To define the guidelines for the design of a Mewis duct for a small bulk cargo ship, a numerical study was carried out to investigate the effects of a fan-shaped Mewis duct on propeller performance ...of a 38,000 t bulk cargo ship with a three-blade propeller. Calculations were performed using the STAR-CCM+ software based on the solution of the Reynolds averaged Navier–Stokes (RANS) equation. Computations were carried out for a wide range of locations from 0.11
D
to 0.21
D
upstream from the propeller disc, a radius of trailing edge from 0.65
R
to 1.05
R
, and angle of attack of the inner fins, where from 0° to 10° of the fan-shaped Mewis duct, where
D
is the diameter of the propeller and
R
is the radius of the propeller. With the installation of the duct, the axial mean wake fraction on the propeller disc decreased, while the tangential mean wake fraction increased significantly. As the installation position was far away from the propeller, the axial mean wake fraction varied in a wave pattern, while the tangential mean wake fraction increased. With the increase of the radius and the angle of attack of the inner fins of the duct, the axial mean wake fraction first decreased and then increased, while the tangential mean wake fraction increased gradually. Correspondingly, the propeller force coefficients and efficiencies with the duct were all more than those without the duct and changed with the variations of the duct parameters. When the propeller and duct were taken as a propulsion system, the efficiency of the system was less than that of only the propeller. At the same time, with variations of the location, radius, and angle of attack of the inner fins of the duct, the efficiency of the system first increased and then decreased gradually, which indicates that there was an optimum design value for all of the duct parameters. Moreover, only when the design of these duct parameters are reasonable can energy savings be achieved; otherwise, the opposite effect is produced. Eventually, with the installation of the fan-shaped Mewis duct, the wake harmonic degree on the propeller disc can be significantly improved.
Full text
Available for:
EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OBVAL, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
To study the flow pattern of the flow coating technology applied to substrates with a complex three-dimensional surface (the lens of an automotive lighting lamp) and the possible defects (mainly ...“wrinkles”), numerical simulation was used to simulate the coating implementation process. The complex geometric surface of the substrate was simplified and partitioned such that the simulation resources were significantly reduced. The k–ε realizable turbulence model and the volume of fluid multiphase model were used for the numerical solution. The flow pattern and moving contact line are analyzed by fluid dynamics, as well as the reasons for the formation of “wrinkles” and the “heel” phenomenon. It can be concluded from the comparison of the results and the actual experiments/videos that the method used in this study can effectively simulate the flow field in the implementation of the flow coating technology and predict the main defects of the flow field caused by the three-dimensional geometric characteristics of the substrate.
Reducing vessel resistance by using ventilated cavities has been a highly researched topic in the marine industry. There is limited literature on ventilated supercavities near the free surface, which ...indicates that their dynamic behavior is more complex than conventional ventilated cavities due to the effect of the free surface. This paper employs numerical simulations to study the dynamic behavior of the ventilated supercavity, taking into account the effect of the free surface. Numerical simulations can predict gas leakage behaviors, cavity geometry, and internal flow structures. The influence of the free surface shortens the length of the ventilated cavity and increases the diameter. The presence of the free surface mainly changes the vertical velocity distribution between the free surface and the cavity. The results show that there are two typical gas leakage mechanisms under different immersion depths: twin-vortex tube leakage mode and re-entrant jet leakage mode. The internal flow field of ventilated supercavity is classified into three regions: the internal boundary layer, the ventilation influence region, and the reverse flow region. As the distance between the free surface and the ventilated supercavity decreases, the ventilated supercavity is affected by both the free surface effect and the gravity effect.
This research aims to extend our understanding of propeller wake dynamics under a light loading condition, thereby laying a foundation for design optimization and flow control of the propeller. ...Dynamic mode decomposition (DMD) and reconstruction are used to analyze the transient vortical wake structures obtained by large eddy simulation. The propeller wake includes stable tip and hub vortices without interacting evolution at the light loading condition, and elliptical instabilities are observed downstream of the tip vortices. DMD describes the most energetic modes and the corresponding dominant frequencies are the blade passing frequency and its multiples. The coherent structures identified via DMD are primarily associated with the ordered convection of the tip vortices and have little correlation with the hub vortices. Additionally, the propeller wake flow is reconstructed using the first four DMD modes, and the primary wake features are well restored with a maximum reconstructed error of 7.98%. This demonstrates that the flow-field reconstruction based on the DMD reduced-order model is promising for predicting the propeller wake and controlling the propeller operation.
