It is prevailingly known that acoustic sound frequency emanated from the trailing edge of two-dimensional airfoil varies with the so-called ladder-like structure and discontinuously jumps to other ladder-like states with increase of the freestream velocity. In order to unravel such critical conditions resulting in the ladder-like structure, two-dimensional jet with no aerodynamic sound emission impinging to still air is used, because acoustic sound is accepted to the flow at the jet exit. That is, when unstable disturbances growing in shear layer of the jet are excited by a loud speaker, the acoustic feedback loop automatically selects one frequency from broad-band frequencies of the shear layer. Ladder-like structures are found to be similar to airfoil trailing-edge noise. It is also found that each rung slope and jump frequency between ladder-like states show good agreement with existing empirical model. Furthermore, when the remnant of the distance between speaker and jet divided by wavelength of a selected acoustic sound is equivalent to half wavelength of sound, selected frequency jumps to other state. Also, hysteresis phenomenon is observed in cases that the speaker approaches to the jet flow or goes away.
Recent research has studied damaged fuselage panels made with CFRP laminate. The damage is removed with a squared hole and repaired with a patch. Two repair methods have been proposed. One is bonding a small CFRP patch as a time-limited repair to withstand the limit load, and the other is bonding a large CFRP patch as a permanent repair to withstand the ultimate load. Test panels repaired with these methods are loaded with tension until fracture. The test results show that a time-limited repair with the bonding small CFRP patch is possible, and that the two types of repair panels have different initial points of fracture. This paper also proposes the repair method of fastening a titanium patch as a time-limited repair. Design techniques of test panels for these three repair methods are presented, and simple methods to predict the fracture load of fastening and bonding repair are proposed. The fastening titanium patch is also shown to be possible as a time-limited repair. The test results of each repair method show that the design techniques are useful, and that simple fracture load prediction methods are highly consistent with the test results under limited test data.
Our experimental studies showed that there were two different combustion regions in the combustion chamber of the swirling-oxidizer-flow-type hybrid rocket engine. The front region near the injector had higher values of the local fuel regression rate than those in the rear region, and the rates in the rear region were almost the same along the length direction. We established the combustion model to predict the engine performance with swirling oxidizer flow, in which the combustion chamber region was divided into the front and rear regions. In the front region, the local fuel regression rates were given as time constant values independent of the grain port area. In the rear region, the regression rates were given by a new formula which included the effect of the swirling oxidizer flow. It was found that the simulation results were well in agreement with the experimental results about the time variations and values of the chamber pressure and thrust. The issues in application of this new prediction method to the other swirling-flow-type engines using a different fuel and an injector were discussed.
In this paper, a method for identifying an impact force acting on a CFRP laminated plate is proposed. The location and force history of the impact force are identified using the sound radiation from the impacted plate. Microphones are used to measure the sound pressures at multiple locations. The impact location is identified from the arrival times of the sound wave to the microphones. For the force history identification, experimental transfer matrices which relate the impact force and the measured sound pressures are used. The force history is identified by minimizing the deviation between the measured and estimated sound pressures where the estimated ones are obtained using the experimental transfer matrices. The validity of the proposed method is verified experimentally by performing impact force identification of CFRP laminated plates. In addition, the effect of background noise on the accuracy of the identification results is also examined. The results reveal that the proposed method is capable of accurately identifying the impact location and force history from the radiated sound.