An improved method for measuring skin friction in two-dimensional flow
s Is described. The instrument, termed a global interferometer skin-fr
iction meter, is used to measure the shape of a thin oil film placed o
n a test model subject to aerodynamic shear. The oil film shape is the
n related to the applied shear through lubrication theory. Through acq
uisition of a single image of the oil film interference pattern, the s
kin-friction distribution can be obtained at any point covered by the
film. Enhancements to the method are presented that may extend the ins
trument's application to fully three-dimensional flows with time-varia
nt model temperatures. Results from tests performed on flat plate boun
dary layers along with numerical simulations show the instrument can a
ccurately measure wall shear over relatively large areas in a single t
est. Results show the method provides numerous advantages over other s
hear measurement techniques. The instrument's configuration, the theor
etical background, and the parameters that influence its accuracy are
presented.