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

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Riblets and Tripwires and their role in reducing drag over underwater aerofoils

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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”.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 aircrafts momentum”.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.The extent of drag reduction is found to increase with the size of riblets. However, for very large riblets,

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1. Walker DB. Viscous Drag Reduction for Slender Surface Craft. A Trident Scholar Project Report. 1989 Dec 29. Naval Academy Annapolis, Maryland. Available from: http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA216264&Location=U2&doc=GetTRDoc.pdf

2. Korvin-Kroukovsky BV, Ross EW, Savitsky D. Turbulence stimulation in the boundary layer of planing surfaces. Davidson laboratory report. 1952 Aug [cited 2011 Aug 6]. Available from: http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19750069299_1975069299.pdf

3. Friedmann E. Optimal Shape Design and its application to microstructures. 6th World Congress on Structural and Multidisciplinary Optimization. 2005 May 30—Jun3 [cited 2011 Aug 6]. Available from: http://numerik.iwr.uni-heidelberg.de/~elfi/ef_rio.pdf

4. Moin P, Kim J. Tackling Turbulence with Supercomputers. Stanford: Center for Turbulence Research. Available from: http://www.stanford.edu/group/ctr/articles/tackle.html

5. Pendergast DR, Mollendorf JC, Termin AC. Application of theoretical principles to swimsuit drag reduction. Sports Eng 2006 [cited 2011 Aug 6]. 65-76. Available from: http://www.teamtermin.com/docs/SwimSuit_Drag_Reduction.pdf

6. Peet Y, Sagaut P. Turbulent Drag Reduction using Sinusoidal Riblets with Triangular Cross-Section. 38th AIAA Fluid Dynamics Conference and Exhibit; 2008 June 23-26. Seattle, Washington. p 1-9. Available from: http://www.mcs.anl.gov/~peet/aiaa_2008.pdf

7. García-Mayoral R, Jiménez J. Drag reduction by riblets. Phil Trans R Soc A. 2011 [cited 2011 Aug 6]. Available from: http://torroja.dmt.upm.es/pubs/2011/rgm_jj_philtrans11.pdf

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preview essay on Riblets and Tripwires and their role in reducing drag over underwater aerofoils
  • Pages: 4 (1000 words)
  • Document Type: Essay
  • Subject: Physics
  • Level: Undergraduate
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