Hydrodynamic Flow Field Research Articles

NUMERICAL SIMULATION OF THE VISCOUS FLOW AROUND SHIPS TURNING IN SHALLOW WATER

Due to the effect of water depth, hydrodynamic characteristics of ships manoeuvring in shallow waters are quite different from those in deep water. In the present study, the CFD method is used to investigate the hydrodynamic behaviour of the ESSO OSAKA model in model-fixed condition in deep and shallow waters by solving RANS equations, which performs steady turning motion with the effect of free surface considered. The governing equations are discretized by the finite volume method (FVM) and the free surface is captured by applying the volume of fluid (VOF) method. Viscous hydrodynamic characteristics and flow fields in a series of cases at different water depths and speeds are investigated respectively. By comparing the numerical results in shallow water with those in deep water, it was found that effects of water depth and free surface are significant on ship’s hydrodynamic forces. The coefficients of drag, lateral force and yaw moment all increase with the decrease of water depth. The drag coefficient in shallow water is about 45% larger than that in deep water. However, effect of water depth on the lateral force coefficient is small, it is about 15% larger in shallow water than that in deep water. While, it has significant effect on the yaw moment coefficient, which is about 25% larger in shallow water than in deep water. And as the water depth increases, the effect of the free surface gradually becomes smaller. Both coefficient of pressure component of drag force and drag coefficient in the present simulation are about 7% smaller than the case without considering the free surface at h/d ≥ 3.0, while that of frictional component is almost the same. When h/d ≥ 3.0, the lateral force and yaw moment coefficients are almost the same as when the free surface is not considered. The error between the present numerical result and the measurements is within 10%, which indicates that RANS method has promising capability to predict the hydrodynamic forces on ships manoeuvring in shallow water.

Research on the effects of complex terrain on the hydrodynamic performance of a deep-sea fishlike exploring and sampling robot moving near the sea bottom

Deep-sea exploring and sampling technologies have become frontier topics. Generally, the movable exploring mode near the seabed with low disturbance is an important way to improve the measurement accuracy and expand the measurement range. Inspired by fish, the fishlike propulsion method has the characteristics of low disturbance and high flexibility, which is very suitable for near-seabed detection under complex terrain conditions. However, the swimming mechanism and surrounding flow field evolution law of the robotic fish under the constraints of complex terrain are still unclear. In this paper, the confined terrain space is constructed with an undulating seabed and a narrow channel, and the hydrodynamic changing law and flow field evolution law of the autonomous swimming process of the fishlike swimmer in the confined space are analyzed. Moreover, the influence mechanism of the terrain on the motion performance of the robotic fish is revealed, and the optimal motion mode of the robotic fish under a complex terrain constraint is discussed. The results show that the propulsion force, Froude efficiency, and swimming stability of the robotic fish vary with the distance from the bottom under the undulating seabed condition lightly. When the distance from the bottom exceeds a certain value, it can be considered that the undulating seabed no longer affects the swimmer. Furthermore, when the robotic fish swims through a narrow channel with certain width, the swimming performance obviously varies with the distance from the boundary surface. During swimming in the confined terrain space, the propulsion force and swimming stability of robotic fish will decrease. In order to maintain the forward speed, the robotic fish should improve the tail-beat frequency in real time. However, considering the swimming stability, the tail-beat frequency is not the larger the better. The relevant conclusions of this paper could provide theoretical support for the development of low-disturbance bionic exploring and sampling platforms for deep-sea resources and environments.

Unveiling the Rolling to Kayak Transition in Propelling Nanorods with Cargo Trapping and Pumping

Magnetic nanorods driven by rotating fields in water can be rapidly steered along any direction while generating strong and localized hydrodynamic flow fields. Here we show that, when raising the frequency of the rotating field, these nanopropellers undergo a dynamic transition from a rolling to a kayak-like motion due to the increase in viscous drag and acquire a finite inclination angle with respect to the plane perpendicular to the bottom surface. We explain these experimental observations with a theoretical model which considers the nanorod as a pair of ferromagnetic particles hydrodynamically interacting with a close stationary surface. Further, we quantify how efficiently microscopic cargoes can be trapped or expelled from the moving nanorod and use numerical simulations to unveil the generated hydrodynamic flow field. These propulsion regimes can be implemented in microfluidic devices to perform precise operations based on the selective sorting of microscopic cargoes.

