Beat Lüthi

Beat Lüthi
ETH Zurich | ETH Zürich · Institute of Environmental Engineering

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70
Publications
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1,960
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Publications

Publications (70)
Chapter
Aggregates grown in mild shear flow are released, one at a time, into homogeneous isotropic turbulence where their breakup is recorded by three-dimensional particle tracking velocimetry (3D-PTV). The aggregates have an open structure with fractal dimension around 2.2, and their size varies from 0.9 to 3.1 mm which is large compared to the Kolmogoro...
Article
Full-text available
Aggregates grown in mild shear flow are released, one at a time, into homogeneous isotropic turbulence where their motion and intermittent breakup is recorded by three dimensional Particle Tracking Velocimetry (3D-PTV). The aggregates have an open structure with fractal dimension of ∼ 2.2 and their size is 1.4 ± 0.4 mm which is large compared to th...
Article
We present an experimental investigation of entrainment and the dynamics near the turbulent/non-turbulent interface in a dense gravity current. The main goal of the study is to investigate changes in the interfacial physics due to the presence of stratification and to examine their impact on the entrainment rate. To this end, three-dimensional data...
Conference Paper
Aggregates prepared under fully destabilized conditions by the action of Brownian motion were exposed to an extensional flow generated at the entrance of the sudden contraction. Two non-invasive techniques were used to monitor their breakup process, i.e. light scattering and 3D particle tracking velocimetry (3D-PTV). While the first one can be used...
Article
Abstract Cardiovascular diseases can lead to abnormal blood flows, some of which are linked to hemolysis and thrombus formation. Abnormal turbulent flows of blood in the vessels with stenosis create strong shear stresses on blood elements and may cause blood cell destruction or platelet activation. We implemented a Lagrangian (following the fluid e...
Article
Parallel to the massive use of image-based computational hemodynamics to study the complex flow establishing in the human aorta, the need for suitable experimental techniques and ad hoc cases for the validation and benchmarking of numerical codes has grown more and more. Here we present a study where the 3D pulsatile flow in an anatomically realist...
Article
A three-dimensional pulsatile aortic flow in a human ascending aorta is studied to investigate the effect of the aortic stiffness on the flow field and turbulent fluctuating velocities in the ascending aorta. A non-intrusive optical measurement technique, 3D Particle Tracking Velocimetry (3D-PTV), has been applied to anatomically accurate phantoms...
Article
We present a newly developed combined scanning particle tracking velocimetry (SPTV) and scanning laser-induced fluorescence (SLIF) technique. The new method allows for the first time to measure the full velocity gradient tensor and the three-dimensional density field simultaneously in a refractive index matched environment. The data thus obtained w...
Article
PurposeTo validate Bayesian multipoint MR velocity encoding against particle tracking velocimetry for measuring velocity vector fields and fluctuating velocities in a realistic aortic model. Methods An elastic cast of a human aortic arch equipped with an 80 or 64% stenotic section was driven by a pulsatile pump. Peak velocities and peak turbulent k...
Chapter
A three-dimensional pulsatile aortic flow in a human ascending aorta is investigated in-vitro in this paper. A non-intrusive measurement technique, 3D Particle Tracking Velocimetry (3D-PTV), has been applied to the anatomically accurate silicon replicas. A compliant and a stiff aortic model were analyzed to better understand the influence of the ar...
Article
Large eddy simulation (LES) of a stirred tank equipped with a Rushton impeller and four cylindrical baffles was used to characterize the flow pattern and to assess the maximum turbulent kinetic energy dissipation rate ε max. While the shorter baffle‐impeller distance significantly affects the radial velocity profile and the trailing vortices expans...
Article
We report on effects of mean shear on the turbulent entrainment process, focusing in particular on their relation to small-scale processes in the proximity of the turbulent/non-turbulent interface (TNTI). Three-dimensional particle tracking velocimetry (3D-PTV) measurements of an axisymmetric jet are compared to data from a direct numerical simulat...
