## 1 Introduction  
In dental clinics, the environment is filled with various sounds [1], particularly the high-pitched discomforing noise of dental drills. Studies show that over half the patients feel uneasy due to this noise, which is closely linked to heightened levels of dental anxiety [2, 3]. This anxiety can result in patients avoiding dental visits, ultimately compromising their oral health [2, 3]. Addressing dental drill noise is therefore crucial for improving patient experience and promoting better oral health outcomes [2, 3]. The dental high-speed air turbine handpiece (ATH), a prominent source of high-pitched sound in dental clinics, is defined by ISO standards as a dental drill operating at 160,000 rotations per minute (rpm) or higher, powered by compressed air [4]. Structurally, an atm consists of a rotor housed in the drill head body, which includes an impeller, bearings, and a spindle [5; 6]. When high-pressure compressed air enters the atm, the impeller rotates rapidly, causing the cutting diamond bar inserted into the spindle to rotate. Given that tooth enamel is the toughest substance in the human body [7], atm can sometimes reach speeds of up to 400,000 rpm for effective drilling [6; 8; 9]. At such high rotational speeds, airflow inside and around the atm generates complex fluid dynamics, producing noise. The characteristic frequency components of the noise generated by atm are primarily determined by the drill's rotational speed [10; 11]. The noise generated during atm idling is different in timbre from the sound of drilling teeth and dental metal, but their frequency components are closely related [10]. Psychoacoustic experiments and acoustic physical measurements have strongly linked both sound pressure levels and high-frequency noise components to patient discomfort caused by atm noise [10; 11]. Therefore, understanding the aeroacoustic mechanisms of atm noise generation is essential for developing solutions to mitigate these unpleasant sounds from devices and improve patient experiences. Computational fluid dynamics (CFD) has been widely applied for numerical analysis in various industries, including aerospace, automotive, construction, and chemical engineering. While previous CFD studies on atm have focused on torque and rotational speed [12; 13; 14], they have been limited to steady-state modeling. Although these studies provide useful insights, steady-state simulations assume fluid properties remain constant over time, which may not capture the transient behaviors critical to aeroacoustic phenomena. Therefore, in systems where transient effects are significant, this approach may lead to incomplete or misleading results. In contrast, transient simulation accurately captures dynamic behavior, transients, and temporal variations, providing a more precise representation of time-sensitive systems and improving the reliability of results. To our knowledge, no previous work has conducted a full transient CFD simulation of atm with compressible flow modeling. Using CFD to assess the transient state for aeroacoustic noise presents unique challenges in simulating airflow in atm. The high rotational speed (up to 320,000 rpm), complex small geometry, and significant scale differences in pressure [ranging from 303,975 Pa (3.0 atm) at the inlet of atm to 100 Pa for noise fluctuations] demand a compressible flow solver and fine computational resolution. Additionally, transient simulations are essential for accurately predicting decibel levels and evaluating noise impacts on patients, requiring supercomputing resources. These challenges have historically limited research on non-steady airflow and aeroacoustics in atm. To address these challenges, we developed a novel transient CFD framework for atm noise analysis. By analyzing the flow field of an atm at high rotational speeds during dental treatment, we aimed to identify the cause of acoustic noise and explore potential improvements.