Resolving High Reynolds Number Turbomachinery and Complex Boundary Layer Flows by Refractive Index Matching

Joe Katz, Johns Hopkins University, USA

A series of experimental studies involving high Reynolds number internal flows within complex domains have been performed by matching the refractive index of the solid boundaries with that of the fluid. This approach provides unobstructed visual access to the sample volume, and prevents disruptive reflections from walls. It has been used for characterizing the inner part of rough wall boundary layer, canopy flows, including the turbulence between branches, as well as turbomachinery flows. Examples of theses applications and challenges involved will be introduced and discussed. However, most of the attention will be paid to tip leakage flows in axial turbomachines, which adversely affect the machine performance, and are major contributors to noise, vibrations, onset of stall in compressors, and cavitation breakdown in pumps. A series of common features in tip leakage flows have been observed in a series of experiments performed within several machines with different sizes, speeds, load distributions and tip-gap sizes. These observations follow the evolution of the backward leakage flow across the narrow tip gap, its rollup into a tip leakage vortex (TLV) near the suction side of the rotor blade, and the dynamics of this vortex within the rotor passage. Several notable phenomena include: (i) in instantaneous realizations, the vicinity of the TLV center contains multiple interlacing structures that never roll up into a single vortex; (ii) the TLV migrates from the suction side of one blade to the pressure side of the neighboring blade under the influence of its “image”; (iii) vortex breakup occurs in regions of adverse pressure gradients in the aft parts of the passage, rapidly spreading TLV fragments over substantial fraction of the tip region; (iv) endwall casing boundary layer separation occurs when the leakage flow meets the main passage flow, feeding counter-rotating vorticity into a layer that surrounds the TLV center; (v) quasi axial vortices extending diagonally upstream, from the suction-side of one blade to the pressure side of the next blade, play prominent roles in the onset of large scale instabilities in the rotor passage, such as cavitation breakdown and stall, and (vi) the turbulence in the tip region is highly anisotropic and inhomogeneous. Its levels are high in the shear layer connecting the TLV to the suction-side corner of the blade, near the TLV center, and in the region of endwall boundary layer separation. Specific mechanisms dominating the turbulence production will be introduced and discussed.

Joseph Katz is the William F. Ward Sr. Distinguished Professor in the Dept. of Mechanical Engineering at the Johns Hopkins University (JHU). He received his BS degree from Tel Aviv University and MS and PhD from Caltech, all in Mechanical Engineering. After 5 years at Purdue University, he joined JHU in 1988. He is a ASME and APS fellow, and a Gilman Scholar at JHU. He was the Technical Editor of the Journal of Fluids Engineering for 10 years, and currently serves as the Chair of the Board of Journal Editors of ASME. He founded the Center for Environmental and Applied Fluid Mechanics at JHU, and presently serves as its Director. He has received several awards including the 2004 ASME Fluids Engineering Award and several "Best Paper" awards. His research focuses on experimental fluid mechanics and development of optical diagnostic techniques. His group has been involved in characterization of turbulent  single  and  multiphase  flows, such as: (i) Breakup of crude oil and droplet dynamics, (ii) cavitation and bubble dynamics, (iii) flow structure and turbulence within turbomachines, (iv) boundary layers, canopy flows, and rapidly strained turbulence, (v) flow induced vibrations, (vi) ship bow waves, (vii) turbulence in the coastal bottom boundary layer, and (viii) swimming behavior of marine plankton both in the laboratory and in the ocean. His group has also been involved in development and applications of Particle Image Velocimetry (PIV) and its derivatives in the laboratory and in the ocean (& cornfields), as well as Holographic PIV, and microscopic digital holography. His work has been published in more than 120 journal papers, six patents, and more than 190 conference papers. He has advised numerous graduate students and post-docs, and has been funded by ONR, NSF, NASA, DOE, AFOSR, NOAA, Bosch LLC, and GOMRI.

