scholarly journals Fan Aerodynamics With a Short Intake at High Angle of Attack

2021 ◽  
Vol 143 (5) ◽  
Author(s):  
Ben Mohankumar ◽  
Cesare A. Hall ◽  
Mark J. Wilson
Keyword(s):  
Pressure Ratio ◽  
Radial Pressure ◽  
High Angle ◽  
High Angle Of Attack ◽  
Ratio Distribution ◽  
High Diffusion ◽  
Work Input ◽  
Velocity Triangle ◽  
Tip Velocity ◽  
High Work

Abstract Fans that are designed to maintain thrust at the high angle of attack (AOA) flight condition could exploit the cruise fuel burn benefit of a shorter intake design. This article considers how the fan rotor radial pressure ratio distribution and tip velocity triangle can be designed to improve thrust when coupled to a short intake operating at high AOA. Two AOA values are investigated using unsteady computational fluid dynamics: 20 deg (attached flow) and 35 deg (separated flow). Thrust at high AOA is governed by three key loss and work input mechanisms. (i) Rotor choking loss: flow is accelerated around the intake bottom lip and enters the rotor with high Mach numbers. Fans designed with a tip-high radial pressure ratio distribution reduced choking loss with a separated intake compared to a mid-high design, particularly when the tip velocity triangle was designed with high diffusion instead of high camber. (ii) Rotor–separation interaction loss: the rotor ingests low mass flow when operating inside the separation and the casing boundary layer separates. High diffusion tip designs strengthened the casing separation, but this penalty did not outweigh improved choking loss. (iii) Work input in radial flows: high AOA generates strong radial flows through the rotor, which alter both the amount and the way work is imparted on the flow. Fans designed with a mid-high radial pressure ratio distribution imparted high work on streamlines migrating toward the hub. Consolidating these findings, we propose two design philosophies for improved thrust at high AOA: high work (mid-high radial pressure ratio distribution) or low loss (tip-high radial pressure ratio distribution with high diffusion tip velocity triangle).

Fan Aerodynamics With a Short Intake at High Angle of Attack

2020 ◽  
Author(s):  
Ben Mohankumar ◽  
Cesare A. Hall ◽  
Mark J. Wilson
Keyword(s):  
Pressure Ratio ◽  
Angle Of Attack ◽  
Radial Pressure ◽  
Separation Mechanism ◽  
Flow Distortion ◽  
Ratio Distribution ◽  
Rotor Design ◽  
Velocity Triangle ◽  
Tip Velocity ◽  
High Work

Abstract Future turbofan engines seek shorter intakes to reduce the cruise fuel burn of a low pressure ratio, large diameter fan. However, shorter intakes increase the level of flow distortion entering the rotor when the aircraft angle of attack (AOA) is high, reducing thrust when critically needed. This paper considers how the fan rotor radial pressure ratio distribution and tip velocity triangle can be designed to improve thrust at high AOA. Full annulus, unsteady CFD is performed on three rotor designs coupled to a short intake. We show that rotor design for high AOA should be guided by three flow mechanisms. Mechanism i) is caused by high Mach number flow over the bottom intake lip, which chokes the rotor leading to high loss. Mechanism ii) is the loss generation in the rotor tip as it passes through an intake separation. Mechanism iii) shows radial flows through the rotor change both the amount and the way work is imparted on the flow. Two comparable rotor design philosophies for high thrust are proposed; high work or low loss. Rotors designed to a mid-high radial pressure ratio distribution impart high work on streamlines that migrate radially towards the hub and exit the rotor at highly cambered sections. Meanwhile, tip-high designs reduce choking losses in the midspan when operating with a separated intake, particularly when the tip velocity triangle is designed to high axial velocity diffusion over high camber. However, such designs suffer with higher tip losses after exiting an intake separation.

Sweep Effects on Fan-Intake Aerodynamics at High Angle of Attack

2021 ◽  
Author(s):  
Ben Mohankumar ◽  
Cesare A. Hall ◽  
Mark J. Wilson
Keyword(s):  
Mach Number ◽  
Pressure Ratio ◽  
Angle Of Attack ◽  
Pressure Rise ◽  
Low Pressure ◽  
Rotating Stall ◽  
High Angle ◽  
High Angle Of Attack ◽  
Mach Numbers ◽  
High Mach

