The invention relates to the method of feeding the air, or fluid of lower density, into the duct (2) by means of tubes separated one from another in such a way that there should be water, or liquid fluid, between them, so that this bathes at least part of its walls. In other words, similar to the stalks of a bouquet submerged in water, and at whose ends, instead of flowers, air bubbles exit, these being produced at the air inlet at the extremity of the stalk or tube. It relates more specifically to the method of feeding the lower-density fluid by means of tubes which at their base are grouped, close to each other or touching, and at their outlet extremities are separated, as if forming a bouquet, in such a way that the separation distance of the extremities of the tubes, between axes of symmetry, is approximately the diameter of the bubble.