Conference Paper
The complex hemodynamics observed in the human aorta make this district a site of election for an in depth investigation of the relationship between fluid structures, transport and pathophysiology. In recent years, the coupling of imaging techniques and computational fluid dynamics (CFD) has been applied to study aortic hemodynamics, because of the...
Article
The special case of entrainment in a stratified flow, relevant to many geophysical flows such as oceanic overflows, so far has not been studied experimentally in terms of small-scale aspects around the turbulent/non-turbulent interface. In view of the fact that existing engineering concepts perform unsatisfactorily in practice, a new gravity curren...
Article
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Article
We report an experimental analysis of the local entrainment velocity in the self-similar region of a turbulent jet. Particle tracking velocimetry is performed to determine the position of the convoluted, instantaneous turbulent/non-turbulent interface and to compute velocity and velocity derivatives in the proximity of the interface. We find that t...
Article
Full-text available
A three-dimensional, pulsatile flow in a realis-tic phantom of a human ascending aorta with compliant walls is investigated in vitro. Three-Dimensional Particle Tracking Velocimetry (3D-PTV), an image-based, non-intrusive measuring method is used to analyze the aortic flow. The flow velocities and the turbulent fluctuations are determined. The velo...
Article
Full-text available
Genuine plane Couette flow is hard to realize experimentally, and no applications of modern spatially resolving measurement techniques have been reported for this flow so far. In order to resolve this shortcoming, we designed and built a new experimental facility and present our first results here. Our setup enables us to access the flow via 3D par...
Article
Full-text available
Particle tracking velocimetry (PTV) was applied to study turbulent entrainment in an axisymmetric jet at Re = 5000. Several single-point flow statistics are used to characterize the general flow field of our newly designed jet facility, proving that a self-preserving axisymmetric jet could be established. An analysis of the Lagrangian evolution of...
Article
In this Letter we present results from particle tracking velocimetry and direct numerical simulation that are congruent with the existence of a laminar superlayer, as proposed in the pioneering work of Corrsin and Kistler (NACA, Technical Report No. 1244, 1955). We find that the local superlayer velocity is dominated by a viscous component and its...
Conference Paper
In this paper, the 3D pulsatile flow in the replica of a human ascending aorta is investigated. The presented experimental results are obtained via three-dimensional Particle Tracking Velocimetry (3D-PTV), an image-based, nonintrusive measuring method. The investigation domain is the ascending aorta. A silicon replica of an aorta is used for the ex...
Article
Full-text available
We present a technique capable of measuring in a Lagrangian manner the three components of the flow velocity simultaneously at two different spatial resolutions. We use two synchronized 3D Particle Tracking Velocimetry (3D-PTV) systems equipped with filters to separate the two types of seeding particles and measure in volumes of 55 × 55 × 50 mm3 an...
Article
Full-text available
Viscous depletion of vorticity is an essential and well known property of turbulent flows, balancing, in the mean, the net vorticity production associated with the vortex stretching mechanism. In this letter we however demonstrate that viscous effects are not restricted to a mere destruction process, but play a more complex role in vorticity dynami...
Article
The two-dimensional space spanned by the velocity gradient invariants Q and R is expanded to three dimensions by the decomposition of R into its strain production −1/3sijsjkski and enstrophy production 1/4ωiωjsij terms. The {Q; R} space is a planar projection of the new three-dimensional representation. In the {Q; −sss; ωωs} space the Lagrangian ev...
Article
This paper presents an analysis of flow properties in the proximity of the turbulent/non-turbulent interface (TNTI), with particular focus on the acceleration of fluid particles, pressure and related small scale quantities such as enstrophy, ω2 = ωiωi, and strain, s2 = sijsij. The emphasis is on the qualitative differences between turbulent, interm...
Article
Lagrangian properties obtained from a particle tracking velocimetry experiment in a turbulent flow at intermediate Reynolds number are presented. Accurate sampling of particle trajectories is essential in order to obtain the Lagrangian structure functions and to measure intermittency at small temporal scales. The finiteness of the measurement volum...
