E. Rathakrishnan

Active control of suddenly expanded flows from overexpanded nozzles

An experimental investigation was carried out to study the effectiveness of micro-jets to control base pressure in suddenly expanded axi-symmetric ducts. Four micro-jets of 1 mm orifice diameter located at 90

Visualization of Axis-Switching of Elliptical Slot Jets

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Control of suddenly expanded flows with micro-jets

interval along a pitch circle diameter of 1.3 times the nozzle exit diameter in the base region was employed as active controls. The Mach numbers of the suddenly expanded flows were studies 2.0, 2.5 and 3.0. The jets were expanded suddenly into an axi-symmetric tube with cross-sectional area 2.56, 3.24, 4.84, and 6.25 times that of nozzle exit diameter. The length-to-diameter ratio of the sudden expansion tube was varied from 10 to 1. The jets were operated at an over expansion level of Pe/Pa = 0.277. The wall pressure distribution in the suddenly enlarged duct was also measured. It is found that the micro-jets can serve as active controllers for base pressure. Also, the wall pressure distribution is not adversely influenced by the micro-jets. From the present investigation it is evident that for a given Mach number and nozzle pressure ratio one can identify the enlargement length to diameter ratio which will result in maximum increase/decrease of base pressure.

Experimental investigation on spiked body in hypersonic flow

Elliptical slot jets of aspect ratio (AR) 2 and 4 were visualized by surface coating technique to understand the physics behind their superiority over circular jets and the influence of aspect ratio on axis-switching phenomenon. The jets were made to impinge on the flat surface of a transparent plate, which was coated with lamp black and pump oil. The plate was kept normal to the jet axis at different axial locations. When the jet impinges on the flat surface of the transparent plate, the surface coating was eroded due to the shearing action. After exposing the coated plate to the jet, it was taken out and the pattern on the plate surface was photographed with a diffused light projected from the uncoated side of the plate. The presence of vortices of various sizes starting from the largest ones at the ends of minor axis and the smallest ones at the major axis ends for Mach 0.6 jet is clearly inferred from the visualization pictures, shown in Fig. 1. For the present aspect ratio elliptical jets at X/D = 1.0, the coating at the minor axis ends are almost intact, indicating that the vortices were large in size, since the large size vortices are good entrainers but poor promoters of mixing. In contrary the small vortices are efficient mixing promoters but weak entrainers. The presence of small vortices is inferred form the significant erosion of the surface coating near the major axis extremities. For the AR 2 jet, axis-switching occurs between X/D = 2 and 3 and for higher AR jet like AR 4, axis-switching occurs between X/D = 5 and 6 (where X is the axial distance from the elliptical slot and D is the equivalent diameter of the elliptical slot). It implies that for AR 2, the entrainment is more compared to AR 4. The visualization pictures demonstrate the generation of vortices from largest to smallest size with continuous variation in their size, owing to the continuous variation of the radius of curvature of the ellipse along its azimuth. These vortices of varying size are responsible for the improved entrainment and enhanced mixing of the elliptical jets compared to circular ones. Among the different aspect ratios tested, AR = 2 is found to be best from jet mixing point of view. Axis-switching X/D = 1.0 X/D = 2.0 X/D = 3.0

Control of suddenly expanded flow

The results of an experimental investigation carried out to control the base pressure in a suddenly expanded axi-symmetric passage are presented in this paper. Active control in the form of micro-jets was employed to control the base pressure. Air injection at four locations at the base, symmetric to the nozzle axis was used as the active control. The jet Mach numbers at the entry to the suddenly expanded duct, studied were 1.87, 2.2 and 2.58. The length-to-diameter ratio of the suddenly expanded duct was varied from 10 to 1. Nozzles generating the above jet Mach numbers were operated with nozzle pressure ratio (NPR) in the range 3 to 11. In addition to base pressure, wall pressure field along the enlarged duct length was also studied. It is found that the active control in the form of blowing through small orifices (micro-jets) are effective in controlling the base pressure field. As high as 95 per cent increase in base pressure was achieved for certain combination of parameters of the present study. From the present investigation it is evident that for a given Mach number and NPR one can identify the L/D which will result in maximum increase/decrease of base pressure.

