1,720,982 research outputs found
Unsteady fluid dynamics around a hovering flat plate wing
Full text linkInsect flight is characterised by complex time-dependent flows in response to the unsteady wing movements. Biological fliers exploit the unsteady flow fields to modulate aerodynamic forces, thereby displaying unmatched flight performance, especially in hover. Naturally, this has inspired the creation of engineering models to replicate the flight behaviour. An in-depth understanding of the flow fields generated during hover and their dependence on the kinematics is paramount to achieve this goal. The two main kinematic components of a hovering wing are the stroke, which refers to the back-and-forth motion, and wing rotation, which refers to the change in angle of attack. The phase relation between stroke and rotation is quantified in terms of phase-shift and is broadly classified into symmetric, advanced, and delayed rotation. The phase-shift and duration of rotation, together referred to as rotational timing, are investigated in this bio-inspired study. The objective is to characterise the effect of rotational timing on the aerodynamic forces and the flow fields generated by a hovering wing. The unsteady flow around a hovering flat plate wing that mimics hoverfly kinematics has been investigated experimentally using particle image velocimetry and direct force measurements. The measurements are conducted at a Reynolds number of Re=220 and a reduced frequency of k=0.32 in order to dynamically match a hoverfly. The Lagrangian finite-time Lyapunov exponent method is used to analyse the unsteady flow fields by identifying dynamically relevant flow features such as the primary leading edge vortex (LEV), secondary vortices, and topological saddles, and their evolution within a flapping cycle. Firstly, the flow and force behaviour was characterised for a typical flapping cycle. The flow evolution in a symmetric, fast rotation is divided into four stages that are characterised by the LEV emergence, growth, lift-off, and breakdown and decay. Tracking saddle points is shown to be helpful in defining the LEV lift-off which occurs at the maximum stroke velocity. The flow fields are correlated with the aerodynamic forces revealing that the maximum lift and drag are observed just before LEV lift-off, which corresponds to the maximum stroke velocity. Secondly, the effect of phase-shift on the formation and evolution of lift-enhancing flow structures are discussed. Two advanced and delayed rotations are compared. The flow development stages and forces are similar for all rotations but the timing of stages varies. The evolution of forces and flow strongly depend on the stroke velocity. Thirdly, the dependence of the flow and force evolution on the stroke velocity was substantiated by doubling the rotational duration in the symmetric rotation. It was found that the timing of the flow stages altered, whereas the flow and forces mostly evolved similarly to that of a fast rotation. The fast rotation, however, produces higher maximum lift and drag compared to the slow rotation. Lastly, the effect of phase-shift on the aerodynamic characteristics of a slow rotation is further explored. The slow rotation cases exhibit distinct flow patterns for varying phase-shifts unlike the fast rotations, in terms of the formation, evolution and breakdown of the flow structures as well as the timing. The forces also show distinct trends for varying phase-shifts and strongly depend on the angle of attack along with the stroke velocity in the slowUNFOL
To tread or not to tread: comparison between water treading and conventional flapping wing kinematics
Full text linkHovering insects are limited by their physiology and need to rotate their wings at the end of each back-and-forth motion to keep the wing's leading edge ahead of its trailing edge. The wing rotation at the end of each half-stroke pushes the leading edge vortex away from the wing which leads to a loss in the lift. Unlike biological fliers, human-engineered flapping wing micro air vehicles have different design limitations. They can be designed to avoid the end of stroke wing rotation and use so-called water-treading flapping kinematics. Flapping wings using conventional flapping kinematics have a designated leading and trailing edge. In the water-treading mode, the role of the leading and trailing edges are continuously alternated throughout the stroke. Here, we compare velocity field and force measurements for a rectangular flapping wing conducting normal hovering and water-treading kinematics to study the difference in fluid dynamic performance between the two types of flapping kinematics. We show that for similar power consumption, the water-treading mode produces more lift than the conventional hovering mode and is 50% more efficient for symmetric pitching kinematics. In the water-treading mode, the leading edge vortex from the previous stroke is not pushed away but is captured and keeps the newly formed leading edge vortex closer to the wing, leading to a more rapid increase of the lift coefficient which is sustained for longer. This makes the water-treading mode a promising alternative for human-engineered flapping wing vehicles
