Experimental and theoretical studies of aeroacoustics damping performance of a bias-flow perforated orifice

Aeroacoustics damping performance of an in-duct perforated orifice with a bias flow in terms of acoustic power absorption Δ and reflection χ coefficients are evaluated in this work. For this, experimental measurements of a cold-flow pipe system with a diameter of 2b with an in-duct perforated plate implemented are conducted over the frequency range of 100 to 1000 Hz first. The effects of (1) the downstream pipe length Ld, (2) porosity η, (3) bias flow Mach number Ma and (4) the orifice thickness lw are experimentally evaluated on affecting the noise damping performance of the in-duct perforated orifice. It is found that decreasing Ld leads to increased Δmax (maximum power absorption). However, the orifice thickness plays a negligible effect at lower frequency, and a non-negligible role at higher frequency range. The maximum power absorption Δmax and reflection coefficients χmax are found to be approximately 80% and 90% respectively. There is an optimum porosity or Mach number corresponding to Δmax. In addition, Δ and χ are periodically changed with the forcing frequency. To simulate the experiments and gain insights on the damping performance of the orifice with a diameter of 2a, an 1D theoretical model that embodies vorticity-involved noise absorption mechanism is developed. It is based on the modified form of the Cummings equation describing unsteady flow through an orifice and the Cargill equation describing acoustically open boundary condition at the end of the downstream duct. It is shown that Δ and χ are strongly related to (1) the bias flow Mach number Ma, (2) forcing frequency ω, (3) porosity η, (4) and the downstream pipe length Ld. Comparing with the experimental measurements reveals that good agreement is obtained. This confirms that the present experimental and theoretical study shed lights on the optimum design of in-duct orifices.National Research Foundation (NRF)This work is supported by the University of Canterbury, New Zealand with Grant No. 452STUPDZ, and National Research Foundation, Prime Minister’s Office, Singapore, with Grant No. NRF2016NRF-NSFC001-102 and National Natural Science Foundation of China with Grant No. 11661141020. This financial support is gratefully acknowledged

Investigation on the stability of the Rijke-type thermoacoustic system with an axially distributed heat source

Due to the incurred damages to the combustors, large-amplitude self-sustained thermoacoustic oscillations are unwanted in many propulsion systems, such as liquid/solid rocket motors and aero-engines. To suppress these thermoacoustic oscillations efficiently, the mechanism of thermoacoustic instability needs to be clarified. Following the previous experimental work, the transitions to instability in a Rijke-type thermoacoustic system with an axially distributed heat source are studied numerically in this paper. The URANS numerical method is utilized and verified by means of a mesh sensitivity analysis. The influences of the axially distributed heater length, the heater location, and the mean flow velocity on the nonlinear dynamic behaviors of thermoacoustic oscillations are evaluated. To explore the corresponding mechanism behind these influences, the principle of acoustic energy conservation has been applied. The acoustic energy gains from the thermal-acoustic coupling are quantified via Rayleigh’s integral, and their phase differences are calculated by the cross-correlation function. The acoustic damping induced by the vortex dissipation is qualitatively analyzed by the characteristics of the flow fields in the Rijke tube. Finally, as the heater length, the heater location, or the mean flow velocity is varied, three mechanisms of the transitions to instability in a Rijke-type thermoacoustic system are identified

Assessing the Suitability of Fractal Dimension for Measuring Graphic Complexity Change in Schematic Metro Networks

Schematization is a process of generating schematic network maps (e.g., metro network maps), where the graphic complexity of networks is usually reduced. In the past two decades, various automated schematization methods have been developed. A quantitative and accurate description of the complexity variation in the schematization is critical to evaluate the usability of schematization methods. It is noticed that fractal dimension (F) has been widely used to analyze the complexity of geographic objects, and this indicator may be appropriate for this purpose. In some existing studies, although F has been employed to describe the complexity variation, the theoretical and experimental basis for adopting this approach is inadequate. In this study, experiments based on 26 Chinese cities’ metro networks showed that the F of all these metro networks have decreased in schematization, and a significant positive correlation exists between the F of original networks and the reduction of F after schematization. The above results were verified to have similar trends with the subjective opinions of participants in a psychological questionnaire. Therefore, it can be concluded that F can quantitatively measure the complexity change of networks in schematization. These discoveries provide the basis for using F to evaluate the usability of schematization methods