Research on broadband aerodynamic noise from wind turbine blades is becoming important in several countries. In this work, computer simulation of acoustic emissions from wind turbine blades are predicted using quasi empirical model for a three-bladed horizontal axis 3 MW turbine with blade length ~47 m. Sound power levels are investigated for source and receiver height of 80 m and 2 m above ground and located at a distance equal to total turbine height. The results are validated using existing experimental data for Siemens SWT-2.3 MW turbine having blade length of 47 m, as well as with 2.5 MW turbine. Aerofoil self-noise mechanisms are discussed in present work and results are demonstrated for wind speed of 8 m/s. Overall sound power levels for 3 MW turbine showed good agreements with the existing experiment data obtained for SWT-2.3 MW turbine. Noise map of single source sound power level, dBA of an isolated blade segment located at 75 %R for single blade is illustrated for wind speed of 8 m/s. The results demonstrated that most of the noise production occurred from outboard section of blade and for blade azimuth positions between 80° and 170°.
Trailing edge surface of aerofoil is an important source of broadband aerodynamic noise production. In this paper, three aerofoil self-noise mechanisms from turbulent boundary layer near trailing edge surface are studied. Numerical computations were performed for a three bladed 2 MW horizontal axis upwind turbine of blade length 37 m and source height of 80 m, for wind speeds of 8-15 m/s. A weighted 1/3rd octave band sound power levels (SPL) are evaluated for receiver located at distance of total turbine height and at 2 m above ground. The results obtained for sound power level using baseline models showed maximum values occurring between 300 Hz and 1 kHz region of spectrum. The trends for BPM model showed a reduction of ~2 dBA near 1 kHz region of spectrum at 10 m/s, but Grosveld’s and Lowson model were identical and agreed over the entire spectrum. The effect of rotational speed on sound power levels using three baseline models are illustrated at a wind speed of 8 m/s for 2 MW turbine. Results showed that for a change of ±10% rotor speed from the rated value, there is an increase of 2 to 6 dBA over the entire sound spectrum due to differences in blade tip speed.
An important aerodynamic noise source from lifting surface occurs from trailing edge of an aerofoil as found in wind turbine blades. In this work, semi-empirical method proposed by Brookes, Pope, Marcolini is applied to evaluate trailing edge bluntness vortex shedding noise source. For low Mach number flows (0.1884) and moderate to high chord Reynolds number, 4.73 × 105 - 3.35 × 106, change in sound power level was assessed for trailing edge thicknesses in terms of 0.1%, 0.5% and 1% chord lengths at wind speeds of 8 m/s, 10 m/s. For overall change of trailing edge thickness from 0.1% to 1% chord, an increase in noise levels up to 50 dB was found at low frequencies, while a decrease up to 30 dB was found between mid-band to high frequencies of spectra.
Natural gas hydrates have been investigated for past several years as a potential resource for commercially producing gas. The stable condition for the formation of gas hydrates sediments i.e., high pressure and low temperature generally occurs in deep-sea and permafrost regions. The methane gas extraction from the hydrate-bearing sediments was carried out by drilling and using various dissociation methods. The dissociation methods include thermal injection, depressurization, and chemical injection, which will cause a significant loss of solids in the pore space of geomaterials after extraction. The loss of solids will eventually reduce the reservoir strength, leading to borewell wall collapse, causing subsurface landslides (Collett et al., 2002). The behavior of gas hydrate sediments is governed by coupled Thermo-Hydro-Chemo-Mechanical (THMC) response during the gas extraction process. In this study, in order to understand the coupled behavioral response of hydrate rich sediments, a 2D hydrate reservoir is simulated (using a multi-phase numerical schema) and analysed under axis symmetric conditions. The initial results suggests that the rate of change of gas pressure near the well bore decreases with the increase in the duration of the extraction. Further, the maximum settlement occurs near the seabed level while the rate of settlement decreases with time. The maximum shear stress generally occurs near the well bore which results in associated maximum volumetric strains. Thus, the continuous gas extraction results in highly porous medium which is stabilized primarily due to the geomechanical changes.
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