Influence Of Free Surface Research Articles

Numerical investigation on the influence of free surface on the hydrodynamic and wake characteristics of submarine

In the process of movement, the submarine inevitably involves free surface navigation, such as floating observation, suction power generation, and rescue. Under various circumstances, the interaction between the submarine and the free surface leads to complex flow fields and wakes, which affects its hydrodynamic performance. In this study, a three-dimensional numerical model of a submarine considering the influence of free surface is established. The volume-of-fluid model with an artificial compression term is used to capture the gas–liquid interface, and the unsteady flow field and hydrodynamics are predicted using the shear stress transport k–ω turbulence model. Based on the analysis of mesh convergence and numerical reliability, the hydrodynamic performance and wake flow field characteristics of submarines under different submergence depths were studied. The results show that the free surface has a significant influence on the resistance, wave wake, wake field, and vortex structure of the submarine, which is closely related to the submergence depth. Compared with the infinite submergence depth, the total resistance of the submarine near the free surface increases by 159.2%, mainly due to the pressure resistance. The surface wave system generated by the interaction between the free surface and the hull will directly affect the distribution of the wave surface wake, the wake flow field, and the vortex structure. As the submergence depth increases, the free surface effect gradually weakens, and it can be ignored when the submergence depth is more than 4 times the diameter of the hull.

Characteristics of flow field and various resistances of sandglass-type FPSO with special shape during towing process

For the new concept of floating production, storage and offloading unit (FPSO), the unique sandglass-type shape of its floating body can effectively reduce heave motion responses. However, this design feature may also generate distinct flow characteristics during towing. In this paper, the numerical method was established and validated to successfully predict the towing performance of the sandglass-type model by comparing it with the model experiment of a polar ocean nuclear energy platform with the sandglass-type shape. On this basis, the analysis incorporates both a local model of the underwater body without a free surface and a global model of the floating body with a free surface to analyze towing resistances. Specifically, the paper focuses on the frictional resistance and pressure resistance of sandglass-type models versus various draughts. Furthermore, the influence of free surface on various kinds of resistances and the towing resistances of sandglass-type FPSO with different speeds were studied. Finally, concluding remarks and future perspectives are presented.

Collaboratively enhancing the strength and ductility of GH3600 Nickel-Based Ultra-Thin strips via the Pre-Precipitation process

Here, the effects of pre-precipitation annealing on the microstructural evolution and room-temperature mechanical properties of nickel-based ultra-thin strips after the subsequent high-temperature annealing were analyzed. By pre-precipitation annealing, discontinuous grain-boundary carbide was introduced into the cold-rolled 67 μm GH3600 nickel-based ultra-thin strips, which hindered the rapid migration of grain boundaries. Thus, the grain size was maintained at only 5.5 μm after holding at 1050 °C for 1 h, indicating that the high-temperature microstructure stability of the strips was significantly improved. Further, the phenomenon of local single-layer crystal and size effect caused by the abnormal coarsened grains, which were more than 100 μm in size during the single-stage high-temperature annealing (1050 °C for 1 h), was eliminated. Owing to the weakened free surface influence and an increase in the strengthening effect, such as grain boundary strengthening, as well as intragranular and intergranular back stress strengthening, the work hardening ability of the strips was enhanced after pre-precipitation annealing, thereby resulting in excellent comprehensive mechanical properties whose flow stress and elongation were 1.9 and 4.8 times those of single-stage high-temperature annealing. This study successfully provides an economical and easy-to-implement effective method to prepare solid-solution-strengthened high-performance ultra-thin superalloy strips.

Resonance with surface waves induces forbidden velocity bands in dislocation glide

We use molecular dynamics simulations to demonstrate a pronounced influence of free surfaces on the mobility of dislocations at near-sonic velocities. By correlating instantaneous velocities v and resolved stresses τ, we show that the mobility relations of both edge and screw characters are composed of multiple ranges of stable uniform motion separated by bands of forbidden velocity. The lower limiting velocity for uniform motion of both edge and screw dislocations is the Rayleigh wave speed, cR. When forced to propagate at an average velocity greater than cR, dislocations exhibit cyclic, intermittent jumps in instantaneous velocity between cR and higher velocity branches of the mobility relation. Bands of forbidden velocity may be calculated directly from the velocity dependence of dislocation drag functions, which contain peaks near cR and—in the edge case—one other characteristic surface wave speed, which we associate with shear-horizontal (SH) waves. We propose that peaks in the drag function are due to resonance of free surface vibrations forced by nearby moving dislocations.

