therein to fix the vane 14 in its longitudinal direction the intermediate part 19 of the connection bar 17 is of a thickness that is less than the width of the side opening 26 of the journaling groove 25 to allow the vane 14 to pivot via its connection bar 17 for this purpose the journaling groove 25 has opposite curved concave pivot surfaces 27 with which the circular pivot element 16 of the vane 14 is in slideable co operation to form an axis of pivot 28 that is at right angle to the machine direction the height of the assembly bar 23 as seen at right angle to the grooved surface 29 of the anchoring element 10 is chosen so that the distance a of the axis of pivot at right angle to the anchoring element 10 is sufficiently great to locate the vane 14 and its connection bar 17 at a sufficient distance from the discharge opening 13 of the turbulence channel 6 without detrimentally affecting the flow of stock which is deflected after the discharge opening 13 of the turbulence channel 6 the width of the assembly bar 23 is chosen so that it acquires sufficient support surface against the surface 29 of the anchoring element 10 to absorb the torque forces arising in the assembly bar 23 the assembly bar 23.