Anomalous Scaling in a Model of Hydrodynamic Turbulence with a Small Parameter

The major difficulty in developing theories for anomalous scaling in hydrodynamic turbulence is the lack of a small parameter. In this Letter we introduce a shell model of turbulence that exhibits anomalous scaling with a tunable small parameter. The small parameter $\epsilon$ represents the ratio between deterministic and random components in the coupling between $N$ identical copies of the turbulent field. We show that in the limit $N\to \infty$ anomalous scaling sets in proportional to $\epsilon^4$. Moreover we give strong evidences that the birth of anomalous scaling appears at a finite critical $\epsilon$, being $\epsilon_c\approx 0.6$.