Riblets and Tripwires and their role in reducing drag over underwater aerofoils - Essay Example

Their optimal specifications, mechanism of functioning and potential applications for drag reduction over underwater aerofoils have also been dealt with. Riblets and tripwires In case of underwater vehicles, fluid mechanical drag (hydrodynamic drag) reduces the speed with which they travel through water. For a specific limit of engine power, the maximum speed that can be attained by the vehicle is drastically reduced due to the formation of eddies very close to the surface of the vehicle. Apart from this, another source of hydrodynamic drag is the development of pressure drag due to laminar flow at the boundary layers.

Riblets can be used to reduce the formation of eddies near the surface of the vehicle, while tripwires can be used to reduce the pressure drag and induce turbulence at the boundary layers. Riblets: These are minute grooves, longitudinally placed on the surface of a vehicle or airfoil, and are in alignment with the direction of flow. They “have been shown by Walsh and Lindemann [1984] at the National Aeronautics and Space Administration (NASA) and Reidy and Anderson [1988] at the Naval Ocean Systems Center (NOSC) to produce as much as 8 percent turbulent flow drag reduction on flat plates”.

1 Studies on “fully submerged axisymmetric bodies”1 conducted by Beauchamp and Philips [1986 and 1987], have shown that the use of riblets reduces skin friction by seven percent.1 Tripwires: Tripwires, also known as turbulators, are thin wires that are attached at the hull of a vehicle or at the nose of a submarine or aircraft. These are used to “trip” or disturb the boundary layer and introduce turbulence, thereby reducing pressure drag. These are “one of the oldest and most used methods of turbulence stimulation”.2 1. Mechanism To comprehend the mechanism of functioning of riblets and tripwires, it is essential to understand the types of drag imposed on a vehicle and the process of their induction.

The total drag experienced by a body, includes a combination of pressure and friction. The frictional drag can be reduced by using smooth surfaces. However, studies have shown that this form of drag is further reduced with the help of riblets. The boundary layer of flow around a vehicle can be distinguished into three parts, “a relatively small viscous sublayer, the middle buffer layer and the logarithmic layer”.3 The riblets that are used to reduce drag “extend into the buffer layer. With this kind of riblets a drag reduction of about 8-10%” has been achieved.

This is because the riblets restrain the movement of eddies, which are prevented from coming very close, within 50 microns, to the surface of the aircraft or vehicle. “By keeping the eddies this tiny distance away, the riblets prevent the eddies from transporting high-speed fluid close to the surface, where it decelerates and saps the aircraft's momentum”.4 To reduce the pressure drag over a vehicle, tripwires are used. Pressure drag occurs when there is a “lack of pressure recovery on the back of the body, due to separation of the flow around the body”.

5 This results from laminar flow in the boundary layer around the vehicle. By using tripwires, it is possible to induce turbulence in this laminar layer, thereby reducing pressure drag. 2. Optimal Specifications The extent of drag reduction is found to increase with the size of riblets. However, for very