Naumov I.V.   Litvinov I.V.   Okulov V.L.   Mikkelsen R.F.   Sorensen J.N.  

Development of far wake behind solid disk and rotating blades

Reporter: Naumov I.V.

   In the present work, we have studied the development of the very far wake behind the solid disk or rotor modeled wind turbine with a horizontal axis. These optical measurements were carried out at the water flume at Technical University of Denmark (DTU).  The initial flume flow was subject to a low turbulence level (less 2%), limiting the influ-ence of external disturbances on the development of the inherent wake instability. The flow regimes with different free flow speeds (0.2, - 0.6 m/s) was investigated. The Reynolds numbers based on disk/rotor diameter and free stream was varied in the range 80.000 <Re< 230.000. As working fluid, tap water at a temperature of 20 ºС was used. The disk or rotor were placed in the middle of the flume and was subject to uniform flow conditions.

   The one- and three-bladed rotors have a diameter 2R = 0.376 m with the blades of length 0.159 m and solid disk has a diameter D = 0.3 m. The rotor blades designed using Glauert’s optimum theory at a tip speed ratio λ =5. The wake development has been studied in the range of tip speed ratios from 3 to 12, and at different cross-sections from 10 to 100 radii downstream from the rotor or disk. By using regression techniques to fit the velocity profiles it was possible to obtain accurate velocity deficits and estimate length scales of the wake attenuation both disk and rotor. 

  LDA (laser Doppler anemometry) and PIV (particle image velocimetry) experiments were carried out to study the development of mean velocity profiles and root-mean-square profiles. Each time history in a measurement point was recorded in period of 60 s, corre-sponding to about 20-50 times of the rotor rotation. Everywhere in the far wakes, the value of the Strouhal number is equal to 0.23 for three-blade rotor and 0.15 for solid disk. This Strouhal number is in good agreement with the well-known constants that usually characterize the oscillation in wakes behind bluff bodies. The data indicate that Strouhal number of this precession is independent of the rotor angular speed. 

It has been found experimentally that velocity deficit for three-blade rotor decreases from 20 % of the free stream velocity at 20R downstream to 1-2 % at 60R behind rotor. The velocity deficit becomes insignificant after 40R and at 60R the velocity profile restores to the initial free flow. After 60R the value of pulsations, as well as the velocity deficit, completely decreased to the pulsation level of the free stream velocity and did not exceed 2 %. For solid disk, the velocity deficit decreases from 10 % of the free stream velocity at 10R to 1-2% at 40R downstream for disk. However, the value of pulsations decreases to the pulsation level of the free stream velocity only behind 60R.

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