Chapter
In preparation of simultaneous large-scale / small-scale 3D Particle Tracking Velocimetry (3D-PTV) experiments in a developing turbulent flow we performed two types of measurements separately: (i) the velocity and coarse-grained velocity derivatives were measured in a large observation volume with focus on the large-scale flow features and (ii) spa...
Article
Lagrangian properties obtained from a Particle Tracking Velocimetry experiment in a turbulent flow at intermediate Reynolds number are presented. Accurate sampling of particle trajectories is essential in order to obtain the Lagrangian structure functions and to measure intermittency at small temporal scales. The finiteness of the measurement volum...
Article
Full-text available
We present a comparative experimental study of a turbulent flow developing in clear water and dilute polymer solutions (25 and 50 wppm polyethylene oxide). The flow is forced by a planar grid that oscillates vertically with stroke S and frequency f in a square container of initially still fluid. Two-component velocity fields are measured in a verti...
Article
Full-text available
From Lie-group (symmetry) analysis of the multi-point correlation equation Oberlack and Günther (Fluid Dyn Res 33:453–476, 2003) found three different solutions for the behavior of shear-free turbulence: (i) a diffusion like solution, in which turbulence diffuses freely into the adjacent calm fluid, (ii) a deceleration wave like solution when there...
Conference Paper
Plane Couette flow has the peculiar property that transition to turbulence occurs despite the fact that its laminar profile is linearly stable. For high enough Reynolds numbers small amplitude perturbations are enough to initiate the growth of turbulent spots. Such spots have been subject to investigation for a few decades, see e.g. [1, 2]. In addi...
Chapter
We present a comparative experimental study of a turbulent flow developing in clear water and dilute polymer solutions (25 and 50 wppm polyethylene oxide). The flow is forced by a planar grid that oscillates vertically in a square container of initially still fluid. The two-component velocity fields are measured in a vertical plane passing through...
Article
Full-text available
We present a collection of eight data sets from state-of-the-art experiments and numerical simulations on turbulent velocity statistics along particle trajectories obtained in different flows with Reynolds numbers in the range R{lambda}in[120:740]. Lagrangian structure functions from all data sets are found to collapse onto each other on a wide ran...
Article
Full-text available
We report an analysis of small-scale enstrophy omega(2) and rate of strain s(2) dynamics in the proximity of the turbulent/non-turbulent interface in a flow without strong mean shear. The techniques used are three-dimensional particle tracking (3D-PTV), allowing the field of velocity derivatives to be measured and followed in a Lagrangian manner, a...
Article
Full-text available
From Lie-group (symmetry) analysis of the multi-point correlation equation Oberlack and Guenther [Fluid Dyn. Res. 33, 453-476 (2003)] found three different solutions for the diffusion of shear-free turbulence: (i) a heat equation like solution when the turbulence diffuses freely into the adjacent calm fluid, (ii) a deceleration wave like solution w...
Article
This is a report on a field experiment in an atmospheric surface layer at heights between 0.8 and 10m with the Taylor micro-scale Reynolds number in the range Reλ = 1.6−6.6 ×103. Explicit information is obtained on the full set of velocity and temperature derivatives both spatial and temporal, i.e. no use of Taylor hypothesis is made. The report co...
Article
This is part 3 of our work describing experiments in which explicit information was obtained on all the derivatives, i.e. spatial derivatives, ∂/∂ x j , and temporal derivatives, ∂/∂ t , of velocity and temperature fields (and all the components of velocity fluctuations and temperature) at the Reynolds number Re λ~10 ⁴ . This part is devoted to the...
Article
We report the first results of an experiment, in which explicit information on all velocity derivatives (the nine spatial derivatives, ∂ u i ∂ x j , and the three temporal derivatives, ∂ u i /∂ t ) along with the three components of velocity fluctuations at a Reynolds number as high as Re λ~10 ⁴ is obtained. No use of the Taylor hypothesis was made...