Prediction of Tunnel Blasting Vibration Velocity Considering the Influence of Free Surface

The blast vibration attenuation will change as the tunnel cut hole is blasted to create a new free surface. To guarantee safe tunnel construction and optimize blast design parameters, it is crucial to understand the impact of the free surface on blast vibration attenuation. Previous studies have often only qualitatively evaluated the effect of the free surface on blast vibration, failing to quantify these impacts on vibration attenuation. In this paper, by analyzing the decay law of the blast vibration velocity under different free surfaces, we quantitatively assessed the effect of the number of free faces and burden distance on the peak vibration velocity. Then, we introduced free-face parameters to enhance the current formula for predicting blast vibration velocity. The results show that the peak vibration velocity decreases with the increasing free surface number and increases with increasing burden distance; the free surface factor cannot be ignored in the decay of vibration velocity and contributes about 21% more to the total decay factor than the scale distance factor. The average tolerance rises from 24.79% in the Sadovsky formula to 13.32% in the correction formula, and the correction formula more precisely predicts the peak vibration velocity. The accuracy of the correction formula provides a good scientific basis for the design of tunnel blasting parameters. The research results were successfully applied to the Jiujiawan proximity tunnel on the Duyun-Anshun highway in Guizhou, effectively guaranteeing blasting safety with a minimum clear distance of 17.2 m.

On ventilated supercavities moving horizontally near free surface

By means of open source CFD tool OpenFOAM platform, a multiphase solver capable to deal with two phases flows is used to investigate ventilated supercavities. The VOF (volume of fraction) method is employed in order to capture cavity interface and the free surface. The main objective of this work is to investigate ventilated supercavity generated after a disk-cavitator which moves horizontally near a free surface. Most of the work in this research has been focused on the shape of supercavity and its asymmetry. Furthermore, two distinct cavity closure types, i.e. the re-entrant jet (RJ) and the twin vortex (TV) and are discussed separately to compare their gas leakage mechanisms at different submergence ratio. Numerical results indicate that the air-water free surface will have a significant influence on the cavity surface curvature and gas shedding process. Under the influence of free surface, the length diameter ratio of ventilated cavitation increases with H‾ decrease with decrease. The influence of free boundary on the upper surface of cavitation interface is greater than that on the lower surface. When Fr number and ventilation volume are constant, when approaching the free surface, the interface curvature of ventilation cavitation will increase. The deflection of cavitation axis is mainly affected by gravity and free surface. When the Fr number is small, the cavitation axis will be slightly disturbed. When the Fr number is large, the cavitation axis deflects reversely. These may give meaningful references for the design of surface vehicles.

Experimental and numerical investigation to evaluate the thermal performance of jet impingement surface cooling with MWCNT/Al2O3-deionized water hybrid nanofluid

The study investigates the influence of free surface by jet impingement of aqueous nanodispersion dominant with hybridized nanoparticles. Hybrid nanofluid (HNF) was prepared by a two-step approach using 7 nm Alumina (γ-Al2O3) and a dominant portion of 3–5 nm Multi-Walled Carbon Nano-Tubes (MWCNT) in the ratio of 10:90 dispersed within deionized (DI) water base fluid. The nanofluid viscosity and thermal conductivity measurements were at varying temperatures (10–60 °C) and volume concentrations (0.025, 0.05, 0.1, and 0.15%). A jet nozzle (1.65 mm inner diameter) impinged the HNFs onto a targeted round surface (42 mm diameter) at an H/Dj ratio of 4. The Nusselt number (Nu) evaluations were at varying fully-developed turbulent regimes: Reynolds number (Re) between 6000 and 16,500, Weber number (We) between 1000 and 7000, and Peclet number (Pe) between 80,000 and 205,000. Besides, we investigated a transient cooling rate for varying volume concentrations (vol%) and the numerical aspect of the work by employing the Eulerian-Eulerian approach to model jet impingement. The maximum augmentation in Nu number is at 0.05 vol% HNF, with a 17% increase compared to DI water. Based on the numerical investigation using Computational Fluid Dynamics (CFD), a maximum improvement of 19.7% in Nu number was with the 0.15 vol% HNF, whereas the improvement in the 0.05 vol% particle concentration fluid was 13.7%. The ambiguity of the results between the experiment and CFD analysis could be connected to the size and shape of nanoparticles, mixing or swirling parameters, surfactant type, or a combination of all the above. In terms of the transient cooling rate investigated, the best-performing fluid is the 0.15 vol% particle concentration fluid with a relaxation time of 1 s in CFD and 1.75 s in the experiment. Besides, the steady time is 23 s in CFD and 33 s in the experiments. A correlation for the Nusselt number is proposed based on the obtained data.

