the invention described below applies particularly to hydraulic machines of the francis turbine type . since these machines are known , they will not be described in detail in the present description . the invention also applies to other types of hydraulic machines in which problems of the formation of eddy , reduced - pressure or cavitation zones occur . in the description , the terms “ upstream ” and “ downstream ” are defined with respect to the direction of flow of the main flow e traversing the hydraulic machine . with reference to fig1 , a member of a hydraulic machine of the blade profile 1 type of a turbine is described . such a blade profile 1 is for example a fixed guide vane , a wicket gate or a turbine blade . the blade profile 1 shown in fig1 is a fixed guide vane . the function of fixed guide vanes and wicket gates is to guide a main flow e which traverses the rotor 4 of the turbine sweeping against the blades 6 , which rotates the rotor 4 about a vertical axis z - z ′. the blade profile 1 has an upstream end 8 and a downstream end 10 connected together by side walls 12 swept by the main flow e . whether it be a fixed guide vane , a wicket gate or a turbine blade , an eddy and / or reduced - pressure zone 14 is created in the vicinity of the downstream end 10 , particularly immediately downstream of the downstream end 10 , which can cause vibrations in the hydraulic machine . the blade profile 1 comprises at least one duct 16 extending inside the profile between an inlet opening 18 and an outlet opening 20 . the inlet opening 18 is placed in the vicinity of the upstream end 8 and emerges for example in one of the side walls 12 of the blade profile 1 . a portion e 1 of the main flow e sweeping the side walls 12 is therefore tapped off in the duct 16 via the inlet opening 18 and is conveyed to the outlet opening 20 . according to another embodiment , the duct 16 taps the flow e 1 on the outside of the blade profile 1 and conveys this flow to the outlet opening 20 . the outlet opening 20 emerges in the downstream end 10 . therefore , the tapped flow e 1 from the main flow e travels in the duct 16 and is injected into the zone 14 via the outlet opening 20 . the effect of this is to modify the properties of the flow e in the zone 14 and thereby to prevent the vibration phenomena . in the case of a blade 6 , there is also a problem of the creation of cavitations on the profile of the blades 6 of the rotor 4 in a zone in the vicinity of the inlet edges or upstream end 8 and / or of the outlet edges or downstream end 10 of the blades . in order to alleviate this drawback , the blade 6 , shown in fig2 , comprises other ducts ( not shown ) extending inside the blade between an inlet opening 22 and an outlet opening 24 , 26 . the inlet openings 22 of the ducts are placed in the vicinity of the upstream end 8 of the blade 6 so as to tap off a flow from the main flow e upstream of the blade . the outlet openings 24 , 26 of the ducts are arranged to inject the tapped flow on the side walls of the blades 6 in the vicinity of the upstream end 8 and / or of the downstream end 10 of the blade 6 . the effect of the tapped and injected flow is to locally modify the main flow e and thereby to prevent the phenomena of forming cavitation on the profile of the blades . certain ducts therefore comprise an outlet opening 24 leading into a side wall of the blade 6 in the vicinity of the upstream end 8 in order to prevent the phenomena of forming cavitation on the blades in the vicinity of the upstream end 8 . other ducts comprise an outlet opening 26 opening into a side wall of the blade 6 in the vicinity of the downstream end 10 in order to prevent the phenomena of forming cavitation on the blades in the vicinity of the upstream end 10 . according to various embodiments , the inlet and outlet openings may be placed in series along the upstream end 8 and the downstream end 10 of the blade 6 in a direction which may be perpendicular to the direction of the main flow e , as shown by the outlet openings 24 of fig2 . according to a particularly advantageous embodiment , the outlet openings are arranged so as to open into the downstream end 10 of the blade 6 in the direction of the main flow e . the openings open , for example , into the base of the blade 6 . the injection of the flow tapped into the downstream end makes it possible to eliminate the eddy zone that is formed in the zone of turbulence of the blades 6 . the blades 6 of the rotor 4 are placed between a ceiling 28 and a belt 30 . according to one embodiment , the phenomena of cavitation on the blades may also be prevented by openings 31 made in the ceiling 28 opposite the blades 6 , as shown in fig3 . these openings 31 communicate with the outlet openings 24 , 26 and / or with the outlet openings 20 opening into the downstream end 10 by means of channels not shown . in this embodiment , a flow e 2 is tapped off from the main flow e and passes through the spaces between the fixed part of the turbine and the moving blades 6 . the tapped flow e 2 is tapped off in the annular space 34 situated above the ceiling 28 . this flow e 2 enters the openings 31 and is then guided to the outlet openings 24 , 26 and / or 20 . in addition to the cavitation phenomena on the blades , there may also be phenomena of forming a vortex in the space 33 between the blades 6 . these phenomena may be alleviated by means of ducts , the inlet and outlet orifices of which are placed between the upstream and downstream ends of the blades and open into the space 33 between the blades . according to one embodiment , the problem of forming a vortex between the blades 6 is solved by means of orifices 32 made in the ceiling 28 , as shown in fig2 . in this embodiment , the flow e 2 tapped off upstream of the blades 6 in the annular space 34 travels into the openings 32 and supplies the spaces 33 between the blades 6 , as shown in fig3 . the openings 32 are distributed in the ceiling 28 facing the spaces 33 separating the blades 6 . therefore , the tapped flow e 2 is injected between the blades 6 and modifies the properties of the flow e in order to prevent the phenomena of forming a vortex between the blades 6 . as a variant , instead of or in addition to travelling via the ceiling 28 , the tapped flow e 2 can travel through the belt 30 by means of openings ( not shown ) made in the latter . according to an embodiment that can be applied to all the injection means described above , the injection means comprise a valve 72 placed in the path of the tapped flow , as shown in fig2 and 5 . the valve 72 can be moved between an open position in which it allows the tapped flow to pass and a closed position in which it prevents the passage of the tapped flow . the valve 72 is for example placed in the vicinity of each inlet opening of the injection means and makes it possible manually or automatically to control the injection of the tapped flow . in the case of the francis turbine , the valve 72 is provided in the vicinity of each opening 32 arranged in the ceiling 28 . the movement of the valve 72 is controlled by control means ( not shown ) which are mechanical or electric in a manner known per se . therefore , during operating conditions of the hydraulic machine causing the formation of eddy or reduced - pressure or cavitation zones , an automatic system or an operator of the machine switches the valve ( s ) to the open position which makes it possible to inject the tapped flow in the said zones and to prevent the formation of these zones , as described above . it should be noted that the tapped flow is not modified relative to the main flow e , that is to say that the water does not sustain any operation to modify its composition during the tapped flow .