High resolution modeling of aluminium transport in a fjord estuary with focus on mean circulation and irregular flow events

Environmental impact assessments of trace metals and radionuclides in estuarine waters will benefit from numerical transport models that can provide detailed and accurate predictions of concentrations of harmful physico-chemical forms of contaminants at adequate spatial and temporal resolution. Aiming to study the potential of aluminium (Al) exposure to biota, a transport model (OpenDrift) including dynamic speciation and transformation processes was improved and applied, using three-dimensional hydrodynamic flow fields from a numerical ocean model (ROMS) at high horizontal resolution (32 m). Al transport and concentration was computed along the Sandnesfjorden Fjord, south-eastern Norway, from river outlet to open coastal waters. Validation of the circulation model with 29 hydrographic profiles from Sandnesfjorden showed substantial improvements compared to previous studies due to optimized model configuration (salinity overestimation decreased from >7 psu to <4 psu). Modeled Al data compared well with observed surface Al concentration from 12 locations and the along-fjord decreasing trend in Al-concentration was well reproduced (error ratios were <2 in Sandnesfjorden). Except in the channel area, both salinity and Al concentration estimates lie well within the expected variability. However, the transport modeling gave a more detailed site-specific picture of the Al concentration, suggesting more scattered and variable fields than indicated by observational data (variations of a factor 3–4 over short spatiotemporal scales). Reversed flow events (surface flow into the fjord) caused considerable mixing and redistribution of water masses, affecting both horizontal mixing of river discharges with coastal water as well as vertically as surface water mixed with deeper water masses. These blocking events strongly changed properties and distribution of the water masses giving rise to local and short-term high Al-exposure episodes (variations of a factor of 10 over a 12 h period) in the fjord that may pose risks to biota and therefore should be taken into account in impact and risk assessments.

A Review on Hydrodynamic Performance and Design of Pump-Jet: Advances, Challenges and Prospects

A pump-jet, which is generally and widely adopted on underwater vehicles for applications from deep sea exploration to mine clearing, consists of a rotor, stator, and duct, with the properties of high critical speed, high propulsion efficiency, great anti-cavitation performance, and low radiated noise. The complex interaction of the flow field between the various components and the high degree of coupling with the appendage result in the requirements of in-depth research on the hydrodynamic performance and flow field for application and design. Due to the initial application on the military field and complicated structure, there is scant literature in the evaluation of pump-jet performance and optimal design. This paper, in a comprehensive and specialized way, summarizes the pump-jet hydrodynamic performance, noise performance, and flow field characteristics involving cavitation erosion and vortices properties of tip-clearance, the interaction between the rotor and the stator and the wake field, as well as the optimal design of the pump-jet. The merits and applications range of numerical and experimental methods are overviewed as well as the design method. It also concludes the main challenges faced in practical applications and proposes a vision for future research. It was found that the compact structure and complex internal and external flow field make the pump-jet significantly different, also leading to higher performance. As the focus of cavitation research, vortices interact with the complex structure of the pump-jet, leading to instabilities of the flow field, such as vibration, radiated noise, and cavitation erosion. The effective approaches are adopted to reduce radiated pump-jet with minimal influence on the hydrodynamic performance, such as eliminating the tip clearance and installing the sawtooth duct. Advanced optimal technology can achieve high performance, cavitation performance, and acoustic performance, possessing good prospects. Further developments in investigation and the application of pump-jets in the multidisciplinary integration of fluid dynamics, acoustics, materials, chemistry, and bionics should be the main focus in future research.

Numerical study on transient four-quadrant hydrodynamic performance of cycloidal propellers

Aiming to investigate the transient four-quadrant hydrodynamics on blades under the circumferential motion of the steering center, an efficient three-dimensional (3D) model with half blades was established in this paper. The 3D model with half blades accounts for the hydrodynamic loss induced by the complicated mechanical structure, predicting the hydrodynamic performance of the high-load cycloidal propeller with high accuracy. On this basis, the open water manoeuvering performance of the cycloidal propellers in fully azimuth angle was simulated by Reynolds-averaged Navier-Stokes (RANS) solver, and the hydrodynamic loads and flow field characteristics were analyzed in detail. The research shows that the variation of azimuth angle φ induces the rapid response of hydrodynamic loads, which directly affects the direction of ship motion. Compared with φ = 0°, the increase in the azimuth angle of the steering center will enhance the negative impact of the inflow velocity on the wake flow field and the internal flow field within the blades. Additionally, the non-uniform flow in the main thrust direction leads to a significant increase in the undesired net lateral force. Our primary findings revealed in present paper should contribute to accurate prediction of hydrodynamic performance for high-load cycloidal propellers during maneuvering process.

