Noise and flow characteristics of variable backwards- and forward-facing steps and gaps in laminar boundary layer

Abstract

The noise and flow characteristics of combined backwards-facing and forward-facing steps (BFS and FFS) are experimentally investigated in an acoustic wind tunnel using a far-field microphone array and Particle Image Velocimetry (PIV), respectively. The step models considered have a step height H of 10∼40 mm and a step gap L of 10∼100 mm, and the Reynolds number is (Re _H =) 2.1 × 10⁵∼1.1 × 10⁶. The incoming flow boundary layer is laminar, and the step height is approximately 3∼28 of the boundary layer thickness. The results show that the noise is closely related to the step type, step height H and the gap L. In the step-only configuration, the BFS noise does not vary with the step height H ₁ and is much lower than the FFS noise, of which the sound pressure level depends on U ₀ ^5.8 ∼U ₀ ^6.2 within the height range H ₂ . As for the combined step model (BFS&FFS), the noise spectrum changes significantly with the step heights and gaps. When H ₁ >H ₂ , the noise spectrum resembles the BFS-only due to the flow shield of the FFS from the upstream BFS. When H ₁ <H ₂ , the noise spectrum is dominated by the FFS, especially at higher H ₂ or longer gap L due to more face area of the FFS exposed to the incoming flow. As H ₁ =H ₂ , the noise resembles the BFS in the low-frequency range and the FFS at high frequencies, respectively. Generally, the step-only and BFS&FFS have broadband noise characteristics, except when the ratio L/H is small, the noise spectrum has the characteristics of a typical cavity dominated by tonal peaks.