Simultaneous eduction of acoustic impedance and in-duct flow velocity for the fused impedance model under the grazing flow condition

For solving the problem of abrupt changes of the acoustic liner impedance at certain frequencies due to the measurement errors in the flow/acoustic field of the grazing flow tube in the acoustic liner impedance inverse eduction method, an improved acoustic impedance eduction method is proposed to synchronize and accurately extract the acoustic liner impedance and flow velocity inside the flow tube. Firstly, the frequency domain acoustic impedance model is introduced into the mode matching method to establish the forward prediction model of the duct acoustic field under the grazing flow condition. Then, a full-band objective error function containing the unknown parameters of the acoustic impedance model and the in-duct flow velocity is constructed by combining the theoretically predicted acoustic field with the measured acoustic field inside the duct. Finally, the interior point method is utilized to optimize the variables to be solved. The validation results of the NASA baseline dataset of the grazing flow tube test with the CT57 acoustic liner show that the proposed method can realize the simultaneous eduction of the acoustic impedance and the flow velocity inside the flow duct while guaranteeing the computational efficiency. The optimized acoustic impedance curve maintains good continuity, and the optimization error of the flow velocity is kept within 10% under the condition of Ma < 0.3.