Numerical Experiments in Modulated Turbulence

Flows in nature are often strongly influenced by time- or space-dependent boundary conditions. Such flows usually do not follow the Kolmogorov K41 predictions. This intriguing fact directly affects the turbulent mixing and motivated the research presented in this thesis. Two general aspects were considered, i.e., controlled non-Kolmogorov turbulence and periodically modulated turbulence. The perturbations that arise from flow through/along complex boundaries were represented by broadband forcing, which was allowed to simultaneously act in a broad range of scales. The consequences of adopting different ranges and strengths of externally forced scales were numerically investigated, both in terms of the quality of mixing of embedded passive scalar fields in physical space as well as in terms of the alterations in the triadic interactions in spectral space. These numerical studies of forcing strategies have established the strong external influence that may be exerted on the turbulent cascading process. Turbulent flows under the influence of periodically modulated forcing were also investigated. A~comprehensive parameter study was conducted to examine the response of turbulent flow to time-modulated forcing at various length scales, forcing strengths and forcing protocols. The existence of response maxima in periodically agitated turbulence was confirmed using Direct Numerical Simulations. These studies demonstrated the utility of forcing methods to represent complicated turbulent flows as may be seen in nature.