Xianying Zhang

A numerical investigation of the noise radiated by a turbulent flow over a cavity

Abstract The tonal noise radiated by a two-dimensional cavity submerged in a low-speed turbulent flow has been investigated computationally using a hybrid scheme that couples numerical flow computations with an implementation of the Ffowcs Williams–Hawkings equation. The turbulent near field is computed by solving the short-time-averaged, thin-layer approximation of the Navier–Stokes equations, with turbulence modelled by the Wilcox k–ω model. Second order spatial and temporal discretization schemes with fine grids in the immediate region of the cavity and a small time step were used to capture the unsteady flow physics. Along all external boundaries, a buffer zone is implemented to absorb propagating disturbances and prevent spurious numerical reflections. Comparisons with experimental data demonstrate good agreement in both the frequency and amplitude of the oscillations within the cavity. The unsteady characteristics of the cavity flow are discussed, together with the mechanisms for cavity noise generation. The influence of freestream flow velocity and boundary layer thickness on the frequency and amplitude of the oscillations within the cavity and the nature of the noise radiated to the far field are also investigated. Results indicate that both the frequency and amplitude of oscillation are sensitively dependent on the characteristics of the shear layer spanning the mouth of the cavity.

Influence of ground impedance on the sound radiation of a railway sleeper

A railway track consists of rails attached to sleepers (cross ties) which are laid in ballast. The sleeper provides support for the rail and transfer loads to the ballast and subgrade. Due to the wheel/rail interaction the rail is induced to vibrate and this vibration is transmitted to the sleepers; both the rail and the sleepers radiate sound. Existing models used to predict the sound radiation from the sleeper consider this to be completely embedded in a rigid ground; in reality, however, the sleeper is surrounded by, or embedded to some extent, in the ballast. It is therefore necessary to take these conditions into account in order to obtain a more realistic model. This paper investigates the influence of the ground in close proximity to the sleeper on its sound radiation. A 1/5 scale concrete sleeper is analyzed by using the boundary element method in 3-D. Ground absorption is introduced in terms of its acoustic impedance using the Delany-Bazley model and its effects on the sleeper radiation are predicted. Finally, the numerical results are validated by experimental results using a 1/5 scale model.

Control of Cavity Flow Oscillation through Leading Edge Flow Modification

The effects of a leading edge ramp and mass injection on supersonic cavity flow oscillations are investigated at Mach 1.5 and 2.5, through solutions of Reynolds-averaged Navier-Stokes equations with the effect of turbulence modelled by a two-equation k — u model. The flow is found to undergo a coupled motion of shear layer flapping in transverse direction and vortex convection in streamwise direction due to non-linear propagation effects, which leads to two different responses with the introduction of a leading edge ramp at Mach 1.5 and 2.5. At Mach 1.5, a strong flapping motion leads to nearly similar Strouhal numbers and sound pressure levels in the cavity compared with a baseline case. The roll-up of the shear layer produces convective vortices, leading to enhanced pressure fluctuations on the downstream surface. At Mach 2.5, a weak shear layer instability produces a reduction in sound pressure level, and the increased distance between the edge of the ramp and the trailing edge produces a reduction in the Strouhal number. When mass injection is introduced, a passive pressure response is observed, leading to local vorticity production and vortex shedding. The flow mechanism remains the same at both Mach numbers, with a weak sitting vortex near the trailing edge. The study has identified an optimal mass injection pressure ratio for flow control.

A New Model for the Prediction of Track Sound Radiation

The TWINS model is a widely used and well-established model for rolling noise which has been validated against field measurements in terms of overall noise spectra and levels. However, there are still some areas that can be improved. In particular, the radiation from the rail is based on a model of a rail in free space and there are also limitations in the model for the sound radiation from the sleepers. This paper draws on recent research into the effects of the proximity of the rail and sleeper to an absorbing ground on their sound radiation. Moreover, the ballast is acoustically absorbing to some extent because of the gaps between the ballast particles and this can affect the noise radiation by the rail and sleeper. The ballast absorption is represented here using the Johnson-Allard model with measured values of flow resistivity and porosity. Additionally, the Delany and Bazley model is introduced with a higher value of flow resistivity for comparison. These are used to produce the normal impedance of the ballast layer which is introduced in boundary element calculations of the radiation from the rail and sleepers. Comparisons are made first with the sound radiation from a 1:5 scale track model which has been measured reciprocally in the reverberation chamber. It is shown that the impedance using the measured flow resistivity is inadequate for use in the numerical models; probably an extended reaction model would be more appropriate. However, the Delany and Bazley model with the higher flow resistivity gives better agreement with the measurements and is a practical solution. The new models have also been used together with TWINS to predict the sound radiation from an operational track and the results have been compared with an example field measurement. The new models are found to give an improvement at low frequencies, where the sleeper is the dominant noise source.