Numerical investigation of the natural transition in boundary layers on underwater axisymmetric bodies with superhydrophobic surfaces

In the present study, the effects of surface slip on the hydrodynamics and flow around a two-dimensional National Advisory Committee for Aeronautics 0012 hydrofoil are systematically investigated by numerical methods. The objective is to fully understand the effects of surface slip on the streamlined body. Three slip positions (both surfaces, the upper surface, the lower surface) and eight slip lengths (in a wide range from 1 to 500 μm) under 0°–10° angles of attack are fully investigated at a moderate Reynolds number of 1.0 × 106. Surface slip has been found to increase lift and reduce drag by postponing the flow transition, laminar separation bubble, and flow separation on the hydrofoil surface under both surfaces and the upper surface slip conditions. Slip has also been found to induce upshift of the mean velocity profile, decrease the displacement thickness, and mitigate the turbulent kinetic energy in the flow field. However, counterintuitive phenomenon occurs under the lower surface slip condition, where the total drag of the hydrofoil is increased compared to that under the no slip condition. Total drag increase is found mainly due to the increase in the pressure drag under small slip lengths and relatively large angles of attack. Flow maps demonstrating the complex interaction between different surface slip conditions and the flow field are further presented. The results suggest that surface slip can not only reduce drag, but also increase the drag of the streamlined body, which shall provide valuable insights for practical applications of slippery materials.

Influence of surface slip on hydrodynamics and flow field around a two-dimensional hydrofoil at a moderate Reynolds number

Zhu,

Huang,

et al. 2024

Influence of bow shape and model scale on the natural transition in boundary layers on underwater axisymmetric bodies with a superhydrophobic surface

Liu,

Zhang,

Liu

2024

This study uses numerical methods to investigate the influence of bow shape and model scale on the natural transition in bow boundary layers on underwater axisymmetric bodies with a superhydrophobic surface. The transition locations for eight bow shapes and four model scales are predicted by using the eN method, which is based on a linear stability theory. The results show that the superhydrophobic surface always stabilizes the boundary layer and delays the transition for different bow shapes and model scales. However, differences are observed in the basic laminar flow, boundary-layer stability, and transition location when different bow shapes are used. These differences are not prominent at the small model scale, but become significant at the medium and large model scales. As the model scale increases, the transition location in the boundary layer on the superhydrophobic surface moves upstream and then downstream, leading to a “dangerous” scale, at which the transition location is closest to the leading edge. Furthermore, this study provides the optimal bow shape with the latest transition location at each model scale, and the optimal bow shape with the longest transition delay distance.

High-fidelity simulations of boundary layer transition on an underwater axisymmetric body with stability theory

Shen,

Qiao,

Xiao

et al. 2024

During the cruise status of an underwater high-speed vehicle, the flow around the vehicle's head typically transitions from a laminar to a turbulent state, triggering flow noise that can interfere with the normal operation of sonar. In order to accurately investigate this flow noise through numerical simulation, a high-fidelity turbulent flow field solution is essential. Common traditional turbulence numerical simulation methods, such as unsteady Reynolds-averaged simulation and large eddy simulation (LES), struggle to capture high-frequency turbulent fluctuations accurately due to their inability to directly resolve small-scale eddy structures, which results in compromising the simulation accuracy of high-frequency flow noise. To address this issue, this paper employs direct numerical simulation (DNS) to achieve high-fidelity resolution of the turbulent flow field, thereby enabling a more accurate assessment of flow noise distribution on the vehicle's surface. Meanwhile, considering significant computational resources required to solve the entire flow field in an underwater high Reynolds number environment, this study also incorporates the fixed transition modeling method and stability theory to confine the DNS computational domain to the vicinity of the transitional zone to improve simulation efficiency. Comparative analysis of flow noise monitoring results in the laminar, transitional, and turbulent zones revealed that the flow noise source in the laminar zone exhibits the lowest amplitude across all frequencies, while the flow noise source in the transitional zone features the highest amplitude, approximately 10 dB higher than that in the turbulent zone. Moreover, significant amplitudes in high-frequency components (above 30 kHz) are detected in both the transition and turbulent zones. Additionally, this study employs LES with the Smagorinsky model to simulate the flow field within the same computational domain as DNS, demonstrating the limitations of the Smagorinsky model-based LES in capturing high-frequency flow noise.