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The dynamic response of floating horizontal axis wind turbines (FHWATs) are affected by the viscous and inertia effects. In free decay motion, viscous drag reduces the amplitude of pitch and roll fluctuation, the quasi-static mooring system overestimates the resonant amplitude values of pitch and roll. In this paper, the quasi-static mooring system is modified by introducing linear damping and quadratic damping. The dynamic response characteristics of the FHAWT modified model of the DTU 10 MW tension leg platform (TLP) were studied. Dynamic response of the blade was mainly caused by wind load, while the wave increased the blade short-term damage equivalent load. The tower base bending moment was affected by inclination of the tower and the misaligned angle βwave between wind and wave. Except the yaw motion, other degrees of freedom motions of the TLP were substantially affected by βwave. Ultimate tension of the mooring system was related to the displacement caused by pitch and roll motions, and standard deviation of the tension was significantly affected by the wave frequency response. Under the action of wave load, the viscous drag would stimulate the mooring system and increase the resonance of the platform motion.

This paper analyses the nonlinear forces on a moored point-absorbing wave energy converter (WEC) in resonance at prototype scale (1:1) and at model scale (1:16). Three simulation types were used: Reynolds Averaged Navier–Stokes (RANS), Euler and the linear radiation-diffraction method (linear). Results show that when the wave steepness is doubled, the response reduction is: (i) 3% due to the nonlinear mooring response and the Froude–Krylov force; (ii) 1–4% due to viscous forces; and (iii) 18–19% due to induced drag and non-linear added mass and radiation forces. The effect of the induced drag is shown to be largely scale-independent. It is caused by local pressure variations due to vortex generation below the body, which reduce the total pressure force on the hull. Euler simulations are shown to be scale-independent and the scale effects of the WEC are limited by the purely viscous contribution (1–4%) for the two waves studied. We recommend that experimental model scale test campaigns of WECs should be accompanied by RANS simulations, and the analysis complemented by scale-independent Euler simulations to quantify the scale-dependent part of the nonlinear effects.

A design of semi-submersible platform is mainly based on the extreme response analysis due to the forces experienced by the components during lifetime. The external loads can induce the extreme air gap response and potential deck impact to the semi-submersible platform. It is important to predict air gap response of platforms accurately in order to check the strength of local structures which withstand the wave slamming due to negative air gap. The wind load cannot be simulated easily by model test in towing tank whereas it can be simulated accurately in wind tunnel test. Furthermore, full scale simulation of the mooring system in model test is still a tuff work especially the stiffness of the mooring system. Owing to the above mentioned problem, the model test results are not accurate enough for air gap evaluation. The aim of this paper is to present sensitivity analysis results of air gap motion with respect to the mooring system and wind load for the design of semi-submersible platform. Though the model test results are not suitable for the direct evaluation of air gap, they can be used as a good basis for tuning the radiation damping and viscous drag in numerical simulation. In the presented design example, a numerical model is tuned and validated by ANSYS AQWA based on the model test results with a simple 4 line symmetrical horizontal soft mooring system. According to the tuned numerical model, sensitivity analysis studies of air gap motion with respect to the mooring system and wind load are performed in time domain. Three mooring systems and five simulation cases about the presented platform are simulated based on the results of wind tunnel tests and sea-keeping tests. The sensitivity analysis results are valuable for the floating platform design. [ABSTRACT FROM AUTHOR]

A design of offshore floating structure is mainly based on the extreme response analysis due to the forces experienced. The extreme response can induce the negative air gap response and potential impact to the deck bottom of floating structure. It is important to predict the slamming load in order to check the strength of local structures which withstand the wave slamming. In recent years, studies of the effects of wind load on air gap response and slamming load are ignored. When the platform suffers the extreme wave, the wind is also harsh. Moreover, the wind load can affect the motion response of the platform. The wind load cannot be simulated easily by model test in towing tank whereas it can be simulated accurately in wind tunnel test. Though the model test results are not accurate enough for air gap and slamming load evaluation due to the loss of wind effect, they can be used as a good basis for tuning the radiation damping and viscous drag in numerical simulation. This paper aims at presenting the sensitivity analysis results of wave slamming load with respect to the wind load for the design of semi-submersible platform. As an example of semi-submersible drilling platform design, the wind tunnel test has been carried out, and the sea-keeping model test is also performed in towing tank, while the wind load effect is ignored. According to the model test results, a numerical model is tuned and validated by ANSYS AQWA. Sensitivity analysis studies of the relative velocity between water particle and platform surface and the wave slamming load with respect to the wind load are performed in time domain by the tuned numerical model. Five simulation cases about the presented platform are simulated based on the results of wind tunnel tests and sea-keeping tests. The sensitivity analysis results are valuable for the floating platform design. [ABSTRACT FROM AUTHOR]

