A heat-radiating system, comprising: - a substrate configured to form a channel for a cooling fluid and to radiate heat by heat exchange between the substrate and the cooling fluid supplied during operation, - a cooling structure, and - a controller configured to operate the heat-radiating system and during operation to control a flow velocity (u) of the cooling fluid, wherein: - the cooling structure comprises a vortex flow generating portion on a surface of the substrate that is configured to be in contact with the cooling fluid during operation, - the vortex flow generating portion is constituted by a plurality of recesses that extend in a direction (β) intersecting a flow direction (α) of the cooling fluid and that are configured to cause a vortex flow depending on a flow condition of the cooling fluid, - a respective recess of the vortex flow generating portion has a depth, - the heat-radiating system has a characteristic length (d) which is defined according to the following expression d = 4 × A / L, with A denoting a minimum channel cross-sectional area of a channel cross-section perpendicular to the flow direction (α) of the cooling fluid and with L denoting a maximum wetted perimeter of the channel, and - the controller is configured to control operation of the heat-radiating system and the flow velocity (u) of the cooling fluid such that: - the depth of a recess and a laminar sub-layer thickness δ b near a wall surface satisfy the following relation: H > δ b = 63.5 / Re 7 / 8 × d, wherein Re is a Reynolds number, d is a characteristic length, and the Reynolds number is defined as Re = u × d / v, v is a kinematic viscosity of the cooling fluid, u is a flow velocity of the cooling fluid, and d is the characteristic length, and - operation of the heat-radiating system is controlled such that the flow condition of the cooling fluid falls within a range in which the following relation is satisfied: u / v ≤ 206 × d 1 / 7. - a substrate configured to form a channel for a cooling fluid and to radiate heat by heat exchange between the substrate and the cooling fluid supplied during operation, - a cooling structure, and - a controller configured to operate the heat-radiating system and during operation to control a flow velocity (u) of the cooling fluid, - the cooling structure comprises a vortex flow generating portion on a surface of the substrate that is configured to be in contact with the cooling fluid during operation, - the vortex flow generating portion is constituted by a plurality of recesses that extend in a direction (β) intersecting a flow direction (α) of the cooling fluid and that are configured to cause a vortex flow depending on a flow condition of the cooling fluid, - a respective recess of the vortex flow generating portion has a depth, - the heat-radiating system has a characteristic length (d) which is defined according to the following expression d = 4 × A / L, with A denoting a minimum channel cross-sectional area of a channel cross-section perpendicular to the flow direction (α) of the cooling fluid and with L denoting a maximum wetted perimeter of the channel, and - the controller is configured to control operation of the heat-radiating system and the flow velocity (u) of the cooling fluid such that: - the depth of a recess and a laminar sub-layer thickness δ b near a wall surface satisfy the following relation: H > δ b = 63.5 / Re 7 / 8 × d, wherein Re is a Reynolds number, d is a characteristic length, and the Reynolds number is defined as Re = u × d / v, v is a kinematic viscosity of the cooling fluid, u is a flow velocity of the cooling fluid, and d is the characteristic length, and - operation of the heat-radiating system is controlled such that the flow condition of the cooling fluid falls within a range in which the following relation is satisfied: u / v ≤ 206 × d 1 / 7. - the depth of a recess and a laminar sub-layer thickness δ b near a wall surface satisfy the following relation: H > δ b = 63.5 / Re 7 / 8 × d, wherein Re is a Reynolds number, d is a characteristic length, and the Reynolds number is defined as Re = u × d / v, v is a kinematic viscosity of the cooling fluid, u is a flow velocity of the cooling fluid, and d is the characteristic length, and - operation of the heat-radiating system is controlled such that the flow condition of the cooling fluid falls within a range in which the following relation is satisfied: u / v ≤ 206 × d 1 / 7.