Control of Suddenly Expanded Flows from Correctly Expanded Nozzles

A spike attached to a hemispherical body drastically changes its flowfield and influences aerodynamic drag in a hypersonic flow. It is, therefore, a potential candidate for drag reduction of a future high-speed vehicle. The effect of the spike length, shape, spike nose configuration and angle-of-attack on the reduction of the drag is experimentally studied with use of hypersonic wind-tunnel at Mach 6. The effects of geometrical parameters of the spike and angle-of-attack on the aerodynamic coefficient are analysed using schlieren picture and measuring aerodynamic forces. These experiments show that the aerodisk is superior to the aerospike. The aerodisk of appropriate length, diameter and nose configuration may have the capability for the drag reduction. The inclusion of an aero disk at the leading edge of the spike has an advantage for the drag reduction mechanism if it is at an angle-of-attack, however consideration to be given for increased moment resulting from the spike is required.

Corrugated Tabs for Supersonic Jet Control

Purpose – This paper presents an experimental investigation to study the effectiveness of micro jets under the influence of Over, Under, and Correct expansion to control the base pressure in suddenly expanded axi‐symmetric ducts.Design/methodology/approach – Four micro jets of 1 mm orifice diameter located at 90° intervals along a pitch circle diameter of 1.3 times the nozzle exit diameter in the base region was employed as active controls. The tests were conducted for Mach numbers 1.5, 1.3, 1.6, 1.8, 2.0, 2.5, and 3.0. The jets were expanded suddenly into an axi‐symmetric tube with cross‐sectional area 2.56 times that of nozzle exit area. The L/D ratio of the sudden expansion tube was varied from 10 to 1.Findings – From the present studies, it was found that the maximum increase in base pressure is 152 percent for Mach number 2.58. It is found that the micro jets do not adversely influence the wall pressure distribution.Research limitations/implications – As a result of developments in space flights and ...

Effect of Ribs on Suddenly Expanded Flows

The effectiveness of micro jets to control base pressure in suddenly expanded axi-symmetric ducts has been presented in this paper. Four micro jets of 1 mm 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 suddenly expanded flows were 1.25, 1.30, 1.48, 1.6, 1.8 and 2.0. The jets were expanded suddenly into an axi-symmetric tube with cross-sectional area 2.56, 3.24, 4.84, and 6.25 times that of nozzle exit area. The length-to-diameter ratio of the sudden expansion tube was varied from 10 to 1. The jets at all Mach numbers were correctly expanded. It is found that the micro jets can serve as active controllers for base pressure. Also, the wall pressure distribution is not adversely influenced by the micro jets.

Drag Reduction for Spike Attached to Blunt-Nosed Body at Mach 6

The efficiency of corrugated tabs in promoting the mixing of Mach 1.8 axisymmetric jet has been investigated experimentally. Two rectangular tabs of 4.2% blockage, with corrugations at the edges, located diametrically opposite at the exit of a Mach 1.8 convergent-divergent nozzle were found to be better mixing promoters than identical rectangular tabs without corrugations, at overexpanded, correctly expanded, and underexpanded states of the jet. Furthermore, the corrugated tabs were found to be more efficient in weakening the shocks in jet core compared with the plain tabs. As high as 78 % of reduction in core length was achieved with corrugated tabs for the jet operated at nozzle pressure ratio of 7, the corresponding reduction with the plain tabs is only 54%. The mixing effectiveness of corrugated tabs increases progressively with increase of nozzle pressure ratio whereas, the maximum mixing effectiveness of the plain tabs is found to be at the correctly expanded state. The shadowgraph pictures for the uncontrolled and controlled jets clearly demonstrate the effectiveness of corrugated tabs in weakening the waves in the jet core. The speculation of smaller vortices generated by the corrugated tab is supported by a preliminary visualization with water flow channel.

Truncated Triangular Tabs for Supersonic Jet Control

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