Resulting Flow Structures Research Articles

Direct numerical simulation of a heat removal configuration for fusion blankets

A series of direct numerical simulations (DNS) are performed to study the natural convection heat transfer between concentric cylinders at several Rayleigh and Hartmann numbers. The buoyant flow is driven by the temperature difference between the inner and the outer walls, with the inner wall being at lower temperature, while an external transverse magnetic field is imposed. Both laminar and turbulent flows are observed depending on the magnitude of the Rayleigh and Hartmann numbers. The resulting flow structures of the cases studied were both laminar and turbulent. The results show the 3D nature of turbulence and the tendency of the magnetic field to form narrow Hartmann layers, 3D jets and wakes at specific azimuthal angles. Some particular features of the turbulent regime as well as the heat transfer are also investigated. The magnetic field effect on the convective heat transfer is assessed via the Nusselt number showing that conduction dominates as the Hartmann number increases.

A simplified model for flows with eddies in symmetrically expanding channels

We derive a simplified model for two-dimensional (2D) channel flows with recirculated regions at moderate Reynolds numbers based on an extension of the boundary layer (BL) theory and averaging across the channel. The model reproduces symmetry-breaking bifurcations and resulting flow structures accurately. Analytical estimates for the decay rates toward the parabolic profile before and after a sudden change in the walls agree well with the full numerical simulations. A seemingly chaotic steady flow is also discovered in a channel with periodic expansions and contractions.

DNS of low Reynolds number turbulent flows in dimpled channels

Direct numerical simulation (DNS) is performed to study turbulent flows over dimpled surfaces in a channel. Results on mean field and second-order quantities are obtained. ‘Horseshoe’ vortices can be observed in the dimples of sparse arrays. As inter-dimple separation is reduced, the ‘feet’ of the horseshoe vortices are gradually lifted off the dimple surface, and the resulting flow structures in the cavities become flattened and stretched to become something akin to two-dimensional separation bubbles. At the higher dimple density, the stream traces near the surface also develop a distinct formation similar to what had been observed in earlier Reynolds-averaged Navier–Stokes (RANS) simulations (Isaev, S.A., Leont'ev, A.I. and Baranov, P.A., 2000, Technical Physics Letters, 26, 15; Lin, Y.L., Shih, T.I.-P. and Chyu, M.K., 1999, ASME paper, 99-GT-263; Lin, Y.L. Shih, T.I.-P., 2001, International Journal of Transfer Phenomena, 3, 1). Regions of high turbulence intensity are found above the downstream half of the dimples and along their side edges. These regions coincide with the locations of vortex shedding found in the experiments of Ligrani et al. (2001, Physics of Fluids, 13, 3442) and the locations of vorticity concentrations observed in Park et al. (2004, Numerical Heat Transfer, Part A (Applications), 45(1), 1) and Won and Ligrani (2004, Numerical Heat Transfer, Part A (Applications), 46(6), 549). For a fixed mean pressure gradient, it is observed that the flow rates through the channels are reduced by the presence of dimples. This indicates that the dimpled channels we have studied so far have larger drag than flat-wall channels. Computed friction coefficients for dimpled channels also confirmed the conclusion.

Transition mechanisms induced by travelling crossflow vortices in a three-dimensional boundary layer

The laminar breakdown induced by purely travelling crossflow vortices in a three-dimensional flat-plate boundary-layer flow is investigated in detail by means of spatial direct numerical simulations. The base flow considered is generic for an infinite swept wing, with decreasing favourable chordwise pressure gradient and a sweep angle of . First, the primary downstream growth and nonlinear saturation state of a single crossflow wave are simulated. Secondly, background disturbance pulses are added, and the subsequent mechanisms triggering transition to turbulence in this scenario are identified and analysed in detail. The saturated travelling crossflow vortex is found to give rise to a co-travelling secondary instability not unlike the instability in the much investigated steady crossflow-vortex case, but with characteristic differences. An analysis method with a spanwise Galilean transformation to travel with the primary wave and a consequently adapted timewise/spanwise Fourier decomposition of the disturbance flow enables unambiguous isolation of the various secondary disturbance modes. The resulting flow structures and their dynamics in physical space are visualized.

Nonlinear evolution of shear instabilities of the longshore current: A comparison of observations and computations

The time dependent nearshore circulation field during 3 days of the SUPERDUCK field experiment is simulated. We consider the generation of nearshore currents due to obliquely incident breaking waves, damping effects due to bottom friction, and diffusion effects due to lateral momentum mixing caused by turbulence and depth‐varying current velocities. Because of uncertainties in the friction and lateral mixing coefficients, numerical simulations are carried out for a realistic range of values for these coefficients. The resulting shear instabilities of the longshore current exhibit unsteady longshore progressive vortices with timescales of O(100 s) and length scales of O(100 m) and longer. The time dependent flow involves the strengthening, weakening, and interaction of vortices. Vortex pairs are frequently shed offshore. During this process, locally strong offshore directed currents are generated. We find that a stronger mean current and faster and more energetic vortex structures result as the friction coefficient is decreased. However, the longshore length scales of the resulting flow structures are not altered significantly. An increase in the mixing coefficient causes relatively small variations in the propagation speeds. However, the resulting flow structures are less energetic with larger longshore length scales. Shear instabilities are found to induce significant horizontal momentum mixing in the surf zone and affect the cross‐shore distribution of the mean longshore current. Mixing due to the presence of the instabilities is found to be dominant over mixing caused by more traditional mechanisms such as turbulence. For values of the free parameters that reproduce the propagation speed of the observed motions, the frequency range within which shear instabilities are observed as well as the mean longshore current profile are predicted well.