  It is remarked that fluxes in conservation laws, such as the Reynolds
stresses in the momentum equation of turbulent shear flows, or the spectral
energy flux in isotropic turbulence, are only defined up to an arbitrary
solenoidal field. While this is not usually significant for long-time averages,
it becomes important when fluxes are modelled locally in large-eddy
simulations, or in the analysis of intermittency and cascades. As an example, a
numerical procedure is introduced to compute fluxes in scalar conservation
equations in such a way that their total integrated magnitude is minimised. The
result is an irrotational vector field that derives from a potential, thus
minimising sterile flux `circuits'. The algorithm is generalised to tensor
fluxes and applied to the transfer of momentum in a turbulent channel. The
resulting instantaneous Reynolds stresses are compared with their traditional
expressions, and found to be substantially different.
