Non-orthogonal plane-marching parabolized stability equations for the secondary instability of crossflow vortices
K.J.Groot, J. Casacuberta, S. Hickel (2026)
Computers & Fluids 306: 106947. doi: 10.1016/j.compfluid.2025.106947
A detailed derivation, analysis, and verification is given for the non-orthogonal, plane-marching Parabolized Stability Equations (PSE) approach. We show that converged solutions can be achieved for a broad frequency range with an existing stabilization method for the line-marching PSE approach. In applying the approach to a flow distorted by a medium-amplitude crossflow vortex, we determine its linear secondary instability mechanisms.
Baroclinic shift of hypersonic heating streaks over an ablating compression ramp
A.O. Başkaya, S. Hickel (2025)
Physics of Fluids 37: 116108. doi: 10.1063/5.0281627
We present novel observations from direct numerical simulations of transitional Mach 8 flow over a 15° compression ramp ablator. Heating streaks over the ramp are seen to undergo a half-wavelength shift near the location of transition from laminar to turbulent boundary layer flow. This phenomenon leads to an intriguing pattern of ablation grooves on the surface. Our analysis shows that the underlying mechanism is driven by the baroclinic torque in the strongly stratified near-wall region. We discuss the impact of this baroclinic shift for a surface undergoing ablative recession and assess its sensitivity to different thermal boundary conditions and perturbation amplitudes.
Transcritical real-fluid effects on dual-fuel combustion of methane and n-dodecane
M. Fathi, D. Roekaerts, S. Hickel (2025)
Applications in Energy and Combustion Science 24: 100398. doi: 10.1016/j.jaecs.2025.100398
A comprehensive unified framework of high-fidelity physical models and numerical simulation techniques for transcritical dual-fuel combustion systems is presented and applied for the analysis of three configurations, each involving injection of an n-dodecane jet into a 6 MPa, 1000 K environment. The cases differ in ambient composition: (1) a single-fuel baseline consisting of air mixed with combustion products, and (2–3) two dual-fuel cases with either methane or a 90:10 (by volume) methane/ethane blend added to the ambient.
Effects of intense strain on flame structure and NOx generation in turbulent counterflow lean-premixed hydrogen flames
M. Fathi, S. Hickel, N.A.K. Doan, I. Langella (2025)
Combustion and Flame 282: 114459. doi: 10.1016/j.combustflame.2025.114459
This work examines for the first time in detail the coupled effects of strain and turbulence in hydrogen flames, for various conditions spanning different signs of the Markstein length and increasing applied strain levels. In particular, it clarifies the different roles of applied strain, turbulence-driven strain, and curvature on both flame structure and NOx generation. Results show for the first time that both in-flame and post-flame NOx can be suppressed at high strain levels under turbulent combustor-relevant conditions by straining the flame.
Large eddy simulations of transcritical e-fuel sprays using real-fluid multiphase flamelet-based modeling
M. Fathi, S. Hickel, D. Roekaerts (2025)
Combustion and Flame 281: 114360. doi: 10.1016/j.combustflame.2025.114360
This work introduces a modeling technique for the use of transcritical counterflow flames in flamelet modeling, expanding the capabilities of large-eddy simulations with multiphase thermodynamics (LES-MT) to accurately modeling transcritical combustion. By incorporating real-fluid effects and two-phase interactions, the transcritical flamelet library provides a high-fidelity representation of the complex behaviors in high-pressure multiphase autoignition scenarios. This calibration-free approach can significantly improve our understanding of the transcritical combustion of emerging fuels such as OME3 or their combination with traditional fuels such as n-dodecane.
Dynamic multi-level load balancing for scalable simulations of reacting multiphase flows
G. van den Oord, V. Azizi, M. Fathi, S. Hickel (2025)
Int. J. of High Performance Computing Applications 39: 519-531. doi: 10.1177/10943420251329199
We have addressed the uneven computational load arising from chemical reactions and multiphase thermodynamics in INCA by dynamically offloading work to underutilized cores. The implementation is highly abstract, enabling the use of independent dynamic load balancers for the chemistry and thermodynamics components within INCA.
Numerical simulation of transcritical multiphase combustion using real-fluid thermochemical and transport properties
M. Fathi, D. Roekaerts, S. Hickel (2025)
Combustion and Flame 275: 114055. doi: 10.1016/j.combustflame.2025.114055
Streaky perturbations in swept-wing flow over forward-facing step
J. Casacuberta, S. Westerbeek, J.A. Franco, K.J. Groot, S. Hickel, S. Hein, M. Kotsonis (2025)
Physical Review Fluids 10: 023902. doi: 10.1103/PhysRevFluids.10.023902
Direct numerical simulations have been performed to scrutinize the stationary velocity-perturbation streaks that form close downstream of two-dimensional forward-facing step of fixed height embedded in laminar incompressible swept-wing boundary-layer flow.
Low-frequency dynamics of turbulent recirculation bubbles
L. Laguarda, S. Hickel (2024)
Physics of Fluids 36: 081708. doi: 10.1063/5.0227332
We revisit the origin of low-frequency unsteadiness in turbulent recirculation bubbles (TRBs), and, in particular, the hypothesis of a dynamic feedback mechanism between unconstrained separation and reattachment locations. Our results demonstrate, for the first time, effective suppression of the low-frequency characteristics of the TRB without reducing its size, strongly supporting our hypothesis.
