To explore the effect of density on large yellow croaker (Larimichthys crocea) under intensive aquaculture conditions and determine the appropriate culturing density, this study investigate the effects of different stocking densities on the nutritional composition, stress, and immune levels of large yellow croaker. Through a long-term aquaculture experiment, conducted under flow-through conditions of intensive aquaculture, three initial density groups were set: a low density group [LD], 4.92 kg/m³; a medium density group [MD], 7.56 kg/m³; and a high density group [HD], 10.08 kg/m³, for a 150-day rearing trial. Large yellow croaker were fed to satiation twice daily (6:00, 17:00). At the end of this trial, the final densities were 10.38 ± 0.50, 14.41 ± 1.06, and 18.71 ± 0.99 kg/m³ in the LD, MD, and HD groups, respectively. The results showed that the growth performances were adversely influenced by a high stocking density. Levels of cortisol in serum, superoxide dismutase (SOD) and catalase (CAT) in liver, Na⁺-K⁺ ATPase and Na⁺-K⁺ ATPase gene in gills, and heat shock protein (HSP70/90) genes and glutathione S-transferase (GST) genes in the liver significantly increased under HD treatment. Results of immune response analyses showed that there was a clear decrease in immunoglobulin M (IgM), complement component 4 (C4), and lysozyme (LZM) in serum, lysozyme (LZM) genes, tumor necrosis factor-alpha (TNF-α) genes and interleukin-1β (IL-1β) genes in the head kidney of large yellow croakers reared in the HD group. An obvious increase in free amino acids and fatty acids in the muscle of large yellow croakers reared in HD group was also observed. Overall, this study showed that the optimal final culturing density of large yellow croaker under flow-through systems should be between 14.41 kg/m³ and 18.71 kg/m³ to improve aquaculture efficiency and product quality.

The present work presents numerical research on the wake flows behind a propeller operating under three advance coefficients. Large eddy simulations are adopted to obtain the viscous flow information behind the propeller. In particular, the study highlights the comparison of the evolution characteristics and the flow physics within the propeller wakes with three advance coefficients. The predicted global force and moment coefficients and phase-average statistics of streamwise velocity agree well with the available experimental data. Compared to all other flow structures in the wake, the tip vortices are found to play the most significant role according to the results. During the pairing process of adjacent tip vortices, the tip vortices diffuse circumferentially, leading to enhanced mutual-induction effects. When the advance coefficient is low, the wake becomes distorted, and the pairing process takes place in the middle region of the flow field. As a result of their unstable motion, the four tip vortices generated by the propeller cannot be distinguished individually in the far field. Instead, they break down into smaller vortices and tend to distribute themselves uniformly in the azimuthal direction. The increase in the advance coefficient delays the pairing process. This study offers valuable insights for the design and optimization of marine propellers.

Propeller wake fields in an open-water configuration were compared between two loading circumstances using large-eddy simulation (LES) with a computational domain of 48 million grids and an overset mesh technique. To validate the results of the numerical simulation, available experimental data are compared, which indicates that the grid systems are suitable for the present study. The results indicate that the present LES simulations describe the inertial frequency range well for both high and low-loading conditions. Under high-loading conditions, the interlaced spirals and secondary vortices that connect adjacent tip vortices amplify the effects of mutual inductance, ultimately triggering the breakdown of the propeller wake systems. At a great distance from the propeller, the vortex system loses all coherence and turns into a collection of smaller vortices that are equally scattered across the wake. In contrast, under light-loading conditions, the wake vortex system exhibits strong coherence and has a relatively simple topology. The elliptic instability and pairing processes are only observed at a far distance from the propeller. The convection velocity transferring tip vortices downstream is larger under the light-loading condition, which leads to the larger pitch of the helicoidal vortices. The larger pitch weakens the mutual inductance or interaction effects among tip vortices, which delays the instability behaviors of the whole vortex system. The results and implications of this study serve as a guide for the development and improvement of next-generation propellers that function optimally when operating behind aquaculture vessels.

The noise analysis of a large-scale aquaculture vessel reveals that during its navigation, the primary equipment noise, particularly from the propeller, exerts a notable influence on the aquaculture environment for large yellow croaker. The free surface greatly impacts the noise performance of propellers, which is a significant factor affecting the fish’s habitat. This study adopts the numerical simulation method to analyze the hydrodynamic and acoustic characteristics of the E1619 propeller operating near the free surface. The open-water performance and noise calculations of the propeller are verified through experiments, and the effects of different immersion depths and advance coefficients on the propeller are explored. The results demonstrate that the free surface significantly affects the thrust, torque, and noise of the propeller, especially at shallow immersion depths and low advance coefficients. Surface wave pattern causes the instability and breakup of tip vortices, causing increased thrust and torque fluctuations, reduced efficiency, and significant overall sound pressure levels in the entire flow field. As immersion depth and advance coefficients increase, the interaction between tip vortices and the free surface weakens, wake vortex instability decreases, and noise levels gradually reduce. These analyses and conclusions can guide the design of next-generation propellers for aquaculture vessels to optimize performance near the free surface.

Large-scale offshore aquaculture vessels provide an effective approach for expanding marine aquaculture; however, the complex flow characteristics induced by water circulation systems and vessel motion remain insufficiently understood. In this study, a numerical model based on the volume of fluid (VOF) method and the k-$\omega$ SST-DES turbulence model was developed to investigate the vortex flow characteristics. The developed numerical model accurately captured the vortex flow characteristics and free-surface fluctuations in the aquaculture tank, and the dominant coherent structures were identified using Q-criterion-based proper orthogonal decomposition (POD). The results show that the mean vortex structure is mainly concentrated around the central axis, while higher-order POD modes reveal vortex-core oscillation, deformation, and interactions with surrounding flow structures. Furthermore, the effects of roll excitation and its coupling with inlet jet flow were investigated. The results demonstrate that roll motion induces significant periodic fluctuations in the flow field, while the inlet jet flow maintains the stability of the central vortex structure and reduces velocity variations. These findings provide new insights into the hydrodynamic mechanisms governing vortex evolution under combined vessel motion and water circulation and offer a scientific basis for optimizing flow conditions in large-scale offshore aquaculture tanks.