4:00 pm Presentation
“Large Eddy Simulation Including Population Dynamics Model for Polydisperse Droplet Evolution in Turbulence”
Presented by ADITYA KANDASWAMY AIYER (Adviser: Prof. Meneveau)
Previous studies have shown that dispersion patterns of oil droplets in the ocean following an oil spill depend critically on droplet diameter. Hence predicting the evolution of the droplet size distribution is of critical importance for predicting macroscopic features of oil dispersion in the ocean. We adopt a population dynamics model of polydisperse droplet distributions for use in Large Eddy Simulation. We generalize a breakup model from Reynolds averaging approaches to LES in which the breakup is modeled as due to bombardment of droplets by turbulent eddies of various sizes. The breakage rate is expressed as an integral of a collision frequency times a breakage efficiency over all eddy sizes. An empirical fit to the integral is proposed in order to avoid having to recalculate the integral at every LES grid point and time step. The fit is tested by comparison with various stirred tank experiments. As a flow application for LES we consider a turbulent jet emanating from a source where oil droplets are released. The advected velocity and concentration fields of the droplets are described using an Eulerian approach. We apply this LES model to study the change of the oil droplet size distribution due to breakup, caused by interaction of turbulence generated by the jet with the oil droplets.
4:25 pm Presentation
“On the Interactions of a Rotor Blade Tip flow with Axial Casing Grooves in an Axial Compressor Near Best Efficiency Point”
Presented by HUANG CHEN (Adviser: Prof. Katz)
Previous studies have shown that axial casing grooves (ACGs) are effective in delaying the onset of stall, but degrade the performance of axial turbomachines around the best efficiency point. Our recent experimental study (GT2017-65099) in the JHU refractive index-matched liquid facility have examined the effects of ACGs on delaying stall of a one and half stage compressor. The semicircular ACGs based on Müller et al. (GT2011-45364) reduce the stall flow rate by 40% with a slight decrease in pressure rise at higher flow rates. Efficiency measurements conducted as part of the present study show that the ACGs cause a 2.4% peak efficiency loss. They are followed by detailed characterizations of the impact of the ACGs on the flow structure and turbulence in the tip region at high flow rates away from stall. Comparisons with the flow structure without casing grooves and at low flow rate are aimed at exploring relevant flow features that might be associated with the reduced efficiency. The Stereo-PIV measurements in several meridional and radial planes show that the periodic inflow into the groove peaks when the rotor blade pressure side (PS) overlaps with the downstream end of the groove, but diminishes when this end faces the blade suction side (SS). The entrainment of the PS boundary layer and its vorticity during the inflow phase generates counter-rotating radial vortices at the entrance to the groove, and a “discontinuity” in the appearance of the tip leakage vortex (TLV). While being exposed to the blade SS, the backward tip leakage flow causes flow separation and formation of a counter-rotating vortex at the downstream corner of the groove, which migrates towards the passage with increasing flow rate. Interactions of this corner vortex with the TLV cause fragmentation of the latter, creating a broad area with secondary flows and elevated turbulence level. Consequently, the vorticity shed from the blade tip remains scattered from the groove corner to the blade tip long after the blade clears this groove. Compared to a smooth endwall, the groove also increases the flow angle near the blade tip leading edge and varies it periodically. Accordingly, the magnitude of circulation shed from the blade tip and leakage flow increase near the leading edge. The insight from these observations might guide the development of ACGs that take advantage of the effective stall suppression by the ACGs but alleviate the adverse effects at high flowrates.
