DNS study of turbulent transport of vorticity and secondary-flow structures in a square duct with wall-mounted longitudinal ribs

Analysis of vortex structures and flow losses in a supersonic expander-rotor

Hydrodynamic instabilities and interface dynamics of two immiscible liquids driven by a rotating disk.

In the rust removal process via ultra-high-pressure (henceforth UHP) water jet, nozzle translation and rotation significantly affect the jet profile, thereby influencing rust removal performance. Unfortunately, the research on UHP water jets in motion is still rare, lacking systematic theoretical analysis. To address this, 3D numerical simulations incorporating cavitation effect, multiphase flow, and liquid compressibility are conducted to investigate the hydrodynamic performance of UHP water jetting under different translational speeds. The water-jet hydrodynamic characteristics under stationary and translating nozzle conditions are compared in terms of velocity, vortex structure, turbulent kinetic energy, pressure distribution, and impact area. Furthermore, five cases related to different translational speeds are analyzed to examine the influence of motion on jet behavior. Results show that the stationary jets exhibit axisymmetric velocity profiles and vortex structures. When the translational motion is introduced, the jet deflects opposite to the nozzle movement, triggering enhanced shear layer instability. This kind instability serves as a primary driver for the emergences of increased turbulence, vortex deformation, and pressure asymmetry. Consequently, the turbulent kinetic energy distribution broadens and decays more rapidly downstream. Pressure distributions become asymmetric, with high-pressure zones forming upstream and more intense pressure decaying downstream. The effective impact area expands significantly under moderate wall-shear-stress thresholds, but becomes less uniform at higher velocities. An optimal translational speed of approximately 20 m/s can be identified, offering the best trade-off between cleaning performance and energy efficiency. This study provides theoretical support and practical guidance for advancing rust removal technology via UHP water jetting.

When pump-turbines operate under off-design conditions, complex internal vortex flows often arise, leading to hydraulic instabilities and increased energy losses. Traditional hydraulic loss evaluation methods fail to quantitatively link vortex structures to energy dissipation mechanisms, limiting their ability to guide design improvements. To address this gap, this study proposes a Rortex-enhanced hydraulic loss evaluation framework to systematically analyze vortex dynamics and associated energy dissipation across wide-load turbine-mode operating conditions. By decomposing the dissipation terms in the mean turbulent kinetic energy equation, the framework identifies rigid vorticity-induced dissipation, shear vorticity-induced dissipation, and rotational-shear coupling losses. Numerical simulations, validated by experimental data, show that shear vorticity-induced dissipation dominates overall hydraulic losses, primarily along blade surfaces and boundary layers. Although rigid vorticity-induced dissipation contributes less overall, it increases significantly at lower loads and is concentrated around vortex peripheries due to shear interactions. Energy losses within vortex cores are predominantly driven by rotational-shear coupling effects, strongly correlated with the enstrophy of the pseudo Lamb vector curl term (ERCT). These findings suggest that suppressing ERCT-driven vortex formation and mitigating rotational-shear interactions through optimized blade designs could effectively reduce energy losses and enhance the hydraulic stability and efficiency of pump-turbines.

Understanding the underlying mechanisms behind cavitation-induced noise plays a crucial role in promoting sustainable marine technologies and mitigating underwater acoustic pollution. To address the difficulty of capturing nonlinear flow-acoustic interactions under cavitating conditions, a novel hybrid method, this study proposes a novel hybrid method based on the DDES turbulence model and developed a numerical prediction method for cavitation noise combining three-dimensional implicit vortex sound theory with cavitation. The method is validated by experiment, with head and efficiency prediction errors less than 4.2% and noise level deviations within 4.8%. Results show that: as the cavitation number drops, vapour cavities extend along the blade passage and contribute to intensified vortex structures near the impeller’s entrance. Pressure pulsation frequencies are 0.167fBPF at the impeller and 1fBPF at the volute, with strong dynamic-static interference near the volute tongue. Flow-induced noise exhibits a discrete distribution, with peak sound pressure levels at 1fBPF in the low-frequency range and 6–7fBPF in the mid-to-high frequencies. Bubble pulsation amplifies noise at σ = 0.07, 0.062, and 0.060, while severe flow blockage at σ = 0.051 reduces noise levels. Noise sources are primarily concentrated near the volute tongue and impeller-volute junction, where impeller rotation induces a ‘jet wake’-like pattern, and impeller motion significantly influences noise source distribution near the volute tongue. The synergy angle analysis reveals efficient acoustic radiation near the impeller inlet. These findings provide theoretical support for low-noise design and cavitation noise control in hydraulic machinery.

