Change of Oscillation Modes of Circular Underexpanded Jet by Impingement on a Small Plate

The frequency characteristics of the discrete tones generated by the impingement of a circular underexpanded jet on a small circular plate were studied experimentally. When the plate was moved along the jet axis at fixed pressure ratios, it was found that the frequencies could basically be divided into two groups. These groups belong to the categories of the impinging tones and the screech tones. Furthermore, it was observed that three types of frequency changes of the screech tones, (sawtooth, stepwise and intermittent ones), are realized periodically with the increasing nozzle-to-plate distance. Therefore, it is considered that the frequency change of the screech tone by the insertion of a small plate into the jet is associated with the self-sustained oscillation of the circular underexpanded free jet. The most interesting phenomenon discovered is the stepwise change of the frequencies of discrete tones. The pressure ratio range of the stepwise change of the frequencies overlaps that for the helical oscillation mode of the free jet and that for the radiation of the strong hole tone

Discrete tones generated by the impingement of a high-speed jet on a circular cylinder

Experimental measurements were made of the frequencies of discrete tones emanating from high subsonic and choked underexpanded jets of air issuing from a circular nozzle and impinging on a slender circular cylinder placed normal to the jet axis. In the experiments, a few types of discrete tones were observed. It is proved that the frequencies of the discrete tones can be calculated by using a feedback model proposed by Powell, and Ho and Nosseir. Schlieren photographs of the flow field along with the near-sound field were taken for various nozzle-to-cylinder distances and various pressure ratios. Close investigation of these schlieren photographs has shown that for the subsonic jet, one strong sound wave is emitted near the cylinder and two large ring vortices are produced near the nozzle exit during one cycle of the feedback loop. These two large ring vortices merge together in a later stage of the loop. For the choked underexpanded jet, one strong sound wave is emitted and one large ring vortex is produced during one cycle of the feedback loop. In this case, a merging of the successive large ring vortices has not occurred

An Ultrasonic Altitude-Velocity Sensor for Airplanes in the Vicinity of the Ground : I. Fundamental Characteristics

This paper shows the possibility of an ultrasonic sensor which detects the altitude from the ground and the vertical velocity of an airplane in the vicinity of the ground. The principle of the present technique depends on the measurement of time in which the ultrasonic wave propagates the distance between the airplane and the ground, because the sonic velocity is approximately constant at the usual atmospheric temperature. Furthermore, by differentiating the altitude signal with respect to time, it is possible to detect the vertical velocity of the airplane, too. The fundamental performances of this sensor are investigated with some experiments carried out in our laboratory, and it is shown that the ultrasonic sensor will be useful in place of the radar altimeter, in particular at very low altitude. As an application of this sensor, the automatic control of a VTOL airplane in hovering flight is investigated by analog simulation studies

Structure of Shock Waves in Bubbly Liquid

In this paper, steady and unsteady shock waves in a bubbly liquid are treated numerically. A new system of model equations describing the bubbly flow is applied and the detailed flow structure behind a shock front is investigated in detail. It is proved that the velocity difference between the liquid and the gas phases induced by a stationary shock wave is of order α1/2, wher α is the void fraction of the gas-phase. Radial oscillation of bubbles tends to produce a oscillatory profile of the translational velocity of the bubbles near the wave fronts. Numerical simulation shows that oscillatory behaviour of the mixture pressure is significantly suppressed by the translational motion of bubbles and that the whole shock structure is remarkably affected by the velocity difference between the phases especially in the case of weak shocks. It is confirmed that the stationary shock wave is realized as an asymptotic solution for a shock tube problem with uniform conditions in the low pressure and high pressure chambers

Numerical and Experimental Studies on Choked Underexpanded Jets

Axisymmetric underexpanded supersonic jets are investigated numerically and experimentally. A time-dependent technique of solution is applied to solve the Euler equations for a compressible ideal gas. The characteristics of the Mach disk obtained by the numerical calculations are compared with the experiments, and a good agreement is obtained. It is shown that the numerical results are very sensitive to the choice of the boundary conditions imposed on the artificially introduced numerical boundaries. The boundary condition giving the best results is found to be the ambient gas condition. It is shown that the global jet structure with a nearly regular shock pattern, wich is stable and steady itself, is destabilized by the vortex rings (Kelvin-Helmholtz roll-up) on the jet boundary. These vortices produce shocks inside the jet, which are convected downstream with the eddies. This strongly suggests that a time-independent or a time-converged solution cannot be expected without making a suitable time-averaging of the time-dependent solutions