Noise Reduction in a Non-Ideally Expanded Round Jet via Steady Blowing

Abstract

An effort to reduce supersonic jet noise was attempted by using steady microjets in a sonic round jet operated at Mj =1.14, 1.3, and 1.5. The corresponding jet oscillation modes are respectively axisymmetric, flapping, and helical modes. Either two or four micronozzles, with an exit diameter of 1mm, were placed at an angle of 45° relative the jet axis on the exit plane of the main nozzle, whose exit diameter is 10 mm. The far-field spectra measured at 90 °, relative to the main jet axis, showed that screech tones can be completely removed and that broadband shock associate noise can be significantly reduced at 4 -6% of the main jet mass flow rate depending on the jet Mach number, when two microjets were used. The reduction of overall sound pre ssure level appeared to depend on the jet oscillation mode and was 3.7, 9.0, and 2.7 dB at M j =1.14, 1.3, and 1.5, respectively. From Schlieren images and Pitot pressure distribution along the main jet centerline, it was found that the distorted and weakened shock cell structures are responsible for the reduced noise in this measuring direction. The shock associated noise was better controlled by two microjets than by four microjets. A greater reduction in the mixing noise, measured at 30 ° relative to the main jet axis, was obtained by four microjets than by two of them. The thrust loss, estimated theoretically, is about 1.2 to 1.8% when the mass flow rate of microjets is 4 -6% of the main jet mass flow rate, which is tenth to seventh of that by mechnical tabs. Therefore, the use of microjets offers a design alternative in controlling supersonic jet noise with relatively little thrust loss.