4:15-4:40 p.m. Presentation
“Simultaneous PLIF and PIV Measurements on Refractive Index Matched Immiscible Buoyant Oil Jet Fragmentation in Water”
Presented by XINZHI XUE (Adviser: Prof. Katz)
Subsurface oil well blowout generate immiscible turbulent buoyant oil jets, which breaks up into a cloud of oil droplets. Understanding of the fragmentation is essential for evaluating the spreading of the oil jet and its interaction with the surrounding water in the near field, and for determining the droplet size distribution needed for modeling the subsequent transport of the oil. There is limited experimental data on the near field behavior of the opaque oil jet because of the inability to perform phase distribution measurements there. Injecting silicone oil into sugar water, which have the same refractive index, as surrogates for crude oil and water, respectively, enables us to observe the breakup process. The dynamic similarity is maintained by keeping nearly the same interfacial tension as well as density and viscosity ratios. The mixing process is visualized by simultaneous applications of planar laser induced fluorescence(PLIF) by premixing the oil with dye, and particle image velocimetry (PIV). The PLIF images are used for measuring the droplet sizes as well. Results show that with increasing Reynolds number, the jet spreading angle evaluated from the PLIF images increases, and its centerline velocity decreases at a faster rate. Beyond about 7 nozzle diameters, the turbulence peaks at the center of the jet, and its magnitudes scales with centerline velocity. As expected, the oil ligament fragmentation occurs primarily in regions of high strain-rate fluctuations in the near field shear layer and at the end of the potential core. This latter moves closer to the nozzle, and the resulting characteristic droplet sizes decrease with increasing Reynolds number. In both cases, the fragmentation process generates compound droplets, each containing multiple layers of oil and water.
4:40-5:00 p.m. Presentation
“Common Features of the Turbulence Structures in the Tip Region of Axial Turbomachines”
Presented by YUANCHAO LI (Adviser: Prof. Katz)
The flow in turbomachines is inherently complex and turbulent. Modern design processes rely heavily on CFD, especially RANS simulations to elucidate the flow, raising the questions about the applicability of popular turbulence models to turbomachines. Several axial turbomachines, including two waterjet pumps and an aviation compressor, have been studied experimentally in the JHU refractive index-matched facility in the past few years. This talk summarizes some common features in turbulence structure observed in these machines, such as the anisotropic distributions of Reynolds stresses and mechanisms (e.g., production, transport) causing it. Among all the machines, elevated turbulent kinetic energy (TKE) is observed to be associated with the characteristic vortical structures, e.g., the tip leakage vortex (TLV) and the shear layer connecting TLV to the blade suction side tip corner. High TKE is also evident near the blade tip corners and in some cases, in the layer separated from the endwall boundary where the leakage flow meets the opposite-directional main passage flow. The dominant terms of turbulence production rates show similar distributions in these machines, such as high shear production in the shear layer and opposite effects by flow contraction/stretching in the production of <u′z2> and <u′r2> near the TLV center. Moreover, eddy-viscosities estimated using individual stress and strain rate components reveal extreme spatial variability and inconsistency, suggesting that the popular eddy-viscosity based models are not applicable for these machines. However, the common turbulence features can serve as useful references for numerical simulations in which local production and anisotropy should be taken into account.
4:00 pm Presentation
“Cavitation Inception in Turbulent Shear Layers”
Presented by KARUNA AGARWAL (Adviser: Prof. Katz)
Cavitation in turbulent shear layers initiates along streamwise vortices. This has been argued to be the cause of Reynolds number dependence of the cavitation index. However, no volumetric pressure and flow-field measurements exist to explain this. Experiments to obtain tomographic PIV data downstream of a backward-facing step in the high speed water tunnel facility are planned. To characterize the turbulent boundary layer at the step, 2D PIV images are recorded. High speed images in wall-normal and spanwise planes are recorded to study the cavitating structures and find the conditions at which they first appear. To better understand cavitation in turbulence, very high speed (5 MHz frame rate) holographic study of injected free stream nuclei will be performed.
4:25 pm Presentation
“An Ensemble-Based Algorithm for Characterization of Scalar Sources in Turbulent Environment”
Presented by QI WANG (Adviser: Prof. Zaki)
An algorithm to determine the location and intensity of a scalar source with a parametrized shape is proposed and tested in a canonical turbulent channel flow at $Re_\tau = 180$. The algorithm uses forward simulations of an ensemble of scalar-source distributions, and can be easily applied to scenarios with a growing time horizon. The history of the scalar concentration at the sensor location due to the true source is compared with predictions from the ensemble members in order to determine the parameters of the source. Prediction errors are due to the approximation of the eigenvectors of the impulse-response matrix, or “eigen-sources”. In order to obtain a better approximation of the eigen-sources, a POD projection is used and is demonstrated to enhance the accuracy of the algorithm. The effect of measurement noise on the quality of reconstruction is quantified using the ratio of the standard deviation in the predicted source parameters and in the observation noise. The results provide a measure of the difficulty of source reconstruction for different relative positioning of sources and sensors.
