31 



and a trip wire have drag forces of their own and a turbulence rod produces a 

 wake which alters the pressure distribution over the bow and also produces 

 some effect on the wave resistance. 



Tests with and without a turbulence stimulator give changes in total 

 measured resistance A R. which may be written 



4R t = 4R f +AR r + D s [15] 



where the A's indicate the actual changes in the various resistances and D 

 is the drag (or undesirable effect on drag) of the stimulator itself. 



For purposes of fairing the data it is convenient to compute a so- 

 called residuary-resistance coefficient by approximating the friction drag of 

 the model with flat-plate friction data. Then 



R. - R-, 



r fsu 2 



where R' f is the resistance of a flat plate at the model Reynolds number and 

 having the same length and wetted surface as the model. The change in the 

 measured total model resistance with turbulence stimulation is then reflected 

 as a change in the computed residuary-resistance coefficient or AC = AC, . 

 Equation [15] may be transposed and put into nondlraensional form to give 



AC, AC AR + D o 



where C- is the Schoenherr frictional-resistance coefficient for flat plates. 

 It is usually assumed that the total increase in resistance with turbulence 

 stimulation is due entirely to a change in the frictional resistance, or 

 AR =0. This is a convenient and necessary assumption since it would be ex- 

 tremely difficult to estimate AR with accuracy. 



The Prandtl assumption that the turbulent boundary layer behaves as 

 though it started at the leading edge may be applied with particular confi- 

 dence in the case of large models since the transition region is relatively 

 close to the bow. Hence the flow over the after portion of the model would 

 not be affected appreciably by the stimulation of turbulence and the residuary 

 resistance will be nearly unaffected. The assumption that AR = is then 

 quite acceptable. An attempt can be made to estimate D , however, at least in 

 the cases of stimulation with trip wire and sand strip. 



