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What is a Model?

Conor Rowan
PhD Student
Aerospace Engineering Sciences
University of Colorado

Abstract: For many engineers, and especially for engineering graduate students, working with models is extremely familiar. For example, researchers studying the mechanical behavior of solids and fluids tend to make use of the overarching model of continuum mechanics to make sense of the relationships between basic phenomena in materials such as stress and strain. More specialized models exist under the umbrella of continuum mechanics to describe complex phenomena such as fracture and aspects of turbulent flows. Within the context of engineering, it is clear what constitutes a model: models are mathematical relationships between physical quantities used to describe the behavior of complex engineering systems. These mathematical relationships often involve simplifying assumptions, empirical quantities, and fundamental laws of physics. Though the concept of a model may appear straightforward to grasp when viewed from this engineering perspective, there are a host of interesting and tricky questions when viewed from the perspective of philosophy. In this seminar, we will expand the definition of models outside of science by asking the questions: how can we think of all knowledge-producing fields such as economics, sociology, biology, psychology, and even religion as working with models of different sorts? Is it possible to formulate a definition of a “model” that is sufficiently general to encompass broader descriptions of the world than engineering models? In addition to surveying some famous concepts from the philosophy of science, I will argue that models are used in disciplines outside science and engineering and are fundamental to the way we understand our world. Goals and values will be discussed as essential components of the process of developing and using models, indicating that our basic epistemic tools have unavoidable subjective elements. As future experts and science communicators in a climate of political polarization and distrust of expertise, it is important that PhD students develop broad, humble, and thoughtful perspectives on the nature and scope of scientific models. As such, students are encouraged to cultivate a perspective on their work in engineering that includes critical thinking about where scientific models come from, and what kinds of questions they are equipped to address.

Passive Attenuation of Tollmien-Schlichting Waves by Phononic Subsurfaces

Thomas Calascione
PhD Student
Aerospace Engineering Sciences
University of Colorado Boulder

Abstract: The drag reduction through the delay of laminar-to-turbulent transition in wall-bounded flows has been a focus of the flow control community for decades. One path previously investigated for achieving these benefits is through active opposition control of Tollmien-Schlichting (TS) waves using a piezoelectrically driven oscillating surface (PDOS, Amitay et al. 2016). However, the dynamic interaction of the fluid and the solid surface can also be exploited through passive coupling. Hussein et al. (2015) computationally demonstrated a solid phononic subsurface (PSub) flow stabilization technique to attenuate or amplify the growth of TS waves. In this method, the interaction between the fluid and the PSub passively generates a spatio-temporal elastic deformation profile at the surface of the solid which counters the growth of the TS-waves. In the current work, this technique is explored experimentally within the University of Colorado Boulder low-speed wind tunnel. TS waves are seeded into the flow by an upstream PDOS within the laminar boundary layer of a 3.5 m long flat plate assembly at a freestream speed of 15 m/s. Time-resolved particle image velocimetry measurements are used to assess the growth and mitigation of the TS waves for both the baseline flow and the case with a PSub installed downstream for comparison with the prior computational results of Hussein et al. (2015).

Characterization of a Novel Inclinable Wind Tunnel
for the Fundamental Study of Wildfire Combustion

Laura Shannon
PhD Student
Department of Mechanical Engineering
University of Colorado Boulder

Abstract: As the frequency and magnitude of wildfires grow worldwide, it is becoming increasingly important to understand the dynamics behind how these fires grow and spread. Wildfire spread is complex and affected by numerous variables. Two of these– cross-flow wind speed and ground slope– have a significant impact on the physical and chemical processes that occur during a burn. A novel inclinable wind tunnel facility (the WindCline) was constructed to study the coupling between the inertial cross-flow and buoyancy vectors in a controlled, combusting flow. The facility allows both the angle and ratio between the two vectors to be modified. The WindCline can operate at wind speeds up to 20 m/s with a freestream turbulence intensity less than 1% and is inclinable from -13 to +15 degrees. The 0.35 m wide by 0.85 m long test-section is highly modular to allow for combustion experiments with both solid fuel arrays and gaseous burners in flexible arrangements. The suite of measurement systems used to investigate both fluid dynamic and combustion processes in these flows includes high-speed particle image velocimetry, dual frequency comb laser spectroscopy, and particulate matter emissions sensors. A detailed characterization of the WindCline facility will be presented in addition to preliminary results for solid fuel combustion arrays of wooden pegs.

