Optimization of subarray beamforming for instantaneous velocity vector measurement using ultrasonic dual-chirp plane waves

Abstract We optimize local steered beamforming for instantaneous velocity vector estimation using ultrasonic dual-chirp plane waves. While this method enables a high-frame-rate measurement without pulse repetition frequency (PRF) limitations, conventional beamforming faces a trade-off: a large aperture improves the signal-to-noise ratio (SNR) but causes “Doppler averaging,” which degrades angle estimation accuracy. Conversely, a small aperture yields insufficient SNR. We evaluated this trade-off via numerical simulations with multiple point scatterers in ideal and clutter-rich environments. The numerical simulations with multiple targets revealed that the angle estimation accuracy, particularly for off-axis targets, is highly sensitive to the aperture size due to averaging bias. The results demonstrate that a subarray size of 15–20 elements provides an optimal balance, achieving a relative velocity error of less than 1% and an absolute angular error of approximately 1° in the presence of background clutter. This clarifies the design criteria for accurate instantaneous vector flow imaging.