Applications of Physics Based Data Restoration from Tomographic PIV

Fulvio Scarano, Delft University of Technology, Netherlands

Tomographic Particle Image Velocimetry (Tomo-PIV) has developed over a decade into a versatile tool for the inspection and visualization of complex three dimensional flows. The principal domain of application has been the study of fundamental turbulent flow phenomena occurring in boundary layers, wakes, jets including pressure evaluation and aeroacoustics. The availability of measurements on an increased number of dimensions in space and time has paved the way for expansion of the techniques for data restoration. In particular to move from signal-processing based (e.g. spatiotemporal filters, proper orthogonal decomposition) to physics based methods. The latter assimilate measurements with the flow governing laws to yield data reconstruction conditioned towards physical consistency. The lecture will provide a survey of such methods as they have appeared in the literature related to PIV and in relation to relevant applications with experiments. The techniques are categorized based on their objective: i) Increasing temporal resolution (pouring Space into Time, time-supersampling); ii) Data regularization (e.g. solenoidal filtering, Lagrangian data filtering); iii) Enhancing spatial resolution (pouring Time into Space, VIC+, Shake-the-Box); iv) Pressure from PIV. The working principles of the methods are briefly discussed, pointing the reader to the relevant literature for a detailed discussion. The impact of each technique is then discussed first by simplified cases that illustrate their potential and limitations. Examples from real-life experiments are then brought forward to illustrate the current level of applicability of these methods.

Fulvio Scarano is Professor of Aerodynamics in the Department of Aerospace Engineering at TU Delft (Netherlands). He graduated in Aerospace Engineering at University of Naples (1996), obtained a Ph.D. in 2000 (von Karman Institute, Theodor von Karman prize) and joined TU Delft at the faculty of Aerospace Engineering in the Aerodynamics Section in the same year. Since 2008 he has been a full professor of Aerodynamics and has acted as head of section since 2010. Starting director of Aerospace Engineering Graduate School (2012). Currently director of the AWEP department (Aerodynamics, Wind Energy, Flight Performance and propulsion). Recipient of Marie-Curie grant (1999), Dutch Science Foundation VIDI grant (2005) and of the European Research Council grant (ERC, 2009). European project coordinator (AFDAR, Advanced Flow Diagnostics for Aeronautical Research, 2010-2013). Promoted and supervised more than 20 PhDs. His research interests cover the development of particle image velocimetry (PIV) and its applications to high-speed aerodynamics in the supersonic and hypersonic regime. Notable developments are the image deformation technique, Tomographic PIV for 3D flow velocity measurements and its use to quantitatively determine pressure fluctuations and acoustic emissions in wind tunnel experiments. Recent works deal with the combination of PIV data with CFD techniques, extension of PIV to large-scale wind tunnel experiments and applications ranging from sport aerodynamics to ground vehicles, from aircraft to rocket aerodynamics. Author of more than 200 publications, he has delivered more than 20 keynote lectures worldwide. He acts as editorial board member of many international conferences and journals, including Measurement Science and Technology and Experiments in Fluids, among others.

Helicopter Rotor Noise Prediction and the Challenges of Noise Abatement

Ken Brentner, Penn State University, USA

This presentation will provide a review of the key helicopter rotor noise source mechanisms, their fluid mechanics origins, and an assessment of the state-of-the-art of current noise prediction approaches. Rotorcraft noise is comprised of several components that originate from distinct physical mechanisms, which must generally be treated separately. Furthermore, these distinct noise mechanisms each have unique acoustic radiation patterns that are important to understand the resulting noise of the vehicle. These noise sources are generally well understood and the individual noise sources can be computed with a range of prediction approaches ranging from semi-empirical approaches to first principles methods. Noise prediction is essential for the design of new rotorcraft which will reduce the noise at the source. The challenge in using prediction tools for reducing rotor noise through both design and abatement is twofold: 1) choosing the level of fidelity that captures the important physics with enough accuracy; and 2) reducing both the learning curve to use the tools and the computational power required by the noise prediction system to make the prediction tools accessible to both aircraft designers, operators, and land use planners.

