# Schedule

*Main Foyer*

*Room 200*

40 Years of Transition Research at NASA – A Personal Perspective

*Room 200*

**Chair:** Peter Schmid (Imperial College London)

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#### Abstract

In this lecture, an overview of the laminar-turbulent transition research conducted by the speaker and his colleagues at NASA during the past few decades will be presented. This includes development of computational methods for compressible stability analysis, linear and nonlinear parabolized stability equations, supersonic and hypersonic boundary layer transition prediction including the effects of nose bluntness and gas chemistry, development of quite supersonic and hypersonic tunnels, resolution of cone-to-flat plate transition controversy, instability and transition of rotating disk flow, secondary instability of crossflow disturbances and instability of the attachment-line boundary layer. Various laminar flow control research efforts will be briefly discussed. The talk will close by addressing current challenges in laminar-turbulent transition prediction.*Room 200*

*Subcritical Laminar-Turbulent Transition on Blunt Cones at Hypersonic Speeds*

Pedro Paredes, Meelan M. Choudhari, Fei Li

*Numerical Investigation of the Nonlinear Transition Stages in a High-Enthalpy Hypersonic Boundary Layer on a Right Cone*

Michelle Bailey, Christoph Hader, Hermann Fasel

*Controlled Stationary/Traveling Cross-flow Mode Interaction in Mach 6 Boundary Layer*

Alexander Arndt, Thomas Corke, Eric Matlis, Michael Semper

*Sensitivity of Boundary-Layer Stability and Transition to Thermochemical Modelling*

Heather L. Kline, Chau-Lyan Chang, Fei Li

*Nonlinear PSE Transition Predictions in Hypersonic Boundary Layers with Finite Rate Chemical Reactions*

Ludovico Zanus, Fernando Miró Miró, Fabio Pinna

*Sensitivity of Hypersonic Shear Flows to Finite-Rate Chemistry Effects and Surface Roughness*

Athanasios Margaritis, Taraneh Sayadi, Olaf Marxen, Peter Schmid

*Main Foyer*

Numerical Simulation Studies of Hypersonic Boundary-Layer Instability Mechanism

*Room 200*

**Chair:** Xuesong Wu (Imperial College London)

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#### Abstract

In this presentation, a brief overview is first given of our past numerical simulation research of hypersonic boundary-layer receptivity and instability mechanism, conducted in collaboration with my past and current graduate students. Subsequently, our recent results will be presented on the computational and theoretical studies of the supersonic mode in hypersonic boundary layers. Since a few years ago, there has been renewed interest in supersonic modes in hypersonic boundary layers, which have previously been thought to be insignificant due to their smaller amplitudes than Mack's traditional second mode. Supersonic modes are associated with an unstable second Mack mode synchronizing with the slow acoustic spectrum, causing sound to radiate outwards from the boundary layer. Because supersonic modes have yet to be observed experimentally, the majority of previous investigations mainly relied on the Linear Stability Theory (LST) to study supersonic modes on a flat plate. We will present our recent results from a combined LST and Direct Numerical Simulation (DNS) approach to investigate the mechanism of supersonic modes under various flow conditions for hypersonic flow over blunt cones with or without the thermochemical nonequilibrium effects.*Room 200*

*Destabilisation of Stationary and Travelling Crossflow Disturbances Due to Forward and Backward Facing Steps over a Swept Wing*

Emma Cooke, Shahid Mughal, Spencer Sherwin, Richard Ashworth, Stephen Rolston

*BiGlobal Stability Analysis of a Swept-Wing Boundary Layer with Forward and Backward Facing Steps*

Thibaut Appel, Emma Cooke, Richard Ashworth, Shahid Mughal

*Effects of 3D Roughness Patch on Transition in High-Speed Boundary Layers*

Meelan M. Choudhari, Fei Li, Pedro Paredes

*Receptivity from Surface Imperfections.*

Michael Gaster

*Influence of superhydrophobic surfaces on the laminar-to-turbulent transition in a channel flow*

Francesco Picella, Jean-Christophe Robinet, Stefania Cherubini

*Transition in a Swept-boundary Layer Subject to Surface Roughness and Free-stream Turbulence*

