Journal of Hydrodynamics最新文献

Internal solitary waves (ISWs) in oceans exist independently with different modes. Differences in ISW wave–flow characteristics with different modes create different structural damage mechanisms. In this work, physical simulations of first- and second-mode ISWs and their interactions with horizontal cylinders in the same stratified environment are carried out in the same physical flume. The experimental results reveal that under the same stratified environment and characteristic wave amplitude, the characteristic wave velocity and wavelength of the first-mode ISW are approximately three times greater than those of the second-mode ISW. The horizontal positive velocity and change rate of the first-mode ISW are lower than those of the second-mode ISW, and the horizontal negative corresponding value is greater than that of the second-mode ISW. In comparison, the vertical velocity is lower than that of the second-mode ISW. The first- and second-mode ISW loads increase with increasing characteristic wave amplitude. The maximum horizontal force of the second-mode ISW on the cylinder near the centre of the pycnocline is more significant than that of the first-mode ISW, whereas the vertical force of the former is smaller than that of the latter. The second-mode ISW has the spatial distribution characteristics of the peak value of the first-mode force exerted by the first-mode ISW of the convex and concave types simultaneously. The vertical and density profiles of the horizontal velocity induced by the propagation of different ISW modes in the same fluid environment are different, which leads to changes in the spatial distribution characteristics of the horizontal and vertical forces on the cylinder for different ISW modes. These results improve the understanding of ISW mechanisms with different modes on marine structures.

With the development of Liutex theory, new applications have emerged, making it an effective tool for identifying and quantifying vortex structures in various fluid flows. The Liutex method provides precise information on vortex strength, size, and location, and enables real-time analysis of vortex evolution and interactions, enhancing the understanding of complex flow patterns. Based on the incompressible Navier-Stokes equations, we derived the Liutex-shear interaction equation by incorporating the Liutex-shear decomposition into the vorticity transport equation to explore the fundamental relationship between Liutex and shear. We first applied it to the two-dimensional Lamb-Oseen vortex, which has an analytical solution, to validate the previously introduced idea of using high dissipation regions to identify vortex boundaries. Then we conducted numerical experiments on a three-dimensional cylindrical flow at Re = 3 900. We computed the key terms in the Liutex-shear interaction equation for comparative analysis, and demonstrate that the Liutex-shear interaction equation offers a clearer insight into the nature of vortices. Utilizing the Sunway TaihuLight supercomputer, we performed large eddy simulations with 2.5×10⁸ grids for the three-dimensional cylindrical flow case, allowing Liutex to reveal more refined vortex structures in turbulence. These findings confirm that the conclusions obtained in two-dimensional cases are consistent with those in three dimensions and show that shear stretching significantly influences the Liutex-shear interaction, enabling us to approximately predict the evolution of the source term.

Multiple orifices set on a dam are commonly used for flood discharge and sediment removal. Different arrangements of orifices may lead to different scalar or sediment transport process in the near dam reservoir region. 3-D numerical simulations are implemented to simulate flow and sediment transport near an idealized rectangular dam region with a focus on the influence of orifice arrangement on scalar and sediment transport under a same flow discharge and concentration condition. It is found that the concentration distribution in the near dam reservoir region and the transport flux at the dam-site may change dramatically due to different arrangements of orifices. The effect of orifice arrangement on scalar and sediment concentration distribution in the near dam reservoir region is found to be limited roughly within a range of ((15-20)sqrt{sum{A}}) away from the dam where ΣA is the total area of all opened orifices on a dam. Under the same inlet water flow and concentration condition and at a steady state, for scalar transport, the transport flux at the dam-site may be independent of orifice arrangement; while for sediment transport with considering the settling effect, the transport flux is generally larger in cases with one orifice except when the orifice centroid elevation is sufficiently high. This work may provide guidance for reservoir regulation, e.g., to discharge more sediment downstream a reservoir or adjust local sedimentation distribution near the dam.

Vortex-induced vibration (VIV) of a dual-step cylinder with a diameter ratio of (D / d = 2) at different coverage ratios (R = 0%–100%) are experimentally investigated at Reynolds number (Re = 490–3 750). The general vibration responses of the dual-step cylinder at different coverage ratios can be classified into three categories based on the dominance of the small or large cylinder section: (1) Dominated by small cylinder (R = 4%), where the response and forces show a similar trend to that of a uniform small cylinder. The vortex shedding frequency and the drag frequency lock-in at the lower branch. (2) Transitional stage (R = 12.5%–25%), where competition between the large and small cylinder sections exists. The vibration responses show two “lock-in” regions in the transition branch and the lower branch. The lift and drag coefficients are double-peaked. (3) Dominated by the large cylinder (R = 50%–75%), where the initial branch of the response disappears, as replaced by the transition branch. The vibration frequency is still featured by two “lock-in” regions while the drag coefficient returns to the single-peak pattern, similar to that of the uniform large cylinder. The effective added mass significantly influences the vibration responses of the dual-step cylinder and it varies with the reduced velocity. A unified lock-in region is identified where the dimensionless vibration frequency consistently equals the unity, indicating a synchronization between the vibration frequency and the real natural frequency.

Simulations of the transitional flow in Taylor-Couette configuration are carried out to study the effect of the gap width on turbulent transition. The research results show that, under the same radius and the rotating speed of the inner cylinder, as the gap width increases, the flow becomes more stable. It is discovered that the average velocity distribution in the gap approaches the free vortex flow as the width increase and the stability of the flow is enhanced. It is found that, as the gap width increases, the maximum of the energy gradient function (from the energy gradient theory) in the gap decreases, which delays the turbulent transition. As such, the larger the gap width, the later the transition occurs. As the gap width increases, the Reynolds number based on the gap width alone is not able to characterize the flow behavior in Taylor-Couette flows, and the effect of the radius ratio should be taken into account.

