The 21 st International Congress on Sound and Vibration 13-17 July, 2014, Beijing/China THE CONTROL METHOD OF THE INFLOW TURBU- LENCE INTERACTION NOISE FOR ROUTER COOLING FAN Yingbo Xu, Xiaodong Li School of Energy and Power Engineering, Beihang University (BUAA), Beijing, 100191, China e-mail: Lixd@buaa.edu.cn The aerodynamic noise of router cooling fans receives much attention due to the increasing router power. In this paper, the characteristics of the flow field and the sound field of the cooling fan are studied experimentally. The interaction of inflow turbulence with the cooling fan is identified as one of the main noise generation mechanism. Based on such understanding, inflow screens are designed to control the inflow turbulence intensity and the noise generation. Experimental results show that both the area of the high-turbulence-intensity region and the turbulence intensity are obviously reduced by the screen. The inflow turbulence intensity can be reduced by 60% in the centre region when the screens are used. The screens can reduce both tunes and broadband noise over a wide frequency range and have litter effect on flow field. 1. Introduction With rapid development of internet technology, the router, which is a key device to connect users with internet, starts to enter the residential area. At present, cooling fans are commonly used to cool the router electronic components. The increasingly powerful but noisy cooling fans become an important environment concern. It is highly desirable to control the router cooling fan noise. Different from the situation of aero-engine fan noise which has been studied for several decades, very few investigations concerning cooling fan noise could be found in available literatures. Huang 1 and Murray & Isabeela 2 analyzed the tonal and broadband noise of the personal computer cooling fan by means of numerical and experimental approaches. Minogue et al 3,4 presented an active method to control cooling fan noise in a short duct. In these researches, the incoming flow was assumed to be uniform. However, the inflow of the router cooling fan could be non-uniform due to complex electronic components upstream. This may lead to additional aerodynamic noise pertaining to inflow turbulence interaction with the rotating fan blades. There are many researches on the inflow turbulence-blade interaction noise. For instance, Jacob et al 5 conducted an experimental research on rod-airfoil interaction noise. Jiang et al 6 also investigated the rod-airfoil interaction noise by experimental and numerical methods. Some other numerical studies also showed the importance of rod-airfoil interaction noise 7-12. However, previous researches all focus on simplified models. Therefore, for the router cooling fan, it would be essential to study the interaction between the inflow turbulence and the fan blades. ICSV21, Beijing, China, 13-17 July 2014 1
The main purpose of this paper is to study the sound generation mechanism of router cooling fans and explore noise control methods. The sound and flow field measurements are carried out in an anechoic chamber to identify the router cooling fan noise source. The relation between the aerodynamic noise and the flow field of the cooling fan is studied and analyzed based on the experimental results. The effect of the screen on the cooling fan noise is also evaluated experimentally. 2. Experimental setup The experiment is carried out in the anechoic chamber of Fluid and Acoustic Engineering Laboratory of Beihang University. The anechoic chamber is approximately 6 m 8 m 4 m in size and of walls that are acoustically treated with foam wedges providing a reflection free environment above 200 Hz. The dimensions of the router are 0.62 m 0.44 m 0.72 m. The impeller diameter of the cooling fan is 0.2 m. The fan has five blades and the rated speed of fan is 2900 rpm. The setup of the noise measurements of the router cooling fan and a single isolated fan are shown in Fig.1. A single microphone is set 1 m downstream from the center of the fan to measure the fan noise under different operate conditions. The microphones are B&K 4939 condenser microphones and the signals were collected with a National Instruments board PXI-4496 at the sampling frequency of 10 105 Hz for the sampling time of 10s. Figure 1. Setup of the cooling fan noise measurements, (a) router cooling fan and (b) single isolated fan. The unsteady flow field in front of the cooling fan was measured with a hot-wire anemometer as sketched in Fig.2 and the sensor is DANTEC55p13. Signals are sampled at frequency of 20 khz for 5s for the mean flow velocity and turbulence intensity. The measuring points are distributed uniformly in the cross section of the inflow and the interval between the measuring points is 0.01 m. Figure 2. Sketch of the flow field measurement with a hot-wire anemometry ICSV21, Beijing, China, 13-17 July 2014 2
