  The interaction of flexible polymers with fluid flows leads to a number of
intriguing phenomena observed in laboratory experiments, namely drag reduction,
elastic turbulence and heat transport modification in natural convection, and
is one of the most challenging subjects in soft matter physics. In this paper
we review our present knowledge on the subject. Our present knowledge is mostly
based on direct numerical simulations performed in the last twenty years, which
have successfully explained, at least qualitatively, most of the experimental
results. Our goal is to disentangle as much as possible the basic mechanisms
acting in the system in order to capture the basic features underlying
different theoretical approaches and explanations.
