Search

Forschungsprojekt SVI 2018/002 auf Antrag der Schweizerischen Vereinigung der Verkehrsingenieure und Verkehrsexperten (SVI) Titelangabe und Zusammenfassung auf Deutsch, Französisch und Englisch, Text auf Deutsch, Bislang wird die lokale Verkehrssicherheit in der Schweiz im Wesentlichen anhand des tatsächlichen Unfallgeschehens beurteilt. Statistisch betrachtet kommen Unfallereignisse in Relation zum gesamten Verkehrsaufkommen nur selten vor und deshalb liegt dieser Methode eine gewisse Unschärfe zugrunde. Zusätzlich kann die Analyse erst im Nachhinein erfolgen und lässt nicht immer konkrete Rückschlüsse auf die Defizite der Infrastruktur zu. Um dem Ziel einer objektiven Bewertung der Sicherheit von beispielsweise einer Strasse, Kreuzung oder Querungsstelle näher zu kommen, wurde im Rahmen dieses Forschungsprojektes nach Sicherheitsindikatoren und Methoden zu deren Messung recherchiert. Schliesslich wurde eine geeignete Methode definiert, welche anschliessend in Feldversuchen an insgesamt acht Untersuchungsorten mit unterschiedlichen Eigenschaften getestet wurde. Die Messergebnisse der angewendeten alternativen Methode (Videoanalyse) wurden einer objektiven Prüfung unterzogen und den Resultaten der Begleituntersuchungen sowie der Unfallanalysen gegenübergestellt. Das methodische Vorgehen ist in diesem Bericht ebenso dokumentiert wie die Möglichkeiten und Limitationen der Methode sowie die aus den Erkenntnissen der Feldversuche ableitbaren Empfehlungen hinsichtlich ihrer Anwendbarkeit.

Image cross‐correlation techniques, such as Particle Image Velocimetry (PIV), can estimate water surface velocity (vsurf) of streams. However, discharge estimation requires water depth and the depth‐averaged vertical velocity (Um). The variability of the ratio Um/vsurf introduces large errors in discharge estimates. We demonstrate a method to estimate vsurf from Unmanned Aerial Systems (UASs) with PIV technique. This method does not require any Ground Control Point (GCP): the conversion of velocities from pixels per frame into meters per time is performed by informing a camera pinhole model; the range from the pinhole to the water surface is measured by the drone‐board radar. For approximately uniform flow, Um is a function of the Gauckler‐Manning‐Strickler coefficient (Ks) and vsurf. We implement an approach that can be used to jointly estimate Ks and discharge by informing a system of 2 unknowns (Ks and discharge) and 2 non‐linear equations: i) Manning's equation ii) mean‐section method for computing discharge from Um. This approach relies on bathymetry, acquired in‐situ a‐priori, and on UAS‐borne vsurf and water surface slope measurements. Our joint (discharge and Ks) estimation approach is an alternative to the widely used approach than relies on estimating Um as 0.85·vsurf. It was extensively investigated in 27 case studies, in different streams with different hydraulic conditions. Discharge estimated with the joint estimation approach showed a mean absolute error in discharge of 19.1% compared to in‐situ discharge measurements. Ks estimates showed a mean absolute error of 3.2 m1/3/s compared to in‐situ measurements.

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 particle tracking velocimetry and therefore to obtain truly three-dimensional flow fields for the first time experimentally in plane Couette flow. Results are analyzed in terms of basic flow properties, and a clear distinction of flow regimes (laminar for Re<320, transitional for 320400) could be made. Comparison with DNS data shows good agreement in the turbulent regime and builds trust in our data. Furthermore, vortical coherent structures are studied in detail with the additional help of kalliroscope imaging, and the typical vortex spacing is determined to be roughly one gap width. As a noteworthy result, we find that the onset of the turbulent regime coincides with the range of Reynolds numbers at which a distance of 100 wall units is comparable to the gap width

