Combined effect of nozzle pressure ratio and screech prone supersonic mach number in a suddenly expanded flow

This paper presents the results of an experimental study to evaluate the effectiveness of the micro jets to control the base pressure in a suddenly expanded flow at supersonic Mach numbers. Four micro jets of 1mm orifice diameter located at 90 intervals along a pitch circle diameter of 1.3 times the nozzle exit diameter in the base region were employed as active controls. The Mach numbers of the present study were 1.8 and 2.0. The jets were expanded suddenly into an axi-symmetric circular tube with cross-sectional area 2.56, 3.24, 4.84 and 6.25 times that of the nozzle exit area. The Length to Diameter ratio of the suddenly expanded duct was varied from 10 to 1 and experiment were conducted for Nozzle Pressure Ratio (NPR) from 3 to 11. Jets were over, under, and correctly expanded depending upon the NPR of the respective runs. When flow from the nozzle was over expanded or under expanded an oblique shock or expansion fan will be positioned at the nozzle lip, which in turn will result in increase or decrease of the base pressure. From the results it was observed that at NPRs 3 the control was not effective, however, at NPR 5, 7, 9, and 11 a significant change in the base pressure for all the area ratios was seen. From the results it was concluded that the level of expansion, Mach number, length-to-diameter ratio, and area ratio played an important role to fix the value of the base pressure and the control effectiveness by the micro jets

Experimental investigation on the effectiveness of active control mechanism on base pressure at low supersonic mach numbers

In the current investigation, the experiments were carried out to evaluate the effectiveness of microjets in controlling the base pressure from a convergentdivergent nozzle at low supersonic Mach at different expansion level. Tests were carried out for low supersonic Mach numbers 1.25, 1.3, 1.48, and 1.6 while nozzle pressure ratio ranges from 3 to 11. The jets are augmented abruptly into an axisymmetric circular channel with different cross-sectional areas as that of nozzle exit area. The results show that the proficiency of the microjets is only marginal in controlling the base pressure even under the influence of favorable pressure gradient at lower NPRs namely 3 and 5. It was also observed that for higher values of the NPRs such as 7, 9, and 11, the dynamic control by very small jets results in rise of base pressure for the different values of the L/D ratios of these investigations. For NPRs 5 and 7, the trend differs due to the level of expansion, nature of waves present in the base region, relief available to the flow, length to diameter ratio of the enlarged duct, and the Mach numbers. It is seen that most of the cases exhibit similar behavior for higher as well as the lower length to diameter ratios, which means; that the back pressure has not adversely influenced the flow field in the base region as well as in the duct. With this it can be stated that the microjets can be an alternative for the experimentalist for base pressure control in the form of microjets

Control of wall pressure flow field with micro jets and control effectiveness

This paper presents an experimental study of airflow from convergent-divergentnozzles discharged into enlarged duct, focusing attention on the flow development in the duct. To investigate the influence of active control on the flow field developed in the duct, the tiny jets of 0.05 mmradiuslocated at 900interval along the base at 6.5 mm from the main jet were employed. The Mach number tested in the present study was 1.48. The area ratio tested was 2.56. The NPR tested are from 3, 5, 7, and 9 respectively. The L/D ratio of the duct was varied from 10 to 1. The level of expansion at the nozzle exit influences the wall pressure very strongly. When the micro jets were activated they found to influence the base region, taking the wall pressure to considerably higher values, for most of the cases. Flow field in the duct with and without control remains the same

Control of suddenly expanded flows from correctly expanded and under expanded nozzles at supersonic mach number for area ratio 2.56

Airflow from convergent-divergent axisymmetric nozzles expanded into circular duct of higher area were studied experimentally, with the main focus on the flow development in the duct. The flow parameters considered in this investigation are the Mach number at the nozzle exit and the level of expansion. The geometrical parameters considered are the area ratio and the L/D ratio of the duct. To control on the flow field developed in the duct, four tiny jets of 0.5 mm radius located at 90 degrees intervals at 6.5 mm radius from the centre of the nozzle exit were employed as the flow regulators. The Mach number in this case was 1.48. The area ratio of the study was 2.56. The nozzle pressure ratio (NPR) used was from 3.27 and 5.35 respectively which corresponds to ideally expanded and under expanded cases. The L/D ratio of the enlarged duct was varied from 10 to 1. From the results it is found that there is a jump in the wall pressure when flow passes through first oblique shock and this trend is repeated for all the cases.From the results it is found that the entire flow field is full of waves and flow remains identical with and without control when the jets were operated under favourable and adverse pressure gradient, it is also observed that when flow regulators in the form of tiny jets are employed they do not disturb the flow of duct

Experimental and numerical studies of porous media combustion in micro burner

Porous media combustion has been key point of interest for researchers from past couple of decades due its numerous advantages, such as remarkable low emission levels without compromising thermal efficiency. In this present work, dual layered micro burner was built with predefined thickness of reaction and preheat layer. Reaction layer was made up of alumina (discrete type) while preheat layer porcelain (foam type) material. Burner was successfully built to undergo both surface and submerged flames. A concept of equivalences ratio was enabled since its premixed combustion with natural air as one of the inlet along with butane. Trials were not just restricted at stoichiometric ratio but also carried out up to ultra-lean region. Additionally, numerical simulation was performed using commercially available computational fluid dynamics package so that porous media combustion phenomenon can be better analyzed and predicted. Finally, Thermal efficiency was calculated at critical equivalence ratios and emission parameters such as NOx and CO was continuously monitored which were under controlled limits

