Shock wave focusing to achieve high energy concentration
Presented by Professor Veronica Eliasson
Structural Engineering, University of California San Diego
A shock wave is a thin discontinuous region over which fluid properties abruptly change from one state to another. Shock wave focusing occurs frequently both in nature and in a variety of man-made applications. It takes place when a shock wave propagates through a non-uniform or moving media, reflects from curved surfaces or through reflections with other shock waves. Extreme conditions created at the focal region – resulting in very high pressures and temperatures – can be either beneficial as in the case of shock wave lithotripsy or inertial confinement fusion or detrimental as in the case of superbooms (a type of sonic boom). As the shock wave emerges from the focal region, after the shock focusing event, the shape of the shock is often fundamentally altered. Therefore, a deeper understanding of the shock focusing process, and how to control it, is critical to fully understand its consequences and how to best enhance or mitigate it as needed depending on the application at hand. In this talk I will introduce our newest experimental setup that has the capability to produce multiple simultaneous shock waves in two or three dimensions with a turn around time between consecutive experiments that is under two minutes. Ultra-high-speed photography coupled with schlieren techniques are used to probe the shock dynamic events, and in particular, the transition from regular to irregular reflections.
Veronica Eliasson received her Ph.D. in Mechanics at the Royal Institute of Technology, Stockholm, Sweden, in 2007. After a postdoctoral appointment at Graduate Aerospace Laboratories, California Institute of Technology, she became a faculty member at University of Southern California in 2009. In 2016 Dr. Eliasson moved from USC to the Structural Engineering Department at University of California, San Diego. Dr. Eliasson’s research interests are in the area of experimental mechanics and include shock wave dynamics, high strain rate impacts and fracture mechanics.
4:10 pm Presentation
“Distinct Modes of Unsteadiness in a Separating Turbulent Boundary Layer”
Presented by WEN WU (Profs. Rajat Mittal & Charles Meneveau)
Flow separation is ubiquitous in external as well as internal aerodynamics: wings and fuselages at high angles-of-attack, flow past external objects, shock-wave/boundary layer interactions, diffusers, corners, and junctions are just a few examples of this kind of flow. The separated flow is typically unsteady across a broad range of frequencies. In addition to high-frequency unsteadiness associated with turbulent fluctuations, the flow also displays a low frequency “breathing” or “flapping” mode and a higher frequency “shedding” mode. These unsteady models lead to many issues in applications, such as degradation of system performance and aerodynamic noise. They also bring additional difficulties for the prediction and control of these flows due to the complexity of intermittency. Better and more sophisticated tools are required for analysis of these unsteady modes. We generate a realistic separation bubble via adverse pressure gradient induced by a prescribed suction profile and perform direct numerical simulation of a turbulent boundary layer (Re=500) separated by this APG. The resulting flow exhibits two distinct unsteady modes frequencies separated by a factor of three. The higher-frequency motion scales well with the characteristics of a canonical plane mixing layer, corresponding to the generation of quasi two-dimensional spanwise vortices by KH instability. The lower frequency mode is more difficult to characterize but the use of dynamic mode decomposition (DMD) indicates that the low-frequency mode is associated with elongated streamwise vortices, the scale of which agrees well with the Görtler vortices generated due to curvature effects in the flow.
