A two-probe approach for hydrodynamic load evaluation in immersed boundary methods

A fast parallel immersed boundary method for fish swimming simulation on GPU-accelerated computing platform

The motility of bacteria in viscoelastic fluids plays a crucial role in diverse biological processes such as infection and biofilm formation. In such environments, the rotation of long helical flagella is often employed as a primary means of bacterial propulsion. In this paper, we present a general version of the immersed boundary method incorporating the finitely extensible nonlinear elastic (FENE) constitutive equations for viscoelastic fluids. We first verify that the method solves correctly the rotational dynamics of a helical flagellum in a FENE fluid by conducting a convergence study. We then investigate the hydrodynamic interactions of single and multiple flagella in Newtonian and FENE viscoelastic fluids. For a single flagellum at the same solvent viscosity, the fluid flux generated by the flagellum is higher in FENE fluid than in Newtonian fluid for nearly identical rotation rates, implying enhanced bacterial swimming speeds in a viscoelastic fluid. The dependence of rotational dynamics on applied motor torque, relaxation time, and polymer viscosity is systematically analyzed. For multiple flagella, the time required for bundle formation is shown to increase with hook rigidity, initial separation, and the number of flagella. In the case of two flagella rotating counterclockwise under different applied torques, bundling occurs only when the torque difference is within a limited range, producing a bundle whose axis is tilted from the flagellar axes. We further examine conditions leading to bundling failure, providing insights into the mechanical and hydrodynamic factors governing flagellar coordination.

Wall stress model-based immersed boundary method for simulating incompressible turbulent flows around airfoils at high Reynolds numbers

A one-domain ghost-point immersed boundary method for conjugate fluid and porous medium flows

ABSTRACT The cavern effect strongly impacts mixing efficiency in pseudoplastic fluids stirred in tanks. Perturbed six‐bent‐blade turbine impellers suppress cavern formation effectively, yet existing models cannot predict cavern size and morphology consistently. To overcome this, we develop a high‐fidelity framework coupling the lattice Boltzmann method with the immersed boundary method, enabling direct numerical simulations of pseudoplastic fluid mixing driven by a rotating perturbed six‐bent‐blade turbine. By varying mass concentration and rotational speed, we identify three distinct flow regimes. Based on these results, we propose an elongated heart‐shaped cavern model that predicts cavern geometry and size across regimes and apparent Reynolds numbers. Incorporating impeller perturbation effects, we further introduce a six‐petal rose model that captures the periodicity of the phase‐averaged flow field, achieving unprecedented accuracy in reproducing cavern morphology. Together, these models provide physical insights and practical tools for optimizing pseudoplastic fluid mixing.

Nonlinear water waves (NWWs) can be generated by the vertical bottom disturbance, which represents the conceptual processes of the rise of seabed rupture under seismic loads. To explore the correlation between the disturbance parameters and the wave features, a Reynolds-averaged Navier–Stokes (RANS) model is applied, with the flow turbulence and fluid–structure interaction (FSI) being resolved by the k–ɛ model and the immersed boundary method (IBM), respectively. The free surface is tracked using the volume of fluid (VOF) method. After validating against the theoretical solutions and experimental results, the effects of disturbance duration and bulk on the wave features at the source region (the generation stage) and offshore direction (the propagation stage) are systematically discussed. The fixed maximal vertical displacement is considered, with four moving durations and five disturbance widths being simulated, resulting in four disturbance velocities and five disturbance bulks. The results indicate that the proposed RANS model can accurately create various wave patterns (including the linear, solitary, and tsunami-like waves) generated by bottom disturbances. Special attentions are paid to the tsunami-like wave. The wave evolution exhibits strong dependence on disturbance duration and width, with shorter durations triggering earlier soliton fission and longer widths accelerating phase celerity. These findings highlight the critical role of disturbance parameters in governing soliton formation and energy propagation patterns, which are vital in disaster forecasting.

Abstract The paper presents a constraint-based collision model for Cosserat rods, able to handle dynamic or static contact between a large number of highly flexible structures. The model provides the required collision impulses prior to updating the solution of the rods, with the impulses accounted for as external loads. The procedure avoids the need to modify the structure solver itself and circumvents any iteration between the collision model and the solver for the Cosserat rods, maintaining the efficiency of any chosen Cosserat solver. The collision model is adopted from Tschisgale et al. (Arch. Appl. Mech. 89(2):167–193, 2019) and extended towards higher stability, which is found necessary in the case of very flexible rods. Furthermore, the model is supplemented with additional terms that arise when the colliding rods are immersed in a fluid. The latter is accounted for by an immersed-boundary method. A large number of tests are conducted to demonstrate the functionality of the final model. Beyond the present study, this set of cases may constitute a suitable test bench for dry and wet collisions of flexible structures.

