# Study on Noise Model of an Automotive Axial Fan Based on Aerodynamic Load Force
## 1 Introduction  
Vehicle axial cooling fans are frequently used to provide enough air through heat exchangers (radiators) to control engine temperature even at low vehicle speeds or in idle conditions [1]. As the core part of the cooling system, the operating noise of the fan greatly affects the ride comfort of the vehicle. Modern vehicles generally use axial flow cooling fans, and the fan noise includes aerodynamic noise, pipe radiation noise, and motor and casing noise, and among these types of noise, aerodynamic noise is the loudest [2]. Aerodynamic noise is generated by the noise caused by flow [3] and consists mainly of two parts, namely broadband noise and discrete noise. Existing studies have shown that the sound pressure pulsations generated by the periodic impacting of the blade wake flow on downstream objects are superimposed on the fundamental and harmonic frequencies of the blade pulsations, so that the sound pressure level of discrete noise is much higher than that of broadband noise [4]. In order to calculate the aerodynamic noise of the fan, the load noise of the fan must be calculated [5]. The load noise is related to the load force on the surface of the fan blade. For the determination of the load force of wind turbine blades, many scholars have conducted research. Under the conditions of a low Mach number, the noise of the rotary airfoil blade is mainly dipole noise, that is, load noise that is related to the load force on the surface of fan blades [6]. [PERSON] [7] pointed out that the aerodynamic noise of the rotating blade is mainly generated by the pulsating pressure distributed on both sidesof the blade. [PERSON] [8] used a 3D CFD model to simulate and analyze the internal flow field of a vortex fan, obtained the pressure pulsation information on the blade surface, used the pressure flow field of the blade surface as the aerodynamic sound source, and calculated the far field aerodynamic noise generated by the blade by solving the FW-H equation. The calculation results showed that the aerodynamic noise of the blade thickness was from 1 mm to 4 mm in the thin-wing range and its aerodynamic noise was dominant compared with the structured noise and remained basically unchanged. The constant load force of the blade can be obtained by the blade element momentum (BEM) theory through which the blade can be divided into many small units and the constant load force of each unit can be related to parameters such as inflow velocity, chord length, and installation angle through the momentum so as to facilitate the calculation of far-field noise using the FW-H equation [9]. [PERSON] [10] used BEM theory to solve the relative inflow velocity and inflow angle of attack of each blade element in a blade wind turbine, and each position corresponded to different airfoil parameters such as velocity, angle of attack, and chord length, which are necessary input parameters for airfoil noise calculation. [PERSON] [11] obtained the lift and resistance characteristic data of an automotive axial flow fan using the table lookup method from the inflow speed to obtain the constant load force. Based on BEM theory, [PERSON] [12] used the lift and resistance characteristic data of a wind turbine, as well as the constant aerodynamic load force, including chord length and installation angle, which was obtained from the inflow velocity, and then the FW-H equation was used to establish a prediction model of the rotating blade noise of a wind turbine. [PERSON] [13] pointed out that axial fan blades need to withstand complex aerodynamic loads in addition to constant forces but did not give a clear expression.