Optical tweezers have become an important instrument in force measurements associated with various physical, biological, and biophysical phenomena. Quantitative use of optical tweezers relies on accurate calibration of the stiffness of the optical trap. Using the same optical tweezers platform operating at 1064 nm and beads with two different diameters, we present a comparative study of viscous drag force, equipartition theorem, Boltzmann statistics, and power spectral density (PSD) as methods in calibrating the stiffness of a single beam gradient force optical trap at trapping laser powers in the range of 0.05 to 1.38 W at the focal plane. The equipartition theorem and Boltzmann statistic methods demonstrate a linear stiffness with trapping laser powers up to 355 mW, when used in conjunction with video position sensing means. The PSD of a trapped particle's Brownian motion or measurements of the particle displacement against known viscous drag forces can be reliably used for stiffness calibration of an optical trap over a greater range of trapping laser powers. Viscous drag stiffness calibration method produces results relevant to applications where trapped particle undergoes large displacements, and at a given position sensing resolution, can be used for stiffness calibration at higher trapping laser powers than the PSD method. [ABSTRACT FROM AUTHOR]

Evaluation of relative permeability coefficients is one of the key steps in reliable simulation of two-phase flow in porous media. An extensive body of work exists on evaluation of these coefficients for two-phase flow under pressure gradient. Oil transport under an applied electrical gradient in porous media is also governed by the principles of two-phase flow, but is less understood. In this paper, relative permeability coefficients under applied electric field are evaluated for a specific case of two- phase fluid flow in water-wet porous media, where the second fluid phase is oil. It is postulated that the viscous drag on the oil phase, exerted by the electro-osmotic flow of the water phase, is responsible for the transport of oil in the absence of a pressure gradient. Reliable prediction of the flow patterns necessitates accurate representation and determination of the relative permeability coefficients under the electrical gradient. The contribution of each phase to the flow is represented mathematically, and the relative permeability coefficients are evaluated through electro-osmotic flow measurements conducted on oil bearing rock cores. [ABSTRACT FROM AUTHOR]

This paper presents a multi-objective optimization design framework that is integrated with the Computer Aided Design (CAD) for geometric variation and Computational Fluid Dynamics (CFD) software for hydrodynamic computations for the design of Autonomous Underwater Vehicles (AUVs). The optimization model utilizes the ‘Non-dominated Sorting Genetic Algorithm (NSGA-II)’. In the present model hull geometric parameters (i.e. length of nose ( L n ) , length of the parallel middle body ( L m ) , length of the tail ( L t ) , maximum diameter ( D max ) , and two shape variation coefficients of nose ( n n ) and tail ( n t ) ) are considered as the design parameters and minimization of viscous drag, and maximization of nominal wake fraction and total volume are considered as the objective functions for the integrated design approach. CFD software (Shipflow ∗ ™) is used to evaluate the viscous drag and it is integrated with the CAD definition. The optimization framework NSGA-II is implemented in MATLAB ∗∗ ™. Finally, we present a design example of an existing AUV Cormoran and show that the integration of NSGA-II with CFD and CAD is effective for AUV hull form design. Our reported results show that for the given bounds on the design parameters, the optimization design framework is able to produce more efficient hull forms than the existing design. [ABSTRACT FROM AUTHOR]

Abstract: A surrogate model is presented to predict viscous drag in the context of conceptual design activities in aeronautics, where the norm is the use of low fidelity methods. The surrogate is developed with the requirement that its computational cost has to be comparable to that of its lower fidelity counterparts. The specific problem that has been chosen to test the method capabilities is the optimization of an unconventional aircraft empennage. The surrogate model is based on the high order singular value decomposition of a multidimensional tensor that contains a limited amount of information on viscous drag as a function of various design parameters defining the configuration of the empennage. This limited amount of information is generated using a medium level fidelity method, namely a panel method plus a boundary layer formulation, but the surrogate only uses the drag information and is independent of both the method used to generate this information and the specific problem that is considered for illustration. Thus, the proposed surrogate model is generic and can be seen as a means to bring information from higher fidelity models into the lower fidelity methods that are commonly used in the conceptual design phases of aeronautic design. [Copyright &y& Elsevier]