“Experimental Methods in Thermal-Fluid Sciences”
Presented by Professor Matthieu Andre, Mechanical and Aerospace Engineering Department, The George Washington University
There exists a wide range of measurement techniques applied to fluid mechanics, each with its advantages and drawbacks. In this seminar, some examples of advanced optical techniques covering very diverse aspects of thermal-fluid sciences are presented.
In the first part, time-resolved particle image velocimetry (PIV) coupled to planar laser induced fluorescence (PLIF) is applied to a free surface flow to study fundamental physics responsible for atomization and air entrainment. High spatio-temporal resolution PIV data in both phases and precise reconstruction of the interface give new understandings of bubble entrainment caused by shear layer instability below the surface.
The second part discusses the use of molecular tagging velocimetry (MTV) to probe gas-cooled nuclear reactors in accident scenarios. This applied research aims at measuring in a large test facility the slow flow transient following a loss of forced circulation of the coolant. The diagnostics capabilities and performances are first assessed in the lab, and then the technique is deployed to perform in-situ measurements, providing valuable validation data for the models used in the design of such reactors.
Finally, a new experimental facility for fluid-structure interaction studies is described, and examples of optical measurements applied to other research areas are presented.
Matthieu Andre is a research professor in the Mechanical and Aerospace Engineering department at The George Washington University in Washington D.C. He received his M.S. degree from the Ecole Centrale de Lille in France in 2010, and obtained his Ph.D. in Mechanical Engineering from the George Washington University in 2014. His work focuses on experimental fluid mechanics and his current research interests include multiphase flows (e.g. cavitation, stratified flows, free surface flows), buoyant flows, and the development of experimental measurement techniques. He has experience with many laser-based diagnostics such as PIV, MTV, PLIF, Rayleigh scattering, and tunable diode laser absorption spectroscopy. His work was published in prominent journals such as Journal of Fluid Mechanics, Physics of Fluids, Experiments in Fluids, Measurement Science and Technology, and International Journal of Multiphase Flow. He received the best presentation award at the Young Professional Thermal Hydraulics Research Competition at the 2013 ANS winter meeting, and was a winner of the 2013 GW SEAS R&D Showcase for his work on free surface flow instabilities.
4:10 pm Presentation
“Experimental Study of Shock Waves Interaction with Rigid Porous Media”
Presented by OMRI RAM (Adviser: Prof. Katz)
It is well known that porous obstacles can cause significant diffraction and attenuate a shock wave propagating through them. Various models were proposed in the past to incorporate the microscopic interaction forces between the fluid and the skeleton of the porous sample into a macroscopic solution of the governing equations. However, these models which are usually based on a multiphase solution approach require identifying multiple properties of the fluid, the solid matrix and its geometry, some of which are notoriously difficult to measure. In this study, silicon carbide porous media with various porosities were placed in a shock tube at a fixed distance from the end-wall. The samples were subjected to a shock wave and the pressure build-up at the end-wall was recorded. An analysis methodology was developed to study the effect of various parameters on the pressure build-up in the confined volume. This methodology addresses the porous medium and the gas in the confined volume behind it as a single mechanical system. Assuming that the flow through the porous sample is close to being isentropic, a constitutive model that enables predicting the pressure profile developing on the end-wall was derived. Furthermore, it was shown that all of the experimental results can be represented in a non-dimensional form, thus revealing the similarity between them. The mechanical system perspective enabled us to better understand the physical mechanisms affecting the pressure pulse transformation while passing through the porous medium and through the air gap between the rear face of the porous sample and the end-wall. The modal response of the system revealed that when an arbitrary pressure pulse is imposed on the front face of the porous medium the high frequency spectral components were attenuated. The system acts as a low pass filter on the pressure profile propagating through it and inhibits the propagation of fast changing pressure pulses.
