Aerospace Engineering Seminar Series: Cristina Riso "Amplitude-Dependent whirl Flutter Analysis for Advanced Vertical Takeoff and Landing Configurations"

Thursday, April 30, 2026; 3:00pm-4:00pm
028 ECoRE
Speaker: Cristina Riso from Georgia Institute of Technology

Abstract: The growing interest in air vehicles capable of vertical takeoff and landing combined with efficient cruise is driving the development of configurations featuring tilting rotors or propellers mounted on flexible fixed wings. These systems are often susceptible to whirl flutter, requiring accurate aeroelastic stability predictions early in the development process.

Whirl flutter prediction is typically based on eigenvalue analyses of linearized aeroelastic models. However, commonly used models do not adequately resolve wing-rotor and wing-propeller aerodynamic interactions. While higher-fidelity models can capture these effects, they are often incompatible with direct eigenvalue analysis, requiring stability boundaries to be estimated from transient simulations using output-based identification methods. These approaches assume linear system behavior, potentially leading to inaccurate predictions. This limitation also complicates experimental flutter testing, which plays a key role in model validation and technology maturation.

This talk presents recent developments in whirl flutter analysis methods that extend output-based identification to account for amplitude effects arising from system nonlinearities. Results demonstrate the ability of these methods to reveal amplitude-dependent trends that are obscured by linear assumptions, enabling more reliable stability estimates and providing deeper insight into nonlinear aeroelastic dynamics relevant to both analysis and testing.

Speaker Bio: Dr. Cristina Riso is an Assistant Professor in the Daniel Guggenheim School of Aerospace Engineering at Georgia Institute of Technology. Her research focuses on the aeroelastic modeling, analysis, and simulation of fixed- and rotary-wing systems, with an emphasis on aeroelastic instabilities and limit-cycle oscillations. Her research has been recognized with an ORAU Ralph E. Powe Junior Faculty Award, a keynote lecture at the International Forum on Aeroelasticity and Structural Dynamics, and a Boeing Visiting Professorship. She is a Senior Member of the AIAA, serving on the Publication Committee and Structural Dynamics Technical Committee, and a member of the VFS, serving on the Dynamics Technical Committee. Prior to joining Georgia Tech, she was a Research Fellow at the University of Michigan and earned her B.S., M.S., and Ph.D. in Aerospace Engineering from Sapienza University of Rome.

Hosted by: Jessica Chhan,  jmc7050@psu.edu

Achieving and Sustaining Liftoff: The Pathway for Commercialization of Hydrothermal Liquefaction in the Circular Economy

Thursday, April 30, 2026; 10:35am
Capone Learning Auditorium (CBEB 001)
Speaker: Michael Timko from Worcester Polytechnic Institute

The U.S. alone generates 240 million tons of municipal solid waste each year, the majority of which is organic waste including yard waste, food waste, and plastics. Hydrothermal liquefaction (HTL) has the potential to convert this waste into a useful energy and chemical products. Since its discovery in the 1930s by Friedrich Bergius, who applied the process to coal, HTL has undergone two waves of innovation; one in the 1970s following the oil crises of that decade and a second that continues to this day following the oil shock and global financial crisis of 2007. My group has emphasized studies that improve the commercial prospects of HTL, and to that end we performed economic analysis that identified product yield, feedstock cost, and factory size as the three most important factors determining profitability. My talk is arranged on how my group has approached these three factors. In terms of product yield, we have developed data-driven approaches to predict product yields based strictly on feedstock composition. Further, we have developed catalytic HTL (C-HTL), mechanochemical HTL, and a new process termed radical initiated HTL (RI-HTL) to boost biofuel precursor yields at minimal incremental cost. Similarly, we have shown that selection of feedstocks can optimize product yields and even product quality in some cases. In terms of feedstock cost, we have focused our work on waste streams, and in particular have shown that HTL and especially RI-HTL can destroy >90% of the highly carcinogenic perfluoralkylated substances in sewage sludge and related waste. When it comes to factor size, we are breaking new ground on the use of HTL for bamboo conversion into renewable diesel and sustainable aviation fuel. As the fastest growing land plant, bamboo has tremendous potential for bioenergy production and we find that HTL conversion of bamboo can complete replace corn ethanol as a biofuel in the U.S. on just 15% of the total land area. Collectively, these advances point us to a sustainable future in which HTL plays a lead role, not just in laboratories but in factories near you.

Hosted by: Angela Dixon,  adc12@psu.edu

Biomedical Impedance Matching, Time-Varying Waveguides, Asymmetrical Transmission with Chiral Medium and Beyond: Research Experiences at a Primarily Undergraduate Institution

Wednesday, April 29, 2026; 121 Earth & Engineering Science Building
3:35-4:25 p.m.
Speaker: Atilla Ozgur Cakmak from Department of Electrical and Computer Engineering at Grand Valley State University

This seminar will highlight several research activities led by Dr. Cakmak at Grand Valley State University (GVSU), focusing on introducing undergraduate students to the research world of Electromagnetics. In this context, Dr. Cakmak will demonstrate how Electromagnetics serves as a powerful enabler for cultivating research habits at a Primarily Undergraduate Institution.

Three main research directions at microwave frequencies will be discussed:

1. Microwave coupling to the human torso,

2. Time-varying microstrip transmission lines, and

3. Asymmetric transmission using Frequency Selective Surfaces (FSS).

Microwave imaging plays an important role in detecting tumors in certain types of cancer. However, efficiently coupling electromagnetic waves into the human body remains challenging due to impedance mismatches at the skin–air interface. Furthermore, anatomical variability among patients and reactive near-field measurement limitations introduce additional complexities. To address these issues, a new type of actively modulated metasurface has been developed as an impedance-matching medium to enhance coupling efficiency and provide robustness against such variations. In a separate study, microwave strip waveguides are employed to explore signal modulation capabilities of microstrips without relying on active components. When loaded with PIN diodes, microstrips can effectively gate the transmission of microwaves, functioning as a time-varying electromagnetic system. A proof-of-principle demonstration shows amplitude modulation that operate at Wi-Fi and Bluetooth frequencies. Another research effort involves cascading chiral media with FSS layers to achieve pronounced asymmetric transmission. It is demonstrated more than 30 dB of one-way transmission, underscoring the potential of such composite systems for advanced microwave control and isolation applications.

Dr. Atilla Ozgur Cakmak is an Assistant Professor in the Department of Electrical and Computer Engineering at Grand Valley State University (GVSU), Michigan. He joined GVSU in 2021, and his primary research interests lie in the areas of metasurfaces and antennas. Dr. Cakmak has authored or co-authored more than 25 peer-reviewed publications and actively contributes to the scholarly community as an associate editor, topical editor, and reviewer in his field.

Dr. Cakmak earned his Ph.D. in Electrical and Electronics Engineering from Bilkent University, Turkey, in 2012. In 2013, he joined The Pennsylvania State University’s Center for Nanotechnology Education and Utilization (CNEU) as a postdoctoral researcher, focusing on solar cell technologies. He later served as an Assistant Teaching Professor at Penn State, beginning in 2018, where he taught courses in nanolithography and nanophotonics within the Department of Engineering Science and Mechanics. As a mentor and educator, Dr. Cakmak is deeply committed to undergraduate and master’s student supervision, fostering hands-on research experiences and professional development through his teaching and research activities.

Hosted by: Lana Fulton,  lub18@psu.edu