This paper deals with noise sources which are centra! to the problem of core engine noise in turbopropulsion systems. The sources dealt with are entropy noise and direct combustion noise, as well as a nonpropagating psuedosound which is hydrodynaniic noise. It is shown analytically and experimentally that a transition can occur from a combustion noise-dominant situation to an entropy noise-dominant case if the contraction of a terminating nozzle to the combustor is high enough. In the conibustor tested, entropy noise is the dominant source for propagationai noise if the combustor is choked at the exit. It is also speculated that there might be another unexplored noise source interior of the combustor. Analysis techniques include spectral, cross spectral, correlation, and ordinary and partial coherence analysis. Measurements include exterior and interior fluctuating and mean pressures and temperatures.
Experimental studies of base pressure manipulation for an axisymmetric model at Mach 3 with cold gas injection and burning are described. Air, nitrogen, helium, and hydrogen are used for cold gas injection tests. Burning tests use hydrogen subsonically injected either radially upstream of the base plane or axially through a porous base plate. These modes of injection yielded base burning that is efficient for base drag reduction. Base burning is also combined with external compression, simulating external burning to show that the separate effects are additive.
NomenclatureA b = model base area 7 = injection mass flow parameter, Fig. 7 sp = specific impulse, Fig. 8 M = Mach number M = molecular weight M a = air molecular weight bieed = injection mass flow Pj = freestream static pressure p b-base pressure p b0 = undisturbed base pressure V l = freestream velocity P 1 = freestream density
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