A heat transfer surface for a heat exchanger has a corrugated member having parallel spaced apart ridges and planar fin surfaces extending the ridges. Tabs are formed in the planar fin surfaces for forming counter-rotating vortices in the fluid flowing over the heat transfer surface, the tabs being lifted out of the surface of the planar fin surface and extending into or nesting within the openings formed by the corresponding tabs in the adjacent planar fin surface so as to achieve high fin density.