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Insufficient understanding of complex flow feature in pipe-type centrifugal pump is a major problem encountered under the design condition. The inlet distorted flow of the pump duct was described, and the tornado-type vortex formation, propagation and its trigging cavitation bubble for the high noise were explained．The description and explanation were based on numerical simulations by RANS model and experiments．The mechanisms dictating the inlet distortion, including the curved duct flow, flatplate flow and secondary vortex impacts on the incidence angle and boundary layer separation, were studied systematically. Particular attention was devoted to the notable influence of the limiting streamlines on the suction surface and pressure changes at the casing. The primary analysis identified a focus separation vortex route to the Q criterion. Like the tornado, the separation vortex had a vortex leg on the blade suction surface, and it was linked to a low pressure bubble locating ahead of the rotor. In detail, the tornado-type vortex was caused by separation at the leading edge due to high incidence. The separation gave rise to shedding of vortex from the suction surface, and consequent formation of vortex moving ahead of the rotor with the reverse flow. Moreover, another tornado-type vortex was captured standing on mid-part of the suction surface and attaching to the corner close to the shroud, due tp breakdown of former tornado-type vortex, disturbance of adjacent blade and rotation motion. Furthermore, the inlet distortion was the major factor for the complex flow in the pump. By cavitation simulation, the former tornado-type vortex and its inducing cavitation could explain the high noise under the design condition.