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BOUT++ turbulence simulations are conducted to capture the underlying physics of the small ELM characteristics achieved by increasing separatrix density via controlling strike points from vertical to horizontal divertor plates for three EAST discharges. BOUT ++ linear simulations show that the most unstable modes change from high-n ideal ballooning modes to the intermediate-n peeling-ballooning modes and eventually to peeling-ballooning stable plasmas in the pedestal. Nonlinear simulations show that the fluctuation is saturated at a high level for the lowest separatrix density. The elm size decreases with increasing the separatrix density, until the fraction of this energy lost during the ELM crash becomes less than 1% of the pedestal stored energy, leading to small ELMs. Simulations indicate that small ELMs can be triggered either by the marginally peeling-ballooning instability near the peak pressure gradient position inside pedestal or by a local instability in the pedestal foot with a larger separatrix density gradient. The pedestal collisionality scan for type-I ELMs with steep pedestal density gradient shows that both linear growth rate and elm size decrease with collisionality increasing. While the pedestal collisionality and pedestal density width scan with a weak pedestal density gradient indicate small ELMs can either be triggered by high-n ballooning mode or by low-n peeling mode in low collisionality region 0.04~0.1. The simulations indicate the weaker the linear unstable modes near marginal stability with small linear growth rate, the lower nonlinearly saturated fluctuation intensity and the smaller turbulence spreading from the linear unstable zone to stable zone in the nonlinear saturation phase, leading to small ELMs.

BOUT++ turbulence simulations are conducted for a 60s steady-state long pulse high \{beta}p EAST grassy ELM discharge. BOUT++ linear simulations show that the unstable mode spectrum covers a range of toroidal mode numbers from low-n (n=10~15) peeling-ballooning modes (P-B) to high-n (n=40~80) drift-Alfv\'en instabilities. Nonlinear simulations show that the ELM crash is trigged by low-n peeling modes and fluctuation is generated at the peak pressure gradient position and radially spread outward into the Scrape-Off-Layer (SOL), even though the drift-Alfv\'en instabilities dominate the linear growth phase. However, drift-Alfv\'en turbulence delays the onset of the grassy ELM and enhances the energy loss with the fluctuation extending to pedestal top region. Simulations further show that if the peeling drive is removed, the fluctuation amplitude drops by an order of magnitude and the ELM crashes disappear. The divertor heat flux width is ~2 times larger than the estimates based on the HD model and the ITPA multi-tokamak scaling (or empirical Eich scaling) due to the strong radial turbulence transport.