Development and application of synthetic turbulence methods for computational fluid dynamics

Synthetic turbulence methods are an important tool for the study of turbulent flows. They allow to reduce the computational effort of numerical simulations of fluid flows and thereby, improve the quality of simulations of complex flow problems. Contributing to the field of turbulence research, this thesis proposes two new methods of simulating turbulent flows using synthetic turbulence. The methods developed in this work were tested for two scenarios of turbulent flow simulations. The first scenario was the numerical simulation of turbulent flow around a wing. For this simulation a synthetic turbulence method was developed, which generated an initial 3D turbulent wind field to initialise the simulation. Using a complex numerical setup it was possible to simulate the interaction of the synthetic turbulence field, representing atmospheric boundary layer (ABL) turbulence, with a wing on a relatively large range of scales. This method allows to simulate the influence of ABL turbulence on the aerodynamics of the wing, for example, at large angles of attack. In the second scenario a new method was developed to generate synthetic turbulence as inflow boundary condition for Large-Eddy Simulation (LES). A new method to generate anisotropy in the turbulence field was introduced, which allowed to prescribe 1D statistics of the turbulent flow independently. This method can be used, for example, for feeding synthetic turbulence into the interface between the Reynolds-Averaged Navier Stokes (RANS) and LES part of a hybrid RANS/LES. For the first scenario, the generated turbulence was tested in a simple LES of decaying turbulence where it was found that the input statistics for the turbulence generator were reproduced very well. It was also shown that the statistical properties were maintained reasonably well during the simulation with the exception of fluctuations observed in the cross-correlations. In order to investigate the quality of the turbulence generator further, the generated turbulence field was compared to data from an LES of the ABL. It was found that the synthetic turbulence was not able to represent the coherent structures present in a convective boundary layer, but apart from that the turbulence statistics from the synthetic turbulence and LES of the ABL agreed very well. After studying the properties of the synthetic turbulence generator in detail, a synthetic turbulence field was generated for the initialisation of the simulation of the flow around a wing. In a complex setup of two different grid types (Cartesian and unstructured) and two different turbulence model types (LES and RANS), the development of the turbulence in the different numerical environments was studied. It was found that the change in grid characteristics led to a stronger dissipation of turbulence on the unstructured grid. No significant effect on the turbulence could be found when the turbulence model switched from LES to RANS mode, most likely due to the short time the turbulence was exposed to the RANS model. For the second scenario, a new approach for generating anisotropic turbulence was developed. An extensive analysis of the statistics of the generated turbulence was carried out and the results showed very good agreement with the reference data from a Direct Numerical Simulation (DNS). The generated turbulence then served as inflow boundary condition in an LES of a channel flow. A strong influence of the statistical properties of the synthetic turbulence on the behaviour of the turbulence in the channel was found. Comparison to two established synthetic turbulence methods showed a similar performance of the new approach, which at the same time caused much less computational costs and allowed better control of the statistical parameters of the synthetic turbulence

Evaluation of fast atmospheric dispersion models in a regular street network

The need to balance computational speed and simulation accuracy is a key challenge in designing atmospheric dispersion models that can be used in scenarios where near real-time hazard predictions are needed. This challenge is aggravated in cities, where models need to have some degree of building-awareness, alongside the ability to capture effects of dominant urban flow processes. We use a combination of high-resolution large-eddy simulation (LES) and wind-tunnel data of flow and dispersion in an idealised, equal-height urban canopy to highlight important dispersion processes and evaluate how these are reproduced by representatives of the most prevalent modelling approaches: (i) a Gaussian plume model, (ii) a Lagrangian stochastic model and (iii) street-network dispersion models. Concentration data from the LES, validated against the wind-tunnel data, were averaged over the volumes of streets in order to provide a high-fidelity reference suitable for evaluating the different models on the same footing. For the particular combination of forcing wind direction and source location studied here, the strongest deviations from the LES reference were associated with mean over-predictions of concentrations by approximately a factor of 2 and with a relative scatter larger than a factor of 4 of the mean, corresponding to cases where the mean plume centreline also deviated significantly from the LES. This was linked to low accuracy of the underlying flow models/parameters that resulted in a misrepresentation of pollutant channelling along streets and of the uneven plume branching observed in intersections. The agreement of model predictions with the LES (which explicitly resolves the turbulent flow and dispersion processes) greatly improved by increasing the accuracy of building-induced modifications of the driving flow field. When provided with a limited set of representative velocity parameters, the comparatively simple street-network models performed equally well or better compared to the Lagrangian model run on full 3D wind fields. The study showed that street-network models capture the dominant building-induced dispersion processes in the canopy layer through parametrisations of horizontal advection and vertical exchange processes at scales of practical interest. At the same time, computational costs and computing times associated with the network approach are ideally suited for emergency-response applications

