Huang and von Kerezek 
APPENDIX 
EXPERIMENTAL TECHNIQUES FOR MEASURING MAGNITUDE AND 
DIRECTION OF SHEAR STRESS 
In order to determine the shear stress vector distribution 
on the hull, it is necessary to measure the distribution of the magni- 
tude of the shear stress vector 7, and its angle 2 with respect toa 
convenient direction on the ship hull. Two useful measuring devices 
are considered in this Appendix : the flush-mounted hot-film shear 
probe, and the Preston tube and the directional Preston probe. 
(1) Hot-Film Shear Probes 
The principle of the hot-film shear probe is that skin fric- 
tion is a function of electrical current required to maintain a metal 
film at the constant temperature placed on the hull surface 3132. The 
output of the hot-film anemometer is a nonlinear power function of 
shear stress. The ideal response of the hot-film is that the output of 
the instrument is directly proportional to the shear stress measured. 
This ideal response can be accomplished by processing the nonlinear 
output from the anemometer through a linearizer which is commer- 
cially available (e.g. DISA type 55D15 linearizer). The functional 
relationship between the output of the linearizer and the shear stress 
is obtained through calibration, and slight nonlinear response is 
tolerable. Most commercial anemometers and linearizers can be 
adjusted to achieve almost perfect linearization. 
Hot-film shear probes designed and built by Ling 31,32 Were 
used in this study. A strip of platinum film about 0.1mm wide and 
0.8 mm long is fused under high temperature to the polished end of 
a pyrex rod 0, 1l-inch in diameter and l-inch long. Figure Al 
shows the outputs of a hot-film anemometer and linearizer before 
and after the test versus the shear stresses measured by a Preston 
tube. A special wall-jet calibration facility, in which the wall shear 
stress on a flat wall two feet from a 1/2 inch jet can be varied from 
0 to 0.5 psf, was built for this study. This facility using towing basin 
water (not to vary chemical properties and temperature), is essent- 
ial for the proper calibration of the hot-film shear probes. The dir- 
ectional response of the hot-film shear probe is calibrated by rotat- 
ing the hot-film element with respect to the flow direction. Typical 
results are shown in Figure A2, The directional response is propor- 
tional to cosine @ upto @ = 65 degrees. The difference between a 
1976 
