Base Pressure Control using Micro-jets in Supersonic Flow Regimes

Base pressure plays a vital role in aerospace-related applications and its control is essential in reduction of drag and improving fuel consumption. Low pressure at the base of Rockets, Missiles, bombs and shells are a very common problem happening at transonic and supersonic speeds. In most of the cases there is a significant dip in pressure at the base region which will have implications on the design of aerospace vehicles. This paper presents an experimental investigation carried out for flow control at supersonic regimes. Experiments were conducted to measure the base pressure in the base region and wall pressure distribution in the enlarged duct at Mach 1.25 and 2.0, for L/D = 10, for four area ratios namely, 2.56, 3.24, 4.84, and 6.25. From the results it is found that active control in form of micro-jets is effective in boosting base pressure only at higher Mach numbers with larger area ratios at under-expanded conditions. Wall pressure flow field reveals that the quality of the flow with and without control is almost identical and there is no adverse effect of the control mechanism on the flow field of the duct. The variation in reattachment point along the downstream of the duct has very minimal effect with micro-jets as control

Oscillating supersonic delta wing with straight leading edges

A Supersonic similitude has been used to obtain stability derivatives in pitch and roll of a delta wing with straight leading edge for the attached shock case. Ghosh’s strip theory is been used in which strips at different span wise locations are independent of each other. This combines with the similitude to give a piston theory which gives the closed form of solution to stability derivatives in pitch and roll. Some of the results obtained have been compared with those of Hui et al ,Ghosh and Lui &Hui. Results have been obtained for supersonic flow of perfect gas over a wide range of Mach numbers, incidences and sweep angles

Control of nozzle flow using microjets at supersonic Mach Regime

This article reports the active control of base flows using the experimental procedure. Active control of base pressure helps in reducing the base drag in aerodynamic devices having suddenly expanded flows. Active control in the form of microjets having 0.5 mm radius placed at forty-five degrees apart is employed to control the base pressure. The Mach numbers of the present analysis are 1.7, 2.3, and 2.7. The length to diameter (L/D) ratio is varied from 10 to 1 and the nozzle pressure ratio (NPR) being changed from 1 to 10 in steps of 1 for base pressure measurements. The area ratio for the entire analysis is fixed at 2.56. Wall pressure distribution along the enlarged duct is also recorded. No change in base pressure increase/decrease is thoroughly analyzed as well. From the experimental investigation, it is found that control plays an important in modifying the base pressure without disturbing the wall pressure distribution. The base pressure variation is entirely different at L/D = 1 compared to a higher L/D ratio due to change in reattachment length and the requirement of the duct length at higher inertia levels. The quality of the flow in the duct in the presence and absence of control remained the same

Aerodynamics investigation of delta wing at low Reynold’s number

A numerical simulation has been carried out to investigate the aerodynamics of an oscillating delta wing and to evaluate the flow structure over the leading edge at different low Reynolds numbers. The numerical results of the coefficient of lift (CL) vs angle of attack are validated with the experimental results from the previous study. Effects of Reynolds number and angle of attack is investigated for the wing lift coefficient and the aerodynamic efficiency. Pressure contour and turbulence kinetic energy are also observed in each case. Vortex formation from each case is noted from pressure contour and lift coefficient. The stall condition is also observed. Considering the three Reynolds number at max CL, which is an angle of attack of 40 degrees, the CL only increased by 0.01, which is not a very significant increase. But L/D ratio has increased significantly for Reynolds number in the range from 8x104 to 1.5x105 and from Re 1.5x105 to 2.68x105. Whereas for each Reynolds number, the lift coefficient CL seems to attain the highest value of 0.4 at the angle of incidence between 10 degrees and -10 degrees to -20 degrees

