U-shaped Duct Research Articles

Characteristics of wave loading and viscous energy loss of a bottom-sitting U-OWC wave energy device: A validated numerical perspective

This study involves numerical simulations of regular waves interacting with a bottom-sitting circular OWC wave energy converter with a U-shaped duct (circular U-OWC). A numerical wave tank is built based on the RANS equation and is validated against experiments. The effect of water depth and the height of the outer tube of the U-shaped duct on capture efficiency, wave loading, and viscous energy loss is investigated numerically. Results show that the height of the outer tube of the U-shaped duct has a significant effect on the capture efficiency of the device. Wave loading analysis reveals a critical mode of stability that may threaten the operation safety of the device. It is found that increasing the height of the U-shaped duct opening increases the viscous energy loss inside the U-shaped duct. A semi-theoretical model between work done by waves to the device and the viscous energy loss in the U-shaped duct is established, numerical data indicates that a significant amount of work done by waves to the device is lost in viscous effects. This suggests that certain optimization for reducing wave loading could possibly also reduce viscous energy loss, thus improving the overall efficiency of the device.

Limiting the available pneumatic power in a U-OWC

A fixed oscillating water column is a hollow structure open to the sea at its submerged part. The water column motion alternately compresses and decompresses the enclosed air above the inner free surface, which drives a self-rectifying air turbine. In 2003, Boccotti proposed the so-called U-OWC having in view the integration into breakwaters. In this configuration, the inner water is connected to the sea by a U-shaped duct whose opening faces upwards rather than sideways or downwards. This allows the OWC length to be extended (and the resonance frequency reduced) without placing the opening too deeply submerged where wave energy is attenuated by distance to the free-surface. An advantage of OWC converters is the capability to control or dissipate the excess energy available to the power take-off system in excessively energetic sea states. This is in general done by limiting the air turbine torque by controlling a by-pass air valve or a valve in series with the turbine. What is proposed here is to take advantage of the water column configuration to limit the energy absorbed from the waves in the more energetic sea states. This requires the tidal amplitude to be small as occurs in coastal areas like the Mediterranean Sea. The distance of the OWC opening to the mid-sea-level should be chosen such that, in the more energetic sea states, that opening is left uncovered by the troughs of the higher waves. This introduces a limitation to the energy absorbed in the more energetic sea states. This paper reports experiments with a 1:40th-scale U-OWC in a wave flume. The results show that limitations to the capture width ratio occurs in the more energetic wave systems.

Experimental and numerical study of a circular OWC with a U-shaped duct for wave energy conversion in long waves: Hydrodynamic characteristics and viscous energy loss

This paper presents an experimental and numerical study of a pile-type circular oscillating water column device with a U-shaped duct (circular U-OWC) for wave energy conversion in long waves. In the experiments, the circular U-OWC model and a conventional OWC model of identical chamber dimension was compared in a wave flume under various wave conditions. Orifices of different sizes are used to simulate the power-takeoff (PTO). The pneumatic power, PTO damping coefficient, and the scattering of the incident waves are measured. A RANS equation based numerical model with a modified k-ω SST turbulence closure is validated using the experiment. The numerical model is utilized to investigate the spatial non-uniformity of the free surface inside the chamber, and the viscous energy loss in the U-shaped duct. It is found that for long waves, the circular U-OWC device is several times more efficient than a conventional OWC device, and the sloshing effect inside the pneumatic chamber is significantly reduced compared to a conventional OWC device. It is also found that a significant amount of energy entering the U-shaped duct is dissipated in viscous loss, recommendations on U-shaped duct design are given to help reduce the viscous loss of the device.

