## 1 Introduction  
The research over the last several years on the fixed-wing unmanned aerial vehicles (UAVs) with turboelectric distributed propulsion (TeDP) has attracted much attention. The most representative ones are X-57, ESAero, GL-10, N3-X, E-airbus, and many other models [1, 2]. The TeDP system is used to provide lift when UAVs take off. Due to the driving of the distributed propulsion to the air, the flow behind the propulsion system grows extremely complex, which is not only accelerated, but also with strong turbulence. Meanwhile, the characteristic Reynolds number of the wing becomes ultra low because the freestream velocity is almost zero. Owing to the influence of the turbulence and low Reynolds number, the three-dimensional effect of the wing becomes more complicated, which means the wing would encounter the coupling of up and down wash flow, flow separation, laminar separation bubble, spanwise vortex and other phenomena [3, 4, 5, 6]. Different from the laminar flow in which the fluid elements keep parallel to each other, the turbulent flow makes the fluid elements in the irregular vortex motion, and the mixing of different size eddies causes strong exchange and transmission of energy and heat, rapid dissipation of mechanical energy [7; 8]. Turbulent flow is three-dimensional, rotational, intermittent, highly disordered, diffusive and dissipative. The actual modes of turbulence are eddies and high-vorticity regions, so that turbulence is often described by eddy viscosity as a local property of the fluid [9]. Generally, it is considered that the turbulence intensity less than or equal to 1% is low-intensity turbulence, and the turbulence intensity greater than 10% is high-intensity turbulence. In the wind tunnel experiment, the turbulence intensity of the inflow is usually within 3%, but in the practical application, the turbulence intensity near the propulsion system is often more than 5%. When the flow is affected by a small fan's wake and atmospheric turbulence, the turbulence intensity could usually be greater than 10% [10]. Due to the existence of a large number of mixed jets in the combustion chamber and complex cooling structure in the combustor liner, the turbulence intensity at the outlet of the aeroengine could be 15%-20% [11]. Therefore, in the face of the complex jet flow behind the UAV with TeDP, it is very important to study the aerodynamic characteristics of the two-dimensional airfoil under varied turbulence intensities and low Reynolds numbers (\(Re<5\times 10^{5}\)). Under a low Reynolds number, the current study of turbulence intensity mainly focuses on the flow near a flat plate. [PERSON] [12] carried out experiments to study the influence of flow homogeneity and isotropy, decay of turbulent kinetic energy, energy spectra, and length scales. [PERSON] [13] investigated the isotropy and homogeneity of turbulence through skewness and kurtosis of the flow velocity distribution function and transverse variation. He also studied the impact of turbulence intensity on bypass laminar-turbulent transition parameters on a flat plate through experiments. [PERSON] [14] confirmed an appreciably nonlinear dependence of the skin-friction coefficient and other boundary layer parameters on RMS freestream turbulence intensity. [PERSON] [15] found that the mean flow-field responds strongly to turbulence intensity but with little effect of integral scale, and fluctuating pressures depend strongly upon both intensity and scale. [PERSON] [16] and [PERSON] [17] observed the formation of streamwise oriented streaks in the separated shear layer, which significantly altered the flat plate boundary-layer transition under high levels of turbulence intensity. [PERSON] [18] analyzed the transition mechanism by observing the development of small disturbances and their role in the final breakdown to turbulence through direct numerical simulation (DNS). [PERSON] [19] conducted a numerical simulation of transition under low and high disturbance.