| 摘要 |
A tubular portion 1 of a bell mouth has on its outer peripheral surface one or a plurality of first projections 21 projecting radially and one or a plurality of second projections 22 projecting at a position phased differently from the first projection 21 in an axial direction and in a circumferential direction, each of the projections 21 and 22 being formed such that its surface on a bell-shaped opening portion 3 side is formed as a sharply angled surface 2a, while its surface on a tubular opening portion 4 side is formed as a gently inclined surface 2b, an axial phase interval w between the first projection 21 and the second projection 22 is set to satisfy the relationship: w</=W with respect to an axial width W of an inner surface portion of a small-diameter portion 32 in the annularly corrugated pipe P, a circumferential phase angle r being substantially equal to a value calculated by dividing a half angle of circumference (180 degrees) by the first projections 21.
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