Semi-open centrifugal pumps are widely used in energy systems, particularly for handling multiphase flows in industrial, municipal, and energy generation applications. While the semi-open impeller design offers advantages in managing solid–liquid mixtures, it also presents challenges such as leakage flow and flow instability, which can significantly affect both energy efficiency and overall operational performance. Despite substantial research utilizing conventional numerical approaches, such as Eulerian-Eulerian and Euler–Lagrange models, a comprehensive understanding of the intricate interactions between solid particles and the fluid remains insufficient, particularly regarding the impact of particle behaviour on fluid dynamics and system energy dissipation characteristics. This work aims to investigate the two-phase flow behaviour and energy dissipation mechanisms under the influence of leakage flow in semi-open impellers by integrating Computational Fluid Dynamics (CFD) with the Discrete Element Method (DEM), coupled with normalized scale-averaged wavelet spectrum and coherence analysis. The results reveal that increasing the particle volume fraction intensifies particle-blade collisions, disrupting vortex structures and compromising hydraulic stability, while larger particles exacerbate flow disturbances, leading to efficiency losses of up to 12.90%. Furthermore, the energy fluctuations generated by particle-fluid interactions predominantly manifest as high-frequency pressure pulsation signals. A mutual suppression effect between the pressure pulsation signals and particle energy dissipation is observed at the blade passing frequency. These findings offer critical insights into the influence of particle dynamics on pump performance and provide valuable guidance for optimizing pump design to improve operational stability and energy efficiency in solid–liquid flow applications.

To meet the underwater drag reduction needs in engineering applications, one can draw on the drag reduction strategies of marine animals such as sharks, utilizing their surface groove structures and the secretion of viscoelastic drag-reducing mucus to achieve drag reduction effects. However, in the field of engineering research, efficient and accurate simulation methods for viscoelastic fluid flow are relatively scarce. There is an urgent need to develop efficient numerical simulation methods for viscoelastic fluids, and to analyse the drag reduction characteristics of the coupling between surface grooves and drag-reducing agents. Through large eddy simulation and re-development of the ANSYS Fluent platform, efficient and accurate simulation of viscoelastic fluid flow as well as wall infiltration method has been successfully realized. Quantitative analysis of drag reduction characteristics of surface structures and viscoelastic fluid additives within the Reynolds number range of 4×104 to 4×105 has been conducted. Moreover, the synergistic effect of micro-grooves and drag reducer infiltration on drag reduction has been deeply explored. It is found that the maximum drag reduction by the groove surface solely is below 20%, whilst the maximum drag reduction by viscoelastic drag-reducing agent infiltration on the smooth flat plate is up to 28.9%. Notably, when combining the groove structure with drag-reducing agent infiltration, the maximum drag reduction reaches up to approximately 70%. The mechanism for the drag reducing effect is analysed from the aspects of vortex structure evolution on the wall, velocity profile and drag-reducing agent diffusion. This establishes an efficient and accurate simulation method for viscoelastic fluid flow suitable for engineering research, providing an important reference and foundation for underwater drag reduction studies.