Stability derivatives of a oscillating wedges in viscous hypersonic flow

In this paper an oscillating wedge has been considered, and the fluid slabs are kept at 900 to the wedge surface. The solutions of the continuity, momentum,and energy equations are obtained. By using the Rankine-Hugoniot relations for shockwaves, we can find the conditions behind the shock.This theory is unsteady one because of the consideration of effect of secondary wave reflections.Solutions are obtained for hypersonic flow over the wedge by varying different wedge semi vertex angles.These results shows extremely good consistency with Hui's predictions. When the effects of unsteadiness are considered then there is considerable change in the magnitude of the damping derivatives near the leading edge or initial 40 percent of the pivot positions and this difference is only marginal when we further down towards the trailing edge. However, this effect of unsteadiness is not visible in case of the stiffness derivatives. It is observed that the stiffness derivative increases with the increase in the wedge angle due to the increase in the plan form area of the wedge, resulting in the variation in the surface pressure distribution of the wedge. Further, due to the increment in the wedge angle the centre of pressure shifts towards the trailing edge. © Published under licence by IOP Publishing Ltd

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

Influence of micro jets on the flow development in the enlarged duct at supersonic Mach number

In this paper, Computational fluid dynamics method is used to simulate the supersonic flow. Convergent-divergent (C-D) nozzle have been used with sudden expansion. The base pressure controlled by using the microjets of 1 mm of orifice diameter is arranged at ninety degrees at PCD 13 mm. The Mach number is 1.87, and the area ratio of 3.24 was considered for the present study. The L/D of the duct was used 10, and the nozzle pressure ratio (NPR) considered for simulation was from 3, 5, 7, 9 and 11. The two-dimensional planar model has been used using ANSYS commercial software. The total wall pressure distribution and Mach number variation from the inlet to the outlet was observed. From the results, it is found that the microjets are capable of controlling the base pressure, the loss of pressure and decreases in the drag. In the present study, the C-D nozzle designed and modeled: K-ε standard wall function turbulence model has been used and validated with the commercial computational fluid dynamics (CFD)

Active control of base pressure with counter clockwise rotating cylinder at Mach 2

The effect of dynamic cylinder as an active controller to control the base pressure for different level of expansion have been experimentally investigated at Mach 2 through CD nozzle for area ratio 9. Solitary counter clockwise rotating cylinder of 2 mm diameter when seen from top, at 2 mm from side wall of square duct and 8 mm from square nozzle exit in the base region is mounted as a controller. Base pressure in the wake area after sudden expansion of jets from the exit of nozzle has been measured. The length-to-width ratio of sudden expansion duct taken is 10. The experiments were carried out by operating jets for different nozzle pressure ratios (NPR). The wall pressure distribution was also measured for with and without control cases in the duct to see that the amplitude of oscillations does not adversely influence the flow field in the duct. Counter clockwise rotating cylinder as an active controller were found to increase the base pressure as high as 62 percent at NPR 8.5 and 53 percent at NPR 7.8. The control effectiveness is marginal for over expanded nozzles. The wall pressure flow field with and without control are identical with minor fluctuations

Modeling and analysis of convergent divergent nozzle with sudden expansion duct using finite element method

n most of the engineering problems, the sudden expansion at the base is encountered in automobile industries, at the base of the fuselage of the aircraft, at the base of the artillery shells, at the base of the unguided rockets and missiles. At transonic Mach numbers, the contribution of the base drag is more than sixty percent. Hence it is mandatory on the part of the researcher to control the base pressure, depending upon the situation. For external aerodynamics, the base pressure should be closed to the ambient pressure to make base drag almost zero. However, for combustion chambers, the base pressure should be very low so that fuel-air mixing is so that the combustor is efficient. With this idea in mind, this study was undertaken. A finite element (FE) the method has been used to investigate the effectiveness of the microjets on wall pressure distribution in a convergent-divergent (CD) nozzle with sudden expansion. For control of the base pressure, four microjets of 1 mm orifice diameter placed at 90- degree intervals along a pitch circle distance (PCD) of 1.3 in the base region were used as a control mechanism. The variables considered are the Mach number, area ratio, and length-to-diameter ratio (L/D) as 2.2, 2.56, and 8. The simulation was done for nozzle pressure ratio (NPR) 3, 5, 7, 9, and 11 with and without the presence of the microjets. The current results for all the cases of this study indicate that one can identify the Mach number, NPR, and the L/D, which will result in a maximum increase in the pressure. Moreover, the results also proved that the influence of microjet control does not affect the flow field in the duct adversely. The two-dimensional planar CD nozzle with sudden expansion duct has been modeled, and analyses using the K-ε turbulence model independently checked with the ANSYS tool