Effect Of Air Compressibility Research Articles

Experimental Study on the Spring-like Effect on the Hydrodynamic Performance of an Oscillating Water Column Wave Energy Converter

The oscillating water column (OWC) wave energy converter has demonstrated significant potential for converting ocean wave energy. The spring-like effect of air compressibility can significantly affect the hydrodynamic behavior of the device, but it has rarely been investigated through experimental studies. In this study, an experimental test on a model-scaled OWC device was carried out in a wave flume using a series of regular and irregular waves. The spring-like effect was taken into account by the combination of the air chamber with an additional air reservoir of appropriate volume, where the total volume was scaled according to the square of the Froude scale. The hydrodynamic performance was compared with the results obtained without considering the spring-like effect. A phase difference between the air pressure and airflow rate was observed when employing the additional air reservoir. The amplitudes of free surface elevation and airflow rate increased, while the air pressure was reduced when the spring-like effect was considered. The results demonstrate that failure to consider the spring-like effect can lead to overestimation of the hydrodynamic efficiencies, and the errors were mainly affected by the incident wave frequency.

Non-linear turbine selection for an OWC wave energy converter

Turbine-induced damping is a critical parameter affecting the performance of oscillating water column (OWC) wave energy converters. Therefore, selecting the appropriate turbine-chamber combination is an essential step in their design. In this work, a methodology is developed to determine the optimum turbine diameter for a given chamber, i.e., the diameter which maximizes the pneumatic energy capture of the chamber under an extensive set of wave conditions—covering virtually the entire range of wave conditions relevant for wave energy exploitation. This novel approach combines physical and numerical modelling with dimensional analysis. Importantly, it results in a turbine diameter that enables the turbine to operate at maximum efficiency. Through the different modelling techniques applied, the methodology accounts for air compressibility effects and other non-linear effects. It is applicable to non-linear turbines, with the study focusing on the promising biradial turbine. The results indicate that using the proposed methodology to select the turbine diameter significantly improves the capture-width ratio of the OWC, with increases of up to 100% for individual sea states. Two turbine diameters were identified as appropriate for the proposed OWC chamber design, 1.1 m for low-energy sites, and 1.4 m for mid- and high-energy sites.

Statistical assessment of the wave loads at walls through two-phase CFD modeling of the effects of air compressibility

The modeling of wave impacts against coastal structures requires the analysis of hundreds or thousands of waves to be statistically meaningful. Long irregular wave attacks, when affordable, can be performed experimentally, but may be inadequate to track the air entrapment and account for air compressibility, which, instead, plays a key role in the wave impacts. On the other hand, long simulations are generally avoided in numerical modeling for computational effort and numerical stability reasons, even more so when two-phase flows and air compressibility are involved. In such a context, this paper presents, for the first time, the application of a plug-in suite developed in the OpenFOAM® environment to the representation of long time series of irregular waves impacting against coastal defenses while solving two compressible fluids. To this purpose, such a plug-in compressible suite was applied to reproduce recent 2D experiments of wave overtopping and wave impacts at smooth dikes with crown walls. The numerical stability of the compressible solver and its adequacy to accurately reproduce the wave reflection and the wave overtopping are first verified by comparing the numerical results with the laboratory tests. Second, the improved representation of the wave pressures and wave forces at the walls obtained with the plug-in compressible suite is shown by comparing its results with the corresponding ones obtained with the incompressible solver. Specifically, the plug-in suite—accounting for the effects of the air compressibility during the impact events—outperforms the incompressible native solver in the capture of the pressure peaks, in the reproduction of the time–pressure trace, and in the statistical analysis of the pressure distribution along the crown wall.