Luca De Vincentiis, Dan Henningson, Ardeshir Hanifi

*Main Foyer*

*Room 200*

*Optimal Force and State Reconstruction*

Eduardo Martini, Andre Cavalieri, Peter Jordan, Lutz Lesshafft, Aaron Towne

*Global Stability Analysis of the JAXA H-II Transfer Vehicle Re-Entry Capsule*

Andrea Sansica, Atsushi Hashimoto, Yuya Ohmichi

*Overview of the PDE-Based Amplification Factor Transport Model as an Engineering Tool for Transition Prediction in Complex Aerodynamic Flows*

James Coder

*Reduced order model of shock-boundary layers interactions*

Guillaume Chauvat, Peter J. Schmid, Daniel J. Bodony, Vassilis Theofilis, Ardeshir Hanifi

*Global Stability of Fluid-Structure Interaction Problems*

Prabal Singh Negi, Ardeshir Hanifi, Dan Henningson

*Numerical Investigations of Laminar to Turbulent Transition on an Oscillating Airfoil Boundary Layer*

Duncan M. Ohno, Jonas P. Romblad, Marwan Khaled, Ulrich Rist

*Transient growth and self-sustained turbulence in Couette-Poiseuille flow*

Benoit Semin, Lukasz Klotz, Alexandr Pavlenko, Tao Liu, José Eduardo Wesfreid

*Main Foyer*

*Main Foyer*

Transition and Decay in Poiseuille, Couette and Couette-Poiseuille flow

*Room 200*

**Chair:** Marcello A. F. Medeiros (University of Sao Paulo)

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#### Abstract

We review results of experimental investigations into transition in pipe and Couette flows. The outcomes of this work are used to interpret our ex- perimental findings on the decay of turbulence in Couette-Poiseuille flow. We have used PIV techniques to help identify the salient structures of the flow field and thereby elucidate the energy distribution. In particular we have identified that the energy distribution decays anisotropically providing new insights into transition processes in shear flows.*Room 200*

*Distributed Excitation of Crossflow Waves Due To Scattering of Freestream Vortices on Surface Waviness*

Yury S. Kachanov, Vladimir I. Borodulin, Andrey V. Ivanov

*Boundary-Layer Receptivity to Entropy Waves in Subsonic and Transonic Flows.*

Anatoly I. Ruban, Marina A. Kravtsova, Sharad Keshari

*Linear and nonlinear stability of forced planar liquid jets*

Simon Schmidt, Kilian Oberleithner

*Acoustic receptivity of Tollmien-Schlichting waves to localised surface roughness*

Marco Placidi, Michael Gaster, Chris Atkin

*On the Effects of the Acoustic Wave Angle of Incidence in Subsonic Acoustic Receptivity*

Henrique Raposo, Shahid Mughal, Richard Ashworth

*Receptivity of unsteady compressible Gortler vortices to free-stream vortical disturbances*

Samuele Viaro, Pierre Ricco

*Main Foyer*

Progress in Development of Amplitude Method of Transition Prediction on Swept Wing

*Room 200*

**Chair:** Yury Kachanov (Khristianovich Institute of Theoretical and Applied Mechanics SB RAS, Novosibirsk, 630090, Russia)

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#### Abstract

Transition location in the boundary layer on the swept wing is rather sensitive to surface roughness and level of free-stream turbulence [1,2]. Conventional e^N method based on computation of linear amplification coefficients of instability modes can not describe dependence of transition Reynolds number from these factors. Alternative amplitude method of cross-flow dominated transition prediction based on computation of amplitudes of steady and traveling cross-flow instability modes is developed here. Initial amplitudes of disturbances in the boundary layer are found by means of decomposition of free-stream turbulence and surface roughness into a set of periodical waves and consideration of generation of Eigen modes in the boundary layer by these elementary waves via non-localized receptivity mechanism [3]. Subsequent evolution of steady and non-steady modes with continuous spectra and random phases is computed by simplified non-linear PSE-method. Transition location is determined as a place where the sum of amplitudes of steady and non-steady modes reaches a threshold value 0.34. This transition criterion was recently introduced from analysis of experimental data for wide range of surface roughness and turbulence level in [4]. The amplitude method developed reproduces satisfactorily the dependence of transition location on the Reynolds number, the surface roughness, and free-stream turbulence level observed in experiments [1, 2]. Moreover, it gives the evolution of almost all measurable characteristics of the base flow and perturbations in the transition region. In particular, it describes saturation of the growth of steady and traveling modes and the deformation of the velocity profiles in the boundary layer initiated by these modes. This amplitude method of transition prediction is rather simple and does not require large amount of computations. It can be used in future for operative prediction of transition location instead of e^N method.References