4:35 pm Presentation
“Instability of Supersonic Boundary Layers and its Sensitivity to Base-Flow Distortion”
Presented by JUNHO PARK (Adviser: Prof. Zaki)
The nonlinear parabolized stability equations (NPSE) can accurately and efficiently predict the amplification of finite amplitude instability waves and transition to turbulence in high-speed boundary layers. The base state is obtained from the similarity solution of the boundary-layer equations, and is distorted by the instabilities. While the NPSE fully accounts for this distortion, it does not account for potential uncertainties in the base state due to the flow environment, and boundary and thermal conditions. These uncertainties alter the transition behavior. In this work, we examine the sensitivity of finite-amplitude boundary-layer instabilities to base-flow distortions using the NPSE framework. We start with a review of the transition in supersonic boundary layers, and formulate the sensitivity analysis via theoretical (adjoint) and numerical techniques. The sensitivity of instability waves and transition onset to modifications in the base velocity and temperature are analyzed, and the uncertainty in transition due to wall heating is discussed.
“Advanced 3D/4D Bioprinting and Nanomaterials for Complex Tissue Regeneration”
Presented by Professor Lijie Grace Zhang, Department of Mechanical and Aerospace Engineering, the George Washington University
As an emerging tissue manufacturing technique, 3D bioprinting offers great precision and control of the internal architecture and outer shape of a scaffold, allowing for close recapitulation of complicated structures found in biological tissue. In addition, 4D bioprinting is a highly innovative additive manufacturing process to fabricate pre-designed, self-assembly structures with the ability to transform from one state to another directly off the bioprinter. The term “4D” refers to the time-dependent dynamic process triggered by specific stimulation according to predesigned requirements. However, current 3D/4D bioprinting based additive manufacturing technologies are hindered by the lack of advanced smart “inks”. Therefore, the main objective of our research is to develop novel biologically inspired nano or smart inks and advanced 3D/4D bioprinting techniques to fabricate the next generation of complex tissue constructs (such as vascularized tissue, neural tissue and osteochondral tissue). For this purpose, we designed and synthesized innovative biologically inspired nanomaterials (i.e., self-assembly materials, and conductive carbon nanomaterials) and smart natural materials. Through 3D/4D bioprinting in our lab, a series of biomimetic tissue scaffolds were successfully fabricated. Our results show that these bioprinted nano or smart scaffolds have not only improved mechanical properties but also excellent cytocompatibility properties for enhancing various cell growth and differentiation, thus promising for complex tissue/organ regeneration.
Dr. Lijie Grace Zhang is an associate professor in the Department of Mechanical and Aerospace Engineering at the George Washington University. She obtained her Ph.D. in Biomedical Engineering at Brown University. Dr. Zhang joined GW after finishing her postdoctoral training at Rice University and Harvard Medical School. She is the director of the Bioengineering Laboratory for Nanomedicine and Tissue Engineering at GW. She has received the ASME Sia Nemat-Nasser Early Career Award, NIH Director’s New Innovator Award, Young Innovator in Cellular and Molecular Bioengineering, John Haddad Young Investigator Award by American Society for Bone and Mineral Research, and Early Career Award from the International Journal of Nanomedicine, etc. Her research interests include 3D/4D bioprinting, nanobiomaterials, complex tissue engineering and breast cancer bone metastasis. Dr. Zhang has authored 3 books, over 109 journal papers, book chapters and conference proceedings, 6 patents and has presented her work on over 280 conferences, university and institutes. She also serves as the Editor of Materials Science and Engineering C: Materials for Biological Applications; Associate Editor-in-Chief of International Journal of Nanomedicine; and Associate Editor of ASME Journal of Engineering and Science in Medical Diagnostics and Therapy.
“Data-driven spectral filters for identifying structure in the streamwise turbulent kinetic energy of turbulent boundary layers”
Presented by Dr. Woutijn Baars
University of Melbourne
Even though flow-induced jet noise and wall-turbulence are highly broadband in nature, both physical phenomena exhibit a strong coherence in the acoustic pressure and velocity fields, respectively. In the first part of this seminar, a short overview will be provided on the acoustic signatures emitted by high-speed jets. Using an acoustic similarity parameter developed for a characteristic jet sound source, we highlight that nonlinear acoustic waveform distortion can be substantial, but, only under certain combinations of operating conditions and geometric scale of the jet.
