Tham khảo tài liệu 'wind tunnels and experimental fluid dynamics research part 8', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 268 Wind Tunnels and Experimental Fluid Dynamics Research K 1 6. The source of turbulence in wind tunnels The source of turbulence of wind tunnel may briefly divide in two parts . turbulence due to eddies vortex shedding boundary layer shear stress secondary flows and noise mechanical vibration and aerodynamic that There is a correlation between them. Manshadi et-al in 11 studied the effects of turbulence on the sound generation and velocity fluctuations due to pressure waves in a large subsonic wind tunnel. The results of this research determine that while the share due to the monopole is dominant the share due to the dipole and quadrupole remains less important. Furthermore it is found that sound waves have a modest impact on the measured longitudinal turbulence and is essentially generated by eddies 11 . On the assumptions that first these sound waves are of plane type and contribute only to the u component and second that the turbulence and sound are statistically independent uberoi 12 has shown that the spatial correlation coefficient at two different points 1 and 2 for large separation of the points is defined by 1 2 M V where Wp and represent the velocity due to the sound and eddy turbulence respectively and denote a time mean value. Since the measured u component is made up Wp and ue one may conclude 2 1 2 1 2 . A comparison of above equations reveals that ùe 1 ụyấù The above equation states that one may determine the velocities due to the sound and eddy turbulence by calculating the correlation coefficient. In a wind tunnel pressure waves may be generated aerodynamically along the tunnel circuit which may be considered as plane sound waves. These pressure waves may enter the test section either from the downstream direction or through the nozzle and thus altering the test condition. Consequently the lowest velocity fluctuation level in the wind tunnel is determined by the abovementioned pressure fluctuations 11 . . manshadi et-al in 11 investigated .
