Schlieren Image Velocimetry (SIV)

Abstract

Particle image velocimetry (PIV) is a quantitative optical method used in experimental fluid dynamics that captures entire 2D/3D velocity field by measuring the displacements of numerous small particles that follow the motion of the fluid. In its simplest form, PIV acquires two consecutive images (with a very small time delay) of flow field seeded by these tracer particles, and the particle images are then cross-correlated to yield the instantaneous fluid velocity field. The nature of PIV measurement is rather indirect as it determines the particle velocity instead of the fluid velocity. It is assumed in PIV that tracer particles “faithfully” follow the flow field without changing the flow dynamics. To achieve this, the particle response time should be faster than the smallest time scale in the flow. The flow tracer fidelity in PIV is characterized using Stokes number, S k , where a smaller Stokes number (S k < 0.1) represents excellent tracking accuracy. Conversely, schlieren and shadowgraph are truly nonintrusive techniques that rely on the fact that the change in refractive index causes light to deviate due to optical inhomogeneities present in the medium. Schlieren methods can be used for a broad range of high-speed turbulent flows containing refractive index gradients in the form of identifiable and distinguishable flow structures. In schlieren image velocimetry (SIV) techniques, the eddies in a turbulent flow field serve as PIV “particles.” Unlike PIV, there are no seeding particles in SIV. To avoid confusion, a quotation mark is used for “particles” when describing the SIV techniques. As the eddy length scale decreases with the increasing Reynolds number, the length scales of the turbulent eddies become exceptionally important. These self-seeded successive schlieren images with a small time delay between them can be correlated to find velocity field information. Thus, the analysis of schlieren and shadowgraph images is of great importance in the field of fluid mechanics since this system enables the visualization and flow field calculation of unseeded flow.