Numerical study on hydrodynamic performance of underwater manipulator in the subcritical region

The hydrodynamic performance of an underwater manipulator is critical for establishing a hydrodynamic model for precise positioning or grasping. However, elucidating the variable hydrodynamic performance is difficult owing to the different working attitudes in the usage of underwater manipulators. Therefore, this study aims to investigate the hydrodynamic coefficients and flow field characteristics of the underwater manipulator with different motion attitudes in Cartesian coordinates. The effects of section shape, spacing, and interaction on the hydrodynamic performance are investigated in the subcritical range. Results indicated that the lift and drag coefficient of the elliptical section were 76.15 and 51.30% lower than that of the circular section, respectively. Furthermore, the amplitude of the downstream elliptical section was affected primarily by the spacing. However, when arm 2 was in the upstream, the lift coefficient of arm 1 was 3.4 times greater than that of arm 2. This implied that the underwater manipulator is more stable when arm 2 was in the upstream. This study will provide theoretical guidance to reveal the hydrodynamic performance of the underwater manipulator and other marine equipment, such as risers and submarine cable.

The importance of being a cube: Active cubes in a microchannel

• The shape anisotropy of cubes matter for their interaction with active hydrodynamic flow fields. • Active colloidal cubes move parallel to planar boundaries and tubes. • The body internal orientation of the active force determines the trajectory of active cubic colloids. Cubic shaped micrometer and nanometer sized colloids have become a popular research subject in experimental and theoretical soft matter physics. However, they are underrepresented in the field of active matter where the most investigated colloidal shapes are spheres or rods, for which hydrodynamic interactions vary from cube ones due to distinct body shape anisotropies. In this article, we are building on this knowledge by performing simulations to elucidate how active cubes in microchannels interact hydrodynamically with planar and curved boundaries through the flow field generated by an active force. We find that cubes tend to not show the same oscillatory behaviour previously found for other particle shapes, but move along a straight line near the channel centre for a range of initial conditions and parameters that can be accounted for in simulations. This effect arises through the cubes unique shape anisotropy interacting with the flow field and demonstrates that a cube constitutes an interesting hydrodynamic shape that deserves more attention in colloidal active matter research, in order to fully utilise its potential as an enrichment to commonly used particle shapes.

Hydrodynamic and Flow Field Characteristics of Water Jet Propulsion under Mooring Conditions

The acceleration characteristics of a water jet-propelled ship during startup are related to its performance under mooring conditions. Water jet propulsion cavitation during startup increases the vibration and noise of the whole ship. Therefore, accurately predicting and analyzing the performance, hydrodynamics and flow field characteristics of water jet-propelled ships under mooring conditions can help elucidate the startup characteristics of the ships and optimize their acceleration strategies. In this study, the hydrodynamic and flow field characteristics of water jet propulsion and water jet propulsion ships under mooring conditions were studied using three-dimensional numerical modeling. First, the hydrodynamic performance of the water jet propeller was analyzed, and the relevant flow field law was derived. Then, the hydrodynamic performance, internal and external flow field characteristics, pulsation pressure and flow rate at the nozzle, and pulsation pressure at the monitoring points around the impeller of the water jet propulsion ship model were analyzed under mooring conditions. We obtained the open-water law for the water jet propeller and the hydrodynamic force and flow field law for a two-pump water jet propulsion ship. The ship model developed in this study provides a good theoretical foundation for further research on water jet propulsion.

