Kevin Knowles , Peter Wilkins, Salman Ansari, Rafal Zbikowski Integrated Computational and Experimental Studies of Flapping-wing Micro Air Vehicle Aerodynamics Kevin Knowles , Peter Wilkins, Salman Ansari, Rafal Zbikowski Department of Aerospace, Power and Sensors Cranfield University Defence Academy of the UK Shrivenham, England 3rd Int Symp on Integrating CFD and Experiments in Aerodynamics, Colorado Springs, 2007

Outline Introduction Flapping-Wing Problem Aerodynamic Model LEV stability Conclusions Knowles et al.

Micro Air Vehicles Defined as small flying vehicles with Size/Weight: 150-230mm/50–100g Endurance: 20–60min Reasons for MAVs: Existing UAVs limited by large size Niche exists for MAVs – e.g. indoor flight, low altitude, man-portable MAV Essential (Desirable) Attributes: High efficiency High manoeuvrability at low speeds Vertical flight & hover capability Sensor-carrying; autonomous (Stealthy; durable) Microgyro Microsensors Knowles et al.

Why insect-like flapping? Insects are more manoeuvrable Power requirement: Insect – 70 W/kg maximum Bird – 80 W/kg minimum Aeroplane – 150 W/kg Speeds: Insects ~ 7mph Birds ~ 15mph Knowles et al.

Wing Kinematics – 1 Flapping Motion Key Phases sweeping heaving pitching Key Phases Translational downstroke upstroke Knowles et al.

Wing Kinematics – 1 Flapping Motion Key Phases sweeping heaving pitching Key Phases Translational downstroke upstroke Rotational stroke reversal high angle of attack Knowles et al.

Wing Kinematics – 2 Knowles et al.

Mechanical Implementation Knowles et al.

Generic insect wing kinematics Three important differences when compared to conventional aircraft: wings stop and start during flight large wing-wake interactions high angle of attack (45° or more) Complex kinematics: difficult to determine difficult to understand difficult to reproduce Knowles et al. 9

Aerodynamics Key phenomena unsteady aerodynamics leading-edge vortex apparent mass Wagner effect returning wake leading-edge vortex [Photo: Prenel et al 1997] Knowles et al.

Aerodynamic Modelling – 1 Quasi-3D Model 2-D blade elements with attached flow separated flow leading-edge vortex trailing-edge wake Convert to 3-D radial chords Robofly wing Knowles et al.

Aerodynamic Modelling – 1 Quasi-3D Model 2-D blade elements with attached flow separated flow leading-edge vortex trailing-edge wake Convert to 3-D radial chords cylindrical cross-planes integrate along wing span Knowles et al.

Aerodynamic Modelling – 2 Model Summary 6 DOF kinematics circulation-based approach inviscid model with viscosity introduced indirectly numerical implementation by discrete vortex method validated against experimental data Knowles et al.

Flow Visualisation Output Knowles et al.

Impulsively-started plate Knowles et al.

Validation of Model Knowles et al.

The leading-edge vortex (LEV) Insect wings operate at high angles of attack (>45°), but no catastrophic stall Instead, stable, lift-enhancing (~80%) LEV created Flapping wing MAVs (FMAVs) need to retain stable LEV for efficiency Why is the LEV stable? Is it due to a 3D effect? Difficulties involved with ascertaining kinematics of insect flight – keeping insect still for observation. Aerodynamics (seeing what the flow is doing) even more difficult – flow visualisation (smoke) problems. Four main aspects: Wing stops/starts regularly in flight – doesn’t happen with either fixed or rotary wing aircraft where (we hope) wing only starts/stops when aircraft is on the ground Wake is not ‘left behind’ but remains in close proximity to wing for relatively long time period. Angle of attack regularly exceeds stall angle but no catastrophic stall – stable leading edge vortex Many insects use specialised techniques to enhance lift – FMAVs may be able to use some of these. Knowles et al. 17

2D flows at low Re Re = 5 Re = 10 Knowles et al. What effect does increasing Reynolds number have for 2D flows? How is the flow affected by changes in section? Explore LEV – look for spanwise flow. Is LEV stable at high Re? FMAVs likely to operate at Re = 15000ish. Knowles et al. 18

Influence of Reynolds number Basic shape of graphs are the same LEV build up/shed then TEV then LEV. Cycle repeats. Increased unsteadiness at higher Re. α = 45° Knowles et al. 19

2D flows Re = 500, α = 45° Knowles et al.

Influence of Reynolds number Basic shape of graphs are the same LEV build up/shed then TEV then LEV. Cycle repeats. Increased unsteadiness at higher Re. α = 45° Knowles et al. 21

Kelvin-Helmholtz instability at Re > 1000 Notice LEV breakdown at high Re. Re 500 Re 5000 Knowles et al. 22

Secondary vortices Re = 1000 Re = 5000 Knowles et al. Notice LEV breakdown at high Re. Re = 1000 Re = 5000 Knowles et al. 23

2D LEV Stability For Re<25, vorticity is dissipated quickly and generated slowly – the LEV cannot grow large enough to become unstable For Re>25, vorticity is generated quickly and dissipated slowly – the LEV grows beyond a stable size In order to stabilise the LEV, vorticity must be extracted – spanwise flow is required for stability Rotational flow produces conical LEV. Knowles et al. 24

Structure of 3D LEV Knowles et al.

Stable 3D LEV Re = 120 Re = 500 Knowles et al.

Conclusions LEV is unstable for 2D flows except at very low Reynolds numbers Sweeping motion of 3D wing leads to conical LEV; leads to spanwise flow which extracts vorticity from LEV core and stabilises LEV. 3D LEV stable & lift-enhancing at high Reynolds numbers (>10 000) despite occurrence of Kelvin-Helmholtz instability. Knowles et al.

Questions? Knowles et al.