Noise Generation and Its Control of Discrete Frequency Noise Emitted from Aerofoils in Consideration of Flow Transition and Large Scaled Coherent Structures

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It is well known that a discrete frequency noise is emitted even from the well-designed aerofoils for practical use in a uniform flow under some specific conditions, but the mechanism of the noise generation is not clear yet. In this research, coherent flow structures near conventional aerofoils have been investigated to study the mechanism of the noise generation as well as to obtain useful informations for its control. The following three coherent flow structures are considered : (a)a coherent flow due to transition from the laminar to the turbulent in a boundary layer, (2)a coherent flow due to a local separation on an aerofoil surface, (3)a large scaled turbulent coherent flow structure formed in the wake region of an aerofoil. The velocity fields around and behind test aerofoils were obtained and analyzed with FFT. They were also analyzed with phase-locked averaging technique using the reference signal made from the noise itself. The procedure allows us to identify the coherent flow which causes the discrete frequency noise. The results obtained can be summarized as follows ;1.Any flow separation or any coherent wake flow is not formed for the aerofoil from which discrete frequency noise is emitted. Then, a coherent flow due to a local separation and a coherent wake flow are not the cause of the discrete frequency noise.2.When a discrete frequency noise is emitted, a flow transition appears in the baoudary layer on the pressure surface near the trailing edge of an aerofoil and a coherent flow with clear correlation to the noise occurs there.3.Coherent flow fields appearing near the trailing edge on the pressure surface obtained with the phase-locked averaging technique, which are the cause of a discrete, frequency noise, are similar for many cases with different aerofoil sections, angles of attack and velocities.