Experimental and numerical investigation on effects of air compressibility on dynamic performance of the damaged ship

This paper presents a detailed test procedure, including the test facilities, the design of the ship model and the releasing mechanism of the damaged opening. The aim is to analyze the effects of air compressibility on the flooding process and motion responses of the damaged ship. To assist in the explanation of the test results, necessary numerical simulations are performed. Through carrying out series of model tests, it was found that the limited ventilation level not only delays the flooding process but also affects the dynamic responses of the damaged ship. In addition, the test results verify the assumption of 10% full ventilation publihsed by the International Maritime Organization (IMO) while analyzing the applicability of this assumption to the single flooding compartment. The results also show that ensuring a proper ventilation level in the flooding compartments can mitigate gas-liquid interactions and thus improve the dynamic response of the damaged ship. Based on the research results in this paper, an 8% ventilation ratio can be preliminarily used as a reference standard for similarventilation designs. Finally, the obtained experimental data in this paper can be used as a benchmark for future reliability analysis of the numerical methods.

Implications of spring-like air compressibility effects in floating coaxial-duct OWCs: Experimental and numerical investigation

The paper analyses the spring-like air compressibility effect in coaxial-duct oscillating-water-column (CD-OWC) wave energy converters (WECs). This is accomplished through a novel non-linear time-domain model for OWC WECs implemented in the object-oriented language Modelica. Good agreement was observed between numerical and physical model testing results. The air chamber volume significantly affected the spring-like air compressibility effect and the converter performance. Another significant factor is the damping level of the power take-off system. Both fixed and floating CD-OWC versions were numerically investigated, focusing on the compressibility effect. Differences were found mainly due to the additional degrees of freedom in floating configurations. In general, numerical and experimental results showed that air compressibility might positively or negatively affect device power performance in regular waves, depending on whether the frequency is within or out of an interval defined by critical frequencies. Some particular points were observed and categorised as Equicompressum Nullum and Equicompressum critical points, representing different characteristics. Knowledge of these critical values might be important in OWC control. Finally, some applications of the CD-OWC concept are discussed, including considerations on the power output level provided by biradial and Wells air turbines.

Study of air compressibility effects on the aerodynamic performance of the IEA-15 MW offshore wind turbine

With the gradual upscaling of offshore wind turbines, the blade size becomes large and the blade tip velocity reaches almost a Mach number of 0.3, the limit of the incompressible flow assumption.To simulate wind turbine flows, an incompressible solver was often a good choice as it gave good predictions and the Mach number of the wind turbine flows were small. When the Mach number is increased to be close to 0.3, the validity of incompressible flow may be questionable and thus needs to be checked. In this paper, an accurate wind turbine flow solution method is proposed to study the compressibility effects. First, the National Renewable Energy Laboratory (NREL) Phase VI wind turbine is used as an example to verify the method at a small Mach number. Through a comparative analysis between the compressible, the incompressible numerical results, and experimental data, the proposed method is found with a good simulation accuracy. Next, the flow fields of the large wind turbine IEA-15 MW, under different wind conditions are simulated. The local Mach number, density and pressure coefficient at different blade cross-sections of the compressible results are analyzed and compared to its incompressible ones, and the influences of compressibility on the thrust, torque and power of the wind turbine are quantified. The results show that at the rated wind speed, the thrust of the compressible flow simulation is 1.40 % higher than that of the incompressible one, and the torque of the compressible flow simulation is nearly 11.07 % higher than that of the incompressible one. As a result, the compressibility effects are recommended to be added in future studies of large wind turbines.

Computations of pressure loads on an oscillating water column with experimental comparison for random waves

This paper presents numerical investigations with experimental comparisons of random wave induced loads on the external walls of a large scale Oscillating Water Column (OWC) device. The objective is to investigate the capability of a numerical model that uses the Reynolds Averaged Navier–Stokes (RANS) equations to accurately reproduce extreme wave loads on an OWC. Both incompresible and compresible approximations for the RANS equations are studied in this work with the aim of investigating the limitations of both approaches. After a first characterization of the random wave propagation processes at the numerical flume, wave loads are analyzed. An OWC power take-off (PTO) and in-chamber air compressibility effects are studied using regular waves. Next, an OWC facing irregular waves is studied and compared with experimental data, putting special emphasis on the extreme loads together with the stochastic nature of random waves. The simulation of extreme wave loads on an OWC is validated and the necessity of a compressible model for correct reproduction of an OWC in-chamber flow for certain operating conditions of the PTO is pointed out from the simulations. • Incompressible and compressible approximations for the RANS equations are used to study an oscillating water column aero-hydrodynamic behaviour when facing an irregular sea state. • Generation and propagation of random waves at the numerical flume. • Validation of extreme wave loads on an oscillating water column. • Necessity of a compressible model for correct reproduction of an oscillating water column in-chamber flow for certain operating conditions of the PTO.

