**1. Introduction**

Bileaflet mechanical heart valves (BMHVs) are designed and used to replicate the function of natural human heart valves to maintain a unidirectional blood flow, depending on the pressure difference in the upstream and downstream sides of the leaflets. More than 170,000 of the valve replacement operations that occur annually around the world use mechanical heart valves [1,2]. Among various mechanical heart valve types, BMHVs are the most popular and are often implanted to replace diseased heart valves because of their longer lifespan and reliable performance.

Despite the widespread clinical use of mechanical valve replacements, the BMHVs are far from perfect. The major potential complications that remain as drawbacks to mechanical heart valves include hemolysis, damage of blood elements, and thrombosis, as a result of the high-velocity jet flow through the narrow passage between the leaflets [3,4]. Another noticeable phenomenon associated with mechanical valves is unsteady, blood flow-induced leaflet vibration, which leads to the complex interaction of flow dynamics and leaflet kinematics. One of the main reasons for leaflet vibration is the unsteady blood flow pressure pulsation, which is induced by the turbulent flow instabilities [5,6]. Although the occurrence of such a phenomenon is quite low, it may lead to the fracture of BMHVs and life-threatening conditions to patients. In order to deal with the potential risks caused by the leaflet

vibration phenomenon, it is of critical importance to understand the mechanism and characteristics of pressure pulsation induced by unsteady blood flow [7,8].

Previous studies mainly focus on flow characteristics and the flow structure interactions of BMHVs. For example, Matteo Nobili et al. [9] investigated the dynamics of a BMHV by means of a fluid–structure interaction method and an ultrafast cinematographic technique. The computational model captured the main features of the leaflet motion during the systole. Iman Borazjani et al. [10] performed high-resolution fluid–structure interaction simulations of physiologic pulsatile flow through a BMHV in an anatomically realistic aorta. Meanwhile, numerous in vitro and in silico studies on the characteristics of leaflet motion and the hemodynamic performances of BMHVs have been conducted. For example, L. Ge et al. [11] analyzed the results of 2D high-resolution velocity measurements, and a full 3D numerical simulation for pulsatile flow through a BMHV mounted in a model axisymmetric aorta, to investigate the mechanical environment experienced by blood elements under physiologic conditions. Redaelli et al. [12] analyzed the opening phase of a bileaflet heart valve under low flow rates and validated the leaflet motion experimentally. Cheng et al. [13] presented a three-dimensional unsteady flow analysis past a bileaflet valve prosthesis in the mitral position incorporating an FSI algorithm for leaflet motion during the valve closing phase.

As mentioned above, few studies have been performed regarding the mechanism and characteristics of pressure pulsation induced by unsteady blood flow. The mechanisms and influencing factors of pressure pulsation in BMHVs are not well understood at the present time. Hence, it is imperative to study the mechanism and characteristics of pressure pulsation induced by unsteady blood flow.

The objective of this study is, therefore, to numerically investigate the characteristics and influence factors of pressure pulsation in BMHVs under different conditions of flow rate and leaflet fully opening angle. Additionally, the non-dimensional coefficient of pressure pulsation was proposed to evaluate the impact on pressure pulsation induced by unsteady blood flow in a BMHV.

#### **2. Numerical Methods and Modeling**
