Analysis of flow around a heated circular cylinder

The objective of this study is to investigate the forced convection from and the flow around a heated cylinder. This work presents experimental and computational results for laminar flow around a heated circular cylinder. The experiments were carried out using Particle Image Velocimetry (PIV) in a wind tunnel, and numerical simulations by an in-house code and a commercial software package, FLUENT. This paper presents comparisons for vorticity and temperature contours in the wake of the cylinder

Analysis of low Reynolds number flow around a heated circular cylinder

The objective of this study is to investigate the forced convection from and the flow around a heated cylinder. Experimental and computational results are presented for laminar flow around a heated circular cylinder with a diameter of 10 mm. The experiments were carried out using Particle Image Velocimetry (PIV) in a wind tunnel, and numerical simulations using an in-house code and a commercial software package, FLUENT. This paper pre-sents comparisons for vorticity and temperature contours in the wake of the cylinder. Experimental and computa-tional results are compared with those available in the literature for heated and unheated cylinders. An equation is suggested for a temperature-dependent coefficient defining a reference temperature to be used in place of the con-stant used in other studies. An attempt is also made to correct differences between average cylinder surface tem-perature and measured interior temperature of the cylinder

Numerical simulations and measurements of a droplet size distribution in a turbulent vortex street

A turbulent vortex street in an air flow interacting with a disperse droplet population is investigated in a wind tunnel. Non-intrusive measurement techniques are used to obtain data for the air velocity and the droplet velocity. The process is modeled with a population balance system consisting of the incompressible NavierStokes equations and a population balance equation for the droplet size distribution. Numerical simulations are performed that rely on a variational multiscale method for turbulent flows, a direct discretization of the differential operator of the population balance equation, and a modern technique for the evaluation of the coalescence integrals. After having calibrated two unknown model parameters, a very good agreement of the experimental and numerical results can be observed

Experimental determination of droplet collision rates in turbulence

Inter-particle collisions in turbulent flows are of central importance for many engineering applications and environmental processes. For instance, collision and coalescence is the mechanism for warm rain initiation in cumulus clouds, a still poorly understood issue. This work presents measurements of droplet–droplet interactions in a laboratory turbulent flow, allowing reproducibility and control over initial and boundary conditions. The measured two-phase flow reproduces conditions relevant to cumulus clouds. The turbulent flow and the droplet size distribution are well characterized, and independently the collision rate is measured. Two independent experimental approaches for determining the collision rate are compared with each other: (i) a highmagnification shadowgraphy setup is employed, applying a deformation threshold as collision indicator. This technique has been specifically adapted to measure droplet collision probability in dispersed two-phase flows. (ii) Corresponding results are compared for the first time with a particle tracking approach, post-processing high-speed shadowgraphy image sequences. Using the measured turbulence and droplet properties, the turbulent collision kernel can be calculated for comparison. The two independent measurements deliver comparable orders of magnitude for the collision probability, highlighting the quality of the measurement process, even if the comparison between both measurement techniques is still associated with a large uncertainty. Comparisons with recently published theoretical predictions show reasonable agreement. The theoretical collision rates accounting for collision efficiency are noticeably closer to the measured values than those accounting only for transport

Planare PTV Messungen in Phantommodellen zerebraler Aneurysmen

Die Ursachen der Ruptur zerebraler Aneurysmen (Ballonartige Erweiterungen von Blutgefäßen) sind noch immer weitgehend ungeklärt. Die Hämodynamik scheint dabei jedoch ein wichtiger Aspekt zu sein, der zunehmend in den Fokus des Interesses wissenschaftlicher Untersuchungen gerät, zumal gerade aus therapeutischer Sicht auf diesem Weg versucht wird, positiv auf das System einzuwirken. Dabei werden beispielsweise so genannte Flowdiverter implantiert, die den Blutfluss in das Aneurysma reduzieren, zu einer Thrombosierung des Lumens führen und letztlich das Gefahrenpotential des Aneurysmas für den Patienten selbst reduzieren sollen. In der vorliegenden Arbeit werden instationäre Particle Tracking Velocimetry (PTV) Messungen in mehreren Ebenen eines inversen Silikonmodels eines maßstabsgetreuen Aneurysmas vorgestellt (siehe Abb. 1 links). Dabei wurde ein Dauerstrich Ar+-Laser zu einem Lichtschnitt aufgeweitet, der zur Anregung fluoreszierender Mikropartikel genutzt wurde. Eine Hochgeschwindigkeitskamera diente zur Aufnahme des emittierten Tracerlichts und ermöglichte instantanes Verfolgen der Bahnlinien in den Quasi-Ebenen (Tracking). Das verwendete Modellfluid wurde empirisch auf bestmögliche Brechungsindexübereinstimmung mit dem Silikonmaterial und Ähnlichkeit mit strömungsmechanischen Eigenschaften von menschlichem Blut angepasst und mittels einer gesteuerten Peristaltikpumpe entsprechend eines typischen Zyklus‘ des Herzens durch den Modellkreislauf gepumpt. Als Ergebnisse werden die planaren Strömungsstrukturen im Aneurysma mit und ohne Implantation eines Flowdiverters durch Visualisierung der Trajektorien aufgezeigt

Investigation of the Velocity Field in a Full-Scale Artificial Martificial Medical Model

