Expert: Paul Soderman - 6/16/2010

Question
Dear Mr.Soderman,

I’m doing an experiment regarding the aerodynamic drag of a vehicle , using a small vehicle-model.In my experiment, i also like to measure the thickness of its boundary layer on the upper surface of the model.

My problem is that I will be doing it at high Re, which will make the boundary layer turbulent. How do you suggest I must do to measure the boundary layer thickness? Can I measure it simply using an anemometer? Is there is an approximation formula to be used?

Thank You in advance,
Akbar

Answer
Akbar
Boundary-layer thickness can be measured a number of ways.  The oldest method is the boundary-layer rake (also called boundary-layer mouse).  The rake is an array of small total head pressure tubes pointed into the wind.  They must be closely spaced yet not overly block the flow and are sometimes put on an incline so the vertical separation of the tubes is less than a tube diameter.  Static pressure is also measured on the rake or nearby to give the velocity distribution at the rake.  The pressures can be read on manometer tubes or with transducers.  (See W. Rae and A. Pope:  Low-Speed Wind Tunnel Testing.)  Similarly, one can traverse a single total head tube normal to the surface or use other measurement devices such as hot wires or split film.  The velocity profile of a turbulent boundary layer is much different than that of a laminar boundary layer as can be seen in any aerodynamics book (e.g., Schlichting:  Boundary-Layer Theory).  Schlichting (and others) also gives the equations describing the boundary-layer profile.

An anemometer would not work unless it is very small compared to the boundary-layer thickness, which is unlikely.
Paul