This study systematically investigates the propagation characteristics of ring-shaped Airy-Gaussian vortex (RAiGV) beams in a 50 m marine turbulent channel. Utilizing a combined angular spectrum-phase screen model, numerical simulations were conducted to analyze the evolution of light intensity, scintillation index (SI), and detection probability (DP) under varying distribution factors b, topological charge l, and turbulence intensity σ2. Results reveal that the SI of RAiGV exhibits a three-stage pattern: initial rise, decline, and subsequent rise. The valley positions of SI correspond one-to-one with self-focusing foci. Smaller b values result in closer foci, with short-range SI reaching its minimum but eventually surpassing long-range SI. At b = 0.15, the beam maintains a flatter SI curve and higher DP over long distances. The l = 1 vortex structure, characterized by its simplicity, demonstrates superior robustness against turbulence compared to higher-order modes. Appropriate selection of b and l enables a trade-off between near-field peak intensity and far-field stability, providing valuable design guidance for underwater OAM multiplexing communications.

A solitary gravitational current over an inclined bottom is being investigated in the thermostratified laboratory tank of the SPb IO RAS. In the framework of a laboratory experiment close to real natural conditions, preliminary studies of complex, nonlinear processes of interaction of the bottom density flow, stratification and internal waves were carried out. The full life cycle of the formed vortex structures is considered: from their origin on a slope, development and propagation in a stratified environment, to their interaction with the field of internal waves. During the experiments, empirical data were obtained to verify a non-hydrostatic model with a spatial resolution that allows explicit reproduction of individual convective jets and vortices.

A detailed non-hydrostatic model of gravitational flow over an inclined bottom is being developed, which is capable of explicitly reproduce convective cells for future generalization and new parameterizations development. To minimize numerical noise, the method of an inclined computational domain and a regular rectangular grid are used. The properties of high-order accurate advection schemes are investigated. The fundamental possibility of explicitly numerical reproduction of relatively large (on the order of a meter or more) ocean turbulent structures, such as convective cells, is demonstrated. A high-resolution digital array of 3-dimensional velocity and tracer fields (active and passive) created based on a physical experiment for a range of Reynolds numbers of 30–300. This array will be used to develop new parameterizations for a large-scale ocean circulation model.

This study investigates the dynamics of free-surface turbulence (FST) using direct numerical simulations (DNS). We focus on the energy exchange between the deformed free-surface and underlying turbulence, examining the influence of high Reynolds ( ${\textit{Re}}$ ) and Weber ( ${\textit{We}}$ ) numbers at low to moderate Froude ( ${\textit{Fr}}$ ) numbers. The two-fluid DNS of FST at the simulated conditions is able to incorporate air entrainment effects in a statistical steady state. Results reveal that a high ${\textit{We}}$ number primarily affects entrained bubble shapes (sphericity), while ${\textit{Fr}}$ significantly alters free-surface deformation, two-dimensional compressibility and turbulent kinetic energy (TKE) modulation. Vortical structures are mainly oriented parallel to the interface. At lower ${\textit{Fr}}$ , kinetic energy is redistributed between horizontal and vertical components, aligning with rapid distortion theory, whereas higher ${\textit{Fr}}$ preserves isotropy near the surface. Evidence of a reverse or dual energy cascade is verified through third-order structure functions, with upscale transfer near the integral length scale, and enhanced vertical kinetic energy in upwelling eddies. Phase-based discrete wavelet transforms of TKE show weaker decay at the smallest scales near the interface, suggesting contributions from gravitational energy conversion and reduced dissipation. The wavelet energy spectra also exhibits different scaling laws across the wavenumber range, with a $-3$ slope within the inertial subrange. These findings highlight scale- and proximity-dependent effects on two-phase TKE transport, with implications for subgrid modelling.

This paper presents a physically motivated model of quasi-two-dimensional vortex structures in turbulent flows. The theory of quasi-two-dimensional turbulence explains many phenomena in geophysical hydrodynamics, since due to the rapid rotation of the Earth, large-scale movements of the atmosphere and ocean almost two-dimensional. Quasi-2D turbulence is approximately two-dimensional and is described by equations containing additional terms. Such additions allow us to take into account weak three-dimensional effects that arise in real conditions, for example, in the atmosphere or ocean. We consider the basic equations for the velocity and pressure fields using the Lagrangian frame and incorporating centrifugal and Coriolis forces, as well as fractal disturbances on the vortex surface. Numerical simulations implemented in MatLab reproduce classical vortex behavior and reveal the influence of fractal corrections on field asymmetry. The model aligns well with existing experimental data and offers a foundation for analyzing energy transport and vortex interactions in stratified or thin-layered turbulent systems. Keywords: quasi- two-dimensional turbulence, fractal boundary, Lagrangian frame, streamfunction, vortex elements, numerical simulations.