3. Measurement of the router cooling fan noise and the flow field 3.1 Acoustic characteristics of the router cooling fan In order to assess the effects of inflow condition on the router cooling fan noise, the noise levels of the router and a single isolated fan are measured. The microphone is placed on the axis of the fan 1 m downstream. The noise spectra of the router and the isolated fan are compared in Fig.3. The maximum amplitude of sound pressure level in both spectra occurs at BPF (242 Hz) and the harmonics also have high amplitude. However, the SPL of the router is obviously higher than that of the isolated fan. This might due to the different inflow conditions between the router cooling fan and the isolated fan. In the isolated-fan case, the inflow is a free stream flow with low turbulence intensity, while the turbulence intensity of the inflow of the router fan is high due to complex electronic components upstream. It can be deduced from the comparison that the inflow condition has a strong effect on the cooling fan noise. Figure 3. Noise spectra of the router and the single fan 3.2 Upstream flow field of the router cooling fan Figure 4. Turbulence intensity of the inflow of the fan Figure 5. The structures in the router in front of the fan The turbulence intensity is measured by a hot-wire anemometer in front of the fan. As shown in Fig.4, the turbulence intensity is nonuniform in the inflow cross section, it is stronger in the central and lower right regions. The maxima of turbulence intensity are 5% at the center and 6% in lower right region, respectively. The high turbulence intensity is caused by the vortices generated in front of these two regions when the air passes the heat exchanger at the center, and the edge of the baffle at the lower right location, as shown in Fig.5. ICSV21, Beijing, China, 13-17 July 2014 3
The comparison of the noise of the router cooling fan with the isolated fan suggests that the inflow condition has a strong effect on the fan noise. And the flow filed measurement confirms that due to the upstream device in the router, the inflow of the router cooling fan is highly turbulent. Therefore, reducing the inflow turbulence intensity would be an effective way to suppress the router cooling fan noise. 4. Control of router cooling fan noise 4.1 Inflow turbulence interaction noise The interaction of the incoming vortices with the leading edge of a blade is sketched in Fig.6. As the vortices impinge on the leading edge of the blade, they are extruded, stretched, and split. Such processes can be significant noise source. Thus, eliminating the incoming vortices, or reducing the turbulence intensity, would dramatically suppress the noise emission. Figure 6. Inflow turbulence interaction noise 4.2 Design of the screen Mehta & Bradshaw 13 pointed out that screens are effective in controlling turbulence intensity. A screen with cells of reasonable size can suppress the turbulence by mashing whirlpool, eliminating the average velocity change of lateral flow, and reducing the lateral component of the turbulence. The size of the cells should be smaller than the wavelength of minimum lateral velocity. With fixed open-area ratio, the screen with smaller cells has better performance. In the present investigation, the screen is made of metal wires interwoven to form square meshes, as shown in Fig.7. The width of the screen cell is 2.5 mm and the diameter of the metal wire is 0.25 mm. Figure 7. Model of screen ICSV21, Beijing, China, 13-17 July 2014 4
4.3 Flow field of router cooling fan with screen The screens, as sketched in Fig.8, are placed in front of the fan, and at the lower half part of the cross section where the incoming turbulence intensity is high. A three-layer screen has been used in the experiments. Figure 8. Screen in front of the fan The turbulence field behind the three-layer screens is shown in Fig.9. Compared with results without control screen in Fig. 4, both the area of the high-turbulence-intensity region and the turbulence intensity are obviously reduced by the screen. The large scale vortices passing though the screen are divided into smaller vortices. Thus, the incoming turbulence intensity is weakened. Figure 9. Turbulence field of three-layer screens Figure 10. Turbulent intensity difference of the three-layer screens and without screen ICSV21, Beijing, China, 13-17 July 2014 5