Minimizing the Distortions Induced by Mean Shear within 3D Reconstructions of Turbulent Flows
from Time-Resolved sPIV Measurements

Samantha Sheppard
PhD Candidate
Aerospace Engineering Sciences
University of Colorado Boulder

Abstract: The efficacy of reconstructing a 3D volume of time-evolving 3 component velocity from planar experimental measurements is explored within strongly shear-distorting turbulent flows. A common approach to convert the temporal dimension into the streamwise spatial dimension is Taylor’s frozen turbulence hypothesis where the mean velocity is imposed as the convective velocity. In flows with a strong mean shear-rate the instantaneous turbulence structure is distorted when a traditional Taylor’s hypothesis method is used to reconstruct 3D volumes. In the current study, we compare existing methods that extend the classical Taylor’s hypothesis approach to retain time-locality in the convective velocity in order to accurately reconstruct a 4D (time-resolved) velocity field for accurate analysis of turbulence structure. Specifically, we analyze a local mean convective velocity approach (Pinton & Labbe 1994) as well as an instantaneous convective velocity approach (Fratantonio et al 2021) using time-resolved sPIV measurements in transverse and longitudinal planes within the near-wall surface layer of a canonical flat-plate turbulent boundary layer at Reθ=7,700. The reconstruction methods are evaluated based on their ability to preserve both the statistical properties of the flow and the instantaneous structure of the turbulence eddies as well as the streamwise extent to which these methods can be applied.

Vorticity Dynamics in Bluff Body Stabilized Premixed Flames
with External Pressure Gradients and Free-Stream Turbulence

Kelsea Souders
PhD Student
Department of Mechanical Engineering
University of Colorado Boulder

Abstract: Previous studies based on direct numerical simulations of statistically planar premixed flames have shown that the fluctuating velocities generated by a flame are small compared to those generated by the flow for highly turbulent (i.e., high Karlovitz, low Damkohler) conditions. However, recent computational and experimental studies have indicated that the flame-generated and flow-generated fluctuations can have the same leading-order magnitude in configurations featuring large background pressure gradients. In this study, we use PeleC, a fully compressible reacting flow code featuring adaptive mesh refinement, to study vorticity dynamics in the near-wake region of a bluff-body stabilized premixed flame at near-blowoff conditions. We consider the combined effects of free-stream turbulence and mean background pressure gradients, as well as their cumulative effect, on the balance between flame-generated turbulence attributed to heat release from combustion and flow-generated turbulence inherent to the flow configuration. We describe how different terms in the vorticity magnitude transport equation vary with turbulence intensity and mean pressure gradient and connect these results with observed flame phenomena. Ultimately, this study will inform the development of improved models for large eddy simulations used to model real-world combustion systems.

Ceramic Composites for High Temperature Aerospace and Propulsion Systems

David Marshall
Research Professor
Aerospace Engineering Sciences
University of Colorado Boulder

Abstract: Fiber-reinforced ceramic-matrix composites (CMCs) offer unmatched performance as lightweight, strong, high-temperature materials for power generation and aerospace. The design and development of composites with 3D fiber architectures optimized for applications in turbine engines, hypersonic vehicles, and rocket engines will be discussed. The most demanding extreme-heat-flux applications use active cooling, enabled by thin textile-based skins that can tolerate high thermal gradients. They can also enable other functionality, including transpiration and film cooling, mitigation of thermal stresses, and shape-morphing structures. The major challenges and strategies for realizing the potential of these materials will be discussed, with emphasis on recent developments in X-Ray CT methods to characterize microstructural features and damage as required for life prediction.

Generalizing the Surface Layer:
Investigating the Physical Mechanisms by which Impermeable Surfaces Alter Turbulence Structure

Samantha Sheppard
PhD Student
Aerospace Engineering Sciences
University of Colorado Boulder

Abstract: In all wall-bounded turbulent flows there is an inertia-dominated region near the surface where the structure of the turbulence is directly modified by the presence of the impermeable surface. Because the behavior of turbulence near boundaries is central to a wide variety of technologies and applications, it is important to develop both fundamental understanding and accurate prediction methods for surface-turbulence interactions, including both statistical and local flow structure near surfaces in wall-bounded turbulent flows. My research program aims to generalize the classical concept of the inertial “surface layer” within canonical shear-driven turbulent boundary layers to include wall-bounded turbulent flows in general, specifically the near-wall inertia-dominated regions where the energetic integral-scale turbulence eddies are directly modified by the surface in such a way that horizontal coherence lengths of vertical velocity fluctuations increase linearly with distance normal to the surface.  In this work, the turbulence interactions with an impermeable surface are explored experimentally both in the presence and in the absence of mean shear. We hypothesis that the effect of surface impermeability, both in the presence and absence of mean shear rate, will be observed in the turbulence structure of vertical turbulent velocity fluctuations. With this view, the classical concept of a surface layer with linear growth in horizontal integral scale of vertical turbulent velocity fluctuations is generalizable to all wall-bounded turbulent flows.

