## 1 Introduction
Although both techniques could synchronize the instantaneous velocity of both liquid and bubble, they require more devices since two resonant frequencies are needed. As a consequence, twice the amount of pulser/receiver and data processing equipment is required. Therefore, for minimizing the measuring equipment, the measurement system that employs only a single resonant frequency is needed. This study proposes a measurement technique for obtaining the velocity profile of two-phase bubbly flow using the UVP method with a single resonant frequency. The system employs only a single resonant frequency transducer, single channel pulser/receiver, and simple data processing unit. Therefore, extra equipment is not required. The velocity profile of both phases can be separately obtained even though the velocity data for liquid and bubble occur on the same measurement channel. Furthermore, the phase separation used in this study can distinguish the velocity of liquid and bubbles, although similarities are observed with both phases. The proposed idea uses the substantial difference in acoustic impedance between the particle and bubble [17]. This leads to the amplitude effect of the Doppler signal. The Doppler frequency is also reflected by the bubble and particle being different due to their velocities. Based on this idea, [PERSON] et al. [18] proposed the technique using Fast Fourier Transform with a fixed window which collaborate with the classification of Doppler amplitude affected by particle and bubble. The behavior of the Doppler signal could be analyzed. The velocity of both phases could be obtained separately. However, the measurement accuracy was weak when the period of Doppler signal influenced by both particle and bubble occurs within the one fixed window. [PERSON] et al. [19] used time-frequency analysis and Doppler amplitude classification to analyze the Doppler signal and classify the liquid and bubble velocities, respectively. Nevertheless, the performance of time-frequency estimator in each type was not been compared. Then, time-frequency parameters of the estimator have not been optimized. Moreover, the detail of the threshold setting was not discussed. In recent work, the performance of several time-frequency estimators is compared. Short-Time Fourier Transform (STFT) which was applied in non-destructive testing and other fields [20; 21; 22], is selected as a time-frequency estimator in this work because it has a good performance in three criteria--readability, resolution, and computational time. The time-frequency parameter of STFT is optimized reasonably. Furthermore, Doppler amplitude classification is performed by setting the threshold. The threshold value is obtained from experimental testing which is based on the statistical data of Doppler amplitude reflected by particles and bubbles. For confirming the applicability of developed technique, the experiment is conducted on vertical pipe flow apparatus. The velocity profile measurement in two-phase bubbly flow is performed experimentally, demonstrating the separation of liquid and bubble velocity. The developed technique guarantees accuracy by comparing both single-phase and two-phase bubbly flows through the PIV methods. Moreover, the effect of void fraction against velocity measurement of both phases was observed.