Numerical Investigation On The Unsteady Separation Flows Around Double-Delta Wings

Advanced combat aircrafts are in increasing requirement to have higher maneuvering performance at high angles of attack beyond post-stall conditions. In the process of maneuverability, rapidly pitch-up and pitching motions are typical. In order to meet the requirements in the wide range of flight speeds, delta /double-delta or similar wing layouts are commonly adopted in the design of the modern aircrafts. In order to achieving super-maneuverability, it is necessary to study the nonlinear phenomena in the separation flow field when the wings are in either static or pitch-motion state to find out the characteristics of the separation flow, and to observe the unsteady phenomena and their correlative mechanism comprehensively.The first objective in this thesis is to develop some numerical methods that are suitable for simulating the unsteady separation flows around two double-delta wings at high angles of attack and to study the evolution laws of the flow characteristics when the wings are in static or pitch-motion states and the influence of different motion parameters on the characteristics of the unsteady flows. The second is to study the numerical methods used to simulate the low-speed flows and to develop the Slightly Compressible Model (SCM) aiming at widening the application areas.This thesis is divided into eight chapters as follows:In the first chapter, the significance of the present research work is briefly introduced. The status in quo and the progress of the researches in the flow fields around delta wing/double-delta wing are reviewed. The innovation works in this thesis are then described in brief.In the second chapter, the numerical methods applied in this thesis are presented in detail. The main contents include: governing equations, boundary conditions, numerical flux schemes, time-marching methods, turbulence models, moving grid generation techniques, multi-grid methods, parallel computing techniques and unsteady aerodynamic force integration formulas.In the third chapter, several flows with abundant experimental data or academic results are numerically simulated. In these research works, the spatial and the temporal precisions are examined to validate the reliability of the program based on the numerical methods introduced in Chap.2. The emphases are placed on the numerical results of the calculated flows around a delta wing and a spheroid at high angles of attack.In the fourth chapter, the flow characteristics around a static double-delta wing at different angles of attack are investigated thoroughly. The characteristics of the spatial distribution of the vortex system in the leeward side of the wing and the correlations among these different vertical flows are analyzed. The evolvement laws of the characteristics of the flows and their aerodynamics are revealed. The ranges of the angles of attack existing different flow types are confirmed. The differences in the flow structure and the aerodynamics performance between two types of double-delta wings are compared and analyzed.In the fifth chapter, the dynamic characteristics of the flow fields around a double-delta wing during pitch-up motion are researched by numerical methods. The evolvement laws of the dynamic flow fields and its aerodynamics performances following the change of the angles of attack are presented. The studies are concentrated on the investigations of the influences of some motion parameters (such as reduced frequency, the initiative angle of attack and the position of rotation axis) on the dynamic flow performance and the investigations of the physical mechanism producing these influences. In this chapter the aerodynamics performances between two double-delta wings with different wing plane shapes are compared.In the sixth chapter, the dynamic characteristics of the flow fields around a double-delta wing during pitching motion are researched. The evolvement laws of the dynamic flow fields and the aerodynamic performances following the change of the angles of attack are presented. The studies are concentrated on the investigation of the influences of some motion parameters (such as reduced frequency, the averaged angle of attack, position of rotation axis and the pitching amplitude) on the time-lag effects of the dynamic flow field and the hysteresis-loops of the aerodynamic coefficients. In this chapter the difference of the aerodynamics performances between two double-delta wings with different wing plane shapes are given.In the seventh chapter, the Slightly Compressible Model is developed and used. First, some former equation types of the SCM model are reviewed. The merits and the drawbacks of the equations are analyzed theoretically. Then, the calculation of pressure in the original SCM equations is replaced by the calculation of the dispersion of the pressure in this thesis. So the truncation error of the computer is reduced. The multi-grid methods and parallel computing techniques are also applied in this SCM model to accelerate the computing speed. In the last section of this chapter, some unsteady separation flows around a delta wing at high angle of attack are simulated by the SCM model.In the eighth chapter, all the works in this thesis are reviewed, and the innovation and the shortages are also described. Moreover, some considerations on the further research work in this area are listed.Finally, the acknowledgements and references are presented.