arXiv subjects

Irmak T. Karpuzcu

Publications and source records attributed to Irmak T. Karpuzcu.

3 recordsLinked to original sources

arXiv2026

A Kinetic Study of Hypersonic Boundary Layer Second Mack Mode Instabilities

A flat-plate laminar boundary layer is simulated at a free stream Mach number of 6 and unit Reynolds number $Re_1 \sim 1.18\times 10^7$ m$^{-1}$ using the Direct Simulation Monte Carlo (DSMC) method to capture and analyze spontaneous second-mode instability growth. Power spectral density (PSD) analysis identifies dominant frequencies of 200-500 kHz, in line with linear stability theory (LST) predictions. Near-wall perturbations remain confined within the unstable regions known from linear theory. An analytic kinetic fluctuation floor was derived to predict the limit of the DSMC predicted fluctuations taking into account the method of window averaging performed in Dynamic Mode Decomposition (DMD) analyses. DMD of unsteady flowfield snapshots reveals wave packets of spatially coherent modes having wavelengths and phase speeds characteristic of the acoustic second-mode; their growth and decay occur within the LST-predicted unstable bounds. It was observed that the DSMC-simulated flowfield naturally developed second Mack mode instabilities without any external seeding, i.e., arising spontaneously from the kinetic fluctuations of the simulated gas and of amplitude higher than the predicted DSMC numerical error. Targeted interaction with these flow instabilities is demonstrated for an acoustic vibrating surface (AVS), where forcing at the unstable frequency of 250 kHz results in amplified waves downstream, while at the stable frequency of 400 kHz AVS-induced disturbances are damped. This further emphasizes the ability of the present kinetic simulations to capture and describe linear perturbations at Reynolds numbers corresponding to those of Mach 6 flight at altitudes of $\sim$20 km and suggests that DSMC will be a tool for understanding theoretically-founded study and control of laminar-turbulent transition in hypersonic boundary layers.

physics.flu-dyn↗

arXiv2024

Loss of Axial Symmetry in Hypersonic Flows over Conical Shapes

The assumption of axial symmetry for hypersonic flows over conically shaped geometries is ubiquitous in both experiments and numerical simulations. Yet depending on the free stream conditions, many of these flows are unsteady and their transition from laminar to turbulent is a three-dimensional phenomena. Combining triple deck theory/linear stability analysis with the kinetic direct simulation Monte Carlo method we analyze the azimuthal eigenmodes of flows over single and double-cone configurations. For Mach 16 flows we find that the strongest amplification rate occurs for the non-axisymmetric azimuthal wavenumber of n = 1. This occurs in regions quite close to the tip of the cone due to the proximity of the conical shock to the viscous shear layer where non-axisymmetric modes are amplified through linear mechanisms. Comparison of triple deck linear stability predictions shows that in addition to the azimuthal wavenumber, both the temporal content and amplification rate of these non-axisymmetric disturbances agree well with the time accurate DSMC flowfield. In addition to the loss of axial symmetry observed at the conical shock, the effect of axial symmetry assumptions on the more complex shock-shock and shock-boundary layer interactions of a flow over a double cone are considered. The results for the separation region show that axisymmetric and three dimensional simulations differ in almost all of the main flow structures. Three dimensional flowfields result in a smaller separation bubble with weaker shocks and three dimensional effects were manifest in the variation in surface parameters in the azimuthal direction as well. Interestingly, the DSMC simulations show that the loss of axial symmetry in the separation region, begins near the cone tip.

physics.flu-dyn↗

arXiv2024

On linear stability of supersonic flow over a short compression corner at large ramp angles

Linear stability of supersonic flow over a short compression corner with ramp angles 30 and 42 is investigated using Direct Simulation Monte Carlo (DSMC) and Linear Stability Theory (LST) at Mach number 3, Reynolds number 11,200 and low Knudsen number, O(10$^{-4}$). The two-dimensional base flows feature nonzero velocity slip and temperature jump and were found to be steady and laminar at both ramp angles. Modal analysis revealed a previously unknown traveling three-dimensional global mode, the amplitude functions of which peak at the leading-edge and separation shocks and extend within the shear layer of the large laminar separation bubble formed on the short compression corner. This mode is linearly unstable at the higher ramp angle and stable at the lower one, while the known stationary three-dimensional global mode which peaks at the laminar separation is also present in the spectrum, but is (strongly) damped at both ramp angles. Three-dimensional DSMC simulations have fully confirmed the LST results, underlined (again) the significance of modeling the shock contribution in linear stability analyses of high-speed flow, and predicted the nonlinear evolution of the flow up to the generation of lambda vortices on the ramp, for the first time in the context of kinetic theory simulations.

physics.flu-dyn↗