4:35 pm Presentation
“Effect of Different Axial Casing Groove Geometries on the Performance and Efficiency of a Compressors”
Presented by SUBHRA SHANKHA KOLEY (Adviser: Prof. Katz)
The impact of varying the geometry of axial casing grooves were investigated by carrying out performance tests and flow measurements. It has been shown in earlier studies that skewed semi-circular grooves installed near the blade leading edge (LE) have multiple effects on the flow structure, including ingestion of the tip leakage vortex (TLV), suppression of backflow vortices, and periodic variations of flow angle. To determine which of these phenomena is a key contributor, the present study examines the impact of several grooves, all with the same inlet geometry, but with outlets aimed at different directions. The “U” grooves that have circumferential exits aimed against the direction of blade rotation, achieve the highest stall margin improvement of well above 60%, but cause a 2.0% efficiency loss near the best efficiency point (BEP). The “S” grooves, which have exits aimed with the blade rotation, achieve a relatively moderate stall margin improvement of 36%, but they do not reduce the BEP efficiency. Other grooves, which are aligned with and against the flow direction at the exit from upstream inlet guide vanes, achieve lower improvements. These trends suggest that causing high periodic variations in flow angle around the blade leading edge is particularly effective in extending the stall margin, but also reduces the peak efficiency. In contrast, maintaining low flow angles near the LE achieves more moderate improvement in stall margin, without the maximum efficiency loss. Hence, of the geometries tested, the S grooves appear to have the best overall impact on the machine performance. In order to further elucidate the flow we are conducting flow visualization experiments using cavitation which will be followed by PIV measurements to get an quantitative estimation of the flow.
Making Waves: Using Dynamics to Understand Behavior of Cells and Tissues
Presented by Professor Philip Bayly
Department of Mechanical Engineering and Materials Science,
Washington University in St. Louis
Waves and oscillations may be intrinsic to a mechanical system, or induced to probe the constitutive relationships between loading (stress) and deformation (strain). The first part of the talk will describe how we measure the mechanical behavior of brain tissue in vivo, using tagged magnetic resonance (MR) imaging and MR elastography (MRE) to visualize shear waves in the brain. The second part of the talk will focus on wavelike oscillations in cilia and flagella: thin, flexible organelles that beat rhythmically to propel cells or move fluid. We have developed new mathematical models of flagella motion, and found new solutions to existing models which can be used to evaluate the plausibility of long-standing hypotheses. Both these projects exploit specialized imaging and image processing techniques, combined with models of the underlying mechanics, to provide new information on the behavior of these important biological systems.
Philip V. (Phil) Bayly is The Lilyan and E. Lisle Hughes Professor of Mechanical Engineering and Chair of the Department of Mechanical Engineering and Materials Science at Washington University in St. Louis. Dr. Bayly earned an A.B. in Engineering Science from Dartmouth College, an M.S. in Engineering from Brown University, and a Ph.D. in Mechanical Engineering from Duke University. Before pursuing his doctorate, he worked as research engineer for the Shriners Hospitals and as a design engineer for Pitney Bowes. Dr. Bayly has been a member of the faculty at Washington University since 1993, and Chair since 2008. His research involves the study of nonlinear dynamic phenomena in mechanical and biological systems. He is particularly interested in imaging waves and oscillations to understand the mechanics of cells and biological tissues. His research has been funded by the National Science Foundation, the Whitaker Foundation, and the National Institutes of Health.
4:10 pm Presentation
“Wall-Modeling in Curvilinear Coordinates: Implementing Wall Shear Stress as Boundary Condition”
Presented by GHANESH NARASIMHAN (Profs. Zaki & Meneveau)
Wall-modeled Large Eddy Simulations (WMLES) determine instantaneous wall stress from a given filtered velocity. This wall stress is applied as a boundary condition (BC) in the LES solver that solves the filtered Navier-Stokes equations. TransFlow is a conservative Navier-Stokes solver in general curvilinear coordinates using volume flux formulation. Performing WMLES in TransFlow requires applying wall shear stress as boundary condition on a curvilinear grid. To this end, expressions for wall shear stresses in curvilinear coordinates are obtained from general coordinate free definition of the velocity gradient tensor. Since the grid is normal to the boundaries, expressions for wall shear stresses are derived for an orthogonal curvilinear coordinate system. Direct Numerical Simulation (DNS) of channel flow is performed with no-slip BC and wall stresses are computed using the obtained expressions. A separate DNS is run by applying the evaluated stresses as the boundary condition. Validation of implementation is done by comparing the velocity fields from DNS with no-slip and wall stress BC. The relative errors between the velocity fields from these two DNS are shown to be accurate to machine precision for at least 25 viscous time units.
