Forward-backward Facing Step Research Articles

Aeroacoustics of turbulent flow over a forward–backward facing step

This paper presents a combined experimental and numerical study on the aeroacoustics of low Mach number turbulent flow over a forward–backward facing step (FBS). The height of the step is fixed at 50% of the incoming boundary layer thickness. Multiple lengths of the step are considered with aspect ratios (step length to height) equal to 1,2,2.4,4, and 8. The Reynolds number based on the step height ranges from 1.2×104 to 2.8×104 in both experiments and simulations. Wall pressure measurement results show that the largest wall pressure fluctuations occur slightly downstream of the step leading edge, and the downstream flow field is influenced by the leading edge flow disturbance. Large-Eddy Simulation (LES) is performed to visualize the instantaneous and time-averaged flow field. Large-scale vortices formed at the leading edge are found to shed downstream directly when the step length is shorter than the shear layer reattachment length, which leads to lower level wall pressure fluctuations on the top surface. Spectral proper orthogonal decomposition (SPOD) is applied to identify the dynamic behaviours of coherent structures downstream of the step leading edge. Using simulation data, the fluctuating wall pressure spectra are also calculated via Poisson’s equation. The results indicate that both turbulence-mean-shear interaction and turbulence–turbulence interaction contribute to the wall pressure fluctuations in the on-step reattachment region. Aeroacoustic beamforming results show that the dominant noise source is located at the step leading edge, where the highest level of fluctuating wall pressure is recorded. Meanwhile, the integrated far-field acoustic spectra show that the levels of FBS noise are lower when there is no flow reattachment occurring on the top surface. A new scaling law for FBS sound spectra is proposed to address the influence of the step aspect ratio on noise production.

Fluid-structure-acoustic coupling for a flat plate

The present work deals with the aeroacoustic sound radiated by a forward–backward facing step in combination with a flexible wall behind the step. A numerical flow computation with coupled aeroacoustic and vibroacoustic simulation was carried out. The structural deformations of the oscillating plate like structure in the wake of the forward–backward facing step were considered to be small and therefore not affecting the flow field. The presented approach enables a separate consideration for the aeroacoustic as well as the structural borne noise. The influence of the interactions of the acoustic medium with the flexible structure on the vibroacoustic sound radiation is investigated. One-sided and two-sided coupling approaches for the vibroacoustic analysis are introduced. The two-sided vibroacoustic computation allows for considering the damping influence of the ambient fluid on the flexible plate vibration and therefore on the sound radiation. Additional to the simulations, aeroacoustic measurements in an acoustic wind tunnel were performed for validation purposes.

Self-sustained oscillations with acoustic feedback in flows over a backward-facing step with a small upstream step

Self-sustained oscillations with acoustic feedback take place in a flow over a two-dimensional two-step configuration: a small forward-backward facing step, which we hereafter call a bump, and a relatively large backward-facing step (backstep). These oscillations can radiate intense tonal sound and fatigue nearby components of industrial products. We clarify the mechanism of these oscillations by directly solving the compressible Navier-Stokes equations. The results show that vortices are shed from the leading edge of the bump and acoustic waves are radiated when these vortices pass the trailing edge of the backstep. The radiated compression waves shed new vortices by stretching the vortex formed by the flow separation at the leading edge of the bump, thereby forming a feedback loop. We propose a formula based on a detailed investigation of the phase relationship between the vortices and the acoustic waves for predicting the frequencies of the tonal sound. The frequencies predicted by this formula are in good agreement with those measured in the experiments we performed.

Large eddy simulation of a forward–backward facing step for acoustic source identification

The feasibility of using a commercial CFD code for large eddy simulation (LES) is investigated. A first test on homogeneous turbulence decay allows a fine-tuning of the eddy viscosity with respect to the numerical features of the code. Then, a flow over forward–backward facing step at Reynolds number Re h =1.7×10 5 is computed. The results found show good agreement with the new LDA data of Leclercq et al. [Forward backward facing step pair: aerodynamic flow, wall pressure and acoustic characterization. AIAA-2001-2249]. The acoustic source term, recorded from the LES and to be fed into a following acoustic propagation simulation, is found to be largest in the separation from the forward step. The source terms structures are similar to the vortical structures generated at the front edge of the obstacle and advected downstream. Structures generated from the backward step rapidly break down into smaller scale structures due to the background turbulence.