Afroz Javed

Damping Coefficient Prediction of Solid Rocket Motor Nozzle Using Computational Fluid Dynamics

Numerical simulations are carried out to evaluate the nozzle damping of rocket motors. A subscale cold flow experimental condition where nozzle damping coefficients are evaluated through the pulse decay method is taken as a validation case. The flowfield of the motor is simulated by solving three-dimensional Reynolds-averaged Navier–Stokes equations using commercial computational fluid dynamics software. The trend of the pressure decay in the head end is well captured for different values of port-to-throat area ratios, and a very good match is obtained between the computed and experimental values of the nozzle decay coefficient. Validated methodology is used to evaluate the damping coefficient of a burning solid rocket motor with composite propellant.

Model-free simulations for compressible mixing layer

Confined supersonic mixing layer is explored through model-free simulations. Both two- and three-dimensional spatio-temporal simulations were carried out employing higher order finite difference scheme as well as finite volume scheme based on open source software (OpenFOAM) to understand the effect of three-dimensionality on the development of mixing layer. It is observed that although the instantaneous structures exhibit three-dimensional features, the average pressure and velocities are predominantly two-dimensional. The computed wall pressures match well with experimental results fairly well, although three-dimensional simulation underpredicts the wall pressure in the downstream direction. The self-similarity of the velocity profiles is obtained within the duct length for all the simulations. Although the mixing layer thicknesses differ among different simulations, their growth rate is nearly the same. Significant differences are observed for species and temperature distribution between two- and three-dimensional calculations, and two-dimensional calculations do not match the experimental observation of smooth variations in species mass fraction profiles as reported in literature. Reynolds stress distribution for three-dimensional calculations show profiles with less peak values compared to two-dimensional calculations; while normal stress anisotropy is higher for three-dimensional case.

Behaviour of turbulent Prandtl/Schmidt number in compressible mixing layer

The behaviour of turbulent Prandtl/Schmidt number is explored through the model-free simulation results. It has been observed that compressibility affects the Reynolds scalar flux vectors. Reduced peak values are also observed for compressible convective Mach number mixing layer as compared with the incompressible convective Mach number counterpart, indicating a reduction in the mixing of enthalpy and species. Prt and Sct variations also indicate a reduction in mixing. It is observed that unlike the incompressible case, it is difficult to assign a constant value to these numbers due to their continuous variation in space. Modelling of Prt and Sct would be necessary to cater for this continuous spatial variation. However, the turbulent Lewis number is evaluated to be near unity for the compressible case, making it necessary to model only one of the Prt and Sct..

Evaluation of side spillage for a hypersonic air intake using computational fluid dynamic techniques

Mass capture ratio of a hypersonic air intake is one of the most important performance parameters. However, no a priori estimate of its value exists for use in initial design exercise of a hypersonic vehicle. In the present work, an air intake of a non-axisymmetric scramjet engine, designed using stream thrust methodology, is studied using computational fluid dynamic techniques. A large amount of air mass flow rate is observed to spill from the sides, which is not accounted for in the initial design phase. In absence of even an approximate estimate of this spillage, computational fluid dynamic studies become the only available tool to evaluate the mass capture ratio. Simulations are also carried out with a side wall at the intake to stop spillage. Although mass capture ratio and static pressure at combustor entry improve, deterioration in other flow parameters such as static temperature, Mach number and total pressure is observed.

Reynolds averaged Navier-Stokes simulations of compressible mixing layers of similar and dissimilar gases: Performance of k- turbulence model

The issue of growth rate reduction of high speed mixing layer with convective Mach number is examined for similar and dissimilar gases using Reynolds averaged Navier-Stokes (RANS) methodology with k–ɛ turbulence model. It is observed that the growth rate predicted using RANS simulations closely matches with that predicted using model free simulations. Velocity profiles do not depend on the modelled value of Prt and Sct; while the temperature and species mass fraction distributions depend heavily on them. Although basic k–ɛ turbulence model could not capture the reduced growth rate for the mixing layer formed between similar gases, it predicts very well the reduced growth rate for the mixing layer for the dissimilar gases. It appears that density ratio changes caused by temperature changes for the dissimilar gases have profound effect on the growth rate reduction.

Exploration of supersonic confined mixing layer: Effect of dissimilar gases at different temperatures

The growth rate of high-speed mixing layer between two dissimilar gases is explored through the model free simulation results. To analyse the cause for the higher mixing layer growth rate in comparison to the existing values reported in literature, the results were compared with the model free simulations of mixing of two high-speed streams of nitrogen (similar gas) at matched temperature and density. The analysis indicates that pressure and density fluctuations no longer remain correlated completely for the mixing layer formed between two dissimilar gases at different temperatures in contrast to the complete pressure density correlation for similar gases. It has been observed that the correlation between temperature and density fluctuations is near −1.0 for dissimilar gases in the mixing layer region and is much higher than for similar gases. It is concluded that mixing layer of similar gases shows a decrease in growth rate due to compressibility effect, while that of dissimilar gases shows a decrease due to dominant temperature effect on density.