Numerical study of circulation control phase based on fluidic flight control technology

[Objective] Circulation control is a typical fluidic flight control technology that is often used to improve the aerodynamics of aircraft and wind turbine blades. The aerodynamic performance of an airfoil can be improved by adjusting the mass flow rate of the jet. The impact of the Coanda effect on the trailing-edge jet produces a wall-attachment effect, decreasing the trend of flow separation. The researchers adjusted the airfoil circulation by controlling the trailing-edge jet separation. However, the relationship between the trailing-edge jet separation position in circulation control technology and the jet position, jet height, and jet intensity is unclear numerically. [Methods] To solve the problem of trailing-edge jet deflection, this paper mainly investigated the influence of the Coanda effect on the flow field distribution and lift-drag characteristics and analyzed the aerodynamic characteristics at different control stages of circulation control to contribute to the promotion of jet flight control technology. Because of the advancements in computer technology, computational fluid dynamics had considerably improved, and the simulation results for the complex structure of the flow field had reflected the real physical laws. In this paper, the NACA0012 airfoil was modified, and two dimensionless parameters, trailing-edge curvature and jet height, were changed to verify the effectiveness of the Coanda effect under circulation control. Furthermore, several key parameters, such as angle of attack, momentum coefficient, and jet velocity, were simulated using the CFD++ aerodynamic simulation software. Referring to other researchers' work, the concept of the Coanda deflection angle was introduced to reveal the triggering and suppressing mechanisms of the Coanda effect numerically. Thus, the separation prediction method was proposed for the modified NACA0012 airfoil and CC-E0020EJ circulation control airfoil (two symmetrical airfoils). For the two control stages in circulation control technology, this paper deconstructed the lift and drag forces through the push-drag decomposition method, explained the supercritical phenomenon of flow field separation and reattachment behind the trailing edge, and proposed a separation region analysis method. [Results] (1) The most important factor affecting the lift-drag ratio of airfoil trailing-edge parameters was jet nozzle height. When the chord position (xjet/c) was 0.900, and the nozzle height (h/r) was 7.16%, the lift coefficient generated by the Coanda effect considerably increased, and the best effective lift-drag ratio increased to 43. (2) At a zero angle of attack, the variation trend of the Coanda deflection angle with momentum coefficient was highly consistent with the logarithmic relation curve and the same change characteristics of turning point and growth rate were detected on the symmetrical airfoil, indicating that the Coanda deflection angle was a physical criterion for judging the jet strength and increment of the lift coefficient. (3) The concept of "step zone" was introduced at a nonzero angle of attack, and the strong linear relationship between the jet influence zone position and momentum coefficient at 4°, 8°, 12°, and 16° angles of attack was demonstrated. The large area separation in the trailing edge caused a surge in drag coefficient and loss of lift growth. [Conclusions] The analysis method proposed in this paper achieves the expected goal and provides an effective basis for judging the Coanda effect in fluidic flight control. © 2024 Press of Tsinghua University. All rights reserved.