Particle imaging through planar shock waves and associated velocimetry errors

G E Elsinga,G C Orlicz

doi:10.1007/s00348-015-2004-9

Abstract

When imaging particles through a shock wave, the resulting particle image appears blurred and at the wrong location, which is referred to as a position error. Particle image doublets are observed if only part of the light scattered by a particle is deflected or reflected by the shock. These optical distortions are due to the jump in the refractive index that occurs over the shock. Within the context of popular particle-based velocimetry techniques, such as particle image velocimetry and particle tracking velocimetry, the position error propagates into an error in the measured velocity. These particle image distortions and associated errors are assessed and quantified in this paper for the case of planar shocks by means of a light ray tracing approach and by experiments. The errors are shown to be most sensitive to the angle between the viewing direction and the plane of the shock. Increasing this angle to modest values (~5°) is a particularly effective way to decrease the relative velocity error. Looking at the shock from the high-density side is recommended when the accurate determination of the particle response to the shock wave is desired.

Highlights

Compressible flows are well known to distort light rays propagating through it
Particle image velocimetry (PIV) and particle tracking velocimetry (PTV) being optical techniques are affected by similar aero-optical distortions resulting in particle image blur and introducing error in the measured particle velocity (Raffel and Kost 1998; Hou et al 2002; Schrijer et al 2005)
The maximum position error magnitude is reached when s = Wtan(θ), which is associated with the maximum distance between the shock and the particle measured along the light ray/optical axis and corresponds to the location where the shock terminates at the tunnel window

Summary

Introduction

Compressible flows are well known to distort light rays propagating through it. for many years, these. The jump in the refractive index across the shock can cause significant PIV measurement error and leads to interesting particle imaging phenomena. Compared to other optical interfaces in fluids, such as occurring in two-phase flow, shock waves are characterized by a small jump in refractive index (order 10−4) and fluid flowing through it at a significant relative velocity. This means light is deflected over small angles so that the recordings still reveal recognizable particle images, albeit clearly distorted. Exp Fluids (2015) 56:129 gives insight into the blurring phenomenon and provides estimates for the particle image position error and velocity error

Theory

Particle imaging: experiment

Velocity error: experiment

Particle response measurements

Findings

Conclusion