## 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 (2024)*Flow, Turbulence and Combustion* (in press) 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 Re_{τ }= 355, Re_{τ }= 1226, and Re_{τ }= 5118.

## 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.

## 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.

## Convective instabilities in a laminar shock-wave/boundary-layer interaction

S.E.M. Niessen, K.J. Groot, S. Hickel, V.E. Terrapon (2023)*Physics of Fluids* 35: 024101. doi: 10.1063/5.0135590

Linear stability analyses are performed to study the dynamics of linear convective instability mechanisms in a laminar shock-wave/boundary-layer interaction at Mach 1.7. In order to account for all two-dimensional gradients elliptically, we introduce perturbations into an initial-value problem that are found as solutions to an eigenvalue problem formulated in a moving frame of reference.

## GPU-accelerated simulations for eVTOL aerodynamic analysis

V. Pasquariello, Y. Bunk, S. Eberhardt, P.-H. Huang, J. Matheis, M. Ugolotti, S. Hickel (2023)*AIAA paper* 2023-2107. doi: 10.2514/6.2023-2107

The demand for fast, high-fidelity, scale-resolving computational fluid dynamics (CFD) simulations is continuously growing. Especially new emerging aviation technologies, such as electrical vertical take-off and landing aircraft (eVTOL), strongly rely on advanced numerical methods to retain development life-cycle costs and achieving design targets more quickly. This paper presents a cutting-edge large-eddy simulations (LES) solver developed to enable over-night turnaround times for full aircraft simulations on advanced graphics processing unit (GPU) architectures.

## Unsteady mechanisms in shock wave and boundary layer interactions over a forward-facing step

W. Hu, S. Hickel, B.W. van Oudheusden (2022)*Journal of Fluid Mechanics* 949: A2. doi: 10.1017/jfm.2022.737

The flow over a forward-facing step (FFS) at Ma_{∞}=1.7 and Re_{?} = 13 718 is investigated by well-resolved large-eddy simulation. To investigate effects of upstream flow structures and turbulence on the low-frequency dynamics of the shock wave/boundary layer interaction (SWBLI), two cases are considered: one with a laminar inflow and one with a turbulent inflow.

## Experimental investigation of shock–shock interactions with variable inflow Mach number

L. Laguarda, J. Santiago Patterson, F.F.J. Schrijer, B.W. van Oudheusden, S. Hickel (2021) *Shock Waves* 3: 457-468. doi: 10.1007/s00193-021-01029-3

Experiments on shock–shock interactions were conducted in a transonic–supersonic wind tunnel with variable free-stream Mach number functionality. Transition between the regular interaction (RI) and the Mach interaction (MI) was induced by variation of the free-steam Mach number for a fixed interaction geometry, as opposed to most previous studies where the shock generator angles are varied at constant Mach number.

## Low-frequency unsteadiness mechanisms in shock wave/turbulent boundary layer interactions over a backward-facing step

W. Hu, S. Hickel, B.W. van Oudheusden (2021)*Journal of Fluid Mechanics* 915: A107. doi: 10.1017/jfm.2021.95

The low-frequency unsteady motions behind a backward-facing step (BFS) in a turbulent flow at Ma=1.7 and Re_{∞}=1.3718×10^{7} m^{−1} are investigated using a well-resolved large-eddy simulation.

## Influence of upstream disturbances on the primary and secondary instabilities in a supersonic separated flow over a backward-facing step

W. Hu, S. Hickel, B.W. van Oudheusden (2020) *Phys. Fluids *32: 056102. doi: 10.1063/5.0005431

*Ma*= 1.7 and Re

_{δ}=13718. Oblique Tollmien–Schlichting (T–S) waves with properties according to linear stability theory (LST) are introduced at the domain inlet with zero, low, or high amplitude (cases ZA, LA, and HA).

## Dynamics of unsteady asymmetric shock interactions

L. Laguarda, S. Hickel, F.F.J. Schrijer, B.W. van Oudheusden (2020) *Journal of Fluid Mechanics* 888: A18. doi: 10.1017/jfm.2020.28

The response of asymmetric and planar shock interactions to a continuous excitation of the lower incident shock is investigated numerically. Incident shock waves and centred expansion fans are generated by two wedges asymmetrically deflecting the inviscid free stream flow at Mach 3.

## Transitional Flow Dynamics Behind a Micro-Ramp

J. Casacuberta, K.J. Groot, Q. Ye, S. Hickel (2020) *Flow Turbulence and Combustion* 104: 533-552. doi: 10.1007/s10494-019-00085-1

Micro-ramps are popular passive flow control devices which can delay flow separation by re-energising the lower portion of the boundary layer. We compute the laminar base flow, the instantaneous transitional flow, and the mean flow around a micro-ramp immersed in a quasi-incompressible boundary layer at supercritical roughness Reynolds number.

