Numerical Study on the Effect of Gap Diffraction on the Hydrodynamic Performance of A Floating Breakwater

Two-dimensional(2D)flume experiments are useful in investigating the performances of floating breakwaters(FBs),including hydrodynamic performances,motion responses,and mooring forces.Designing a reasonable gap between the flume wall and the FBs is a critical step in 2D flume tests.However,research on the effect of the gap on the accuracy of 2D FB experimental results is scarce.To address this issue,a numerical wave tank is developed using CFD to estimate the wave-FB interaction of a moored dual-cylindrical FB,and the results are compared to experimental data from a previously published work.There is good agreement between them,indicating that the numerical model is sufficiently accurate.The numerical model is then applied to explore the effect of gap diffraction on the performance of FBs in2D experiments.It was discovered that the nondimensional gap length L_(Gap)/W_(Pool)should be smaller than 7.5%to ensure that the relative error of the transmission coefficient is smaller than 3%.The influence of the gap is also related to the entering wave properties,such as the wave height and period.

Structure Optimization of a Tesla Turbine Using an Organic Rankine Cycle Technology

The so-called ORC(Organic Rankine Cycle)heat recovery technology has attracted much attention with regard to medium and low temperature waste heat recovery.In the present study,it is applied to a Tesla turbine.At the same time,the effects of the disc speed,diameter and inter-disc gap on the internal flow field and output power of the turbine are also investigated by means of CFD(Computational Fluid Dynamics)numerical simulation,by which the pressure,velocity,and output efficiency of the internal flow field are obtained under different internal and external conditions.The highest efficiency(66.4%)is obtained for a number of nozzles equal to 4,a disk thickness of 1 mm,and a gap of 1 mm between the disks.The results of the study serve as a theoretical basis for the structural design and optimization of Tesla turbines.

Impact of Blade-Flapping Vibration on Aerodynamic Characteristics of Wind Turbines under Yaw Conditions

Although the aerodynamic loading of wind turbine blades under various conditions has been widely studied,the radial distribution of load along the blade under various yaw conditions and with blade flapping phenomena is poorly understood.This study aims to investigate the effects of second-order flapwise vibration on the mean and fluctuation characteristics of the torque and axial thrust of wind turbines under yaw conditions using computational fluid dynamics(CFD).In the CFD model,the blades are segmented radially to comprehensively analyze the distribution patterns of torque,axial load,and tangential load.The following results are obtained.(i)After applying flapwise vibration,the torque and axial thrust of wind turbines decrease in relation to those of the rigid model,with significantly increased fluctuations.(ii)Flapwise vibration causes the blades to reciprocate along the axial direction,altering the local angle of attack and velocity of the blades relative to the incoming wind flow.This results in the contraction of the torque region from a circular shape to a complex“gear”shape,which is accompanied by evident oscillations.(iii)Compared to the tangential load,the axial load on the blades is more sensitive to flapwise vibration although both exhibit significantly enhanced fluctuations.This study not only reveals the impact of flapwise vibration on wind turbine blade performance,including the reduction of torque and axial thrust and increased operational fluctuations,but also clarifies the radial distribution patterns of blade aerodynamic characteristics,which is of great significance for optimizing wind turbine blade design and reducing fatigue risks.

A NUMERICAL AND ANALYTICAL STUDY ON A TAIL-FLAPPING MODEL FOR FISH FAST C-START

The force production physics and the flow control mechanism of fish fast C-start are studied numerically and theoretically by using a tail-flapping model.The problem is simplified to a 2-D foil that rotates rapidly to and fro on one side about its fixed leading edge in water medium.The study involves the simulation of the flow by solving the two-dimensional unsteady incompressible Navier- Stokes equations and employing a theoretical analytic modeling approach.Firstly,reasonable thrust magnitude and its time history are obtained and checked by fitting predicted results coming from these two approaches.Next,the flow fields and vortex structures are given,and the propulsive mechanism is interpreted.The results show that the induction of vortex distributions near the trailing edge of the tail are important in the time-averaged thrust generation,though the added inertial effect plays an important role in producing an instant large thrust especially in the first stage.Furthermore,dynamic and energetic effects of some kinematic controlling factors are discussed.For enhancing the time- averaged thrust but keeping a favorable ratio of it to time-averaged input power within the limitations of muscle ability,it is recommended to have a larger deflection amplitude in a limited time interval and with no time delay between the to-and-fro strokes.

