Presentation on theme: "Kevin Knowles , Peter Wilkins, Salman Ansari, Rafal Zbikowski"— Presentation transcript:
1 Kevin Knowles , Peter Wilkins, Salman Ansari, Rafal Zbikowski Integrated Computational and Experimental Studies of Flapping-wing Micro Air Vehicle AerodynamicsKevin Knowles , Peter Wilkins, Salman Ansari, Rafal ZbikowskiDepartment of Aerospace, Power and SensorsCranfield UniversityDefence Academy of the UKShrivenham, England3rd Int Symp on Integrating CFD and Experiments in Aerodynamics, Colorado Springs, 2007
2 Outline Introduction Flapping-Wing Problem Aerodynamic Model LEV stabilityConclusionsKnowles et al.
3 Micro Air Vehicles Defined as small flying vehicles with Size/Weight: mm/50–100gEndurance: 20–60minReasons for MAVs:Existing UAVs limited by large sizeNiche exists for MAVs – e.g. indoor flight, low altitude, man-portableMAV Essential (Desirable) Attributes:High efficiencyHigh manoeuvrability at low speedsVertical flight & hover capabilitySensor-carrying; autonomous(Stealthy; durable)MicrogyroMicrosensorsKnowles et al.
4 Why insect-like flapping? Insects are more manoeuvrablePower requirement:Insect – 70 W/kg maximumBird – 80 W/kg minimumAeroplane – 150 W/kgSpeeds:Insects ~ 7mphBirds ~ 15mphKnowles et al.
9 Generic insect wing kinematics Three important differences when compared to conventional aircraft:wings stop and start during flightlarge wing-wake interactionshigh angle of attack (45° or more)Complex kinematics:difficult to determinedifficult to understanddifficult to reproduceKnowles et al.9
10 Aerodynamics Key phenomena unsteady aerodynamics leading-edge vortex apparent massWagner effectreturning wakeleading-edge vortex[Photo: Prenel et al 1997]Knowles et al.
11 Aerodynamic Modelling – 1 Quasi-3D Model2-D blade elements withattached flowseparated flowleading-edge vortextrailing-edge wakeConvert to 3-Dradial chordsRobofly wingKnowles et al.
12 Aerodynamic Modelling – 1 Quasi-3D Model2-D blade elements withattached flowseparated flowleading-edge vortextrailing-edge wakeConvert to 3-Dradial chordscylindrical cross-planesintegrate along wing spanKnowles et al.
13 Aerodynamic Modelling – 2 Model Summary6 DOF kinematicscirculation-based approachinviscid model with viscosity introduced indirectlynumerical implementation by discrete vortex methodvalidated against experimental dataKnowles et al.
17 The leading-edge vortex (LEV) Insect wings operate at high angles of attack (>45°), but no catastrophic stallInstead, stable, lift-enhancing (~80%) LEV createdFlapping wing MAVs (FMAVs) need to retain stable LEV for efficiencyWhy 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 groundWake 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 vortexMany insects use specialised techniques to enhance lift – FMAVs may be able to use some of these.Knowles et al.17
18 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
19 Influence of Reynolds number Basic shape of graphs are the sameLEV build up/shed then TEV then LEV. Cycle repeats.Increased unsteadiness at higher Re.α = 45°Knowles et al.19
21 Influence of Reynolds number Basic shape of graphs are the sameLEV build up/shed then TEV then LEV. Cycle repeats.Increased unsteadiness at higher Re.α = 45°Knowles et al.21
22 Kelvin-Helmholtz instability at Re > 1000 Notice LEV breakdown at high Re.Re 500Re 5000Knowles et al.22
23 Secondary vortices Re = 1000 Re = 5000 Knowles et al. Notice LEV breakdown at high Re.Re = Re = 5000Knowles et al.23
24 2D LEV StabilityFor Re<25, vorticity is dissipated quickly and generated slowly – the LEV cannot grow large enough to become unstableFor Re>25, vorticity is generated quickly and dissipated slowly – the LEV grows beyond a stable sizeIn order to stabilise the LEV, vorticity must be extracted – spanwise flow is required for stabilityRotational flow produces conical LEV.Knowles et al.24
27 ConclusionsLEV is unstable for 2D flows except at very low Reynolds numbersSweeping 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.