Papers by Bernhard Wieneke
ABSTRACT A novel approach to the evaluation of time resolved particle-based tomographic data is i... more ABSTRACT A novel approach to the evaluation of time resolved particle-based tomographic data is introduced. By seizing the time information contained in such datasets, a very fast and accurate tracking of nearly all particles within the measurement domain is achieved at seeding densities comparable to (and probably above) the thresholds for tomographic PIV. The method relies on predicting the position of already tracked particles and refining the found position by an image matching scheme (‘shaking’ all particles within the measurement ‘box’ until they fit the images: ‘Shake The Box’ - STB). New particles entering the measurement domain are identified using triangulation on the residual images. Application of the method on a high-resolution time-resolved experimental dataset showed a reliable tracking of the vast majority of available particles for long time-series with many particles being tracked for their whole length of stay within the measurement domain. The image matching process ensures highly accurate particle positioning. Comparing the results to tomographic PIV evaluations by interpolating vector volumes from the discrete particles shows a high conformity of the results. The availability of discrete track information additionally allows for Lagrangian evaluations not possible with PIV data, as well as easy temporal smoothing and a reliable determination of derivations. The processing time of a not fully optimized version of STB proved to be a factor of 3 to 4 faster compared to the fastest methods available for TOMO-PIV.
ABSTRACT This work focuses on tomographic image reconstruction in experimental fluid mechanics (T... more ABSTRACT This work focuses on tomographic image reconstruction in experimental fluid mechanics (TomoPIV), a recently established 3D particle image velocimetry technique. Corresponding 2D image sequences (projections) and the 3D reconstruction via tomographical methods provides the basis for estimating turbulent flows and related flow patterns through image processing. TomoPIV employs undersampling to make the high-speed imaging process feasible, resulting in an ill-posed image reconstruction problem. We address the corresponding basic problems involved and point out promising optimization criteria for reconstruction based on sparsity maximization, that perform favorably in comparison to classical algebraic methods currently in use for TomoPIV.
A Schroeder C E Willert Particle Image Velocimetry Topics in Applied Physics 112 P 103 125, 2008
Tomographic particle image velocimetry is a 3D PIV technique based on the illumination, recording... more Tomographic particle image velocimetry is a 3D PIV technique based on the illumination, recording, reconstruction and analysis of tracer-particle motion within a three-dimensional measurement volume. The recently developed technique makes use of several simultaneous views of the illuminated particles, typically 4, and their three-dimensional reconstruction as a light-intensity distribution by means of optical tomography. The reconstruction is performed with the MART algorithm (multiplicative algebraic reconstruction technique), yielding a 3D distribution of light intensity discretized over an array of voxels. The reconstructed tomogram pair is then analyzed by means of 3D crosscorrelation with an iterative multigrid volume-deformation technique, returning the three-component velocity vector distribution over the measurement volume. The implementation of the tomographic technique in time-resolved mode by means of high repetition rate PIV hardware has the capability to yield 4D velocity information. The first part of the chapter describes the operation principles and gives a detailed assessment of the tomographic reconstruction algorithm performance based upon a computer-simulated experiment. The second part of the chapter proposes four applications on two flow cases: 1. the transitional wake behind a circular cylinder; 2. the turbulent boundary layer developing over a flat plate. For the first case, experiments in air at ReD = 2700 are described together with the experimental assessment of the tomographic reconstruction accuracy. In this experiment a direct comparison is made between the results obtained by tomographic PIV and stereo-PIV. Experiments conducted in a water facility on the cylinder wake shows the extension of the technique to timeresolved measurements in water at ReD = 540 by means of a low repetition rate PIV system. A high data yield is obtained using high-resolution cameras (2k × 2k pixels) returning 650k vectors per volume. Measurements of the turbulent boundary layer in air at Re θ = 1900 provide a clear visualization of streamwise-aligned low-speed regions as well as hairpin vortices grouped into packets. Finally, in similar flow conditions the boundary layer is measured using a high repetition rate PIV system at 5 kHz, where the spatiotemporal evolution of the flow structures is visualized revealing a mechanism for the rapid growth of a Q2 event, possibly associated to the generation of hairpin-like structures.
Ieee Transactions on Image Processing, Dec 23, 2011
A variational approach is presented to the estimation of turbulent fluid flow from particle image... more A variational approach is presented to the estimation of turbulent fluid flow from particle image sequences in experimental fluid mechanics. The approach comprises two coupled optimizations for adapting size and shape of a Gaussian correlation window at each location and for estimating the flow, respectively. The method copes with a wide range of particle densities and image noise levels without any data-specific parameter tuning. Based on a careful implementation of a multiscale nonlinear optimization technique, we demonstrate robustness of the solution over typical experimental scenarios and highest estimation accuracy for an international benchmark data set (PIV Challenge).
ABSTRACT Particle Image Velocimetry (PIV) has been used for instantaneous recordings of velocity ... more ABSTRACT Particle Image Velocimetry (PIV) has been used for instantaneous recordings of velocity fields within a volume of a flow. The volume information was gained by multiple cameras and a tomographic approach based on the Multiplicative Algebraic Reconstruction Technique (MART). Volume cross-correlation enabled the calculation of instantaneous 3D-3C velocity vectors. The usability of this measurement tool has been proven in different applications including setups at wind and water tunnels.
