Previous Talks

Speaker: Dr. Taeseok Kim (Assistant Professor, Jeju National University) [GS]

Abstract: The trend toward downsizing nuclear reactors is gaining significant traction due to enhanced safety and operational flexibility. The thermal components of reactors must also be reduced in size to meet compact design requirements. Among these components, the steam generator plays a critical role in nuclear reactor systems. Current small modular reactor (SMR) designs predominantly employ once-through helical steam generators; however, this approach faces limitations when adapting SMRs for maritime applications or low-power systems. The printed circuit heat exchanger (PCHE) has emerged as a promising alternative for steam generator applications due to its compact and efficient design. However, the characteristics of two-phase flow and boiling heat transfer in mini-channels, a key feature of PCHEs, remains insufficiently understood for direct application. In mini-channels, pressure drop and heat transfer coefficients significantly differ from those observed in conventional steam generator pipes. Two-phase flow parameters such as void fraction, flow regimes, and channel geometry can strongly influence these parameters. We are conducting research on the effects of PCHE design on two-phase flow pressure drops, heat transfer, and superheated steam generation.

Speaker: Mr. Yuta Iwatani (Ph.D. student, Tohoku University) [GS]

Abstract: Wall temperature can affect the dynamics of compressible flows through the dynamic viscosity and the nonlinear coupling of kinetic and internal energy, and the laminar-to-turbulent transition of the boundary layer (BL) is no exception. In this study, we investigate the effects of wall heat fluxes on the subharmonic transition of the boundary layer at Mach number 0.8 using direct numerical simulation (DNS), ultimately aiming to achieve drag reduction by controlling the BL in aircraft and other fluid machinery with wall temperature. The DNS results show that wall heating promotes the transition while cooling delays it. Notably, wall cooling impedes the growth of the two-dimensional linearly unstable mode (Mack’s first mode), while the subharmonic secondary instability of the first mode emerges, leading to flow behavior similar to oblique transition, distinct from the H-type transition observed in the heated and adiabatic cases. This shift in the predominant scenario, or the dominant modes, alters the nonlinear mode interactions in the transitional BL. We examine these nonlinear interactions of modes in the transitional BL using bispectral mode decomposition (BMD) and discuss the connection of the nonlinear mode interactions to the skin friction coefficient with the aid of the angular momentum integral analysis.

Speaker: Dr. Chungil Lee (Postdoctoral Research Associate, Tohoku University) [GS]

Abstract: Supersonic jets generated from the engine of the rocket and supersonic aircraft emit very strong noise. Screech tones are dominant noise source of jets and can cause structural fatigue in rockets and supersonic aircraft. Therefore, a precise understanding of screech dynamics is essential for both predicting and reducing screech tones. While many studies have been conducted to understand screech dynamics, the 3D unsteady dynamics of screech tones have not been experimentally reported due to the low temporal resolution of high-speed cameras. In the present work, we develop a 3D spatiotemporal super-resolution measurement technique to reconstruct time-resolved (TR) 3D flow fields from sensor data*. This approach simultaneously conducted the non-time-resolved 3D background oriented schlieren (3D-BOS) and TR microphone measurements. A linear regression model between 3D-BOS and microphone data is constructed to estimate TR 3D flow fields associated with screech tones from the microphone data. Using the proposed method, the intermittent events and azimuthal switching of flow structures associated with screech tones can be analyzed. (*Reference: Lee et al., Phys. Fluids, 2023)

Speaker: Dr. Shuji Otomo (Assistant Professor, Tokyo University of Agriculture and Technology) [GS]

Abstract: Accurate, non-intrusive force measurement remains challenging in many scenarios, particularly those involving animals or vehicles. Estimating forces from flowfields obtained via particle image velocimetry (PIV) has shown promise but remains highly complex. This talk presents the vortex force map (VFM) method as a solution for computing unsteady forces from PIV data. Beyond force computation, the VFM method also visualises the contribution of individual vortical structures to the overall force. The VFM method is applied to three kinematic cases: surging flat plates and pitching NACA 0018 aerofoils, at Reynolds numbers on the order of 10,000. These flows are characterised by massive separation, with coherent leading-edge and trailing-edge vortex shedding. In all cases, the VFM method demonstrates strong agreement with direct force measurements. Additionally, it proves robust to noise, a critical consideration in experimental fluid mechanics*. (* Reference: Otomo et al., Exp. Fluids, 2025, accepted)