Satwindar Singh Sadhal

A headshot of Professor Raj M. Manglik. It is with great sadness that we share the news of the passing of our colleague and friend Raj M. Manglik, Professor of Mechanical Engineering at the University of Cincinnati on August 22, 2025, at age 71. Raj leaves a legacy of many path-breaking contributions in heat transfer enhancement, interfacial phenomena with surfactant and polymeric solutions, thermal energy storage (TES), and a record of distinguished service to ASME and the broader heat transfer community.Raj was born in Prayagraj, India, on September 30, 1953. He received his B.Tech. in Mechanical Engineering from the Indian Institute of Technology, Madras in 1976. He then worked for Triveni Turbines for 8 years, gaining valuable industrial experience. In 1984, he came to Iowa State University to pursue his M.S. under the tutelage of Professor Arthur E. Bergles. After completing his M.S., he moved to Rensselaer Polytechnic Institute (RPI) with Dr. Bergles and received his PhD in 1991.His doctoral work focused on enhanced heat transfer and compact heat exchangers, especially the tubular longitudinal swirl flows with twisted-tape inserts. Through judicious experimentation, flow visualization, and computational simulations, Raj established the flow-mixing mechanisms that characterize the influence of the tape inserts. By identifying the recirculation structure and its parametric scaling, along with a fundamental decomposition and balance of the fluid-force field, a new Swirl parameter was devised that universally scales confined helical vortex flows [1,2]. The consequent physical phenomena- or mechanism-based generalized correlations that have been devised provide unique and easy-to-use design equations for the practicing engineer that cover laminar, transition, and turbulent flow regimes. These correlations, apart from being highly cited in the archival literature, have become a standard part of numerous thermal engineering design handbooks, reference books, and textbooks. They are included as the preferred design equations in virtually all commercial software and routinely used for designing heat exchangers with twisted-tape inserts for the process, power, and chemical industry. Likewise, his friction factor and Nusselt number correlations for offset strip fins [3] became the industry standard. Working with his collaborators and students, he recently updated [4] these correlations to expand their range of applicability in both flow and geometric parameters.Raj joined the faculty of the University of Cincinnati in 1991 and developed a strong collaboration with Procter & Gamble. His initial focus was to address the need for high-performance and microscale exchangers that guided his research on compact heat exchangers that are characterized by high-density (surface area-to-volume) cores. Raj tackled the complex enhanced convection in cross-corrugated channels with sinusoidal trough profiles of plate-and-frame heat exchangers (PHEs) through extensive experiments to extract the parametric influence of the cross-corrugated flow geometry [5]. The augmented transport mechanisms include periodic disruption of boundary layers at the corrugated peak and trough regions, with re-circulating fluid cells in the resulting separation zones, which evolve into helical vortices with increasing acuteness of cross-corrugation angles. By appropriately scaling these convective transport features, he devised the first-of-their-kind friction factor and Nusselt number correlations for chevron PHEs [6].Thermal processing of non-Newtonian media in both single- and two-phase flows quickly became the second theme of his research focus at the University of Cincinnati. In thermal processing of rheologically complex media in foods, biochemical, and pharmaceutical manufacture, anomalous convection behaviors manifest and lack of close thermal control leads to product loss and degradation. Raj addressed the underlying transport phenomena in both a fundamental and mechanistic manner [7–9]. For example, he published ground-breaking results for heat transfer in nucleate boiling of aqueous, semi-dilute polymeric solutions, which are at once surface active and shear thinning in nature [10]. Besides effects of rheology and interfacial properties on the heat transfer, adsorption of macromolecules or agglomerates of small monomers onto the heater surface favors formation of new nucleation sites, which together with decreased dynamic surface tension promote ebullience growth. Contrarily, high viscosity effects tend to suppress the near-wall microconvection and bubble growth, thereby weakening boiling. The consequent interplay of pseudoplasticity and interfacial tension relaxation provides an effective control mechanism for thermal processing of a variety of viscous non-Newtonian media. He extensively examined bubbling and boiling of aqueous surfactant solutions to understand the effect of liquid–gas interfacial molecular adsorption leading to dynamic surface tension relaxation and physisorption on the liquid–solid interface, resulting in changing surface wettability. A paper on this work [8] earned him ASME's Melville Medal in 2006.More recently, he worked on developing a novel dry cooling system for thermal power plants that incorporated a daytime peak load shifting TES system, with an enhanced air-cooled condenser (ACC) design, to significantly increase power plant efficiency [11]. Extensive work on different fin cores led to optimizing the ACC with highly enhanced heat transfer surfaces [12–14]. For TES, it was shown that plate-fin and tube-fin type grids can be judiciously used to encase the phase change material (PCM) between heating–cooling surfaces to produce very high surface area density cores [15]. In doing so, especially when the finned-core-side is employed for encasing the PCM, the latter's limitations of low thermal-conductivity-driven thermal resistance are overcome [16,17].Raj received several awards for his research contributions, including the American Society of Mechanical Engineers (ASME) Heat Transfer Division 75th Anniversary Medal in 2013, the Heat Transfer Memorial Award of ASME in 2016, and the ASME James Harry Potter Gold Medal for advancing fundamental thermodynamics in research and education in 2018. He also received the AIChE Donald Q. Kern Award in 2020. He was elected as a Fellow of ASME in 2004 and a Fellow of ASHRAE in 2017. In 2025, ASME Board of Governors selected him as an ASME Honorary Member, which is a distinction recognizing his lifetime of service to the profession.Raj was an outstanding teacher and a dedicated mentor. He received several awards recognizing his teaching excellence at the University of Cincinnati. He was the recipient of the 2009 Engineering Tribunal Professor of the Quarter Award, the 2004 BP Faculty Excellence Award, the 2000 Robert Hundley Award for Excellence in Teaching, the 2001 Neil Wandmacher Teaching Award, and 1993 Professor of the Year Award. Besides his excellent classroom instruction, Raj was active in engaging students in design practice through participation in professional societies. He founded and established the ASHRAE Student Chapter at the University of Cincinnati in 1992–1993 and was the faculty advisor throughout his career. Many senior design teams won international student design competitions under his mentorship and coaching. Raj treated his graduate students as his family, and his warmth and guidance created a home away from home for them in his lab.His pedagogical contributions include the well-known undergraduate textbook “Principles of Heat Transfer” that he co-authored with Frank Kreith. The book not only covers physical principles and technical details very clearly but also connects the fundamentals of heat transfer with applications in new emerging technologies. Similarly, his monograph on plate-and-frame heat exchangers and other handbook chapters on enhanced heat transfer provide students and engineering practitioners with important modeling, analysis, and design tools.Raj had a lasting impact on the heat transfer community through his lifelong dedicated service to ASME, AIChE, ASHRAE, and to the broader thermal-fluids community. He served as Editor-in-Chief for the Journal of Enhanced Heat Transfer for over 10 years; he was Associate Editor for the Journal of Heat and Mass Transfer for two terms and Guest Editor for five different special issues. He was Chair of the Heat Transfer Division (HTD) (2018–2019), Chair of K-10 (2013–2016), and served on various HTD committees over the years. He was instrumental in increasing the involvement of AIChE and JSME members in ASME conferences and activities of HTD. As chair of the awards committee, he streamlined the nomination process and was instrumental in developing the Boelter-McAdams Prize. He made a strong mark and left deep footprints on the functioning and vision for the ASME Heat Transfer Division.On a more personal note, Raj is survived by his wife, Vandana; their children, Aditi and Animaesh; and four grandchildren. Raj cherished his time with friends and family and enjoyed theater and music. He will be sorely missed.

