Hi, all.
We know boundary layer thickness of flat plate (Blasius solution) as 0.99 U.My question is can we apply the same phenomena to airfoil (flat plate approximation) or is there any formulae or terminology/relation to estimate boundary layer thickness of an airfoil.Please, someone, explain and also the influencing parameters. Thanks all
Go with CFD and Proceed on to wind tunnel testing. Contact IRTDC for more detail.
Simply move your probe across the flow at any section of airfoil till you see mean free stream velocity reaches tunnel velocity. That is approximate boundary layer thickness. This is true till point of separation.
@mohsen sir .I s there any analytical method to evaluate boundary layer thickness?please do help
Take XFLR5 software. Read the models it uses. Simulate some flows over airfoils.
Links: http://www.xflr5.com/xflr5.htm ; http://m-selig.ae.illinois.edu/ads/coord_database.html
We use two methods in this case: If you have the velocity field, just calculate the vorticity omega, it decays exponentially near the boundary-layer edge because outside you have irrotational potential flow. Then you may state that delta is where the vorticity has decayed to a small fraction of the maximum value that is closer to the wall. In the second method you use the vorticity that is, by large, approximately du/dy, und you integrate a new u*=Integral(omega times dy) normal to the local wall, and that u* reaches a constant value outside the boundary layer because omega is zero there. Then use just the .99 criterion.
Please find the attachment ( Boundary Layer Theory, Hermann Schlichting 1979 )
In case of an airfoil there is a transverse pressure gradient across the boundary layer (b.l.). It means that the b.l. edge is located where the real (computed or measured) velocity profile reaches 99.5% of the ideal profile (assuming no total pressure losses). Fo a flat plate the ideal profile is just a constant (Umax). For a curved wall, airfoil or helicopter rotor blade this is another story...
Oskar
You can use the traditional definition of boundary layer thickness for flat plates. There are some criteria to calculate boundary layer thicknesses for airfoils. You'd better define a line that is perpendicular to the airfoil surface. Then check these parameters:
1) vorticity: flow is rotational in the boundary layer; therefore, the vorticity value is not zero. However, the vorticity reaches zero out of the boundary layer (inviscid flow).
2) Total pressure: if the flow becomes inviscid, total pressure will be constant.
To calculate other boundary layer parameters such as the displacement thickness and the momentum thickness, Deuse and Sandberg have proposed new formulas based on the traditional definition. If you are interested, you can read the 3.2 section of Article Different noise generation mechanisms of a controlled diffus...
http://www.aerodynamics4students.com/subsonic-aerofoil-and-wing-theory/2d-boundary-layer-modelling.php
Flat plate and airfoil are two different cases, But the method for calculating boundary layer thickness remains same, i.e. distance or height above the boundary of flat plate or airfoil equals to 99% of free stream velocity in wind tunnel representing thickness,
In your question, however, standard flow . What do you mean by it ? Laminar, turbulent flows or any other type ? In each case the thicknesses differ from each other and dependent on several flow conditions
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