Passive Control of Shock-Wave/Turbulent Boundary-Layer Interaction Using Spanwise Heterogeneous Roughness
W. Wu, L. Laguarda, D. Modesti, S. Hickel (2025)
Flow, Turbulence and Combustion 115: 29-49. doi: 10.1007/s10494-024-00580-0
A novel passive flow-control method for shock-wave/turbulent boundary-layer interactions (STBLI) is investigated. The method relies on a structured roughness pattern constituted by streamwise-aligned ridges. Its effectiveness is assessed with wall-resolved large-eddy simulations of the interaction of a Mach 2 turbulent boundary layer flow with an oblique impinging shock with shock angle 40°. A parametric study is performed to investigate the effect of the spacing between the ridges. We find that ridges with small spacing effectively mitigate the low-frequency unsteadiness of STBLI and slightly reduce total-pressure loss.
Reynolds number effects in shock-wave/turbulent boundary-layer interactions
L. Laguarda, S. Hickel, F.F.J. Schrijer, B.W. van Oudheusden (2024)
Journal of Fluid Mechanics 989: A20. doi:10.1017/jfm.2024.361
We investigate Reynolds number effects in strong shock-wave/turbulent-boundary layer interactions (STBLI) by leveraging a new database of wall-resolved and long-integrated large-eddy simulations (LES). The database encompasses STBLI with massive boundary-layer separation at Mach 2.0, impinging-shock angle 40° and friction Reynolds numbers Reτ = 355, Reτ = 1226, and Reτ = 5118.
Passive stabilization of crossflow instabilities by a reverse lift-up effect
J. Casacuberta, S. Hickel, M. Kotsonis (2024)
Physical Review Fluids 9: 043903. doi: 10.1103/PhysRevFluids.9.043903
A novel mechanism is identified, through which a spanwise-invariant surface feature (a two-dimensional forward-facing step) significantly stabilizes the stationary crossflow instability of a three-dimensional boundary layer. The mechanism is termed here as reverse lift-up effect, inasmuch as it acts reversely to the classic lift-up effect; that is, kinetic energy of an already-existing shear-flow instability is transferred to the underlying laminar flow through the action of cross-stream perturbations.
Fluid Ablation Interactions on a Compression Ramp at Mach 8
A.O. Başkaya, S. Hickel, S.D. Dungan, C. Brehm (2024)
AIAA SciTech, NATO AVT-346: Instability and Transition in Hypersonic Separated Flows, Orlando. AIAA paper 2024-0501. doi: 10.2514/6.2024-0501
Direct numerical simulations (DNS) are performed over a 15° compression ramp undergoing ablation at Mach 8 to examine fluid-ablation interactions (FAI) on transitional high-speed boundary layers. The experiments at these conditions with a rigid wall are first numerically replicated for a laminar baseflow. Heating streaks are introduced by adding perturbations in the baseflow informed by prior stability calculations. The ramp is then replaced by a low-temperature ablator in our DNS and the interaction of the streaks with the recessing ablator surface are examined. Different approaches from two independently developed solvers are used to study this problem.
Assessment of immersed boundary methods for hypersonic flows with gas–surface interactions
A.O. Başkaya, M. Capriati, A. Turchi, T. Magin, S. Hickel (2024)
Computers & Fluids 270: 106134. doi: 10.1016/j.compfluid.2023.106134
The efficacy of immersed boundary (IB) methods with adaptive mesh refinement (AMR) techniques is assessed in the context of atmospheric entry applications, including effects of chemical nonequilibrium (CNE) and gas–surface interactions (GSI). We scrutinize a conservative cut-cell IB method and two non-conservative IB methods, comparing their results with analytical solutions, data from the literature, and results obtained with a reference solver that operates on body-fitted grids.
Shock-wave/turbulent boundary-layer interaction with a flexible panel
L. Laguarda, S. Hickel, F.F.J. Schrijer, B.W. van Oudheusden (2024)
Physics of Fluids 36: 016120. doi: 10.1063/5.0179082
The dynamic coupling between a Mach 2.0 shock-wave/turbulent boundary-layer interaction (STBLI) and a flexible panel is investigated. Wall-resolved large-eddy simulations are performed for a baseline interaction over a flat-rigid wall, a coupled interaction with a flexible panel, and a third interaction over a rigid surface that is shaped according to the mean panel deflection of the coupled case.
Assessment of Reynolds number effects in supersonic turbulent boundary layers
L. Laguarda, S. Hickel, F.F.J. Schrijer, B.W. van Oudheusden (2024)
International Journal of Heat and Fluid Flow 105: 109234. doi: 10.1016/j.ijheatfluidflow.2023.109234
Wall-resolved large-eddy simulations (LES) are performed to investigate Reynolds number effects in supersonic turbulent boundary layers (TBLs) at Mach 2.0. The resulting database covers more than a decade of friction Reynolds number Reτ from 242 to 5554, which considerably extends the parameter range of current high-fidelity numerical studies. Reynolds number trends are identified on a variety of statistics for skin-friction, velocity and thermodynamic variables. The efficacy of recent scaling laws as well as compressibility effects are also assessed.