As the medium for intravitreal drug delivery, the vitreous body can significantly influence drug delivery because of various possible liquefaction geometries. This work innovatively proposes a varying-porosity approach that is capable of solving the pressure and velocity fields in the heterogeneous vitreous with randomly-shaped liquefaction geometry, validated with a finite difference model. Doing so enables patient-specific treatment for intravitreal drug delivery and can significantly improve treatment efficacy. A physics-informed neural network (PINN) model is also established for the simulation, and three cases are used for validation. Despite limited information, the PINN model, together with the varying-porosity approach, captured fluid and drug transport in the partially liquefied vitreous. This opens the possibility for optimizing intravitreal drug delivery based on ultrasonography in clinical practice.

The purpose of this study is to systematically examine the basic fluid dynamics associated with a fully liquid region within a porous material. This work has come about as a result of our investigation on the ocular fluid dynamics and transport process in a partially liquefied vitreous humor. The liquid is modeled as a sphere with Stokes flow while the surrounding infinite porous region is described by Brinkman flow. The development here provides basic three-dimensional axisymmetric results on flow characterization and also serves to evaluate the limits of validity of Darcy flow analysis for the same geometry. In the Darcy flow model, the liquid region is also treated as a porous region with a much higher permeability. Therefore, both liquid and porous regions are modeled by Darcy's law. Besides the analytical results from Brinkman-Stokes model, the simpler case of Darcy-Darcy flow for the same geometry has been provided. The results of both cases are compared and the differences between the two sets of results provide the range of validity of our computational model (Khoobyar et al. in J Heat Transf 144:031208, 2022). Some interesting fluid-dynamical aspects of the system are observed through the analysis. For the Darcy-Darcy system, the liquid region velocity is uniform throughout, as expected for potential flow. With the Brinkman-Stokes model, the liquid region has a paraboloidal profile with the maximum possible peak value of six times the far-field velocity in the porous medium. With the liquid region having a lower resistance, the flow tends to converge there for both models as it seeks the path of least resistance. As for the validation of the Darcy-Darcy model, it is a good approximation as far as the exterior flow is concerned. However, the liquid region flow profiles for the two models are different as noted. The current Brinkman-Stokes model has led to explicit analytical solutions for the flow field for both regions. This has permitted an asymptotic analysis giving deeper insight into the flow characterization.