Investigation on the prominence of abrupt expansion on the base pressure of an axi-symmetric body

This investigation presents the outcome of the tests conducted to control the base flows at supersonic Mach numbers. Also the efficiency of the flow controllers to govern the pressure in the base region in a rapidly expanded duct has been exercised. Four tiny jets of 1mm diameter are positioned at 90 intervals at a distance of 6.5 mm from the central axis of the main jet. The inertia levels of the abruptly expanded flows are 1.25, 1.3, 1.48, 1.6, 1.8, 2.0, 2.5 and 3.0. These jets are connected by an axi-symmetric circular brass tube whose cross-sectional area was 2.56, 3.24, 4.84 and 6.25 respectively. The L/D ratio of the enlarged duct was varied from 10 to 1 and NPR was varied from 3 to 11. However, the results presented were for Low Length to Diameter ratio, equal to 4. It was found that when the flow was discharged to the ducts of the given area ratios, it remained attached with the duct wall for all the inertia levels and the NPRs tested in the present case. It was found that the expansion level plays a significant role to decide the pressure at the base and its control efficacy. Whenever, the flow is over expanded, an oblique shock is formed at the nozzle lip, which in turn leads to enhancement of the pressure in the base region. The formation of the shock waves, reflection and recombination continued till the pressure becomes atmospheric. It was observed that the flow remains attached even for low length-to-diameter ratio, equal to 4. No adverse effect of back pressure was observed during the test. It was found that the micro jets can serve as controllers for the base pressure

Investigation on Three-Dimensional CFD Validation for a Variable Span Morphing Wing

This paper presents the computational design validation of a morphing wing profile that allows a change in the wing aspect ratio. Generally, the aerodynamic performance can be enhanced by changing the aspect ratio of the wing. Variable span morphing wing allows two distinct airfoil sections of the wing to extend its span using another section employing strap driven mechanism. The primary airfoil model section is maintained by NACA 4412 with span of 1.05 meters with full extension to 1.50 meters. Therefore, both two dimensional and three dimensional computational studies are carried out in a span morphed wing and un-morphed wing for various angles of attack respectively. It is found that the extended wing gives better aerodynamic performance and improved range. However, the separation phenomenon and turbulence occurs in the transition regions of wing profile. CFD FLUENT are used to understand the different flow patterns and pressure variations over the various sections of the wing. This study also demonstrates the potential effectiveness of a model meshing on the simulation and emphasizes the sensitivity of the solution outcome to the model solution setu

Energy harvesting from aerodynamic instabilities: Current prospect and future trends

This paper evaluates the layout and advancement of energy harvesting based on aerodynamic instabilities of an aircraft. Vibration and thermoelectric energy harvesters are substantiated as most suitable alternative low-power sources for aerospace applications. Furthermore, the facility associated with the aircraft applications in harvesting the mechanical vibrations and converting it to electric energy has fascinated the researchers. These devices are designed as an alternative to a battery-based solution especially for small air-crafts, wireless structural health monitoring for aircraft systems, and harvester plates employed in UAVs to enhance the endurance and operational flight missions. We will emphasize on various sources of energy harvesting that are designed to come from aerodynamic flow-induced vibrations, specific attention is then given at those technologies that may offer, today or in the near future, a potential benefit to reduce both the cost and emissions of the aviation industry. The advancements achieved in the energy harvesting based on aerodynamic instabilities show very good scope for many piezoelectric harvesters in the field of aerospace, specifically green aviation technology in the future

Dynamic stability of unguided projectile with 6- DOF trajectory modeling

The paper presents the study of unguided rolling projectiles at varying Mach numbers using aerodynamic coefficients. The aerodynamic coefficients are estimated using an aerodynamic prediction code, Missile DATCOM. The predicted dynamic derivatives will determine the design criteria, and also their effect on the design aspects (stability and accuracy) of the projectile. To satisfy the condition of stability for the trajectory of projectile, a model of 6 DOF equations has been used. The result parameters, such as static margin of the projectile, force and moment derivatives in all the three modes, spin rate, angle of attack, and time rate are shown. The accuracy of the rolling moment derivatives is uncertain due to the limitation of DATCOM, but it is the general method applicable at the initial design stage of any projectile and therefore falls in the realm of aerodynamic database. The results address the problems of static and dynamic stability by giving initial perturbations, effect of spin on thrust misalignment, and effect of variation in the geometric parameters during the power on and off cases. Therefore, there is a need for full aerodynamic characterization of the projectile, in which sufficient derivatives are computed for accurate results

Estimation of stability derivatives in pitch for an oscillating 2-D wedge in supersonic Flow

A similitude has been obtained for a pitching oscillating two dimensional wedge with attached bow shock at high angle of incidence in supersonic flow. A strip theory given by Ghosh [1] for flow in spanwise location has been used. For the two-dimensional flow, a normal shock is formed directly in front of the body and becomes a curved oblique shock as it extends around the body. This integrates with the similitude model leading to one-dimensional piston theory reformulated by Ghosh [1] from initial works of Lighthill [2] and Miles [3]. Therefore, precise form of simple relations for damping and stiffness derivatives in pitch has been obtained. The present theory is valid only for shock wave which is attached with the nose of the wedge. From the combination of theory developed, we can evaluate the derivatives for stiffness and damping in pitch for a wide range of supersonic Mach numbers, for various pivot positions, and angle of incidence. Substantially the same results are being validated with the analytical results of Liu and Hui [4] with great simplicity