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

Niessen S.E.M., Groot, K.J., Hickel, S., Terrapon, V.E. (2023)Niessen S.E.M., Groot, K.J., Hickel, S., Terrapon, V.E. (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.

## Experimental and numerical investigation into the drag performance of dimpled surfaces in a turbulent boundary layer

van Campenhout, O.W.G., van Nesselrooij, M., Lin, Y.Y., van Oudheusden, B.W., Hickel, S. (2023)*International Journal of Heat and Fluid Flow* 100: 109110. doi: 10.1016/j.ijheatfluidflow.2023.109110

Although several previous studies have reported a potential drag-reducing effect of dimpled surfaces in turbulent boundary layers, there is a lack of replicability across experiments performed by different research groups. To contribute to the dialogue, we scrutinize one of the most studied dimple geometries reported in the literature, which has a dimple diameter of 20 mm and a depth of 0.5 mm.

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

Hu, W., Hickel, S., van Oudheusden, B.W. (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_{𝛿} = 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.

## Direct numerical simulation of interaction between a stationary crossflow instability and forward-facing steps

Casacuberta, J., Hickel, S., Westerbeek, S., Kotsonis, M. (2022) *Journal of Fluid Mechanics* 943: A46. doi: 10.1017/jfm.2022.456

The interaction between forward-facing steps of several heights and a pre-existing critical stationary crossflow instability of a swept-wing boundary layer is analysed.

## Secondary instabilities in swept-wing boundary layers: Direct Numerical Simulations and BiGlobal stability analysis

Casacuberta, J., Groot, K.J., Hickel, S., Kotsonis, M. (2022)

SciTech Forum and Exposition, San Diego. AIAA paper 2022-2330, doi: 10.2514/6.2022-2330

The evolution of secondary instabilities in a three-dimensional stationary-crossflow-dominated boundary layer is investigated by means of Direct Numerical Simulations (DNS) and linear spanwise BiGlobal stability analysis. Single-frequency unsteady disturbances and a critical stationary crossflow mode are considered.

## Mechanisms of interaction between stationary crossflow instabilities and forward-facing steps

Casacuberta, J., Hickel, S., Kotsonis, M. (2021)

AIAA Scitech paper 2021-0854. doi: 10.2514/6.2021-0854

We study the interaction between a stationary crossflow instability and forward-facing steps in a swept-wing boundary layer using Direct Numerical Simulations (DNS). The station- ary primary crossflow mode is imposed at the inflow. Steps of several heights are modeled.

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

Hu, W., Hickel, S., van Oudheusden, B.W. (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.

## Inertia gravity waves breaking in the middle atmosphere: energy transfer and dissipation tensor anisotropy

Pestana, T., Thalhammer, M., Hickel, S. (2020) *Journal of the Atmospheric Sciences *77: 3193-3210. doi: 10.1175/JAS-D-19-0342.1

We present direct numerical simulations of inertia–gravity waves breaking in the middle–upper mesosphere. We consider two different altitudes, which correspond to the Reynolds number of 28 647 and 114 591 based on wavelength and buoyancy period. While the former was studied by Remmler et al., it is here repeated at a higher resolution and serves as a baseline for comparison with the high-Reynolds-number case.

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

Hu, W., Hickel, S., van Oudheusden, B.W. (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

Laguarda, L., Hickel, S., Schrijer, F.J., van Oudheusden, B.W. (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

Casacuberta, J., Groot, K.J., Ye, Q., Hickel, S. (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.

## Effectivity and efficiency of selective frequency damping for the computation of unstable steady-state solutions

Casacuberta, J., Groot, K.J., Tol, H.J., Hickel, S. (2018)*Journal of Computational Physics *375: 481-497. doi: 10.1016/j.jcp.2018.08.056

Selective Frequency Damping (SFD) is a popular method for the computation of globally unstable steady-state solutions in fluid dynamics. The approach has two model parameters whose selection is generally unclear. In this article, a detailed analysis of the influence of these parameters is presented, answering several open questions with regard to the effectiveness, optimum efficiency and limitations of the method.

## Validation of large-eddy simulation methods for gravity wave breaking

Remmler, S., Hickel, S., Fruman, M.D., Achatz, U. (2015) *Journal of the Atmospheric Sciences* 72: 3537-3562. doi: 10.1175/JAS-D-14-0321.1

To reduce the computational costs of numerical studies of gravity wave breaking in the atmosphere, the grid resolution has to be reduced as much as possible. Insufficient resolution of small-scale turbulence demands a proper turbulence parameterization in the framework of a large-eddy simulation (LES). We consider three different LES methods—the adaptive local deconvolution method (ALDM), the dynamic Smagorinsky method (DSM), and a naïve central discretization without turbulence parameterization (CDS4)—for three different cases of the breaking of well-defined monochromatic gravity waves.

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

Matheis, J., Hickel, S. (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.

## Benchmarking in a rotating annulus: A comparative experimental and numerical study of baroclinic wave dynamics

Vincze, M., Borchert, S., Achatz, U., Von Larcher, T., Baumann, M., Liersch, C., Remmler, S., Beck, T., Alexandrov, K.D., Egbers, C., Fröhlich, J., Heuveline, V., Hickel, S., Harlander, U. (2015) *Meteorologische Zeitschrift* 23: 611-635. doi: 10.1127/metz/2014/0600

The differentially heated rotating annulus is a widely studied tabletop-size laboratory model of the general mid-latitude atmospheric circulation. The two most relevant factors of cyclogenesis, namely rotation and meridional temperature gradient are quite well captured in this simple arrangement. The radial temperature difference in the cylindrical tank and its rotation rate can be set so that the isothermal surfaces in the bulk tilt, leading to the formation of baroclinic waves.

## On the construction of a direct numerical simulation of a breaking inertia-gravity wave in the upper-mesosphere

Fruman, M.D., Remmler, S., Achatz, U., Hickel, S. (2014) *Journal of Geophysical Research *119: 11613-11640. doi: 10.1002/2014JD022046

A systematic approach to the direct numerical simulation (DNS) of breaking upper mesospheric inertia-gravity waves of amplitude close to or above the threshold for static instability is presented. Normal mode or singular vector analysis applied in a frame of reference moving with the phase velocity of the wave (in which the wave is a steady solution) is used to determine the most likely scale and structure of the primary instability