Numerical analysis on a self-rectifying impulse turbine with U-shaped duct for oscillating water column wave energy conversion

In this paper, a novel self-rectifying impulse turbine with U-shaped duct used for oscillating water column wave energy conversion is proposed. It has a pair of fixed, symmetrical and radial-flow guide vane rows, and an axisymmetric, U-shaped duct with increasing cross-sectional area from the rotor to the guide vane rows, therefore it is expected to be relatively simple, reliable and efficient. A three-dimensional (3D) steady-state numerical model and a fully transient model are established for the present turbine. These two models are validated by comparison with experimental results from the published works, respectively, and then used to investigate the steady performance and transient characteristics of the impulse turbine. Effect of the geometry of the guide vanes, the number of the guide vanes, and the hub-to-tip ratio on turbine performance under steady airflows and transient airflows are studied, and the difference between these two models in predicting turbine performance is discussed.

A new species of Orthobula Simon, 1897 (Araneae: Trachelidae) from South America.

Orthobula Simon, 1897 is a group of very small litter-dwelling spiders with a tropical and subtropical distribution. The genus comprises 18 species, without any records in the Neotropical realm yet. Here we describe O. sudamericana sp. nov., distributed in Argentina and Paraguay. The new species appears to be most closely related to O. chayuensis Yang, Song and Zhu, 2003. The male also resembles O. charitonovi (Mikhailov, 1986). Orthobula sudamericana sp. nov. females differ from these species by the straight and parallel insemination ducts, and males by the centrally located, U-shaped sperm duct. Further, we summarize details on its natural history and habitat characteristics.

Attenuation of low-frequency sound in U-shaped duct with membrane coupled acoustic resonator: Modeling and analysis

This study proposes a new solution to control low-frequency noise in acoustic duct. The control device is in the shape of a Helmholtz resonator combined with a tensioned membrane in the air cavity, referred to as a membrane resonator. The membrane resonator is connected to the main waveguide through two necks. A theoretical model is developed to evaluate its acoustic performance when attached to a U-shaped duct as the host structure. Numerical simulation shows that the proposed resonator generates effective transmission loss in the very low-frequency region, as a result of the vibro-acoustic coupling between the integrated membrane and the two-end Helmholtz resonator. Since the attenuation band is deep sub-wavelength, a lumped-mass model is developed to capture the main physics involved in the acoustical-mechanical coupling. With the developed models, the tuning range of the proposed system is further investigated. Several tuning mechanisms are demonstrated by changing the membrane tension, thickness, material type and combining multiple resonators. The transmission loss has fairly large variation range and the desired frequency band is fully controllable. The proposed resonator has simple geometry and mechanical design, whilst the deep sub-wavelength characteristics and the offered tunability are promising. It has the potential to be used solely or as the basic building block in a wide range of noise control applications.

Modelling and field testing of a breakwater-integrated U-OWC wave energy converter with dielectric elastomer generator

This paper introduces a theoretical and experimental study of a wave energy converter (WEC) that combines the two innovative concepts of U-oscillating water column (U-OWC) and dielectric elastomer generator (DEG) power take-off (PTO). The U-OWC is a type of oscillating water column that features a U-shaped duct that is introduced to tune its dynamics to a certain wave period without active means of phase-control. The DEG is a compliant polymeric generator that makes it possible to convert mechanical energy into electrical energy by exploiting the large deformations of elastomeric membranes.A lumped-parameter mathematical model of the proposed WEC has been set-up and a small-scale model/prototype has been preliminary tested in a benign real-sea environment. During experiments, relevant experimental data have been collected and used for assessing the reliability of the modelling approach. Beside the model validation, specific experiments have been conducted to test a simple but yet effective load shedding system based on the progressive opening of an air valve. Finally, a preliminary design of a full-scale U-OWC equipped with DEG-PTO has been studied through wave-to-wire analysis. The obtained numerical results show an overall performance that is comparable with that of more conventional, expensive and complex PTO technologies.

