Volume 16, Issue 4, April 1945
Index of content:
16(1945); http://dx.doi.org/10.1121/1.1916285View Description Hide Description
16(1945); http://dx.doi.org/10.1121/1.1916286View Description Hide Description
The supersonic beam radiated from a one‐centimeter radius X‐cut quartz disk is examined experimentally as a function of the spherical curvature of the crystal, at a frequency of 1110 kilocycles. The crystals of large curvature produce a definite focusing action as has been reported from an earlier investigation. The experimental data show that it is possible to obtain a much greater excess pressure amplitude close to the generator from a curved crystal than is possible from a flat crystal having the same area and thickness using the same input power. The quartz plates of small curvature do not produce a marked focusing action but give a larger supersonic amplitude at large distances from the generator. The flat crystal gives the smallest angle of spread of the supersonic beam. The resonant frequency for any of the five crystals studied did not differ by more than 20 kilocycles (2 percent) from that to be expected from the thickness.
16(1945); http://dx.doi.org/10.1121/1.1916287View Description Hide Description
16(1945); http://dx.doi.org/10.1121/1.1916288View Description Hide Description
The flames considered are those which maintain vibration of the air in a surrounding “air tube.” Earlier theories are reviewed. Rayleigh's theory involves standing waves in the gas supply tube and leads to certain lengths of gas tube for which singing does not occur. It is now found that maintenance of vibration in the air tube may be excellent when the gas tube is so long that no appreciable standing waves are formed in it, and that under suitable conditions there are no lengths of gas tube for which a flame will not sing. Rayleigh's theory is modified by taking into account (1) the increase in viscosity which the flame brings about in the orifice from which it burns and (2) the progressive waves that run downward through the gas tube. These waves involve a maximum rate of efflux of gas near the phase of least pressure in the air tube, whereas Rayleigh's theory involves a maximum rate of efflux a quarter of a period earlier or later, depending on the length of the gas tube. If the most rapid efflux is near the phase of least pressure, and if the flame gases rise from the orifice to the top of the flame in approximately half a period, maintenance is possible. In thirteen cases where the flame sang, values obtained for the time of rise run from 0.6T to 0.9T; and in sixteen cases where the flame was silent, they run from 1.2T to 3.3T. In view of approximations made in reaching these results they are regarded as satisfactory. When the gas tube is of only moderate length, standing waves in it may be superposed on the running waves. This superposition leads to the possibility of lengths of gas tube for which the flame will not sing. Regions of silence are more likely to occur when the flame is small. The size of the flame is also a factor in determining the distance to which it must be inserted in the air tube before maintenance is possible. The distance is a minimum for flames of moderate size. The amended theory gives explanations of these facts and of several others. The initiation of the vibration is attributed to the slight disturbances that are always present in the atmosphere.
16(1945); http://dx.doi.org/10.1121/1.1916290View Description Hide Description