Abstract: The aerodynamics of a sailing yacht with different sail trims are presented, derived from simulations performed using computational fluid dynamics. A Reynolds-averaged Navier–Stokes approach was used to model sixteen sail trims first tested in a wind tunnel, where the pressure distributions on the sails were measured. An original approach was employed by using two successive simulations: the first one on a large domain to model the blockage due to the wind tunnel walls and the sails model, and a second one on a smaller domain to model the flow around the sails model. A verification and validation of the computed aerodynamic forces and pressure distributions was performed. The computed pressure distribution is shown to agree well with the measured pressures. The sail surface pressure was correlated with the increase of turbulent viscosity in the laminar separation bubble, the flow reattachment and the trailing edge separation. The drive force distribution on both sails showed that the fore part of the genoa (fore sail) provides the majority of the drive force and that the effect of the aft sail is mostly to produce an upwash effect on the genoa. An aerodynamic model based on potential flow theory and a viscous correction is proposed. This model, with one free parameter to be determined, is shown to fit the results better than the usual form drag and induced drag only, even if no friction drag is explicitly considered. [Copyright &y& Elsevier]

Abstract: Monte Carlo simulation of the rupture of multiple receptor–ligand bonds between two PMN cells suspended in a Newtonian fluid is performed. We demonstrate via micro-mechanical model of two cells adhered by multiple receptor–ligand bonds that viscous drag caused by relative motion of cell suspended in a Newtonian fluid modulates transmission of an applied external load to bonds. Specifically, it is demonstrated that at any time the intermolecular bond force is not equivalent to the instantaneous applied force. The difference in the instantaneous applied force and the intermolecular bond force depends on the viscosity of fluid, the size of cell, the applied loading rate, and the number of bonds at any instant of time. Viscous drag acting on cell reduces average bond rupture forces. [Copyright &y& Elsevier]

The deep sea oil-filled brushless DC (BLDC) motor operates in an underwater environment. The pressure varies with the working depth. The viscosity of the oil in the motor is sensitive to the pressure and hence the viscous drag loss varies with the pressure. The high pressure has influence on the loss characteristics of silicon steel and results in the increase of the core loss in the motor. Computational fluid dynamics (CFD) method is adopted to calculate the flow field in the air gap and the viscous drag loss at different pressures is obtained. Finite element method (FEM) is adopted to obtain the core loss at high pressure. The motor design considering the high pressure effect is compared with the motor design neglecting the pressure effect. The comparison results show that high pressure has large influence on viscous drag loss and core loss, resulting in the decrease in the efficiency and increase in the temperature rise. The experiment is done and the results verify the effectiveness of the calculated results. [ABSTRACT FROM PUBLISHER]

The aim of this work is to develop a simple nonlinear model of a wave energy converter (WEC) for capturing power from ocean waves and converting it into electrical power. A generic axisymmetric device is considered, which consists of a vertical circular cylinder surrounded by a circular annulus. The nonlinear system of equations of motion of this generic WEC are derived; these include the nonlinear term arising from viscous drag due to boundary layer separation. The expressions for radiation damping and added mass are determined by dimensional analysis. These equations are then solved numerically and the results are displayed graphically in a number of figures. Consideration of these graphs leads to conclusions that should be taken into account by the design engineer. [ABSTRACT FROM AUTHOR]

This article presents a porous media transport approach to model the performance of an air-cooled condenser. The finned tube bundles in the condenser are represented by a porous matrix, which is defined by its porosity, permeability, and the form drag coefficient. The porosity is equal to the tube bundle volumetric void fraction and the permeability is calculated by using the Karman–Cozney correlation. The drag coefficient is found to be a function of the porosity, with little sensitivity to the way this porosity is achieved, i.e., with different fin size or spacing. The functional form was established by analyzing a relatively wide range of tube bundle size and topologies. For each individual tube bundle configuration, the drag coefficient was selected by trial and error so as to make the pressure drop from the porous medium approach match the pressure drop calculated by the heat exchanger design software ASPEN B-JAC. The latter is a well-established commercial heat exchanger design program that calculates the pressure drop by using empirical formulae based on the tube bundle properties. A close correlation is found between the form drag coefficient and the porosity with the drag coefficient decreasing with increasing porosity. A second order polynomial is found to be adequate to represent this relationship. Heat transfer and second law (of thermodynamics) performance of the system has also been investigated. The volume-averaged thermal energy equation is able to accurately predict the hot spots. It has also been observed that the average dimensionless wall temperature is a parabolic function of the form drag coefficient. The results are found to be in good agreement with those available in the open literature. [ABSTRACT FROM AUTHOR]

