APPENDIX A 

 EFFECTS OF TURBULENCE STIMULATORS 



The need for artificial stimulation of turbulence on ship models in order to avoid the 

 spurious results obtained in ship predictions based upon model experiments in which some 

 laminar flow persisted was recognised in some tanks, including Hamburg and Wageningen, 

 before 1930. Its importance was not generally appreciated, however, until around 1948 when 

 it was realised that on some hull forms, notably those with a full forebody and raked stem, 

 laminar flow could persist to an alarming extent. Thus experiments with LIBERTY ship 

 models showed that the effect of stimulation over the lower speed range could amount to 15 

 or o\en 20 percent. 



A number of methods of stimulating turbulence have been devised from time to time, 

 Kempf early proposed a "comb" which made a pattern of grooves in the wax hull around a 

 station about 5 percent of the length aft of the stem, while a "trip-wire" placed around the 

 hull at the same place was adopted very early in the work and has maintained its place as 

 one accepted method to the present time. Sand strips down the stem and along the LWL for 

 a short distance from the fore end are also used. All these devices add some parasitic drag 

 to the hull, and to avoid this use has been made of struts ahead of the model attached to 

 the towing carriage. Some experiments of this kind made with fine models in which no lami- 

 nar flow effects could be detected suggested that the wake from the strut could actually 

 reduce the measured resistance of the hull. In order to avoid some of these effects, studs 

 similar to those developed on aircraft models were tried; they have the stimulating effect of 

 trip wires or sand strips but a very low parasitic drag. These were described by Hughes and 

 Allan in 1951."*^ 



The original Series 57 models were run with and without turbulence stimulation. The 

 standard method used on all models was a sand strip i^ in. wide down each side of stem and 

 along the LWL for a distance of 4 ft or one-fifth of the length of the model. In addition, 

 some models were run with a trip wire, 0.04 in. in diameter placed around a station at — 

 from the stem. Others were fitted with studs, as described in Reference 46; these have a 

 diameter of 1/8 in., were 1/10 in. high and spaced 1 in. apart along a line parallel to the 

 stem profile. The distance of the line from the stem depends on the half angle of entrance 

 on the LWL (1/2 a^). For the three Series 57 models of 0.70, 0.75, and 0.80 Cg, the 1/2 «£ 

 values were 13.3, 27.1, and 44.0 deg, the distance of the studs from the stem being, respec- 

 tively, 2.13, 2.70, and 3.25 in. 



The effects upon resistance were somewhat erratic but never very serious. 



A-l 