It is shown in Fig.10 that the screen is remarkably effective in high-turbulence-intensity region. In the high-turbulence-intensity region, the large-scale inflow vortices are divided into small ones. The turbulence intensity can be reduced by 40% at lower right corner and 60% at the center. On the other hand, the screen is less effective in low-turbulence-intensity region, where the flow field is smooth and the scale of the vortices are so small, that most of them can pass through the orifices of the screen without being disturbed. The interval of the measuring point is 10mm (Fig.11). The speed distribution of without screen and three-layer screens in the vertical direction is shown in Fig.12. The speed of three-layer screens is slower comparing with without screen at the bottom. The maximum difference of the speed is 3%. The speed is different along the vertical direction and there are two humps. The speed drops sharply at axis y=-3, because in front of this part there are lots of heat exchangers, which block the air and cause the speed to drop suddenly. The results show that mean velocity field is changed slightly when the screen is installed, so the flow of the cooling fan can be kept. Figure 11. Measurement point of the vertical direction Figure 12. Speed distribution along the vertical direction 4.4 Acoustic characteristics of router cooling fan with screen The spectra of router cooling fan noise with three-layer screens and without screen are shown in Fig.13. The screen is effective in suppressing the cooling fan noise. The SPL at the first, second and third harmonics is obviously decreased when the three-layer screens is used. The noise reduction is up to 10 db at the first harmonic, 4 db at the second harmonic and 8 db at the third harmonic. Figure 13. Spectral of three-layer screens and without screen The screens crush the large vortices into smaller ones and reduce the scale and intensity of inflow turbulence in the whole flow filed. The flow field becomes uniform and the interaction of the vortices with fan is suppressed. Therefore, fluctuating aerodynamic forces on the leading edge of ICSV21, Beijing, China, 13-17 July 2014 6
blade is reduced, and thus the intensity of noise source at the leading edge are decreased. The result shows that the screen is effective to reduce the inflow interaction noise at low frequency range. 5. Conclusions The characteristics and generation mechanism of aerodynamic noise from a router cooling fan are studied by flow visualization and sound field measurement. The cooling fan is found to be the most important aerodynamic noise source of the router and the inflow turbulence-blade interaction noise is the major part of the cooling fan noise. Screens are effective in suppressing the router cooling fan noise by reducing the inflow turbulence intensity. The screen has better performance at the blade passing frequency (BPF) and its harmonics. The screen has slight influence, no more than 3 percent, on the mean velocity field. Acknowledgments This work is supported by grants from the 973 Program (2012CB720202) and the 111 project B07009. REFERENCES 1 Huang, L. X. Characterizing Computer Cooling Fan Noise. Journal of Acoustical Society of America, 114, 3189-3200, (2003). 2 Murray, H., Isabeela, L. Experimental Study of the Noise Emission of Personal Computer Cooling Fans, Applied Acoustics, 67, 849-863, (2006). 3 Minogue, P., Rankin, N., Ryan, J. Adaptively Canceling Server Fan Noise, Analog Dialogue, 2, 34-40, (2000). 4 Minogue, P., Rankin, N., Ryan, J. Short Duct Server Fan Noise Cancellation, Active99, 539-550, (1999). 5 Jacob, M. C., Boudet, J., Casalino, D., Michard, M. A Rod-Airfoil Experiment as Benchmark for Broad Noise Modeling, Theoretical and Computational Fluid Dynamics, 19, 171-196, (2005). 6 Jiang, M., Li, X. D., Zhou, J. J. Experimental and Numerical Investigation on Sound Generation from Airfoil-Flow Interaction, Applied Mathematics and Mechanics, 32(6), 765-776, (2011). 7 Greschner, B., Thiele, F., Jacob, M. C., Casalino, D. Prediction of Sound Generated by a Rod- Airfoil Configuration Using EASM DES and the Generalized Lighthill/FW-H Analogy, Computers and Fluids, 37(4), 402-413, (2008). 8 Casalino, D., Jacob, M. C., Roger, M. Prediction of Rod Airfoil Interaction Noise Using the FWH Analogy, AIAA Paper 2002-2543, (2002). 9 Boudet, J., Grosjean, N., Jacob, M. C. Wake-Airfoil Interaction as Broadband Noise Source: A Large-Edgy Simulation Study, International Journal of Aeroacoustics, 4(1), 93-115, (2005). 10 Creschner, B., Thiele, F., Casalino, D., Jacob, M. C. Influence of Turbulence Modeling on The Broadband Noise Simulation for Complex Flows, AIAA Paper 2004-2926, (2004). 11 Gerolymos, G. A., Vallet, I. Influence of Temporal Integration and Spatial Discretization on Hybrid RSM-VLES Computations, AIAA Paper 2007-4094, (2007). 12 Carani, M., Dai, Y., Carani, D. Acoustic Investigation of Rod Airfoil Configuration with DES and FWH, AIAA Paper 2007-4016, (2007). 13 Mehta, R. D., Bradshaw, P. Design Rules for Small Low Speed Wind Tunnels, Journal of the Royal Aeronautical Society, 718, 443-449, (1979). ICSV21, Beijing, China, 13-17 July 2014 7