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The vortex organization of cylinder wakes is experimentally studied by time-resolved tomographic Particle Image Velocimetry at Reynolds numbers ranging from 180 to 5,540. Time resolved measurements are performed at Re = 180, 360 and 540, whereas the transitional (Re = 1,080) and turbulent regimes (Re = 5,540) are investigated by snapshots separated in phase by more than p/4. The vortex structure evolution is visualized by the 3D vorticity field, revealing a regular shedding at the lowest Reynolds, whereas at Re [ 500 the Bénard-Kármán vortex street exhibits counter-rotating stream-wise vortex pairs (characteristic of Mode B) dominating the 3D motion. The regime at Re = 360 produces a transitional pattern where the counter-rotating vortex pairs (Mode B), coexist with profoundly distorted shedding of oblique elements forming a chain of rhombus-like vortex cells. In the turbulent flow regime (Re = 5,540) a large increase in the range of flow scales is directly observed with the appearance of Kelvin-Helmholtz type vortices in the separated shear layer consistently with what is abundantly reported in literature. The statistical description of the secondary structures is inferred from a 3D autocorrelation analysis yielding two span-wise wavelengths for the counter-rotating pairs, an inner length given by (twice) the distance between counter-rotating elements and an outer one given by the distance between pairs. The uncertainty analysis of the present tomographic PIV experiments reveals that this approach is suited for the investigation of vortex wakes with a typical error of 2 and 10% on the velocity and vorticity vectors, respectively.

The wall shear stress plays a key role in the interaction between blood flow and the surrounding tissue. To obtain quantitative information about this parameter, velocity measurements are required with sufficient spatial (and temporal) resolution. We present a methodology for the determination of the wall shear stress in vivo in the vitelline network of a chick embryo. Velocity data is obtained by microscopic particle image velocimetry using correlation ensemble averaging; the latter is used to increase the signal-to-noise ratio of the measurements. The temporal evolution of the pulsatile flow is reconstructed by sorting the image pairs based on a phase estimate. From these flow measurements, the wall shear stress can be derived either directly from the magnitude of the gradients or from fits to velocity profiles. Both methods give results that are in good agreement with each other, while the former method is significantly easier to implement. For more accurate studies, the full threedimensional velocity field may be required. It is demonstrated how this velocity field can be obtained by scanning the measurement volume.

In order to study the role of blood -tissue interaction in the developing chicken embryo heart, detailed information about the haemodynamic forces is needed. In this study, we present the first in vivo measurements of the three-dimensional distribution of wall shear stress (WSS) in the outflow tract (OFT) of an embryonic chicken heart. The data are obtained in a two-step process: first, the three-dimensional flow fields are measured during the cardiac cycle using scanning microscopic particle image velocimetry; second, the location of the wall and the WSS are determined by post-processing flow velocity data (finding velocity gradients at locations where the flow approaches zero). The results are a three-dimensional reconstruction of the geometry, with a spatial resolution of 15-20 mm, and provides detailed information about the WSS in the OFT. The most significant error is the location of the wall, which results in an estimate of the uncertainty in the WSS values of 20 per cent.

The effect of small particles on decaying grid-generated turbulence is studied experimentally. Using a two-camera system, instantaneous fluid-phase and particlephase measurements can be obtained simultaneously. The data obtained with this system are used to study the decay behaviour of the turbulent flow. The role of particle size, particle density and volume load is studied in a number of different cases. These cases are chosen so that the individual role of these parameters can systematically be evaluated. Addition of particles to the flow has significant effects on the decaying turbulence: first, the onset of the turbulent decay appears to shift upstream; second, the flow becomes anisotropic as it develops downstream. The latter is observed as an increase in integral length scale in the vertical direction. The rate at which the flow becomes anisotropic can be predicted using a new parameter: the product of the non-dimensional number density and the Stokes number (referred to as the 'Stokes load'). This parameter, combining the relevant fluid and particle characteristics, is a measure for the energy redistribution leading to anisotropy. In addition to redistributing energy, the particles also produce turbulence. However, this only becomes evident when the grid-generated turbulence has decayed sufficiently, relatively far downstream of the grid. The turbulence production by particles can also account for the observed decrease in slope of the power spectrum, which leads to a 'cross-over' effect. The production of turbulence by the particles can be predicted using a model for the momentum deficit of the particle wakes. The validity of this approach is confirmed using conditional sampling of the fluid velocity field around the particles.

In micro-Particle Image Velocimetry, the requirement of a large field-of-view often results in a large depth-of-correlation, i.e. large depth of the measurement volume. When the velocity varies substantially over the depth-of-correlation, special attention should be paid to a correct interpretation of the measured velocities. When a specialized microscope is needed to meet the requirements of a setup, the resulting more complex optical arrangements can have additional effects on the measurement results. In order to determine flow parameters such as the flow rate, it is sufficient to have a robust estimate of the maximum velocity when the flow is Poiseuille flow. In this paper, an interpretation of the results from particle image velocimetry measurements with low magnification in a round capillary is given for two types of microscopes: a conventional and a specialized microscope. The measured velocity appears to be lower than the maximum velocity, yet is still above the average velocity. The interpretation of the measured velocity differs for the two types of microscopes. The under-estimation of the maximum velocity obtained from the conventional microscope remains small (within 6%) for low-magnification measurements, while the under-estimation of the maximum velocity obtained from the specialized microscope increases up to 25% for a large depth-of-correlation. The images of the in-and outof-focus particles turn out to play a crucial role in this difference between the two microscopes. Validation of the optical properties of a microscope is important, especially for specialized microscopes where particle images deviate substantially from the existing theory, and this theory is also used to derive the analytical expression for the depthof-correlation. A procedure is recommended to obtain a correct interpretation of the measured velocity. This procedure is generally applicable, but mainly of importance for specialized microscopes.

The zebrafish embryo is a small, cheap, whole-animal model which may replace rodents in some areas of research. Unfortunately, zebrafish embryos are commonly cultured in microtitre plates using cell-culture protocols with static buffer replacement. Such protocols are highly invasive, consume large quantities of reagents and do not readily permit high-quality imaging. Zebrafish and rodent embryos have previously been cultured in static microfluidic drops, and zebrafish embryos have also been raised in a prototype polydimethylsiloxane setup in a Petri dish. Other than this, no animal embryo has ever been shown to undergo embryonic development in a microfluidic flow-through system. We have developed and prototyped a specialized lab-on-a-chip made from bonded layers of borosilicate glass. We find that zebrafish embryos can develop in the chip for 5 days, with continuous buffer flow at pressures of 0.005-0.04 MPa. Phenotypic effects were seen, but these were scored subjectively as 'minor'. Survival rates of 100% could be reached with buffer flows of 2 µL per well per min. High-quality imaging was possible. An acute ethanol exposure test in the chip replicated the same assay performed in microtitre plates. More than 100 embryos could be cultured in an area, excluding infrastructure, smaller than a credit card. We discuss how biochip technology, coupled with zebrafish larvae, could allow biological research to be conducted in massive, parallel experiments, at high speed and low cost.

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