Experimental localization of an acoustic sound source in a wind-tunnel flow by using a numerical time-reversal technique

Abstract

The possibility of using the time-reversal technique to localize acoustic sources in a wind-tunnel flow is investigated. While the technique is widespread, it has scarcely been used in aeroacoustics up to now. The proposed method consists of two steps: in a first experimental step, the acoustic pressure fluctuations are recorded over a linear array of microphones; in a second numerical step, the experimental data are time-reversed and used as input data for a numerical code solving the linearized Euler equations. The simulation achieves the back-propagation of the waves from the array to the source and takes into account the effect of the mean flow on sound propagation. The ability of the method to localize a sound source in a typical wind-tunnel flow is first demonstrated using simulated data. A generic experiment is then set up in an anechoic wind tunnel to validate the proposed method with a flow at Mach number 0.11. Monopolar sources are first considered that are either monochromatic or have a narrow or wide-band frequency content. The source position estimation is well-achieved with an error inferior to the wavelength. An application to a dipolar sound source shows that this type of source is also very satisfactorily characterized.

Multiple line arrays for the characterization of aeroacoustic sources using a time-reversal method

“…3(b) and 3(c) are p radian out of phase. 1,16 This result may also be confirmed by inspecting the TR simulations in Mm. 2 Figure 3(d) shows the variation of relative phase /(x, y) in the acoustic pressure histories with respect to the focal point (x f 1 , y f 1; ) over the small region enclosing the two focal points in Fig.…”

“…The / between thep(x, y,t) timehistories at (1) (x f 1 , y f 1 ) and (x f 2 , y f 2 ) is equal to 0.32 p radian, indicating a significant deviation from in-phase radiation of the maxima points f 1 and f 2 due to the convective effect of mean flow, 1 (2) (x f 3 , y f 3 ) and (x f 4 , y f 4 ) is zero indicating that the maxima points f 3 and f 4 are exactly in-phase, (3) (x f 3 , y f 3 ) and (x f 1 , y f 1 ) is 0.83p radian, and (4) (x f 3 , y f 3 ) and (x f 2 , y f 2 ) is 0.84p radian, signifying small deviation from out-of-phase radiation between the two adjacent pairs of maxima points in (3) and (4) due to the convective effect of mean flow. 1 Nonetheless, the / values indicate that four focal points in Fig. 4(c) have alternating phase, hence representing a lateral quadrupole source.…”

“…3,4 Naturally, this LA configuration yields the most accurate prediction of the source location, and the shape and strength may also be recovered exactly. 2,4 However, setting up of such a LA configuration may be difficult in an aeroacoustics experiment conducted in an anechoic wind tunnel 1 due to the number of microphones required and because the microphone locations may be restricted to regions outside the flow field. Padois et al…”

“…1 Therefore, this Letter presents new, multiple LA simulations that quantifies the improvements in both accuracy and source characterization for monopole, dipole, and quadrupole sources in subsonic cross-flow. The practical importance of the work is seen in obtaining guidelines for designing an aeroacoustic experiment involving acoustic pressure measurements over microphone LAs in a TRM located at the boundaries of an anechoic wind tunnel shown by a schematic in Fig.…”

“…1,2 There are many uncertainties in the use of TR in this application, one of which concerns the number and type of microphone line arrays (LAs) that can be used. TR simulations performed using a limited angular aperture line array (LA) in a Time-Reversal Mirror (TRM) that only partially encloses a source, captures only a small fraction of the acoustic pressure radiated.…”