The aim of this contribution was to experimentally characterize the flow in the aneurysm of a full-scale medical phantom model for validation of companion numerical simulations. Due to its practical importance, a non-intrusive treatment of brain aneurysm attracts growing interest. To develop suitable treatment options, a better knowledge of the blood flow pattern in the complex geometry of aneurysms and cerebral vasculature is very important. To get information on the flow around the aneurysm, Laser Doppler Velocimetry (LDV) measurements had already been carried out by our group in a similar model at several cross-sections, considering a pulsating flow. As a complement, the present experimental series were carried out using large-field imaging measurement techniques. First, 2D-Particle Tracking Velocimetry (PTV) images were recorded and our in-house PTV-algorithm was optimized for the measurement configuration. The flow was seeded by fluorescent tracers and the excitation wave length was filtered out to avoid undesirable Mie scattering. In order to validate the PTV algorithm, the recorded images were also evaluated using a conventional cross-correlation method, like in Particle Image Velocimetry (PIV). Due to the different nature of the two evaluation methods, an interpolation of the Lagrangian (PTV) data into an Eulerian coordinate system (PIV) was required, in order to make a proper comparison between the two evaluation algorithms. The experimental setup, methods, results and conclusions are presented in this work

Investigation of the velocity field in a full-scale model of a cerebral aneurysm

Due to improved and now widely used imaging methods in clinical surgery practise, detection of unruptured cerebral aneurysms becomes more and more frequent. For the selection and development of a low-risk and highly effective treatment option, the understanding of the involved hemodynamic mechanisms is of great importance. Computational Fluid Dynamics (CFD), in vivo angiographic imaging and in situ experimental investigations of flow behaviour are powerful tools which could deliver the needed information. Hence, the aim of this contribution is to experimentally characterise the flow in a full-scale phantom model of a realistic cerebral aneurysm. The acquired experimental data will then be used for a quantitative validation of companion numerical simulations. The experimental methodology relies on the large-field velocimetry technique PTV (Particle Tracking Velocimetry), processing high speed images of fluorescent tracer particles added to the flow of a blood-mimicking fluid. First, time-resolved planar PTV images were recorded at 4500 fps and processed by a complex, in-house algorithm. The resulting trajectories are used to identify Lagrangian flow structures, vortices and recirculation zones in two-dimensional measurement slices within the aneurysm sac. The instantaneous inlet velocity distribution, needed as boundary condition for the numerical simulations, has been measured with the same technique but using a higher frame rate of 20,000 fps in order to avoid ambiguous particle assignment. From this velocity distribution, the time-resolved volume flow rate has been also derived. In this manner, a direct comparison between numerical simulations and PTV measurements will be possible in the near future, opening the door for highly accurate computational predictions

Model results, link to archive file

This study deals with the comparison of numerically and experimentally determined collision kernels of water drops in air turbulence. The numerical and experimental setups are matched as closely as possible. However, due to the individual numerical and experimental restrictions, it could not be avoided that the turbulent kinetic energy dissipation rate of the measurement and the simulations differ. Direct numerical simulations (DNS) are performed resulting in a very large database concerning geometric collision kernels with 1470 individual entries. Based on this database a fit function for the turbulent enhancement of the collision kernel is developed. In the experiments, the collision rates of large drops (radius > 7.5 µm) are measured. These collision rates are compared with the developed fit, evaluated at the measurement conditions. Since the total collision rates match well for all occurring dissipation rates the distribution information of the fit could be used to enhance the statistical reliability and for the first time an experimental collision kernel could be constructed. In addition to the collision rates, the drop size distributions at three consecutive streamwise positions are measured. The drop size distributions contain mainly small drops (radius < 7.5 µm). The measured evolution of the drop size distribution is confronted with model calculations based on the newly derived fit of the collision kernel. It turns out that the observed fast evolution of the drop size distribution can only be modeled if the collision kernel for small drops is drastically increased. A physical argument for this amplification is missing since for such small drops, neither DNSs nor experiments have been performed. For large drops, for which a good agreement of the collision rates was found in the DNS and the experiment, the time for the evolution of the spectrum in the wind tunnel is too short to draw any conclusion. Hence, the long-time evolution of the drop size distribution is presented in Riechelmann et al. 2015 (doi:10.1127/metz/2015/0608)

Influence of turbulence on the drop growth in warm clouds, Part I : comparison of numerically and experimentally determined collision kernels

This study deals with the comparison of numerically and experimentally determined collision kernels of water drops in air turbulence. The numerical and experimental setups are matched as closely as possible. However, due to the individual numerical and experimental restrictions, it could not be avoided that the turbulent kinetic energy dissipation rate of the measurement and the simulations differ. Direct numerical simulations (DNS) are performed resulting in a very large database concerning geometric collision kernels with 1470 individual entries. Based on this database a fit function for the turbulent enhancement of the collision kernel is developed. In the experiments, the collision rates of large drops (radius > 7.5μm) are measured. These collision rates are compared with the developed fit, evaluated at the measurement conditions. Since the total collision rates match well for all occurring dissipation rates the distribution information of the fit could be used to enhance the statistical reliability and for the first time an experimental collision kernel could be constructed. In addition to the collision rates, the drop size distributions at three consecutive streamwise positions are measured. The drop size distributions contain mainly small drops (radius < 7.5μm). The measured evolution of the drop size distribution is confronted with model calculations based on the newly derived fit of the collision kernel. It turns out that the observed fast evolution of the drop size distribution can only be modeled if the collision kernel for small drops is drastically increased. A physical argument for this amplification is missing since for such small drops, neither DNSs nor experiments have been performed. For large drops, for which a good agreement of the collision rates was found in the DNS and the experiment, the time for the evolution of the spectrum in the wind tunnel is too short to draw any conclusion. Hence, the long-time evolution of the drop size distribution is presented in a companion paper (Riechelmann et al., 2014)