Effectiveness of Micro-Jets at Different Level of Expansion

Abstract

An experimental investigation was conducted to study the effectiveness of micro jets to control base pressure in suddenly expanded axi-symmetric duct. Since active control is used to control the base pressure and hence the base drag the air is drawn from the main settling chamber to the control chamber to control the base pressure, hence it doesn't require additional source of energy for the control mechanism. The flow parameters considered in this investigation are the Mach number (1.2, 1.3, 1.5, 1.9, 2.2, 2.6, and 3.0) at the exit of the nozzle, and the ratio of the pressure in the main settling chamber to the ambient pressure (NPR). The geometrical parameters considered are the area ratio between the sudden expansion duct cross-section and the nozzle exit area, and the duct length-to-diameter ratio (10 to 1). The area ratio considered was 3.24. The experiments were done for NPRs in the range from 3 to 11 in a step of 2 for all the Mach numbers. The wall pressure distribution in the suddenly expanded duct was also measured and it is seen from the static wall that the control does not affect the wall static pressure adversely. When the micro jets were activated they were found to influence the base region, taking the base pressure to considerably higher values compared to that for without control, for most of the cases. However, there are certain combinations of parameters for which the active control results in decrease of base pressure. Due to the importance of the problem connected with fluid dynamic drag, a vast number of investigations, both theoretical and experimental, considering base pressure and base drag have been performed. The rapid growth of interest in supersonic drag problems associated with the development of supersonic aircraft, projectiles, missiles, and spacecraft was one reason for the fact that the theory made greater advances at high Mach numbers than at low speeds and that more experiments were performed in this velocity range to verify the theoretical results. Because of its vide applicability, suddenly expanded flows have studied extensively. Many researchers attempted to control the base pressure with passive means and some of the works relevant to the present study are reviewed in the section to follow. Therefore, in the present study an attempt is made to investigate the effect of level of expansion, Mach number, Nozzle Pressure Ratio (NPR), L/D ratio and area ratio on base pressure with and without control in the form of micro jets. Flow field of sudden expansion is very complex which is characterized by the separation of the flow at the base, flow re-circulation at the base and the reattachment in the downstream of the enlarged duct. A shear layer which is exiting from the nozzle may be divided into two main regions, one is being the flow recirculation region at the base of the duct and the other is the main flow region as shown in (Fig. 1). II. Literature Review Nusselt (1929) appears to be one of the first to study experiments with high speed flow through ducts with abrupt increase in area. From his more extensive experiments with sonic and supersonic flow he concluded that the base pressure will be equal to the inlet pressure if the entrance velocity is subsonic but if the entrance flow is supersonic, the pressure could be equal to, lower than, or more than the entry pressure. If the entrance Mach number is unity, no area of the jet will be affected by the expansion waves from the opposite corner. Wicks (1953) studied experimentally the effect of boundary layer on flow through sudden expansion at critical Mach number. From their studies they found that the base pressure at the base region is function of the boundary layer thickness and type before the expansion. He assumed boundary layer as a source of fluid in the base region. Korst (1956) studied the problem of base flow for Mach number equal to one and greater than one for cases in which the flow downstream of the base is sonic or supersonic downstream of the wake. He developed a model based on the concepts of interface between the dispersive shear stream and the neighboring free stream and the conservation of mass in the wake.