But === Define the characteristics necessary for the dynamic study of a flowing structure. By choosing one of the key factors, the user specifies the type of "flow structure" configuration studied: bundle of tubes under transverse or axial flow, control cluster, co-axial shells. Depending on the type of configuration, the data provided makes it possible to carry out downstream: * a fluid/structure coupling study: estimation of fluid-elastic force coefficients and calculation of new modal parameters of the structure using the operator CALC_FLUI_STRU [:external:ref:`U4.66.02 `], * the projection on a modal basis under flow of one or more turbulent excitations defined by their spectral density (s): operators DEFI_SPEC_TURB [:ref:`U4.44.31 `] and PROJ_SPEC_BASE [:ref:`U4.63.14 `]. It is then possible to calculate the response of the structure to the turbulent excitation, in time and in frequency. The "DEFI_FLUI_STRU" operator produces a type_flui_stru concept intended to be used downstream by operators FONC_FLUI_STRU [:ref:`U4.35.02 `], CALC_FLUI_STRU [:external:ref:`U4.66.02 `] or, indirectly, PROJ_SPEC_BASE [:ref:`U4.63.14 `].