Transit-time jitter and bandwidth-limited uncertainty in ultrasonic flow sensing under valve-induced turbulence in pump-controlled systems

Upstream flow disturbances degrade ultrasonic transit-time flow measurement by introducing transit-time jitter along the acoustic path. This paper characterizes how butterfly valve closure angle affects the uncertainty of a dual-path transit-time sensor (DN250, water, 7.5D downstream) when a variable-speed pump maintains a constant flow rate. Eleven valve angles (0 to 81 degrees), a baseline, and an Asymmetric Swirl Generator were tested. Formal hypothesis testing showed that the linear blockage coefficient is not statistically significant at the 95% confidence level, yielding a reduced two-parameter model dominated by vortex shedding with sine-squared scaling of the valve angle. The saturation flow rate of (95 ± 9) m 3 /h remained constant across configurations, confirming bandwidth-limited uncertainty behavior governed by the sensor Nyquist-frequency limit. A transition near 63 degrees defines a practical operating limit of 60 degrees. The Asymmetric Swirl Generator produced approximately 15% higher turbulence than a fully open valve. The single experimental configuration and the two-point exponential region are discussed as constraints on generalizability.