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Vol. 13, No. 3 (1957)

1957-02-05
Topics
CONTENT
ON THE PARAMAGNETIC ANISOTROPY OF SINGLE CRYSTALS OF CHROME ALUMS
HSIANC JEN-SHENG
1957, 36 (3): 177-180. doi: 10.7498/aps.13.177
Abstract +
The existence of anisotropy in paramagnetic cubic crystals has never been observed and it was only vaguely conjectured by Schlapp and Penney in 1932 for chrome alums. In this paper, a calculation based on P. R. Weiss's theory (1948) shows that the paramagnetic anisotropy does exist in the single crystals of chrome alums. Percentage differences (γ ) between the susceptibility along direction (χ) and that along direction (χ) in KCr alum single crystal at room temperature are given for several values of magnetic field. It is pointed out that the ordinary concept of principal susceptibilities along respectively three mutually perpendicular magnetic axes is no longer applicable to describe the paramagnetic anisotropy of the cubic crystals of chrome alums.
THE ELECTRIC MULTIPOLE RADIATION EMITTED BY A NEUTRON
TZU HUNG-YUAN
1957, 36 (3): 217-219. doi: 10.7498/aps.13.217
Abstract +
It is pointed out, that that estimation made by Stech about the transition probability of a neutron emitting electric multipole radiation is not correct. The actual value of the transition matrix is not larger than that estimated by Weisskopf.
MODERN DEVELOPMENTS IN THE THEORY OF TURBULENCE
CHOU PEI-YUAN
1957, 36 (3): 220-244. doi: 10.7498/aps.13.220
Abstract +
This is a critical exposition and analysis of the modern developments in the theory of turbulent motion of an incompressible fluid. We begin with the review of the mixture length theories based upon Reynolds' equations of mean motion. Secondly, we analyze the principal contributions to the theory of homogeneous isotropic turbulence. Thirdly, we discuss the treatment of the general turbulent shear flow by means of Reynolds' equations of mean motion and the dynamical equations of velocity correlations which are derived from the equations of turbulent velocity fluctuation. We also point out at the same time that although this method yields theoretical results which are in better agreement with experiment than the results of the mixture length theories and furthermore the theory also leads to the theoretical distributions ot the mean squares of velocity fluctuation, on account of the presence of the higher order velocity correlations in the equations, it continuously leads to unclosed systems of differential equations and hence meets difficulties which are difficult to overcome. Therefore, based upon the above retrospect of the developments of the theory of turbulence and the recent work on the vorticity structure of the homogeneous isotropic turbulence in its final period of decay, we finally propose a new approach to the turbulence problem: The basic component motion of turbulence is vortex motion due to the action of viscosity of the fluid. The dynamical equations which govern the vortex motion of turbulence are Reynolds' equations of mean motion and the equations of velocity fluctuation derived from the Navier-Stokes equations by the averaging process. We also emphasize the importance of Reynolds' recognition that the turbulent motion of a fluid can be separated into the mean motion and fluctuation. The future theoretical investigation is to look for the vortex motions which are solutions of these two sets of equations. In order to make the solutions of the problem unique and comparable with experimental measurements, they should also satisfy statistical conditions on the distribution of vortices analogous to Kol-mogoroffs condition in his statistical theory of locally isotropic turbulence at high Reynolds number turbulent flows.