Tag Archives: Turbulence

Experimental study of the turbulence intensity effects on marine current turbines behaviour – Part I: One single turbine


Paul Mycek, Benoît Gaurier, Grégory Germain, Grégory Pinon, and Elie Rivoalen – Renewable Energy, February 2014

Abstract

The ambient turbulence intensity in the upstream flow plays a decisive role in the behaviour of horizontal axis marine current turbines. Experimental trials, run in the IFREMER flume tank in Boulogne-Sur-Mer (France) for two different turbulence intensity rates, namely 3% and 15%, are presented. They show, for the studied turbine configuration, that while the wake of the turbine is deeply influenced by the ambient turbulence conditions, its mean performances turn out to be slightly modified. The presented conclusions are crucial in the view of implanting second generation turbines arrays. In addition, complete and detailed data sets (wake profiles and performance graphs) are made available to the scientific community in order to encourage further comparisons.

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Experimental study of the turbulence intensity effects on marine current turbines behaviour – Part II: Two interacting turbines


Paul Mycek, Benoît Gaurier, Grégory Germain, Grégory Pinon, and Elie Rivoalen – Renewable Energy, February 2014

Abstract

The future implantation of second generation marine current turbine arrays depends on the understanding of the negative interaction effects that exist between turbines in close proximity. This is especially the case when the turbines are axially aligned one behind another in the flow. In order to highlight these interaction effects, experiments were performed in a flume tank on 3-bladed 1/30th scale prototypes of horizontal axis turbines. Continue reading

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Turbulent inflow characteristics for hydrokinetic energy conversion in rivers


V.S. Neary, B. Gunawan, and D.C. Sale – Renewable and Sustainable Energy Reviews, October 2013

Abstract

Marine and hydrokinetic technologies, which convert kinetic energy from currents in open-channel flows to electricity, require inflow characteristics (e.g. mean velocity and turbulence intensity profiles) for their siting, design, and evaluation. The present study reviews mean velocity and turbulence intensity profiles reported in the literature for open-channel flows to gain a better understanding of the range of current magnitudes and longitudinal turbulence intensities that these technologies may be exposed to. We compare 47 measured vertical profiles of mean current velocity and longitudinal turbulence intensity (normalized by the shear velocity) that have been reported for medium-large rivers, a large canal, and laboratory flumes with classical models developed for turbulent flat plate boundary layer flows. The comparison suggests that a power law (with exponent, 1/a=1/61/a=1/6) and a semi-theoretical exponential decay model can be used to provide first-order approximations of the mean velocity and turbulence intensity profiles in rivers suitable for current energy conversion. Over the design life of a current energy converter, these models can be applied to examine the effects of large spatiotemporal variations of river flow depth on inflow conditions acting over the energy capture area. Significant engineering implications on current energy converter structural loads, annual energy production, and cost of energy arise due to these spatiotemporal variations in the mean velocity, turbulence intensity, hydrodynamic force, and available power over the energy capture area.

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Characteristics of the turbulence in the flow at a tidal stream power site


I. A. Milne, R. N. Sharma, R. G. J. Flay and S. Bickerton – Phil. Trans. R. Soc. A, 2013

Abstract

This paper analyses a set of velocity time histories which were obtained at a fixed point in the bottom boundary layer of a tidal stream, 5 m from the seabed, and where the mean flow reached 2.5 m s−1. Considering two complete tidal cycles near spring tide, the streamwise turbulence intensity during non-slack flow was found to be approximately 12–13%, varying slightly between flood and ebb tides. The ratio of the streamwise turbulence intensity to that of the transverse and vertical intensities is typically 1 : 0.75 : 0.56, respectively. Velocity autospectra computed near maximum flood tidal flow conditions exhibit an f−2/3 inertial subrange and conform reasonably well to atmospheric turbulence spectral models. Local isotropy is observed between the streamwise and transverse spectra at reduced frequencies of f>0.5. The streamwise integral time scales and length scales of turbulence at maximum flow are approximately 6 s and 11–14 m, respectively, and exhibit a relatively large degree of scatter. They are also typically much greater in magnitude than the transverse and vertical components. The findings are intended to increase the levels of confidence within the tidal energy industry of the characteristics of the higher frequency components of the onset flow, and subsequently lead to more realistic performance and loading predictions.

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Navy oceanographers delve deeper in wave data to improve forecasts


W. Ramos – Consortium for Ocean Leadership, November, 2012

Around the globe, mariners and navies alike have long observed and included weather and sea states in navigational planning when plotting course or developing military strategy.  And although forecasting had become an integral function by the start of the 20th century, these predictions were often crude and qualitative.  For the United States Navy, the years 1941 through 1946 provided an unusual stimulus to ocean wave research, according to pioneer World War II oceanographer Charles Bates.  During this brief five-year period, theory, observation, and prediction of sea, swell, and surf made the greatest strides in their history.  As a result, the U.S. Navy has one of the most active and vital operational oceanography programs in the world.  Naval oceanography provides critical information to such combat disciplines as anti-submarine warfare, mine warfare and countermeasures, naval special warfare, amphibious operations, and ship transit planning.  Today, U.S. Naval Research Laboratory physicists at the Remote Sensing Division continue to improve the integrity of these forecasts by developing a means to include the effects of the amorphous near-surface phenomenon of turbulence generation by nonbreaking waves.

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Measurements of Turbulence at Two Tidal Energy Sites in Puget Sound, WA


J. Thomson, B. Polagye, V. Durgesh, and M. Richmond – IEEE J. Ocean. Eng., July, 2012

Abstract

Field measurements of turbulence are presented from two sites in Puget Sound, WA, that are proposed for electrical power generation using tidal current turbines. Time series data from multiple acoustic Doppler instruments are analyzed to obtain statistical measures of fluctuations in both the magnitude and direction of the tidal currents. The resulting turbulence intensities (i.e., the turbulent velocity fluctuations normalized by the deterministic tidal currents) are typically 10% at the hub heights (i.e., the relevant depth) of the proposed turbines. Length and time scales of the turbulence are also analyzed. Large-scale, anisotropic eddies dominate the turbulent kinetic energy (TKE) spectra, which may be the result of proximity to headlands at each site. At small scales, an isotropic turbulent cascade is observed and used to estimate the dissipation rate of TKE, which is shown to balance with shear production. Data quality and sampling parameters are discussed, with an emphasis on the removal of Doppler noise from turbulence statistics. The results are relevant to estimating the performance and fatigue of tidal turbines.

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