Large-scale 4D-PTV measurements of a thermal plume using helium-filledsoap-bubbles (HFSB) and Shake-The-Box

Kurzfassung

Shake-The-Box (STB) is a 4D-PTV technique [1, 2] based on time-resolved illumination, recording and reconstruction of densely seeded particles in a volume of a flow. It is an intrinsic feature of the STB evaluation scheme that particles are reconstructed as trajectories in space and time by using the time series of particle images captured by typically three to six cameras from different viewing directions. In contrast to tomographic particle reconstruction using multiplicative algorithms like MART [3] the STB algorithm do not need a voxel space and related intensity distribution for an accurate reconstruction of particle positions. After initialization STB identifies the 3D position of each particle and time-step essentially by a two-step approach: predicting and shaking. While the prediction step relies on the extrapolation of single particle trajectories to an estimated 3D position of the particle at time instant n+1 by using a polynomial fitting function, the shaking step is correcting the 3D particle position estimation by an image matching scheme implemented according to the Iterative Particle Reconstruction (IPR) technique developed by Wieneke [4] and the usage of the local optical transfer function (OTF) [5]. Especially for deep volumes the data sizes needed for a STB evaluation delivering only a list of x-y-z-particle positions and related velocity distributions after temporal fitting is much smaller than a respective voxel representation. As one consequence the computing time for a STB reconstruction is about 3 to 20 times faster than the fastest actual tomo PIV algorithm (the latter value for low particle numbers) and is mainly depending on the number of particles within the measurement volume. Therefore, STB is a very sparse reconstructing scheme and predesignated for dense particle trajectories in flows with a large and deep measurement volume. On the other hand new hardware developments based on HFSB for use in large-scale tomographic PIV experiments [6] and pulsed LED illuminations are making significant progress recently.