Influence of Spray Gun Position and Orientation on Liquid Film Development along a Cylindrical Surface

A method combining computationalfluid dynamics(CFD)and an analytical approach is proposed to develop a prediction model for the variable thickness of the spray-induced liquidfilm along the surface of a cylindrical workpiece.The numerical method relies on an Eulerian-Eulerian technique.Different cylinder diameters and positions and inclinations of the spray gun are considered and useful correlations for the thickness of the liquidfilm and its distribution are determined using various datafitting algorithms.Finally,the reliability of the pro-posed method is verified by means of experimental tests where the robot posture is changed.The provided cor-relation are intended to support the optimization of spray-based coating applications.

Experimental and CFD investigation of flow behavior and sand erosion pattern in a horizontal pipe bend under annular flow

The internal erosion of pipelines in oil and gas storage and transportation engineering is highly risky.High gas velocity of annular flow entrained sand will cause damage to the pipelines,and may further result in thinning of the wall.If this damage lasts for a long time,it may cause pipeline leakage and cause huge economic losses and environmental problems.In this research,an experimental device for studying multiphase flow erosion is designed,including an erosion loop and an experimental elbow that can test the erosion rate.The annular flow state and pipe wall erosion morphology can also be tested by the device.The computational fluid dynamics(CFD)method is combined with the experiment to further study the annular flow erosion mechanism in the pipeline.The relationship between gas-liquid-solid distribution and erosion profile was studied.The results show that the most eroded region occurs be-tween 22.5° and 45° in the axial angle direction and between 90° and 135° in the circumferential angle direction of the elbow.The pits and deep scratches form on the surface of the sample after the sand collision.

Impact Analysis of Fluid-structure Coupling Embedded Weapon Bay

The coupling behavior of the imbedded weapon store occurring between the local unsteady flow field round the store and the structure response on the processing of opening its bay-door is simulated by using numerical method based on computational fluid mechanics(CFD).The transient aerodynamic behaviors when opening door under various flight altitudes and the corresponding structure deformation evolution in the unsteady flow fields are analyzed respectively and presented.The rules of aircraft attitude parameters′impacting to the responses of structure and the bay-door′s opening process are obtained by comparing with the analysis results.These rules can be applied to the structure design of bay-door and route specification of missile when disengaged and launched from within store.

Process Modeling of Ferrofluids Flow for Magnetic Targeting Drug Delivery

Among the proposed techniques for delivering drugs to specific sites within the human body, magnetic targeting drug delivery surpasses due to its non-invasive character and its high targeting efficiency. Although there have been some analyses theoretically for magnetic drug targeting, very few researchers have addressed the hydrodynamic models of magnetic fluids in the blood vessel of human body. This paper presents a mathematical model to describe the hydrodynamics of ferrofluids as drug carriers flowing in a blood vessel under the applied magnetic field. A 3D flow field of magnetic particles in a blood vessel model is numerically simulated in order to further understand clinical application of magnetic targeting drug delivery. Simulation results show that magnetic nanoparticles can be enriched in a target region depending on the applied magnetic field intensity. Magnetic resonance imaging confirms the enrichment of ferrofluids in a desired body tissue of Sprague-Dawley rats. The simulation results coincide with those animal experiments. Results of the analysis provide the important information and can suggest strategies for improving delivery in favor of the clinical application.

In-situ remediation of deep petroleum-contaminated soil injection

A computational fluid dynamics(CFD)numerical simulation and field experiment were used to investigate optimal operating parameters of high-pressure jet grouting equipment and clarify the boundary law of the injection area in the remediation process.The response surface optimization design results show that the optimal injection pressure is 30 MPa,rotation speed is 23 r/min,commission speed is 30 cm/min,and the optimal injection diameter is 147.3 cm.Based on the CFD numerical simulation,the ratio of the injection core,turbulent zone,and seepage zone is approximately 1∶4∶2.The distribution law of jet core,turbulence zone and seepage zone at different cross-sections under 30 MPa operating conditions is as follows:The jet core radius is approximately 100 mm,the turbulence zone is mainly distributed at 100 to 500 mm,the seepage zone is mainly distributed at 500 to 700 mm,the seepage zone could be completed within 2 h,and the proportion of the three boundary zones in the injection zone is similar to that of the numerical simulation.This study provides theoretical parameters and practical reference for the remediation of deep pollution via in-situ chemical oxidation in the Loess Plateau soil environment.