Kenneth S. Brentner is a Professor of Aerospace Engineering at The Pennsylvania State University in University Park, Pennsylvania. He received his BS in Aeronautics and Astronautics from Purdue University, his MS in Aeronautics from The George Washington University, Joint Institute for the Advancement of Flight Sciences (JIAFS, located at NASA Langley Research Center in Hampton, Virginia), and his PhD in Acoustics from the University of Cambridge, England (Corpus Christi College, Cambridge). Upon graduation from Purdue, he joined the Aeroacoustics Branch at NASA LaRC where he performed computational and theoretical research on propeller and helicopter rotor noise. It was during this time that he developed the helicopter rotor noise prediction program WOPWOP based upon Farassat's formulation 1A. While at NASA he earned a PhD as a student of Professor Shon Ffowcs Williams while on graduate study leave. Upon return to NASA LaRC, Brentner was one of several Aeroacoustics Branch researchers that were involved in numerical work which they called "Computational Aeroacoustics" or CAA. Professor Brentner's research has continued to be focused on computational aeroacoustics and the Ffowcs Williams - Hawkings (FW-H) equation in particular. In 2000, Dr. Brentner joined the Department of Aerospace Engineering at The Pennsylvania State University and he continued his research on the noise of maneuvering rotorcraft, computational aeroacoustics, and novel uses of the FW-H equation. His research group has developed the new code PSU-WOPWOP, which is a very general FW-H solver. Professor Brentner served on several technical committees of both the American Helicopter Society and American Institute of Aeronautics and Astronautics; as Associate Editor and Editor in Chief of the Journal of the American Helicopter Society; as the 2011 Technical Chair of the AIAA/CEAS Aeroacoustics meeting held in Portland, OR and several other responsibilities. He is an Associate Fellow of the AIAA and has received several awards from NASA, Penn State, AIAA and AHS. Professor Brentner also regularly serves as a consultant on computational aeroacoustic and rotor acoustics.

Velocity and Vorticity in the Right Human Heart

Jean Hertzberg, University of Colorado, USA

Recent advances in time-resolved 3D cardiac magnetic resonance imaging (4DMRI) have allowed for the 3-dimensional, 3-component characterization of blood flow in the right ventricle (RV) and right atrium (RA) of the human heart.  In this talk, an overview of a large, ongoing, multi-disciplinary investigation of 4D right heart hemodynamics in normal and pathologic subjects is given. The image processing workflow is described, including visualization techniques for understanding and communicating complex right heart flow structures throughout the cardiac cycle. Finally, a qualitative visual comparison of 3D flow structures in the vena cava, RA, and RV between healthy subjects and pulmonary hypertensive patients is presented.

Jean Hertzberg is currently Associate Professor of Mechanical Engineering at CU-Boulder. She teaches graduate and undergraduate courses in measurement techniques, thermodynamics, fluid mechanics, heat transfer, design and computer tools. She has pioneered a spectacular course on the art and physics of flow visualization, and is conducting research on the impact of the course with respect to visual perception and educational outcomes. Her disciplinary research centers around pulsatile, vortex dominated flows with applications in both combustion and bio-fluid dynamics. She is also interested in a variety of flow field measurement techniques. Current projects include electrospray atomization of jet fuel and velocity and vorticity in human cardiac ventricles and large vessels.

Experiments on acoustic transport in thermal turbomachinery – sound generation, acquisition, and analysis

Jörg Seume, Leibniz Universität Hannover, Germany

In thermal turbomachinery, reducing sound emissions and preventing failure due to acoustically excited vibrations contribute to the overall goals of improving environmental impact and reliability. The former is particularly important in aircraft engines, while acoustic resonance is less written about but has been a real-life problem in the axial compressors in aircraft engines and stationary gas turbines as well as radial compressors in process applications. A prerequisite for reducing noise and acoustic resonance is understanding the transport of tonal noise in turbomachines. The lecture covers recent analytical and experimental work on the instrumentation and data analysis for experiments on sound transport in axial turbomachinery. Progress in this area also requires contributions to the generation of synthetic sound fields as well as analytical models for data analysis. Contributions in both areas are reported along with some sensitivity analyses for instrumentation choices and experimental results from an axial turbine. Finally, some recommendations are made for further improvements of the methodology for acoustic research in turbomachinery.