A comparative numerical study of bank effects on a cruise ship in crabbing motion based on URANS method

The crabbing motion of a cruise ship perpendicularly towards a quay wall is investigated by an unsteady Reynolds-averaged Navier-Stokes (URANS) method. In the numerical simulations, the cruise ship is approaching the quay wall at a constant lateral speed and stops at a rather small ship-quay distance. The crabbing motion with multiple degrees of freedom is simulated by the dynamic overset mesh technique, while the free surface elevation is simulated by the Volume of Fluid method. To obtain reliable temporal discretization for the crabbing process, the time-step dependence study is conducted first, from which a suitable time step is determined. From the computations, the hydrodynamic performances of the cruise ship including: longitudinal force, lateral force, roll moment and yaw moment, as well as surge, heave, roll, pitch and yaw motions are predicted. To evaluate the importance of numerical models used in the URANS method, a comparative investigation is carried out to evaluate the impacts of boundary condition, turbulence model and temporal discretization scheme on the numerical results. From the comparisons, notable influences by the turbulence model and the temporal discretization scheme on both hydrodynamic quantities and flow field features can be observed, especially when the bank effects tend to be pronounced; while the boundary conditions are shown to be less significant. The numerical results indicate remarkable variations of the hydrodynamic quantities in the crabbing process under the influences of ship speed and bank effects. Given that the surge motion is one of the most principal crabbing performances, the initial drift angle with a set of deviations is also computed, coupled with two typical ship speeds in terms of practical navigation. It suggests that more attentions should be paid to the low-speed crabbing to minimize the surge variations. The present study demonstrates promising abilities of CFD methods to deal with the hydrodynamic predictions of harbour manoeuvres in restricted waters.

Emergence of active turbulence in microswimmer suspensions due to active hydrodynamic stress and volume exclusion

Microswimmers exhibit an intriguing, highly-dynamic collective motion with large-scale swirling and streaming patterns, denoted as active turbulence – reminiscent of classical high-Reynolds-number hydrodynamic turbulence. Various experimental, numerical, and theoretical approaches have been applied to elucidate similarities and differences of inertial hydrodynamic and active turbulence. We use squirmers embedded in a mesoscale fluid, modeled by the multiparticle collision dynamics (MPC) approach, to explore the collective behavior of bacteria-type microswimmers. Our model includes the active hydrodynamic stress generated by propulsion, and a rotlet dipole characteristic for flagellated bacteria. We find emergent clusters, activity-induced phase separation, and swarming behavior, depending on density, active stress, and the rotlet dipole strength. The analysis of the squirmer dynamics in the swarming phase yields Kolomogorov-Kraichnan-type hydrodynamic turbulence and energy spectra for sufficiently high concentrations and a strong rotlet dipole. This emphasizes the paramount importance of the hydrodynamic flow field for swarming motility and bacterial turbulence.

Hydrodynamic analysis of fish schools arranged in the vertical plane

Schools of fish can provide individuals with hydrodynamic advantages, thereby improving the swimming efficiency. Fish schools in nature are mostly spatial configurations, not just limited to the horizontal plane. Through three-dimensional numerical simulations, this paper discusses the hydrodynamic characteristics and flow field structure of fish schools in various vertical patterns. The results show that a school of fish arranged vertically can improve the thrust and swimming efficiency of individuals as well as those arranged horizontally. There are two ways to significantly enhance hydrodynamic advantages. One is to maximize the channeling effect in multiple planes. In a vertical circular pattern, the fish school forms multiple coupled channels, which hinder the free expansion of flow in both vertical and horizontal planes, thereby obtaining higher energy-saving benefits. The other is the combined exploitation of the channeling effect and wake energy, which is illustrated in the vertical rectangular pattern. The following fish can use the channeling effect to increase the thrust due to the presence of parallel companion. Meanwhile, the high speed region of the following fish can merge with the jet flows of the preceding fish, thereby capturing the wake energy and further improving swimming efficiency.

Hydroacoustic and hydrodynamic investigation of bio-inspired leading-edge tubercles on marine-ducted thrusters