[1] Radeztsky R.H., Reibert M.S., Saric W.S. Effect of micron-sized roughness on transition in swept-wing flows. AIAA J. 37(11):1370–1377. 1999

[2] Deyhle H., Bippes H. Disturbance growth in an unstable three-dimensional boundary layer and its dependence on initial conditions// J. Fluid Mech. 316: 73-113., 1991

[3] Crouch J.D. Non-localized receptivity of boundary layers. J. Fluid Mech. 224:567–581, 1992.

[4] Crouch J.D., Ng L.L., Kachanov Y.S., Borodulin V.I., Ivanov A.V. Influence of surface roughness and free-stream turbulence on crossflow-instability transition. Procedia IUTAM, 2015. V. 14. IUTAM_ABCM Symposium on Laminar Turbulent Transition / Eds.: M.A.F. Medeiros & J.R. Meneghinipp, P. 295–302.

*Room 200*

*Experimental Investigation of Spanwise Periodic Surface Heating for Control of Crossflow-Dominated Laminar-Turbulent Transition*

Hans Peter Barth, Stefan Hein

*Real-time feedback control of TS waves*

Jonathan Morrison, Hari Vemuri, Richard Bosworth, Eric Kerrigan

*Nonlinear Optimal Control in Shear Flows using Deep Reinforcement Learning*

Onofrio Semeraro, Michele Alessandro Bucci, Alexandre Allauzen, Guillaume Wisniewski, Laurent Cordier, Lionel Mathelin

*Nonlinear Optimal Control of Transition due to a Pair of Vortical Perturbations using a Receding Horizon Approach*

George Papadakis, Dandan Xiao

*Active Attenuation of a Trailing Vortex Inpsired by Stability Analysis*

Louis Cattafesta, Ross Richardson, Adam Edstrand, Yiyang Sun, Kunihiko Taira, Peter Schmid

*Delay of bypass transition via data-driven reduced order modeling and control theory*

Pierluigi Morra, Kenzo Sasaki, Ardeshir Hanifi, André V. G. Cavalieri, Dan S. Henningson

*Main Foyer*

*Room 200*

*Experimental Investigation of Crossflow Instabilities*

Tariq Saeed, Jonathan Morrison

*Investigation on the freestream turbulence amplification approaching the attachment-line of swept circular cylinders using multi-component LDA*

Isabella Fumarola

*Instabilities and transition on a rotating cone – old problems and new challenges*

Kentaro Kato, Antonio Segalini, P. Henrik Alfredsson, R. J. Lingwood

*Interaction of free-stream turbulence and discrete roughness on boundary-layer transition*

Shumpei Hara, Santhosh Babu Mamidala, Jens Henrik Mikael Fransson

*Reynolds number dependence on very-large-scale features in transitional and turbulent channel flows*

Masaharu Matsubara, Yu Imanishi, Yuya Tanada, Sattaya Yimprasert, Yutaro Endo, Tatsuya Tsumura

*On the importance of free-stream turbulence length scale in boundary-layer transition*

Jens Henrik Mikael Fransson

*Experimental characterization of the effects of two-dimensional surface defects on the laminar-turbulent transition of a sucked boundary layer*

