100-kHz Rate Rayleigh Imaging for Combustion and Flow Diagnostics

Two-dimensional (2D) Rayleigh scattering (RS) imaging at an ultrahigh repetition rate of 100 kHz is demonstrated in non-reacting and reacting flows employing a high-energy burst-mode laser system. Image sequences of flow mixture fraction were directly derived from high-speed RS images. Additionally, a 2D instantaneous flow velocity field at 100 kHz was obtained through optical-flow-based analysis of the RS images. The technique was also applied to study turbulent flames having a near-constant Rayleigh cross section. The demonstrated high-speed RS technique in conjunction with optical-flow-based analysis provides non-intrusive, simultaneous measurements of the flow mixing and velocity field, extending the measurement capability of the RS technique to high-speed non-reacting and reacting flows

Application of acoustic techniques to fluid-particle systems – A review

Acoustic methods applied to opaque systems have attracted the attention of researchers in fluid mechanics. In particular, owing to their ability to characterise in real-time, non-transparent and highly concentrated fluid-particle systems, they have been applied to the study of complex multiphase flows such as fluidised beds. This paper gives an overview of the physical principles and typical challenges of ultrasound and acoustic emission AE methods when applied to fluid-particle systems. The principles of ultrasound imaging are explained first. The measurement techniques and signal processing methodologies for obtaining velocity profiles, size distribution of the dispersed phases, and solid volume fraction are then discussed. The techniques are based on the measurement of attenuation, sound speed, frequency shift, and transit time of the propagated sound wave. A description of the acoustic emission technique and applications to fluid-particle systems are then discussed. Finally, extensions and future opportunities of the acoustic techniques are presented

Hybrid Particle Image Velocimetry with the Combination of Cross-Correlation and Optical Flow Method

Particle Image Velocimetry (PIV) has been of relevant discussions lately as the equipment used to obtain temporally and spatially resolved flow fields have advanced rapidly. Despite these advancements, the accuracy of evaluating these images have yet to exceed expectations. Current techniques typically utilize one method, either correlation based (frequently) or optical flow (non-frequently), and both are vulnerable to specific conditions incorporated in the PIV images. Only through the combination of two methods, cross correlation and optical flow, can a technique benefit from the strengths of each method while concealing the flaws each individual method contains. The Hybrid Particle Image Velocimetry method utilizes the fairly unrestricted cross-correlation method, which can process images that contain particles with relatively large displacements, and the high resolution analysis of the Optical Flow method. Susceptible to large displacements, the Optical flow method is restricted to images with particularly small displacements. Combining the two methods requires the constraints set forth on the Optical flow method to be conserved. Meaning that the Cross-correlation results have to be manipulated into a form applicable for the Optical Flow method. Thus steps such as interpolation, shifting the image, and filtering the image are crucial for transitioning cross-correlation results to optical flow analysis. Validating the accuracy of the Hybrid method was conducted through standard PIV images that encompassed various parameters encountered in PIV. Each set of images were analyzed by the hybrid method and three other commonly-used correlation techniques in order to compare the hybrid method\u27s accuracy with current methods. Results confirmed that the Hybrid method is consistently more accurate than the other methods, especially near the boundaries. Additionally, for cases dealing with large particles or small displacement, the Hybrid method attains more accurate results