arXiv subjects

Mohsen Lahooti

Publications and source records attributed to Mohsen Lahooti.

3 recordsLinked to original sources

arXiv2026

Direct Numerical Simulation of Vertical-Axis Wind Turbine Near-Wake Dynamics

Geometrically-resolved Direct Numerical Simulations of vertical-axis wind turbines are presented. Simulations were performed using the spectral/hp element method framework Nektar++ with a moving reference frame formulation. Three turbine geometries are considered with one, two and three blades. The full dynamic stall process is resolved, including the formation of large-scale vortices, the separation from the blade, and interaction with the near wake. Increasing blade number introduces blade-vortex interactions that interact with the dynamic stall process. For the three-bladed configuration, these interactions coincide with the early phase of dynamic stall during which the laminar separation bubble develops, and the resulting dynamic stall vortex is reduced in size. Further, the DNS outcomes identified a novel complementary mechanism, where direct vortex impingement causes the premature breakup of the dynamic stall vortices. With the dynamic stall vortex a defining feature of the VAWT near-wake, its accelerated breakdown removes the flow structures that distinguish the VAWT wake from that of a bluff body. Hence the near-wake transitions more rapidly towards bluff-body dynamics, with shear-layer-associated recovery. Self-similarity analysis is extended into the near-wake to quantify this transition, capturing the downstream rate at which the wake loses its dependence on blade-generated coherent structures and collapses onto a self-similar solution. Blade number is shown to be more influential than tip-speed ratio in setting the rate of this transition. The results have implications for closely-spaced turbine arrays and coupled-pair configurations, where the inflow experienced by a downstream rotor is shown to be blade-number-dependent.

physics.flu-dyn↗

arXiv2023

Wall-bounded periodic snap-through and contact of a buckled sheet

Fluid flow passing a post-buckled sheet placed between two close confining walls induces periodic snap-through oscillations and contacts that can be employed for triboelectric energy harvesting. The responses of a two-dimensional sheet to a uniform flow and wall confinement in both equilibrium and post-equilibrium states are numerically investigated by varying the distance between the two ends of the sheet, gap distance between the confining walls, and flow velocity. Cases with strong interactions between the sheet and walls are of most interest for examining how contact with the walls affects the dynamics of the sheet and flow structure. At equilibrium, contact with the wall displaces the sheet to form a nadir on its front part, yielding a lower critical flow velocity for the transition to snap-through oscillations. However, reducing the gap distance between the walls below a certain threshold distinctly shifts the shape of the sheet, alters pressure distribution, and eventually leads to a notable delay in the instability. The contact between the oscillating sheet and the walls at post-equilibrium is divided into several distinct modes, changing from sliding/rolling contact to bouncing contact with increasing flow velocity. During this transition, the time-averaged contact force exerted on the sheet decreases with the flow velocity. The vortices generated at the extrema of the oscillating sheet are annihilated by direct contact with the walls and merging with the shear layers formed by the walls, resulting in a wake structure dominated by the unstable shear layers.

physics.flu-dyn↗

arXiv2019

Equal bi-Vectorized (EBV) method to high performance on GPU

Due to importance of reducing of time solution in numerical codes, we propose an algorithm for parallel LU decomposition solver for dense and sparse matrices on GPU. This algorithm is based on first bi-vectorizing a triangular matrices of decomposed coefficient matrix and then equalizing vectors. So we improve performance of LU decomposition on equal contributed scheme on threads. This algorithm also is convenient for other parallelism method and multi devices. Several test cases show advantage of this method over other familiar method.

cs.DC↗