Water jet cutting is a novel machining technology. Effective jet cutting requires a highly coherent jet to achieve a perfect surface finish. However, the jet breaks up just in a short distance downstream the nozzle exit. Interfacial shear is considered to have a dominate effect on the jet surface instability and its breakup. This work focuses on the turbulence both in the jet and its ambient air. The Fluent code is employed to simulate the air-water flow inside and outside a jet cutting nozzle. We get insight into the original interfacial turbulence near the nozzle exit. The contribution of the interfacial flow to the jet coherency is analyzed. It is expected the original turbulence can be reduced by changing the shape of nozzle exit. Several nozzles with different configurations in their outlets are tested both experimentally and numerically. Through these work, it is found that the jet is more coherent as the air flow is more consistent with the jet flow in the nozzle tip. The cone-down with modest cone angle in nozzle outlet is an appealing design for creating a fine jet.

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