Publication Date

2015-05-05

Availability

Open access

Embargo Period

2015-05-08

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PHD)

Department

Mechanical Engineering (Engineering)

Date of Defense

2014-11-07

First Committee Member

Gecheng Zha

Second Committee Member

Weiyong Gu

Third Committee Member

Bertrand Dano

Fourth Committee Member

Amir Rahmani

Fifth Committee Member

Na Li

Abstract

This thesis investigates the performance of co-flow jet (CFJ) flow control and its applications using experimental testing and computational fluid dynamics (CFD) simulations. First, the study examines the CFJ energy expenditure, lift enhancement, drag reduction, stall margin increase, dynamic stall removal, and performance variation with Mach number. These investigations are conducted for a variety of stationary airfoils, pitching airfoils, and 3D CFJ wings. Then, the CFJ airfoil is applied to design an ultra-high wing loading general aviation electric airplane (EA). For a stationary airfoil and wing, CFJ increases the lift coefficient (CL), reduces the drag and may produce thrust at a low angle of attack (AoA). The maximum lift coefficient is substantially increased for a 2D CFJ airfoil and reaches a value of 4.8 at Cµ = 0.30. The power consumption of the CFJ pump, measured by the power coefficient (Pc), is influenced by a variety of parameters, including themomentum coefficient (Cµ ), the AoA, the injection slot location, and the internal cavity configuration. A low Cµ of 0.04 produces a rather small Pc in the range of 0.01 - 0.02 while a higher Cµ rapidly increases the Pc. Due to the stronger leading edge suction effect, increasing the AoA decreases the Pc. That is until the flow is near separation, within about 2°- 3° of the stall AoA. An injection slot location within 2% - 5% chord from the leading edge very effectively reduces the power coefficient since the leading edge suction effect is typically the strongest within this range. An internal cavity design with no separation is crucial to minimize the CFJ power consumption. When the Mach number is increased from 0.03 to 0.3, the suction pressure behind the airfoil leading edge is lowered due to the compressibility effect. This increases the CFJ airfoil maximum lift coefficient and decreases the power coefficient because of the lower required jet injection pressure. The drag coefficient remains fairly stable within this range of Mach numbers. At Mach 0.4, as the AoA increases, the flow on the suction surface becomes transonic. Consequently, a strong shock wave interrupts the jet and triggers a boundary-layer separation. The shock wave boundary-layer interaction and wave drag increase the total drag and the power coefficient significantly due to a large increase in entropy. Overall, the CFJ effectiveness is enhanced with an increasing Mach number as long as the flow remains subsonic, typically with free stream Mach number less than 0.4. For a pitching airfoil, CFJ is able to remove the dynamic stall with a substantial lift increase and drag decrease. Two pitching airfoil oscillations with dynamic stall are studied in this thesis, namely the mild dynamic stall and the deep dynamic stall. At Mach 0.3, the CFJ with a relatively low Cµ of 0.08 removes the mild dynamic stall. Thereby, the timeaveraged lift is increased by 32% and the time-averaged drag is decreased by 80%. The resulting time-averaged aerodynamic (L/D)ave, which does not take the pumping power into account, reaches 118.3. When Cµ is increased, the time-averaged drag becomes negative, which demonstrates the feasibility of a CFJ to propel helicopter blades using its pump as the only source of power. The deep-stall is mitigated at Cµ = 0.12 and completely removed at Cµ = 0.20 with a great (L/D)ave increase. At Mach 0.4, the CFJ mitigates the mild dynamic stall. However, the energy consumption is higher than at Mach 0.3 due to the appearance of shock waves in the flow. A 3D CFJ wing based on NACA 6415 airfoil with an aspect ratio of 20 produces a maximum L/D of 38.5 at a remarkably high cruise CL of 1.20 with an AoA of 5.0° and a low Cµ of 0.04. The takeoff and landing performance is also excellent with a maximum CL of 4.7 achieved at Cµ of 0.28 and AoA of 40.0°. When the wing thickness is increased from 15% to 21%, not only the lift is increased by about 5% but the structural strength is also improved. Overall the CFJ wing efficiency is found to be similar to that of conventional wings, but the lift coefficient at cruise condition is much higher, typically by 2-3 times. Hence CFJ is particularly suitable to design a compact wing with high wing loading. In the final study of this thesis, a CFJ Electric Aircraft (CFJ-EA) is designed for the general aviation. The aircraft has a high wing loading so that it can carry more battery and reach a longer range with a relatively small wing size. The CFJ-EA mission is to carry 4 passengers at a cruise Mach number of 0.15 with a range of 315nm. The CFJ-EA cruises at a very high CL of 1.3, which produces a wing loading of 182.3kg/m2, about 3 times higher than that of a conventional general aviation airplane. To determine the aircraft range and endurance, we introduce the corrected aerodynamic efficiency (L/D)c defined as (L/D)c = L/(D+P/Vinf), where the L and D are the aerodynamic lift and drag, P is the CFJ pumping power and Vinf is the free stream velocity. The (L/D)c of the CFJ-EA is excellent with a cruise value of 23.5 at a low Cµ of 0.04. Takeoff and landing distances are also good due to a very high maximum CL of 4.8, achieved with a high Cµ of 0.28. During takeoff and landing, the wing pivots around its 1/4 chord axis so that it can achieve an AoA of 25.0° with the fuselage rotated by only 5.0°. Based on a measure of merit defined as MPS=Miles*Passengers/S, where S is the wing planform area, the MPS of the present EA design is about half that of a conventional reciprocating engine general aviation airplane, and is 1.5 to 2.5 times greater than the MPS of the state of the art EA. This suggests that, compared to the conventional EA, a same size CFJ-EA has a far greater range, or a smaller CFJ-EA achieves the same range. Therefore, the CFJ-EA concept may open the door to a new class of general aviation EA designs. The same CFJ airfoil flow control technology is also suitable for airplanes and rotorcraft using conventional propulsion systems including high altitude planform, general aviation, commercial aviation or military transport to improve the range, reduce the wing size and/or reduce the takeoff and landing distances.

Keywords

Aerodynamics; Aircraft Design; Flow Control; Aerospace Engineering; Fluid Mechanics

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