Belyaev and Levin (Journal of Aerosol Science, 5 (1974) 325) (B&L) empirically derived a function for aspiration efficiencies A of thin-walled inlets operated at low velocity ratios U/U-0 (U flow velocity inside the inlet, U-0 velocity of the free flow). This function, which is especially applicable for particle sampling on fast, high-altitude aircraft, has not been experimentally or theoretically verified. By means of the computational fluid dynamics model (CFX), we evaluate this B&L function by explicitly determining inlet aspiration efficiencies over a wide range of high-altitude airborne applications. We find good agreement in the range where B&L performed their measurements (U/U-0 = 0.18-0.2, particle Stokes numbers Stk > 0.1), but for lower U/U-0 and smaller Stk the B&L function deviates from the CFX results. Therefore, we provide here a modified B&L function covering the ranges U/U-0 = 0.007-0.2 and Stk > 0.001: A(Stk) = (a Stk + b)/(a Stk + 1 + b) (U-0/U) + c(U/U-0) a = 1.26, b = 0.27, c = 0.50, (a) U/U-0 < 0.2, (b) Stk > 0.001. In addition, we used CFX to analyze the performance of a short inlet and demonstrate the strong sensitivity of the sampling characteristics to the inlet design, i.e. the particle transmission efficiency changes from zero for a standard inlet to 30% for a short inlet and the aspiration efficiencies of smaller particles are lower by approximately a factor of two when sampled with the short in comparison to the standard inlet. (C) 2003 Elsevier Ltd. All rights reserved.