Nonsteady Load Responses on the Main Shaft by the Passage of Daytime Atmospheric Turbulence Eddies through the Rotor of Utility-scale Wind Turbines

Jarred Kenworthy
PhD Candidate
Centre for Doctoral Training in Wind & Marine Energy Systems & Structures
Strathclyde University, Glasgow, Scotland

Abstract: As the structural component on the drivetrain closet to the rotor, the main bearing responds directly to fluctuating forces and moments on the main shaft, that are driven by aerodynamic interactions between the rotor blades and highly energetic turbulence eddies within the atmospheric boundary layer (ABL) that pass through the rotor. The implication is that the main bearing is forced at multiple time scales during the passage of typically strong daytime atmospheric turbulence eddies which take place continuously during the day over the lifetime of the wind turbine. The current study has performed analysis of nonsteady moment and force components on the main bearing from the passage of energy-dominant atmospheric turbulence eddies through the rotor plane of utility scale wind turbines using high-fidelity LES of the daytime ABL with an advanced actuator line model (ALM) for the blades within the turbine rotor. The two-way coupled nature of the ALM captures the small-scale transients. The aim is to use these quantifications to determine the nonsteady forcings that lead to premature failure on the main bearing, with the aim to understand the mechanisms that drive the detrimental nonsteady forcings.

Aerothermochemistry of Hypersonic Flight:
Advanced Modeling Approaches and Experimental Verification

Maninder Grover
Air Force Research Laboratory (AFRL)
Dayton, OH

Abstract: Energetic shockwaves generated during hypersonic flight can heat up the gas enveloping a vehicle to temperatures that cause excitation of internal energy modes and chemical reactions in the gas-phase. As hypersonic flight trajectories usually occur at high altitude, the high speed and low density of the flow can cause the characteristic flow times to be comparable to rates of molecular excitation and chemical reactions in the gas. Therefore, the gas enveloping a hypersonic body can be in thermal and chemical nonequilibrium. Traditional modeling of flow in thermochemical nonequilibrium is based on empirical relations and experimental data from the Apollo era, which causes the predictive models to have large uncertainties. This can influence flow features like shock stand-off and the composition of the shock-heated gas in predictive simulations, and consequently can affect estimation of mission critical quantities such as surface heating experienced by the vehicle. In recent years, advancement in theoretical chemistry has led to the development of high fidelity molecular interaction potentials using quantum mechanics. These potentials have been used to simulate molecular interactions which have shed light on molecular level mechanisms for macroscopic phenomena - like excitation of internal energy modes and chemical reactions. As computational capability of HPC systems increases we are now able to embed these molecular interactions within time accurate flow field simulations of hypervelocity experiments. An atomic level understanding of hypersonic experiments paves a path for accurate and verified models for lower fidelity computations.

Bio: Dr. Grover graduated with his doctorate in Aerospace from the University of Minnesota in 2018. Since then he has been a contracted researcher for the Air Force Research Laboratory. Dr Grover specializes in particle methods, nonequilibrium flow modeling and high fidelity methods for simulation of thermochemical nonequilibrium. He is a recipient of the Department of Energy's INCITE award for 2022 and 2023 and has received a US Air Force directorate level medal of merit for his work

Crystallography of Lattice Structures under Geometric Frustration:
Dimples, Bees, And Lightweight Structures

Francisco López Jiménez
Assistant Professor
Aerospace Engineering Sciences
University of Colorado Boulder

Abstract: Lattice structures are found in several natural processes that produce self-organized patterns, as well as in engineering applications that require a lightweight structure. In this talk, we will explore the patterns that that appear in cases in which geometric constraints prevent the formation of a regular crystal, focusing on two different systems. First, we look at the dimples that appear as an instability in bilayer elastomers under compression. We explore the effect of curvature on the system and compare the patterns to those observed in other crystal systems. Second, we study how honeybees adapt the construction of their honeycomb under different constraints. We 3D-print experimental frames with imprinted foundations, that the bees extend as they construct the comb. The panels impose a variety of constraints, ranging from crystal misalignment to cells of different sizes.  We then discuss how these natural systems can serve as inspiration on the design of man-made cellular solids.

