Obtaining the unsteady drag coefficient of micron-sized particles when the diameter is uncertain

The Los Alamos National Laboratory Horizontal Shock Tube (HST) studies response of shock-accelerated particles and droplets in a gas. When subjected to the highly unsteady flow of a shock, normal drag coefficients may not properly predict the motion of particles. The current experiments are done as a part of an ongoing campaign to improve drag laws in this regime, and are performed in conjunction with validation and computational efforts. Individual particle positions were obtained using a high speed eight-pulse particle tracking technique. Particles of several nominal diameters selected from the range of 1-10 μm were subjected to 1.2, 1.3, and 1.4 Mach number shocks in air. The nominal particle diameter was obtained from measurement of the size distribution before injection into the shock tube. While care is taken to narrow the width of the particle size distribution, averaging of the results is necessary to account for the polydispersity. We will describe the required averaging, and demonstrate how careful control of the uncertainty introduced by this diameter error was shown to reduce the bias in the resultant computed drag coefficients.