# Stefan HickelDelft University of Technology | TU · Faculty of Aerospace Engineering (AE)

Stefan Hickel

Dr.-Ing.habil.

## About

200

Publications

41,790

Reads

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3,107

Citations

Introduction

- Modeling of turbulence and scalar mixing for large-eddy simulation (LES).
- Adaptive numerical methods: models, solvers, adaptive grids, immersed interfaces, ...
- Wall modeling for LES.
- Atmospheric gravity waves: Instability growth and transition to turbulence.
- Stratified and rotating turbulence.
- Cavitation: Interaction of turbulence and phase change.
- Mixing and heat transfer in supercritical fluids.
- High-speed flows and shock-wave/turbulence interaction.
- Combustion.

Additional affiliations

April 2015 - present

February 2007 - April 2015

Education

April 2005 - June 2008

June 2003 - March 2005

October 1998 - May 2003

## Publications

Publications (200)

We derive and analyze a model for implicit Large Eddy Simulation (LES) of compressible flows that is applicable to a broad range of Mach numbers and particularly efficient for LES of shock-turbulence interaction. Following a holistic modeling philosophy, physically sound turbulence modeling and numerical modeling of unresolved subgrid scales (SGS)...

We present and evaluate a two-phase model for Eulerian large-eddy simulations (LES) of liquid-fuel injection and mixing at high pressure. The model is based on cubic equations of state and vapor-liquid equilibrium calculations and can represent the coexistence of supercritical states and multi-component subcritical two-phase states via a homogeneou...

Many geophysical and man-made fluid flows are affected by the interaction between system rotation and turbulence. When the Rossby number, i.e. the ratio of the rotational time scale and the turbulent time scale, is sufficiently small, the energy dissipation rate is reduced with respect to the non-rotating isotropic case, and the typical cloud of is...

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. The excitations mechanisms considered are (i) pitching of the lowe...

We present a new family of fast and robust methods for the calculation of the vapor‐liquid equilibrium at isothermal‐isobaric (PT‐flash), isothermal‐isochoric (VT‐flash), isenthalpic‐isobaric (HP‐flash), and isoenergetic‐isochoric (UV‐flash) conditions. The framework is provided by formulating phase‐equilibrium conditions for multi‐component mixtur...

Proper Orthogonal Decomposition (POD) plays an important role in the analysis of complex nonlinear systems governed by partial differential equations (PDEs), since it can describe the full-order system in a simplified but representative way using a handful of dominant dynamic modes. However, determining a POD from the results of complex unsteady si...

The flow over a forward-facing step (FFS) at 𝑀𝑎∞=1.7 and 𝑅𝑒𝛿=13718 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...

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. Direct numerical simulations (DNS) are performed of the incompressible three-dimensional laminar base flow and the stationary distorted flow that arise from the interaction between...

We present a novel framework for high-fidelity simulations of inert and reacting sprays at transcritical conditions with highly accurate and computationally efficient models for complex real-gas effects in high-pressure environments, especially for the hybrid subcritical/supercritical mode of evaporation during the mixing of fuel and oxidizer. The...

We perform direct numerical simulations of turbulent flow at friction Reynolds number Reτ≈500-2000\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$Re_\tau \approx 500{-}...

During atmospheric entry, capsules and space debris are exposed to a flow environment with complex fluid thermochemistry and gas-surface interactions (GSI) that may lead to mass loss and shape change. A promising approach for the numerical simulation of such challenging flows is the use of immersed boundary (IB) and adaptive mesh refinement (AMR) t...

View Video Presentation: https://doi.org/10.2514/6.2022-3276.vid During atmospheric entry, the flow environment around capsules or space debris is characterized by complex fluid thermochemistry and gas-surface interactions (GSI). Computational fluid dynamics (CFD) simulations of these conditions are crucial in the design process of such objects. A...

Accurate simulations of high-pressure transcritical fuel sprays are essential for the design and optimization of next-generation gas turbines, internal combustion engines, and liquid propellant rocket engines. Most important and challenging is the accurate modelling of complex real-gas effects in high-pressure environments, especially the hybrid su...

We propose and analyse an optimization method that uses a machine learning approach to solve multi-objective, constrained propeller optimization problems. The method uses an online learning strategy where explainable supervised classifiers learn the location of the Pareto front and advise search strategies. The classifiers are trained with orthogon...

This paper introduces a machine learning approach for optimizing propellers. The method aims to improve the computational cost of optimization by reducing the number of evaluations required to find solutions. This is achieved by directing the search towards design clusters with good performance, i.e. high propulsive efficiency and low cavitation. T...

We perform direct numerical simulations of turbulent flow at friction Reynolds number $Re_\tau \approx 500-2000$ grazing over perforates plates with moderate viscous-scaled orifice diameter $d^+\approx40$--$160$ and analyse the relation between permeability and added drag. Unlike previous studies of turbulent flows over permeable surfaces, we find...

