An Investigation of Bend Connection Effect on the Finite Amplitude Oscillation in Circular Ducts

Abstract

This paper presents an experimental and theoretical study of finite amplitude oscillations in circular ducts ended with/without bend with connections. The acoustic oscillations are created by reciprocating piston at the closed end of the circular duct while the other end is left free end or it is fitted with constant cross section area bend with different turning angles. Three different turning angles for bends of 450, 900, and 1350 degrees were used for experimental study. The pressure time histories along the duct were recorded at three different axial locations using a capacitance pressure transducer connected to a data acquisition system and DAQ VIEW software. The results showed that, the bend angle has a significant effect on wave deformation inside the duct. Also, it is observed that, close to the piston, the free end duct (duct without bending) has a larger acoustic wave amplitude than the same duct fitted with the bend and the vice versa for the duct close to its exit exactly before the attached bend entrance. Also, the amplitude of the wave in the duct close to the piston increases with rising the bend angles from 450 to 1350, while it decreases close to the duct exit exactly before the bend entrance with increasing the bend angle. That is due to the wave reflections from the bend at the duct exit and wave interaction between different wave phases at the duct exit. Two different piston reciprocating speeds have been used and the results are compared for the different bend turning angles. The numerical simulation and analytical solution are compared with the experimental results for qualitative validation. It is noticed that the experimental results were in agreement with the numerical and analytical results within an accepted error. Also, fast Fourier transform is performed for the experimental results and higher harmonics are noticed for duct fitted with bends at its exit. These higher harmonics are significantly noticed for duct fitted with 1350 bend.