•The propeller wake is simulated using the LES method under a light loading condition.•DMD identifies the most energetic modes and the associated dominant frequencies.•DMD analyzes contribution of tip and hub vortices to the wake coherent structures.•The propeller wake flow is well reconstructed by a DMD reduced-order model.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The effects of the bionic tubercle leading-edge on the cavitating wake dynamic with cavitaion number σ = 1.76, 2.016 is calculated and discussed using numerical techniques based on the detached eddy ...simulation method. The propeller loads, cavitation extents, instantaneous wake topology, and the kinetic energy spectra are analyzed to investigate the influence mechanism of the bionic tubercles on propeller performance and wake flow field. The results indicated that the effect of bionic tubercle comes from the formation of the counter-rotating vortex pairs which change the distribution of streamlines on leading edge. The bionic tubercles are conductive to the cavitation and blade shed vortex reduction independent of advance coefficient. Analysis of power spectral density of kinetic energy demonstrates that the mean values of propeller with bionic leading-edge are generally smaller than those of original one, especially in the far field.
•The effects of the tubercles on propeller cavitating wake dynamic is calculated and discussed.•The action mechanism of the tubercles is analyzed from the perspective of flow field.•The tubercles are conductive to the reduction of blade cavitation and blade shed vortex.•The mean values of KE of tubercles propeller are smaller than those of original propeller.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
This paper investigates the ventilation elimination mechanisms during the deceleration process of a surface-piercing hydrofoil using the unsteady Reynolds-averaged Navier-Stokes (RANS) method ...together with a Volume of Fluid (VOF) model. The numerical results are in good agreement with the experimental data. The ventilation elimination mechanism of the surface-piercing hydrofoil is analyzed from the perspectives of the hydrofoil hydrodynamic performance, the ventilated cavity evolution, vortex structures, and re-entrant jets. The results indicate that the ventilation elimination includes three stages, i.e. a decrease in the ventilated cavity, washout, and reattachment. The decrease in the ventilated cavity is due to the hydrofoil speed decrease in the FV flow. Washout is the transition from fully ventilated to partially ventilated flow, and reattachment is the transition from partially ventilated to fully wetted flow. The underwater vortex structures around the surface-piercing hydrofoil are composed of a tip vortex, an unstable vortex induced by the shear layer, and a Karman vortex caused by the vortex shedding from the trailing edge of the hydrofoil. Ventilation stability strongly depends on the re-entrant jet. When Φ (the angle between the flow direction and the closure line of the ventilated cavity) is greater than 45°, the re-entrant jet impinges on the ventilated cavity's leading edge and destabilizes the ventilated cavity.
•Ventilation elimination mechanisms are revealed during the hydrofoil deceleration.•Ventilation elimination includes three stages, i.e., cavity reduction, washout and reattachment.•Vortices include a tip vortex, a vortex induced by the shear layer and a Kaman vortex.•Ventilation stability strongly depends on the re-entrant jet.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The development of a strong absorption and low filler loading microwave absorbers that aims to solve the electromagnetic pollution is still a serious challenging. Herein, novel dual-configuration ...nanoporous Co3O4–C and Co/CoO–C composites were prepared via hydrothermal and high-temperature pyrolysis methods. Results demonstrated that micromorphology and electromagnetic properties of as-synthesized composites could be regulated by tuning the pyrolysis temperature, and the formed porous structure can be explained by kirkendall effect. Furthermore, the reflection loss of Co3O4–C material annealed in air at 400 °C (A-400) and Co/CoO–C material annealed in N2 at 900 °C (N-900) with an ultra-low filler content of 5 wt% can be reached to −51.8 dB at 15.8 GHz and −55.7 dB at 6.3 GHz, respectively. The exceptional microwave absorption performances can be attributed to multiple reflection and scattering, interface polarization effect and good impedance matching. Such achievements indicate that cobalt (oxide)-carbon nanocomposites could be a promising candidate for designing and fabricating lightweight and high-efficient microwave absorbers.
The dual-configuration porous Co3O4–C and Co/CoO–C composites respectively show the superior microwave absorption capability with a RLmin of −51.8 dB at 2.0 mm and −55.7 dB at 3.8 mm which are promising as a candidate for next-generation high-performance absorbing materials. Display omitted
•Glucose as carbon source and PVDF as matrix contribute to economy and environmental protection.•The RLmin of Co3O4–C is −51.8 dB at 2.0 mm with an effective bandwidth of 4.6 GHz.•The RLmin of Co/CoO–C can be reached −55.7 dB at 3.8 mm with an effective bandwidth of 4.4 GHz.•The novel dual-configuration porous structure is good to microwave absorption.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
PDF