Magnetohydrodynamic flows in u-shaped ducts under a uniform transverse magnetic field

In this study, the three-dimensional liquid metal (LM) magnetohydrodynamic (MHD) flows in a u-shaped duct under a uniform magnetic field applied perpendicular to the flow plane are numerically analyzed with the use of commercial software CFX. The u-shaped duct system is made up of an inflow channal, an outflow channal and a connecting channel located between the inflow channel and outflow channel. In the current study, the effects of the length of the connecting channel and the conductance ratio on the flow characteristics are investigated. Higher velocities are observed in the side layers of the inflow, outflow and connecting channels, forming “M-shaped” velocity profiles. In addition, the velocity recirculations are created in the inner regions of the two right-angle segments just after turning due to the inertial force therein, yielding complicated distributions of the current and electric potential, and found are the diverging and converging of the velocity component parallel to the magnetic field when fluids are turning. The results show that the pressure drop is in close relationship with the conductance ratio and the length of the connecting channel. The characteristics of the fluid velocity, current, electric potential and pressure gradient of LM MHD flows in a u-shaped duct are examined in detail.

On the modelling of water column oscillations in U-OWC energy harvesters

The objective of this paper is to investigate the reliability of the mathematical model used for estimating the U-Oscillating Water Column (U-OWC) response in the time domain. This device is a wave energy harvester belonging to the family of Oscillating Water Columns. It comprises a water column, an air chamber with a self-rectifying turbine and a vertical U-shaped duct connecting the water column to the open wave field. The model used for describing the water column oscillations is based on the application of the unsteady Bernoulli equation for a real fluid and involves the determination of energy losses occurring over the entire duct length. The reliability of this mathematical model is assessed against experimental data. For this purpose, a U-OWC experimental model is installed in a benign natural basin, the Natural Ocean Engineering Laboratory NOEL. The experimental model is exposed to waves stochastically characterized. Thus, its response is described by relevant statistics. The paper shows that the current mathematical model may yield relevant overestimations of the system response. This relates to the approximate representation of the head losses, which are based on the Darcy-Weisbach formula traditionally employed in the analyses of steady flows. For overcoming these limitations, the paper proposes the utilization of the instantaneous acceleration-based model, which includes unsteady head losses proportional to the water column inertia. In this context, the model shows a general good agreement with the experimental measurements and demonstrate a superior capability of predicting the U-OWC response in all the processed records.

A numerical investigation of flow and heat transfer in rotating U-shaped square ducts

Three-dimensional incompressible viscous flow and heat transfer in a rotating U-shaped duct with a square cross-section are studied numerically. The governing equations including full Navier–Stokes and energy equations are solved using a finite volume method based on the SIMPLER algorithm in an orthogonal uniform grid. The effects of wall curvature and rotation on the flow field and heat transfer considering different directions of rotation are presented in detail. The results show that in the bend region, the centrifugal forces due to curvature are significant, performing an intensive outward secondary flows resulting in an increase of heat transfer on the outer wall. But in the straight parts of the duct, secondary flows due to Coriolis forces are dominant. The comparison of numerical results at different rotation angles indicates that the maximum heat transfer rate occurs when the duct rotates about an axis parallel to the axis of duct curvature, at which both the Coriolis and the centrifugal forces intensify each other, while the minimum heat transfer rate takes place in the negative diagonal axis of rotation with angles of α = 180 and β = 135 .

A Numerical Study of Flow and Heat Transfer in a Smooth and Ribbed U-Duct With and Without Rotation

Computations were performed to study the three-dimensional flow and heat transfer in a U-shaped duct of square cross section under rotating and non-rotating conditions. The parameters investigated were two rotation numbers (0, 0.24) and smooth versus ribbed walls at a Reynolds number of 25,000, a density ratio of 0.13, and an inlet Mach number of 0.05. Results are presented for streamlines, velocity vector fields, and contours of Mach number, pressure, temperature, and Nusselt numbers. These results show how fluid flow in a U-duct evolves from a unidirectional one to one with convoluted secondary flows because of Coriolis force, centrifugal buoyancy, staggered inclined ribs, and a 180 deg bend. These results also show how the nature of the fluid flow affects surface heat transfer. The computations are based on the ensemble-averaged conservation equations of mass, momentum (compressible Navier-Stokes), and energy closed by the low Reynolds number SST turbulence model. Solutions were generated by a cell-centered finite-volume method that uses second-order flux-difference splitting and a diagonalized alternating-direction implicit scheme with local time stepping and V-cycle multigrid.