Large-eddy simulation of low frequency oscillations of the Dean vortices in turbulent pipe bend flows

Large-eddy simulations are performed to investigate turbulent flows through 90° pipe bends that feature unsteady flow separation, unstable shear layers, and an oscillation of the Dean vortices. Single bends with curvature radii of one- and three-pipe diameters are considered at the Reynolds number range 5000–27 000. The numerically computed distributions of the time-averaged velocities, Reynolds stress components, and power spectra of the velocities are validated by comparison with particle image velocimetry measurements. The power spectra of the overall forces onto the pipe walls are determined. The spectra exhibit a distinct peak in the high frequency range that is ascribed to vortex shedding at the inner side of the bends and shear layer instability. At the largest Reynolds number the spectra also exhibit an oscillation at a frequency much lower than that commonly observed at vortex shedding from separation. It turns out that the associated flow pattern is similar to the swirl switching phenomenon earlier found in experimental studies with which the present results are compared. It is shown that the low frequency oscillation perceptible on the entire wall is caused by the two Dean vortices whose strength vary in time and which as such alternately dominate the flow field.