Pan Han

Geopolitical risk has become an increasingly important source of financial uncertainty, with implications for asset prices, financial institutions, capital flows, and financial stability. This review examines how geopolitical risk can be incorporated into financial risk management by synthesizing the literature on measurement, transmission channels, and risk governance. It discusses major approaches to measuring geopolitical risk, including news-based indices, country-specific indicators, broader uncertainty measures, and firm- or industry-level exposure measures. It then reviews how geopolitical risk is transmitted through asset pricing, volatility, tail risk, sovereign and funding channels, commodities, banking stability, and systemic spillovers. A central argument of the review is that geopolitical risk should not be treated as a standalone uncertainty variable. Rather, it is a cross-cutting risk driver that can affect market risk, credit risk, liquidity risk, counterparty credit risk, operational and legal risk, model risk, and systemic risk at the same time. Building on this synthesis, the paper proposes an integrative framework that links geopolitical risk indicators to exposure mapping, scenario translation, stress quantification, and governance action. The review highlights counterparty credit risk, collateral stress, reverse stress testing, and scenario governance as important areas for future research.

The leading-edge vortex (LEV) formation on the caudal fin (CF) has been identified as playing a key role in efficient lift-based thrust production of fish-like propulsion. The enhancement of the CF LEV through its interaction with vortices formed upstream due to a median fin with a distinct shape is the focus of this paper. High-speed, high-fidelity videos and particle imaging velocimetry (PIV) were obtained from rainbow trout during steady forward swimming to visualize the undulatory kinematics and two-dimensional flow behavior. Body kinematics are quantified using a traveling-wave formulation that is used to prescribe the motion of a high-fidelity three-dimensional surface model of the fish body for a computational fluid dynamics (CFD) study. The pressure field of the CFD result is compared and validated with the PIV result from the experiment. Using CFD, the vortex forming and shedding behaviors of the anal fin (AF) and their capturing and interaction with the trunk (TK) and the CF are visualized and examined. Coherent AF-bound LEVs are found to form periodically, leading to thrust production of the AF. The vortices subsequently shed from the AF are found to help stabilize and reinforce the LEV formation on the CF by aiding LEV initiation at stroke reversal and enhancing LEV during a tail stroke, which leads to enhancement of lift-based thrust production. The CF is found to shed vortex tubes (VTs) that create backward-facing jets, and the ventral-side VT and the associated backward jets are both strengthened by vortices shed by the AF. An additional benefit of the AF is found to be reduction of body drag by reducing the lateral crossflow that leads to loss of beneficial pressure gradient across the body. Through varying AF-CF spacing and AF height, we find that CF thrust enhancement and TK drag reduction due to the AF are both affected by the position and size of the AF. The position and area of the AF that led to the most hydrodynamic benefit are found to be the original, anatomically accurate position and size. In this paper, we demonstrate the important effect of vortex interaction among propulsive surfaces in fish-like propulsion.

Given growing interest in emulating dolphin morphology and kinematics to design high-performance underwater vehicles, the current research effort is dedicated to studying the hydrodynamics of dolphin-like oscillatory kinematics in forward propulsion. A computational fluid dynamics method is used. A realistic three-dimentional surface model of a dolphin is made with swimming kinematics reconstructed from video recording. The oscillation of the dolphin is found to enhance the attachment of the boundary layer to the posterior body, which then leads to body drag reduction. The flapping motion of the flukes is found to generate high thrust forces in both the downstroke and the upstroke, during which vortex rings are shed to produce strong thrust jets. The downstroke jets are found to be on average stronger than the upstroke jet, which then leads to net positive lift production. The flexion of the peduncle and flukes is found to be a crucial feature of dolphin-like swimming kinematics. Dolphin-inspired swimming kinematics were created by varying the flexion angle of the peduncle and flukes, which then resulted in significant performance variation. The thrust benefits and propulsive efficiency benefits are associated with a slight decrease and slight increase of the flexion of the peduncle and flukes, respectively.

Recent studies have shown that by utilizing the interactions among median fins (the dorsal, anal, and caudal fins), fishes can achieve higher propulsion performance at the caudal fin. This work aims at a systematic study of the effects of dorsal/anal fin shape and flapping phase on the hydrodynamic performance due to median-fin interactions (MFI) in underwater propulsion using a three-dimensional bluegill sunfish model. Flow simulations were conducted on stationary Cartesian grids using an immersed-boundary-method-based incompressible Navier-Stokes flow solver. The results showed that, due to the collision between the posterior body vortices (PBVs) and caudal fin leading edge vortices (LEVs), the latter is strengthened. As a result, the thrust and efficiency of the caudal fin are improved simultaneously, by 25.6% and 29.2%, respectively. Increases in the dorsal/anal fin area result in stronger caudal fin LEVs, and thus further caudal fin performance enhancement. On the other hand, changing the dorsal/anal fin flapping phase affects the collision time between the PBVs and the LEVs, and results in caudal fin performance changes. Phase-leading dorsal and anal fins are found to improve caudal fin efficiency, whereas phase-lag dorsal and anal fins maintain caudal fin thrust at a higher level. Compared to trunk-synchronized dorsal and anal fins, 60° phase-leading dorsal and anal fins increase the propulsive efficiency of the caudal fin from 77.9% to 90.1%. In addition, it is found that the presence of the dorsal and anal fins greatly reduces drag on the fish body by preventing the PBVs from crossing the body midline and debilitating interactions between the left- and right-stroke PBVs. Results of this work improve our understanding of MFI in fishlike swimming and demonstrate the benefits of optimal MFI for the design of high-performance bioinspired underwater vehicles.