Jeanne Cam-Tu Methel, Maxime Forte, Olivier Vermeersch, Grégoire Casalis

*Main Foyer*

Unraveling Transition and Turbulence Using Nonlinear Optimization

*Room 200*

**Chair:** Yongyun Hwang (Department of Aeronautics, Imperial College London)

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#### Abstract

Transitional and turbulent flows are characterized by coherent structures, fluid motions highly correlated over both space and time in the form of small- and large- scale streaks and hairpin vortices, which carry a much larger momentum than the chaotic motion at small scales. It is now known that streaks arise due to a linear mechanism of transient energy growth, which is one of the main mechanisms allowing the self-sustainment of exact invariant solutions of the Navier-Stokes equations (equilibria, travelling waves, periodic orbits), which populate the state space capturing most of the organized flow structures recurrently observed. However, the commonly observed hairpin vortices still not have a clear place in this dynamical system view of turbulence, their origin being mostly unclear. Using a nonlinear optimization approach, in this talk we will identify and characterise highly energetic transient events such as the growth of hairpin vortices and the recurrence of bursts in transitional and turbulent flows as optimal flow structures. We will show that these optimal coherent structures reproduce well the spatial spectra and the probability density function of the velocity typically measured in turbulent flows. Finally, we use the nonlinear optimization in the framework of the dynamical system view of turbulence, showing how hairpin vortices emerge along a strongly amplified path in the stable manifold of an exact invariant traveling wave solutions. Stable manifolds, although exponentially contracting for infinite time, allow excursions on a finite time horizon and may thus provide the necessary connectivity between invariant solutions supporting turbulence. These results imply that hairpin vortices, even if inherently transient coherent structures, are robust features of transitional and turbulent shear flows, resulting from strong nonlinear transient growth that repeats in time as a by-product of the self-sustained wall cycle.*Room 200*

*Numerical / experimental investigations of the effect of transpiration cooling on second mode instabilities in hypersonic, conical flows*

Viola Wartemann, Giannino Ponchio Camillo, Alexander Wagner

*Roughness-Induced Laminar-Turbulent Transition in the Boundary Layer of a Capsule-Like Geometry at Mach 20 Including Non-Equilibrium*

Christian Stemmer, Antonio Di Giovanni

*Influence of High-Temperature Effects on the Stability of the Wake Behind an Isolated Roughness Element in Hypersonic Flow*

Iván Padilla Montero, Fernando Miró Miró, Fabio Pinna

*Instability analysis of under-expanded supersonic impinging jets*

Shahram Karami, Vassilis Theofilis, Julio Soria

*Effect of the streaky structures on the instabilities in supersonic boundary layers*

Jianxin Liu, Xuesong Wu

*Direct Numerical Simulations (DNS) of “natural” transition in high-speed boundary layers using a broadband random forcing approach*

Christoph Hader, Hermann Fasel

*The Role of Receptivity in Prediction of High-Speed Laminar-Turbulent Transition*

Ivan Egorov, Alexander Fedorov, Andrey Novikov

*Main Foyer*

*Main Foyer*

Tackling the Uncertainty of Transition Prediction in High-Speed Boundary Layers

*Room 200*

**Chair:** Spencer J Sherwin (Imperial College London)

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#### Abstract

Transition to turbulence in high-speed boundary layers is very sensitive to the environmental conditions. Even seemingly negligible changes in the spectral content of free-stream disturbances can appreciably shift transition location, alter the heat-transfer rate on the wall and significantly affect drag. This sensitivity, combined with the wealth of possible mechanisms for transition to turbulence, present a challenge: How can any simulation provide meaningful predictions when flight conditions are often uncertain? In order to address this challenge, we set out to determine the earliest possible transition location for a given flow, independent of the spectral makeup of the environment. We call this condition the “nonlinearly most dangerous disturbance”. The problem is formulated as a constrained optimization, where the objective is to identify the inflow disturbance that leads to the lowest transition Reynolds number in a Mach 4.5 zero-pressure-gradient boundary layer. The constraints are the initial disturbance total energy and that the flow satisfies the full nonlinear Navier-Stokes equations. The results are surprising and cannot be ascribed to classical transition scenarios. Through a series of nonlinear energy exchanges, our inflow disturbance modifies the base state and spurs new instabilities that cause transition to turbulence upstream of any other inflow condition. We have also devised new approaches to enhance the fidelity of our simulations, so they are more representative of experimental and flight conditions. Our methodologies are robust, and applicable with any computational approach.*Room 200*

*What can we learn from the Edge about bypass transition?*

Miguel Beneitez, Yohann Duguet, Philipp Schlatter, Dan S. Henningson

*The minimal seed for wall-bounded transition in the frequency domain*

Georgios Rigas, Denis Sipp, Tim Colonius

*Nonlinear optimal disturbances in compressible shear flows*

M. J. Philipp Hack, Zhu Huang, Tim Flint

*Amplitude-Dependent Three-Dimensional Neutral Modes in Plane Poiseuille-Couette Flow at Large Reynolds Number*

