Aerospace Engineering

In fluid dynamical systems, it is not known a priori whether disturbances grow either in space or in
time or as spatio-temporal structures. However, for boundary layers, it is customary to treat it as a
spatial problem and some limited comparison between prediction and laboratory experiments exist.
In the present work, the receptivity problem of a zero pressure gradient boundary layer excited by
a localized harmonic source is investigated under the general spatio-temporal framework, using the
Bromwich contour integral method. While this approach has been shown to be equivalent to the
spatial study, for unstable systems excited by a single frequency source T. K. Sengupta, M. Ballav,
and S. Nijhawan, Phys. Fluids 6, 1213
1994, here we additionally show, how the boundary layer
behaves when it is excited
i at a single frequency that corresponds to a stable condition
given by
spatial normal-mode analysis and
ii by wideband frequencies, that shows the possibility of flow
transition due to a spatio-temporally growing forerunner or wave front. An energy based receptivity
analysis tool is also developed as an alternative to traditional instability theory. Using this, we
reinterpret the concept of critical layer that was originally postulated to explain the mathematical
singularity of inviscid disturbance field in traditional instability theory of normal modes.

Receptivity of a Bickley jet to a time harmonic symmetric (S- class) and anti-symmetric (AS- class) vortical excitation is reported. Unlike wall bounded flows, the eigen-spectrum
of jets reveal the presence of multiple dominant modes. The S- class displays the presence of upstream propagating disturbances. It is reasoned that due to limited streamwise extent of the domain, experiments and computations on round jets do not always correlate with the linear stability properties. For DNS, a new compact scheme (OUCS4), introduced in [6], along with RK4 time stepping is used. A new filtering procedure is advocated in the radial direction, which removes the numerical instability at the core (due to a mathematical singularity) and allows us to study receptivity of round jets to different classes of excitation.

Here, the fundamental problem of Rayleigh–Taylor instability (RTI) is studied by direct numerical simulation (DNS), where the two air masses at different temperatures, kept apart initially by a non-conducting horizontal interface in a 2D box, are allowed to mix. Upon removal of the partition, mixing is controlled by RTI, apart from mutual mass, momentum, and energy transfer. To accentuate the instability, the top chamber is filled with the heavier (lower temperature) air, which rests atop the chamber containing lighter air. The partition is positioned initially at mid-height of the box. As the fluid dynamical system considered is completely isolated from outside , the DNS results obtained without using Boussinesq approximation will enable one to study non-equilibrium thermodynamics of a finite reservoir undergoing strong Int J Thermophys (2016) 37:36 irreversible processes. The barrier is removed impulsively, triggering baroclinic instability by non-alignment of density, and pressure gradient by ambient disturbances via the sharp discontinuity at the interface. Adopted DNS method has dispersion relation preservation properties with neutral stability and does not require any external initial perturbations. The complete inhomogeneous problem with non-periodic, no-slip boundary conditions is studied by solving compressible Navier–Stokes equation, without the Boussinesq approximation. This is important as the temperature difference between the two air masses considered is high enough (T = 70 K) to invalidate Boussinesq approximation. We discuss non-equilibrium thermodynamical aspects of RTI with the help of numerical results for density, vorticity, entropy, energy, and enstrophy.

Hypersonic waverider configurations are promising candidates for future space and defence applications as they meet the stringent demands of aerodynamic efficiency at high Mach numbers. An attempt has been made here to study the high angle of attack characteristics of such configurations at hypersonic Mach numbers using HiFUN. At high angle of attacks the simulations compares well with the Newtonian theory as the upper surface pressures are negligible compared to the lower surface pressures. The aerodynamic efficiency is maximum at 4 degree and the configuration stalls at 45 degree.

Waverider is a lifting body tailored to create high lift to drag ratio in high speed flow. Present work includes optimization of power law derived waverider using TLBO for a design altitude of 30km at zero design angle of attack and for a range of angle of attack from -2 to 6 for inviscid, laminar and turbulent cases at design Mach number from 5 to 8. Optimised waverider geometry parameters for all three cases are compared for two objective functions; (a) maximization of lift to drag ratio and (b) maximization of lift to drag ratio and volume along with minimization of heat flux (represented by wetted area in present study). Dependence of n, δ and λ (design variables) on lift to drag ratio is discussed in detail. Numerical and experimental validation of the analytical model shows a close match with the data available in the literature. Despite having some limitations, this model serves as a valuable design tool in the conceptual phase to arrive at optimised waverider configurations. Optimization function can be changed according to the designer’s requirements.

Low enthalpy turbulent flows pertaining to 2D scramjet inlets are characterized by strong entropy gradients in the flow field, massive viscous dissipation, complicated shock-shock interactions and shock-wave boundary layer interactions. Prediction of these phenomena using numerical simulations still remains a major hurdle. In the present study, turbulent RANS simulations were carried out over standard test cases pertaining to two dimensional scramjet inlets using Spalart - Allmaras (SA) and k-ω turbulence models. Qualitative and quantitative comparisons were made with the experimental Results and other CFD solvers. Results indicate that, k-ω SST turbulence model can predict, the aerodynamic heating phenomena better compared to SA and is recommended for use in such flow regimes.