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The sound generated or absorbed by short circular holes with a mean flow passing through them is relevant in many practical applications. Analytical models for their acoustic response often ignore the fact that such holes open to a confined or finite space either side, or account for this effect simply by adding an end mass inertial correction. The vortex–sound interaction within a short hole has been recently shown to strongly affect the acoustic response at low frequencies (D. Yang, A.S. Morgans, J. Sound Vib. 384 (2016) 294–311 [ 19] ). The present study considers a semi-analytical model based on Green's function method to investigate how the expansion ratios either side of a short hole affect the vortex–sound interaction within it. After accounting for expansions to confined spaces using a cylinder Green's function method, the model is substantially simplified by applying a half-space Green's function for expansions to large spaces. The effect of both the up- and downstream expansion ratios on the acoustics of the hole is investigated. These hole models are then incorporated into a Helmholtz resonator model, allowing a systematic investigation into the effect of neck-to-cavity expansion ratio and neck length. Both of these are found to affect the resonator damping. [ABSTRACT FROM AUTHOR]

The acoustic characteristics of bluff-body burners play a critical role in the combustion stability for combustors using this type of burners. The acoustic modelling of an axisymmetric bluff-body burner entails properly capturing the sound-vortex interaction for the flow through the annular aperture of the burner. Such a problem pertaining to annular apertures can also be of relevance to other engineering applications, such as acoustic dampers or turbofan duct systems. The methodology of combining suitable acoustic Green's functions with a vortex sheet model has been applied extensively in previous theoretical studies of the acoustic response of a short circular orifice with a mean flow passing through it. In this work, the Green's function and vortex sheet model theory is generalised in order to efficiently predict the acoustic characteristics of thin annular apertures sustaining a mean flow, which effectively emulate the typical axisymmetric bluff-body burner configurations in realistic combustors. This requires the incorporation of multiple Kutta conditions for modelling the vortex shedding and multiple vortex sheets for modelling the interaction of the shed vorticity and the acoustics. A high-resolution compressible Large Eddy Simulation (LES) of a simplified representative geometry is performed for validation; the analytical prediction and numerical findings show very good agreement, and the LES further provides key insights into the speed with which vortical disturbances convect downstream. [ABSTRACT FROM AUTHOR]

One of the most important parameters influencing the acoustic response of holes that sustain a low-Mach-number bias flow is their length-to-diameter ratio. For sufficiently short holes, the bias flow is detached within the hole's length, while in long holes the bias flow reattaches. The acoustic behaviour of each class is different and separate modelling approaches exist in the literature. For many technical applications, however, the length-to-diameter ratio falls in the range 1. 5 < L h / D h < 3. 0 , where is not clear if the holes behave acoustically as short or long holes. In this work, the acoustics of such medium holes are explored numerically and analytically. The numerical approach is based on the linearisation of the compressible Navier–Stokes equations (LNSE) around a Reynolds-averaged mean flow. Medium holes are shown to whistle at higher Strouhal numbers than short holes although the mean flow reattaches within them. The underlying physics are further investigated by incorporating selected flow features of the LNSE results into a semi-analytical model accounting for vortex-sound interaction. It is shown that the perturbation field is determined by the three-way coupling of the two vortex sheets shed from the inlet and outlet edges of the hole with the acoustic field. Furthermore, the modelling of the growth of vorticity inside the hole is shown to be crucial to enable whistling in the semi-analytical model. • Short holes with reattached bias flow generate acoustic energy (whistle). • Whistling frequency significantly higher than for holes with detached bias flow. • New semi-analytical model captures the acoustics of medium holes. • Two vortex sheets shed from the hole edges couple with the acoustic field. • Shear layer instability within the hole is crucial for whistling to occur. [ABSTRACT FROM AUTHOR]