Gas Density Measurements in a Jet Using Raman Scattering

was also calculated by an alternate method. From the schlieren photographs, a line was extended from the initial bump in the shock to the burst trailing edge; the angle which this line makes with the cone surface is the local relative Mach angle. Values of local burst trailing edge speeds determined from these Mach angles are given in Table I in parenthesis. The velocities of the burst leading and trailing edges were found to be fairly consistent for each observation, with the leading edge of the burst travelling at about local edge velocity while the trailing edge of the burst moved supersonic relative to local speed, UT EJue « 0.82. This result is consistent with James,5 where MT^EJMe was about 0.79 for Me = 7.0 in air. It should be noted that the present study conducted in helium represents an equivalent air Mach number6 of 9.8, since Mair = ^helium (helium - 1/y.i, - I) 1'2 for both the same wall to total temperature ratio and density ratio across the boundary layer. Location of boundary-laye r transition at the cone surface, detected from heattransfer ^measurements and reported in Ref. 7, is also given in Table 1. For the present conditions, bursts were observed as far forward of surface transition as STE/Str = 0.675 (Re/m = 30.9 x 10 6) and as far rearward of transition as S^JS^ = 1.50 (RJm = 37.3 x 10 6). Calculated burst growth rates listed in Table 1 were about 300 w/sec. The disclosure of turbulent bursts upstream of surface transition (STE/Str = 0.675) implies that the outer portion of a hypersonic boundary layer upstream of the surface transition location may be in a developed transitional state. The investigation of Ref. 8 for a 12° half-angle cone at Me = 7.2 also observed turbulent bursts ahead of the surface transition location (ST E /$tr = 0.77). Previously, a 2.87° half-angle cone with Me « 14 was tested in the same facility and over the same Reynolds number range as the present study.9 Mean flow profiles in the outer portion of this hypersonic boundary layer were transitional long before evidence of transition was observed at the surface. Further analysis9'10 disclosed that the turbulent disturbance spreading angle relative to the wall was very shallow (about 0.60°) and remained essentially constant over a wide Mach number range. Spark schlierens of the cone in Ref. 9 revealed a wavy disturbance structure at the boundary-layer edge ahead of the surface transition location (see Fig. 1 of Ref. 9) with no evidence of turbulent bursts. The frequency of these wavy disturbances was approximately 90 KHz (ue « 1830 ra/sec, wave length w 26). From the work of Mack, 11 the frequency of the most unstable two-dimensional disturbance for the conditions of Ref. 9 was approximately 80 KHz for the 2nd mode and 46 KHz for the 1st mode. This result suggests that Tollmien-Schlichting wave forms characteristic of the 2nd mode could be present in the sound dominated disturbance environment (p/p^ « 4% from Ref. 7) of Ref. 9. For the present study, where p/p^ was about 4% (see Ref. 7), turbulent bursts were the dominant boundary-layer feature. However, a wavy structure ahead of and downstream of bursts was evident (see Fig. 2), with a frequency of about 250 KHz (ue « 1700 m/sec, wave length « 38) although this represents a crude frequency estimate since the wave form was not clearly established. From Mack11 for the conditions of the present study, the characteristic frequencies for the 1st and 2nd modes were approximately 225 KHz and 370 KHz, respectively. This result implies that Tollmien-Schlichting waves characteristic of the 1st mode could be present at this lower Mach number.