Experimental and numerical evaluation of aeroacoustic damping performance of in-duct double-layer orifice plates
Date of Issue2017-02-28
School of Mechanical and Aerospace Engineering
With more stringent noise regulation, more and more researches have been conducted to improve the design of acoustic dampers. As one of the typical acoustic dampers, perforated plates/liners have been widely applied in practice. In this work, aeroacoustic damping performances of in-duct double-layer perforated plates are experimentally studied. For this, a cold flow pipe system with double-layer perforated plates implemented is designed and tested in the Anechoic Chamber at Nanyang Technological University. A centrifugal pump is implemented to generate mean flow through the perforated plate. The mean flow Mach number M_a is varied by changing the flow rate of the pump. Two arrays of microphones are flush mounted to the pipe system in order to determine sound absorption coefficient α characterizing the aeroacoustic damping performance. The noise damping performance of these perforated plate is found to depend on 1) mean flow (also known as bias flow) Mach number〖 M〗_a, 2) porosity σ (also known as open air ratio), 3) flow direction, 4) the distance Lc between two layers of the plates, 5) the distance dv of the orifice relative to the plate’s centre point. When mean flow Mach number M_a is low, the sound absorption coefficient α is oscillating with increased frequency. However, when Mach number M_a is high, the sound absorption troughs are more separated and made shallower, at the meantime, the absorption peaks are widened and lowered. As the Mach number is further increased, general downward trend of sound absorption coefficient α came into view. When there is no mean bias flow, perforated plates with a smaller porosity σ is found to be associated with a large sound absorption coefficient α. However, when mean bias flow is present, perforated plates with a smaller porosity σ are associated with smaller sound absorption coefficient α. Furthermore, when the flow direction is changed, the damping performance of the double-layer perforated plates is varied. Increasing L_c gives rise to increased sound absorption coefficient α as the duct flow is present. However, varying d_(v )is shown to have little effect on the damping performance. Finally, Double-layer perforated plates’ damping performances are involved with large sound absorption coefficient α than that of single-layer perforated plates. However, when duct flow Mach number is low, the double-layer orifice plate has the potential to generate noise itself, which weakens the sound absorption ability. Because in practical application the orifices are not always widely separated, so holes interaction effect is also investigated in present work by varying the ratio ξ of orifice diameter D to the orifice centre distance b and orifices distribution pattern. The results show that with the increase of ξ the sound absorption capability is increased, however, an exactly converse trend is appeared as the excitation frequency is beyond 350 Hz and it is also shown that the presence of the bias flow can enhance this effect. When it comes to the orifices distribution pattern, the diamond distribution exhibits more promising results. To benchmark the experiment, a corresponding flow borne acoustic (acoustic field does not alter the mean flow field) simulation is conducted. In order to reduce the computational cost a hybrid approach in frequency domain is adopted. Firstly, the background flow field is calculated using k-ω turbulent model with finer mesh and in acoustic field the Euler equation is linearized around the mean value obtained from the CFD with relatively coarser mesh. To stabilize the linearized Euler equation, the streamline upwind Petrov-Galerkin (SUPG) is considered. The problem is solved via a direct solver named MUMPS (Multifrontal Massively Parallel sparse) based on LU (lower and upper) decomposition. After comparison, it is found that the numerical results agree very well with the experimental results. In summary, parametric measurements and numerical simulations are conducted to gain insight on the noise damping performance of double- and single-layer in-duct perforated plate in the presence of a mean flow. The critical roles of 1) M_a, 2) σ, 3) flow direction, 4) L_c and 5) d_v are examined one at a time. Also the effect of holes interaction is investigated. The experimental and numerical findings help to design effective acoustic liners.
DRNTU::Engineering::Mechanical engineering::Fluid mechanics