CTR Seminars Archive 2020
Flow and Ignition Dynamics Following Laser-induced Breakdowns
Date and Time: Friday, November 20, 2020 - 16:30
Location: Zoom
Event Sponsor: Parviz Moin, Director of Center for Turbulence Research
Speaker(s): Dr. Jonathan Wang, Postdoctoral Fellow in the PSAAP III project at the Center for Turbulence Research at Stanford University
Laser-induced breakdown is a versatile means of depositing energy in a fluid and a promising alternative to conventional electrode-spark ignition for combustion systems. Using numerical simulations we analyze the flow dynamics following the laser pulse and show that it is sensitive to even subtle alterations in the plasma kernel, which lead to qualitative changes in the flow pattern and ejections of hot gas from the laser focal region. This sensitivity is leveraged in a dual-pulse configuration, in which the timing and positioning of the pulses can be controlled to enhance dispersal of hot gas and increase the burning rate of nascent flames. In an inhomogeneous mixture, it is further shown that the rapid plasma expansion can produce ignition-suppressing flow so pronounced in some cases that ignition fails. The dependence of these hydrodynamics on electron recombination and diffusion is also assessed.
Speaker Bio:
Dr. Jonathan Wang is a Postdoctoral Fellow in the PSAAP III project at the Center for Turbulence Research at Stanford University. He received his B.S. at the University of California Berkeley and completed his PhD at the University of Illinois Urbana−Champaign, where he worked in the PSAAP II Center and was advised by Jonathan Freund. His research interests include high-speed compressible flows, combustion dynamics, and chemically reacting flows.
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Investigation of direct combustion noise in turbulent premixed jet flames using DNS
Date and Time: Friday, November 6, 2020 - 16:30
Location: Zoom
Event Sponsor: Parviz Moin, Director of Center for Turbulence Research
Speaker(s): Dr. Davy Brouzet, Postdoctoral Researcher at the Stanford Center for Turbulence Research
Direct combustion noise plays a key role in the triggering and dynamics of thermo-acoustic instabilities of modern gas turbines. Moreover, many combustion devices can produce high levels of noise, while being subjected to stringent noise regulations. Achieving a better understanding of the sound production by premixed flames is important for designing safer and quieter combustion devices. The presented research has for goals to study the mechanisms involved in the sound generation process of turbulent premixed jet flames using DNS. State of the art post-processing methods are used to analyze the importance of shear layer structures and flame `annihilation events’ to the generated sound. The impact of chemistry modelling on combustion noise is also studied and differences between simple and complex mechanisms are investigated.
Speaker Bio:
Dr. Davy Brouzet is a postdoctoral researcher at the Stanford Center for Turbulence Research. He obtained his Bachelor and Master degree from the Swiss Federal Institute in Lausanne (EPFL) in 2012 and 2014, respectively. Then, he pursued his PhD at the University of Melbourne under the supervision of Prof. Michael Brear and Dr. Mohsen Talei, studying combustion noise in turbulent premixed flames using direct numerical simulations. During this time, he was fortunate to collaborate with researchers from Caltech and CERFACS. His research interests include turbulent combustion, aero-acoustics and combustion noise, and numerical methods for reacting flows.
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Transcritical diffuse-interface hydrodynamics of propellants in high-pressure combustors of chemical propulsion systems
Date and Time: Friday, October 23, 2020 - 04:30
Location: Zoom
Event Sponsor: Parviz Moin, Director of Center for Turbulence Research
Speaker(s): Dr. Javier Urzay, Sr. Research Engineer at the Stanford Center for Turbulence Research
Rocket engines and high-power new generations of jet engines and diesel engines oftentimes involve the injection of one or more reactants at subcritical temperatures into combustor environments at high pressures, and more particularly at pressures higher than those corresponding to the critical points of the separate components, which typically range from 13 to 50 bars for most propellants. This class of trajectories in the thermodynamic space has been traditionally referred to as transcritical. However, the fundamental understanding of fuel atomization, vaporization, mixing, and combustion processes at such high pressures remains elusive. A theory of the transcritical hydrodynamics of propellants in high-pressure combustors will be presented in this talk. This theory couples the multicomponent Navier-Stokes conservation equations with an extended version of the diffuse-interface theory of van der Waals. Technological factors that motivate the investigation of this problem will be outlined, and fundamental transcritical flow structures revealed by this theory will be discussed.
