Daniel Schanz

Non-uniform optical transfer functions in particle imaging: calibration and application to tomographic reconstruction

A new approach to the weighting function, which describes particle imaging in tomographic reconstruction, is introduced. Instead of assuming a spatially homogeneous mapping function of voxels to the images, a variable optical transfer function (OTF) is applied. By this method, the negative effects of optical distortions on the reconstruction can be reduced considerably. The effects of these improvements in reconstruction quality on the methods of tomographic particle imaging velocimetry, as well as 3D particle tracking are investigated. A method to calibrate the OTF to experimental circumstances is proposed as an additional step to the volume self-calibration. It is shown that this kind of calibration is able to capture the predominant particle imaging both for simulated as well as experimental data. The most common distortions of particle imaging are blurring due to a small depth of field and astigmatism due to imaging optics. The effects of both of these distortions on reconstruction and correlation quality are investigated via simulated data. In both cases, a strong influence on relevant parameters can be seen. Reconstructions using a spatially varying OTF, calibrated to the imaging conditions, show a significant improvement in reconstruction quality and the accuracy of the particle peak position, as well as in the accuracy of the gained displacement vector field when using two time steps. Evaluation of experimental data by PTV methods shows a reduction in ghost particle intensity and improvements in peak position accuracy. A computationally efficient method of applying the OTF to tomographic reconstruction is introduced.

Molecular dynamics simulations of cavitation bubble collapse and sonoluminescence

The dynamics of the medium within a collapsing and rebounding cavitation bubble is investigated by means of molecular dynamics (MD) simulations adopting a hard sphere model for the species inside the bubble. The dynamics of the surrounding liquid (water) is modelled using a Rayleigh–Plesset (RP)-type equation coupled to the bubble interior by the gas pressure at the wall obtained from the MD calculations. Water vapour and vapour chemistry are included in the RP-MD model as well as mass and energy transfer through the bubble wall. The calculations reveal the evolution of temperature, density and pressure within a bubble at conditions typical of single-bubble sonoluminescence and predict how the particle numbers and densities of different vapour dissociation and reaction products in the bubble develop in space and time. Among the parameters varied are the sound pressure amplitude of a sonoluminescence bubble in water, the noble gas mixture in the bubble and the accommodation coefficients for mass and energy exchange through the bubble wall. Simulation particle numbers up to 10 million are used; most calculations, however, are performed with one million particles to save computer run time. Validation of the MD code was done by comparing MD results with solutions obtained by continuum mechanics calculations for the Euler equations.

The complex aerodynamic footprint of desert locusts revealed by large-volume tomographic particle image velocimetry

Particle image velocimetry has been the preferred experimental technique with which to study the aerodynamics of animal flight for over a decade. In that time, hardware has become more accessible and the software has progressed from the acquisition of planes through the flow field to the reconstruction of small volumetric measurements. Until now, it has not been possible to capture large volumes that incorporate the full wavelength of the aerodynamic track left behind during a complete wingbeat cycle. Here, we use a unique apparatus to acquire the first instantaneous wake volume of a flying animals entire wingbeat. We confirm the presence of wake deformation behind desert locusts and quantify the effect of that deformation on estimates of aerodynamic force and the efficiency of lift generation. We present previously undescribed vortex wake phenomena, including entrainment around the wing-tip vortices of a set of secondary vortices borne of Kelvin–Helmholtz instability in the shear layer behind the flapping wings.

Investigation of scaling laws in a turbulent boundary layer flow with adverse pressure gradient using PIV

We present an experimental investigation and data analysis of a turbulent boundary layer flow at a significant adverse pressure gradient at Reynolds number up to Reθ = 10, 000. We combine large-scale particle image velocimetry (PIV) with microscopic PIV for measuring the near wall region including the viscous sublayer. We investigate scaling laws for the mean velocity and for the total shear stress in the inner part of the boundary layer. In the inner part the mean velocity can be fitted by a log-law. In the outer part of the inner layer the log-law ceases to be valid. Instead, a modified log-law provides a good fit, which is given in terms of the pressure gradient parameter and a parameter for the mean inertial effects. Finally we describe and assess a simple quantitative model for the total shear stress distribution which is local in wall-normal direction without streamwise history effects.

MOLECULAR DYNAMICS APPROACH TO SONOLUMINESCENT BUBBLES

Bubbles in a standing sound field can be trapped at a pressure antinode and driven to strongly nonlinear oscillations with fast collapse, whereby shock waves and also faint, short light pulses are emitted. The physical and chemical processes in the interior of the bubbles associated with these phenomena are still not completely understood and also defy spatially resolved experimental investigation. Numerical modeling primarily relies on continuum methods by solving partial differential equations. Here an alternative approach is presented. The processes within collapsing sonoluminescing bubbles are investigated by molecular dynamics (MD) simulations with several million particles.

