Tracer Particle Response in High-Gradient Flow

Many laser-based fluid velocity measurements depend on the motion of tracer particles seeded into the flow. In most cases, the tracers are assumed to follow the flow exactly. However, this is not always the case. The actual motion of a tracer particle is dependent on the properties of both the particle and the fluid surrounding it. Previous analysis for spherical particles in the Stokes regime (assumes Re $\ll 1$) shows that the absolute difference between the particle and fluid velocity exponentially decays in time, with the relaxation time constant dependent on particle diameter, free stream velocity, Reynolds number, and both particle and fluid mass density. For all cases, it is necessary to accurately describe the physics of the tracer particle motion to perform rigorous quantitative studies with particle-based techniques. This study aims to measure and describe particle response to a step change in velocity in a uniform flow. Velocity profiles of solid tracer particles ranging from 300 to 3800 nm in diameter, with initial particle Reynolds numbers up to 100, were measured in a shock tube using particle image velocimetry. The goal of this study is to assess velocity relaxation estimates and assumptions for particle-based velocimetry techniques.