Speaker Bio:
Dr. Javier Urzay is a Sr. Research Engineer at the Stanford Center for Turbulence Research, where he has worked for nearly a decade. He received his B.Sc./M.Sc. degree in Mechanical Engineering in 2005 from the Carlos III University of Madrid (Spain), and his M.Sc. and Ph.D. degrees in Aerospace Engineering in 2006 and 2010 from the University of California San Diego working on theoretical combustion physics and fluid mechanics. His research interests include high-speed, chemically reacting, multi-phase turbulent flows, hypersonic aerothermodynamics, supersonic combustion, high-pressure propulsion systems, chemical rockets, and their applications to aeronautics and astronautics. He is currently a member of the U.S. Air Force Reserves under the Air Mobility Command at Travis Air Force Base.
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Performance & improvements of wall-modeled large-eddy simulation for external aerodynamics
Date and Time: Friday, October 9, 2020 - 06:30
Location: Zoom
Event Sponsor: Parviz Moin, Director of Center for Turbulence Research
Speaker(s): Dr. Adrian Lozano-Duran, Postdoctoral Research Fellow at the Center for Turbulence Research at Stanford University
The use of computational fluid dynamics for external aerodynamic applications has been a key tool for aircraft design in the modern aerospace industry. In the last decades, large-eddy simulation with near-wall modeling (wall-modeled LES) has gained momentum as a cost-effective approach for both scientific research and industrial applications. In this talk, we discuss current challenges of wall-modeled LES to become a design tool for the aerospace industry. We focus first on the working principles and performance of wall-modeled LES for external aerodynamic applications, with emphasis on realistic commercial aircrafts. In particular, we examine the computational cost to predict mean flow features and forces for a given degree of accuracy using theory and numerical simulations of the NASA Juncture Flow. It is shown that current models might underperform in out-of-equilibrium conditions and we provide some tentative corrections based machine learning techniques.
Speaker Bio:
Dr. Adrian Lozano-Duran is a Postdoctoral Research Fellow at the Center for Turbulence Research at Stanford University hosted by Prof. Moin. He received his PhD in Aerospace Engineering from the Technical University of Madrid in 2015 at the Fluid Mechanics Lab. advised by Prof. Jimenez. The overarching theme of his research is physics and modeling of wall-bounded turbulence via theory and computational fluid mechanics. His work covers a wide range of topics, such as turbulence theory and modeling by machine learning, large-eddy simulation for external aerodynamics, geophysical and multiphase flows, among others.
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Shock-induced phenomena in turbulent flows through numerical simulation and diagnostics
Date and Time: Friday, September 25, 2020 - 16:30
Location: Zoom
Event Sponsor: Parviz Moin, Director of Center for Turbulence Research
Speaker(s): Professor Ivan Bermejo-Moreno
Shock-induced scalar mixing and ignition under canonical shock-turbulence interactions (STI) will be considered first, by means of Direct Numerical Simulation in 3D and 2D, respectively. The effects of relevant physical parameters (shock and turbulence Mach numbers, and Reynolds number) will be highlighted on statistical changes along the shock-normal direction of scalar variance and dissipation-rate budgets, flow topology, and alignments of the scalar gradient with vorticity and strain-rate eigendirections. Shock-induced scalar mixing will also be addressed by tracking the downstream evolution of the geometry and physics of scalar structures initialized with a well-defined shape as they are transported and diffused by the background turbulence in STI, and compared with decaying homogeneous isotropic turbulence.
Flow-structure interactions of shock waves reflecting off turbulent boundary layers that develop along flexible walls will be addressed next, comparing results from ongoing numerical simulations with prior wind tunnel experiments. The calculations couple wall-modeled large-eddy simulation for the fluid flow, using an Arbitrary Lagrangian-Eulerian formulation, with an elastic solid structural solver that accounts for geometric nonlinearities, and a mesh deformation module based on a spring-system analogy. Strong shock/boundary-layer interactions resulting in mean flow separation and low-frequency unsteadiness that can interact with the natural frequencies of the structure will be emphasized.
Speaker Bio:
Ivan Bermejo-Moreno received his Ph.D. in aeronautics in 2008 from the California Institute of Technology. Afterwards, he held a postdoctoral research fellowship at the Center for Turbulence Research, Stanford University/NASA Ames Research Center from 2009 to 2014. He joined the Aerospace and Mechanical Engineering Department at the University of Southern California in 2015. His research combines numerical methods, physical modeling and high-performance computing for the simulation and analysis of turbulent fluid flows involving multi-physics phenomena. He is a recipient of the Fulbright Fellowship, the Rolf D. Buhler Memorial Award, the William F. Ballhaus Prize and the Hans G. Hornung Prize.
