(VIC) interpolation without the material derivative. A visible increase in resolved details in the turbulent structures is obtained with the VIC+ approach, both in numerical simulations and experiments. This results in a more accurate determination of the turbulent stresses distribution in turbulent boundary layer investigations. Data from a jet experiment, where the vortex topology is retrieved with a small number of tracers indicate the potential utilization of VIC+ in low-concentration experiments as for instance occurring in large-scale volumetric PTV measurements.

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The present work investigates the use of a numerical approximation to the Navier-Stokes equations to increase the temporal resolution of time-resolved PIV data for general flows. The solution of the governing equations is applied to 3D data obtained from tomographic PIV and is based on the vortex-in-cell method (Christiansen in J Comput Phys 13: [363][364][365][366][367][368][369][370][371][372][373][374][375][376][377][378][379] 1973) under the hypothesis of incompressible flow. The principle of time-supersampling is that the spatial information can be leveraged to increase the temporal resolution. The unsteady numerical simulation of the dynamic evolution of the flow is applied within the 3D measurement domain and time integration is performed between each pair of consecutive measurements. Initial conditions are taken from the first measurement field and time-resolved boundary conditions are approximated between the two fields. Temporal continuity of the velocity field is obtained by imposing a weighted average of forward and backward time integration. The accuracy of this time-supersampling method is studied for two experimental datasets obtained from time-resolved tomographic PIV measurements: a turbulent wake and a circular jet. The results are compared to linear interpolation, advectionbased supersampling, and measurement data at high sampling rate. In both flows, we demonstrate the ability to reconstruct detailed temporal dynamics using data subsampled at a rate far below the Nyquist frequency.

This study describes the working principles of the coaxial volumetric velocimeter (CVV) for wind tunnel measurements. The measurement system is derived from the concept of tomographic PIV in combination with recent developments of Lagrangian particle tracking. The main characteristic of the CVV is its small tomographic aperture and the coaxial arrangement between the illumination and imaging directions. The system consists of a multi-camera arrangement subtending only few degrees solid angle and a long focal depth. Contrary to established PIV practice, laser illumination is provided along the same direction as that of the camera views, reducing the optical access requirements to a single viewing direction. The laser light is expanded to illuminate the full field of view of the cameras. Such illumination and imaging conditions along a deep measurement volume dictate the use of tracer particles with a large scattering area. In the present work, helium-filled soap bubbles (HFSB) are used. The fundamental principles of the CVV in terms of dynamic velocity and spatial range (DVR and DSR) are discussed. Maximum particle image density is shown to limit tracer particle seeding concentration and instantaneous spatial resolution. Time-averaged flow fields can be obtained at high spatial resolution by ensemble averaging. The use of the CVV for time-averaged measurements is demonstrated in two wind tunnel experiments. After comparing the CVV measurements with the potential flow in front of a sphere, the near-surface flow around a complex wind tunnel model of a cyclist is measured. The measurements yield the volumetric time-averaged velocity and vorticity field. The measurements of the streamlines in proximity of the surface give an indication of the skin-friction lines pattern, which is of use in the interpretation of the surface flow topology.