Gravity-induced virtual clocking effect in large-capacity/low-head pumped hydro energy storage system with horizontal shaft

•Gravity-induced virtual clocking effect is found in low-head tubular PHES system.•Partial flow separation is observed under different virtual clocking positions.•Distributions of pressure fluctuation amplitudes at guide-vane inlet show a phase difference.•Differences in flow deviation ratio between the two sides of guide-vane outlet can reach 10%.•The optimal virtual clocking scheme reducing potential stability risks is recommended.