4:35 pm Presentation
“Extending a Row-Averaged Model to a Turbine-Specific Model for Wind Farm Control”
Presented by GENEVIEVE STARKE (Profs. Gayme & Meneveau)
This study builds upon a recently proposed model-based receding horizon control approach that enables wind farms to follow a reference power signal. The wake model used in the controller is extended from a one-dimensional row-averaged model to encompass more two-dimensional effects such as wakes. This enables the control of individual turbines which generalizes the application to arbitrary wind farm configurations. The wake model is also adjusted to incorporate the changes in the freestream velocity across the spanwise component of the farm, which allows the wind turbines to be more responsive to local rather than aggregate wind conditions.
Aeronautics at Bio-Scales
Presented by Professor Geoffrey Spedding
University of Southern California
Aeronautics is a mature and powerful engineering discipline, and great success has been achieved in predicting flows and designing aircraft configurations at large scales, where the effects of viscosity can be modeled as minor modifications to basically inviscid dynamics. That is not the case at smaller scales, those of the new generation of drones, and of smaller birds and bats. Here the competing inertial and viscous terms in the governing equations lead to a delicate balance in solutions that have extreme sensitivity to variations in boundary and initial conditions. In this talk we will show how, in a Reynolds number regime that is only now becoming of practical interest, nominally simple problems do not necessarily have simple solutions, and how seemingly modest computational and experimental goals remain elusive. Nevertheless, with a little persistence, one can perhaps exploit these flow sensitivities for efficient and novel control strategies.
Geoffrey Spedding received his Ph.D. in 1981 from the University of Bristol, England. He began work as a Research Associate in the Department of Aerospace Engineering at the University of Southern California in the same year, where he worked on models of insect wings and models of atmospheres and oceans. He became a full Professor in 2005, and Chair of the Aerospace and Mechanical Engineering Department in 2010. His current research has three themes: (i) Geophysical Fluids: particularly the evolution of turbulence in oceans and atmospheres, and its relation to the persistence of wakes of islands and underwater vehicles; (ii) Advanced imagining and data analysis including accurate particle imagining velocimetry (PIV) techniques and novel 2D wavelet transforms and interpolation routines for scattered data; (iii) Aerodynamics of small flying devices, especially those where birds and bats coexist in engineering design space. In 2010 he was elected Fellow of the American Physical Society. In 2013 he was awarded the Chaire Joliot at ESPCI, Paris.
4:10 pm Presentation
“On the Promotion of Instabilities for Hydro-Kinetic Energy Harvesting Through Flexible Materials and Wave-Like Analogs”
Presented by DIEGO F. MURIEL (Prof. Katz)
The implementation of flexible materials for micro-energy harvesting is limited by the required high flow velocities to start exploitable oscillations. Initial configurations consist of membranes or plates with minimal or no induced curvature. Upon the action of an external flow, the harvester will oscillate presenting attractive possibilities for energy extraction. We demonstrate that forcing a plate into a wave-like deformation reduces minimum required flow velocities and provides consistent large amplitude oscillatory motions. First, we present the capacity to extract energy through a self-powered water pump, and second, we build a reduced-order model to explore the underlying physics. We show that the main driving mechanisms are a marginally stable deformation, a destabilizing effect of the compressive force, and an unsteady flow field that exists prior to the onset of large amplitude oscillatory motions. Our results open possibilities for new configurations of energy harvesters, and we argue the physical concepts can be translated to areas such as locomotion and propulsion.
