arXiv subjects

Graham V. Candler

Publications and source records attributed to Graham V. Candler.

6 recordsLinked to original sources

arXiv2025

Modified Marrone-Treanor model: parameterization and benchmarking for five-species air

We present updated parameters for five-species air (N2, O2, NO, N and O) reactions to be used with the Modified Marrone-Treanor two-temperature model. The vibrational relaxation and chemical reaction rates are derived from quasiclassical trajectory calculations and direct molecular simulations using ab initio potential energy surfaces. The resulting model enables efficient computational fluid dynamics simulations of nonequilibrium air chemistry in hypersonic flows. We show that the model reproduces direct molecular simulation benchmark solutions with high accuracy in zero-dimensional heat baths representative of strong nonequilibrium post-shock conditions. The model's analytical expressions for dissociation rate coefficient and vibrational energy change per reaction ensure that the correct amount of energy is transferred between the vibrational and trans-rotational modes. Detailed balance is imposed for three-body recombination reactions and our simulations exhibit quasi-steady-state dissociation rates and proper approach to thermochemical equilibrium. In direct comparison with the Park TTv model, the Modified Marrone-Treanor model predicts significantly slower conversion of N2 into N below 10000 K and significantly more NO production at all temperatures. This is likely due to its significantly higher Zeldovich reaction rates compared to Park.

physics.chem-ph↗

arXiv2025

Modified Marrone-Treanor dissociation model: formulation and benchmarking for diatom/atom mixtures

We present a modified Marrone-Treanor model for dissociation with rate parameters derived exclusively from quasiclassical trajectory calculations on ab initio potential energy surfaces. Analysis of the trajectory dataset for reactant O2 and N2 diatoms sampled from Boltzmann internal energy distributions over a wide T,Tv range indicates that a modified version of the classical Marrone-Treanor two-temperature model captures the most relevant physics of shock-heated dissociating diatomic species very well. We find that simple correction factors account for non-Boltzmann depletion effects observed in direct molecular simulations employing the same potentials. The concentration-dependent functional form proposed for these correction factors ensures that depletion effects vanish at chemical equilibrium. Based on comparisons in isothermal and adiabatic heat baths we verify that the resulting two-temperature dissociation model accurately reproduces all major features observed in the direct molecular simulations, while remaining computationally inexpensive enough for large-scale computational fluid dynamics simulations.

physics.chem-ph↗

arXiv2023

Linear Analysis of Boundary-Layer Instabilities on a Finned-Cone at Mach 6

Boundary-layer instabilities for a finned cone at Mach=6, $Re=8.4 \times 10^6$ [m$^{-1}$], and zero incidence angle are examined using linear stability methods of varying fidelity and maturity, following earlier analysis presented in [doi.org/10.2514/6.2022-3247]. The geometry and laminar flow conditions correspond to experiments conducted at the Boeing Air Force Mach 6 Quiet Tunnel (BAM6QT) at Purdue University. Where possible, a common mean flow is utilized among the stability computations, and comparisons are made along the acreage of the cone where transition is first observed in the experiment. Stability results utilizing Linear Stability Theory (LST), planar Parabolized Stability Equations (planar-PSE), One-Way Navier Stokes (OWNS), forced direct numerical simulation (DNS), and Adaptive Mesh Refinement Wavepacket Tracking (AMR-WPT) are presented. A dominant three-dimensional vortex instability occurring at $\approx$ 250 kHz is identified that correlates well with experimental measurements of transition onset. With the exception of LST, all of the higher-fidelity linear methods considered in this work were consistent in predicting the initial growth and general structure of the vortex instability as it evolved downstream. Some of the challenges, opportunities, and development needs of the stability methods considered are discussed.

physics.flu-dyn↗

arXiv2020

Transient growth analysis of oblique shock wave/boundary-layer interactions at Mach 5.92

We study physical mechanisms that trigger transient growth in a high-speed spatially-developing laminar boundary layer that interacts with an oblique shock wave. We utilize an approach based on power-iteration, with the global forward and adjoint linearized equations, to quantify the transient growth in compressible boundary layers with flow separation. For a Mach 5.92 boundary layer with no oblique shock wave, we show that the dominant transient response consists of oblique waves, which arise from the inviscid Orr mechanism, the lift-up effect, and the first-mode instability. We also demonstrate that the presence of the oblique shock wave significantly increases transient growth over short time intervals through a mechanism that is not related to a slowly growing global instability. The resulting response takes the form of spanwise periodic streamwise elongated streaks and our analysis of the linearized inviscid transport equations shows that base flow deceleration near the reattachment location contributes to their amplification. The large transient growth of streamwise streaks demonstrates the importance of non-modal effects in the amplification of flow perturbations and identifies a route for the emergence of similar spatial structures in transitional hypersonic flows with shock wave/boundary-layers interaction.

physics.flu-dyn↗

arXiv2019

Reattachment streaks in hypersonic compression ramp flow: an input-output analysis

We employ global input-output analysis to quantify amplification of exogenous disturbances in compressible boundary layer flows. Using the spatial structure of the dominant response to time-periodic inputs, we explain the origin of steady reattachment streaks in a hypersonic flow over a compression ramp. Our analysis of the laminar shock/boundary layer interaction reveals that the streaks arise from a preferential amplification of upstream counter-rotating vortical perturbations with a specific spanwise wavelength. These streaks are associated with heat flux striations at the wall near flow reattachment and they can trigger transition to turbulence. The streak wavelength predicted by our analysis compares favorably with observations from two different hypersonic compression ramp experiments. Furthermore, our analysis of inviscid transport equations demonstrates that base flow deceleration contributes to the amplification of streamwise velocity and that the baroclinic effects are responsible for the production of streamwise vorticity. Finally, the appearance of the temperature streaks near reattachment is triggered by the growth of streamwise velocity and streamwise vorticity perturbations as well as by the amplification of upstream temperature perturbations by the reattachment shock.

physics.flu-dyn↗

arXiv2017

Simulation and stability analysis of oblique shock wave/boundary layer interactions at Mach 5.92

We investigate flow instability created by an oblique shock wave impinging on a Mach 5.92 laminar boundary layer at a transitional Reynolds number. The adverse pressure gradient of the oblique shock causes the boundary layer to separate from the wall, resulting in the formation of a recirculation bubble. For sufficiently large oblique shock angles, the recirculation bubble is unstable to three-dimensional perturbations and the flow bifurcates from its original laminar state. We utilize Direct Numerical Simulation (DNS) and Global Stability Analysis (GSA) to show that this first occurs at a critical shock angle of $θ= 12.9^o$. At bifurcation, the least stable global mode is non-oscillatory, and it takes place at a spanwise wavenumber $β=0.25$, in good agreement with DNS results. Examination of the critical global mode reveals that it originates from an interaction between small spanwise corrugations at the base of the incident shock, streamwise vortices inside the recirculation bubble, and spanwise modulation of the bubble strength. The global mode drives the formation of long streamwise streaks downstream of the bubble. While the streaks may be amplified by either the lift-up effect or by Görtler instability, we show that centrifugal instability plays no role in the upstream self-sustaining mechanism of the global mode. We employ an adjoint solver to corroborate our physical interpretation by showing that the critical global mode is most sensitive to base flow modifications that are entirely contained inside the recirculation bubble.

physics.flu-dyn↗