Effects of Mach number on the development of a subsonic rectangular jet

Abstract

ASUBSONIC jet of air emanating from a rectangular nozzle of intermediate aspect ratio has been investigated using hot-wire anemometry. The purpose of this investigation was to extend our knowledge of a rectangular jet from incompressible to compressible flow. Results of the hot-wire measurements show that a compressible subsonic rectangular jet behaves in a manner similar to that of an incompressible jet. It exhibits three distinct regions characterized by the decay of the mean axial velocity along the axis of the jet. These three regions are a potential core region, a two-dimensiona l type region, and an axisymmetric type region. The principal effect of increasing the Mach number of the rectangular jet was found to be the extension of the two-dimensional type region, and the jet behaves more nearly like a two-dimensional jet at higher Mach numbers. Contents The experiment was carried out on a rectangular nozzle, 50 mm long (L) and 3 mm wide (D), which was preceded by a 40 mm long smooth channel (50x3 mm). A Cartesian coordinate system was used with its origin located at the center of the nozzle and the X axis oriented along the centerline of the jet. The Y and Z axes are parallel to the long and short dimensions of the nozzle, respectively. Four exit Mach numbers (M^=0.18, 0.3, 0.5, and 0.8) were studied in this experiment. The corresponding Reynolds numbers based on the width of the nozzle were 12,000-52,000. For all the Mach numbers studied, a top-hat mean velocity profile with a centerline turbulence intensity of about 0.4% was obtained at the nozzle exit. Measurements were made with standard constant temperature anemometer system (DISA 55P11, 55M10, and 55M25). The mean hot-wire output voltage was linearized for a velocity range of 20-280 m/s, with the voltage directly proportional to the velocity. The variation of the (squared) normalized mean velocity along the centerline of the jet for four exit Mach numbers is shown in Fig. 1. The centerline mean velocity Uc is normalized with respect to the mean velocity at the center of the nozzle exit U0. The three regions of an incompressible rectangular jet as described above can be identified in this figure. It appears that the jet decay rate has a fairly weak dependence on the Mach number.