In this thesis, the effects of surface-mounted V-shaped ribs and spanwise system rotations on fully-developed turbulent square duct flows were investigated using the particle image velocimetry (PIV) measurement, large-eddy simulation (LES) and direct numerical simulation (DNS). The PIV measurements were conducted in a water channel, and three different angled V-shaped (60°, 45° and 30°) and perpendicular (90°) ribs were considered. As a complement for the PIV experiments, LES was also used to simulate the measured cases under the same experimental conditions. To perform LES, a parallel finite volume method (FVM) code adopting the generalized curvilinear coordinate system was developed. The first- and second-order moments of the turbulence flows obtained from LES were validated against the PIV measurement data. Both the PIV and LES results showed that strong secondary flows in the pattern of a pair of counter-rotating streamwise-elongated vortices exist in all three V-shaped rib cases. The impacts of rib geometry on turbulent coherent structures were investigated using vortex identifiers, temporal autocorrections, spatial two-point autocorrelations, and velocity spectra. To study the effects of system rotations on turbulent square duct flows, DNS was performed for the turbulent flows confined within a square duct subjected to at a wide range of spanwise system rotations. The DNS was conducted using a modified open-source parallel spectral-element method (SEM) code. The influences on turbulent duct flows by the system rotation were investigated by analyzing the transport equations of Reynolds stresses and vorticity correlations. Turbulent structures under different system rotations were also systematically studied using the energy spectra, autocorrelations of vorticity fluctuations, and linear stochastic estimation. It was observed that in response to the system rotation, secondary flows appear as streamwise counter-rotating vortices. At sufficiently high rotation numbers, a Taylor-Proudman region appears and complete laminarization is almost reached near the top and sidewalls. It was also observed that the Coriolis force dominates the transport of Reynolds stresses and turbulent kinetic energy, and forces the spectra of streamwise and vertical velocities to synchronize within a wide range of streamwise length scales.