Tripwires: Tripwires, also known as turbulators, are thin wires that are attached at the hull of a vehicle or at the nose of 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”.
T 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 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, 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 aircrafts momentum”. T 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. Te extent of drag reduction is found to increase with the size of riblets. However, for very large riblets,...
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