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An experimental study of a granular surface submitted to a circular fluidmotion is presented. The appearance of an instability along the sand-waterinterface is observed beyond a critical radius $r_c$. This creates ripples witha spiral shape on the granular surface. A phase diagram of such patterns isconstructed and discussed as a function of the rotation speed $\omega$ of theflow and as a function of the height of water $h$ above the surface. The studyof $r_c$ as a function of $h$, $\omega$ and $r$ parameters is reported.Thereafter, $r_c$ is shown to depend on the rotation speed according to a powerlaw. The ripple wavelength is found to decrease when the rotation speedincreases and is proportional to the radial distance $r$. The azimuthal angle\az of the spiral arms is studied. It is found that \az scales with $h\omegar$. This lead to the conclusion that \az depends on the fluid momentum.Comparison with experiments performed with fluids allows us to state that thespiral patterns are not the signature of an instability of the boundary layer.Comment: 7 pages, 10 figures, 1 table, using RevTeX4, submitted for  publication (2002

Granular spirals on erodible sand bed submitted to a circular fluid motion