Supersonic jet shock noise reduction

T JShock-cell noise has been identified as a p densitypotentially significant problem for advanced super- _ wavelength• sonic aircraft at takeoff. Therefore NASAhasconducted fundamental studies of the phenomena e polar angle from inlet axis, deginvolved and model-scale experiments aimed at ea apparent angle of attack 25, degdevelopingreviewsthe meansresultsof ofnoisea seriesreduction.of studiesThis papercon- eM Mach angle, 180°-sin-l(I/Mj) degducted to determine means by which supersonic jet Subscripts:shock noise can be reduced to acceptable levels foradvanced supersonic cruise aircraft. Theoretical a ambientstudies were conducted on the shock associated c convectionnoise of supersonic jets from convergent-divergent d design point(C-D) nozzles. Laboratory studies were conductedon the influence of narrowband shock screech on D downstreambroadband noise and on means of screech reduction.e equivalentThe usefulness of C-D nozzle passages was investi-gated at model scale for single-stream and dual- ef effectivestream nozzles. The effect of off-design pressure Ex exitratio was determined under static and simulatedflight conditions for jet temperatures up to 960 K. h hydraulicAnnular and coannular flow passages with center ISA international standard atmosphere (288 Kplugs and multielement suppressor nozzles wereevaluated, and the effect of plug tip geometry was and 101.3 kN/m2)established. In addition to the far-field acousticdata, mean and turbulent velocity distributions i innerwere obtained with a laser velocimeter, and shadow- j fully-expanded jetgraph images of the flow field were obtained.n shock cell number (downstream of nozzleNomenclature exit)(All symbols are in SI units unless noted.) o outerA area P premergedc speed of sound p plugD nozzle diameter 0 aircraftF functional relation I inner streamf I/3-octave-band center frequency 2 outer streamh annulus heightk ratio of convection velocity to jet Introductionvelocity Shock-cell noise has been identified as aL shock cell spacing potentially significant problem for advanced super-sonic aircraft at takeoff. Therefore, the NASAM Mach number, V/c Lewis and Langley Research Centers have conductedOASPL overall sound pressure level, dB re 20 _N/m2 fundamental studies of the phenomena involved andmore applied studies aimed at developing means ofP pressure noise reduction. Early studies indicated that itR source-to-observer distance might be necessary to employ convergent-divergent(C-D) nozzle passages in order to reduce or elimi-S nondimensional frequency parameter nate shock noise. Some benefits have been demon-SPL I/3-octave-band sound pressure level strated statically for single stream C-D circular' nozzles at design jet Mach numbers of 1.5 and 2.0; IdB re 20 _N/m2 however, lower jet Mach numbers will probably beT total temperature required to achieve acceptable jet noise levels.In addition, more complicated single- or dual-V velocity stream nozzle geometries, possibly including multi-X distance downstream of nozzle exit element suppressor exhaust passages will probablybe required, and the effect of flight should beturbulent length scale ratio determined. Existing shock noise theories werefound to apply to a limited degree, but only to*Member AIAA. convergent circular nozzles, thus indicating a needfor further theoretical advances. Experimental data