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Overview of transition prediction methods
Date and Time: Friday, February 14, 2020 - 16:30
Location: CTR Conference Room 103
Event Sponsor: Parviz Moin, Director of Center for Turbulence Research
Speaker(s): Dr. Michael Karp, Postdoctoral Fellow in the Center for Turbulence Research at Stanford University
An overview of several available methods for prediction of the transition location is presented. Two particular methods are discussed – the eN method, and the empirical, correlation-based, model of Langtry & Menter (2009). The role of stability theory is explored, including local stability theory, incorporation of non-parallel effects (parabolized stability equations), and extensions into the nonlinear regime. Limitations and drawbacks are pointed out and possible future paths for improvement are highlighted. An area of particular interest is the receptivity stage, where environmental disturbances and surface imperfections trigger instability mechanisms in boundary layers.
Speaker Bio:
Dr. Michael Karp is a Postdoctoral Fellow in the Center for Turbulence Research at Stanford University since 2017. Michael received all of his degrees from the Faculty of Aerospace Engineering, Technion (BSc 2007, Msc 2013, PhD 2017). His research combines theory and computations for understanding transition to turbulence. His research interests include Aerodynamics, Fluid Mechanics, Flow Instabilities, Transition to turbulence, Flow Control and Flight Mechanics.
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Hydrodynamics of migrating zooplankton aggregations
Date and Time: Friday, January 24, 2020 - 16:30
Location: CTR Conference Room 103
Event Sponsor: Parviz Moin, Director of Center for Turbulence Research
Speaker(s): Dr. Isabel Houghton, Postdoctoral Fellow at the Data Institute of University of San Francisco (USF)
Biologically generated turbulence has been proposed as an important contributor to nutrient transport and ocean mixing. However, for swimming animals to produce non-negligible transport and mixing, they must produce eddies at scales comparable to the length scales of stratification in the ocean. It has previously been argued that biologically generated turbulence is limited to the scale of the individual animals involved, which would make turbulence created by highly abundant centimeter-scale zooplankton such as krill irrelevant to ocean mixing. Their small size notwithstanding, zooplankton form dense aggregations tens of meters in vertical extent as they undergo diurnal vertical migration over hundreds of meters. In this work, we investigate the potential for this behavior to introduce additional length scales — such as the scale of the aggregation — that are of relevance to animal interactions with the surrounding water column. Utilizing laboratory experiments, we show that the collective vertical migration of centimeter-scale swimmers generates aggregation-scale eddies that mix a stable density stratification, resulting in a significantly enhanced effective turbulent diffusivity. The large-scale fluid transport similarly enhances mixing of other relevant scalars, such as dissolved oxygen, leading to cascading biogeochemical effects upon the water column. Altogether, the results illustrate the potential for marine zooplankton to considerably alter the physical structure of the water column, with potentially widespread effects owing to their frequent vertical migrations and high abundance in climatically important regions of the ocean.
Speaker Bio:
Dr. Isabel Houghton is currently a postdoctoral fellow at the Data Institute of University of San Francisco (USF) utilizing data science techniques to conduct research on observing oceanic dynamics. Prior to USF, she received her Ph.D. in Environmental Engineering from Stanford University in 2019. Her research interests broadly include experimental and computational approaches to understanding fluid dynamics relevant to the ocean.
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Time-Accurate and highly-Stable Explicit (TASE) operators for stiff differential equations
Date and Time: Friday, January 10, 2020 - 16:30
Location: CTR Conference Room 103
Event Sponsor: Parviz Moin, Director of Center for Turbulence Research
Speaker(s): Dr. Maxime Bassenne, Postdoctoral Research Fellow in the Laboratory of Artificial Intelligence in Medicine and Biomedical Physics in the Stanford Radiation Oncology department
Unconditionally stable implicit time-marching methods are powerful in efficiently solving stiff differential equations. In this talk, I will present a novel unified framework for handling both physical and numerical stiffness based on Time-Accurate and highly-Stable Explicit (TASE) operators.
The proposed TASE operators act as preconditioners on the stiff terms and can be readily deployed to most existing explicit time-marching methods. The resulting time integration method remains the original explicit time-marching schemes, yet with nearly unconditional stability. The TASE operators can be designed to be arbitrarily high-order accurate such that the original explicit time-marching accuracy order is preserved. I will illustrate the performance of the TASE method on a set of benchmark problems with strong stiffness. Numerical results demonstrate that the proposed framework preserves the high-order accuracy of the explicit time-marching methods with very-large time steps for all the considered cases.
Speaker Bio:
Dr. Maxime Bassenne is a Postdoctoral Research Fellow in the Laboratory of Artificial Intelligence in Medicine and Biomedical Physics in the Stanford Radiation Oncology department. He received his PhD degree in Mechanical Engineering from Stanford University in 2019. His research interests broadly revolve around advancing computational science to solve problems in engineering and medicine.