Joerg Seume is a Professor in Turbomachinery and Fluid Mechanics at Leibniz Universitaet Hannover. He is the Head of Institute of Turbomachinery and Fluid Dynamics and Dean of the Faculty of Mechanical Engineering. Joerg Seume received his Ph.D. in Mechanical Engineering for a NASA-sponsored thesis with Prof. T.W. Simon from the University of Minnesota in 1988 on “Laminar-to-Turbulent Transition in Oscillating Flow” as it occurs in Stirling engines. He received his MS in Mechanical Engineering from the University of Wisconsin at his native Madison. In industry, he worked as a research engineer at Sunpower Inc. of Athens, Ohio to develop solar powered Stirling engines. His career in gas turbines started when joining Siemens in 1991 where he went from the fluid mechanics laboratory to testing prototype gas turbines, a position of project-leader in gas turbine development, and management positions in quality and production plant integration. In 2000, Dr. Seume accepted the professorship in Turbomachinery and Fluid Mechanics at Leibniz Universitaet Hannover focusing his research on unsteady effects in turbomachinery and wind turbines, including aeroelasticity and aeroacoustics. He currently serves as Dean of the Faculty of Mechanical Engineering.

Image-based Methods to Measuring and Modeling Flow Phenomena of Gases and Liquids

Marcus Magnor, Iowa State University, USA

Because the flow of gases and liquids is affected by and interacts with solids brought into the stream, invasive measurement techniques are not suitable for all application scenarios. Non-invasive techniques enable taking measurements from a distance but may measure only integrated quantities along rays of projection. Optical techniques, in particular, are constrained by the minimal interaction of light with most gases and liquids as well as the fact that the visual appearance of flow phenomena is only indirectly related to the physical quantities of density, pressure, and velocity that underlie flow behavior. On the other hand, image-based measurement approaches enable acquiring millions of data points simultaneously to sample complex flow fields densely in space and time. In my talk I will present several image-based techniques to capture and model surfaces, volumes, and velocity fields of different gaseous and liquid flow phenomena in nature. For most techniques, we rely on standard video cameras, and some approaches work with as few as a single camera. By making use of simple physical effects during acquisition like fluorescence or Schlieren in combination with state-of-the-art reconstruction algorithms including optical tomography, weighted minmal hypersurfaces, and compressed sensing, the geometry of complex flows can be modeled in 3D or even 4D.

Marcus Magnor heads the Computer Graphics Lab of the Computer Science Department at Technische Universität Braunschweig (TU Braunschweig).  He received his BA (1995) and MS (1997) in Physics from Würzburg University and the University of New Mexico, respectively, and his PhD (2000) in Electrical Engineering from Erlangen University. For his post-graduate studies, he joined the Computer Graphics Lab at Stanford University. In 2002, he established the Independent Research Group Graphics-Optics-Vision at the Max-Planck-Institut Informatik in Saarbrücken. He completed his habilitation in 2005 and received the venia legendi for Computer Science from Saarland University. In 2009, he was Fulbright Scholar at the University of New Mexico, USA, where he holds an appointment as Adjunct Professor at the Physics and Astronomy Department. He is an awardee of a ERC Starting Grant. His research interests concern visual computing, i.e. visual information processing from image formation, acquisition, and analysis to image synthesis, display, perception, and cognition. Areas of research include, but are not limited to, computer graphics, computer vision, visual perception, image processing, computational photography, astrophysics, imaging, optics, visual analytics, and visualization.