Rishi Kumar, Andrew Walton

*Distributed VWI arrays and the emergence of self-similarity in turbulent shear flows.*

philip hall

*Transition in Rotating Plane Couette Flow, Revisited*

Masato Nagata, Baofang Song, Darren P. Wall

*Main Foyer*

Linear instability mechanisms in turbulent flows

*Room 200*

**Chair:** Ardeshir Hanifi (KTH Royal Institute of Technology)

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#### Abstract

Large-scale coherent structures have been observed in turbulent flows for some decades. Their behaviour has been modelled using linear stability analysis of the mean flow; for instance, models for jets predict the appearance of a hydrodynamic wavepacket, due to spatial amplification, saturation and decay of Kelvin-Helmholtz modes. However, early comparisons between stability results and experiments were often of qualitative nature, due to the intrinsic difficulty of isolating wavepackets amidst a turbulent flow. This talk reviews linear analysis for turbulent flows, focusing on more recent developments, where coherent turbulent structures are modelled as the most amplified flow response to non-linear excitation via resolvent analysis. Spectral proper orthogonal decomposition (SPOD) appears as the natural approach to obtain coherent structures from spatio-temporal data from experiment or simulation; such structures can in turn be quantitatively compared to the most amplified responses from resolvent analysis. The approach is exemplified by comparisons between theoretical and experimental wavepackets in turbulent subsonic jets. Moreover, the same linear methods are used for the analysis of recently discovered phenomena within turbulent jets: acoustic modes trapped in the potential core, and large-scale streaks.*Room 200*

*Use of instabilities for optimal laminar separation delay*

Michael Karp, M. J. Philipp Hack

*Transient growth analysis of the flow around an elastically mounted circular cylinder*

Daiane I. Dolci, Bruno S. Carmo

*Instabilities in laminar shock boundary layer interactions*

Nathaniel Hildebrand, Anubhav Dwivedi, Sidharth GS, Joseph Nichols, Mihailo Jovanovic, Graham Candler

*On a new non-modal instability in a two-dimensional disk-type flow*

Tim Gebler, Judith Kahle, Dominik Plümacher, Martin Oberlack

*Instability of tilted shear flow in a strongly stratified and viscous medium*

Yongyun Hwang, Lloyd Fung

*Taking advantage of randomness to empower resolvent analysis*

Jean Helder Marques Ribeiro, Chi-An Yeh, Kunihiko Taira

*Room 200*

*Time-evolving network analysis of two-dimensional turbulence*

Chi-An Yeh, Muralikrishnan Gopalakrishnan Meena, Kunihiko Taira

*Study of Transition to Turbulence using Discrete Directed Percolation Model*

Kouta Watanabe, Hideki Shiiba, Yoshio Ishii

*Spatially localized states and their dynamics in transitional plane Couette flow*

Anton Pershin, Cedric Beaume, Steven Tobias

*On Subharmonic Resonance and Other Nonlinear Mechanisms in Wavepackets in Boundary Layers*

Marcello A. F. Medeiros, Fernando H. T. Himeno, Marlon S. Mathias, Andrés G. Martinez

*Nonlinear evolution of multiple helical modes on subsonic circular jets with a large radius: a weakly nonlinear critical-layer theory*

Zhongyu Zhang, Xuesong Wu

*On the role of actuation for the control of streaky structures in boundary layers*

André V. G. Cavalieri, Kenzo Sasaki, Pierluigi Morra, Ardeshir Hanifi, Dan S. Henningson

*Senior Common Room*

*Main Foyer*

*Room 200*

*Receptivity of a hypersonic blunt cone boundary layer to slow acoustic wave: role of entropy-layer disturbance*

Caihong Su, BingBing Wan

*Receptivity of inviscid mode in supersonic boundary layers due to interaction of freestream acoustic waves and wall roughness*

Ming Dong

*Global stability analysis and DNS of a swept airfoil section in subsonic flow*

Neil D. Sandham, Nicola De Tullio

*Experimental Study of Distributed Receptivity Coefficients at Excitation of Crossflow Waves by Freestream Vortices*

Vladimir I. Borodulin, Andrey V. Ivanov, Yury S. Kachanov

*Effect of discrete widely spaced suction on a transitioning flow at high suction rates*

Barry Crowley, Chris Atkin

*Main Foyer*

*Room 200*