Underwater radiated noise (URN) has a negative impact on the marine acoustic environment where it can disrupt marine creature's basic living functions such as navigation and communication. To control the ambient ocean noise levels due to human activities, international governing bodies such as the International Maritime Organization (IMO) have issued non-mandatory guidelines to address this issue. Under such framework, the hydroacoustic performance of marine vehicles has become a critical factor to be evaluated and controlled throughout the vehicles' service life in order to mitigate the URN level and the role humankind plays in the ocean. This study aims to apply leading-edge (LE) tubercles of the humpback whales’ pectoral fins to a benchmark ducted propeller to investigate its potential in noise mitigation. This was conducted using CFD, where the high-fidelity improved delayed detached eddy simulations (IDDES) in combination with the porous Ffowcs-Williams Hawkings (FW-H) acoustic analogy was used to solve the hydrodynamic flow field and propagate the generated noise to the far-field. It has been found that the LE tubercles have shown promising noise mitigation capabilities in the far-field, where the OASPL at J = 0.1 was reduced to a maximum of 3.4 dB with a maximum of 11 dB reduction in certain frequency ranges at other operating conditions. Based on detailed flow analysis researching the fundamental vortex dynamics, this noise reduction is shown to be due to the disruption of the coherent turbulent wake structure in the propeller slipstream causing the acceleration in the dissipation of turbulence and vorticity-induced noise.

Inertial effects on the flow near a moving contact line

The wetting or dewetting of a solid substrate by a liquid involves the motion of the contact line between the two phases. One of the parameters that govern the dynamics of the flow near a moving contact line is the local Reynolds number, $\rho$ . At sufficient proximity to the moving contact line, where $\rho \ll 1$ , the flow is dominated by viscous forces over inertia. However, further away from the contact line, or at higher speeds of motion, inertia is also expected to be influential. In such cases, the current contact line models, which assume Stokes flow and neglect inertia entirely, would be inaccurate in describing the hydrodynamic flow fields. Hence, to account for inertia, here we perform a regular perturbation expansion in $\rho$ , of the streamfunction near the Stokes solution. We, however, find that the leading-order inertial correction thus obtained is singular at a critical contact angle of $0.715 {\rm \pi}$ . We resolve this spurious mathematical singularity by incorporating the eigenfunction terms, which physically represent flows due to disturbances originating far from the contact line. In particular, we propose a stick slip on the solid boundary – arising from local surface heterogeneities – as the mechanism that generates these disturbance flows. The resulting singularity-free, inertia-corrected streamfunction shows significant deviation from the Stokes solution in the visco-inertial regime ( $\rho \sim 1$ ). Furthermore, we quantify the effect of inertia by analysing its contribution to the velocity at the liquid interface. We also provide the leading-order inertial correction to the dynamic contact angles predicted by the classical Cox–Voinov model; while inertia has considerable effect on the hydrodynamic flow fields, we find that it has little to no influence on the dynamic contact angles.

Solution of Parapolic Navier-Stokes Equations for Laminar Forced Convective Flow in Entrance Region of a Flat Passage.(Dept.M)

Due to dependence of forced convective heat transfer on the hydrodynamic flow field; any improvement on the analysis of this flow field is of great importance to understand the heat transfer process. In this work the flow field is described by the parabolic Navier-Stokes equations; which are solved by local non-similarity solution-method. According to this method; with suitable definition of the problem variables, a set of ordinary differential equations are produced. This set is solved, numerically, by Runge-Kutta method accompanied with shooting method of boundary value problems. The values of local Nusselt number and local coefficient of friction are calculated for fluids of prandtl number of one. Some passages of Reynolds number (Re1) of 100, 200 & 300 are studied here. Two formulate of Nusselt number for different passage heights are proposed.

Experimental and Numerical Investigations of the Hydrodynamic Characteristics, Twine Deformation, and Flow Field Around the Netting Structure Composed of Two Types of Twine Materials for Midwater Trawls

Experimental and Numerical Investigations of the Hydrodynamic Characteristics, Twine Deformation, and Flow Field Around the Netting Structure Composed of Two Types of Twine Materials for Midwater Trawls

Simulation and Experimental Study of Ion Concentration Polarization Induced Electroconvective Vortex and Particle Movement.

Ion concentration polarization (ICP) has been widely applied in microfluidic systems in pre-concentration, particle separation, and desalination applications. General ICP microfluidic systems have three components (i.e., source, ion-exchange, and buffer), which allow selective ion transport. Recently developed trials to eliminate one of the three components to simplify the system have suffered from decreased performance by the accumulation of unwanted ions. In this paper, we presented a new ICP microfluidic system with only an ion-exchange membrane-coated channel. Numerical investigation on hydrodynamic flow and electric fields with a series of coupled governing equations enabled a strong correlation to experimental investigations on electroconvective vortices and the trajectory of charged particles. This study has significant implications for the development and optimization of ICP microfluidic and electrochemical systems for biomarker concentration and separation to improve sensing reliability and detection limits in analytic chemistry.