The effectiveness of pressurized metered-dose inhalers (pMDIs) relies on correct inhalation technique. While prior studies investigated idealized breathing, the impact of real-life irregularities remains less understood. This study explores how real-life irregularities-pausing, coughing, and premature exhalation-alter aerosol transport and deposition in the airways. Large-eddy simulations combined with a discrete phase model were performed on a realistic male airway geometry extending from the oral cavity to the fourth bronchial generation. Computational predictions were validated against in vitro experiments conducted under constant inhalation. Breathing irregularities substantially modified airflow dynamics and shifted deposition toward the upper airways. Coughing generated the strongest vortical structures and turbulence, followed by premature exhalation. Deposition in the left lung decreased from 19.9% during standard COPD inhalation to 2.1% during exhalation and 0.9% during coughing, while mouth-throat deposition increased to 35.2% during coughing compared to 14.5% under the COPD baseline condition. Exhalation caused higher overall particle loss (27.9%) than coughing (24.1%), but coughing produced more pronounced inertial impaction in the upper airways. Fine particles (< 2µm) were largely exhaled (approximately 80%), whereas particles in the 2-5µm range-considered optimal for deep lung delivery-were redirected and lost under disturbed flow conditions. Irregular breathing patterns markedly decrease deep lung deposition and increase upper airway losses. Repeated puffs without adequate intervals may exacerbate this problem, leading to excessive upper-airway deposition and increasing the likelihood of side effects. These findings provide guidance for physicians to tailor puff number and timing, improving therapeutic efficacy while minimizing risks to patient safety.

Global energy shortage has increasingly become a major issue of global concern. Since the pump is the largest component in the energy conversion device, it is very important to improve the energy characteristics. The existence of tip clearance leads to the formation of leakage vortices, which causes flow instability and performance degradation, especially in the hump region. This paper investigates the internal flow characteristics of a mixed-flow pump in the hump performance region. The boundary vorticity flow (BVF) diagnosis theory and the vorticity transport equation are utilized to analyze the influence of complex vortex structures on pressure pulsation and flow patterns. It can be found the vortex structure in the impeller is divided into primary tip leakage vortex (PTLV) and secondary tip leakage vortex (STLV) under the hump condition (0.7 Q dm ). The vortex structure near the leading edge (LE) of the blade have a tendency to move to the next blade, and the scale increases first and then decreases. On the contrary, the vortex structure near the trailing edge (TE) does not change significantly with time. In the spectrum analysis of the leakage vortex and pressure pulsation, the complete evolution frequency of the leakage vortex is 1.1 f n , while the pressure pulsation is 1.0 f n . This study provides a theoretical basis and technical support for the design of the mixed-flow pump.

Abstract The unsteady tip leakage flows are accountable for aerodynamic losses that inherently hinder the performance of unshrouded turbines. Abatement of these penalties through the tightening of the operating tip clearance is highly relevant for new generations of small-core high-speed turbines, whose characteristic low aspect-ratio passages result in significant influence of secondary flow structures. A deep understanding of the development of tip leakage flows is paramount to defining a suitable transition towards tight clearance turbine configurations. In-situ experimental measurements provide insights to build such comprehension and further serve to improve the numerical resolution fidelity of these highly detached unsteady flows delivered by commercially available computational tools. This manuscript presents the experimental resolution of the unsteady pressure fields experienced in the over-tip casing of a small-core high-speed turbine. Fast response miniature pressure transducers captured at 2MHz the casing's static pressure of the TRL6 small-core turbine demonstrator “STARR”, located in the Purdue Experimental Turbine Aerothermal Laboratory. The versatility of the facility for engine-representative testing allowed identifying the independent influence of the operating pressure ratio and rotational speed on the squealer tip leakage flow structures along a 60% tip clearance reduction. Besides the pressure differential across the blade, the resulting phase-locked average fields revealed two vortical structures along the squealer tip cavity as a secondary mechanism driving the tip leakage flows, with different shifting and combination trends along the operational envelope and tip clearance reduction.

