Fluid Mechanics Applications/B32 submarine lift and drag

Introduction

Control surfaces of a typical submarine.

A submarine is a watercraft which can be assumed to be the combination of geometrical shapes like hemisphere, cone and cylinder. The nose and tail of the submarine can be a hemisphere or cone and the body of the submarine is cylindrical. All the exterior components of a submarine are streamlined for efficiency. It is capable of independent operations underwater.

The submarine was introduced in the early 16th century. The effective use of the submarine for naval warfare on a large scale started from the first World War I. Submarines are also used by some civilian applications.

The major components of submarines are as follow:-

Pressure Hull

How do submarines survive where people can't? One of the reasons is the pressurized hull. The hull of a standard ship is the metal outside that keeps the water out. Most submarine have two hulls, one inside to help them survive .The other hull is water proof while the inner one( called the pressure hull) is much stronger and resistant to water pressure. The strongest submarine have hulls made from tough steel and titanium.

Blast tank

Whether a submarine is floating or submerging depends on the submarine buoyancy. Buoyancy is controlled by the blast tanks which are found between the submarine's inner and outer hulls. A submarine resting on the surface has positive buoyancy which means it is less dense than the water around it and will float at this time the blast tanks are mainly full of air. To submerge, the submarine must have negative buoyancy . Vent on the top of the blast tanks are opened sea water coming through the flood ports forces air out the vents and the submarine begin to sink.

The submarine blast tanks now filled with sea water is denser then the surrounding water . The exact depth can be controlled by adjusting the water to air ratio in blast tanks. Submerged marine can obtain neutral buoyancy that means the weight of the submarine equal the amount of water is displaced . The submarine will neither rises nor sink in this state. To make the submarine rise again ,compressed air is simply blown into the tanks forcing the seawater out the submarine gain positive buoyancy became less dense than the water and rises.

Engine

Most submarines use diesel engines, with some larger subs utilizing an onboard nuclear reactor, and some smaller unmanned submersibles using purely battery power. Early submarines sometimes employed gasoline engines, and the earliest submarines were man powered.

Diesel-Electric

Diesel and gasoline engines require oxygen to operate. The engine is run when the submarine is at the surface or near it through the use of a snorkel. In this design the propeller is not driven by the engine directly, and the engine is used to charge the batteries. Once the diesel engine has fully charged the batteries, the vessel can submerge and use battery power for propulsion.

Tower/sail

Submarine are cigar- shaped so they can slip smoothly through the water but in the very center . There is a tall tower packed with navigation and other equipment known as the conning tower . It is also regarded to simply as the tower or the sail.

Planes

Just as shark have fins on their bodies to help them swim and dive so submarines have fins called diving plane or hydroplanes . As the submarine propeller push forward water rushes over the planes creating an upward or downward forces that helps the submarine gradually rise or fall . The fins can be tilted to change the angle at which it climbs or dives through sea.

There are some more auxiliary components such as Navigation system Life-support system

Lift and Drag

Submarine under water fells a resistance force against motion when it moves forward by the motion of propeller. The forces are lift and drag which can be defined as:- A body moving with velocity "V" in a fluid making some angle with the stream line of the fluid flow experiences a force "F" acting on the body . The component of the force along the stream line is called the DRAG and the component of the force orthogonal to the stream line is called the LIFT. The drag and lift forces are expressed in terms of co efficient of drag and lift respectively.The co efficient are defined as the ratio of corresponding forces to the dynamic forces on the projected areas or the planform areas. The co efficient of drag

    CD=D/(1/2)¶u2A

Such that drag force D is given by

    D=Cd(1/2)¶u2A

The co efficient of lift CL is defined as

    Cl=L/((1/2)¶u2A)

Such that the lift force L is given by

    L=Cl(1/2)¶u2A

A symmetric and non symmetric body moving with velocity in a fluid making an angle of attack '&' with the stream line of the fluid or moving along the stream lines. The stream lines separates and moves around the moving body. The fluid had to move in different stream lines around the moving body. The two stream lines will cover different distance ,so the stream line which would cover more distance would have to move with more speed than the stream line which had to cover short distance in account of CONSERVATION OF MASS. The fast moving stram line will produce less pressure then the stream line moving with less speedBERNOULLI EQUATIONS. This pressure difference across the moving body which ultimately results in a net force'F' which produce lift and drag. Also due to the some angle of attack the direction of flow changes which result in the change in momentum of the body taking a control volume around the body and applying the MOMENTUM EQUATIONS in control volume. A net resultant force is found to act on the body. Taking both the aspect of conservation of mass and momentum equation together in a control volume . A net resultant force is found to act on body which is more general in practice. A submarine moving with speed 'v' having a small angle of attack will produce a lift in the submarine which is not necessary when ship is moving under the water because if lift is produced in the submarine it will come up in the surface ultimately which we don't desire. submarine are used to move underwater and perform operations. So there is an auxiliary arrangement of hydroplane in the conning tower to avoid this unnecessary effect of producing lift. This hydroplane has shape like aerofoil and acts as a flap. By tilting the fins and changing the angle of attack of hydroplane with the streamline of water produces a lift negative to unnecessary lift produced due to the motion of the pressurized hull. Now submarine is able to move under water and can perform operations as per the signal. Hydroplane is also used in coordination with the navigation system to control the submarine direction in motion.

References

Fluid Mechanics F.M.White Engineering Fluid Mechanics K.L.Kumar