Publication Date

2018-05-02

Availability

Open access

Embargo Period

2018-05-02

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PHD)

Department

Mechanical Engineering (Engineering)

Date of Defense

2018-03-15

First Committee Member

Ge-Cheng Zha

Second Committee Member

Hongtan Liu

Third Committee Member

Weiyong Gu

Fourth Committee Member

Wangda Zuo

Abstract

The objective of this dissertation is to investigate the super-lifting performance of Co-Flow Jet (CFJ) flow control airfoil and its applications to electric aircraft. The CFJ airfoil is promising to transform future aircraft design with extremely short takeoff/landing (ESTOL) and ultra-high cruise efficiency due to its substantial lift enhancement and drag reduction with very low energy expenditure. To resolve turbulent vortical structures for super-lifting CFJ airfoil flows, the improved delayed detached eddy simulation (IDDES) with high order schemes is developed and implemented in the in-house CFD code, FASIP. The high order schemes used in this study include a fifth-order weighted essentially non-oscillatory (WENO) scheme for the inviscid fluxes reconstruction and a fourth order conservative central differencing scheme for the viscous fluxes. An efficient and low diffusion E-CUSP (LDE) scheme as a Riemann solver designed to minimize numerical dissipation is utilized. The comparative study of the S-A URANS, DES, DDES, and IDDES simulation is performed on the turbulent boundary layer flows over the flat plate and the stalled flows of the NACA0012 airfoil. The validation study indicates that IDDES method can predict the law of the wall accurately for different mesh sizes, Reynolds numbers, and Mach numbers whereas the DES and DDES obtain the velocity profile in the boundary layer with model stress depletion and log layer mismatched at certain conditions. The 2D RANS simulation of a CFJ-NACA6421 airfoil discovers for the first time that CFJ airfoil is able to achieve the super-lift coefficient (SLC), which is defined as a lift coefficient that exceeds the theoretical limit based on potential flows. For the CFJ-NACA6421 airfoil, a maximum lift coefficient of 12.6 is achieved at the angle of attack (AoA) of 70 deg and jet momentum coefficient of Cμ = 0.60. It is 66% higher than the theoretical limit of 7.6 for an airfoil of 21% thickness (t/c = 0.21). The circulation achieved around the CFJ airfoil is so large that the stagnation point is detached from the airfoil solid body and the Kutta condition does not apply anymore. For the superlift condition at AoA of 70 deg, the vortex structures on the CFJ airfoil suction surface appear to have four counter-rotating vortex layers next to each other from the airfoil wall surface to the far field freestream. The 2D simulation of CFJ airfoil indicates that the CLmax appears to have no limit. The CLmax limit from the potential flows is the result of imposing Kutta condition, which is necessary for potential flows, but not a true physical condition. In reality, CLmax depends on how much energy can be added to the flow to overcome the severe adverse pressure gradient. To further verify the super-lift coefficient and the vortical structures of the CFJ airfoil, a 3D unsteady IDDES investigation of the CFJ-NACA6421 airfoil is performed with a span length of 10% of the chord. The IDDES results verify that the CFJ airfoil is able to achieve the super-lift coefficient at ultra-high AoAs with attached flow. The 3D steady RANS simulation of a finite-span super-lift CFJ wing is carried out with different aspect ratios (AR) without using any flaps. The RANS simulation results indicate that the CFJ wing can achieve the maximum lift coefficient of 7.8 at a very high AoA of 70 deg with good aerodynamic efficiency. At high AoAs, the outer 25% wingspan is affected more by the wingtip vortex that contributes the lift reduction and drag increase. The ultra-high lift coefficient does not appear to increase the penalty of induced drag due to the negative drag at zero lift. The Oswald efficiency is increased with the AR decreased from 20 to 5 at the same AoA and Cμ. It achieves the value as high as 0.967 at AR of 5, Cμ of 0.25 and AoA of 25 deg, indicating that the penalty of induced