  A growing body of evidence suggests that the solar wind is powered to a large
extent by an Alfv\'en-wave (AW) energy flux. AWs energize the solar wind via
two mechanisms: heating and work. We use high-resolution direct numerical
simulations of reflection-driven AW turbulence (RDAWT) in a fast-solar-wind
stream emanating from a coronal hole to investigate both mechanisms. In
particular, we compute the fraction of the AW power at the coronal base
($P_{\rm AWb}$) that is transferred to solar-wind particles via heating between
the coronal base and heliocentric distance $r$, which we denote $\chi_{\rm
H}(r)$, and the fraction that is transferred via work, which we denote
$\chi_{\rm W}(r)$. We find that $\chi_{\rm W}(r_{\rm A})$ ranges from 0.15 to
0.3, where $r_{\rm A}$ is the Alfv\'en critical point. This value is small
compared to~one because the Alfv\'en speed $v_{\rm A} $ exceeds the outflow
velocity $U$ at $r<r_{\rm A}$, so the AWs race through the plasma without doing
much work. At $r>r_{\rm A}$, where $v_{\rm A} < U$, the AWs are in an
approximate sense "stuck to the plasma", which helps them do pressure work as
the plasma expands. However, much of the AW power has dissipated by the time
the AWs reach $r=r_{\rm A}$, so the total rate at which AWs do work on the
plasma at $r>r_{\rm A}$ is a modest fraction of $P_{\rm AWb}$. We find that
heating is more effective than work at $r<r_{\rm A}$, with $\chi_{\rm H}(r_{\rm
A})$ ranging from 0.5 to 0.7. The reason that $\chi_{\rm H} \geq 0.5$ in our
simulations is that an appreciable fraction of the local AW power dissipates
within each Alfv\'en-speed scale height in RDAWT, and there are a few
Alfv\'en-speed scale heights between the coronal base and $r_{\rm A}$.

Solar and Stellar Astrophysics;Plasma Physics;Space Physics