Re-entrant jets and bubbly shock waves as mechanisms for sheet-to-cloud transition in partial cavitation

Steve Ceccio, University of Michigan, USA

Using time resolved X-ray densitometry, time resolved 2-D void fraction flow fields for a variety of cavitating flows are examined.  Example results from the cavitating flows formed at the apex of a wedge, the flow over a hydrofoil, and the flow in the wake of a cylinder are presented.  In these cavities are obtained to identify the mechanisms of transition from closed partial cavities to open cavities exhibiting periodic shedding of large gas pockets. From the void fraction field measurements, two distinct types of cavity shedding mechanisms are identified: shedding associated with a re-entrant jet in the cavity closure region that produces intermittent shedding of smaller scale cavities, and large scale, periodic cloud shedding caused by the formation of a condensation shock within the high void-fraction flow in the separated region of partial cavitation. A discussion of the observed occurrence and properties of the shock wave, and its role in causing periodic shedding is presented based on the one-dimensional model of shock propagation in bubbly mixtures and the relationship between the transition to strong shedding as a function of the Mach number of the bubbly flow in the cavity.

Steven L. Ceccio is the ABS Professor of Marine and Offshore Design and Performance and the Department Chair of Naval Architecture and Marine Engineering at the University of Michigan. He has appointments in Naval Architecture and Marine Engineering, Mechanical Engineering, and Applied Mechanics. He received his B. S. degree in mechanical engineering from the University of Michigan in 1985. He received his M. S. degree in 1986, and his Ph. D. in 1990 both in mechanical engineering from the California Institute of Technology. Upon completion of post-doctoral studies, also at the California Institute of Technology, he was appointed as an Assistant Professor in Mechanical Engineering at the University of Michigan, Ann Arbor in 1990. He was promoted to Associate Professor with tenure in 1996, and Professor in 2003. He served as an Associate Vice President for Research at the University of Michigan from 2004 to 2009 and as the Director of the Naval Engineering Education Center from 2010 to 2015. Prof. Ceccio’s research focuses on the fluid mechanics of multiphase flows and high Reynolds number flows, including flow in propulsors and turbomachinery, cavitating flows, vortical flows, friction drag reduction, the dynamics of liquid-gas, gas-solid, and three-phase disperse flows, and the development of flow diagnostics. He has served as an Associate Editor of the Journal of Fluids Engineering. He has also acted as a consultant to government and industry. Prof. Ceccio is a fellow of the American Society of Mechanical Engineers and of the American Physical Society, and he was named the 2014 Freeman Scholar by A.S.M.E.

Fractal generated turbulence in round jets: flow topology and heat transfer

Tommaso Astarita, University of Naples "Federico II", Italy

Numerous studies on fractal generated turbulence in free shear flows have demonstrated outstanding capabilities of such grids in producing a tunable turbulence intensity profile, depending only on the grid geometry parameters. Indeed, these grids lead to a more elongated turbulence production region, a local peak and a fast decay. Furthermore, the local Reynolds number value that characterizes the decay region is sensitively larger than that characteristic of regular grids. This paved the way to a variegate portfolio of possible applications, such as to exploit fractal generated turbulence for convective heat transfer enhancement in impinging jets. As demonstrated via targeted IR Thermography experiments, the introduction of these grids is indeed capable of producing a noticeable increment of the convective heat transfer rate, especially at moderately short nozzle to plate distances, with respect to regular grids and jet without turbulence promoter. The Tomographic PIV investigation of the flow field shows that, in addition to the higher turbulence intensity level, this increment is addressed to the capability of producing streamwise vorticity, which sensitively enhances the jet entrainment rate, thus leading to a more efficient convective heat transfer. On the downside, in the impingement region, the presence of counter-rotating wall vortices is responsible for a local upwash of the wall jet, thus leading to a lower uniformity in the convective heat transfer distribution.

Tommaso Astarita received his master degree in Aeronautical Engineering in 1993, summa cum laude, and a PhD in Aerospace Engineering in 1997, both from the University of Napoli “Federico II”. He was Post-doctoral Fellow at the von Kàrmàn Institute for Fluid Dynamics, Rhode–Saint–Genèse (Belgium 1997) and at the University of Naples (1998-2000). He was winner of various fellowship and prizes. He then joined the University of Naples Staff as a researcher and now he is Associate Professor of Fluid mechanics and has taught: Experimental fluid dynamics; Fluid dynamics, Gasdynamics I and II and Theoretical methods in Gasdynamics. The main research activities are dedicated to the experimental study of problems in the fields of fluid dynamics and convective heat transfer: Application of infrared thermography to the quantitative measurements of convective heat transfer coefficients and flow visualization; Measurement of instantaneous and mean flow field with Particle Image Velocimetry (PIV); Development of the stereo and Tomo PIV techniques.

An Exploration of Model Validation Applications in Robust Control Framework

Jerzy Sawicki, Cleveland State University, USA

Abstract: Robust control techniques have allowed engineers to create more descriptive models through the inclusion of uncertainty in the form of both plant perturbations and additive noise. This additional information allows for the creation of models which are robust to any deviations from the physical system, provided the uncertainties properly define these differences. Model validation techniques were developed to answer the question: Given an experimental data set and a model with plant perturbations and additive noise, could the model reproduce the observed input-output data? As a result, model validation provides a guarantee that the uncertain model is able to account for all experimental data. In addition to traditional robust control applications, model validation has seen applications in other areas, particularly in the field of structural health monitoring. This presentation will provide an overview of the current state of the art for model validation in the context of robust control, as well as a few illustrating examples and possible directions for future research.

Jerzy Sawicki is the D. E. Bently and A. Muszynska Endowed Chair, professor in mechanical engineering and director of the Center for Machinery Dynamics and Control in the Washkewicz College of Engineering at Cleveland State University (CSU). He received his PhD in Mechanical & Aerospace Engineering from Case Western Reserve University, BS and MS in Applied Mathematics from the University of Gdansk, and BS and MS in Mechanical Engineering from Gdansk University of Technology. Dr. Sawicki joined the faculty of Cleveland State University as an Assistant Professor in 1993 and since 2004 holds the Bently and Muszynska Endowed Chair position. His research interests are in the area of machine dynamics and control, mechatronics, and machine diagnostics. He has published over 250 peer-reviewed journal papers and conference articles, one research monograph, co-edited three books, and has advised numerous graduate students, post-docs, and research scientists. Dr. Sawicki currently serves as an Associate Editor for ASME Journal of Engineering for Gas Turbines and Power. He is a U.S. representative to the ISO/TC 108/SC 2/WG 7 international committee, a Fellow of ASME, recipient of several Best Paper awards, Ohio Outstanding Engineering Educator Award, and licensed Professional Engineer in the State of Ohio. Dr. Sawicki also serves as Vice President for Research at Cleveland State University.