Influence of nozzle lip geometry on the Strouhal number of self-excited waterjet

•The nozzle lip geometry including the straight length, expansion length and angle had a significant influence on the Strouhal number of self-excited waterjet.•ith the increase of expansion angle, the Strouhal numbers rise rapidly at a relatively small angle. After that, the Strouhal numbers decrease and then increase slightly.•The Strouhal numbers are monotone decrease with the increase of the expansion length. There will be no self-excited oscillation if the straight length is too long. The influence of nozzle lip geometry on the Strouhal number of the self-excited waterjet is studied. Jets emanating from the self-excited nozzle with various lip parameters are investigated in a high pressure cell with Reynolds number 3×105 and 4 cavitation numbers (0.04, 0.06, 0.08, 0.1). Strouhal numbers are obtained from the pressure oscillation measured by a transducer. The variables of lip geometry affecting the Strouhal number include: expansion angles (0-40°), expansion length (L2/d = 1–3) and straight length (L1/d = 0.15–0.75). The Strouhal numbers of all cases show qualitatively similar behaviour as the expansion angle increases. The general trend is that the Strouhal numbers rise rapidly at a relatively small angle. After the sudden rise, the Strouhal numbers decrease first and then increase slightly. This is due to the balance of effects from the local cavitation number changed by expansion angle and the angle itself. The Strouhal numbers monotone decrease with the increase of expansion length. Even though the straight length does not influence the value of Strouhal number during the experimental range, we can infer that there will be no self-resonating phenomenon if the straight length is too long.