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Wind Tunnels and Experimental Fluid Dynamics Research Part 3

Tham khảo tài liệu 'wind tunnels and experimental fluid dynamics research part 3', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 68 Wind Tunnels and Experimental Fluid Dynamics Research microns Stokes relations will be valid as indicated by the low Reynolds number 0.1 . Dust grains can typically be assumed to follow the fluid flow well the same wind speed and turbulence . Under the action of external forces F such as gravitation electrostatic or magnetic affects the particulates can be assumed to accelerate within a short period of time ls and therefore given a uniform field achieve terminal velocity that is to say reach a drift velocity at which the external force is balanced by the drift induced drag Ur Hall 1988 Fay Sonwalker 1991 . Values of the electric field induced drift velocity Ur have typically been calculated to be around lcm s for fields of order 10KV m. The value of this terminal velocity is given in equation 1 Equation 1 field induced drift velocity UT F X Ị2 Here LI is the molecular viscosity for air LI 1.8x10-5 kg ms and r is the dust grain radius. At the low pressures used in most Mars simulation studies the suspended particulate size is smaller than the collision distance between gas molecules the scattering length Ấ . Here an empirical relation can be used to correct for the non uniform nature of the gas called the slip factor s l - 1.257 r v 0.4 exp -l.ly This factor is of the order of 10 in most Mars simulation studies though is typically negligible at ambient pressure. The simplicity of this relation allows the accumulation of dust within a wind tunnel environment to be used to quantify estimate the force s applied to the dust particulates. This simplicity however relies on the assumption of average particulate properties such as size mass morphology as well as the external forces applied. It also neglects effects of the flow such as turbulence. 4.4 Dust deposition At low turbulent flows dust deposition is dominated by gravitational settlement as turbulent wind speed is small compared to the gravitational terminal velocity. In this case dust deposition within a wind .