3D numerical investigations of the swirling flow in a straight diffuser for the variable speed values of the rotor obtained with a magneto-rheological brake

The self-induced instabilities developed in decelerated swirling flows lead to pressure fluctuations. The swirling flows generated under variable speeds of the rotor of a swirl generator using a magneto-rheological brake are numerically and experimentally investigated. Three-dimensional turbulent flow computation is performed for several rotor speeds. Two mean velocity components numerically computed are validated against LDV data on a survey axis located downstream to the rotor for two speed values. The torque on the rotor blade is examined for several speeds. The numerical results are checked against the design values to identify the benefits and limitations of the concepts applied to the design stage. The flux of moment of momentum is modified when the rotor speed is slow down. The circulation, the total pressure and the swirl free velocity are examined on a cross section located downstream to the rotor for all investigated speeds. As a result, the changes of the hydrodynamic flow field are quantified. The unsteady pressure measured on the cone wall at four levels is acquired. The Fourier spectra associated with different self-induced instabilities developed in the cone are examined revealing the distribution of both plunging and rotating components. A low frequency plunging component in a straight diffuser is determined on a speed range of the rotor.

Impact of the prequench state of binary fluid mixtures on surface-directed spinodal decomposition

Using lattice Boltzmann simulations we investigate the impact of the amplitude of concentration fluctuations in binary fluid mixtures prior to demixing when in contact with a surface that is preferentially wet by one of the components. We find a bicontinuous structure near the surface for an initial, prequench state of the mixture close to the critical point where the amplitude of concentration fluctuations is large. In contrast, if the initial state of the mixture is not near the critical point and concentration fluctuations are relatively weak, then the morphology is not bicontinuous but remains layered until the very late stages of coarsening. In both cases, it is the morphology of a depletion layer rich in the nonpreferred component that dictates the growth exponent of the thickness of the fluid layer that is in direct contact with the substrate. In the early stages of demixing, we find a growth exponent consistent with a value of 1/4 for a prequench state away from the critical point, which is different from the usual diffusive scaling exponent of 1/3 that we recover for a prequench state close to the critical point. We attribute this to the structure of a depletion layer that is penetrated by tubes of the preferred fluid, connecting the wetting layer to the bulk fluid even in the early stages if the initial state is characterized by concentration fluctuations that are large in amplitude. Furthermore, we find that in the late stages of demixing the flow through these tubes results in significant in-plane concentration variations near the substrate, leading to dropletlike structures with a concentration lower than the average concentration in the wetting layer. This causes a deceleration in the growth of the wetting layer in the very late stages of the demixing. Irrespective of the prequench state of the mixture, the late stages of the demixing process produce the same growth law for the layer thickness, with a scaling exponent of unity usually associated with the impact of hydrodynamic flow fields.

Study on the 3D Hydrodynamic Characteristics and Velocity Uniformity of a Gravity Flow Circular Flume

Flumes have been widely used in water conservancy science and environmental science research. It is of great significance to obtain the hydrodynamic characteristics and flow field uniformity in the flume. In this study, a new type of annular flume was taken as an example. The 3D flow field was simulated by using a commercial computational fluid dynamics (CFD) code, and was also measured by acoustic doppler velocimeter (ADV) to verify the simulation results. The average relative error range was between 8.37% and 9.95%, the simulated results basically reflected the actual situation of the flow field. On this basis, the structural characteristics of flow field were analyzed. A new calculation method of flow velocity uniformity was presented according to the flow characteristics of natural open channels. The velocity uniformity in the straight channel was calculated and analyzed based on this method, and the influence of speed on the velocity uniformity was further discussed. The length of uniform section was negatively correlated with the rotational speed (average velocity), which was between 39 cm and 101 cm in the straight, and the uniformity coefficient was less than 10%. Finally, the water flow characteristics in the straight channel without wheel were compared with the natural open channel flow. The longitudinal velocity was well fitted with the Prandtl logarithmic distribution formula (R2 &gt; 0.977), and the application feasibility of the flume was analyzed. This study can provide technical support for the development and application of annular flume.