Theoretically based correction to model test results of OWC wave energy converters to account for air compressibility effect

The oscillating-water-column (OWC) wave energy converter with air turbine has been object of extensive development. The spring-like effect of air compressibility in the chamber is related to the density–pressure relationship. It is known to significantly affect the power performance of the full-sized converter, and is rarely accounted for in model testing. A method is presented to correct results from physical model testing for effects of air compressibility. It combines linear theory in the frequency domain and hydrodynamic coefficients, together with air thermodynamics. A new concept of linear turbine simulator aerodynamically equivalent to the orifice simulator is introduced in the theory. Corrections for non-linear real-fluid effects may be accounted for from comparisons with data from wave tank testing. The method was validated in a case study involving published data from OWC model testing with regular waves of different periods and amplitudes in a wave flume. Theoretically based corrections were found to well predict the air compressibility effects upon power performance. Air compressibility may negatively or positively affect power conversion depending on whether the wave period is larger or smaller than a critical value accurately predicted by the theory.

Finite element simulations of free reed instrument operation

To facilitate research into the acoustical properties of free reed instruments, there is a need for a quick and inexpensive way to prototype new geometries. It is here that a computational model with easily adjustable parameters offers a substantial advantage over an experimental apparatus if it can sufficiently resolve the essential acoustic features. Simulations using the finite element method have been utilized to solve the fluid dynamics, pressure acoustics, structural mechanics, and the multi-physics couplings underpinning various free reed systems. These phenomena include attack transients and frequency-pressure dependence of Western free reeds, fluid-structure interaction of turbulent airflow in an Asian free reed, effects of air compressibility on reed vibration, and the acoustic impedance of a reed-driven khaen pipe. Results consistent with experimental and analytical models were obtained and methods of optimizing the computational cost while maintaining realism are proposed. The models fail, however, to capture the essential feature of free reed sound production via the periodic interruption of airflow through the reed plate. Ways for future research to achieve this benchmark are suggested. [Research supported by National Science Foundation, Grant NSF-REU-1950337.]

Ocean Energy Systems Wave Energy Modeling Task 10.4: Numerical Modeling of a Fixed Oscillating Water Column

This paper reports on an ongoing international effort to establish guidelines for numerical modeling of wave energy converters, initiated by the International Energy Agency Technology Collaboration Program for Ocean Energy Systems. Initial results for point absorbers were presented in previous work, and here we present results for a breakwater-mounted Oscillating Water Column (OWC) device. The experimental model is at scale 1:4 relative to a full-scale installation in a water depth of 12.8 m. The power-extracting air turbine is modeled by an orifice plate of 1–2% of the internal chamber surface area. Measurements of chamber surface elevation, air flow through the orifice, and pressure difference across the orifice are compared with numerical calculations using both weakly-nonlinear potential flow theory and computational fluid dynamics. Both compressible- and incompressible-flow models are considered, and the effects of air compressibility are found to have a significant influence on the motion of the internal chamber surface. Recommendations are made for reducing uncertainties in future experimental campaigns, which are critical to enable firm conclusions to be drawn about the relative accuracy of the numerical models. It is well-known that boundary element method solutions of the linear potential flow problem (e.g., WAMIT) are singular at infinite frequency when panels are placed directly on the free surface. This is problematic for time-domain solutions where the value of the added mass matrix at infinite frequency is critical, especially for OWC chambers, which are modeled by zero-mass elements on the free surface. A straightforward rational procedure is described to replace ad-hoc solutions to this problem that have been proposed in the literature.