View Video Presentation: https://doi.org/10.2514/6.2022-2330.vid The evolution of secondary instabilities in a three-dimensional stationary-crossflow-domina- ted boundary layer is investigated by means of Direct Numerical Simulations (DNS) and linear spanwise BiGlobal stability analysis. Single-frequency unsteady disturbances and a critical station...

The low-frequency unsteady behaviour of the flow over a forward-facing step (FFS) and backward-facing step (BFS) at Ma∞ = 1.7 and Re∞ = 1.3718 ×10^7m^−1 is investigated using a well-resolved large eddy simulation (LES). The mean and instantaneous flow field illustrate the unsteady features of the shock wave/boundary layer interaction (SWBLI) system...

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 w...

Adaptive Mesh Refinement (AMR) is potentially an effective way to automatically generate computational meshes for high-fidelity simulations such as Large Eddy Simulation (LES). When combined with adjoint methods, which are able to localize error contributions, AMR can generate meshes that are optimal for computing a physical quantity of interest (e...

The low-frequency unsteady motions behind a backward-facing step (BFS) in a turbulent flow at Ma=1.7 and Re∞=13.7×10^6 are investigated using a well-resolved large-eddy simulation. The instantaneous flow field illustrates the unsteady phenomena of the shock wave/boundary layer interaction (SWBLI) system, including vortex shedding in the shear layer...

Multi-fidelity optimization methods promise a high-fidelity optimum at a cost only slightly greater than a low-fidelity optimization. This promise is seldom achieved in practice, due to the requirement that low- and high-fidelity models correlate well. In this article, we propose an efficient bi-fidelity shape optimization method for turbulent flui...

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. Particular emphasis is placed on ensuring a fully stationary solution, in order...

We present well-resolved RANS simulations of two generic asymmetrically heated cooling channel configurations, a high aspect ratio cooling duct operated with liquid water at $$Re_b = 110 \times 10^3$$ and a cryogenic transcritical channel operated with methane at $$Re_b = 16 \times 10^3$$ . The former setup serves to investigate the interaction of...

The low-frequency unsteady motions behind a backward-facing step (BFS) in a turbulent flow at Ma = 1.7 and Re∞ = 1.3718 × 10^5 is investigated using a well-resolved large-eddy simulation (LES). The instantaneous flow field illustrates the unsteady phenomena of the shock wave/boundary layer interaction (SWBLI) system, including vortex shedding in th...

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 the buoyancy period. While the former was studied by Remmler et al. (2013), it is here repeated at a higher resolution an...

The development of primary and secondary instabilities is investigated numerically for a supersonic transitional flow over a backward-facing step at Ma = 1.7 and Re=13 718. 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 Z...

Multi-fidelity optimization methods promise a high-fidelity optimum at a cost only slightly greater than a low-fidelity optimization. This promise is seldom achieved in practice, due to the requirement that low- and high-fidelity models correlate well. In this article, we propose an efficient bi-fidelity shape optimization method for turbulent flui...

The low-frequency unsteady motions behind a backward-facing step (BFS) in a turbulent flow at $Ma=1.7$ and $Re_\infty=1.3718\times 10^5$ is investigated using a well-resolved implicit large-eddy simulation (LES). The instantaneous flow field illustrates the unsteady phenomena of the shock wave/boundary layer interaction (SWBLI) system, including th...

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. Resu...

We present a new family of fast and robust methods for the calculation of the vapor-liquid equilibrium at isobaric-isothermal (PT-flash), isochoric-isothermal (VT-flash), isenthalpic-isobaric (HP-flash), and isoenergetic-isochoric (UV-flash) conditions. The framework is provided by formulating phase-equilibrium conditions for multi-component mixtur...

Flow reconstruction from DMD mode $\phi_2$. The animation shows the contours of pressure gradient at 32 phase angles. The green solid and dashed lines indicate the mean reattachment shock and boundary layer edge. The while solid line is the instantaneous isoline of $u=0$. The black solid and dashed lines denote the instantaneous ioslines of $u^\pri...

Flow reconstruction from DMD mode $\phi_3$. The animation shows the contours of pressure gradient at 32 phase angles. The green solid and dashed lines indicate the mean reattachment shock and boundary layer edge. The while solid line is the instantaneous isoline of $u=0$. The black solid and dashed lines denote the instantaneous ioslines of $u^\pri...

Flow reconstruction from DMD mode $\phi_2$. The animation shows the contours of pressure gradient at 32 phase angles. The green solid and dashed lines indicate the mean reattachment shock and boundary layer edge. The while solid line is the instantaneous isoline of $u=0$. The black solid and dashed lines denote the instantaneous isolines of $u^\pri...

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 shearlayer...

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 shearlayer...

The path of laminar-to-turbulent transition behind a backward-facing step (BFS) in the supersonic regime at $Ma=1.7$ and $Re_{\delta_0}=13718$ is investigated using a very well-resolved large eddy simulation (LES). Five distinct stages are identified in the transition process by the visualisation of instantaneous flow. The transition is initiated b...

