As shown in Figure 7, analysis of the hot film, output signals revealed that four different 

 types of boundary-layer flow could be distinguished: 



1. Laminar flow, when hot film signals were of steady amplitude. 



2. A smooth wavelike disturbance that had identifiable frequency content, superimposed 

 on a laminar flow, and no doubt associated with Tollmien-Schlichting waves. 9 (An early 

 (Type 2a) and advanced stage (Type 2b) of wavelike flow is illustrated in Figure 7.) 



3. An intermittent turbulent bursting flow, reported by Schubauer and Klebanoff, 10 

 preceded and followed in time by periods of Type (2) flow. 



4. A fully turbulent flow of random nature. 



Under fixed test conditions, probe outputs indicated that Types (2) or (3) and Types (3) or 

 (4) could alternately occur on repeat-run, tape-recorded, time samples of the signal at a given 

 probe location. For a given location of a hot film probe on a forebody, the speed at which 

 a particular type of flow occurred could be determined to within 1/4 m/s. 



TURBULENCE STIMULATORS 



Resistance of each model was measured both with and without turbulence stimulators. 

 In the Series 1 experiments. Model 4620-3 was used to evaluate stimulators of the following 

 types: sand strips, wires, and studs. The results of these experiments have been reported in 

 Reference 1 and will not be discussed here. The 0.61-mm wire was chosen for all of the 

 remaining resistance tests because it proved to be an effective trip in the present experimental 

 arrangement, having a precisely defined geometry consistent from test to test and being easy 

 to install and remove. More parasitic drag data were also available for the wire than for the 

 other types of trips. 



Table 4 gives the wire sizes used and the locations at which the wires were installed on 

 the nine models. With the exception of Model 4620-3, only 0.61-mm diameter wires were 

 used. Whenever possible, experiments were conducted with wires installed at x/L= 0.05, the 

 traditional location used at the Center for trip wires, where L is the model length. Exceptions 

 occurred when laminar separation, which acts as a natural trip, or transition took place forward 

 of this location. When laminar separation occurred forward of x/L= 0.05, a trip was placed a 

 short distance ahead of the separation location. 



9 Schlicting, H., "Boundary Layer Theory," McGraw-Hill, Inc., New York, Chapters 12, 16, and 17 (1955). 



Schubauer, G.B. and P.S. Klebanoff, "Contributions on the Mechanics of Boundary Layer Transition," 

 National Advisory Committee for Aeronautics TN 3849 (1955). 



11 



