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1. Lagrangian vs. Eulerian: An Analysis of Two Solution Methods for Free-Surface Flows and Fluid Solid Interaction Problems

   https://doi.org/10.3390/fluids6120460  

   Rakhsha, Milad; Kees, Christopher E.; Negrut, Dan (December 2021, Fluids)

   As a step towards addressing a scarcity of references on this topic, we compared the Eulerian and Lagrangian Computational Fluid Dynamics (CFD) approaches for the solution of free-surface and Fluid–Solid Interaction (FSI) problems. The Eulerian approach uses the Finite Element Method (FEM) to spatially discretize the Navier–Stokes equations. The free surface is handled via the volume-of-fluid (VOF) and the level-set (LS) equations; an Immersed Boundary Method (IBM) in conjunction with the Nitsche’s technique were applied to resolve the fluid–solid coupling. For the Lagrangian approach, the smoothed particle hydrodynamics (SPH) method is the meshless discretization technique of choice; no additional equations are needed to handle free-surface or FSI coupling. We compared the two approaches for a flow around cylinder. The dam break test was used to gauge the performance for free-surface flows. Lastly, the two approaches were compared on two FSI problems—one with a floating rigid body dropped into the fluid and one with an elastic gate interacting with the flow. We conclude with a discussion of the robustness, ease of model setup, and versatility of the two approaches. The Eulerian and Lagrangian solvers used in this study are open-source and available in the public domain. 

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2. Computational Investigation of Thrust Production of a Dolphin at Various Swimming Speeds

   Wang, J. (August 2021, ASME Fluids Engineering Division Summer Meeting)

   Dolphins are known for their outstanding swimming performance. However, the difference in flow physics at different speeds remains elusive. In this work, the underlying mechanisms of dolphin swimming at three speeds, 2 m/s, 5 m/s, and 8 m/s, are explored using a combined experimental and numerical approach. Using the scanned CAD model of the Atlantic whitesided dolphin (Lagenorhynchus acutus) and virtual skeleton-based surface reconstruction method, a three-dimensional high-fidelity computational model is obtained with time-varying kinematics. A sharp-interface immersed-boundary-method (IBM) based direct numerical simulation (DNS) solver is employed to calculate the corresponding thrust production, wake structure, and surface pressure at different swimming speeds. It is found that the fluke keeps its effective angle of attack at high values for about 60% of each stroke. The total pressure force coefficient along the x-axis converges as the speed increase. The flow and surface pressure analysis both show considerable differences between lower (2 m/s) and higher (5 m/s and 8 m/s) speeds. The results from this work help to bring new insight into understanding the force generation mechanisms of the highly efficient dolphin swimming and offer potential suggestions to the future designs of unmanned underwater vehicles. 

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3. Effects of body shape on aerodynamic performance and wake structures in snake-like gliding

   Gong, Y. (January 2022, AIAA)

   Flying snakes are the only snakes on Earth capable of aerial gliding, taking advantage of fluid dynamic principles to leap from point to point among the trees. During their gliding, the locomotion of aerial undulation is observed. We hypothesize that this locomotion and its associated unsteady vortex dynamics are critical to their aerodynamic performance. However, there is a lack of detailed three-dimensional flow field information around the snake body in gliding due to the difficulties in experimental flow visualizations of live animals. In this study, a computation fluid dynamics (CFD) study has been conducted to study the fluid dynamics of a snake-like gliding. A mathematical equation describing the horizontal undulation motion was applied for constructing snake-like 3D computational models and a series of flow simulations were conducted. An immersed-boundary-method (IBM)-based direct numerical simulation (DNS) flow solver along with adaptive mesh refinement (AMR) was used in the simulation. Specifically, different head positions, corresponding to different horizontal wave shapes and their effect on aerodynamic performance, flow field and wake structures behind the body will be studied. In addition, the dynamic undulating motion is introduced in the model and a CFD simulation is also conducted. Results from this study are expected to bring a step stone to understanding snake-inspired locomotion. 

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4. Body Shape Effects on the Hydrodynamic Performance of Bio-Inspired Undulating Swimmers

   https://doi.org/10.1115/FEDSM2022-87645  

   Kelly, J.; Pan, Y.; Dong, H. (August 2022, Fluids Engineering Division Summer Meeting)

   In this study, numerical simulations are performed to study the effects of body shape on propulsive performance in a carangiform-like swimming motion. A focus is given to the variation in performance due to changes in the maximum thickness, maximum thickness location, leading-edge radius, and boattail angle of an undulating foil. An immersed boundary method-based incompressible flow solver is implemented to solve for the propulsive performance of two-dimensional undulating foils. The resulting flow simulations yield the thrust, drag, efficiency, and flow for each body shape. From this study, we have found that better propulsive performance comes from a thinner maximum thickness, a maximum thickness location closer to the head of the fish, a narrower boattail angle, and a larger leading-edge radius. Particular care is given to the analysis of the boattail angle, because of the surprising and significant results. In changing only the boattail angle the efficiency is shown to vary by 10.3%. Changes in the leading-edge radius varies the efficiency by 4.4%, the maximum thickness by 4.0%, and the maximum thickness location along the body by 5.0%. The large improvement observed in the thinner boattail angle cases are caused by the increased curvature around the middle of the fish body leading to a high-pressure region at the tail that improves the thrust performance. The results can be used to improve understanding of fish body shapes observed in nature as well as better informing the design of bioinspired underwater robots. 

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5. Computational Modeling and Hydrodynamic Analysis of Fish Schools in Three-Dimensional Arrangements

   https://doi.org/10.1115/FEDSM2022-87690  

   Pan, Y.; Zhang, W.; Dong, H. (August 2022, Fluids Engineering Division Summer Meeting)

   In this work, numerical simulations are employed to study hydrodynamic interactions in trout-like three-dimensional(3D) fish bodies arranged in vertical and horizontal planes. The fish body is modeled on a juvenile rainbow trout (Oncorhynchus mykiss) and is imposed on a traveling wave to mimic trout swimming. Three typical minimal schools are studied, including the in-line, the side-by-side, and the vertical school. A sharp interface immersed-boundary-based incompressible Navier-Strokes flow solver is then used to quantitively simulate the resulting flow and hydrodynamic performance of the schools. The results show that the hydrodynamic efficiency of the leading fish in the in-line school increases by 5.28%, and the thrust production and efficiency of the side-by-side school are enhanced by 2.28% and 3.86%, respectively. Besides, the thrust production of the vertical school increases by 21.6%. The results suggest great potential in exploiting the hydrodynamic benefits in fish schools arranged in three-dimensional space. 

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