4:35 pm Presentation
“Effect of Free-Stream Vortical Disturbances on Thermal Turbulent Boundary Layer”
Presented by JIHO YOU (Prof. Zaki)
Direct numerical simulations are performed to examine the impact of free-stream vortical forcing on a thermal turbulent boundary layer. When the boundary layer is buffeted by the external perturbations, the wall heat-transfer rate is increased relative to the canonical configuration where the free stream is quiescent. The change in the Stanton number is attributed to the distortion of the base temperature profile and enhanced production of the scalar variance. Both terms are increased due to the wall-normal heat flux, which is itself a result of the enhanced Reynolds stresses in response to the free-stream vortical forcing. The enhanced production of temperature variance leads to the formation of high thermal fluctuations that are maintained in the forced flow. In addition, the free-stream disturbances modify the spectral content of the boundary layer, and enlarge the scales of the hydrodynamic and thermal structures in the logarithmic layer. Near the near wall, the thermal structures are also strengthened due to their modulation by the outer velocity motions.
Uncovering new mechanisms in biological and engineering architectured materials
Presented by Professor Pablo D. Zavattieri
Lyles School of Civil Engineering, Purdue University
Our ability to improve more than one mechanical property in most engineering materials has been somewhat limited in the past by the inherent inverse relation between these desired properties often found in man-made materials. On the other side, Nature has evolved efficient strategies to synthesize materials that often exhibit exceptional mechanical properties that significantly break those trade-offs. In fact, most biological composite materials achieve higher toughness without sacrificing stiffness and strength in comparison with typical engineering material. Interrogating how Nature employs these strategies and decoding the structure-function relationship of these materials has opened up a new set of concepts in materials engineering. Considering the current progress in material synthesis and manufacturing, these new concepts have converged to the field of architectured materials. In this talk, I will describe some interesting mechanics problems that we encountered as we studied some extraordinary species, and how we can translate these lessons learned to architectured materials. In particular, I will focus on two different examples: One is related to Bouligand architectures, a naturally-occurring architecture typically found in arthropods such as the Mantis Shrimp, and its capability to promote delocalization to mitigate catastrophic failure. The second example is related to a family of architecture materials whose unit cells have multiple stable configurations inspired by competing auxetic mechanisms found in Nature. Implementation of some of those ideas to cellular architectured material guided us to the development of reusable energy absorbing materials.
Dr. Pablo Zavattieri is a Professor of Civil Engineering and University Faculty Scholar at Purdue University. Zavattieri received his BS/MS degrees in Nuclear Engineering from the Balseiro Institute, in Argentina and PhD in Aeronautics and Astronautics Engineering from Purdue University. He worked at the General Motors Research and Development Center as a staff researcher for 9 years, where he led research activities in the general areas of computational solid mechanics, smart and biomimetic materials. His current research lies at the interface between solid mechanics and materials engineering. His engineering and scientific curiosity has focused on the fundamental aspects of how Nature uses elegant and efficient ways to make remarkable materials. He has contributed to the area of biomimetic materials by investigating the structure-function relationship of naturally-occurring high-performance materials at multiple length-scales, combining state-of-the-art computational techniques and experiments to characterize the properties. His current research program includes the study of naturally-occurring architectures and the translation to engineering materials. Prof. Zavattieri is the recipient of the NSF CAREER award, the Roy E. & Myrna G. Wansik Research Award, he is a National Academy of Engineering Frontiers of Engineering Alumnus and a National Academy of Science Kavli Frontier of Science Fellow. He was also appointed a Purdue University Faculty Scholar for the period 2015-2020.
4:10-4:35 p.m. Presentation
“Bubble Rising Velocity in Strong Turbulence”
Presented by ASHWANTH SALIBINDLA (Prof. Rui Ni)
For bubbly ship wakes and breaking waves, the mean rising velocity of bubbles determines their residence time inside the carrier phase and the resulting surface bubble concentration over time. We carried out an experimental study of the rising velocity of bubbles with size ranging from 1 mm to 10 mm. A vertical water tunnel V-ONSET has been developed to generate strong turbulence with a large energy dissipation rate (ϵ~0.1 m2/s3) and a controllable mean flow. In this presentation, I will introduce the facility and also talk about some results regarding the rise velocity of bubbles in turbulence. We will compare our results with previous experimental and numerical works in this area.