Flow over Broad Crested Weirs： Comparison of 2D and 3D Models

The flow over broad-crested weirs was simulated by computational fluid dynamic model. The water surface profile over broad crested weir was measured in a laboratory model and validated using two and three dimensional Fluent programs. The Reynolds Averaged Navier-Stokes equations coupled with the turbulent standard （k-ε） model and volume of fluid method were applied to estimate the water surface profile. The results of numerical model were compared with experimental results to evaluate the ability of model in describing the behaviour of water surface profile over the weir. The results indicated that the 3D required more time in comparison with 2D results and the flow over weir changed from subcritical flow at the upstream （U/S） face of weir to critical flow over the crest and to supercritical flow at downstream （D/S）. A reasonable agreement was noticed between numerical results and experimental observations with mean error less than 2 %.

Channel Slope Effect on Energy Dissipation of Flow over Broad Crested Weirs

The main purpose of broad crested weir used in open channels is to raise and control upstream (U/S) water level. In this study, a new performance was added to this weir, by making a step at downstream (D/S) of weir. The energy dissipation, the height of the weir/the upstream water height ratio and Froude number relationships (E% – P/h – Fr) for three range of flume slop S = 0.0, 0.002 and 0.004 were simulated. The experiments were performed in a laboratory horizontal channel of 4.6 m length, 0.3 m width and 0.3 m depth for a wide range of discharge. The D/S step height of the weir was 7.5 cm. FLUENT software was used as numerical model which represent a type of Computational Fluid Dynamics (CFD) model in order to simulate flow over weirs. The Volume of Fluid (VOF) method with the Standard k – ε turbulence model was used to estimate the free surface profile and the structured mesh with high concentration near the wall regions. The experimental results of the water surface profile gave a high agreement with the results of the numerical models. The maximum value 28.78 of E% was obtained in single step broad crested weir in the experimental result and 27.35 in numerical result at S = 0.004. Finally, the range of the relative error of the energy dissipation between experimental and numerical results was achieved and the maximum was 6.76 in all runs.

Comprehensive Euler/Lagrange modelling including particle erosion for confined gas-solid flows

The present research aims to assess the capability of a comprehensive Euler/Lagrange approach for predicting gas-solid flows and the associated solid particle erosion.The open-source code OpenFOAM®4.1 was used to carry out the numerical simulations,where the standard Lagrangian libraries were substantially extended to account for all necessary models.Particles are tracked considering both translational and rotational motion as well as all relevant forces,such as gravity/buoyancy,drag and transverse lift due to shear and particle rotation.The tracking time step was dynamically adapted ac-cording to the locally relevant time scales,which drastically reduces computational times.Stochastic approaches are adopted to model particle turbulent dispersion,particle collisions with rough walls and particle-particle interactions.Five solid particle erosion models,available in the literature,were considered to estimate pipe bend erosion.Three study cases are provided to validate the adopted nu-merical approach and erosion models extensively.The first case intends to evaluate the ability of the extended CFD code to predict the behaviour of gas-solid flows in pneumatic conveying systems.This goal is achieved by comparing the numerical results with the experimental data obtained by Huber(1997)and Huber and Sommerfeld(1994,1998)in a pneumatic conveying system.Here,the importance of considering inter-particle collisions and surface roughness for predicting particle velocity,mass flux and mean diameter distributions in gas-solid flows is highlighted.The second and third case intend to evaluate the ability of the erosion models in estimating bend erosion in diluted gas-solid flows.The erosion data obtained experimentally by Mazumder et al.(2008)and Solnordal et al.(2015)in very dilut pneumatic conveying systems is used for validating the numerical results,neglecting now inter-particle collisions and two-way coupling.Besides a comprehensive analysis of the different influential properties on erosion,the innovation of the present study is as follows.For the first time also a temporal modifi-cation of the surface roughness due to the erosion was considered in the simulations obtained from previous measurements(Novelletto Ricardo&Sommerfeld,2020).As the surface roughness is increased due to erosion,eventually erosion rate becomes lower.This is the result of diminishing wall collision frequency.Simulations for several degrees of surface roughness showed that larger roughness is coupled with a drastic reduction of erosion.Hence,numerical simulations neglecting wall surface roughness are not realistic.The consideration of a particle size distribution instead of mono-sized computations showed a possible reduction of erosion rate.The detailed analysis of the different single-particle erosion models revealed that the model proposed by Oka et al.(2005)and Oka and Yoshida(2005)yields the best agreement with the measurements,however particle and wall properties are needed.

