Articles written in Sadhana

    • Aerodynamic characteristics of an ogive-nose spinning projectile


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      This work experimentally investigates the Robins–Magnus effect on a 5-caliber spinning projectile at a low subsonic Mach number of 0.1 corresponding to a Reynolds number of 3.2 × 10⁵ based on the model length. The model configuration tested was a cylinder with spherically blunt tangent ogive nose portion. The normal, axial and side force coefficients were obtained for various angles of attack (α) ranging from 0° to 34° with non-dimensional spin rates (Ω) of 0–0.05. Results indicate that the side force coefficient increases with α up to a value of around 28° and decreases thereafter. Interestingly, in the range of spin rates considered in the present study, normal and axial force coefficients are not affected due to spin. However, the side force coefficients are seen to increase with spin rates at higher α. Flow visualization studies are demonstrated to explain the underlying mechanism behind the variation of these aerodynamic coefficients.

    • Vortex ring propagation and interactions studies


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      This paper experimentally investigates the vortex ring propagation and interactions with thin cylindrical and flat surfaces. Dye-based visualization technique is adopted for the interaction studies. Vortex rings are generated from a circular nozzle of 19 mm diameter with the stroke length ratios (length of the fluid slug to nozzle diameter, LN/DN) of 1 to 5, and ejection velocities in the range of 0.05 to 0.2 m/s. Vortex interaction studies are carried out with two different bodies; firstly, with the circular cylinders having the diameters of 0.2, 0.6, 1.5 and 2.5 mm, and secondly with a flat solid surface. Results indicate that the trails in the vortex ring start following at LN/DN = 4. The influence of the initial velocity is found to be insignificant on the vortex ring diameter, however, found to depend on stroke length ratio. Vortex-cylinder interaction studies indicated that vortex velocity decreases with increase in cylinder diameter after the interaction. Reconnections of vortex rings are observed in lower cylinder diameter cases. In case of vortex ring interaction with the flat surface, stretching of the vortex core is observed leading to a considerable increase in the vortex ring diameter.

    • Shock structures of underexpanded non-circular slot jets


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      Flow visualization studies using shadowgraph technique are carried out to investigate the shock evolution from non-circular slot jets at various under-expansion levels. The non-circular topologies considered are triangular, square and elliptic, and the circular jet is taken as the baseline case for the study. These jets are underexpanded in the pressure ratio (R) range of 2–6 corresponding to a fully expanded jet Mach number up to 1.85. Results indicate that the shock cell structures of non-circular jets strongly depend upon the initial shape ofthe topology. The shock structures of triangular jet have additional secondary oblique shocks that are distinct from those of other non-circular jets. Mach disk is almost absent in a shock cell structure of triangular jet, which is unlike the case of other jets used in the study. The study suggests that square jet undergoes faster diffusion process compared with the triangular jet. Axis-switching phenomenon is predicted for the elliptical jet at a distance of 3–7 equivalent diameters.

    • Performance analysis of a vortex chamber under non-reacting and reacting conditions


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      A series of non-reacting and reacting flow experiments are performed in a vortex combustion coldwall (VCCW) chamber using gaseous oxygen and gaseous hydrogen as propellants. Oxidizer is injected tangentially at the aft end of a combustion chamber from four ports. Hydrogen is injected axially from the top centre of the chamber. The oxidizer to fuel mixture ratios considered for the experimental studies are in the range of 4.2–6.0 for non-reacting case, and 6.38 for reacting flow experiments. Numerical simulations under non-reacting conditions are conducted to understand the flow behaviour in the chamber at a mixture ratio of 4.2 considering the same propellants used in the experiment. Results from non-reacting flow cases indicated that the chamber pressure increased by 0.8 bar with an increase in the mixture ratio from 4.2 to 6.0. The chamber pressure developed under the reacting flow conditions is found to be higher by around 1.3 bar compared with the non-reacting flow condition. The oxidizer concentration is found to be higher along the inner chamber wall, thus limiting the wall surface temperature to 360 K in the reacting conditions.

    • Visualisation studies on plane jets at low Reynolds numbers


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      Flow visualisation of plane jets in the Reynolds number (Re) range of 90 to 750 is carried out experimentally, considering water as fluid. A conventional rectangular nozzle with the exit dimensions of 20 mm 9 2 mm is used. Various visualisation experiments are carried out using a colour dye mixed or injected with thejet flow. Images are processed using image processing software to estimate the characteristics of the jets. Besides, to assess the growth of the shear layer in free jets, a nozzle with an extension of its base wall by 5 mm is considered. The resulting images indicate that the mean spread rate of a turbulent plane jet is around 0.152. In the case of laminar jets, a vortex street with large size vortices is observed at lower Reynolds numbers. The Strouhal number of the vortices formed in the laminar jet is observed to increase with an increase in theReynolds number. The amplitude of the oscillations of a laminar jet column at Re & 90 is observed to increase with an increase in the axial distance. The dissipation and spread rates are lower for smaller Reynolds number laminar jet. The flow emerging from a nozzle with an extended base-wall is found to be asymmetric along its nozzle centreline, attributed to the delay in the shear layer formation at the wall side

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