Flow characteristics comparison of PIV and numerical prediction results for an unmanned underwater vehicle positioned close to the free surface

In the present paper, flow characteristics of an Unmanned Underwater Vehicle (UUV) with a commonly used Myring profile were investigated numerically and experimentally using Computational Fluid Dynamics (CFD) and the Particle Image Velocimetry (PIV) technique under the influence of free surface. The 3-D and two-phase flow simulation generated using the Volume of Fluid (VOF) were carried out using the Large Eddy Simulation (LES) turbulence model for high accuracy in both near free-surface and almost uniform flow conditions. Due to the presence of the free-surface effect, dynamics and unsteady instantaneous flow characteristics such as force and moment coefficients, streamlines topology, and pressure values on the body surface along with vorticity structures were found to be very chaotic and have irregular motion in the wake while the followable variation trend of the time-averaged properties was obtained to show critical immersion ratio. The immersion ratios of 0.75≤ h/D≤3.50 were examined at Reynolds numbers Re = 2.0 × 104 and 4.0 × 104. Jet-like flow between the UUV body and the free surface of the water was detected at the immersion ratio of h/D = 0.75, which caused a substantial asymmetry in flow structures, resulting in highest drag and lift values. Increased surface disturbance at Re = 4.0 × 104 caused air introduction into via jet-like flow in h/D = 0.75, which caused positive lift. Hydrodynamic coefficients and isosurfaces shown that the free-surface effect decreased significantly up to h/D = 1.50 at constant Reynolds numbers. Further investigation of time-averaged velocity components, streamlines, vorticity and turbulence statistics revealed that h/D = 1.50 acted as a transitional immersion ratio as the flow structure changed significantly with Reynolds numbers. The utilized CFD approach yielded especially excellent agreement with the PIV measurements with the discrepancy which varies from 1% to 15% in near wake for streamwise velocity components to simulate the essential unmeasured flow features needed in the research and development process of UUVs when they move below the free surface.

Influence of free-surface on wake flow characteristics of a torpedo-like geometry

Influence of free-surface on wake flow characteristics of a torpedo-like geometry

Failure Characteristics of Rock Sample during Penetration Tests with the Assistance of Different Free Surface Combinations

Abstract Free surface-assisted TBM disc cutter for rock breaking is an important direction for the optimizing of TBM cutter structure. To study the influence of free surface on TBM excavation, the free surfaces of limited-width opening and unlimited-width opening were presented, and six different free surface combinations were proposed. The penetration tests of sandstone with different free surface combinations were carried out. The results showed that the rock mass adjacent to the free surface of unlimited-width opening fell off after sample fracture. The rock mass adjacent to the free surface of limited-width opening caused the free surface to close after the rock broke. Under different free surface combinations, the auxiliary effect for rock breaking was not obvious when the free surface depth was shallow, and the failure morphology of the sample changed from vertical splitting to inclined fracture with the increase of free surface depth. The free surface-assisted rock-breaking method can reduce energy consumption during rock breaking. Among all combinations, the cumulative energy before peak load of the SDL sample dropped the most to 37.62% of the intact sample. Finally, the free surface combinations can affect the design of the cutter layout of TBM.

Analysis of Pressure Fluctuation of Tubular Turbine under Different Application Heads

The vigorous development of low-head hydraulic resources and tidal energy with greater stability and predictability is drawing attention to tubular turbines. However, many problems, such as incorrect unit association relationship, insufficient unit output, and severe vibration, occur frequently in tubular turbines, particularly when the water head is low. These phenomena cannot be known through model machine tests and numerical studies. Therefore, this study takes the tubular turbine with different water heads as the research object, in accordance with the actual boundary conditions. The unsteady numerical research for the prototype machine is conducted while considering the free surface in the reservoir area and water gravity. The internal flow characteristics of the tubular turbine with different water heads and the influence of free surface on its performance are analyzed. The research indicates the following: affected by the free surface and the water gravity, the pressure in the entire flow passage of the horizontal tubular turbine increases with the increase in the submerged depth. In addition, the short water diversion section allows the water flow from the reservoir area to still have a certain asymmetry before reaching the runner. During the rotation process of the runner, the surface pressure and torque of the blade have evident periodic fluctuations, and the amplitude of the fluctuations will increase significantly with the decrease in H/D1. Moreover, in the case of small H/D1, the amplitude of pressure pulsation in the draft tube is larger, and concentrated high-frequency pressure pulsation occurs. These factors will lead to the occurrence of material fatigue damage, unstable output, and increased vibration in low-head tubular turbines.