Abstract: In this paper a physical-based constitutive relation for defining the thermo-viscoplastic behaviour of FCC metals with dependence on strain on thermal activation processes is presented. The model, based on previous considerations reported by Rusinek and Klepaczko [Rusinek A, Klepaczko JR. Shear testing of sheet steel at wide range of strain rates and a constitutive relation with strain-rate and temperature dependence of the flow stress. Int J Plasticity 2001;17:87–115], is founded on physical aspects of the material behaviour. The proposed constitutive relation is applied to define the behaviour of oxygen-free high conductivity (OFHC) copper using the experimental data reported in Nemat-Nasser and Li [Nemat-Nasser S, Li Y. Flow stress of FCC polycrystals with application to OFHC copper. Acta Mater 1998;46:565–77]. The description of the material behaviour provided by the model gets satisfactory agreement with the experiments. The analytical predictions of this constitutive description are compared with those obtained from the models due to Voyiadjis and Almasri [Voyiadjis GZ, Almasri AH. A physically based constitutive model for fcc metals with applications to dynamic hardness. Mech Mater 2008;40:549–63], and Nemat-Nasser and Li. This comparison reveals that the original formulation proposed in this paper is a suitable alternative to other physically based relations for modeling OFHC copper. [Copyright &y& Elsevier]

Abstract: In this paper are presented two extensions of the Rusinek–Klepaczko constitutive relation [Rusinek A, Klepaczko JR. Shear testing of sheet steel at wide range of strain rates and a constitutive relation with strain-rate and temperature dependence of the flow stress. Int J Plasticity 2001;17:87–115] to define the behaviour of aluminium alloys at wide ranges of strain rate and temperature. The formulations reported extend the validity of the Rusinek–Klepaczko model for the definition of particular aspects of the behaviour of aluminium alloys, namely the negative strain rate sensitivity and the viscous drag. Such formulations are applied to describe the thermo-viscoplastic behaviour of two commercial aluminium alloys (AA 5083-H116 and AA 7075) using experimental data available in the literature [Clausen AH, Børvik T, Hopperstad OS, Benallal A. Flow and fracture characteristics of aluminium alloy AA5083-H116 as function of strain rate, temperature and triaxiality. Mater Sci Eng A 2004;364:260–72; El-Magd E, Abouridouane M. Characterization, modelling and simulation of deformation and fracture behaviour of the light-weight wrought alloys under high strain rate loading. Int J Impact Eng 2006;32:741–58]. Their analytical results are compared with those obtained from the phenomenological constitutive relations reported in Clausen et al. (2004) and El-Magd and Abouridouane (2006). The best agreement with experiments is achieved by the predictions provided by the extended Rusinek–Klepaczko models. Moreover, the formulations presented may be implemented into FE code using for example the algorithm reported in [Zaera R, Fernández-Sáez J. An implicit consistent algorithm for the integration of thermoviscoplastic constitutive equations in adiabatic conditions and finite deformations. Int J Solids Struct 2006;43:1594–612]. [Copyright &y& Elsevier]

To estimate the strain rate dependence of the dynamic flow stress of aluminum alloys, 2017-O and - T4, high strain rate and rate reduction tests are performed at strain rates ranging from about 1000/sec to 30000/sec. A steep increase in the flow stress is observed for 2017-O at the strain rate of about 3000/sec. For 2017-T4, the above phenomenon is observed at the strain rate above about 10000/sec. A simplified model for dislocation kinetics under dynamic plastic deformation is employed which can represent a transition in the rate controlling mechanism of dislocation motion from a thermally activated process to a viscous drag. It is expected that the steep increase in the flow stress observed at high strain rates is attributed to the rate dependence of the viscous drag on the dislocation motion. [ABSTRACT FROM AUTHOR]

Abstract: We compare two-dimensional and three dimensional lattice-Boltzmann (LB) simulations of fluid flow in a pseudo-2D micromodel used in experimental work. We show that 2D LB simulations can be used to compute the average velocity field in 3D systems in which the third dimension is small compared with the other two dimensions, and where the velocity component along the third dimension is zero. Correct results are obtained provided a viscous drag force, representing the effect of the third dimension, is used in the 2D model. The use of 2D simulations allows to significantly reduce the computational cost of the calculations, and hence to increase the size and resolution of the systems that can be studied using the LB method. [Copyright &y& Elsevier]