This paper presents a front-tracking immersed boundary method (IBM) for simulating the dynamics of three-dimensional capsules in multiphase viscoelastic flows. The approach couples the lattice Boltzmann method for solving fluid and constitutive equations with the finite element method for capsule membrane mechanics, while fluid–structure interaction and interface tracking are handled using the immersed boundary (IBM) and front-tracking (FTM) techniques, respectively. The solver is validated through several benchmark cases—including spherical and biconcave capsules in Newtonian shear flows, capsules in Newtonian/Oldroyd-B multiphase flows, and compound capsules in Newtonian shear—showing excellent agreement with existing numerical results. The method is further applied to investigate biconcave and compound capsules in Newtonian/Oldroyd-B multiphase shear flows. For biconcave capsules (modeling red blood cells), results show that internal viscoelasticity negligibly affects tumbling motion but significantly alters swinging dynamics, reducing deformation at moderate Weissenberg numbers while increasing it at high values. For compound capsules, the outer membrane exhibits non-monotonic deformation with increasing viscoelasticity, while the inner membrane shows monotonic reduction (low volume fraction) or minimal response (high volume fraction). This work establishes a robust numerical framework for studying capsule dynamics in viscoelastic multiphase environments and provides valuable benchmarks for biomedical and microfluidic applications.

This study proposes an active flow control strategy for an airfoil by integrating the immersed boundary–lattice Boltzmann method (IB-LBM) with the deep reinforcement learning (DRL) algorithm of Proximal Policy Optimization (PPO). The flow field is simulated using LBM, while the immersed boundary method is employed to accurately capture the interaction between the fluid and the moving airfoil. A PPO agent is trained to optimize the airfoil's motion in real time, with a reward function defined based on aerodynamic performance metrics, such as lift and drag coefficients. Numerical experiments are conducted under both steady and sinusoidal inflow conditions to assess the effectiveness and adaptability of the proposed control strategy. The results show that the PPO-controlled airfoil achieves substantial improvements in aerodynamic efficiency compared with uncontrolled cases, and the learned policy demonstrates robust transferability across different flow regimes. Overall, this work underscores the potential of coupling advanced computational fluid dynamics with DRL to tackle complex flow control problems and provides new insight for the intelligent optimization of wind energy systems.

An accurate Ghost Cell Immersed Boundary Method for compressible flows with heat transfer

A ghost-cell immersed boundary method for reacting flow simulations with conjugate heat transfer

Fully GPU-accelerated, matrix-free immersed boundary method for complex fiber-reinforced hyperelastic cardiac models

An improved immersed boundary method for investigating flows over multiple irregular geometries with fractal interpolation

In modern spinning technologies, airflow serves as a flexible and efficient driving force for fiber motion during yarn formation. To better understand yarn formation mechanisms, numerical simulation has become a primary method for analyzing fiber dynamics in airflow. This review focuses on the numerical simulation of fiber motion in airflow during yarn formation. The evolution of fiber models is systematically introduced, including rigid particle models, multiple-rigid-body chain models, and finite-element models, along with their respective modeling methods and applicability. In addition, a comparative evaluation of three common numerical methods, namely the arbitrary Lagrangian–Eulerian method, the immersed boundary method, and the lattice Boltzmann method, is presented, highlighting their strengths and limitations in addressing fluid–structure interaction problems. Finally, the research progress in fiber motion simulation is reviewed for representative airflow-assisted spinning technologies, such as pneumatic compact spinning, rotor spinning, air-jet spinning, and vortex spinning. Existing challenges in simulation accuracy are highlighted, and potential directions for future research are proposed. The review indicates that significant progress has been achieved in refining fiber models and improving the accuracy of airflow simulations. However, challenges remain in multifiber coupling and large deformation simulations. Future studies should focus on developing high-accuracy multiscale simulation methods and enhancing the integration between simulation results and experimental validation.

This study addresses the trade-off between accuracy and efficiency in predicting the aerodynamics and wakes of large wind turbines. We developed a unified immersed boundary–actuator line framework with large-eddy simulation. The actuator line efficiently represents blade loading, while the immersed boundary method (IBM) with a wall model resolves near-blade turbulence. The solver uses a staggered Cartesian discretization and is accelerated by a hybrid CPU/GPU implementation. An implicit signed-distance geometry treatment and a ghost cell wall function based on Spalding’s law reduce near-wall grid requirements and eliminate body-fitted meshing. Flow past a three-dimensional cylinder at Re = 3900 validates the accuracy and good grid convergence of the IBM. For the wind turbine, three meshes show converged thrust and torque, with differences below 1% between the two finer grids. At the rated condition (U∞ = 11.4 m/s), thrust and torque agree with STAR-CCM+ and FAST, with deviations of 6.3% and 1.2%, respectively. Parametric cases at 4–10 m/s show thrust and torque increasing nonlinearly with inflow, approximately quadratically, in close agreement with reference models. As wind speed rises, the helical pitch tightens, the wake broadens, and breakdown occurs earlier, consistent with stronger shed vorticity. The framework delivers high fidelity and scalability without body-fitted meshes, offering a practical tool for turbine design studies and extensible wind plant simulations.

