PIV Analysis of Ludwig Prandtl's Historic Flow Visualization Films

Around 1930 Ludwig Prandtl and his colleagues O. Tietjens and W. M\"uller published two films with visualizations of flows around surface piercing obstacles to illustrate the unsteady process of flow separation. These visualizations were achieved by recording the motion of fine particles sprinkled onto the water surface in water channels. The resulting images meet the relevant criteria of properly seeded recordings for particle image velocimetry (PIV). Processing these image sequences with modern PIV algorithms allows the visualization of flow quantities (e.g. vorticity) that were unavailable prior to the development of the PIV technique. The accompanying fluid dynamics video consists of selected original film sequences overlaid with visualizations obtained through PIV processing.Comment: Contribution to the "Gallery of Fluid Motion", 63rd Annual APS-DFD Meeting 2010, Long Beach (CA

A Rapid, Empirical Method for Detection and Estimation of Outlier Frames in Particle Imaging Velocimetry Data using Proper Orthogonal Decomposition

This paper develops a method for detection and removal of outlier images from digital Particle Image Velocimetry data using Proper Orthogonal De-composition (POD). The outlier is isolated in the leading POD modes, removed and a replacement value re-estimated. The method is used to estimate and replace whole images within the sequence. This is particularly useful, if a single PIV image is suddenly heavily contaminated with background noise, or to estimate a dropped frame within a sequence. The technique is tested on a synthetic dataset that permits the effective acquisition frequency to be varied systematically, before application to flow field frames obtained from a large-eddy simulation. As expected, outlier re-estimation becomes more difficult when the integral time scale for the flow is long relative to the sampling period. However, the method provides a systematic improvement in predicting frames compared to interpolating from neighbouring(1) frames

Multiphase measurement of blood flow in a microchannel

This paper was presented at the 4th Micro and Nano Flows Conference (MNF2014), which was held at University College, London, UK. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute, ASME Press, LCN London Centre for Nanotechnology, UCL University College London, UCL Engineering, the International NanoScience Community, www.nanopaprika.eu.Blood is a complex fluid comprising red blood cells (RBCs) suspended in a continuous medium. Recent studies have shown that the spatial concentration distributions of the RBCs have a considerable impact on their velocity distributions. By extending this analysis, we present the first multiphase experimental analysis of microscale blood flow to include local velocity and concentration distributions of both phases of the blood. Human blood is perfused though a PDMS microchannel comprising a sequentially bifurcating geometry with a 50×50μm cross-section. The flow rate and the proportion of flow entering the branches of the bifurcation are varied, and the effects on the velocity and concentration distributions of the RBCs and suspending medium are analysed. In addition, the influence of RBC aggregation is investigated. The relative velocity between the two phases of the blood is shown to be dependent to varying degrees on all of the independent parameters examined in this study. A mechanism for the observed trends based on collisions of RBCs with the channel walls in the bifurcation is proposed

Flow velocity mapping using contrast enhanced high-frame-rate plane wave ultrasound and image tracking: methods and initial in vitro and in vivo evaluation

Ultrasound imaging is the most widely used method for visualising and quantifying blood flow in medical practice, but existing techniques have various limitations in terms of imaging sensitivity, field of view, flow angle dependence, and imaging depth. In this study, we developed an ultrasound imaging velocimetry approach capable of visualising and quantifying dynamic flow, by combining high-frame-rate plane wave ultrasound imaging, microbubble contrast agents, pulse inversion contrast imaging and speckle image tracking algorithms. The system was initially evaluated in vitro on both straight and carotid-mimicking vessels with steady and pulsatile flows and in vivo in the rabbit aorta. Colour and spectral Doppler measurements were also made. Initial flow mapping results were compared with theoretical prediction and reference Doppler measurements and indicate the potential of the new system as a highly sensitive, accurate, angle-independent and full field-of-view velocity mapping tool capable of tracking and quantifying fast and dynamic flows