Abstract: Spacers are designed to generate significant secondary flow structures and create directional changes in the flow through membrane modules. Shape of the spacers used in membrane modules strongly influences the resulting flow and therefore performance of the module. In this work fluid dynamics of rectangular channels similar to membrane modules and containing different spacers was simulated using a three-dimensional computational fluid dynamics (CFD) model. A ‘unit cell’ approach was evaluated and used for this purpose. In addition to predicting the pressure drop, the simulated results provided significant insight into fluid dynamics of spacer filled channels. The validated CFD model was used to evaluate performance of different spacer shapes and understand the role of spacer shape and resulting fluid dynamics. The models were extended for the first time to simulate flow in spacer filled curvilinear channels, which could be useful in understanding the fluid behavior in spiral modules. The results were compared with those obtained with the flat channel. The approach and results presented in this work will have significant implications for identifying improved spacers with higher propensities to reduce fouling in membrane modules. [Copyright &y& Elsevier]

Drag/thrust analysis of jet-propelled transonic transport aircraft on the basis of calculated viscous flow is discussed. Unique definitions for viscous drag plus wave drag and for induced drag are established. The concept of additive throughflow drag is introduced. Drag/thrust bookkeeping is given attention. All drag components can be calculated in the flow region adjacent to the aircraft, where numerical accuracy is expectedly highest. Uniform handling of complex aircraft configurations is brought within reach. Near-field/far-field drag balances are exact. Computational aspects are discussed, in particular the elimination of spurious drag sources. Numerical examples are given for a wing-body and for a wing-body-pylon-nacelle configuration. In either case, the spurious drag sources are eliminated. Acceptable agreement is obtained for the total drag in the first case, and for the installation drag in the second case. Extension of the analysis presented to propeller-driven transport aircraft is straightforward. [Copyright &y& Elsevier]

Investigations into the drag of ellipsoids in proximity have been carried out experimentally using a low-speed wind tunnel and numerically using a commercial computational fluid dynamics (CFD) code (CFX™). The purpose of the investigations was to improve the understanding of the viscous resistance and viscous interaction effects between twin bodies in proximity, such as the hulls of a catamaran, and consequently to improve the techniques for estimating the resistance and powering of commercial catamarans. The wind tunnel tests were carried out on a single ellipsoid with a length-diameter ratio (L/D) of 6.0 and a pair of similar ellipsoids in proximity at separation-length ratios (S/L) of 0.27, 0.37, 0.47 and 0.57 at Reynolds number values up to 3.2 × 10[sup 6]. The ellipsoids thus represented a reflex (or reflected) model of a catamaran hull. In the numerical work, investigations were carried out on ellipsoids with the same geometry as those tested in the wind tunnel and ellipsoids with a larger length-diameter ratio. Results of the wind tunnel tests and numerical investigations are presented and compared. It is found from the investigations that viscous form effects and viscous interactions are present for such bodies and that CFD techniques can make very useful contributions to the investigations of these effects. [ABSTRACT FROM AUTHOR]

This paper describes a one-dimensional model that predicts the minimum static pressure in a rotating transfer passage taking into account both the centrifugal and the viscous forces, the latter having been neglected in previous studies. The minimum pressure is important as it determines the engine lubricating capacity and whether aero-cavitation may occur in the passage. The flow is considered laminar and the fluid is incompressible and Newtonian with constant viscosity. The current model predicts a lower minimum pressure within the transfer passage due to the inclusion of the viscous forces in the model. [ABSTRACT FROM AUTHOR]

The present study involves the thermodynamic performance and entropy production analysis of an infrared suppression (IRS) system. The IRS device is usually used in ocean liners, warships, and cargo ships to reduce the turbine exhaust temperature. First, the governing differential equations (Navier-Stokes, energy, turbulence, radiation equations) are solved using ANSYS Fluent 19.0 to elucidate the thermodynamic performance of the IRS device. Then entropy production and irreversibilities are calculated in post-processing. The pertinent parameters, viz. Reynolds number at nozzle inlet (6.12 × 105 to 3.11 × 106), the nozzle outlet fluid temperature (400 to 600 K), nozzle overlap, funnel overlap, and the types of guide vanes are varied to analyze the impact of these parameters on entropy generation. The effect of different boundary conditions for funnel walls (i.e., adiabatic, diathermic with or without radiation) on entropy generation is discussed. A comparison of various types of guide vanes is also performed. It is found that entropy production (or irreversibility) is less when mass entrainment is more, i.e., device outlet temperature is less. Moreover, entropy production caused by heat transfer has the main contribution in total entropy production than that caused due to fluid friction. The entropy generation is found to be minimum for zero nozzle overlapping, zero funnel overlapping, diathermic funnel wall with radiation, and QCGV (quarter circular guide vane) type. The mass entertainment is maximum, and device outlet temperature is minimum with the mentioned conditions. [ABSTRACT FROM AUTHOR]