lecture19 - Lecture 19- Marine Hydrodynamics Lecture 19...

Info iconThis preview shows pages 1–3. Sign up to view the full content.

View Full Document Right Arrow Icon

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: Lecture 19- Marine Hydrodynamics Lecture 19 Turbulent Boundary Layers: Roughness Effects So far, we have assumed a hydraulically smooth surface. In practice, it is rarely so, due to fouling, rust, rivets, etc. . . . U o Viscous sublayer v k = characteristic roughness height To account for roughness we first define an equivalent sand roughness coecient k (units: [ L ]), a measure of the characteristic roughness height. The parameter that determines the significance of the roughness k is the ratio k We thus distinguish the following two cases, depending of the value of the ratio k on the actual surface- e.g., ship hull. 1. Hydraulically smooth surface For k < v << , where v is the viscous sub-layer thickness, k does not affect the turbulent boundary layer significantly. k << 1 C f C f , smooth C f = C f ( R e L ) 1 2.20 - Marine Hydrodynamics, Spring 2005 2.20 2. Hydraulically rough surface For k >> >> v , the ow will resemble what is sketched in the following figure. separation v k In terms of sand grains: each sand grain can be thought of as a bluff body. The ow, thus separates downstream of each sand grain. Recalling that drag due to separation = form drag >> viscous drag we can approximate the friction drag as the resultant drag due to the separation behind each sand grain....
View Full Document

This note was uploaded on 02/27/2012 for the course MECHANICAL 2.20 taught by Professor Dickk.p.yue during the Spring '05 term at MIT.

Page1 / 7

lecture19 - Lecture 19- Marine Hydrodynamics Lecture 19...

This preview shows document pages 1 - 3. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online