Adaptive Detached Eddy Simulation

Paul Durbin, Iowa State University, USA

In simulations that resolve turbulent eddies, the requirement for fine grid resolution near walls is a major expense. Mitigating this expense has led to the idea of hybrid simulation, in which a layer near the wall is treated by a Reynolds averaged model (RANS), transitioning to eddy resolved simulation away from the surface. Detached Eddy Simulation (DES) is an attractive version of hybrid modeling. But, rather than thinking of it as a combination of RANS and LES regions, per se, it should be considered as a length scale prescription that is used in an eddy resolving simulation. As a hybrid method, DES invokes a length scale prescription that reverts to RANS or LES formulas, within regions of a turbulence simulation. In this approach the behavior of the simulation morphs from being similar to RANS, representing turbulent transport primarily with an eddy viscosity, to resolving the energetic scale, turbulent eddies, as the distance from the surface increases. The switch in behaviour is effected by placing a limit on the length scale. In previous two-equation DES models, an upper bound was placed on a dissipation length scale. The present two-equation approach, is based on the k- model. The eddy viscosity is written T = l2, so the upper bound on l directly reduces the eddy viscosity to a subgrid viscosity. From another perspective, production of k (P = 2l22) is decreased by the bound. k is the unresolved turbulent energy. The topic of this talk, Adaptive DES, has the virtue of seamlessly adapting to grid resolution and flow conditions, either permitting eddy resolving simulation, or invoking the RANS formulation. The length scale formula contains a coefficient CDES. The similarity between the l2- model and the Smagorinsky LES model, suggests applying the dynamic procedure to determine this coefficient locally, via test filter stresses. However, the dynamic procedure fails if the grid is too coarse. This is prevented by introducing a lower bound that compares grid spacing to the Kolmogoroff scale, and restricts CDES if the grid is too coarse. The model constant adapts to the grid and flow. If the grid is fine enough CDES can become nearly zero. On a coarse grid it reverts to a default value of 0.12. Additionally, it turns out that when the grid is fine enough, the RANS region becomes thin and the eddy simulation region starts low in the boundary layer. Then a larger part of the flow is computed by eddy resolving simulation and predictions are more accurate than with a fixed constant. In channel flow, the RANS region can shrink below the log-layer. Adaptive DES will be illustrated by a variety of simulations, including separation in two and three dimensional geometries, rotating channel flow and a combustor swirler nozzle.

Paul Durbin is a professor in aerospace engineering at Iowa State University. He received his BSE in Aerospace and Mechanical Sciences from Princeton University and his PhD from the Dept. of Applied Mathematics and Theoretical Physics (DAMTP) at Cambridge University. After a post-doc at Cambridge, he became a research engineer at NASA Lewis (now NASA Glenn). He spent two years as a visiting associate professor at the University of Arizona, then went to the Center for Turbulence Research, at Stanford, as a senior fellow. After that he became Professor in the Mechanical Engineering Dept. at Stanford. 10 years ago, he moved to Iowa State University, as the Martin C. Jischke professor. He is a fellow of the American Physical Society. Professor Durbin's research is in theory, analytical modeling and simulation of turbulence and transition, especially on bypass transition. He has developed various models for Reynolds averaged prediction of turbulence and transition, and for detached eddy simulation of turbulent flow. Working with his students, he has explored the basic mechanisms and phenomenology of bypass transition and mixed mode transition by direct numerical simulations. He is the author of two books on fluid dynamics & turbulence, and he regularly serves as a consultant on turbulence modeling.

Recent Advances in Measurement Technology for Turbomachinery Applications

Anestis Kalfas, Aristotle University of Thessaloniki, Greece

In recent years, progress in measurement technology enabled refined turbomachinery research and advanced technology development. The present work aims to review recent advances in turbomachinery measuring techniques. In particular, attention is focussed on intrusive measurement technologies, including point measurement techniques. Various types of probes, traditional and novel, are being examined with respect to their capacity to provide accurate reference data. In the dawn of the new millennium, the need for unsteady measurement techniques, suitable for turbomachinery applications, became more intense. The ascent of the modern design methodologies, requiring unsteady data for reference, made the development of suitable measurement technologies, urgently required. At the same time, progress in numerical modelling of complex turbomachinery geometries as well as turbomachinery modelling accentuated the need for more accurate reference data to facilitate further development. Over the last 25 years, pioneering research work at ETH Zurich lead to technology development, which made possible the development of the Fast Response Aerodynamic Probe (FRAP) technology. Inspirational work in Cambridge has lead scientists and engineers, to develop unsteady numerical methods and measurement techniques, aiming to evaluate the loss generation mechanisms by means of the entropy function. The work at a number of leading institutions worldwide has contributed to establishing the need for advanced and reliable experimental techniques. The advent of fast response sensor technology and precision manufacturing enabled the development of fast and accurate aerodynamic probes, which operate reliably in a wide range of realistic turbomachinery applications. Overall, the application of these techniques extents to both compressors and turbines ranging from small and low speed air conditioning fans to large multistage aeroengine and power generation turbomachinery units. Recent advances have also, addressed measurements in the wet steam areas of large multistage steam turbines. The development of miniaturised on board electronics enabled rotating frame of reference measurements, enhancing the previously limited range of application. Furthermore, progress in artificial intelligence methods, applied to probe navigation through complex turbomachinery flow fields, have enabled the optimisation of data acquisition processes, minimising the impact of measurements to the final product. Finally, averaging techniques applied to cloud-of-point-measurements are also under consideration. This work concludes with an outlook of future developments in measurement technology for turbomachinery applications.

