A method to estimate the dipole source noise of a rotor was presented. The method utilizes the Local Momentum Theory for the prediction of the angle of attack distribution on the disk. The influence of the angle of attack distribution on the rotor disk upon the acoustic waveform was discussed. In the present method, a new parameter which specifies the non-linear acoustic directivity caused by large blade pitch angle, twist, camber, or thickness ratio was introduced. This parameter has shown to give the noise level more than 20% larger than the previous result for the rotor having zero-twist angle, 4° pitch angle, and NACA 0010 section shaped blade. The noise level and the acoustic waveform computed from the present theory were compared with those computed from the previous theory for the same rotor thrust. The most distinctive feature of the present method comparing with the previous one is to use the variable angle of attack distribution on the disk instead of the uniform angle of attack distribution. The difference in noise level was not so big but the waveform computed from the present theory has shown many peaks caused by blade-vortex interactions whereas the previous one was smooth and simple. Strong correlation between the sudden increase in the angle of attack and the negative peak in the observed waveform was shown for the rotor in the upwash field where the interaction effect was intense especially when the direction of the tip vortex shedded from the former blade coinside with the span direction of the following blade. Then it was concluded that the non-uniform distribution of the angle of attack on the rotor disk should be adopted for the estimation of the rotor noise of helicopter in forward flight or in descending flight to account intense blade-vortex interactions.
The generation and breakdown of longitudinal vortices in the boundary layer along a concave wall has been studied experimentally. The experiments have been carried out using the concave wall 1m in radius of curvature at air velocity range 2 to 5m/s. The air flow has been made visible by evaporated-gas-oil smoke, and the flow patterns have been stereographically photographed from upper, lateral and rear sides with Strobo. It is noticeable that the vortices are formed into pairs of ones downstream, and that arch shaped vortices which lie across each of these pairs generate periodically and gradually break down. Such phenomena are compared with velocity distributions and frequency of the arch shaped vortices. The mechanism of these vortices is also investigated.
The wall structure of the turbulent boundary layer in adverse pressure gradient with U1∝(x-x0)-0.20 has been investigated to study the bursting phenomenon and the relationship between the wall structure and a large scale motion of the outer layer. Conditional sampling of the fluctuation of wall pressure and velocities has been made and the space-time correlations between the wall pressure and the velocity components and those of velocities have been measured. In the wall region, conditional sampling shows that, the wall pressure fluctuation becomes low when the normal velocity is directed outwards in region of streamwise momentum deficit, and that, when it is directed inwards in region of streamwise momentum excess, the wall pressure fluctuation exceeds its mean value. These results give a plausible explanation to the observed behaviour of the space-time correlation between the wall pressure and the velocity components in the wall region. Space-time correlation of the velocities reveals that the coherent structure with the oblique angle to the wall exists in the boundary layer.
Nonsteady load distributions on two-dimensional thin airfoils oscillating in an incompressible inviscid flow are governed by POSSIO'S integral equation, which is solved numerically through appropriate manipulation of logarithmic singular- ity. When a small gap is introduced onto a lifting airfoil, the airfoil is divided into two parts by a method of two semi-circles. Then, the integral equation can be solved numerically by the similar procedure as in reference 5). As an alternative method, this problem is attacked analytically through application of the method of matched asymptotic expansions. The outer solution is expressed assuming the flow rate through the gap to be a form of DIRAC'S delta function. The flow near the gap is govetned by an inner solution. The ratio of wing thickness to gap is assumed to be small. The results obtained by two approaches, numerical and matched asymptotic expansions, agree closely each other. In the appendix a simplified method is also presented, where the inner solution need not be considered. The strengths of gap flow are obtained by the KUTTA condition which is imposed at the front edge of the gap.
The fracture process of a brittle beam under transverse impulsive loading is studied. The mechanisms and time sequence of deformation and fracture are determined by high-speed framing camera photographs, strain gages and terminal observation. The method of characteristics is used to solve the TIMOSHENKO beam equations to predict the response of a brittle beam, including fracture. Comparisons between computed and measured strains show that the effect of an initial, central fracture in a beam on the subsequent structural response can be approximated by a two-stage fracture model which specifies how the bending moment at the fracture point is reduced to zero after the initiation of fracture.