The effect of roughness and form was investigated for toroidal-throat sonic nozzles, using both experimental and computational techniques. Eight, 10 mm diameter nozzles were specially made to investigate the effects of deviating from ISO 9300; these nozzles were measured for diameter, axial profile, roughness and roundness in order to assess the effects on discharge coefficient, Cd. Flow calibrations have revealed that roughness effects are negligible for Ra/d ≤ 4.5×10−5 (three times the limit of ISO 9300) in the turbulent regime (Re > 106), in keeping with previously published data taken in the laminar regime. Test data from three unpolished nozzles, of average Ra of ∼ 0.3 μm, also demonstrated that excellent reproducibility in Cd vs. Re (≤ 0.04%) might be achieved without the need for polishing. Data is also presented from recent primary gravimetric calibrations of NEL’s 16 reference nozzles. The data supports the proposed equation for normally machined nozzles over the Reynolds number range tested (0.17×106 < Re < 17×106), with the standard deviation from the ISO/CD 9300 curve being 0.27% (k = 2; 95% confidence) over the range 0.35×106 < Re < 17×106, provided d ≥ 3.8 mm.

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