drag for 3D CFJ wing is small even though ultra high lift coefficient is obtained. Furthermore, the super-lifting CFJ flow control concept is applied to a 2D circular cylinder as a general lifting system to study the fundamental physics of the superlifting phenomenon. The 2D RANS simulation indicates that the CFJ cylinder can achieve a maximum lift coefficient of 28 at Cμ=0.8, far exceeding the potential limit of 4π where the stagnation point is on the bottom of the cylinder. A trade study of injection and suction slot configurations is performed to obtain the optimum injection and suction slot locations. An experimental investigation of CFJ airfoil with embedded compressors was conducted at the Low Speed Wind Tunnel of Texas A&M University. The wind tunnel experiment for the first time proves that an airfoil can achieve a lift coefficient exceeding the theoretical limit by CFJ flow control. A high thrust coefficient of the CFJ airfoil was also observed. Both the high lift and thrust are attributed to the super-suction effect with very low pressure at the airfoil leading edge induced by the injection jet. The CLmax achieved in the experiment varies from 8.0 to 8.6, substantially exceeding the theoretical limit of 7.6. A very large thrust coefficient (negative drag) of 1.0 is achieved at low AoAs. A thrust is maintained up to the angle of attack of 40 deg when the airfoil is about to get stalled. To improve the performance of the CFJ electric airplane (EA), a super-lifting CFJ takeoff airfoil and modified high-efficiency CFJ cruise airfoil are applied to the improved design, CFJ-EA2. The CFJ-EA2 wings are designed to be pivotable to take advantage of ultra-high lift coefficient at high AoA. The 3D steady RANS simulation of CFJ-EA2 at AoA of 30, 40, and 50 deg is performed using the Cμ from 0.2 to 0.6. The simulation results indicate that using the super-lifting wings, the CFJEA2 can achieve a maximum lift coefficient of 6.9 at a very high AoA of 50 deg with a good aerodynamic efficiency. For the cruise performance enhancement, an improved CFJ cruise airfoil is applied on the CFJ-EA2 to have a higher wing loading and better cruise efficiency. The cruise lift coefficient of CFJ-EA2 wings is 1.59 and the corrected aerodynamic efficiency (L/D)c is increased to 31. The productivity efficiency of (C2L/CD)c is 50, which is 50% higher than CFJ-EA. The wing loading of the CFJ-EA2 airplane is increased to 214 kg/m2. In addition to its higher MTOW of 2289 kg because of its high wing loading, the CFJ-EA2 has a range of 531 nm because more batteries are installed. Finally, a conceptual design of CF Hybrid Electric Regional Aircraft (CFJ-HERA) is conducted. The main purpose of CFJ-HERA airplane is to use CFJ flow control and hybrid electric propulsion to achieve better fuel efficiency and higher payload than the baseline ATR72-500 regional aircraft. The CFJ-HERA airplane uses CFJ wings with an aspect ratio of 12 based on CFJ-NACA6421-INJ13-SUC20 airfoil with the original fuselage and empennage design of ATR72-500. The lift coefficient of 1.158 is achieved at AoA of 4◦ and Cμ of 0.02 based on a RANS simulation of the CFJ wing. The CFJ-HERA cruises at the Mach number of 0.46 with the range of 2500 nm. The MTOW is increased to 39500 kg since the CFJ wing a high wing loading of 660 kg/m2. The hybrid propulsion system is analyzed with various components including electric motors, inverter/converter, electric cable, and CFJ micro-compressor. In conclusion, this dissertation demonstrates that the CFJ airfoil can achieve both the super lift coefficient during takeoff and landing and ultra-high cruise efficiency during cruise. The super-lift coefficient of CFJ airfoil with embedded micro-compressor was proved in the wind tunnel testing. It expands the high lift theory of fluid mechanics to a new area, which may foster industrial applications that are very different from today’s technology. The novel CFJ aircraft design combined with the benefits of distributed electric propulsion (DEP) has the potential to transform the aviation industry.

Keywords

CoFlow Jet; Flow Control; Electric Aircraft; Hybrid Electric Propulsion; Aerodynamics; Aircraft Design

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