arXiv subjects

Nek Sharan

Publications and source records attributed to Nek Sharan.

4 recordsLinked to original sources

arXiv2026

Mach-disk formation and shock-structure transitions in underexpanded coflowing jets

The near-field shock structures of underexpanded sonic jets exiting into a subsonic coflow are investigated over a range of nozzle pressure ratio (NPR) and coflow-to-nozzle-exit velocity ratio ($U_c$), representative of a propulsive nozzle in subsonic flight. Time-averaged statistics from fully-resolved axisymmetric simulations and inviscid method-of-characteristics (MOC) analysis are used to understand how coflow alters the shock-cell structures, in particular the Mach-disk formation. It is well established that increasing NPR transitions the centerline reflection from regular (characterized by oblique shocks) to Mach reflection (characterized by a near-normal Mach disk). We find that coflow has the opposite influence: a strong coflow shrinks the Mach disk until it vanishes, reverting Mach reflection to regular reflection, so the NPR for this transition increases with $U_c$. This effect has previously been attributed to a reduction in the jet-boundary inclination at the nozzle lip, which confines the lip Prandtl-Meyer fan to a smaller angle and weakens the embedded shock. We show instead that this inclination is determined by the non-uniform pressure the coflow imposes along the jet boundary, which is the primary driver of the shock-structure transitions in coflowing jets. The non-uniform pressure weakens the boundary-reflected compression waves and orients them at shallower angles, so the embedded shock reflects regularly or fails to form. A simulation-informed MOC analysis with this non-uniform pressure boundary condition reproduces the transition behavior with increasing coflow. Coflow also lengthens the first shock cell linearly, which is accurately estimated by a simple correction to Prandtl classical shock-cell length scaling.

physics.flu-dyn↗

arXiv2026

Curvilinear Moving Overset Method for High-order Non-dissipative Schemes

This paper presents a non-dissipative, high-order, moving overset method for curvilinear grids to simulate unsteady compressible flows in complex geometries with moving components. Centered finite-difference schemes that are up to sixth-order accurate in the interior are used with a weak moving overset interface treatment. The novel aspects of the proposed approach compared to conventional overset methods are: (i) instead of overwriting all conservative or primitive variables at the interface (or fringe) points with the interpolated values, a characteristic decomposition is performed and only the incoming characteristic variables are imposed for inviscid flows, consistent with the hyperbolic character of the Euler equations; for viscous flows, the viscous fluxes are imposed in addition to the incoming characteristics variables, (ii) instead of using multiple layers of fringe points at the interface, the proposed approach ensures high-order accuracy and stability with a single layer, thus minimizing the parallel communication costs at each timestep, and (iii) the proposed approach ensures long time stability with non-dissipative schemes without introducing artificial dissipation explicitly (using numerical filters) or implicitly (using upwind schemes). The stability is demonstrated by an eigenvalue analysis of the time-dependent (semi-discrete) system matrix for moving grids, proving the eigenvalue spectra remains confined to the left half of the complex plane with grid motion. The proposed approach is validated over a range of canonical and practical unsteady flow problems involving moving grids: 1-D scalar advection, 2-D isentropic vortex convection, flow past rotating 2-D circular cylinder, pitching 2-D and 3-D airfoil/wing flow, and flow past 2-D and 3-D oscillating circular cylinder, demonstrating high-order accuracy and long time stability for inviscid/viscous flows.

physics.flu-dyn↗

arXiv2021

Investigation of high-pressure turbulent jets using direct numerical simulation

Direct numerical simulations of free round jets at a Reynolds number ($Re_{D}$) of $5000$, based on jet diameter ($D$) and jet-exit bulk velocity ($U_{e}$), are performed to study jet turbulence characteristics at supercritical pressures. The jet consists of $\mathrm{N_{2}}$ that is injected into $\mathrm{N_{2}}$ at same temperature. To understand turbulent mixing, a passive scalar is transported with the flow at unity Schmidt number. Two sets of inflow conditions that model jets issuing from either a smooth contraction nozzle (laminar inflow) or a long pipe nozzle (turbulent inflow) are considered. By changing one parameter at a time, the simulations examine the jet-flow sensitivity to the thermodynamic condition (characterized in terms of the compressibility factor ($Z$) and the normalized isothermal compressibility), inflow condition, and ambient pressure ($p_{\infty}$) spanning perfect- to real-gas conditions. The inflow affects flow statistics in the near-field (containing the potential core closure and the transition region) as well as further downstream (containing fully-developed flow with self-similar statistics) at both atmospheric and supercritical $p_{\infty}$. The sensitivity to inflow is larger in the transition region, where the laminar-inflow jets exhibit dominant coherent structures that produce higher mean strain rates and higher turbulent kinetic energy than in turbulent-inflow jets. Decreasing $Z$ at a fixed supercritical $p_{\infty}$ enhances pressure and density fluctuations (normalized by local mean pressure and density, respectively), but the effect on velocity fluctuations depends also on local flow dynamics. When $Z$ is reduced, large mean strain rates in the transition region of laminar-inflow jets significantly enhance velocity fluctuations (normalized by local mean velocity) and scalar mixing, whereas the effects are minimal in jets from turbulent inflow.

physics.flu-dyn↗

arXiv2019

Direct numerical simulation of high-pressure mixing in turbulent jets

Combustion in automotive and aerospace applications employing diesel, gas turbine and liquid rocket engines is preceded by injection and mixing of fuel and oxidizer at high pressures, often exceeding mixture critical values. Experimental observations indicate that the jets injected at supercritical pressures exhibit significantly different dynamics than the jets at subcritical conditions, owing to the lack of distinct liquid and gas phases in supercritical state. As a result, the averaged flow quantities such as the potential core length, jet spatial growth rate and velocity decay profiles differ in the two conditions, resulting in different mixed-fluid distributions. In this study, turbulent jet direct numerical simulations (DNS) are performed to examine the variations in statistics between injection of Nitrogen ($\mathrm{N_{2}}$) in Nitrogen ($\mathrm{N_{2}}$) at subcritical (perfect-gas) and supercritical conditions. Isothermal round jets at Reynolds number ($Re_{D}$), based on jet diameter ($D$) and jet orifice velocity ($U_{0}$), of $5000$ and Mach number of $0.6$ are considered. For mixing analyses, a passive scalar transported with the flow is examined.

physics.flu-dyn↗