Simulating collisions of charged cloud drops in an ABC flow

&amp;lt;p&amp;gt;Calculating the electric force between cloud drops is not straightforward. Since water drops are conducting, the electric force is not just simply the force between point charges, but instead the charge in each drop induces an infinite number of image charges in the other drop. The effect of these image charges can cause the electric force between two like charged cloud drops to become attractive on very short distances, when only applying Coulomb's law would result in a repulsive force. The attractive effect of image charges could potentially increase the collision rate of cloud drops. Within the United Arab Emirates Rain Enhancement Program (UAE REP) we are investigating the potential for rain enhancement by charging clouds.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;Simulating the behaviour of cloud drops is numerically very expensive. A large number of drops needs to be simulated to obtain stable collision statistics. Additionally, the drops move in a complex turbulent environment with eddies spanning several orders of magnitude in size. Simulating the turbulent flow alone is an expensive task. Because of the typical sizes of cloud drops, their motion is predominantly influenced by the smallest turbulent scales in the flow. Therefore, Direct Numerical Simulation (DNS) is necessary and used to simulate the influence of turbulent flow on drop motion. In this work, instead of using DNS, we use an ABC flow to simulate the turbulent effect on cloud drops. This simple approximation for the turbulent flow allows to simulate the drop motion using much less computational resources then needed by DNS and therefore, allows to include the very expensive effect of electrical drop charge in our simulation of colliding drops in a turbulent environment.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;To investigate the effect of electrical charge on drop collisions, a Lagrangian particle code for the interaction of cloud drops is used. It calculates the motion of individual drops based on the aerodynamical force due to the ABC flow and the gravitational force and registers drop collisions from which collision statistics can be calculated. In the cloud model all drops carry positive charges. The effect of the electric force is calculated by an approximation which uses Coulomb's law for the effect of the point charges and an additional term to approximate the effect of image charges which produce an attractive force on short distance.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;Results for the collision kernel with and without charge will be presented. The effect of the additional term to Coulomb's law will be shown for different drop sizes and drop charges. It will be discussed if the attractive force for like charged drops on short distances can lead to an enhancement in drop collisions and under which conditions the effect is the largest.&amp;lt;/p&amp;gt; </jats:p

An anisotropic synthetic turbulence method for Large-Eddy Simulation

The method for generating anisotropic synthetic turbulence by Auerswald and Bange (2015) is extended to account for the integral length scales in y - and z -direction. This extension leads to more realistic tur- bulent structures. The method reproduces the given turbulence statistics very well and allows to set a number of turbulence parameters independently. In four Large-Eddy Simulations of a channel flow the synthetic turbulence is used as inflow boundary condition. The performance of the synthetic turbulence is tested and compared to the Direct Numerical Simulation (DNS) results by Moser et al. (1999). In these simulations the synthetic turbulence shows good performance in recovering realistic turbulence down- stream in the channel. The skin-friction coefficient converges to the level of the DNS. The profiles of the Reynolds stresses are very similar in the LES and the DNS except for the profiles of R + ww where large deviations occur

Development and Evaluation of a Community Health Program to Promote Physical Activity Among Vulnerable Community-Dwelling Older Adults: Protocol for a Prospective Cohort Study

Background: Vulnerable older adults have a high risk of morbidity and mortality. Regular physical activity (PA) can have a positive effect on the health and health-related behavior of this specific target group. However, evidence of the impact and feasibility of community-based PA promotion interventions for vulnerable older adults is still limited. Objective: The BeTaSen (Bewegungs-Tandems in den Lebenswelten Chemnitzer Seniorinnen und Senioren: ein Beitrag zur kommunalen Gesundheitsförderung) study aims to evaluate the (1) impact as well as the (2) feasibility, acceptance, and usefulness of a 12-month low-threshold PA intervention program for community-dwelling vulnerable older adults. Methods: For our population-based prospective observational cohort study, a total of 120 vulnerable older adults (aged 75 years or older) in the area of Chemnitz (Germany) will be recruited to participate in (1) weekly neighborhood-based low-threshold PA meetings with trained mentors (activity tandems) and (2) monthly exercise meetings led by trained exercise instructors. Within the intervention, participants will be encouraged to perform the PA independently. Participants will complete assessments, which will include questionnaires as well as objective measurements of their physical, cognitive, and psychosocial health at 3 different time points (baseline, 6 months after the start, and 6 months after the end of the intervention). Additionally, a process evaluation will be performed, including questionnaires and qualitative interviews, involving the participants, mentors, and municipal project partner representatives. Results: The BeTaSen project process began in October 2021, with the start of data collection and intervention in August 2022 in the first neighborhoods of the city of Chemnitz. A total of 86 participants were recruited at the time of submission of the manuscript. Longitudinal results are expected by 2025. Conclusions: This study’s results will provide insights on (1) the PA behavior of vulnerable older adults as well as the impact of PA interventions on health-related outcomes such as cognitive, physical, and psychosocial health, and (2) the feasible and useful components of community-based PA interventions. Thus, this pilot study contributes to future recommendations and provides a basis for further research, such as the development of feasible and sustainable target group–specific interventions in community settings