MA4: Instability, Transition and Turbulence in Fluids: from Fundamentals to Applications

Room: Old Main Academic Center 3090

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OrganizerAdrian Sescu, Mississippi State University

Daniel Foti, University of Memphis

Time: 10:00 am - 10:25 am (CST)

Title: Quantification of the Scale and Energy Transfer of Coherent Structures in Turbulent Wakes

Abstract: Coherent structures are an inherent stochastic feature of many turbulent flows that persist relative to certain time scales of the flow. The structures are observed to convect in the flow and are associated with large-scale momentum and energy transport. With the viability of high-fidelity simulations, vast amounts of data are available to comprehensively investigate the flow field to (i) quantify spatio-temporal behavior of coherent structures and (ii) elucidate details of the energy transport and transfer. Triple decomposition and mode decomposition provide generalized approaches for classifying coherent structures and identifying their scale and evolution in terms of velocity fluctuations. Individual coherent structure scales are separated, and an inter-connect system of coherent kinetic energy budgets specific to each scale is obtained. Each budget contains terms that represent inter-scale energy transfer. The scale-specific kinetic energy in wakes are examined and reveal that inter-scale transfer occurs over a wide range of scales while transport is dominated by the largest scales. This provides new approaches towards reduced-order modeling of turbulent flows. Preliminary analysis of wake meandering and other related energetic coherent structures in wind turbine wakes are analyzed to identify scale interactions towards reduced-order wind turbine models.

Xiaowen Wang, National Institute of Aerospace, VA

Time: 10:20 am - 10:50 am (CST)

Title: Transition Control of Hypersonic Boundary Layers by Using Ultrasonically Absorptive Porous Coatings

Abstract: The design of hypersonic vehicles is constrained by the considerable drag and heat transfer. These constraints are significantly enhanced as the boundary-layer transition happens. Strategies to maintain laminar boundary layers or delay transition can dramatically reduce drag and surface heating. For small amplitude disturbance, the transition of a boundary layer over a smooth surface consists of three stages: receptivity, modal growth of unstable waves, and breakdown to turbulence. According to the three-stage transition, modal growth of unstable boundary-layer waves is critical to transition control.In the past two decades, the stabilizing effect of ultrasonically absorptive porous coatings on hypersonic boundary layers has been demonstrated by theoretical analyses, experiments, and numerical simulations. In this work, a review of the existing literature on the topic is firstly reported. It is noticed that researches on stabilization efficiency of porous coating are limited and almost all previous work consider either felt-metal porous coating or regular porous coating. Here, the stabilities of two hypersonic boundary layers are investigated with a combination of linear stability theory and numerical simulation, which shows the physics of transition control using porous coating. Three approaches are then proposed for the application of porous coating to achieve high stabilization efficiency.

James Coder, University of Tennessee Knoxville

Time: 10:50 am - 11:15 am (CST)

Title: PDE-Based Methods for Transition Prediction in Complex Aerodynamic Flows

Abstract: Predicting laminar-turbulent transition from first principles is a difficult under taking for practical aerodynamic flows. These flows often feature high Reynolds numbers, geometric complexities, and inherent unsteadiness, limiting the ability to perform either DNS or detailed stability calculations against a steady base flow.Consequently, there has been increased community interest in PDE-based transition models that are not restricted by discretization strategy or geometry topology. Such models can be fully integrated within a flow solver without an excessive increase in grid resolution requirements or sacrificing parallelization, and they can be applied to general three-dimensional configurations with minimal user intervention. This presentation will provide an overview of these PDE-based models, including the amplification factor transport (AFT) model developed by the author. The AFT model is rooted in linear stability theory and it has been successfully applied to a wide range of engineering applications. The model formulation and coupling with PDE-based turbulence models will be discussed, along with efforts made to ensure its suitability next-generation, finite-element flow solvers. Predictions obtained using the model will be presented along with quantitative and qualitative comparisons to experimental baselines.

Praveen Ramaprabhu, University of North Carolina at Charlotte

Time: 11:15 am - 11:40 am (CST)

Title: Shock-driven Hydrodynamic Instabilities: From Clean Energy to Stellar Explosions

Abstract: This talk will describe recent developments in our understanding of shock-driven instabilities that are observed when an interface between disparate media is impulsively accelerated. Even miniscule undulations at the material interface grow under the influence of baroclinic vorticity deposited by the shock, and eventually serve to vigorously mix the two (or more) layers of pure fluids. Such a shock-driven flow is termed the “Richtmyer-Meshkov” instability, and can be observed in many situations in nature and in engineering applications. For example, in laser-driven nuclear fusion, instability-driven mixing is harmful to the objective of maximizing yield, while the opposite is true for Scramjet combustion where instabilities can drive the process to higher combustion efficiencies. The Richtmyer-Meshkov instability is also an important ingredient in explaining supernovae detonations, where stellar models that neglect mixing fail to reproduce the observed light curves. I will highlight recent numerical simulations that have expanded our understanding of how the instability progresses under a variety of conditions including convergent geometries, inclusion of material strength effects, and in chemically reacting flows.

Denis Aslangil, University of Alabama

Time: 11:40 am - 12:05 pm (CST)

Title: Buoyancy-Driven Variable-Density Turbulence

Abstract: In the real world, turbulence occurs in multi-material/phase flows, which in most cases involve materials with large density differences. Unlike incompressible single-fluid flows, the velocity field in multi-material flows with compositional variations is tightly coupled with the density field, and these flows are called variable density (VD) flows. Such flows are of broad interest due to their occurrence in the form of Rayleigh-Taylor and Richtmyer-Meshkov instabilities and VD jets, mixing layers in high-speed air vehicles, combustion applications in ramjet engines, high energy density processes like inertial confinement fusion (ICF). Moreover, they are observed in convection regions in the atmosphere, oceans, and the Earth’s mantle. In this talk, the results of extremely large-resolution (domain size up to 20483) direct numerical simulations (DNS) of buoyancy-driven homogeneous VD turbulence will be presented to explore the turbulent mixing and transitions of the flow with large density variations. In addition, the talk will discuss the highly asymmetric nature of the VD flow development at large density ratios.