Flow field investigations in the free bypass jet flow of a V2527 engine at Ground Operation using SPIV

The investigation of the flow field at the nozzle exit of a full scale aero-engine using Stereo Particle Image Velocimetry (SPIV) aims at providing important reference and validation data required for reliable modeling and prediction. Furthermore, the turbulent jet engine exhaust flow is of prominent interest for the aerodynamic and -acoustic characterization of an aircraft engine. Especially the development of the free shear-layer between fan-bypass flow and outer flow is an important source of high-frequency noise as well as of big interest for modeling the turbulence of a jet flow correctly by advanced RANS methods. The SPIV measurements have been performed at free running fan phase positions and with a large number of statistically independent samples enabling a turbulence characterization of the fan-bypass and corresponding shear layer flow based on mean- and RMS-fields of all three velocity components, POD analysis and two-point-correlation functions.

Investigation of Transitional Flow Structures Downstream of a Backward-Facing-Step by Using 2D-2C- and High Resolution 3D-3C- Tomo- PIV

Transitional flow structures in the shear layer of a laminar separation bubble downstream of a backward facing step (BFS) have been investigated by means of 2D-2C- und highly resolved 3D-3C- tomographic Particle Image Velocimetry (PIV) for Reynolds numbers between Reh = 1420 and 3000 based on free-stream velocity U and step height h. By using an external acoustic excitation of the shear layer it was possible to arrange phase locked measurements of the wavy flow structures which emanate from instabilities according to Kelvin-Helmholtz [1] (KH). Snapshots of fully 3D-3C velocity vector volumes show complex flow topologies of the non-linear part of the laminar-turbulent transition scenario. This part seems to be governed by hairpin-like, streamwise elongated vortices on top of the classical spanwise oriented 2-D waves. These vortices organize a rapid fluid exchange normal to the shear layer leading to turbulent reattachment of the flow and subsequent development of a turbulent boundary layer.

Large-scale density gradient visualization of the V2527 engine jet flow at Ground Operation using Background Oriented Schlieren (BOS)

A high-speed BOS (Background Oriented Schlieren) system has been installed at the Airbus A320 aircraft engine in the sound-attenuating hangar during the DLR project SAMURAI. The density gradient structures of the free jet flow of a V2527 jet engine at maximum continuous thrust (MCT) have been recorded in a large domain with high temporal resolution. The measurements disclose the qualitative temporal evolution of these structures as well as an estimate of the time-averaged density. A cross-correlation analysis between the BOS measurements and a far-field microphone signal was used to examine the density structures connected to the noise generating processes.

News from bubble dynamics: High static pressures, shock waves and interior dynamics

Three topics from the vast area of bubble dynamics are addressed: (i) the influence of high static pressure on the oscillation of a spherical bubble in a sound field (calculations), (ii) the response of a bubble to a shock wave (experiments), and (iii) the interior dynamics as seen by molecular dynamics calculations. The notion of a bubble habitat is used for positionally and spherically stable, not dissolving bubbles in some two-dimensional space of parameters (mainly bubble radius at rest and sound pressure amplitude) for presenting the results. An elevated static pressure substantially enlarges the bubble habitat and thus the range, where bubbles can be stably trapped. Shock wave - bubble interaction is investigated in the context of laser induced bubbles collapsing in the neighborhood of a solid, plane boundary. Jet development as influenced by both the boundary and by a shock wave propagating parallel to the boundary is studied by high speed photography. The interior of a bubble is explored by molecu...

Inspection of the interior of a collapsing bubble via molecular dynamics

Strongly collapsing bubbles emit shock waves, faint light flashes and induce chemical reactions. At present, the interior of a collapsing bubble is not yet accessible experimentally to reveal the processes behind this behavior. Thus our knowledge has to be advanced theoretically. The standard method relies on solving a set of partial differential equations. However, in the case of small sonoluminescent bubbles, the number of molecules may become too small for a continuum approach. Therefore, the processes within collapsing sonoluminescent bubbles are investigated by molecular dynamics simulations. The temperature, density and pressure distribution within the bubble are calculated. Whereas mass and heat diffusion inside the bubble are automatically accounted for, they must be explicitly introduced across the boundary to the liquid. In particular, also the dissociation of water vapor and chemical reactions of the dissociation products are taken into account. Results are presented for different acoustic driv...