In the previous measurements of the aerodynamic sound generated from an inclined circular cylinder, it is reported that the sound pressure level (SPL) changes with the aspect ratio and the inclined angle. Therefore, we have investigated the changes in the vortex structure of the wake considered as one of the causes of the SPL variation. Using the low-noise wind tunnel, the velocity fluctuation in the wake is measured to obtain the correlation length. Moreover, the flow visualization is performed with a hydrogen bubble method and a numerical analysis method in order to clarify how the wake structure changes by variations of aspect ratio and inclined angle. As a result of this investigation, it is shown that the spanwise structure of Karman's vortex is highly influenced by the interference of Karman's vortex with the bottom endplate, and that the influence on the spanwise structure in the wake becomes greater as the aspect ratio decreases and the inclined angle increases.

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The major causes of the sound pressure level (SPL) variation due to the changes of the inclined angle of a circular cylinder and the aspect ratio (ratio of distance between endplates to cylinder diameter) were investigated. The velocity fluctuation of the wake and the surface pressure fluctuati6n of the cylinder were measured in a low noise wind tunnel to fred the correlation length, coherent output power and Strouhal number. The results show that the changes of these values qualitatively correspond with the change of SPL. Flow visualization tests are performed to clarify the variation of wake in relation with inclined angle and aspect ratio. It is found that the spanwise structure of Karman's vortex street is broken down by the upward flows generated around the bottom endplate, and the degree of the interference between the upward flow and Karman's vortex street is smaller when the aspect ratio is larger.

<div class="section abstract"><div class="htmlview paragraph">5 belt wind tunnels are the most common facility to conduct the experimental aerodynamics development for production cars. Among aerodynamic properties, usually drag is the most important development target, but lift force and its front/rear balance is also important for vehicle dynamics. Related to the lift measurement, it is known that the “pad correction”, the correction in the lift measurement values for the undesirable aerodynamic force acting on wheel belt surface around the tire contact patch, must be accounted.</div><div class="htmlview paragraph">Due to the pad correction measurement difficulties, it is common to simply subtract a fixed amount of lift values from measured lift force. However, this method is obviously not perfect as the pad corrections are different for differing vehicle body shapes, aerodynamic configurations, tire sizes and shapes. As an alternative, CFD is a possible solution to estimate pad correction, but it is still not practical to calculate all cases of the development due to the calculation cost and time.</div><div class="htmlview paragraph">One existing wind tunnel test measurement procedure is to utilize special arrangements in the car restraint apparatus. But this method requires a preparation test apart from the actual aerodynamic test run, which is not ideal in terms of wind tunnel test productivity.</div><div class="htmlview paragraph">In this paper, a new method to estimate the pad correction for 5 belt wind tunnel is proposed, introducing a newly developed mathematical model of pressure distribution on the belt surface. With this model, it is possible to estimate pad correction values without any other preparation test.</div><div class="htmlview paragraph">In order to develop this method, CFD analysis with tire rotation condition was made for 10 cases/6 vehicles. For the validation of CFD itself, these CFD results were compared with the pressure distribution data measured in a single belt wind tunnel.</div><div class="htmlview paragraph">The pressure distribution model was created ensuring its mathematically generated distribution can reproduce 10 cases of CFD results with a unified expression, by changing its 68 parameters. Also, these parameters were reduced to 7, in order to enable parameters estimation by multiple linear regression (MLR) from 42 pressure tap point data around the 5 belt wind tunnel wheel belt.</div><div class="htmlview paragraph">For the verification of the method, CFD results were used. The pad correction values were estimated from 42 pressure tap point data taken from the CFD results, and compared with directly calculated pad correction values from the same CFD results.</div><div class="htmlview paragraph">The verification of the estimation accuracy showed within error variation widths of 0.007 ~ 0.008 (widths between maximum and minimum errors) of pad correction for <i>CL_front</i> or <i>CL_rear</i> correction values. Also, the error variation widths for the same car were 0.001 ~ 0.003 for <i>CL_front</i> pad correction, which is enough to predict its configuration difference trend.</div></div>

We measured the aerodynamic noise generated from an inclined circular cylinder with endplates in experimental conditions using a low noise wind tunnel. The inclination-angle dependency of the aerodynamic sound is related to the aspect ratio A that is the ratio of the distance between the endplates to the circular cylinder diameter. To analyze the dependency in detail we derived the correlation length from the velocity fluctuation in the wake. As a result, when the inclined angle changed from 0 degrees to 20 degrees, the correlation length got smaller at A = 10. On the other hand, in the case of A = 30, the correlation length got longer. So we visualized the flow around the cylinder using numerical simulation and the hydrogen bubble method in order to analyze these phenomena.