Article
We present a new stochastic model for relative two-particle separation in turbulence. Inspired by material line stretching, we suggest that a similar process also occurs beyond the viscous range, with time scaling according to the longitudinal second-order structure function S2(r), e.g.; in the inertial range as ε−1/3r2/3. Particle separation is mo...
Article
Combined measurements of the Lagrangian evolution of particle constellations and the coarse grained velocity derivative tensor \( \partial \tilde u_i /\partial x_j \) are presented. The data is obtained from three dimensional particle tracking measurements in a quasi isotropic turbulent flow at intermediate Reynolds number. Particle constellations...
Article
A Particle Tracking Velocimetry experiment has been performed in a turbulent flow at intermediate Reynolds number. We present experimentally obtained stretching rates for particle pairs in the inertial range. When compensated by a characteristic time scale for coarse-grained strain we observe constant stretching. This indicates that the process of...
Article
From particle tracking velocimetry we present an experimental measure of the ratio between backwards and forwards relative dispersion in an intermediate Reynolds number turbulent flow. Lack of time-reversal symmetry implies that their ratio may be different from 1. From a stochastic model, this has recently been studied by Sawford et al [Phys. Flui...
Article
Full-text available
The Lagrangian evolution of infinitesimal material lines is investigated experimentally through three dimensional particle tracking velocimetry (3D-PTV) in quasihomogeneous turbulence with the Taylor microscale Reynolds number Re(lambda)=50. Through 3D-PTV we access the full tensor of velocity derivatives du(i)/dx(j) along particle trajectories, wh...
Article
Full-text available
In this article, we present an experimental setup and data processing schemes for 3D scanning particle tracking velocimetry (SPTV), which expands on the classical 3D particle tracking velocimetry (PTV) through changes in the illumination, image acquisition and analysis. 3D PTV is a flexible flow measurement technique based on the processing of ster...
Article
Full-text available
The evolution of material lines, $l$, and vorticity, $\omega$, is investigated experimentally through three-dimensional particle-tracking velocimetry (3D-PTV) in quasi-homogeneous isotropic turbulence at $Re_{\lambda }\,{=}\,50$. Through 3D-PTV data the full set of velocity derivatives, $\partial u_{i}/\partial x_{j}$, is accessible. This allows us...
Article
Full-text available
The full set of velocity derivatives, $\partial u_{i}/\partial x_{j}$, is measured experimentally in a Lagrangian way in quasi-homogeneous isotropic turbulence. This is achieved by applying the three-dimensional particle tracking velocimetry (3D-PTV) technique to an electromagnetically forced flow with $\hbox{\it Re}_{\lambda}\,{\thickapprox}\,50$....
Article
Full-text available
The work reported below is a comparative study of the properties of turbulence with weak mean flow in a Newtonian fluid and in a dilute polymer solution with an emphasis on the small scale phenomena. The main tool used is a three-dimensional particle tracking system allowing to measure and follow in a Lagrangian manner the field of velocities, as w...
Article
We present an experimental, comparative study of the turbulent quantities of a weak mean turbulent flow in water and in a dilute polymer solution with an emphasis on phenomena at small scales. The experiment is performed by using a three-dimensional particle tracking velocimetry (3D-PTV) system. 3D-PTV allows to measure in a Lagrangian manner field...
Article
Full-text available
The Lagrangian evolution of infinitesimal material lines is investigated experimentally through three dimensional measurements of particle trajectories (3D-PTV) in quasi homogeneous isotropic turbulence with Re λ = 50. By postprocessing 3D-PTV data obtained by Lüthi et al. (2005) we access the full set of velocity derivatives ∂u i/∂x j which allows...
Article
Full-text available
Three-dimensional particle tracking velocimetry (3D-PTV) was applied to measure acceleration and spatial velocity gradients in quasi-homogeneous and isotropic turbulent ow. Structural aspects such as: spatial distribution of extreme values of acceleration in respect to the elds of vorticity and/or rate-of-strain, decomposition of the total accelera...

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