Latest papers in fluid mechanics

Author(s): Serge Mora, Martine Le Berre, and Yves Pomeau

During floods caused by a continuous increase in river flow, the water level often rises suddenly, while the recession takes much longer. This behavior is consistent with the subcritical instability highlighted in this article. This instability emerges for uniform, quasi-stationary flows at high Reynolds numbers in a channel. With increasing flow, a sudden jump in water level occurs when a well-defined flow rate is reached. If the flow rate subsequently decreases, the water level drops again suddenly, but at a flow rate well below the previous one.

[Phys. Rev. Fluids 9, 084607] Published Thu Aug 15, 2024

Author(s): Byjesh N. Radhakrishnan, Ahana Purushothaman, Ranabir Dey, and Sumesh P. Thampi

Using lattice Boltzmann simulations and an analytical framework based on far-field approximations and method of images, we study the trajectories of microswimmers inside three-dimensional channels of square and rectangular cross-sections. We find that pusher-type microswimmers move helically inside the square tube, weak pullers slide through the center of the channel while strong pullers exhibit a trajectory which is off-center. The trajectories of the neutral swimmers are challenging to generalize due to the sensitivity to the initial conditions. Finally a method of superposition is used to construct three dimensional trajectories, thus explaining the origin of their apparent complexity.

[Phys. Rev. Fluids 9, 083302] Published Wed Aug 14, 2024

Author(s): Min Lu, Zixuan Yang, Guowei He, and Lian Shen

Wave breaking is recognized as one of the most violent air-sea interaction processes, significantly enhancing the transfer of heat, mass, and momentum between the oceans and the atmosphere. In this study, we investigate heat transfer in wind turbulence over breaking waves using direct numerical simulation, with a particular focus on the effects of wave age. Our findings suggest that temperature responds in a more complex way to wave age than velocity does, emphasizing the need to incorporate this phenomenon into air-sea interaction and weather forecasting models.

[Phys. Rev. Fluids 9, 084606] Published Wed Aug 14, 2024

Author(s): Mariam Dynar, Hamid Ez-Zahraouy, Chaouqi Misbah, and Mehdi Abbasi

The presence of platelets near vessel walls is crucial for clot formation to stop bleeding. We examine platelet margination influenced by red blood cell (RBC) aggregation through numerical simulation. Our findings indicate that moderate to strong RBC aggregation enhances platelet margination in microcirculation, thereby improving the capacity to stop bleeding. This mechanism offers a natural counteraction against major bleeding in conditions such as diabetes, where strong RBC aggregation is commonly observed.

[Phys. Rev. Fluids 9, 083603] Published Tue Aug 13, 2024

Author(s): Artur Gesla, Yohann Duguet, Patrick Le Quéré, and Laurent Martin Witkowski

In the present study the axisymmetric flow in an aspect ratio R/H=10 cavity is revisited. The base state is shown to lose stability in a supercritical Hopf bifurcation. Branches of periodic and chaotic self-sustained solutions are computed using harmonic balance method and time integration. In addition, edge states separating the steady laminar and chaotic regimes are identified using a bisection algorithm. All the self-sustained solutions found are shown to exist only for a high enough Reynolds number and are therefore disconnected from the experimentally observed circular rolls.

[Phys. Rev. Fluids 9, 083903] Published Tue Aug 13, 2024

Author(s): Someshwar Sanjay Ade, Pavan Kumar Kirar, Lakshmana Dora Chandrala, and Kirti Chandra Sahu

Our study experimentally investigates the morphology and breakup of a droplet descending into an airstream, analyzing child droplet size distributions through high-speed shadowgraphy and in-line holography. We found that varying the droplet’s release height results in different breakup modes — from vibrational to retracting bag-stamen breakup at the same Weber number — each with distinct size distributions. Our theoretical model, which incorporates the effective Weber number, accurately predicts these distributions, underscoring the critical impact of droplet dynamics and aerodynamic interactions on breakup behavior.

[Phys. Rev. Fluids 9, 084004] Published Tue Aug 13, 2024

Author(s): V. Srinivasan, X. Tan, E. Whitely, I. Wright, A. Dhotre, and J. Yang

When a jet of fluid emerges into an ambient medium of the same density but higher viscosity, the dominant mode of instability transitions from axisymmetric at low viscosity ratio M to a helical mode at high M. It is shown that these helical modes are unstable global modes associated with enhanced mixing and the emergence of a single dominant frequency observed everywhere in the near field. These frequencies align well with predictions of absolute instability from spatiotemporal linear stability theory.

[Phys. Rev. Fluids 9, 083902] Published Mon Aug 12, 2024

Author(s): Mitesh Thakor, Yiyang Sun, and Datta V. Gaitonde

The perturbation dynamics in a supersonic shear layer are characterized using large-eddy simulations and linear-operator-based input-output analysis. The Kelvin-Helmholtz instability emerges as the primary mechanism for disturbance energy amplification. To identify feasible actuator placement locations, we conduct a state-space restricted input-output analysis, revealing the splitter plate trailing surface as the most receptive region. Furthermore, simulations with applied forcing demonstrate that the coherent structures predicted by linear analysis remain active within a highly nonlinear turbulent flow.

[Phys. Rev. Fluids 9, 084603] Published Mon Aug 12, 2024

Author(s): Yunpeng Wang, Zhijie Li, Zelong Yuan, Wenhui Peng, Tianyuan Liu, and Jianchun Wang

The implicit U-Net enhanced Fourier neural operator (IUFNO) combines the loop structure of implicit FNO (IFNO) with U-Net, leading to enhanced long-term predictive ability in the large-eddy simulations (LES) of turbulent channel flow. It is found that the IUFNO outperforms the traditional dynamic Smagorinsky model (DSM) and the wall-adapted local eddy-viscosity (WALE) model at coarse LES grids. The predictions of both the mean and fluctuating quantities by IUFNO are closer to the filtered direct numerical simulation (fDNS) benchmark compared to the traditional LES models, while the computational cost of IUFNO is much lower.

[Phys. Rev. Fluids 9, 084604] Published Mon Aug 12, 2024

Author(s): Mostafa Kamal and Perry L. Johnson

This paper addresses the artificial bottleneck effect in large-eddy simulations (LES), causing erroneous energy spectrum overshoots due to residual stress model inaccuracies. We use Stokes Flow Regularization (SFR) to improve LES models with detailed residual kinetic energy considerations. Adding a nonlinear gradient component to the residual stress closure accurately captures local stress structures, reduces kinetic energy over-predictions, and enhances energy cascade representation. The mixed model produces vortex tube-like structures similar to those in filtered direct numerical simulations (DNS), while the eddy viscosity model yields shear layer-like features.

[Phys. Rev. Fluids 9, 084605] Published Mon Aug 12, 2024