Semi-empirical prediction of tone noise due to counter-rotating open rotors
Counter-rotating open rotors (CROR) were extensively studied to power medium-size aircraft in the 80s aft er the first increases in fuel costs. Indeed, their efficiency i s greater than that of turbofans and of single prop fans. They have again become a topical subject due to the new incre ase of fuel costs and to the risk of oil shortage. They however raise a serious acoustic issue because noise is gen erated by the two rotors and by their interactions, and due to the lack of any shielding nacelle. An original and fast semi-empirical method is proposed to predict radia ted sound levels. The objective is to rapidly assess if certification rules can be fulfilled, and to estimate the possib le impact of a future fleet on noise contours around airports. This work is thus focused on takeoff and approach conditions (advancing Mach number lower than 0.3). It is shown that direc tivity of a tone is mainly determined by a Bessel fu nction, and a parabolic pattern is suggested for overall sound pr essure levels. The shape of third-octave spectra is based on the large number of interaction tones which are present in each frequency band. Finally, some information is given on overall sound levels.
Installation effects on counter rotating propeller noise
General Electric has developed an Ultra-high bypass open rotor engine(UDF@’ engine)When Boeing was studying the use this engine with counter rotating propulsors, possible noise increase due to installation on the aft end of the fuselage was one of the concerns. Such noise increase could be due to the inflow distortion and acoustic diffraction. To determine the installation effects, a test of a scale model of the U D e r o puisor was conducted at the German-Dutch wind tunnel (DNW) in Holland. To simulate installation effects, models of pylon, fore-shortened-aft-fuselage and empennage were used. These could be added in various combinations to enable effects of individual items to be determined. A pylon trailing edge blowing technique was tested to reduce the impact of installation effects. The test results showed that the addition of the pylon upstream of the propulsor could increase the front rotor blade passage frequency(BPF) by as much as 10-12 dB with negligible effect on interaction tones. Addition of the fuselage increased the front rotor BPF by an additional 2-3 dB and also impacted the interaction tones. The root-flow at the attachment point of the pylon to the fuselage was also seen as a source of noise increase. BPF of the front rotor was seen to increase monotonically as the pylon was brought close to the propulsor and when the fuselage was moved closer to the propulsor tip. Pylon trailing edge blowing technique was found to be successful in reducing the pylon wake and the root-flow distortion and was capable of bringing the installed configuration front BPF levels back down close to the isolated propulsor levels. Comparison of data taken at Boeing Transonic Wind Tunnel facility with that taken at DNW agreed reasonably well providing confidence in repeatability of model propulsor data and in noise data taken at BTWT with acoustically treated tunnel walls.
Optical absorption and carrier‐induced bleaching effect in quantum wire and quantum box structures
The optical absorption spectra of quantum wires and boxes are studied theoretically to evaluate the carrier‐induced bleaching effect. It is found that the absorption coefficients of these highly confined systems (HCSs) and their changes with the introduction of carriers can be far greater than those of less‐confined systems (LCSs) such as quantum wells (films) and bulk semiconductors. In addition, the carrier‐induced bleaching effect of HCS is found to remain very high at room temperature while that of LCS is substantially weakened by the thermal distribution of carriers.