The second, main part of this seminar focuses on the appearance of organized motions in wall-bounded turbulence. An organization is evidenced by the classification of distinctly different flow structures, including large-scale motions, such as hairpin packets, and very large-scale motions. In conjunction with less organized turbulence, all these flow structures contribute to the streamwise turbulent kinetic energy. Since different class structures comprise dissimilar scaling behaviors of their overlapping imprints in the velocity spectra, their coexistence complicates the development of models for the wall-normal trend of the energy statistics. Via coherence analyses of two-point data we derive spectral filters for stochastically decomposing the velocity spectra into sub-components, representing different types of statistical flow structures. In the process we reveal a Reynolds-number invariant wall-scaling for a portion of the outer-region turbulence that is coherent with the near-wall region; this supports the existence of a wall-attached self-similar structure embedded within the logarithmic region. It is also explored how these findings affect our ongoing work in the unique high-Reynolds-number boundary layer facility at Melbourne, including real-time control of the coherent scales to investigate their responsiveness to wall-based actuation.
Dr. Woutijn Baars received his B.Sc. (2006) and M.Sc. (2009) degrees from Delft University of Technology, where he experimentally studied the effects of icing on the stability of light aircraft. In 2013, Dr. Baars received his Ph.D. degree in Aerospace Engineering and Engineering Mechanics from the University of Texas at Austin. At UT Austin, his research investigations included the acoustic signatures generated by high-speed jets and the unsteady wall-pressure induced by shock wave boundary layer interactions in overexpanded nozzle flows. Currently he is a Post-Doctoral Research Fellow at the University of Melbourne, where he focuses on high-Reynolds-number wall-bounded flows. His ongoing research interests include the stochastic structure of wall-turbulence and how this organisation can assist active flow control for skin-friction drag reduction.
“Dynamics of buoyant particles in turbulent flows”
Presented by Dr. Varghese Mathai, University of Twente
Particle suspensions in turbulent flows occur widely in nature and industry. In most situations, the particles have a density which is different from that of the carrier fluid. This density difference can affect their motion through flows, and offers potential for changing the flow properties in many multiphase settings. In this talk, we will discuss the use of Lagrangian particle-tracking techniques to study the dynamics of light (buoyant) particles in turbulent flows.
In the first part, we address the acceleration dynamics of tiny buoyant particles (100-micron air bubbles) in a turbulent water flow. We examine the role of gravity on the bubble acceleration statistics. We find that microbubbles experience very different accelerations as compared to fluid tracers, and these occur despite their small size and minute Stokes number (small response time). Some implications of these findings to particle tracking experiments will be discussed.
In the second part, we move to the case of buoyant particles of finite size (particle size is large compared to the smallest turbulent flow length-scales). For spherical particles, buoyancy produces interesting variability in particle dynamics. In addition to buoyancy, we reveal the role of a largely ignored control parameter, the particle’s moment of inertia. Using experiments and direct numerical simulations, we demonstrate that the moment of inertia can be tuned to trigger distinctly different wake-induced-motions for both spherical and cylindrical particles. We draw some interesting analogies to the motions observed for anisotropic particles.
Dr. Varghese Mathai is a postdoctoral researcher in the Physics of Fluids group at University of Twente, the Netherlands. He received his Master’s in Mechanical Engineering from Indian Institute of Science, Bangalore, and PhD in Applied Physics from University Twente, the Netherlands (2017). His PhD research was focused on the dynamics of buoyant particles and air bubbles in turbulent flows by using Lagrangian Particle Tracking and Particle Image Velocimetry techniques. His research interests lie in dispersed multiphase flows, bluff body flows, and free surface flows. His work has appeared in journals such as Physical Review Letters, Journal of Fluid Mechanics, Experiments in Fluids, and Journal of Vascular Surgery. Varghese’s work was selected among the top five PhD theses in fluid mechanics by the European Research Committee on Flow, Turbulence, and Combustion (ERCOFTAC). In 2018, he received the Best Research Prize by the European Cooperation in Science and Technology (Eu-COST).