Ways that Animal Wings Produce Sound

There are at least eight ways that wings potentially produce sound. Five mechanisms are aerodynamic sounds, created by airflow, and three are structural sound created by interactions of solid surfaces. Animal flight is low Mach (M), meaning all animals move at less than 30% of the speed of sound. Thus in aerodynamic mechanisms the effects of air compressibility can be ignored, except in mechanism #1. Mechanism #1 is trapped air, in which air approaches or exceeds Mach 1 as it escapes a constriction. This mechanism is hypothetical but likely. #2 is Gutin sound, the aerodynamic reaction to lift and drag. This mechanism is ubiquitous in flight, and generates low frequency sound such as the humming of hummingbirds or insect wing tones. #3 is turbulence-generated atonal whooshing sounds, which are also widespread in animal flight. #4 are whistles, tonal sounds generated by geometry-induced flow feedback. This mechanism is hypothetical. #5 is aeroelastic flutter, sound generated by elasticity-induced feedback that is usually but not always tonal. This is widespread in birds (feathers are predisposed to flutter) but apparently not bats or insects. Mechanism #6 is rubbing sound (including stridulation), created when bird feathers or insect wings slide past each other. Atonal rubbing sounds are widespread in bird flight and insects; tonal stridulation is widespread in insects. #7 is percussion, created when two stiff elements collide and vibrate, and is present in some birds and insects. Mechanism #8 are tymbals and other bistable conformations. These are stiff elements that snap back and forth between two conformations, producing impulsive, atonal sound. Tymbals are widespread in insects but not birds or bats; insect cuticle appears predisposed to form tymbals. There are few examples of bat wing sounds: are bats intrinsically quiet, or just under-studied? These mechanisms, especially Gutin sound, whooshes, and rubbing (#2, #3, and #6) are prominent cues in ordinary flight of all flying animals, and are the 'acoustic substrate' available to be converted from an adventitious sound (cue) into a communication signal. For instance, wing sounds have many times evolved into signals that are incorporated into courtship displays. Conversely, these are the sounds selected to be suppressed if quiet flight is selected for. The physical mechanisms that underlie animal sounds provides context for understanding the ways in which signals and cues may evolve.

Comparison of impulsive wave forces on a semi-submerged platform deck, with and without columns and considering air compressibility effects, under regular wave actions

The decks of oil-extracting platforms could be damaged by great wave impact loads under harsh ocean conditions. Our understanding of the pressure distribution characteristics under the deck, especially those influenced by columns, is incomplete. A series of experiments are carried out to study the spatial and instantaneous distribution of the impact pressure generated by regular waves acting on the deck, and two cases are considered: a flat plate (FP for short) and a plate with four vertical columns (CP for short). Three-dimensional numerical simulations using a modified wave generating tool are conducted and compared to experimental data. Both compressible and incompressible flow solvers are applied to further quantitatively analyse the effect of air compressibility on impacting forces. The results show marked changes in both the pressure distribution scatter under the deck and the magnitude of the impact pressure when comparing the two cases. It has been demonstrated that run-up along the columns can cause intense localized pressure on the deck, and a close relationship exists between this increased pressure and fluid velocity. Additionally, the preliminary results showed that the phase compressibility increases the peak pressure compared with that of an incompressible solver, and the former matches the experimental measurements better.

Numerical analysis of the influence of air compressibility effects on an oscillating water column wave energy converter chamber

The most studied device used for extracting wave energy is the Oscillating Water Column (OWC). In general, numerical simulations of these cases by means of models based on Reynolds Averaged Navier-Stokes equations adopt the Volume of Fluid method to deal with the free surface flow which is considered incompressible in both water and air. The aim of this study is to investigate the influence of the compressibility effect on the air inside the OWC chamber by the FLUENT® numerical model. A methodology is implemented, taking into account both water and air flows incompressible, but, at every instant, a pressure condition is imposed on the top boundary of the chamber to consider the compressibility effect. This pressure condition is based on an analytical equation that considers the isentropic transformation of the air and effects of Wells and impulse turbines. Results of compressible and incompressible numerical models are compared. The amplification factor, the root mean square of air pressure inside the chamber and OWC efficiency in relation to incident wave period, wave height and turbine characteristic relation are analyzed. Results show that air compressibility effects can diminish the predicted OWC efficiency up to about 20% in both Wells and impulse turbines.

