This thesis concerns the numerical simulation and the remote sensing of aircraft wake vortex flows. Due to its lift force, an aircraft releases large scale swirling flows (vortices) in its wake. As these vortices can impact significantly the trajectory of a following aircraft, their study is of great importance for practical applications concerning safety of air traffic management. The investigation carried here is twofold: it concerns, on one hand, the physics and the numerical simulation of aircraft wake vortices and, on the other hand, the technique to detect those vortices and measure their properties.The numerical simulation of aircraft wake vortices requires high order and energy conserving codes. Moreover, as aircraft wake vortex flows are turbulent, subgrid scale (SGS) models are required to perform Large Eddy Simulation (LES) of these flows. In the first part of this work, the numerical codes used (here spectral and high order finite differences) are presented and validated. Several SGS models are presented and their ability to perform LES of vortical flows, also in presence of a ground is assessed. In particular a new “multiscale” model with a natural wall damping behaviour has been developed and investigated: its performance is very good. In the second part, two flows relevant to the wake vortex problem are analyzed. The LES of a wake vortex system in a weakly turbulent atmosphere allowed highlighting the turbulent equilibrium state of such a flow. LES of wake vortices in ground effect, with and without wind, were also carried out. These simulations help to better understand the physics of wake vortices. In the last part, we simulate the remote sensing of aircraft wake vortices using a ground based LIDAR (Light Detection And Ranging) system. The aim of this LIDAR is to sense aircraft wake vortices and turbulent winds. As the LIDAR signals are simulated using realistic parameters and realistic turbulent flows, this work serves as support in the design of an airport based LIDAR system.

Experiments and direct numerical simulations (DNS) have been performed to examine the e¤ects of initial conditions on the dynamics of a Rayleigh-Taylor unsta- ble mixing layer. Experiments were performed on a water channel facility to measure the interfacial and velocity perturbations initially present at the two-fluid interface in a small Atwood number mixing layer. The experimental measurements have been parameterized for use in numerical simulations of the experiment. Two- and three- dimensional DNS of the experiment have been performed using the parameterized initial conditions. It is shown that simulations implemented with initial velocity and density perturbations, rather than density perturbations alone, are required to match experimentally-measured statistics and spectra. Data acquired from both the exper- iment and numerical simulations are used to examine the role of initial conditions on the evolution of integral-scale, turbulence, and mixing statistics. Early-time turbu- lence and mixing statistics are shown to be strongly-dependent upon the early-time transition of the initial perturbation from a weakly-nonlinear to a strongly-nonlinear flow.

Zugl.: Giessen, Universiẗat, Diss., 2009.

Zugl.: Wien, Techn. Univ., Diss., 2009

Zugl.: Giessen, Universiẗat, Diss., 2009. / Auch im Internet unter der Adresse http://geb.uni-giessen.de/geb/volltexte/2009/7265/pdf/ZieleniewiczDaniela_2009_07_21.pdf verfügbar.

As more applications rely on distributed systems (peer-to-peer services, content distribution networks, cloud services), it becomes necessary to identify hosts that return content to the user with minimal delay. A large scale map of delays would aid in solving this problem. Existing methods, which deploy devices to every region of the Internet or use of a single vantage point have yet to create such a map. While services such as PlanetLab offer a distributed network for measurements, they only cover 0.3% of the Internet. The focus of our research is to increase the speed of the single vantage point approach so that it becomes a feasible solution. We evaluate the feasibility of performing large scale measurements by performing an experiment using more hosts than any previous study. First, an efficient scanning algorithm is developed to perform the measurement scan. We then find that a custom Windows network driver is required to overcome bottlenecks in the operating system. After developing a custom driver, we perform a measurement scan larger than any previous study. Analysis of the results reveals previously unidentified drawbacks to the existing architectures and measurement methodologies. We propose novel meth- ods for increasing the speed of experiments, improving the accuracy of measurement results, and reducing the amount of traffic generated by the scan. Finally, we present architectures for performing an Internet scale measurement scan. We found that with custom drivers, the Windows operating system is a capable platform for performing large scale measurements. Scan results showed that in the eleven years since the original measurement technique was developed, the response patterns it relied upon had changed from what was expected. With our suggested improvements to the measurement algorithm and proposed scanning architectures, it may be possible to perform Internet scale measurement studies in the future.

In a world that relies heavily on data, protection of that data and of the motion of that data is of the utmost importance. Covert communication channels attempt to circumvent established methods of control, such as rewalls and proxies, by utilizing non-standard means of getting messages between two endpoints. The Domain Name System (DNS), the system that translates text-based resource names into machine-readable resource records, is a very common and e ective platform upon which covert channels can be built. This work proposes, and demonstrates the e ectiveness of, a novel technique that estimates data transmission throughput over DNS in order to identify the existence of a DNS tunnel against the background noise of legitimate network tra c. The proposed technique is robust in the face of the obfuscation techniques that are able to hide tunnels from existing detection methods.

In this research, pulsatile blood flow through a modeled arterial stenosis assuming Newtonian and non-Newtonian viscous behavior is simulated using direct numerical simulation (DNS). A serial FORTRAN code has been parallelized using OpenMP to perform DNS based on available high performance shared memory parallel computing facilities. Numerical simulations have been conducted in the context of a channel with varying the degree of stenosis ranging from 50% to 75%. For the pulsatile flow studied, the Womersley number is set to 10.5 and Reynolds number varies from 500 to 2000, which are characteristic of human arterial blood flows. In the region upstream of the stenosis, the flow pattern is primarily laminar. Immediately after the stenosis, the flow recirculates and an adverse streamwise pressure gradient exists near the walls and the flow becomes turbulent. In the region far downstream of the stenosis, the flow is re-laminarized for both Newtonian and non-Newtonian flows.

Würzburg, University, Diss., 2007.

Zugl.: Giessen, Univ., Diss., 2005