This is an in-memoriam honoring Professor Darrell W. Pepper as an exceptional researcher, educator, and engineer.

This study uses the singular perturbation method to analyze the streaming flow around a pulsating bubble at the velocity antinode of a standing wave. The bubble radially and laterally oscillates with small nondimensional amplitudes of ε` and ε, respectively. The momentum equation is expanded using ε. The frequency parameter M, which is the ratio of the bubble radius to the viscous length, is included in the expanded equations as OM−1. Four boundary conditions are solved: non-pulsating and pulsating assuming no-slip and shear-free boundaries. For the non-pulsating bubble, the streaming is on the order of OM−1 for the shear-free boundary. The flow has a quadrupole pattern, with direction from the equator to the poles. However, for the non-pulsating bubble with the no-slip boundary, the flow pattern is from the poles to the equator and the direction reverses after a critical value of M=13.3. When bubble pulsation is introduced, the intensity of the streaming increases and is proportional to M. The flow pattern is dipole with a direction from the south to the north pole for the shear-free boundary. For the non-slip boundary, the flow is quadrupole for small values of M and varies with the phase shift ϕ. As M increases, the flow intensifies and becomes dipole. For both cases, the maximum velocity is at the phase shift angle ϕ=135° and M=10.

The internal flows within single drops levitated in air have been experimentally examined on Earth and under low-gravity conditions. The motivation for this study is provided by the need to assess the impact of the levitation fields: Can a quiescent undisturbed state be reached when a liquid sample is electrostatically or ultrasonically levitated on Earth? The usefulness of the containerless experimentation methods for free drops and bubbles can only be rigorously established if the potential interfering effects associated with levitation do not significantly alter the characteristics of the phenomena under investigation. For example, in the case of the thermocapillary flows generated within a free drop by laser spot heating, the background flow within the unheated drop in an isothermal environment must be characterized first. Using both ultrasonic and electrostatic levitation techniques, we have developed the ability to stably hold single drops and to observe the internal flows under laser spot-heating. The fluid motion under the action of both natural buoyancy and surface tension gradients are three dimensional and asymmetrical, even though the heating is centered on the equator of the levitated drop. In addition, the asymmetrical heat distribution also induces rotation of the drop, especially when ultrasonic levitation is used. In order to eliminate the buoyant contribution, low-gravity experiments are under consideration. Initial space-based investigations using an ultrasonic device have revealed that any residual interference from acoustically-induced stresses and flows are eliminated when the sound power is reduced to a very low level. These data also show, however, that a 6 mm diameter drop remains very sensitive to aerodynamic drag exerted by even very slow circulation. This drag constitutes an effective torque driving the drop into slow solid-body rotation.

Ultrasonic and electrostatic levitation methods allow the detailed experimental investigation of the dynamics and transport processes associated with free single drops stably positioned in a fluid host. The response of these drops to artificially induced controlled stimuli can be recorded and analyzed to deduce some of their thermophysical properties, or to study the characteristics of the nonlinear dynamics of free three-dimensional liquid-gas interfaces. Earth-based levitation, however, requires high field intensities that introduce additional artifacts interfering with the analysis of the drop response. We have carried out experimental studies both at 1-G and in microgravity in order to refine techniques to quiescently position free drops in a gaseous host, and to examine the thermocapillary flows resulting from spot heating. Both spherical and drastically flattened levitated drops have been investigated at 1-G using electrostatic/ultrasonic hybrid levitation systems and spot heating from a focused CO₂ laser. Thermocapillary flows in spherical drops at 1-G invariably couple with the overall drop rotational motion, while drastically flattened drops reveal more regular flow patterns in agreement with results from theoretical predictions for axisymmetric flows. Low-gravity investigations using a low-cost and compact experimental apparatus in the Shuttle Glovebox Facility have allowed the development of experimental methods for the quiescent positioning of free drops. A better understanding control of simple and compound drop rotation and of the impact of acoustic field positioning on drop internal flows has been obtained. Preliminary data on thermocapillary flows in free drops in microgravity have also been gathered.