Abstract
The objective of this dissertation is to numerically investigate the performance of transonic Co-Flow Jet (CFJ) active flow control and its application to aircraft. The motivation is to advance the CFJ flow control to the transonic regime that is little studied. The major research subjects in this dissertation to study CFJ performance enhancement include: 1) Low-speed high-lift performance of CFJ supercritical airfoil; 2) Transonic CFJ supercritical airfoil efficiency at cruise; 3) CFJ transonic wings with different aspect ratio and sweep angles at cruise; 4) 3D wing tip vortex effect with CFJ; 5) 3D CFJ swept cylinder controllability; 6) a transonic high-efficiency CFJ-VTOL aircraft design and simulation at hover and cruise. The in house high order accuracy FASIP CFD code with RANS and IDDES turbulence models is used for the simulation.
RANS simulation is performed to investigate the low-speed performance of CFJ supercritical airfoils for extremely short takeoff/landing (ESTOL) and vertical takeoff/landing (VTOL) performance. The airfoils studied are NASA SC(2)-1010, RAE-2822, and NASA SC(2)-0714. The effects of slot location and size, airfoil thickness, and jet intensity are investigated. It is found that for the CFJ supercritical airfoils, very high maximum lift coefficient at low speed can be obtained while improving the aerodynamic efficiency at cruise at low angle of attack(AoA). The CFJ-NASA-SC(2)-0714 supercritical airfoil achieves super-lift coefficient of 9.1 at Mach 0.1, attributed to its large leading-edge radius and airfoil thickness, whereas the CFJ-RAE-2822 and CFJ-NASA SC(2)-1010 airfoils obtain lower maximum lift coefficient of 5.4 and 5.9 respectively. The overall low-speed performance of supercritical CFJ airfoils is significantly improved compared to conventional super-critical airfoils. The results are promising to achieve high lift coefficient for takeoff/landing without using the conventional flap systems. The same airfoils and configurations are simulated at cruise condition, under which they are shown to increase aerodynamic efficiency (CL/CDC) by 10% for the CFJ-NASA SC(2)-0714 airfoil and productivity efficiency by 24% for the CFJ-RAE-2822 airfoil over their baseline airfoils.
The three CFJ supercritical airfoils are used to form 3D wings to study transonic supercritical wing performance with different aspect ratios and sweep angles. For zero sweep and aspect ratio of 10, the CFJ-NASA SC(2)-1010 wing is shown to increase cruise-efficiency by 10% and lift coefficient by 10% simultaneously over the baseline wing. When the sweep angle is greater than 15◦, the drag increase outweighs the lift enhancement. Similarly, wings formed by CFJ-RAE-2822 are able to increase aerodynamic efficiency at zero sweep and aspect ratio of 10, but not to the extent of its 2D counterpart. Decreasing the CFJ jet strength from root to tip is beneficial to reduce drag and power consumption while maintaining lift enhancement.
To guide 3D wing design with CFJ, a study is conducted to investigate the wing tip vortex behavior and its interaction with the shear layer by comparing with the baseline wing at the same aspect ratio of 10. The baseline wing is formed using NACA 6421 airfoil, whereas the CFJ wing is formed by CFJ-NACA 6421 airfoil previously designed. The total pressure of the injection slot is held constant for the entire angle of attack sweep from α = −1◦ to α = 26◦. The maximum aerodynamic efficiency occurs at an AoA of 2◦, across which the vortex core axial velocity remains mostly wake-like in the near wake region. However, the axial velocity in the vortex core edge increases with jet-like axial velocity when the AoA is at 5◦. With the tip vortex growing in size while propagating downstream, an adverse pressure gradient is created as predicted by Batchelor’s model. At this point the axial velocity decreases to a wake-like profile. This phenomenon is
observed for both the CFJ and baseline wings. The CFJ wing expectedly produces more lift than its baseline counterparts at the same angle of attack. This results in a smaller wake-momentum deficit at lower angles of attack, and a stronger jet-like axial velocity profile at higher angles of attack. The vortex core axial velocity profile and vortex tangential velocity profiles are linked. When greater axial velocity is observed in the core region, higher tangential velocity is also observed. The strength of the tip vortex is greater for the CFJ wing, compared to the baseline wing at the same angle of attack. The free-shear layer roll-up is also examined and indicates a corresponding increase in absolute magnitude for the CFJ when compared to the baseline wing.
