arXiv subjects

Christian Küchler

Publications and source records attributed to Christian Küchler.

6 recordsLinked to original sources

arXiv2024

Lagrangian Particle Tracking at Large Reynolds Numbers

Particle tracking in turbulent flows is fundamental to the study of the transport of tracers, inertial particles or even active objects in space and time, i.e. the Lagrangian frame of reference. It provides experimental tests of theoretical predictions (e.g. for the statistics of fluid accelerations and particle dispersion) and helps to understand important natural processes where particle inertia is important (e.g. cloud microphysics). While the spatial (Eulerian) properties of turbulent flows have been studied for high, atmospheric Reynolds numbers ($R_λ> 10^4$), the profound difficulties in accurately tracking particles in turbulent flows have limited the Reynolds numbers in the Lagrangian reference frame to the Taylor scale Reynolds numbers $R_λ\lesssim 10^3$. Here we describe a setup that allowed Lagrangian particle tracking at $R_λ$ between 100 and 6000 in the Max Planck Variable Density Turbulence Tunnel (VDTT). We describe the imaging setup within the pressurised facility, the laser illumination, the particles and the particle dispersion mechanism. We verify that the KOBO Cellulobeads D-10 particles are suitable tracers. They carry negligible charge and their Stokes number is small over the full range of experimental conditions. We present typical data from the experiment and discuss the challenges and constraints of the setup.

physics.flu-dyn↗

arXiv2022

Flight of a honeybee in turbulent wind

In windy conditions, the air is turbulent. The strong and intermittent velocity variations of turbulence are invisible to flying animals. Nevertheless, flying animals, not much larger than the smallest scales of turbulence, manage to maneuver these highly fluctuating conditions quite well. Here we quantify honeybee flight with time-resolved three-dimensional tracking in calm conditions and controlled turbulent winds. We find that honeybee mean speed and acceleration are only weakly correlated with the strength of turbulence. In flight, honeybees accelerate slowly and decelerate rapidly, i.e., they break suddenly during turns and then accelerate again. While this behavior is observed in both calm and turbulent conditions, it is increasingly dominant under turbulent conditions where short straight trajectories are broken by turns and increased maneuvering. This flight-crash behavior is reminiscent of turbulence itself. Our observations may help the development of flight strategies for miniature flying robotics under turbulent conditions.

physics.flu-dyn↗

arXiv2022

Scaling in Decaying Turbulence at High Reynolds Numbers

The way the increment statistics of turbulent velocity fluctuations scale with the increment size is a centerpiece of turbulence theories. We report data on decaying turbulence in the Max Planck Variable Density Turbulence Tunnel (VDTT), which show an approach of the inertial range statistics toward a nontrivial shape at small scales. By correcting for the contributions of energy decay to the large-scale statistics with a model, we find the scaling exponent of the second-order velocity increment statistics to be independent of the Reynolds number and equal to $0.693\pm0.003$ for $2000\lesssim R_λ \lesssim 6000$. This is evidence of a universal inertial range at high Reynolds numbers.

physics.flu-dyn↗

arXiv2021

Universality in Decaying Turbulence at High Reynolds Numbers

A hallmark of fluid turbulence theory is the universal power law scaling of the velocity difference statistics between two points in space in the inertial range between the large energy injection scale and the small energy dissipation scale. Even at the highest Reynolds numbers available, laboratory and natural flows such universal power laws have not been convincingly demonstrated. Here we show for the decaying active grid turbulence of the Max Planck Variable Density Turbulence Tunnel that the velocity difference statistics at high Reynolds numbers do not exhibit a power law, but have a universal functional form independent of the Reynolds number. We separate this functional form from the power law exponent and discuss potential consequences for turbulence modelling.

physics.flu-dyn↗

arXiv2021

Fabrication of free-standing Pt nanowires for use as thermal anemometry probes in turbulence measurements

We report a robust fabrication method for patterning free-standing Pt nanowires for the use as thermal anemometry probes for small-scale turbulence measurements. Using e-beam lithography, high aspect ratio Pt nanowires (~300 nm width, ~70 $μ$m length, ~100 nm thickness) were patterned on the surface of oxidized silicon (Si) wafers. Combining precise wet etching processes with dry etching processes, these Pt nanowires have been successfully released free-standing between two silicon dioxide (SiO2) beams supported on Si cantilevers. Moreover, the unique design of the bridge holding the device allowed to release the device gently without damaging the Pt nanowires. The total fabrication time was minimized by restricting the use of e-beam lithography to the patterning of the Pt nanowires while standard photolithography was employed for other parts of the devices. We demonstrate that the fabricated sensors are suitable for turbulence measurements when operated in a constant-current mode. A robust calibration between output voltage and fluid velocity was established over the velocity range from 0.5 m s-1 to 5 m s-1 in an SF6 atmosphere at a pressure of 2 bar and a temperature of 21°C. The sensing signal from the nanowires showed negligible drift over a period of several hours. Moreover, we confirmed that the nanowires are able to withstand high dynamic pressures by testing them in air at room temperature velocities up to 55 m/s.

physics.app-ph↗

arXiv2018

Experimental Study of the Bottleneck in Fully Developed Turbulence

The energy spectrum of incompressible turbulence is known to reveal a pileup of energy at those high wavenumbers where viscous dissipation begins to act. It is called the bottleneck effect. Based on direct numerical simulations of the incompressible Navier-Stokes equations, results from Donzis & Sreenivasan (2010) pointed to a decrease of the strength of the bottleneck with increasing intensity of the turbulence, measured by the Taylor micro-scale Reynolds number $R_λ$. Here we report first experimental results on the dependence of the amplitude of the bottleneck as a function of $R_λ$ in a wind-tunnel flow. We used an active grid in the Variable Density Turbulence Tunnel (VDTT) (see Bodenschatz et al. (2014)) to reach $R_λ$ > 5000, which is unmatched in laboratory flows of decaying turbulence. The VDTT with the active grid permitted us to measure energy spectra from flows of different $R_λ$, with the small-scale features appearing always at the same frequencies. We relate those spectra recorded to a common reference spectrum, largely eliminating systematic errors which plague hotwire measurements at high frequencies. The data are consistent with a power law for the decrease of the bottleneck strength for the finite range of $R_λ$ in the experiment.

physics.flu-dyn↗