The invention relates to the production of a composite material of a lobed structure (10) of a flow mixer which comprises a portion having a plurality of lobes (17, 19). For this purpose, a fibrous preform containing a resin will be produced in a simple geometric configuration of developable revolution. This is an intermediate state. By exploiting the thermoformable or thermosetting nature of the resin, the geometry of this intermediate preform is modified to deform it with limited offsets towards the final geometry.