  The impact of turbulent fluctuations on the forces exerted by a fluid on a
towed spherical particle is investigated by means of high-resolution direct
numerical simulations. The measurements are carried out using a novel scheme to
integrate the two-way coupling between the particle and the incompressible
surrounding fluid flow maintained in a high-Reynolds-number turbulent regime.
The main idea consists in combining a Fourier pseudo-spectral method for the
fluid with an immersed-boundary technique to impose the no-slip boundary
condition on the surface of the particle. Benchmarking of the code shows a good
agreement with experimental and numerical measurements from other groups. A
study of the turbulent wake downstream the sphere is also reported. The mean
velocity deficit is shown to behave as the inverse of the distance from the
particle, as predicted from classical similarity analysis. This law is
reinterpreted in terms of the principle of "permanence of large eddies" that
relates infrared asymptotic self-similarity to the law of decay of energy in
homogeneous turbulence.
  The developed method is then used to attack the problem of an upstream flow
that is in a developed turbulent regime. It is shown that the average drag
force increases as a function of the turbulent intensity and the particle
Reynolds number. This increase is significantly larger than predicted by
standard drag correlations based on laminar upstream flows. It is found that
the relevant parameter is the ratio of the viscous boundary layer thickness to
the dissipation scale of the ambient turbulent flow. The drag enhancement can
be motivated by the modification of the mean velocity and pressure profile
around the sphere by small scale turbulent fluctuations.