Modelling Air Compressibility in OWC Devices with Deformable Air Chambers

Air compressibility effects play an important role in large-scale Oscillating Water Column (OWC) wave energy converters. Air compressibility is however not scalable with Froude similarity law. An existing scaling method enables correctly reproducing the air compressibility at the model scale, but its implementation is effortful and becomes cumbersome for floating devices and tests at relatively large scales (1/15th–1/2th). Air compressibility is therefore commonly ignored in model-scale tank testing of conventional OWC devices, which can lead to substantially unrealistic results on the device performance relative to the full-scale device. In the case of the Tupperwave device, which is a closed circuit OWC device, correctly modelling air compressibility during tank testing is however essential because the device relies on air compressibility to work. In this paper, a new method for modelling air compressibility at the model scale is presented. The method uses variable volume chambers, which mimic air compressibility by storing energy under the form of strain energy. This method reduces the difficulties of implementation and enhances the application of the existing method to larger scales. Various applications to this method are identified and described, including the presentation of a novel OWC concept.

The spring-like air compressibility effect in oscillating-water-column wave energy converters: Review and analyses

The oscillating-water-column (OWC) wave energy converter with air turbine has been object of extensive research and development effort, including the deployment of floating and fixed-structure full-sized prototypes into the sea. It consists of a hollow (fixed or floating) structure, open to the sea below the water surface. Wave action alternately compresses and decompresses the air trapped above the inner water free-surface in a chamber, which forces air to flow through a turbine coupled to an electrical generator. The spring-like effect of air compressibility in the chamber is related to the density-pressure relationship. It is known to significantly affect the power performance of the full-sized converter, and is rarely accounted for in theoretical modelling, and even more rarely in physical model testing at reduced scale, as appears from the literature review. Three theoretical models of increasing complexity are analysed and compared: (i) the incompressible air model; (ii) the isentropic process model; (iii) and the (more difficult and rarely adopted) adiabatic non-isentropic process model in which losses due to the imperfectly efficient turbine are accounted for. The air is assumed as a perfect gas. The hydrodynamic modelling of wave energy absorption is based on linear water wave theory. The validity of the various simplifying assumptions, especially in the aero-thermodynamic domain, is illustrated by a case study with numerical results for a fixed-structure OWC equipped with a Wells turbine. Results are shown for regular and irregular waves, and for a theoretical simulation of model testing in wave tank at small scale.

Site-specific wave energy conversion performance of an oscillating water column device

The energy conversion performance of oscillating water column (OWC) wave energy converters at a specific site is often studied by means of analytical models. Based on linear theory, these models lose accuracy when viscous losses and turbulence become significant—more generally, when nonlinear effects play a role, as they often do in real operating conditions. In this work we apply a novel methodology based on a combination of numerical modelling and laboratory tests to investigate OWC performance without these shortcomings. First, high-resolution wave resource characterisation matrices are obtained by means of numerical modelling. Second, the resource matrices are combined with the OWC efficiency matrices obtained through laboratory tests and, importantly, including the effects of turbine-induced damping and air compressibility—usually disregarded in small-scale laboratory tests, but relevant for full-size (prototype) devices. The combined matrices thus obtained express, through a wave height-period distribution, the energy captured by the OWC for different values of the damping coefficient. On this basis, developers can select the most appropriate value of turbine-induced damping for a given site, based on performance values. The implementation of the novel methodology is illustrated through a case study in Galicia (NW Spain), in which three deployment sites are considered. We find that the turbine-induced damping must be matched to the wave climate of the site for an OWC device to achieve high performance; indeed, changes in damping cause variations in the total annual energy captured of up to 11%, which increase to 25% for specific sea states.