“Engineering Matter with Photons for Advanced Technologies”
Presented by Dr. Kitty Kumar, Carnegie Mellon University
Photons are central to many of the forefront trends in science and technology today, serving as a powerful nanofabrication tool or a delicate laser tweezer to manipulate nanoparticles, or an insightful spectroscopic probe for unraveling the structure of large protein molecules. I will present how I have developed light (photons) as the tool to encode functionality into materials and reset the state-of-the-art in flexible silicon-based and soft matter electronics. The work addresses the key challenges in the advancement of emerging technologies by studying the fundamental laser-material interactions and bridges the gap between research and commercialization.
Dr. Kitty Kumar is a postdoctoral associate at Carnegie Mellon University. Her research interests are focused on fundamental principles and practices in ultrafast laser science, soft condensed matter, laser material processing, nanofabrication, biomimetics, and programmable soft matter to address emerging scientific questions and key technical bottlenecks in advanced soft matter technologies for sensing, analysis, space exploration and biomedicine. Kitty received her Ph.D. from the University of Toronto, where she focused on the laser-assisted fabrication of flexible solar cells and developed a novel laser processing technique for three-dimensional structuring of dielectric thin films for flexible electronics. During the postdoctoral position at the Wyss Institute for Biologically Inspired Technologies, Harvard University, she concentrated on the design and fabrication of bio-inspired advanced soft robotic systems for biomedical applications.
“Control of Wind Turbines and Wind Farms”
Presented by Professor Lucy Pao, Electrical, Computer, and Energy Engineering Department, University of Colorado at Boulder
Wind energy is recognized worldwide as cost-effective and environmentally friendly and is among the world’s fastest-growing sources of electrical energy. However, science and engineering challenges still exist. For instance, in order to further decrease the cost of wind energy, wind turbines are being designed at ever larger scales, especially for offshore installations. We will overview a two-bladed downwind morphing rotor concept that is expected to lower the cost of energy more at wind turbine sizes beyond 13 MW compared to continued upscaling of traditional three-bladed upwind rotor designs. We will highlight some of the control systems issues for such wind turbines at these extreme scales and outline selected advanced control methods we are developing to address these issues. In the second part of the talk, we will discuss the growing interest in the coordinated control of wind turbines on a wind farm. Most wind farms currently operate in a simplistic “greedy” fashion where each turbine optimizes its own power capture. Due to wake interactions, however, this greedy control is actually suboptimal to methods in which the collective wind farm is considered. We will overview recent work in wind farm control and show selected results that demonstrate the performance improvements possible when carefully accounting for the wake interactions in coordinating the control of the wind turbines on the farm. We shall close by discussing continuing challenges and on-going and future research avenues that can further facilitate the growth of wind energy.
Lucy Pao is a Professor in the Electrical, Computer, and Energy Engineering Department at the University of Colorado Boulder. She has completed sabbaticals at Harvard University (2001-2002), the University of California, Berkeley (2008), the US National Renewable Energy Laboratory (2009), the Hanse-Wissenschaftskolleg Institute for Advanced Study in Delmenhorst, Germany (2016-2017) and the ForWind Center for Wind Energy Research at Oldenburg University (2016-2017). She earned B.S., M.S., and Ph.D. degrees in Electrical Engineering from Stanford University. Her research has primarily focused on combined feedforward and feedback control of flexible structures, with applications ranging from atomic force microscopy to disk drives to digital tape drives to megawatt wind turbines and wind farms. She is a Fellow of the International Federation of Automatic Control (IFAC) and the Institute of Electrical and Electronics Engineers (IEEE). Selected recent awards include the 2012 IEEE Control Systems Magazine Outstanding Paper Award (with K. Johnson), the 2015 Society for Industrial and Applied Mathematics (SIAM) Journal on Control and Optimization Best Paper Prize (with J. Marden and H. P. Young), the 2017 Control Engineering Practice Award from the American Automatic Control Council, and the Scientific Award 2017 from the European Academy of Wind Energy. Selected professional society activities include being a Fellow of the Renewable and Sustainable Energy Institute (2009-present), General Chair of the 2013 American Control Conference, member of the IEEE Control Systems Society (CSS) Board of Governors (2011-2013 and 2015), IEEE CSS Fellow Nominations Chair (2016-present), and member of the IFAC Executive Board (2017-2020).