CFD를 이용한 덕트 프로펠러의 자유수면 영향도 연구

The influence of free surface for the ducted propeller was investigated based on the numerical simulation in this paper. The ducted propeller consists of a duct with 19A section and the KA4-70 propeller. The immersion ratio was defined as the non-dimensional value between the distance from propeller axis to free surface and diameter of propeller. In this study, 4 immersion ratios were considered as 0.75, 1, 1.25, and 1.5, and the results from each condition were compared to those of fully immersed condition, respectively. As the immersion ratio is decreased, the total thrust and torque of the ducted propeller tend to be decreased, in particular, the portion of thrust reduction on the duct was greater than that of the propeller. It was observed that the pressure on duct is asymmetrically distributed due to the effect of free surface, on the contrary, it is axis-symmetrical in fully immersed condition. And the free surface behind the ducted propeller system is observed in the shape of “V”. As the immersion ratio decreases, the difference between crest and though of free surface was increased.

Attenuation model of tunnel blast vibration velocity based on the influence of free surface

In tunnel blasting excavation, it is important to clarify the attenuation law of blast wave propagation and predict the blast vibration velocity effectively to ensure safe tunnel construction and protection design. The effects of the free surface area its quantity on the blast vibration velocity are considered, and free surface parameters are introduced to improve the existing blast vibration velocity prediction formula. Based on the Tianhuan railway Daqianshiling tunnel project, field blast vibration monitoring tests are performed to determine changes in the peak blasting vibration velocity based on the blast distance and free surface area. LS-DYNA is used to establish tunnel blasting excavation models under three operating conditions; subsequently, the attenuation law of blast vibration velocity and changes in the vibration response spectrum are analysed. Results show that the free surface area and number of free surfaces enable the blast vibration velocity to be predicted under various operating conditions: a smaller free surface area results in a narrower frequency band range, whereas more free surfaces result in a narrower frequency band range. The improved blast vibration velocity prediction formula is validated using field and numerical test data. It is indicated that the improved formula is applicable to various tunnelling conditions.

Numerical study on rock damage characteristics of multi hole blasting

In this study, a two-dimensional finite element numerical simulation method was used to study the damage characteristics of rock mass under multi-hole blasting load. In the analysis, Drucker-Prager constitutive model was used to characterize the strength of rock mass. The stress acting on the blasting hole was simulated by exponential functions, that the stress could reach the maximum value in a short time and then drop to zero in a relative long time. The damage characteristics of rock under different confining pressures, and the influence of free surface on the damage of rock were examined. The results show that under the same blasting load, increasing the confining pressure of surrounding rock can inhibit the development of rock cracks; The free surface made the explosion stress wave reflect, the rock mass between the blasting hole and the free surface produced a phenomenon of stratification crack which restrained the crack propagation below the blasting hole.

Free surface impact on radiation damage in pure nickel by in-situ self-ion irradiation: can it be avoided?

A major issue of in-situ radiation damage studies is related to the influence of free surfaces. Free surfaces in thin foils are indeed strong sinks for radiation-induced defects. Nevertheless, in-situ irradiation is a powerful tool to study real-time microstructural evolution and obtain insight into dynamic mechanisms of radiation damage. Thus, a detailed evaluation of surface effects is essential to validate existing results and provide guideline for future comparative experiments. In this work, nickel is chosen as model material to conduct systematic studies on surface effects due to the high mobility of its self-interstitials. Ultra-high purity Ni thin foils are in-situ irradiated by 2 MeV Ni2+ ions at high temperatures (400-700°C). Microstructural evolution analysis and detailed characterization of dislocation loops are performed in function of specimen thickness.The present work shows: (i) a drastic influence of thickness on the microstructural evolution and irradiated microstructure with the existence of a critical thickness depending on temperature; (ii) a good prediction of an adequate irradiation thickness with a vacancy concentration calculation model; (iii) an impact of free surfaces on the fine distribution of loop Burgers vectors; (iv) a first determination of migration energy of vacancies in Ni considering the temperature dependence of the loop-depleted zones; (v) a production bias model showing that a loss of 10% interstitials favors the growth of vacancy loops observed for the first time in Ni at 510°C.