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 current facility was designed with the goal to gain understanding of how stratification affects interfacial physics. Here, we present the design of the new setup and give details on the turbulence enhancement in the inflow and the refractive index matching technique used. Validation measurements ensure that there is negligible backflow and an essentially irrotational flow outside the current. Measurements via particle image velocimetry of a flow with inflow Reynolds and Richardson numbers of $$Re_0\approx \hbox{4,000}$$ and Ri 0=0.22 are reported. An analysis in a laboratory frame agrees well with flow features reported in the literature, i.e., a streamwise invariant top-hat velocity scale and a Reynolds stress distribution are matched closely by a mixing length model. In a second step, the instantaneous interface position is determined based on a threshold on the normal enstrophy component. An investigation in a frame of reference conditioned on the interface position reveals a strong interfacial shear layer that is much more pronounced than the one observed in jet flows. Its thickness is about two times the Taylor microscale. The data moreover suggest the existence of a fairly strong interfacial density jump across the shear layer. The entrainment parameter is estimated at $$E \approx 0.04$$ congruently from the evaluations in laboratory and conditioned frame, respectively

A three-dimensional, pulsatile flow in a realistic 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 velocity profile at the inlet of the ascending aorta is relatively flat with a skewed profile toward the inner aortic wall in the early systole. In the diastolic phase, a bidirectional flow is observed with a pronounced retrograde flow developing along the inner aortic wall, whereas the antegrade flow migrates toward the outer wall of the aorta. The spatial and temporal evolution of the vorticity field shows that the vortices begin developing along the inner wall during the deceleration phase and attenuate in the diastolic phase. The change in the cross-sectional area is more distinct distal to the inlet cross section. The mean kinetic energy is maximal in the peak systole, whereas the turbulent kinetic energy increases in the deceleration phase and reaches a maximum in the beginning of the diastolic phase. Finally, in a Lagrangian analysis, the temporal evolution of particle dispersion was studied. It shows that the dispersion is higher in the deceleration phase and in the beginning of the diastole, whereas in systole, it is smaller but non-negligible

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 stereoscopic images of flow tracer particles. The technique allows obtaining Lagrangian flow information directly from measured 3D trajectories of individual particles. While for a classical PTV the entire region of interest is simultaneously illuminated and recorded, in SPTV the flow field is recorded by sequential tomographic high-speed imaging of the region of interest. The advantage of the presented method is a considerable increase in maximum feasible seeding density. Results are shown for an experiment in homogenous turbulence and compared with PTV. SPTV yielded an average 3,500 tracked particles per time step, which implies a significant enhancement of the spatial resolution for Lagrangian flow measurements

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 sets of the density and the velocity fields are obtained through a combined scanning particle tracking velocimetry/laser-induced fluorescence approach for two different stratification levels with inflow Richardson numbers of $\mathit{Ri}_{0}=0.23$ and $\mathit{Ri}_{0}=0.46$ , respectively, at a Reynolds number around $\mathit{Re}_{0}=3700$ . An analysis conditioned on the instantaneous position of the turbulent/non-turbulent interface as defined by a threshold on enstrophy reveals an interfacial region that is in many aspects independent of the initial level of stratification. This is reflected most prominently in matching peaks of the gradient Richardson number $\mathit{Ri}_{g}\approx 0.1$ located approximately $10{\it\eta}$ from the position of the interface inside the turbulent region, where ${\it\eta}=({\it\nu}^{3}/{\it\epsilon})^{1/4}$ is the Kolmogorov scale, and ${\it\nu}$ and ${\it\epsilon}$ denote the kinematic viscosity and the rate of turbulent dissipation, respectively. A possible explanation for this finding is offered in terms of a cyclic evolution in the interaction of stratification and shear involving the buildup of density and velocity gradients through inviscid amplification and their subsequent depletion through molecular effects and pressure. In accordance with the close agreement of the interfacial properties for the two cases, no significant differences were found for the local entrainment velocity, $v_{n}$ (defined as the propagation velocity of an enstrophy isosurface relative to the fluid), at different initial stratification levels. Moreover, we find that the baroclinic torque does not contribut

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 particle tracking velocimetry and therefore to obtain truly three-dimensional flow fields for the first time experimentally in plane Couette flow. Results are analyzed in terms of basic flow properties, and a clear distinction of flow regimes (laminar for Re < 320, transitional for 320 < Re < 400, and turbulent when Re > 400) could be made. Comparison with DNS data shows good agreement in the turbulent regime and builds trust in our data. Furthermore, vortical coherent structures are studied in detail with the additional help of kalliroscope imaging, and the typical vortex spacing is determined to be roughly one gap width. As a noteworthy result, we find that the onset of the turbulent regime coincides with the range of Reynolds numbers at which a distance of 100 wall units is comparable to the gap width., Swiss National Science Foundation (Grant 2-77898-10)