Figure 4 Wind Tunnels and Experimental Fluid Dynamics Research
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Fig. 2. External balance example on the left. Calibration facility example on the right. Images courtesy of STARCS. There are several wind tunnel balances manufacturers, who have solved the problems presented in section 3 and they produce internal, external and rotary balances from the range of some Newtons up to some thousands of Newtons. But expensive commercia. balances are not the only option to get an accurate wind tunnel loads measurement system Taking into account our university environment, we, together with the ITER (www.iter.es) decided to manufacture our own balance for the ITER wind tunnel. The following chapter: will show, as example to clearly explain the already mentioned key points, our experience the problems we found during the calibration process and the solutions we have implemented, that will be beneficial for those who intent to build its own balance, as well a: for those who want to understand the complexity of wind tunnel balance systems. The commercial balances are used to have 6 complete coupled channels and a 6x6 decoupling matrix is provided by the vendor. This means that the forces and momentums measurement: are obtained through a complex combination of the 6 channel signals by the use of < completely full decoupvline matrix. The main inconvenient of this concept is that a recalibratior balance in its fixed final position in the test chamber. The second action has to be an accurate determination of the local wind flow direction inside the test chamber. The flow direction determination can be done with the help of different instruments; the most common are the cobra, the wedge, the five holes and the cylindrical probes. All of these probes based their detection method on the same concept. The probes are a bundle of tubes where the leading edge of one of them is perpendicular to the flow direction while the leading edge of the others, one or two pairs, are cut at a known angle to the flow direction. The probe is placed in the test chamber, making an angle with the flow direction, then the pressure measured in the external probes (see Figure 3) are different, and through a precalibrated curve or by reorienting the probe, the angle of the flow can be determined. Components of a Wind Tunnel Balance: Design and Calibration The procedure to be shown here takes into account, that we use standard and calibrated load cells to measure the forces on each bar. Nevertheless, we apply known forces in the direction of the three axes to check not only the calibration of the load cells, but also the signal amplifier and the data acquisition system. As we expected, the manufacturer cell calibration and our calibration had a very good agreement, differences were less than 0,1%. It probes as well, that our philosophy of mechanical decoupling is a very good approach for the design of external balances. It is important to point out that, at this moment, we were interested only in one direction each time. So small errors in the vertical or both horizontal directions (wind direction and Fig. 6. Plots of a signal force mean calculated each second. Five plots for different signal sampling rates: 2.500, 500, 100, 20 and 10 Hz. Plot of the whole averaged force mean value versus sampling frequency. As shown in Figure 6, for low sampling rates, 10 Hz and 20 Hz, which have the same order of magnitude than the Nyquist sampling rate for this signal, the accuracy and reliability of the mean value is poor. The linear regression and the mean value have important deviations. However, a sampling rate of 100 Hz gives good results although this frequency, as shown in the plot of the averaged force mean value, is at the limit. For higher sampling rates as 500 Hz and 2500 Hz the accuracy of the force mean is very good. Fig. 7. Representation of a measured signal, in voltage, at 2.500 Hz sampling rate of an aerodynamic model in the ITER-LSWT, at a wind speed of 20 m/s and at angle of attach of 8°. Time domain signal plot during 20 sec. Frequency domain signal plots with different amplitude scales, with and without the 0 Hz component of the signal. Components of a Wind Tunnel Balance: Design and Calibration Fig. 9. Representation of channel 1 signal at a 2.500 Hz sampling rate of an aerodynamic model introduce in the ITER-LSWT, at a wind speed of 40 m/s and at angle of attach of 8°. Time domain signal plot during 20 sec. Frequency domain signal plots with and without the 0 Hz component of the signal. Components of a Wind Tunnel Balance: Design and Calibration Fig. 10. Representation of channel 2 signal at a 2.500 Hz sampling rate of an aerodynamic model introduce in the ITER-LSWT, at a wind speed of 40 m/s and at angle of attach of 8°. Time domain signal plot during 20 sec. Frequency domain signal plots with and without the 0 Hz component of the signal. Wind Tunnels and Experimental Fluid Dynamics Research The book a€ceWind Tunnels and Experimental Fluid Dynamics Researcha€ is comprised of 33 chapters divided in five sections. The first 12 chapters discuss wind tunnel facilities and experiments in incompressible flow, while the next seven chapters deal with building dynamics, flow control and fluid mechanics. Third section of the book is dedicated to chapters discussing aerodynamic field measurements and real full scale analysis (chapters 20-22). Chapters in the last two sections deal with turbulent structure analysis (chapters 23-25) and wind tunnels in compressible flow (chapters 26-33). Contributions from a large number of international experts make this publication a highly valuable resource in wind tunnels and fluid dynamics field of research.
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