Computation of vortical flow and flow induced noise by large eddy simulation with FW-H acoustic analogy and Powell vortex sound theory

The sound generated by a NACA0012 airfoil in the wake of a rod is numerically simulated by two approaches, one is the large eddy simulation （LES） with the FW-H acoustic analogy and the other is the LES with the Powell vortex sound theory, in order to compare the accuracies of their predictions. The vortical structures around the rod-airfoil are computed by the LES and captured by the vortex identification （Q）. The acoustic predictions are verified by the measurements. It is shown that the computed results by the two hybrid approa- ches （LES and FW-H, LES and Powell） are very similar. Both are shown to be satisfactory in the prediction of the noise generated by an unsteady flow. Subsequently, the numerical simulations of the wall pressure fluctuations and the flow-induced noise of a NACA0015 airfoil are made by the two hybrid approaches. At two angles of attack （ 0~ and 8~ ）, the wall pressure fluctuations of the NACA0015 airfoil are computed. The obtained power spectra of the wall pressure fluctuations are analyzed and compared with the measured data. And the vortical structures around the airfoil at two angles of attack are simulated and analyzed. After that, the flow induced noises of the NACA0015 airfoil at two angles of attack are predicted by the two hybrid approaches （LES and FW-H, LES and Powell）. The radiated sound spectra are analyzed and compared with the experimental data. Comparisons show that both are robust, credible and satisfactory in the numerical prediction of the flow induced noise. All numerical simulations are carried out by parallel processing in the Wuxi supercomputing center.

Sound Source Measurement of a Semi-Circular Cylinder in a Uniform Flow by Particle Image Velocimetry

In this paper, the measurement of an aerodynamic sound source for a semi-circular cylinder in a uniform flow is described using Particle Image Velocimetry (PIV). This experimental technique is based on vortex sound theory, where the time derivative of vorticity is evaluated with the aid of two sets of standard PIV systems. The experimental results indicate that the sound source for the semi-circular cylinder is located around the shear layer near the edge of the semi-circular cylinder. The sound source intensity and the area are reduced in the semi-circular cylinder compared with those of a circular cylinder. This result indicates that the aerodynamic sound of the semi- circular cylinder is smaller than that of the circular cylinder, which supports the microphone measurement result.

Investigation on the Changing Characteristics of Flow-Induced Noise in a Centrifugal Pump

Centrifugal pumps are widely used in engineering for a variety of applications.A known drawback of these devices is the high-level noise generated during operations,which can affect their stability and adversely influence the entire working environment.By combining the Powell vortex sound theory,numerical simulations and experimental measurements,this research explores the trends of variation and the corresponding underlying mechanisms for the flow-induced noise at various locations and under different operating conditions.It is shown that the total sound source intensity(TSSI)and total sound pressure level(TSPL)in the impeller,in the region between the inlet to the outlet and along the circumferential extension of the volute,are much higher than those at pump inlet and outlet.Additionally,under various rotational speeds with the design flow rate(Condition 1),the TSSI and TSPL at pump inlet and outlet are higher than those obtained with the opening of the valve kept unchanged(Condition 2);vice versa when these two parameters are evaluated at various locations in the impeller and the volute under the Condition 2,they exceed the equivalent values obtained for the other Condition 1.

Theory of sound field in a room

In the normal-mode theory of Morse, it gives a series of normal modes as the solution of forced vibration in a room. But actually there is always the direct radiation besides the normai modes which represent the reverbrant sound field only. The reason is that the normai modes were assumed only in the source, and naturally normai modes only are obtained in the solution. A theory of double source is proposed, that the sound source is both the source of the direct radiation as if in free space before the boundary surfaces were reached by the direct radiation, and after the first reflection from the boundary surfaces, the source of the reflected wavelets, randomly distributed both in space an in time on the boundary surfaces that build up the normai modes after further reflections. The wave equation is formed accordingly, and the solution of the wave equation, the sound field in a room, contains explicitly both the direct radiation and the reverberant sound formed of normai modes. The approximate mean square sound pressure is found to be the dircet sound determined by the sound power of the source, and reverberant sound determined by the sound power reduced by a factor of 7T/2, different slightly from the result obtained from energy consideration, if the source is pure tone. There is essentially no difference for a source of band noise.

Modification of Flow Structure and Sound Source by Hybrid Porous-serrated Trailing Edge

The effects of hybrid porous-serrated trailing edge on flow structure and sound source of NACA65(12)-10 at moderate Reynolds number(Rec=5×1O^5)have been investigated by Delayed Detached Eddy Simulation(DDES).Compared with conventional serrated trailing edge,the pressure fluctuation in the vicinity of hybrid porous-serrated trailing edge is further decreased significantly.The typical necklace vortex structures stretching across adjacent serrations are suppressed by the porous additive.It is found that porous media changes the shear stress distribution along the serration edge and inside the serration gap,which consequently eliminates the generation of necklace vortex.Therefore,the deformation of vortex tube caused by velocity vector is weakened.The underlying mechanisms associated to the sound source modification are analyzed based on vortex sound theory.The magnitude of Lamb vector and the angle between vorticity and velocity vectors are synchronously reduced by the porous additive,which implies that the present hybrid porous-serrated trailing edge has important influence on the further attenuation of far-field aerodynamic noise.