The Stories of Women in Fluids: Persevere, Survive, Thrive
No full textPresentation at invited MiniSymposium 3
Aeroacoustics of lift + cruise eVTOL configurations during transition
No full textDuring the transition of a lift + cruise aircraft from hover to forward flight, the thrust generated by the vertical lifting propellers compensates for the deficit in wing lift at low forward velocities. Predicting the noise generated during this transition phase remains one of the key challenges in the aeroacoustic analysis of these configurations. This study aims to characterize part of the aeroacoustic performance of a lift + cruise aircraft during transition. Anechoic wind tunnel testing was conducted on a wing with vertical lift propellers arranged in a lift + cruise layout, simulating the transition from hover to forward flight. Replicating realistic transition behaviour in an open-jet wind tunnel proved challenging, likely due to jet deflection effects altering the effective angle of attack on the wing. However, by maintaining zero net pitching moment and zero excess vertical thrust across increasing freestream velocities, the study investigated the acoustic contributions of the fore and aft vertical lifting propellers. Results indicate that the rear propeller increasingly dominates the acoustic signature at higher flow velocities, with a corresponding rise in broadband noise. The study further concludes that while rear propeller overlap with the wing benefits aerodynamic performance, it is detrimental to acoustic characteristics
Effect of motor-rotor Geometry on the Performance of Electric VTOL UAVs
No full textThis paper aims to quantify the impact of the orientation and position of a vertical lifting motor-rotor system on the aerodynamic performance of a wing. The analysis was conducted on the scaled wing of an electric uncrewed aerial vehicle using wind tunnel measurements to quantify the difference in performance between configurations. The wind tunnel model was produced by applying a set of non-dimensional parameters to scale an existing quad-plane design, conserving important features to make the result both feasible and generalised. The design was analysed at a Reynolds number of 430,000 to be comparable to the operating conditions of the existing vehicle. The analysis is conducted using two-bladed rotors that are prevented from rotating in the oncoming flow (braked). The influence of the braked angle of the front and rear rotors on the aerodynamic performance of the wing was tested at 30◦ brake angle increments between 0-150◦ over wing angles of attack between -4◦ and 10◦. Results show that the braked angles of the front and rear rotor have a significant impact on the aerodynamic drag. In particular, aligning both rotors so that the blades are perpendicular to the wing chord can result in a 25-30% increase in drag when compared to the case of both rotors aligned parallel to the chord direction. Additionally, the aerodynamic penalty incurred by the vertical lifting subsystem (motors and supporting structure) was also measured
Aerodynamic performance of aircraft wings with stationary vertical lift propellers
No full textImprovements in battery and motor technology are facilitating innovative aircraft configurations capable of vertical take-off and landing. Of these configurations, lift+cruise is popular for its inherent redundancy and the option to tailor separate propulsion systems for each flight regime. During cruise, the vertical flight propellers of a lift+cruise design are inactive and often exposed. Increasing the projected area of a body is understood to increase the drag, but the aerodynamic performance of an edgewise, stationary propeller and its influence on neighboring bodies is less clear. This study aimed to quantify the impact of two, tandem, edgewise and stationary propellers on the aerodynamic performance of a wing using wind tunnel measurements. The stationary position of the front and rear propellers were varied in 30
∘ increments between 0-150
∘, at angles of attack between -4
∘ and 10
∘. Results at Re = 4.3×10
5 showed the propellers and supporting systems had negligible influence on the lift. However, a drag increase of up to 30% was recorded when propeller positions were aligned perpendicular to the wing chord instead of parallel. Variations in the stationary position of the propeller altered the lift to drag ratio by up to 36% in a typical cruise configuration.
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