## Dynamics of a supersonic transitional flow over a backward-facing step

W. Hu, S. Hickel, B.W. van Oudheusden (2019) *Phys. Rev. Fluids* 4, 103904. doi: 10.1103/PhysRevFluids.4.103904

The transition mechanism and unsteady behavior behind a backward-facing step (BFS) in the supersonic regime at Ma = 1.7 and Re_{δ} = 13718 is investigated using large-eddy simulation (LES). The visualization of the flow field shows that the transition process behind the step is initiated by a Kelvin-Helmholtz (K-H) instability of the separated shear layer, followed by secondary modal instabilities of the K-H vortices, leading to lambda-shaped vortices, hair-pin vortices and finally to a fully turbulent state.

## Unsteady effects of strong shock-wave/boundary-layer interaction at high Reynolds number

V. Pasquariello, S. Hickel, N.A. Adams (2017) *Journal of Fluid Mechanics* 828: 617-657. doi: 10.1017/jfm.2017.308

We analyse the low-frequency dynamics of a high Reynolds number impinging shock-wave/turbulent boundary-layer interaction (SWBLI) with strong mean-flow separation. The flow configuration for our grid-converged large-eddy simulations (LES) reproduces recent experiments for the interaction of a Mach 3 turbulent boundary layer with an impinging shock that nominally deflects the incoming flow by 19.6° . The Reynolds number based on the incoming boundary-layer thickness of Re_{δ} ≈ 203 000 is considerably higher than in previous LES studies.

## On the transition between regular and irregular shock patterns of shock-wave/boundary-layer interactions

J. Matheis, S. Hickel (2015)*Journal of Fluid Mechanics* 776: 200-234. doi: 10.1017/jfm.2015.319

The reflection of strong oblique shock waves at turbulent boundary layers is studied numerically and analytically. A particular emphasis is put on the transition between regular shock-wave/boundary-layer interaction (SWBLI) and Mach reflection (irregular SWBLI). The classical two- and three-shock theory and a generalised form of the free interaction theory are used for the analysis of well-resolved large-eddy simulations (LES) and for the derivation of stability criteria.

## Wall modeled large-eddy simulation of the VFE-2 delta wing

C. Zwerger, S. Hickel, C. Breitsamter, N.A. Adams (2015)*AIAA paper* 2015-2572. doi: 10.2514/6.2015-2572

We performed wall-modeled large-eddy simulation of the flow field around the VFE-2 delta wing, focusing on two aspects: (1) leading-edge bluntness effects on the primary vortex separation and (2) vortex breakdown above the wing and its control. Regarding aspect (1), the VFE-2 delta wing with sharp leading-edge (SLE) and medium radius round leading-edge (MRLE) are considered for three angles of attack α = {13°, 18°, 23°} leading to different overall flow characteristics.

## Subgrid-scale modeling for implicit Large Eddy Simulation of compressible flows and shock turbulence interaction

S. Hickel, C.P. Egerer, J. Larsson (2014)*Physics of Fluids* 26: 106101. doi: 10.1063/1.4898641

## Large-eddy simulation of a pseudo-shock system in a Laval nozzle

J.F. Quaatz, M. Giglmaier, S. Hickel, N.A. Adams (2014) *International Journal of Heat and Fluid Flow* 49: 108-115. doi: 10.1016/j.ijheatfluidflow.2014.05.006

Well-resolved Large-Eddy Simulations (LES) of a pseudo-shock system in the divergent part of a Laval nozzle with rectangular cross section are conducted and compared with experimental results. The LES matches the parameter set of a reference experiment. Details of the experiment, such as planar side walls, are taken into account, all wall boundary layers are well-resolved and no wall model is used.

## Large-eddy simulation of passive shock-wave/boundary-layer interaction control

V. Pasquariello, M. Grilli, S. Hickel, N.A. Adams (2014) *International Journal of Heat and Fluid Flow *49: 116-127. doi: 10.1016/j.ijheatfluidflow.2014.04.005

We investigate a passive flow-control technique for the interaction of an oblique shock generated by an 8.8° wedge with a turbulent boundary-layer at a free-stream Mach number of Ma_{∞ }= 2.3 and a Reynolds number based on the incoming boundary-layer thickness of Re_{δ} = 60 500 by means of large-eddy simulation (LES).