4:35-5:00 p.m. Presentation
“Spatio-Temporal Dynamics of Turbulent Flows in the Presence Of Waves”
Presented by PATRICIO CLARK DI LEONI (Profs. Tamer Zaki & Charles Meneveau)
Waves, eddies, horizontal winds, vortices, and many other structures can coexist and interact in a turbulent flow. Their identification and extraction in simulations and experiments is a major challenge. We show how, with the aid of spatiotemporal spectra, we can study the interplay between waves and eddies in rotating, free surface, stratified and quantum flows. We present results on the existence of a mixed regime of waves and solitons in surface wave turbulence, links between inertial waves and the development of large-scale horizontal winds in stratified turbulence, quantification of bounds for the validity of wave turbulence in rotating flows, and links between the depletion of helicity and kelvin waves in quantum turbulence.
Harnessing thermal expansion in architected metamaterials
Presented by Professor Damiano Pasini
Mechanical Engineering, McGill University
For technology called to function under harsh temperature swings, e.g. satellite antennas, thermal expansion can be an enemy to fight against. But for others, e.g. deployable systems, thermal expansion can be an ally. In this seminar, I will contribute to address challenges currently existing on both fronts: i) how to meet strict requirements of thermal expansion in ultralightweight stiff materials, ii) how to engage temperature in morphable materials that deploy in situ under extreme conditions. The approach that I will follow draws from concepts of mechanics, geometry, materials, and structural optimization, through a combination of theory, computation and mechanical testing for performance validation.
Damiano Pasini is the Louis Scholar of the Faculty of Engineering at McGill University and Professor of Mechanical Engineering. His research interests lie in solid mechanics, advanced materials and structural optimization with current focus on mechanical metamaterials. He is fully engaged in understanding their mechanics, introducing reliable predictive models, and using them to engineer, build and test architected materials with optimally tuned functional properties that are of practical use in aerospace and other disciplines.
4:10-4:35 p.m. Presentation
“A Novel Particle Tracking Technique using a Scanning Laser Setup Tested via Numerical Experiment”
Presented by MELISSA KOZUL from NTNU (Host: Prof. Tamer Zaki)
Lagrangian particle tracking relying on line-of-sight based volumetric methods is challenged by high particle densities, required for the adequate spatial resolution of high Reynolds-number flows. This presentation will introduce a novel robust 3D particle tracking technique based on a scanning laser setup. We have developed an effective triangulation greatly reducing ghost particle reconstruction using images from only two cameras. Following successful reconstruction of a time series of 3D particle fields, Lagrangian velocities and accelerations are calculated using particle tracking. The method was developed via numerical experiment using the Johns Hopkins Turbulence Database.
4:35-5:00 p.m. Presentation
“Scale Separation in Restricted Nonlinear Wall-Bounded Turbulence”
Presented by BENJAMIN MINNICK (Adviser: Prof. Dennice Gayme)
Numerical and experimental studies have revealed the significance of streamwise coherent structures in wall-bounded turbulence, both near the wall where energy is dissipated and far from the wall where energy is carried. Engineering applications have prompted the study of wall-bounded turbulent flows however, the computational expense of resolving the necessary scales has limited our ability to interrogate the mechanisms underlying the flow. Recently the restricted nonlinear (RNL) model has been proposed. Motivated by these streamwise coherent structures inherent in wall-bounded turbulence, the RNL model neglects nonlinear interactions between nonzero streamwise Fourier modes thereby reducing the order of the streamwise varying dynamics. At low Reynolds number, the RNL model has been shown to accurately predict first-and second-order statistics while retaining as few as one nonzero Fourier mode. Extending to more moderate Reynolds numbers, this model correctly captures log-law behavior, provided the streamwise dynamics are band-limited to dissipative scales. In this work, we move the RNL modeling paradigm to even higher Reynolds numbers, in a regime where a separation of scales is expected. We present results of current efforts and identity additional phenomena to properly capture scale separation.