Numerical study of air entrainment mechanisms and vortical structures in breaking waves

This paper investigates the noise reduction mechanism of a three-element airfoil with a swept angle. It analyzes the impact of the swept angle on airfoil noise through numerical simulations validated by wind tunnel experiments. An in-house high-precision numerical scheme based on the lattice Boltzmann method combined with the parallel computing capabilities of graphics processing units is used to directly simulate the unsteady vortices around the airfoil. The study finds that the noise reduction effect of the swept angle is not solely attributed to the decrease in effective incoming flow velocity, the sweepback effect introduces a spanwise phase difference during vortex shedding, which weakens the spanwise coherence of the vortex structures. Meanwhile, the breakdown into smaller vortex structures leads to an increase in broadband noise which compensates exactly the additional reduction of the low frequency spectrum pressure level. Based on the Lamb vector ω×u in the vortex-acoustic theory and combined with the dynamic mode decomposition method, this paper conducts a study on the quantitative relationship between the evolution of vortex morphology and noise changes.

Abstract Sanitary centrifugal pumps are extensively used in food, pharmaceutical, and bioengineering industries due to their high cleanliness and structural compactness. However, conventional circular volutes, with constant cross-sectional area, exhibit limited diffusion capability and poor hydraulic performance. To address this, an eccentricity-based volute optimization method is proposed to develop an approximately spiral volute, which is further refined into a spiral configuration. Circular, approximately spiral, and spiral volutes were integrated into the same pump for comparative numerical analysis. Results show that, under design conditions, the approximately spiral volute increases head by 2.5 m and efficiency by 5.4%. At large flow rates, total entropy generation is reduced by 0.73 W·K -1 , indicating lower energy losses. Under low flow conditions, vortex structures in the blade passages and volute cross-section are effectively suppressed. The proposed eccentric optimization yields an approximately spiral volute with superior hydraulic performance and flow stability, offering theoretical support for structural optimization of sanitary centrifugal pumps.

As a flexible regulating power source, pumped storage units play a crucial role in enabling the large-scale integration of renewable energy into power grids. Through interregional regulation, they balance the generation and consumption of renewable energy across different regions, thereby enhancing overall utilization efficiency. However, during this process, units frequently traverse or directly operate within the S-shaped characteristic region (S region), which may induce operational instability. When a high-head pump turbine enters the S region, large-scale flow separation and complex vortex inevitably develop within the flow components. The dynamic evolution of these unsteady structures induces high-amplitude pressure fluctuations and force pulsations, posing significant threats to the safe and stable operation of the unit. To elucidate the mechanisms of pressure fluctuation and force evolution in the S region, this study investigates a high-head model pump turbine through unsteady numerical simulations at representative operating points with a 12° guide vane opening. The analysis focuses on internal flow characteristics, pressure fluctuations, and runner force pulsations. The results show that unstable vortex structures, including circumferential and cross flows, develop near the runner inlet, leading to pronounced pressure fluctuations. Time-frequency analysis reveals that cross flow within the runner generates low-frequency components at 0.3 fn and 0.4 fn, while rotating stall in the vaneless space causes low-frequency fluctuations at 0.6 fn and 0.7 fn. In contrast, rotor–stator interaction produces high-frequency components at 9.0 fn. Further analysis confirms that rotating stall and cross flow are the primary sources of flow instability in the S region of pump turbines. Moreover, the runner force results demonstrate that vortex structures induced by cross flow significantly affect the radial force distribution, whereas the axial force is predominantly governed by rotor–stator interaction. These findings provide a theoretical basis for optimizing the hydraulic design and enhancing the operational stability of pumped storage units.