Homogeneity and isotropy in a laboratory turbulent flow

We present a new design for a stirred tank that is forced by two parallel planar arrays of randomly actuated synthetic jets. This arrangement creates turbulence at high Reynolds number with low mean flow. Most importantly, it exhibits a region of 3D homogeneous isotropic turbulence that is significantly larger than the integral lengthscale. These features are essential for enabling laboratory measurements of turbulent suspensions. We use quantitative imaging to confirm isotropy at large, small, and intermediate scales by examining one-- and two--point statistics at the tank center. We then repeat these same measurements to confirm that the values measured at the tank center are constant over a large homogeneous region. In the direction normal to the symmetry plane, our measurements demonstrate that the homogeneous region extends for at least twice the integral length scale $L=9.5$ cm. In the directions parallel to the symmetry plane, the region is at least four times the integral lengthscale, and the extent in this direction is limited only by the size of the tank. Within the homogeneous isotropic region, we measure a turbulent kinetic energy of $6.07 \times 10^{-4}$m$^2$s$^{-2}$, a dissipation rate of $4.65 \times 10^{-5}$m$^2$s$^{-3}$, and a Taylor--scale Reynolds number of $R_\lambda=334$. The tank's large homogeneous region, combined with its high Reynolds number and its very low mean flow, provides the best approximation of homogeneous isotropic turbulence realized in a laboratory flow to date. These characteristics make the stirred tank an optimal facility for studying the fundamental dynamics of turbulence and turbulent suspensions.Comment: 18 pages, 9 figure

Wind Energy and the Turbulent Nature of the Atmospheric Boundary Layer

Wind turbines operate in the atmospheric boundary layer, where they are exposed to the turbulent atmospheric flows. As the response time of wind turbine is typically in the range of seconds, they are affected by the small scale intermittent properties of the turbulent wind. Consequently, basic features which are known for small-scale homogeneous isotropic turbulence, and in particular the well-known intermittency problem, have an important impact on the wind energy conversion process. We report on basic research results concerning the small-scale intermittent properties of atmospheric flows and their impact on the wind energy conversion process. The analysis of wind data shows strongly intermittent statistics of wind fluctuations. To achieve numerical modeling a data-driven superposition model is proposed. For the experimental reproduction and adjustment of intermittent flows a so-called active grid setup is presented. Its ability is shown to generate reproducible properties of atmospheric flows on the smaller scales of the laboratory conditions of a wind tunnel. As an application example the response dynamics of different anemometer types are tested. To achieve a proper understanding of the impact of intermittent turbulent inflow properties on wind turbines we present methods of numerical and stochastic modeling, and compare the results to measurement data. As a summarizing result we find that atmospheric turbulence imposes its intermittent features on the complete wind energy conversion process. Intermittent turbulence features are not only present in atmospheric wind, but are also dominant in the loads on the turbine, i.e. rotor torque and thrust, and in the electrical power output signal. We conclude that profound knowledge of turbulent statistics and the application of suitable numerical as well as experimental methods are necessary to grasp these unique features (...)Comment: Accepted by the Journal of Turbulence on May 17, 201

Studying magnetic turbulence with radio polarimetry

Magnetised plasma permeates the Universe. It is present in stars, accretion discs, galaxies and clusters of galaxies. This dissertation investigates the characteristics of diffuse magnetised plasma in the inter-stellar-medium (ISM) and the intra-cluster-medium (ICM). To probe turbulence in the ISM and ICM, we develop an estimator for the magnetic tension-force spectrum. It is based on polarised synchrotron emission data in the Faraday-rotation-free frequency regime. The tension force is the dynamically most important magnetic force in subsonic magnetic turbulence. We consider two magnetic field scenarios: a statistically isotropic field distribution, and a statistically isotropic field upon a small mean field. In the latter case the magnetic power spectrum can also be obtained in addition to the tension-spectrum. The method is exact in the ideal case of a homogeneous cosmic ray electron distribution with a power law energy spectrum with a spectral index p=3, and isotropic magnetic fields. The method is applied to mock observations based on magneto-hydrodynamic simulations as a prelude to an application to real observations. We confirm the robustness of the estimator by comparing its output to the corresponding quantities directly computed from the simulation. Furthermore, to investigate the global Galactic field we developed hammurabi. This software tool takes three-dimensional theoretical models for the ISM components and generates mock observations from them. It focuses on the main tracers of our Galactic magnetised plasma such as synchrotron flux, Faraday rotation, dispersion measure and ultra-high-energy-cosmic-ray delflections. Hammurabi therefore permits us to confront models of the Galactic magnetised plasma with a broad range of real observations. We report on a number of works based on this code. In particular, due to simultaneous constraints of rotation measure, polarised and total synchrotron intensity, as well as theoretical predictions on the magnetic field and cosmic-ray electron distribution, the Galactic electron density scale height was suggested to be about twice as large as previously thought. This result obtained with hammurabi was recently confirmed independently by other authors using pulsar dispersion measure data. Finally, we consider the role of the Galactic kinetic Sunyaev Zeldovich (kSZ) effect as a CMB foreground. We analyse the detectability of the Galactic kSZ effect by means of an optimally matched filter technique applied to a simulation of an ideal observation. We obtain a S/N ratio of 0.1, and demonstrate thereby that the Galactic kSZ effect can safely be ignored as a CMB foreground. Furthermore, we rule out any significant contamination of the polarised CMB signal by second scattering of galactic kSZ photons