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In this article, we are interested in flow downstream a horizontal axis wind turbine. First the experimental investigations are carried out by means of the PIV measurements. To obtain the flow field in a rotating frame of reference in which the rotor appears fixed, the phase-locked technique is applied. To ensure synchronization between the camera, the lasers and the angular position of the blade an optical sensor is used. At each turn of the rotor the sensor gives a signal, which permits to detect the vertical position of reference blade. This signal is introduced in a delay circuit so that image acquisition takes place according to desired azimuth angles. Explorations are carried out in planes with the angles 0, 30, 60 and 90deg. Due to the camera resolution and the laser power the dimension of one single window of examination is equal to 300 x 300 mm. Because of large dimensions of the flow field studied, each azimuthal plane is divided into 6 windows. Then for each window, the velocity field is obtained by averaging the results of 95 pairs of image. Next, the 3D velocity field is reconstituted by treating the results of images corresponding to the explored azimuth planes. These results are compared with RANS numerical simulation. Generally the numerical results are coherent with experiments, but some discrepancy concerning wind turbine power is revealed.