Abstract Sweep in a transonic fan is conventionally used to reduce design point losses by inclining the passage shock relative to the incoming flow. However, future low pressure ratio fans operate to lower Mach numbers meaning the role of sweep at cruise is diminished. Instead, sweep might be repurposed to improve the performance of critical high Mach number off-design conditions such as high angle of attack (AOA). In this paper, we use unsteady computational fluid dynamics to compare two transonic low pressure ratio fans, one radially stacked and one highly swept, coupled to a short intake design, at the high AOA flight condition. The AOA considered is 35°, which is sufficient to separate the intake bottom lip. The midspan of the swept fan was shifted upstream to add positive sweep to the outer span. Based on previous design experience, it was hypothesised the swept fan would reduce transonic losses when operating at high AOA. However, it was found the swept fan increased the rotor loss by 24% relative to the radial fan. Loss was increased through two key mechanisms. i) Rotor choking: flow is redistributed around the intake separation and enters the rotor midspan with high Mach numbers. Sweeping the fan upstream reduced the effective intake length, which increased the inlet relative Mach number and amplified choking losses. ii): Rotor-separation interaction (RSI): the rotor tip experiences low mass flow inside the separation, which increases the pressure rise across the casing to a point where the boundary layer separates. The swept fan diffused the casing streamtube, causing the casing separation to increase in size and persist in the passage for longer. High RSI loss indicated the swept fan was operating closer to the rotating stall point.

Fan-Intake Coupling With Conventional and Short Intakes

2021 ◽  
Author(s):  
E. J. Gunn ◽  
T. Brandvik ◽  
M. J. Wilson
Keyword(s):  
Flow Field ◽  
Potential Flow ◽  
Pressure Ratio ◽  
Current Trend ◽  
Angle Of Attack ◽  
High Angle ◽  
High Angle Of Attack ◽  
Field Distortion ◽  
Intake Flow ◽  
The Impact

Abstract The current trend in civil engine fans towards lower pressure ratio and larger diameter is accompanied by a need to shorten the engine intake length to reduce weight and drag. This paper uses full-annulus, unsteady CFD simulations of two coupled fan-intake configurations to explain the impact of flow field coupling and intake length on fan and intake performance. On-design and off-design operating points are considered at cruise and high angle of attack, respectively. The fan efficiency at cruise is shown to be determined by a trade-off between two effects. Cruise efficiency is reduced by 0.11% with a short intake due to increased potential flow field distortion, which alters the incidence and diffusion of the rotor. This is partially offset by a reduction in casing boundary layer thickness due to lower intake wetted area. At high angle of attack conditions, a short intake leads to increased potential flow field distortion and an earlier onset of intake flow separation due to a higher adverse pressure gradient approaching the fan. Both effects combine to reduce the fan thrust at such conditions, although the fan is shown to remain stable at attack angles up to 35°. The reduction in performance is shown to be dominated by flow separations in the rotor, which increase in size and severity for a given attack angle as the intake length is decreased. The fan is also shown to have a stronger influence on the form of the intake flow field in a short intake, suggesting that it is necessary to model the fan in the intake design process for a successful design.

scholarly journals Leading Edge Blowing to Mimic and Enhance the Serration Effects for Aerofoil

2021 ◽  
Vol 11 (6) ◽  
pp. 2593
Author(s):  
Yasir Al-Okbi ◽  
Tze Pei Chong ◽  
Oksana Stalnov
Keyword(s):  
Boundary Layer ◽  
Angle Of Attack ◽  
Leading Edge ◽  
Boundary Layer Separation ◽  
Shear Instability ◽  
Vortical Flow ◽  
High Angle ◽  
High Angle Of Attack ◽  
Layer Separation ◽  
Aerodynamic Performances

Leading edge serration is now a well-established and effective passive control device for the reduction of turbulence–leading edge interaction noise, and for the suppression of boundary layer separation at high angle of attack. It is envisaged that leading edge blowing could produce the same mechanisms as those produced by a serrated leading edge to enhance the aeroacoustics and aerodynamic performances of aerofoil. Aeroacoustically, injection of mass airflow from the leading edge (against the incoming turbulent flow) can be an effective mechanism to decrease the turbulence intensity, and/or alter the stagnation point. According to classical theory on the aerofoil leading edge noise, there is a potential for the leading edge blowing to reduce the level of turbulence–leading edge interaction noise radiation. Aerodynamically, after the mixing between the injected air and the incoming flow, a shear instability is likely to be triggered owing to the different flow directions. The resulting vortical flow will then propagate along the main flow direction across the aerofoil surface. These vortical flows generated indirectly owing to the leading edge blowing could also be effective to mitigate boundary layer separation at high angle of attack. The objectives of this paper are to validate these hypotheses, and combine the serration and blowing together on the leading edge to harvest further improvement on the aeroacoustics and aerodynamic performances. Results presented in this paper strongly indicate that leading edge blowing, which is an active flow control method, can indeed mimic and even enhance the bio-inspired leading edge serration effectively.

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