Numerical study of air compressibility effects in breaking wave impacts using a CIP-based model

Abatract The prediction of breaking wave loads is challenging in ocean engineering due to air entrapment and compressibility. In order to investigate air compressibility effects in different modes of breaking wave impact, a two-phase compressible flow model is proposed. A high-order finite difference method, Constrained Interpolation Profile (CIP) method is employed to discretize the Navier-Stokes equations, and the interface is captured by Tangent of Hyperbola for Interface Capturing (THINC) scheme. The present compressible model is validated against several tests: 1-D gravity-induced liquid piston motion, idealized free drop of water patch impact and dam-break experiments. Special attention of this study is paid to breaking wave impacts on a vertical wall in a liquid sloshing tank. For air entrapped impact, the results of CIP-based compressible model are in good agreement with experimental data. It is shown that the air compressibility leads to high-frequency oscillation of the impact pressure, which cannot be reproduced by the incompressible model. It is found that the oscillation of the impact pressure is synchronized with the compression and expansion of the entrapped air pocket, and the oscillation frequency is related to the size of air pocket. For air-water mixing impact, the oscillation of the pressure may be more intense.

A dual-functional wave-power plant for wave-energy extraction and shore protection: A wave-flume study

The fundamental roadblock toward commercial-scale wave power operations is cost. The main objective of this work was to address the cost challenge facing wave energy commercialization through cost-sharing with pile breakwaters to be built for shore protection. This was achieved in this study through a dual-functional wave-power plant for generation of wave-power electricity and protection against coastal erosion for sustainable coastal development. The dual-functional wave-power plant was formed by integrating oscillating-water-column (OWC) devices into a pile breakwater, with each pile being an OWC-pile equipped with a power take-off device. The power extraction efficiency and hydrodynamic characteristics of the dual-functional wave-power plant were measured in a wave flume under various wave conditions. An orifice was used at the top of the pneumatic chamber of each OWC-pile to simulate the power take-off device. To evaluate the performance of the power plant in wave power extraction and shore protection, the surface elevation and pressure inside the OWC chamber, as well as the scattered waves, were measured. It was found that comparing to a standalone OWC-pile device with an identical design and geometric characteristics, an OWC-pile in the dual-functional wave-power plant could achieve significantly larger power-extraction efficiency. Comparing to a pile breakwater with the same dimensions, the wave transmission and reflection of the dual-functional wave-power plant were both weaker, especially the wave reflection, which is beneficial for structure safety and shore protection. Based on the Froude’s law of similarity and an estimation of the effect of air compressibility at full scale, an evaluation of the performance of a dual-functional wave-power plant at full scale was also provided. The findings of this work promote close collaboration between wave-energy utilization community and the shore-protection community for commercial-scale deployment of wave energy converters and contribute to making wave energy economically competitive.

Analytical and Experimental Investigation on Sound Transmission of Double Thin Plates with Magnetic Negative Stiffness

Transmission loss of acoustic metamaterials (AM) made of double thin plates with magnetic (negative) stiffness was analyzed using theory, finite element analysis and experimental techniques. The theoretical formulation was done using a rectangular duct below the first cut off frequency, the model is then validated against finite element method and experiment. Two cubic magnets were used, their interaction force and the resulted magnetic stiffness were calculated. The sound transmission loss (STL) of the structure is calculated for plane wave condition, the addition of magnetic mass shifts STL peaks to the lower frequency compared to a structure without mass. The slight increase in STL for small negative stiffness in experiment is not enough to cancel the effect of air compressibility. However, a significant enhancement could be expected if negative stiffness can be made large enough in the double thin plates. The developed AM can be employed as a prospective sound engineering control at low frequency.

Scale effects in physical modelling of a generalized OWC

Physical modelling is extensively applied in the study of Oscillating Water Column (OWC) devices since it furnishes a reliable evaluation of nonlinear effects, as those induced by the interaction between surface waves and air inside the pneumatic chamber. In this paper, a small scale generalized device is compared to a similar large scale model under random waves, in order to evaluate the main scaling issues on (i) hydrodynamics of the water column, (ii) wave reflection and (iii) loadings at the outer front wall. The small scale model tested allowed to investigate the effects of air compressibility as well.Natural oscillation period is analysed first, which is obtained from the delay between the oscillating motions inside the device and those outer the front wall. Such a period increases in the small scale with the height of the chamber due to the “spring” effect of the air compressibility. Furthermore, the downscale of the OWC causes a reduction of the reflection coefficient, which is in part recovered by increasing the height of the device. Extreme loadings on the front wall can be underestimated by the small scale but safe conditions are always achieved for the high-chamber model.