The Art of Flexible Science Communication

Marisa Petrusky
PhD Student
Aerospace Engineering Sciences
University of Colorado Boulder

Abstract: Whether chatting with colleagues, friends, or family, sharing progress with funding sources, or clarifying misconceptions about world events, science communication remains an essential skill for scientists and engineers at all stages of their career. While many techniques for simplifying one's research and breaking down complex concepts are readily available, it is also important to learn how to first analyze an audience, develop a reasonable objective for the interaction, then adapt your science communication techniques to best suit your audience. In this workshop, we will first learn to identify the values of an audience (both in terms of their personal interests and what they respond well to in a communicator). Then, we will discuss how to narrow your message and develop a concrete science communication objective. Finally, we will discuss communication techniques for explaining scientific concepts to persons of all levels of background knowledge. Participants will have the opportunity to create a one minute "elevator-pitch" explaining their work to a general audience.

The University of Colorado Magnetoaerodynamics and Aerospace Plasmas Laboratory

Hisham Ali
Assistant Professor
Aerospace Engineering Sciences
University of Colorado Boulder

Abstract: I will present an overview of current and future research objectives as well as facility design and construction progress in the Magnetoaerodynamics and Aerospace Plasmas Laboratory by Assistant Professor Hisham Ali. The laboratory will specialize in studying the physics of aerospace plasmas and related applications, such as hypersonics. Key laboratory capabilities include various radio-frequency (RF) plasma torches (1 - 80 kW, 13.56 MHz - 27.1 MHz) with a unique optically clear variable nozzle design enabling access for flow visualization. The plasma torches can function in a table-top format and can also be coupled to a large 1.5m x 2m vacuum chamber and high pumping speed (~20,000 m³/hr), and with temperature-capable mechanical vacuum pumping capability to simulate high altitude environments—maintaining low chamber base pressure at high test gas flow rates. The laboratory will be among less than five RF plasma wind tunnel facilities for hypersonics within US academia. The laboratory includes access for various plasma, fluid, and thermal diagnostics to facilitate hypersonics experimental investigations with a unique focus on magnetohydrodynamics for hypersonics and aerospace plasmas—magnetoaerodynamics.

Experimental Measurements of Nonequilibrium for Hypersonic and Atmospheric Entry Flows

Jesse Streicher
Postdoctoral Research Scientist
Department of Mechanical Engineering
Stanford University

Abstract: I highlight recent progress in Stanford shock-tube experiments using advanced laser absorption diagnostics to probe nonequilibrium reaction rates relevant to hypersonic flows in air. The design of advanced thermal protection systems requires a combination of high-fidelity computational models and low-uncertainty ground test data. Recent advances in high-fidelity computational models can now predict many internal energy excitation, chemical reaction, and ionization rates, but experimental validation data for these models have been severely lacking, especially at high temperatures. Advances in shock tube operation and laser absorption diagnostics have recently obtained quantum-state-specific data to aid in model validation. Experimental studies – augmented with laser absorption diagnostics – enable measurements of quantum-state-specific time-histories of many high-temperature air species, including molecular oxygen (O2), nitric oxide (NO), atomic oxygen (O) and atomic nitrogen (N). The quantum-state-specific time-histories can then be used to infer internal energy excitation and chemical reaction rates for important nonequilibrium processes for shock-heated air. Experiments were performed in a pressure-driven shock tube, with measurements made behind reflected shocks. Experimental temperatures range from 2,000 - 14,000 K and pressures from 0.022 - 1.524 atm. These measurements extend to higher temperatures than any past studies, and measurements at 14,000 K represent a significant advancement in the use of reflected-shock experiments to isolate key vibrational relaxation, chemical reaction, electronic excitation, and ionization reactions in high-temperature nonequilibrium in air.

Bio: Jesse Streicher is a research scientist from Prof. Ron Hanson's shock tube laboratory at Stanford University. He graduated with his PhD in 2022, and his research focuses on the chemical kinetics of high-temperature air with an emphasis on vibrational and chemical nonequilibrium. Through studies completed at Stanford and the NASA Ames electric arc shock tube (EAST), his research has inferred temperature and number density time-histories, vibrational relaxation times, and reaction rate coefficients for comparison to advanced computational models.