In order to improve the efficiency of the centrifugal pump, this paper employs numerical simulation to study the internal flow of a centrifugal pump model with a specific speed ns=65.6. Based on the flow analysis results, flow control strategies are proposed, and a “bulged” impeller with a special blade pressure surface profile is designed. Research is conducted on the internal flow of the “bulged” impeller centrifugal pump, while the entropy production method is employed for a quantitative analysis of energy losses in various flow components. The results indicate that compared with the original impeller centrifugal pump, the total entropy production in the front chamber, back chamber, volute, and outlet pipe of the “bulged” impeller centrifugal pump is reduced. Additionally, the separation vortex on the pressure surface of the “bulged” impeller blade disappears, and the entropy production on the blade wall surface decreases. The study of the internal pressure fluctuation intensity in the centrifugal pump found that the pressure fluctuation intensity at various monitoring points within the impeller and volute of the “bulged” impeller centrifugal pump is reduced, leading to improved dynamic performance. Experimental results of the centrifugal pump show that the “bulged” impeller centrifugal pump exhibits a significant increase in efficiency from low flow to design flow conditions.

Analysis of the flow field of tropical cyclones in the mature stage by the Liutex methods, such as the Omega Liutex and the Liutex core line, is reported. This is the first known study analyzing tropical cyclones with the Liutex method. Liutex has the potential to enable greater understanding of tropical cyclone dynamics and to improve forecasts of their track, intensity, and structure.

The fact that the staggered impeller of a double-suction centrifugal pump can effectively suppress pressure fluctuations has been proved by engineering practice, but the flow mechanism behind it is still not fully understood. In this study, numerical simulations with a proof experiment were conducted, and the vortex dynamics analyses were performed using the newly developed rigid vorticity (Liutex) theory. The following valuable results are obtained: (1) In terms of the intuitive vortex structure, each blade of the impeller induces a trailing vortex rope with a strong rigid vorticity, which gradually evolves inside the volute casing with the rotation of the impeller. The trailing vortex ropes of the symmetric impeller are symmetrically distributed, while those of the staggered impeller present a staggered distribution, and the latter corresponds to a relatively lower rigid vorticity. (2) In terms of the correlation between the vortex and the pressure, the high rigid vorticity zone corresponds to the low-pressure zone. For a fixed point in the volute casing, there is a major “falling-rising” fluctuation in pressure as the symmetric vortex ropes transit it simultaneously, and a minor “falling-rising” fluctuation in pressure as the staggered vortex ropes transit it successively, corresponding to a lower peak-to-peak value of the pressure fluctuations. (3) In terms of the relation between the vortex and the velocity, the vortex ropes induced by the left and right impellers are counter-rotating and develop along the radial direction. This pattern results in high-speed zones at the middle part of the cross-section of the volute casing, both in the streamwise and radial directions, and contributes to velocity fluctuations due to the evolving vortex rope. However, the staggered distribution of vortex ropes can weaken the coupling of vortex pairs, thereby causing lower velocity and pressure pulsations, but can make the main frequency twice that of the symmetric impeller. This study enriches our physical knowledge by revealing the vortex dynamics mechanism of the staggered impeller of a double-suction centrifugal pump to suppress pressure fluctuations.

This study experimentally examines the coupling response of an S-shaped flexible riser exposed to internal gas-liquid two-phase flow and external shear current using the optical non-intrusive measurement. The vibration tests were conducted with the internal fluid transported at a constant velocity of v_i = 0.9 m/s with three representative gas-liquid ratios (Q_g/Q_l) and the depth-averaged reduced velocity of external current (U_rm) ranging from 9.32 to 23.19. The riser top was hanged underneath an oscillating cylindrical platform. The out-of-plane response is enhanced in the presence of external current, while the in-plane response is suppressed due to the more even distribution of liquid slugs. The response competition among the riser, buoyancy module and platform is quantified in terms of dominant frequencies and interaction lengths, which are sensitive to Q_g/Q_l, U_rm. The shift of zero value among the coupling length, affecting length and affected length indicates the switching of dominant role. In general, the response competition in the in-plane direction is more intense than that in the out-of-plane direction.

To clarify the influence of the hydrofoil characteristic thickness on the distribution characteristics and mechanisms of clearance cavitation erosion risk, a large eddy simulation (LES) is conducted to study the clearance cavitating flow around NACA0012 and NACA0024 hydrofoils under identical conditions. The study predicts cavitation erosion risk using three methods: The erosive power method (EPM), the improved gray level method (IGLM) and the energy conservation method (ECM). The numerical results are in good agreement with the experiment data and the ECM is applied due to its simplicity in parameter adjustment and low sensitivity. The results indicate that the characteristic thickness significantly influences the flow field, leading to variations in the position and intensity of cavitation collapse, ultimately resulting in notable differences in cavitation erosion risk distribution. The high cavitation erosion risk region on the clearance surface of NACA0012 is concentrated around the midsection, while it is concentrated in the upstream region for the NACA0024, with a lower frequency of extreme events. Tip separation vortex (TSV) cavitation is the main cause of the differences in cavitation erosion risk distribution. On the clearance surface of the NACA0012, TSV cavitation primarily collapses in the central region, whereas for the NACA0024 hydrofoil, TSV cavitation occurs only in the upstream region of the clearance surface and exhibits more stability. The differences in vorticity distribution near the clearance surface partially influence the distribution of TSV cavitation, thereby affecting the characteristics of cavitation erosion risk distribution. The larger characteristic thickness of the NACA0024 reduces the effects of the stretching term and the baroclinic torque term, weakening the effect of vorticity on TSV cavitation, resulting in more stable patterns of the TSV cavitation.