Weaver, 1993, Ziada and Oengören, 1993). The vortex excitation has been shown to result from the formation of periodic vortices in the space between the tubes (Weaver, 1993, Ziada and Oengören, 1993). Recently, finned tubes have been used in heat recovery steam generator to enhance heat transfer. The fins are mounted spirally around a bare tube surface so as to simplify the production process. It is generally believed that vortex-induced vibration can be suppressed by mounting helical strakes around a bare tube (Blevins, 1986, Zdravkovich, 1981). Zdravkovich (1981) showed that helical strakes consisting of several fins acting as surface protrusions around a bare tube can also reduce vibration. However, acoustic resonance and vortex shedding phenomena still occur occasionally in the
In the present paper the attention is focused on correlation between fan noise and velocity fluctuations of tip leakage vortex around rotor blade of a low pressure axial flow fan at the maximum pressure operating point. We measured time fluctuating velocity near the rotor tip around the rotor blades by using a hot-wire sensor from a relative flame of reference fixed to the rotor blades. As the results, it is clear that the velocity fluctuation due to tip leakage vortex has weak periodicity and the hump portion appeared in its spectrum. If the flow rate was lower than the design condition, the tip leakage flow became to attach to the following blade and the sound pressure level at frequency of velocity fluctuation of this flow was increased. The correlation measurements between the velocity fluctuation of tip leakage flow and the aerodynamic noise were made using a rotating hot-wire sensor near the rotor tip in the rotating frame. The correlation between the velocity fluctuation due to tip leakage flow and acoustic pressure were increased due to generation of weak acoustic resonance at the maximum pressure operating point.
In the present paper, the attention is focused on the effect of local porous material on aerodynamic sound radiated from two-dimensional airfoil. We measured the aerodynamic sound radiated from the airfoil with porous material, tripping wire and porous plate which are mounted locally on the surface of the airfoils near the leading edge. At the normal airfoil, discrete frequency noise is clearly observed at small attack angle. However, it is clear that its noise generated from the airfoil decreased with the local porous material on the surface of pressure side of the airfoil. The porous material is effective to reduce this noise compared with the others. And the sound absorbing coefficient and the air permeability were measured for test porous material. The sound absorbing coefficient increased at the high frequency band, and the air permeability became small for porous materials. As the attack angle increased, the discrete frequency noise was not generated from the normal airfoil. The broadband noises were almost same for all test airfoils.
In the present paper the attention is focused on the characteristics of aerodynamic sound radiated from two or three finned cylinders exposed to cross-flow. We measured the spectrum of SPL for the cylinder spacing ratios ranged from 0 to 1.05 in the transverse direction or the ratios from 0 to 6.8 in the flow direction at Reynolds number of 1.7×10 5 . As a result, the peak SPL for two finned cylinders at the cylinder spacing ratios less than about 2.0 in the flow direction was smallest in the tandem finned cylinders. And the peak SPL was largest at the cylinder spacing ratio of 0.72 in the transverse direction. The peak SPL at the cylinder spacing ratios of 0.72 in the transverse direction and 1.24 in the flow direction for three finned cylinders was smaller than those of the two finned cylinders.
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