EFFECTS OF NOZZLE BODY ON JET NOISE

Most models for jet noise assume that the turbulence producing the noi se is far enough removed from any solid surface that the presence of s urfaces (such as the nozzle itself) has negligible effect on the sound field. This paper addresses the validity of this assumption. Experime nts were performed on a low Mach number jet in which the dominant soun d source, the pairing of vortex rings, was well documented by previous work [1]. The vortex rings were stabilized spatially and temporally b y artificial excitation at St(Dj) = 1.14 and became coplanar (one insi de the other) at x/D-j congruent to 2.5 with a frequency of occurrence St(Dj) = 0.285. In the current study, the directivity of this source was measured for various external nozzle geometries. The external nozz le shape was changed from a conventional conic shape to a flat plate w hose diameter was then changed by a factor of three to determine how e xternal nozzle shape and size affected the sound of the vortex pairing in the jet. To explain the variations in directivity observed with th e different nozzle geometries, a simple model of the vortex ring pairi ng was created using Biot-Savart vortex simulations. Vortex sound theo ry, including surface dipole terms, was applied to this estimate of th e vorticity field to calculate the resulting dipole and quadrupole sou nd sources. The dipole sound was of the same order as the freestream q uadrupole sound. When the phase-average sound field measured in the ex periments was decomposed into multipole components, the relative stren gths of the low frequency dipole and quadrupole components were in goo d agreement with those of the simulation, supporting the general concl usion that the dipole produced by the presence of the nozzle is not ne gligible for vortex motions within the first few diameters of the jet, and supporting the validity of the vortex sound theory itself. The de composition also unveiled a weaker monopole source, which is seen as e vidence of subharmonic feedback from the pairing to the jet nozzle, he lping stabilize successive pairings even though no excitation was prov ided as these subharmonic frequencies. (C) 1995 Academic Press Limited