The numerical simulation of the water flow through the semicircular duct is presented. The results of the simulation show that by implementing such duct into ultrasonic time-of-flight flowmeters it is possible to get an almost flat and linear function of hydrodynamic correction. The optimal choice of the dimension of ultrasonic transducers and the optimal positioning of them allows us to achieve a hydrodynamic function with the slope ~ 4%. It is expected that flat hydrodynamic calibration function gives the possibility to expand the dynamic measurement range of ultrasonic flowmeters and to increase stability and repeatability of the measurement results.