الفهرس 
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Abstract
The main objective of this thesis is to better understand the nonlinear dynamical processes of hydrodynamic (HD) and magnetohydrodynamic (MHD) turbulence. These processes play an important role in understanding the nature of energy transfer and structure formation in a turbulent flow. For this purpose, the spectral properties of transfer functions, triad interactions, and cascade directions of ideal quadratic invariants of incompressible two and three-dimensional HD and MHD turbulent flows are studied by analyzing data from highresolution direct numerical simulations. High resolution is necessary to resolve the inertial range of a turbulent flow, where the nonlinear dynamics of turbulence are dominant and nonuniversal large- and small-scale effects can be neglected. All turbulent systems studied in this thesis are investigated using analysis of fully-developed, driven turbulence produced by large direct numerical pseudospectral simulations. The main advantage of the DNS method is that it does not use any additional physical approximations to solve the underlying equations. This point is of fundamental importance for turbulence investigations because no simplification of the nonlinear terms is employed. In addition, the DNS approach has the possibility of comprehensive diagnostics since calculated turbulent fields are known at all grid points in a computional box at all time steps. The nonlinear terms in the differential equations of Navier- Stokes and MHD turbulent flows lead to the property that three wavevectors k, p and q are involved in any basic triad interaction between turbulent fluctuations with k + p + q = 0, k = jkj, p = jpj, and q = jqj. The transfer of an ideal invariant is typically between two Fourier modes with the third mode being regarded as responsible for the transfer; thus the triad interaction takes place between three wavenumbers.