Underexpanded Circular Jet Research Articles

Evolution characteristics of under-expanded accompanying jet under extreme heat condition

This work is focused to study heat and mass transfer characteristics as well as evolution mechism of the wake flow field of the aerodynamic vehicle (ADV) with base-bleed unit (BBU) under extreme heat condition. Here, we have used a new type of annular nozzle for BBU to improve the structure of the wake flow field and provide more mass and energy for the wake flow field. Based on H2-CO reaction mechanism for the post-combustion from the base-bleed gas and supersonic inflow, numerical models of unsteady accompanying jet coupled with the burning of the propellant are established for annular and circular nozzles. Comparisons of chamber pressure and jet shape of computational results are done with experimental studies, and a reasonable match has been obtained in these comparisons. Further, an under-expanded supersonic annular jet with containing central jet develops into a subsonic annular flow, presenting some distinguishing features compared with the decay process of an under-expanded circular jet. The base pressure coefficient for the annular nozzle increases by 21.8% and the peak of average base temperature is improved by 15.4%, which means this new type of annular nozzle is more beneficial to the increase of heat energy and pressure of the ADV base.

Three-Dimensional Visualization of Shock Cell Structure and Screech Tone Emission in Underexpanded Circular Jet

The three-dimensional visualization of flow and screech-tone emission from an underexpanded circular jet is first investigated experimentally using high-speed cross-schlieren imaging and microphone measurements in the cross-sectional planes along the jet axis. This experimental technique allows the visualization of the shock-cell structure, directivity of sound intensity, and frequency spectrum in the cross-sectional planes of the screeching jet. The high-speed cross-schlieren observation of the screeching jet shows the occurrence of an asymmetrical shock-cell structure that is generated by the flapping mode in the screeching jet. This contributes to the generation of a screech tone propagating upstream along the jet axis and non-uniform sound intensities around the jet in circumferential direction. These observations by high-speed cross-schlieren imaging were validated by the microphone measurements.

The evolution of the initial flow structures of a highly under-expanded circular jet

The three-dimensional flow characteristics of the compressible vortex ring generated by under-expanded circular jets with two typical pressure ratios, i.e. $n=1.4$ (moderate) and 4.0 (high), are investigated numerically with the use of large-eddy simulations. Our results illustrate that these two pressure ratios correspond to different shock structures (shock cell and Mach disc, respectively) within the jet. These two typical types of flow structures and characteristics are discussed and validated with experiments, and the different generation mechanisms of the secondary vortex rings are compared. Moreover, detailed information about the evolution of the secondary vortex ring, primary vortex ring and turbulence transition features, including the radial and azimuthal modes, is investigated. The geometric features and mixing effects of the jets are also explored.

The secondary vortex rings of a supersonic underexpanded circular jet with low pressure ratio

Study of the secondary vortex ring (SVR) is essential to understand the complicated flow structures of supersonic impulsive jets. In the present study, the main characteristics of compressible secondary vortex ring and the primary vortex ring (PVR) in the starting three-dimensional (3D) flow field of a supersonic underexpanded circular jet are investigated numerically for Ma=1.2, 1.4 and 1.8, at a low pressure ratio (jet flow pressure/ambient pressure) of 1.4. The governing equations of large eddy simulation (LES) for compressible flow have been employed and are solved numerically with the combination of high-order hybrid schemes. Our results illustrate the reason for generation of the SVRs by supersonic underexpanded jets, and it is the rolling up of the shear layer which is resulted from the combination of two slip lines when their two triple points on the embedded shock wave interact with each other. After formation, the SVR interacts with the PVR, and rolls over the periphery of PVR and moves upstream. For a higher Mach number of 1.8, multiple SVRs form during the evolution.

Jet Screech Noise Computation

The near-field screech-tone noise of a typical underexpanded circular jet issuing from a sonic nozzle is simulated numerically. The self-sustained feedback loop is automatically established in the simulation. The computed shock-cell structure, acoustic wave length, screech tone frequencies, and sound pressure levels in the near field are in good agreement with existing experimental results.

2008 ヘリカル・モードで振動する不足膨張円形ジェットの周りで周回するマッハ・コーン

Existence of Mach cone and helical vortical structure in the helical oscillation mode of underexpanded circular jet was confirmed by using schlieren instantaneous photographs and drawings of envelopes of the Mach cones by the superposition of spherical sound waves radiated from two moving sound sources about the jet axis at a supersonic speed. The enveloes of a Mach cone are observed as a V-shaped pattern composed of a pair of clear fine lines starting from a prominent point, which rotates about the jet axis. The helical vortical structure is observed as bright pattern of the gathering of the tiny specks around the jet. It always apperars to overlap on the envelopes of the moving Mach cones.