We investigate the inertial migration of slender, axisymmetric, neutrally buoyant filaments in planar Poiseuille flow over a wide range of channel Reynolds numbers ( ${\textit{Re}}_c \in [0.5, 2000]$ ). Filaments exhibit complex oscillatory trajectories during tumbling, with the lateral migration velocity strongly coupled to their orientation. Using a singular perturbation approach, we derive a quasi-analytical expression for the migration velocity that captures both instantaneous and period-averaged behaviour. Finite-size effects are incorporated through solid-phase inertia and the influence of fluid inertia on the orientation dynamics. To validate the theory, we develop a fully resolved numerical framework based on the lattice Boltzmann and immersed boundary methods. The theoretical predictions show good agreement with simulation results over a wide range of Reynolds numbers and confinement ratios. Our model outperforms previous theories by providing improved agreement in predicting equilibrium positions across the investigated range of ${\textit{Re}}_c$ , particularly at high values. Notably, it captures the inward migration trend toward the channel centreline at high ${\textit{Re}}_c$ and reveals a new dynamics, including the cessation and resumption of tumbling under strong inertial effects. These findings provide a robust foundation for understanding filament migration and guiding inertial microfluidic design.

This work presents the development of a novel approach to model the dynamics of cancer cells in microcirculation. We investigate the role the membrane elasticity, and cancer cell shape on deformation dynamics under the shear and pressure forces in a micro-channel. The proposed numerical model is based on a hybrid continuum-particle approach. The cancer cell model includes the cell membrane, nucleus, cytoplasm and the cytoskeleton. The Dissipative Particle Dynamics method was employed to simulate the mechanical components. The blood plasma is modeled as a Newtonian incompressible fluid. A Fluid-Structure Interaction coupling, leveraging the Immersed Boundary Method is developed to simulate the cell's response to flow dynamics. We quantify how subtle variations in these biophysical properties alter deformation indices such as sphericity and aspect ratio, and stress distributions on the membrane of the cancer cell. Our findings align well with existing computational and experimental studies. Results reveal that increased membrane stiffness reduces overall deformation as well as the total distance traveled. Similarly, cell geometry strongly influences flow-structure interactions: near-spherical morphologies exhibit stable deformation with minimal sensitivity to shear variations, whereas elongated geometries show pronounced orientation and stretching effects. Collectively, these findings highlight the critical importance of cell-specific heterogeneity in governing cell dynamics in microvascular flows. Furthermore, the intracellular and extracellular dynamics response of the cancer cell are intrinsically linked to their shape, in which certain morphologies displayed strong resistance to the fluid-induced forces and the ability to migrate in various directions. The insights obtained provide a mechanistic framework for understanding circulating tumor cell transport in shear-dominated environments during metastasis. Our work may inform the design of biomimetic microfluidic systems and therapeutic strategies targeting cancer cell detection and cancer prognosis.

A novel high-order spectral incompressible smoothed particle hydrodynamics (ISPH) scheme with an immersed boundary method (IBM)

This study employs direct numerical simulation to investigate the two-degree-of-freedom vortex-induced vibration (VIV) of a circular cylinder via the immersed boundary method. The wake structure characteristics and hydrodynamic coefficients of VIV are compared with those of forced vibration under the identical cross-flow and in-line amplitudes as well as the same phase difference between the cross-flow and in-line vibrations, to validate the similarity between the forced vibration and VIV. The results demonstrate that the characteristics of forced vibration align closely with those of VIV at the same Reynolds number when the amplitudes and phase difference are consistent. In addition, the effect of phase difference is further examined in the forced vibration. When the dimensionless cross-flow amplitude exceeds 0.59, varying the phase difference leads to the alteration of vortex shedding mode. Four vortex shedding modes are identified: 2S (two single vortices are released per shedding cycle), 2P (two pairs of counter-rotating vortices are released from the upper and lower sides of the cylinder per shedding cycle), P+S (a single vortex and a pair of counter-rotating vortices are released from the cylinder per shedding cycle), and P+S− (two pairs of counter-rotating vortices are released from the upper and lower sides of the cylinder per shedding cycle, and the secondary vortex on one side is weaker than the other side and dissipates within the subsequent 1–2 cycles, which is a newly observed mode). The existence of P+S− mode is further verified using dynamic mode decomposition method. The curve of the mean energy transfer coefficient of VIV coincides with that of forced vibration, providing further evidence that the forced vibration can be effectively used to predict VIV.