On the Existence of Subharmonic Screech in Choked Circular Jets from a Sharp-Edged Orifice

Experiments are performed in choked circular hot and cold nitrogen jets issuing from a 2.44 cm diameter sharp-edged orifice at a fully expanded jet Mach number of 1.85 in an effort to investigate the character of screech phenomenon. The stagnation temperature of the cold and the hot jets are 299 K and 319 K respectively. The axial distribution of the centerline Mach number was obtained with a pitot tube, while the screech data (frequency and amplitude) at different axial and radial stations were measured with the aid of microphones. The fundamental screech frequency of the hot jet is slightly increased relative to that of the cold jet. It is concluded that temperature effects on the screech amplitude are manifested with regard to the fundamental and the subharmonic even at relatively small temperature range considered.

Optical superoscillatory waves without side lobes along a symmetric cut

Optical superoscillation refers to an intriguing phenomenon of a wave packet that can oscillate locally faster than its highest Fourier component,which potentially produces an extremely localized wave in the far field.It provides an alternative way to overcome the diffraction limit,hence improving the resolution of an optical microscopy system.However,the optical superoscillatory waves are inevitably accompanied by strong side lobes,which limits their fields of view and,hence,potential applications.Here,we report both experimentally and theoretically a new superoscillatory wave form,which not only produces significant feature size down to deep subwavelength,but also completely eliminates side lobes in a particular dimension.We demonstrate a new mechanism for achieving such a wave form based on a pair of moonlike sharp-edge apertures.The resultant superoscillatory wave exhibits Bessel-like forms,hence allowing long-distance propagation of subwavelength structures.The result facilitates the study of optical superoscillation and on a fundamental level eliminates the compromise between the superoscillatory feature size and the field of view.

Two－Phase Critical Discharge of Initially Saturated or Subcooled Water Flowing in Sharp－Edged Tubes at High Pressure

The transient critical flow experiment with sharp-edged tubes as the break geometries is conducted in high pressure convective circulation test loop of Xi'an Jiaotong University. The initial steady operation pressure is up to 22.OMPa. An empirical correlation was made to obtain the critical mass flow rates, the critical pressure ratio and the thermal nonequilibrium number were correlated as the functions of the tube length to tube diameter ratio L/D. The predicted critical mass now rate gets a higher accuracy for short tubes with L/D <12, the prediction accuracy is higher than that of Henry model prediction for long tubes with L/D > 12.

AN IMPROVED NON-TRADITIONAL FINITE ELEMENT FORMULATION FOR SOLVING THE ELLIPTIC INTERFACE PROBLEMS

We propose a non-traditional finite element method with non-body-fitting grids to solve the matrix coefficient elliptic equations with sharp-edged interfaces. All possible situations that the interface cuts the grid are considered. Both Diriehlet and Neumann boundary conditions are discussed. The coefficient matrix data can be given only on the grids, rather than an analytical function. Extensive numerical experiments show that this method is second order accurate in the L∞ norm.