Particle Image Velocimetry (PIV) denotes a prevailing technique for imaging turbulent fluids with... more Particle Image Velocimetry (PIV) denotes a prevailing technique for imaging turbulent fluids with high-speed cameras. Corresponding image sequences provide the basis for estimating such flows and related flow patterns through image processing. While so far this technique has been applied in two dimensions (2D) in terms of an illuminated plane inter- secting the volume, recent research focuses on imaging fluids
Lecture Notes in Computer Science, 2008
In particle image velocimetry (PIV) a temporally separated image pair of a gas or liquid seeded w... more In particle image velocimetry (PIV) a temporally separated image pair of a gas or liquid seeded with small particles is recorded and analysed in order to measure fluid flows therein. We investigate a variational approach to cross-correlation, a robust and well-established method to determine displacement vectors from the image data. A "soft" Gaussian window function replaces the usual rectangular correlation frame. We propose a criterion to adapt the window size and shape that directly formulates the goal to minimise the displacement estimation error. In order to measure motion and adapt the window shapes at the same time we combine both sub-problems into a bi-level optimisation problem and solve it via continuous multiscale methods. Experiments with synthetic and real PIV data demonstrate the ability of our approach to solve the formulated problem. Moreover window adaptation yields significantly improved results.
Dans le domaine de la mécanique des fluides, la plupart des écoulements qui sont intéressants dan... more Dans le domaine de la mécanique des fluides, la plupart des écoulements qui sont intéressants dans le cadre de la recherche académique ou représentatifs de situations industrielles révèlent l'existence de structures tridimensionnelles instationnaires. Même si la modélisation numérique a fait des progrès substantiels ces dernières années, les approches expérimentales de ces phénomènes complexes restent essentielles afin de comprendre la physique des écoulements.
The momentum exchange mechanisms in a turbulent boundary layer (TBL) flow are of manifold tempora... more The momentum exchange mechanisms in a turbulent boundary layer (TBL) flow are of manifold temporal and spatial scales and governed by the organization of self-sustaining coherent flow structures driven by entrained high momentum fluid. Generic flow structures such as hairpin-like vortices and spanwise alternating wall bounded low-and high-speed streaks have been observed and extensively analyzed with both experimental and numerical methods, e.g. by means of PIV ) or DNS (Spalart 1988. In many studies the role of these structures for the wall normal and spanwise fluid exchange has been highlighted mostly within an Eulerian reference frame. But for a full understanding of the momentum exchange in turbulent wall flows a step towards a spatially resolved Lagrangian frame of reference would be advantageous. The data achieved from the present application of time-resolved tomographic PIV to a flat plate turbulent boundary layer flow in a water tunnel at Re θ~ 2460 enables for the first time a topological investigation of the flow structures and related particle motions within a temporally and spatially highly resolved Lagrangian and Eulerian reference frame.
Notes on Numerical Fluid Mechanics and Multidisciplinary Design, 2010
Turbulent boundary layer (TBL) flow consists of manifold temporal and spatial scales and is gover... more Turbulent boundary layer (TBL) flow consists of manifold temporal and spatial scales and is governed by the organization and decay of self-sustaining coherent flow structures driven by entrained high momentum fluid. Generic flow structures such as hairpin-like vortices and spanwise alternating wall bounded low- and high-speed streaks have been observed and extensively analyzed with both experimental and numerical methods. The role of these structures for the wall normal and spanwise fluid exchange has been highlighted mostly within an Eulerian reference frame. But for an understanding of the momentum exchange in turbulent wall flows a step towards a spatially resolved Lagrangian frame of reference would be advantageous. The data achieved from the present application of time-resolved tomographic PIV to a flat plate turbulent boundary layer flow enables for the first time the investigation of the flow structures and related particle motions within a temporally and spatially resolved Lagrangian and Eulerian frame of reference.
Cross-correlation constitutes the state-of-the-art method for measuring motion in PIV data. We pr... more Cross-correlation constitutes the state-of-the-art method for measuring motion in PIV data. We present a novel variational approach to correlation-based fluid flow estimation, which determines the displacements via continuous optimisation. A Gaussian shaped window function is used, whose size, eccentricity and orientation can freely be chosen. We adapt the shape to the noise level and displacement gradients by minimising an error
Lecture Notes in Computer Science, 2008
ABSTRACT This work focuses on tomographic image reconstruction in experimental fluid mechanics (T... more ABSTRACT This work focuses on tomographic image reconstruction in experimental fluid mechanics (TomoPIV), a recently established 3D particle image velocimetry technique. Corresponding 2D image sequences (projections) and the 3D reconstruction via tomographical methods provides the basis for estimating turbulent flows and related flow patterns through image processing. TomoPIV employs undersampling to make the high-speed imaging process feasible, resulting in an ill-posed image reconstruction problem. We address the corresponding basic problems involved and point out promising optimization criteria for reconstruction based on sparsity maximization, that perform favorably in comparison to classical algebraic methods currently in use for TomoPIV.
Uploads
Papers by Bernhard Wieneke