# Investigation and Design of the Transonic Laminar Flow Characteristics in a Laminar Aircraft
###### Abstract
The research on flow control by thermal and vortical excitations shows the relative advantage of the latter over the former [30]. Thus, the characteristics of the transonic boundary layer are still complex, especially the effects of various conditions on it, such as _Ma_ and _Re_. The _Re_ also has an important influence on the transition. Prediction of laminar flow and accurate assessment of drag requires consideration of turbulence models and transition effects [31; 32]. In short, various factors have a certain impact on the laminar flow characteristics in the state of transonic flight. Therefore, it is necessary to deeply understand the specific effects of various flight parameters on the characteristics of laminar flow. However, wind tunnel experiments are difficult to accurately reflect the laminar flow characteristics in the real flight state because of the influence of wind tunnel wall interference and the Reynolds number effect in wind tunnels. Although the three-dimensional laminar flow sensitivity based on swept-back wings may have been analyzed by many, the interaction between the sensitivity factors has rarely been studied, which is one of the novelties of this paper. In this paper, a preliminary study of the transonic laminar flow characteristics in real flight conditions was carried out based on this aircraft using CFD (computational fluid dynamics) methods. Additionally, the airfoil of the central experimental wing was designed to reduce the load burden on the outboard wing without affecting the experimental study of the central experimental wing. This will provide a reference for the subsequent laminar flow study. The detailed research process is as follows: Firstly, the RANS method combining the transition prediction model \(\gamma-\text{Re}_{\theta}\) based on local variables and the wind tunnel tests was used to analyze the transonic laminar flow parameter sensitivity for the aircraft. The RANS-based transition prediction method was validated and analyzed by the wind tunnel tests. Through the comparative analysis of numerical simulation results of the central experimental wing of the laminar flow aircraft and the corresponding test data, the calculation ability and accuracy of the transition prediction method were verified. The numerical simulation research in this paper focused on the laminar characteristics near the cruise state, the transition position of the central experimental wing and the length of the laminar flow zone of the central experimental wing under different flight states. Through the calculation, the influence of key flow parameters such as _FSTI_ (free stream turbulence intensity), _Re_ (Reynolds number), _Ma_ (Mach number) and \(\alpha\) (angle of attack, degree) on the transition position of the airfoil surface was summarized. Finally, to conduct high-speed laminar flight experiments, the aircraft design needs to consider the coordination of the aerodynamic characteristics between the outboard wing and the central experimental wing. There is a large area in the central experimental wing, so the reasonable design of the wing can reduce the load burden of the outboard wing without affecting the laminar flow experimental study. Therefore, based on the sensitive investigation, an improved design of the airfoil of the central experimental wing was carried out. In the future, the central experimental wing models based on forward-swept wings and backward-swept wings with different airfoils will be further studied to reveal more complex laminar flow mechanisms deeply.