Influence of free surface and bottom on flow around a cylinder

Doing research on vortex-induced vibration (VIV), not only should we do some numerical simulation but also need to experiment. The existence of the free surface and bottom will influence the drag. We analyzed the drag under different velocity of water under different flow fields, then compared the results. The results show that the water depth will influence vortex shedding thus increase the drag. With a decrease of velocity, the effect of drag will be reduced. Nevertheless, it’s not always decreasing, when velocity lower than 0.8m/s, the effect would be steady.

Influences of free surface jump conditions and different [formula omitted] SST turbulence models on breaking wave modelling

A two-phase flow Computational Fluid Dynamics (CFD) model is employed for both spilling and plunging breaking wave simulations. A numerical wave tank is built to evaluate the performance of two different free surface modelling approaches (i.e., continuous free surface conditions and free surface jump conditions) and three k−ω SST turbulence models (i.e., the original k−ω SST, the buoyancy modified k−ω SST and the free surface k−ω SST (a newly proposed model)). The influences of the free surface modelling approaches and the turbulence models are investigated by comparing with the published experimental measurements in terms of breaking point, free surface elevations, time-averaged velocity field, time-averaged turbulent kinetic energy and general turbulent behaviour, respectively. Overall, the free surface k−ω SST with the free surface jump conditions performs best in terms of the accuracy of the predictions against the experimental data and the capture of the physical breaking phenomena, for instance, the trapped air pocket and the development of the free surface in the surf zone.

A mixed analytical-numerical method for the acoustic radiation of a double elastic spherical shell in finite-depth water

A mixed analytical-numerical method is proposed to evaluate the acoustic radiation of a double elastic spherical shell in finite-depth water by combining the analytical method with the numerical Wave Based Method (WBM) [or Wave Superposition Method, WSM]. The effects of reflection from both the free surface and the seabed are considered. The correctness of this method is verified through several numerical examples. Theoretically, this method can be extended to address the acoustic radiation problem of a double spherical shell in more complex hydroacoustic waveguide environment, because only a change of Green's function used in the WBM is needed. The method presented in this work can provide a benchmark, which is quite scarce at present, for the fluid-structure coupled vibration, acoustic radiation and sound propagation of three-dimensional bodies of an arbitrary shape in the hydroacoustic waveguide environment. Furthermore, the influence of free surface and seabed boundaries on the behaviors of structural vibration, acoustic radiation and sound propagation is presented. Some physical images are also included to illustrate the mechanism more clearly.

Hydrodynamic Forces on a Cylinder With a Flexible Splitter Plate Near the Free Surface

Abstract Experiments were conducted on two splitter plates of different structural rigidities, attached to a rigid cylinder and exposed to cross-flow at different submergence depths in order to study the influence of free surface. It was found that the presence of flexible splitter plate results in drag reduction at all submergence depths when compared with a plain cylinder, with the reduction being dependent on the rigidity of the splitter plate. The lift force was also found to increase abruptly at low depths for the cases involving splitter plates. The results from the analyses of tip kinematics on flexible splitter plate indicated that it performs vertical oscillations symmetrically about its mounting position at high submergence depths but is displaced in one direction up to an order of cylinder diameter and undergoes minimal oscillations when located near the free surface.

Dynamic stall simulation of a pitching hydrofoil near free surface by using the volume of fluid method

In the present research, the turbulent fluid flow around a pitching hydrofoil is studied near the water free surface, numerically. The k−ω SST model is used for turbulence modeling. The finite volume method (FVM) is used for discretizing the Reynolds Averaged Navier–Stokes (RANS) equations and the free surface is captured by the volume of fluid (VOF) method. In order to validate the present simulations, the current results are compared with those of given by experimental test cases. The first experimental test case is a pitching airfoil in the single phase flow and the second one is a stationary hydrofoil near the free surface. Furthermore, the influence of free surface on hydrofoil drag and lift coefficients are investigated. Different submergence depths and oscillation frequencies are investigated. It is found that both submergence depth and frequency of rotation are affected the lift and drag coefficients, significantly.