A matrix for a heat exchanger to exchange heat between a first fluid and a second fluid, the first fluid being for instance air and the second fluid being for instance oil. The matrix comprises: a channel for the first fluid. an array of passages for the second fluid, the passages extending in the channel. The array supports at least two cooling fins. The matrix is made by a process of additive manufacturing. The fins are inclined with respect to each other along the direction of the flow of the first fluid. The array defines rectangular corridors for the first fluid.

A passive vortex formed or induced from a temperature difference across a cavity or void aggregates and supports a horizontal flow over the top of the cavity. A cavity of a suitable depth and width exhibits a small difference in temperature, or heat source, along the sides or bottom of cavity. A resulting convective flow tends to form a rising current along a warmer side, and a complementary downward current on an opposed side of the cavity. The formed vortex tends to draw the cooler downward flow across the warmer, heated surface, enhancing the vortex flow. The vortex aligns with a horizontal flow across the top of the cavity as the upward current complements the downward current on an opposed side of the cavity. A plurality of adjacent cavities tend to align with an aggregate horizontal flow contributed from each cavity.