METHOD AND SYSTEM FOR DETERMINING FLOW PROPERTIES OF A FLUID FLOWING ALONG A SURFACE

The present invention provides a method (M) for determining flow properties of a fluid flowing along a surface (2), the method (M) comprising: modulating (M1) the current through a heating element (5) of a thermal flow sensor (3) located on the surface (2) with at least one modulation frequency (ω), thereby emitting a thermal wave (7) into the fluid flowing along the surface having a penetration depth (L) inversely proportional to the modulation frequency (ω); measuring (M2) an amplitude (U3ω) of a voltage (U) across the heating element (5) at the third harmonic (3ω) of the at least one modulation frequency (ω), the voltage (U) depending on the thermal conductivity and the volumetric heat capacity of the fluid flowing along the surface (2); determining (M3) a flow velocity (VL) at the penetration depth (L) from the attenuation value of the determined amplitude (U3ω) of the voltage (U); and estimating (M4) whether the flow at the penetration depth (L) is laminar or turbulent on the basis of the determined flow velocity (VL). Further the present invention provides a use of such a method, a system and an aircraft.