VOL. 11, NO. 17, SEPTEMBER 2016 ISSN 1819-6608 ARPN Journal of Engineering and Applied Sciences © 2006-2016 Asian Research Publishing Network (ARPN). All rights reserved. www.arpnjournals.com 10624 CONTROL OF SUDDENLY EXPANDED FLOW AT LOW L/D RATIO AND HIGH MACH NUMBERS Zakir Ilahi Chaudhary 1 and Vilas B. Shinde 2 1 MED, DMCE, Airoli, Navi Mumbai, MHSSCE, Mumbai, India 2 NHITM, Department of Mechanical Engineering, DMCE, Airoli, Navi Mumbai, Maharashtra, India E-Mail:zakirilahi@gmail.com ABSTRACT In this paper results reported are the outcome of investigation carried to regulate the suddenly expanded flow through a converging-diverging nozzle at high supersonic Mach numbers and at lower length to diameter ratios. The geometrical and the flow variables assessed in this investigation are the diameter ratio, length to diameter (L/D) ratio, expansion level, and the Mach number. The step heights of the present case are 1.6, 1.8, 2.2 and 2.5. The tests were conducted for P0/Patm in the range of 3 to 11. The (L/D) ratio of the suddenly expanded duct of the test was from 10 to 1, however, the outcome are presented for L/D 4, 3, 2, and 1. The Mach numbers tested in the investigation are 1.9, 2.1, 2.4, and 2.8. The results show that the efficacy of the active control is considerable in regulating the base pressure. Further, it found that for Mach numbers 1.9 and 2.1 the control is very effective for NPRs 9 and 11 but, for NPRs 3, 5, and 7 the control efficiency is only marginal. At NPRs 7 and 9 for some cases peculiar trends are observed and the flow becomes oscillatory. It is seen that majority of the outcomes represent similar behaviour for duct length up to L = 3D, which means; that the back pressure has not negatively affected the flow field in the base region as well as in the duct. The lowest duct length needed for the flow to remain latched on with the duct is L/D = 3 in general for all the cases of the flow variables of the current investigation, however, for few combinations of the flow and geometrical parameters the flow remains attached even for L/D =2 as well as 1. With this it can be stated that the micro jets can be used as one of the methods for flow in the base area control without having any untoward effect in the flow field at the base region. Keywords: L/D ratio, sudden expansion, base pressure, NPR, Mach no. 1. INTRODUCTION The control of base pressure on nozzle base is a very important field of study and finds application in many areas. Some of the several applications are the rocket nozzle base pressure field, flow field at the base of the fuselage of the aircraft, and the blunt base of the projectiles. The base pressure for a rocket nozzle is reduced due to expansion fans sitting at the edge of the base. Thus the hot gases coming out of the nozzle tends to fill this area. This is undesirable since the high temperature of the gases is continuously felt at the base area. This experiment does not simulate the flow conditions on a rocket nozzle completely because the model is stationary. Still the results can give a very good idea on how to control the base pressure. 2. LITERATURE SURVEY Rathakrishnan [1] studied the influence of ribs on the flow quality in suddenly expanded duct. He found that regulators as ribs results in the reduction of the base pressure up to a large extent, in comparison to that for without rib case. Annular ribs with L/D ratio 3:1 suit to be most appropriate combination for the given variables. Further, it is found that the ribs in the form of passive control do not cause any unsteadiness in the flow field of the suddenly expanded duct and any hike in pressure loss compared to without passive control at duct is < 6%. Khan and Rathakrishnan [2-6] conducted experiments to control the base pressure for Mach numbers 2.0 to 3.0. The studies were conducted at an over-expansion level of (Pe/Pa) =0.277. From their research they concluded that the base pressure attains the lowest value for the lowest area ratio namely 2.56 and the L = 6D for all the inertia levels of Mach numbers (2.0, 2.5 and 3.0). The active control seems to be to deliver encouraging outcome at M = 3.0 and A2 = 2.56A1. It was observed that the flow regulators enhance the pressure in the base region for these combinations of variables, amounting in an 83% reduction in base pressure. Ashfaq et al. [9] presented the outcome of the investigation to control the base pressure from a conical nozzle to discover the influence of NPR with sudden expansion at M = 1.0. From the investigation it was concluded that favourable pressure gradient need not give positive results every time. Zakir Ilahi Chaudhary et al. [11-13] from their test observed that the repercussion of Micro jets is very small in some cases in manipulating the pressure in the base area even under the influence of encouraging pressure variation at lower NPRs such as 3 and 5. Further they discussed that the micro jets affect the base pressure in positive as well as negative manner. The favourable and adverse nature of influence was found to be administered by the NPR and therefore the level of expansion. For lesser Mach numbers the base pressure tends to increase with the increase in NPR. The useful increase in base pressure is in the range of 10 to 40 per cent, which is very considerable. These outcomes will be handy for bodies moving at transonic Mach number, where the base drag is huge even a little reduction will give significant increase in the range and hence saving in the energy.