A study of CFJ flow control for 3D swept cylinder is conducted to demonstrate its flight controllability for transonic flow. Due to the sweep and Reynolds number effect for potential wind tunnel testing, the simulated Mach number for its corresponding 2D and 3D configuration is subsonic. For an optimized 2D cylinder at Mach number of 0.25, a lift coefficient of 10.6 is achieved with very low power consumption. A 60◦ swept 3D cylinder at Mach number of 0.45 with an aspect ratio of 10 is configured to investigate two flight control aspects: 1) side force enhancement by CFJ for lateral motion control; 2) Side force cancellation by CFJ to enhance the pitching moment for longitudinal control. By applying a set of CFJ flow control for 15% of the cylinder span, a very large side force coefficient of 3 is achieved. It is more than required to control the cylinder lateral motion. By using two sets of CFJ oriented in opposing injecting directions, the side force can be canceled out, while enhancing the pitching moment for longitudinal control. The CFJ injection and suction slot location affect the direction of the resultant force vector. CFJ active flow control can be manipulated to vector the aerodynamic force on the CFJ cylinder. This study indicates that it is feasible to control a 3D swept cylinder lateral and longitudinal motion using CFJ without a V-tail control surface.
Based on the knowledge learned and developed from the fundamental transonic CFJ flow control, a conceptual aerodynamic design of a transonic CFJ-VTOL air vehicle with cruise Mach number of 0.6 is conducted. The vehicle has a tandem wing tailless configuration with the fuselage designed for 1.5 ton payload or 15 passenger seats. The overall aspect ratio of the tandem wing system is 11.65. The CFJ-VTOL concept has both the propeller and wing generating lift at hovering condition to reduce the required power. At cruise, the CFJ is able to enhance the aerodynamic and productivity efficiency due to increased lift and reduced drag at low energy expenditure. A 2D airfoil study is conducted and selects the NACA6415 based CFJ airfoil to form the 3D unswept and untapered wing. At cruise, a very good aerodynamic efficiency (CL/CD)c of 14.6 is achieved with a lift coefficient of 0.812. At hovering static condition, the CFJ wing is positioned at an angle of attack of 80◦ that keeps the flow attached and generates high lift making use of the flow pulled by the propeller. The CFJ wings generate 19% of the total lift, but consume only 1.5% of the total hovering power. This substantially reduces the disk loading and potentially its associated noise. As a result, the system hovering power is decreased by 21.7%, which reduces the propulsion system weight and benefit the whole mission efficiency. The conceptual aerodynamic design and trade study with high fidelity CFD simulation indicates that the CFJ-VTOL concept is not just feasible to cruise at Mach number 0.6 and higher in transonic regime, it is also possible to increase the mission productivity efficiency substantially (e.g. by 100% or higher) compared to the State of the Art conventional VTOL aircraft.
Using the CFJ-VTOL tandem wing configuration as the starting point for longitudinal static stability control, a trade study of aspect ratio distribution between the front wing and rear wing is performed to find a statically stable design. The fuselage is redesigned from the one in Chapter 9 to improve the fuselage-wing interaction to reduce drag and the sensitivity of the fuselage pitching moment to angle of attack variation. In order to achieve longitudinal stability, the front wing lift slope needs to be shallower than that of the rear wing so that the front wing is less sensitive to the variation of angle of attack than the rear wing. This is achieved by having the aspect ratio of the front wing to be 25% of the rear wing. The front wing incidence angle is 3◦ and the rear wing is 0◦. The large contribution of the fuselage toward the overall pitching moment requires the fuselage incidence angle to be 5◦ at cruise, which allows a positive pitching moment at α = −5◦. This configuration fulfills the requirement of CMα < 0 for the range of −5◦ ≤ α ≤ 2◦. The tandem wing vehicle achieves an excellent aerodynamic efficiency of 15.3. Future work needs to be done to further increase the longitudinal stability beyond the α range above including using dynamic stability control.
The study of this dissertation indicates that a high speed VTOL vehicle using tandem wing configuration at cruise Mach number 0.6 can achieve very high aerodynamic efficiency while having sufficient longitudinal static stability and substantially reduced hover power.