Anestis Kalfas is an Associate Professor in Turbomachinery. He belongs to the teaching staff of the Laboratory of Fluid Mechanics and Turbomachinery of the Department of Mechanical Engineering at the Aristotle University of Thessaloniki. Anestis Kalfas received his PhD in Turbomachinery Aerodynamics from Cranfield University in 1994 and his Dipl.-Ing. in Mechanical Engineering from the Aristotle University of his native Thessaloniki. He worked as a Research Associate at the Whittle Lab., University of Cambridge and as an Aircraft Engineer at the Hellenic Air Force. He has been a Senior Scientist at the Turbomachinery Laboratory of the Swiss Federal Institute of Technology in Zurich since July 2000 where he lectured in Turbomachinery Design. Dr Kalfas is active in the areas of axial steam and gas-turbine aerodynamics, gas-turbine performance and power plant optimization, boundary layer transition and turbulence and novel aerodynamic probe technology.

Leveraging PIV Methods to Interrogate Environmental Flows at the Micro- and Macro-Scales

Kenneth Christensen, University of Notre Dame, USA

Flows of environmental significance abound in nature across a broad range of scales, from the macro-to micro-scales. These flows are oftentimes key drivers of physical and biogeochemical processes on Earth and thus have a tremendous impact on the environment. While direct observations of such processes would be preferable, many of them occur in natural systems for which the environmental conditions limit or completely impede access via modern flow diagnostics due to geometry and/or coexistence of multiple phases (solid, liquid, gas and/or multiple of each). Because of the broad range of scales typically present in such flows, modeling at small scales is required so that predictive simulations are possible. It is at these scales where experiments can inform the development of models that accurately reflect the underlying physics of such processes which, in turn, would yield more reliable predictions at system scales. This lecture will highlight two model problems in this regard, specifically environmental flow systems that challenge flow interrogation with optical diagnostics: turbulent flow associated with interacting barchan dunes at the macroscale and the pore-scale dynamics of CO2 injection into geologic storage sites at the microscale. Novel implementations of PIV methods are being used to overcome these challenges. In the case of flow associated with interacting barchan dunes, we are leveraging refractive index matching coupled with planar and volumetric PIV to access the near-dune flow physics that is inaccessible otherwise.  In the case of geologic CO2 sequestration, we are capturing for the first time the multi-phase flow dynamics of supercritical CO2 and resident water in heterogeneous rock formations at reservoir pressures (80-100 bar) utilizing fluorescent microscopy coupled with microscopic PIV. The details of these unique measurement approaches will be discussed as will the new insight gained into the flow physics that govern these environmental flow systems.

Kenneth T. Christensen is a Professor and Collegiate Chair in Fluid Mechanics at the University of Notre Dame, with a joint appointment in the Departments of Aerospace & Mechanical Engineering and Civil & Environmental Engineering & Earth Sciences. He also presently serves as Assistant Dean of Faculty Development in the College of Engineering. Christensen directs a research group that pursues experimental studies of turbulence, geophysical flows and microfluidics and is a WPI Principal Investigator in the Carbon Dioxide Storage Division of the International Institute for Carbon-Neutral Energy Research (I2CNER) based at Kyushu University in Japan. He is a Fellow of both APS and ASME, an associate Fellow of AIAA and serves on the Editorial Boards of Experiments in Fluids and Measurement Science and Technology. Past recognition includes the AFOSR Young Investigator Award (2006), the NSF CAREER Award (2007), the Francois Frenkiel Award for Fluid Mechanics from APS-DFD (2011) and the Dean’s Award for Excellence in Research (2012) from the College of Engineering at Illinois.