For a flow of air to remain stremlined the volume passing a given point in unit time (the mass flow) must remain constant." If a venturi tube is positioned in such an air stream it obeys the above law. In obeying this law,the flow increase speed while flowing through the throat and in consequence the pressure drops at the throat. This increase or decrease is gradual as the tube slopes. So, at the narrowest portion of the tube the speed of the flow is maximum and the pressure in minimum.
The flat plate produces most drag , the cylinder about 50% drag (due to its shape) and streamlined shape produces only 5%(due to its position).
The performance of aerofoil is governed by its contour(area of the aerofoil).Generally aerofoils can be divided into three classes:
High lift aerofoils:
Features- 1. Smooth leading edge.
2. Lift is the prime factor, not the speed.
3. Thickness/chord is more.
4. Thickness is almost 15% of the chord.
General purpose aerofoils:
Features- 1. Sharp leading edge in respect of high lift aerofoil.
2. Lift as well as the speed is the important factor.
3. Thickness/chord is less than high lift aerofoil.
4. Thickness is almost 10% of the chord.
High speed aerofoils:
Features- 1. Sharp leading edge.
2. Speed is the prime factor.
3. Thickness/chord is very less.
4. Thickness is almost 7% of the chord.