1. Cavity technique - The shock motion produced by a cavity embedded in a turbulent wall boundary layer is of two types: (.a) oscillation of the separation and reattachment shock systems (Ref. 88), and (b) a convecting shock system generated by vortices shed into the downstream boundary layer in the reattachment region (Ref. 89). Interesting oscillation amplitudes occur at various cavity modal resonances whose frequencies are documented in Figure 3 of Reference 88 and Figure 4 of Reference 90 (see also Refs. 91-93). Of particular interest are the following experimenLa 1 findings from Reference 90 (a) the cavity resonant frequency is first order dependent only on Nach number, other parameters such as length of cavity, Reynolds number, wall temperature, upstream bonndary layer state and thickness are at best second order influences (in partial disagreement with Ref. 94), (b) the levelof the discrete component in the spectrum decreases with Mach number and at H -3.5 essentially disappears! The rationale for the disappearance of cavity resonance at high Mach number is probably connected with (1) the increasing percentage of the Cavity shear layer which is supersonic. thereby interfering with cavity feedhack mechanisms and (2) decreasing amplification of free-shear layer instabilities at the higher Mach number (Ref. 95). Therefore, the use of wall cavities for production of turbulence-enhancing shock oscillations in hypervelocity seramjes is problematical (for other than low amplitude shock jittering) as the local Mach number within the combustor is larger than 3.5.
2. stream fuel injection has much to commend it and B major drawback. The drawback is the retro-action of the injection momentum. Major benefits include automatic device cooling/survivability and production of large amplitude shock motions at high frequencies (Refs. 96-99). Shock oscillations produced i n this manner would obviously have to reflect from the wall regions before Intersecting the mixing zones unless mltipleltnteracting injection struts were used. Also, the struts could be located forward (in the "inlet" region) and the fuel injected either in liquid or gaseous farm. A further variation on the same theme is lateral or "weeping" injection from a nose sp 1.ke. In fact, a simple nose spike by itself has, from References 51 and 10, a very interesting shock oscillation frequency range with a strouhal number OC.2) even at high Mach number (see Figure 30 of Ref. 102 and Fig. 3 of Ref. 103 for geometric/frequency specification). Obvtously, some mass addition may be required to ensure spike survivability in high speed, high enthalpy flows. The reason for the high-stream Mach number high frequency performance of this forward spikelinjection (as opposed to the wall cavity case which sputters out around Mach 3.5 as previously discussed) may be speculatively ascribed, at least partially, to the existence of low Mach number flow in the reattachment region on the body shoulder regardless of stream Mach number due to the hypersonic freeze phenomena for blunt body flows. This is further bolstered by the
3. Jet engine combustion noise: Pressure, entropy and vorticity perturbations produced by unsteady combustion or heat addition