We investigate the underlying assumptions of Explicit Algebraic Subgrid-Scale Models (EASSMs) for Large- Eddy Simulations (LESs) through an a priori analysis using data from Direct Numerical Simulations (DNSs) of homogeneous isotropic and homogeneous rotating turbulence. We focus on the performance of three models: the dynamic Smagorinsky (DSM) and...

Nonlinear Explicit Algebraic Subgrid-scale Stress Models (EASSMs) have shown high potential for Large Eddy Simulation (LES) of challenging turbulent flows on coarse meshes. A simplifying assumption made to enable the purely algebraic nature of the model is that the Subgrid-Scale (SGS) kinetic energy production and dissipation are in balance, i.e.,...

Two aspects of homogeneous rotating turbulence are quantified through forced Direct Numerical Simulations in an elongated domain, which is in the direction of rotation about $340$ times larger than the typical initial eddy size. First, by following the time evolution of the integral length-scale along the axis of rotation $\ell_{\|}$, the growth ra...

Transition from a split to a forward kinetic energy cascade system is explored in the context of rotating turbulence using direct numerical simulations with a three-dimensional isotropic random force uncorrelated with the velocity field. Our parametric study covers confinement effects in high-aspect-ratio domains and a broad range of rotation rates...

Micro-ramps are deployed to prevent boundary layer separation by creating a momentum excess close to the wall. Through Direct Numerical Simulations (DNS) of the base, instantaneous and mean flow, we identify that the perturbation dynamics in the wake of the micro-ramp play an essential role in creating the near-wall momentum excess. To identify the...

The unsteadiness of shock wave-boundary layer interactions is investigated in a transitional backward-facing step flow at $Ma=1.7$ and $Re_{\delta_0}=13718$ using large eddy simulation. The mean and instantaneous flow shows that the laminar inflow undergoes a laminar-to-turbulence transition in which Kelvin-Helmholtz vortices form, distort and even...

We present well-resolved large-eddy simulations of turbulent flow through a straight, high aspect ratio cooling duct operated with water at a bulk Reynolds number of $Re_{b}=110\times 10^{3}$ and an average Nusselt number of $Nu_{xz}=371$ . The geometry and boundary conditions follow an experimental reference case and good agreement with the experi...

Transition from a split to a forward kinetic energy cascade system is explored in the context of rotating turbulence using direct numerical simulations with a three-dimensional isotropic random force uncorrelated with the velocity field. Our parametric study covers confinement effects in large aspect ratio domains and a broad range of rotation rate...

Design and Optimisation (D&O) of propellers can be cost effective if lucrative regions in the design space are explored efficiently. This requires insight into the sensitivities of performance w.r.t design variables in all regions of the design space. Using the sensitivities it is possible to reduce the number of design variables and/or number of s...

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 t...

We present an Eulerian three-component two-phase model for the large-eddy simulation (LES) of the cavitating flow within liquid-fuel injectors and the primary atomization of injected fuel jets. The model is applied to a generic nozzle and jet flow at different cavitation numbers and Reynolds numbers. We find that the LES correctly reproduce experim...

Delta wing configurations are commonly employed for high agility supersonic aircraft and aerodynamic devices such as vortex generators, and have thus been a focus of extensive investigations over the past decades.

Experiments and numerical simulations were carried out in order to contribute to a better understanding and predic-tion of high-pressure injection into a gaseous environment. Specifically, the focus was put on the phase separation processes of an initially supercritical fluid due to the interaction with its surrounding. N-hexane was injected into a...

We present numerical simulations of a reacting shock–bubble interaction with detailed chemistry. The interaction between the Richtmyer-Meshkov instability and shock-induced ignition of a \( {\mathrm{H}}_2-{\mathrm{O}}_2 \) gas mixture is investigated. Shock wave Mach numbers in the range of \( Ma=2.13-2.50 \) at a constant initial pressure of \( {p...

We investigate a reacting shock–bubble interaction through three-dimensional numerical simulations with detailed chemistry. The convex shape of the bubble focuses the shock and generates regions of high pressure and temperature, which are sufficient to ignite the diluted stoichiometric H2−O2 gas mixture inside the bubble. We study the interaction b...

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...

Experiments and numerical simulations were carried out in order to contribute to a better understanding and prediction of high-pressure injection into a gaseous environment. Specifically, the focus was put on the phase separation processes of an initially supercritical fluid due to the interaction with its surrounding. N-hexane was injected into a...

We present an adaptive reduced-order model for the efficient time-resolved simulation of fluid-structure interaction problems with complex and non-linear deformations. The model is based on repeated linearizations of the structural balance equations. Upon each linearization step, the number of unknowns is strongly decreased by using modal reduction...

We investigate the effectiveness of the semi-local Reynolds number Re τ to parametrize wall-bounded flows with strong density, ρ, and viscosity, µ, gradients. Several cases are considered, namely, volumetrically heated low-Mach-number turbulent channel flows, a simultane