Numerical study on the characteristics of natural supercavitation by planar symmetric wedge-shaped cavitators for rotational supercavitating evaporator

With the application of supercavitation effect, a novel device named rotational supercavitating evaporator(RSCE) was recently designed for desalination. In order to improve the blade shape of rotational cavitator in RSCE for performance optimization and then design three-dimensional blades, numerical simulations are conducted on the supercavitating flows(with cavitation number ranging from 0.055 to 0.315) around two-dimensional planar symmetric wedge-shaped cavitators with different wedge angles varied from 10 to 180 degrees. Proper numerical method for simulating supercavitating flows around planar symmetric cavitator is established, and assessment of k-ε-v2 -f turbulence model in simulating cavitating flows is conducted. It shows that the size of computational domain would affect the simulation result. Empirical formulae for supercavity dimensions about cavitation number at different wedge angles are obtained, which are of significant importance in the subsequent design of three-dimensional blade. The characteristics of resistance at different wedge angles are discussed, which, together with the characteristics of supercavity dimensions, play important roles in the optimal design of RSCE.

Numerical study on morphological characteristics of rotational natural supercavitation by rotational supercavitating evaporator with optimized blade shape

In view of the supercavitation effect, a novel device named the rotational supercavitating evaporator (RSCE) has been designed for the desalination. In order to improve the blade shape of the rotational cavitator in the RSCE for the performance optimization, the blade shapes of different sizes are designed by utilizing the improved calculation method for the blade shape and the validated empirical formulae based on previous two-dimensional numerical simulations, from which the optimized blade shape with the wedge angle of 45° and the design speed of 5 000 r/min is selected. The estimation method for the desalination performance parameters is developed to validate the feasibility of the utilization of the results obtained by the two-dimensional numerical simulations in the design of the three-dimensional blade shape. Three-dimensional numerical simulations are then conducted for the supercavitating flows around the rotational cavitator with the optimized blade shape at different rotational speeds to obtain the morphological characteristics of the rotational natural supercavitation. The results show that the profile of the supercavity tail is concaved toward the inside of the supercavity due to the re-entrant jet. The empirical formulae for estimating the supercavity size with consideration of the rotation are obtained by fitting the data, with the exponents different from those obtained by the previous two-dimensional numerical simulations. The influences of the rotation on the morphological characteristics are analyzed from the perspectives of the tip and hub vortices and the interaction between the supercavity tail and the blade. Further numerical simulation of the supercavitating flow around the rotational cavitator made up by the blades with exit edge of uniform thickness illustrate that the morphological characteristics are also affected by the blade shape.

The effects of caudal fin deformation on the hydrodynamics of thunniform swimming under self-propulsion

To investigate the effects of the caudal fin deformation on the hydrodynamic performance of the self-propelled thunniform swimming,we perform fluid-body interaction simulations for a tuna-like swimmer with thunniform kinematics.The 3-D vortices are visualized to reveal the role of the leading-edge vortex(LEV)in the thrust generation.By comparing the swimming velocity of the swimmer with different caudal fin flexure amplitudes fa,it is shown that the acceleration in the starting stage of the swimmer increases with the increase of fa,but its cruising velocity decreases.The results indicate that the caudal fin deformation is beneficial to the fast start but not to the fast cruising of the swimmer.During the entire swimming process,the undulation amplitudes of the lateral velocity and the yawing angular velocity decrease as fa increases.It is found that the formation of an attached LEV on the caudal fin is responsible for generating the low-pressure region on the surface of the caudal fin,which contributes to the thrust.Furthermore,the caudal fin deformation can delay the LEV shedding from the caudal fin,extending the duration of the low pressure on the caudal fin,which will cause the caudal fin to generate a drag-type force over a time period in one swimming cycle and reduce the cruising speed of the swimmer.