{"id":20540,"date":"2018-12-14T16:01:35","date_gmt":"2018-12-14T16:01:35","guid":{"rendered":"http:\/\/sitepourvtc.com\/?page_id=20540"},"modified":"2023-02-15T13:08:25","modified_gmt":"2023-02-15T13:08:25","slug":"skin-friction-friction-drag","status":"publish","type":"page","link":"https:\/\/sitepourvtc.com\/nuclear-engineering\/fluid-dynamics\/what-is-drag-air-and-fluid-resistance\/skin-friction-friction-drag\/","title":{"rendered":"Skin Friction – Friction Drag"},"content":{"rendered":"
The friction drag<\/strong> is proportional to the surface area. Therefore, bodies with a larger surface area will experience a larger friction drag. This is why commercial airplanes reduce their total surface area to save fuel. Friction drag<\/strong> is a strong function of viscosity.<\/div><\/div>\n

As was written, when a fluid flows over a stationary surface<\/strong>, e.g., the flat plate, the bed of a river, or the pipe wall, the fluid touching the surface is brought to rest<\/strong> by the shear stress<\/strong> at the wall. The boundary layer<\/strong><\/a> is the region in which flow adjusts from zero velocity at the wall to a maximum in the mainstream of the flow. Therefore, a moving fluid exerts tangential shear forces on the surface because of the no-slip condition<\/strong> caused by viscous effects. This type of drag force<\/strong>\u00a0depends especially on the geometry, the roughness of the solid surface (only in turbulent flow<\/a>), and the type of fluid flow<\/a>.<\/p>\n

\"Drag<\/a>
Source: wikipedia.org License: CC BY-SA 3.0<\/figcaption><\/figure>\n

The friction drag<\/strong> is proportional to the surface area. Therefore, bodies with a larger surface area will experience a larger friction drag. This is why commercial airplanes reduce their total surface area to save fuel. Friction drag<\/strong> is a strong function of viscosity, and an \u201cidealized\u201d fluid with zero viscosity would produce zero friction drag since the wall shear stress would be zero.<\/p>\n

Skin friction<\/strong> is caused by viscous drag in the boundary layer around the object. Basic characteristics of all laminar and turbulent boundary layers<\/strong> are shown in the developing flow over a flat plate. The stages of the formation of the boundary layer are shown in the figure below:<\/p>\n

 <\/p>\n

Boundary layers<\/strong> may be either laminar<\/strong>\u00a0or turbulent,<\/strong> depending on the value of the Reynolds number<\/a><\/strong>.<\/p>\n

The boundary layer is laminar for lower Reynolds numbers<\/strong>, and the streamwise velocity changes uniformly as one moves away from the wall, as shown on the left side of the figure. As the Reynolds number increases<\/strong> (with x), the flow becomes unstable<\/strong>. Finally, the boundary layer is turbulent for higher Reynolds numbers, and the streamwise velocity is characterized by unsteady (changing with time) swirling flows inside the boundary layer.<\/p>\n

The transition from laminar to turbulent<\/strong> boundary layer occurs when Reynolds number at x exceeds Re<\/strong>x<\/sub><\/strong> ~ 500,000<\/strong>. The transition may occur earlier, but it is dependent especially on the surface roughness<\/strong>. The turbulent boundary layer thickens more rapidly than the laminar boundary layer due to increased shear stress at the body surface.<\/p>\n

\"Boundary<\/a><\/p>\n

There are two ways to decrease friction drag<\/strong>:<\/p>\n