The closely spaced twin-jet configuration often seen in military aircraft is distinct from the single jet in both the flowfield and acoustic field. The twin-jet plumes interact with each other weakly or strongly, depending upon the distance between the two jets, the jet operating flow regime, and the Mach number. In the current study, the interaction mechanisms and coupling of a supersonic twin jet exhausting from biconical converging–diverging nozzles with a design Mach number of 1.23 at a separation distance of two nozzle exit diameters are investigated using near-field pressure measurements and phase-locked flow visualization. Across a series of jet operating Mach numbers, the twin-jet plumes feature three major jet azimuthal modes: axisymmetric, helical, and flapping. The jet flapping mode is strongly augmented in the twin-jet configuration compared to the single-jet case. Along the twin-jet plane, which is also the plane of the jet flapping direction, jet plumes with coherent flow structures are obse...

Exploring Physics and Control of Twin Supersonic Circular Jets

Measurement of acoustic shielding by a turbulent jet

Many jet noise suppression devices employ tubes, lobes, and similar devices to break up the flow. To understand the suppression mechanisms of these devices, a two-parallel-jet model experiment was conducted. The three-dimensional noise field was mapped for the case when one jet was at a velocity of 549 m/s and a total temperature of 538°C (jet 1) and the other was at 351 m/s and 93°C (jet 2). Mean flow profiles were also obtained at three axial stations. The experiments showed that noise from a high-velocity jet is reduced by placing a second jet of lower velocity parallel to it on the same side as the listener. The noise reduction is maximum on the lower velocity jet side of the plane containing the common diameter of the two jets. The noise reduction decreases as one moves azimuthally around the jets from this plane. No noise reduction is obtained at approximately 75 deg to this plane. Flow profiles showed insignificant mean flow interaction up to five nozzle diameters of the first jet. From a detailed examination of the noise results and earlier jet noise source location studies, it is concluded that for the present configuration with a spacing of about 1.5 diameters, acoustic shielding is the dominant mechanism responsible for the observed noise reduction as compared to mean flow interaction. Application of the two-parallel-jets results to bypass engine jet noise reduction is demonstrated through the example of a suppressor called the four-tube nonsymmetric nozzle.

Noise characteristics of two parallel jets with unequal flow

A numerical study, using the Direct Simulation Monte Carlo (DSMC) approach, of the interaction effects between two identical sonic under-expanded nitrogen jets under rarefied conditions is reported. In this paper, the effects of the jet stagnation Knudsen number (Kns), the ratio of the stagnation-to-background pressure (Ps/Pb), and the distance between the jet orifices (L/D) were investigated for a range of these parameters. The background pressure has a very significant effect on the physics of flow. The value of L/D affects the locations of the Mach disk in both primary and secondary jets. But When Ps/Pb is relatively large (such as Ps/Pb=220) and L/D is relatively small (L/D≤6.0), the separation distance L/D has only a mild effect on the location of the Mach disk in the secondary jet. A study of the rotationaltranslational non-equilibrium showed that large deviations between the translational and rotational temperatures can be found in the secondary jet, especially in the vicinity of the orifice plate.

Analysis of Interacting, Underexpanded, Rarefied Jets

The closely spaced twin-jet configuration often seen in military aircraft is distinct from the single jet in both the flow and acoustic fields. The twin-jet plumes interact with each other weakly or strongly varying with the jet operating flow regime and Mach number. The interaction mechanisms of the supersonic twin-jet exhausting from bi-conical convergingdiverging nozzles with design Mach number of 1.23 at a separation distance of two nozzle exit diameters are investigated using near-field pressure measurements and phase-locked flow visualization to explore the twin-jet coupling. Across a series of jet operating Mach numbers, the twin-jet plumes feature three major jet motions: axisymmetric, helical, and flapping modes. The jet flapping motion is strongly augmented in the twin-jet configuration compared to the single jet case. Along the twin-jet plane, which is also the jet flapping direction, the coherent flow structures are observed to displace back and forth laterally. Because of the nature of the jet flapping motion, the near-field pressure fluctuations are markedly amplified at low Strouhal numbers, particularly along the twin-jet plane for far downstream locations. Localized arc filament plasma actuators were implemented as an active flow control tool on the twin-jet plumes to explore the coupling mechanisms. The results show that decoupling and coupling of twin-jet plumes can both be achieved, depending on the jet operating condition and the excitation parameters employed.

An Investigation of Twin Supersonic Jet Coupling

An engine arrangement for a supersonic cruise vehicle (SCV) has been developed that shows promise for jet noise reduction without the performance penalties associated with mechanical suppressors and engine oversizing. This arrangement, wherein two engines are placed on top of the wing directly above two similarly-mounted engines below the wing, can produce 3 to 5 dB less noise below the aircraft flight path than when four engines are installed under the wing. This noise reduction is due to acoustic shielding of the upper jets by the lower jets. Test data are reviewed to verify this acoustical shielding phenomenon, and detailed takeoff trajectories are calculated to show the effects of sideline and flyover noise levels on constraining maximum range SCV configurations. Engine placement variation and differential throttling, wherein thrust is unloaded from the lower engine and added to the upper engine, are to be explored as means for maximizing the shielding effect.

J. S. Clauss

Papers

Twin jet noise shielding for a supersonic cruise vehicle

Twin-Jet Noise Shielding for a Supersonic Cruise Vehicle