Aeromechanical performance of compliant aerofoils

The aeromechanics of compliant aerofoils are studied. Several experimental techniques including hot-wire anemometry, particle image velocimetry, high speed photogrammetry and strain gauge force measurements are used. Tests are performed at a chord based Reynolds number of Re= 4x10^4 and angles of attack between 0º and 25º . They explore the impact of the geometry of the leading- and trailing-edge supports as well as the rigid- ity of the aerofoil on the aeromechanics and aerodynamics of membrane aerofoils. Tests on latex membrane wings subjected by four different types of supports are performed. Firstly, the study focuses on the structural performance by evaluating detailed measure- ments of membrane deflections and lift and drag forces. It will be shown that the use of lower bending stiffness supports results in noticeable deformations, both static and dy- namic, especially at mid-to-high incidences. Moreover, the conjunction of hot-wire results with photogrammetry imagery of the membrane deformation indicates that the membrane vibration is coupled with the vortex shedding. This, when coupled with a low-stiffness rectangular cross-section leading- and trailing-edge, results in large amplitude vibrations affecting the membrane, the support and the wake. Hence, a more detailed study of the vortex shedding and the wake attributes is presented. The findings indicate that for low angles of attack the wake characteristics are highly affected by the leading- and trailing- edge geometry; as incidence increases the wake characteristics become less dependant on the support's geometry, eventually reaching a point in which they are fully independent of it and closely resembling a fully stalled rigid aerofoil. Finally, the effects of the aero- foil rigidity are analysed. Tests of varying thickness but constant Young's modulus on unidirectional carbon fibre composite plates are performed. Results show that the Weber number is a crucial parameter when defining the properties and performance of the wing. Furthermore, the study will show that lift and drag forces are higher for membrane wings than for composite plates and that the dynamic motions of the composite plates increase as the plate thickness is decreased resulting in earlier wing stall and worse post-stall be- haviour than membrane wings. The results of this study should provide valuable insight for future use of membrane wings in micro air vehicles.Open Acces

Experimental study of the dominant modes and its contributions to the detached internal flows in a TIC nozzle

During the transient phases of start-up and shut-down, liquid engine nozzles operate under severe over-expanded conditions. Furthermore, these transient phases result in rocket nozzles exhibiting dynamic off-axis loads caused by asymmetric internal separation. There are two recognisable types of flow separation, Free-Shock Separation, FSS, and Restricted-Shock Separation, RSS. It has been shown that the resultant separation type depends largely on nozzle geometry. Many studies have analysed these distinct separation patterns concluding that FSS topologies result in lower magnitude side forces than RSS ones. Moreover, it has been shown that Truncated Ideal Contour, TIC, nozzles only experience FSS separation. Thus, this work presents an experimental study of a TIC nozzle developed in a cold supersonic wind tunnel at PPrime Institute in Poitiers. The study consists of a series of pressure measurements as well as PIV and Schlieren Photography for a range of pre-selected pressure ratios which permit the characterisation of the regime where the nozzle undergoes the most significant jet fluctuations as it is believed that they are strongly associated with the side-loads

Aeromechanics of membrane and rigid wings in and out of ground-effect at moderate Reynolds numbers

Wind tunnel experiments are conducted using membrane wings and rigid flat-plates in ground-effect at a moderate Reynolds number of Re = 56 000 with ground clearances from 1% to 200% chord length measured from their trailing-edge. A six-axis load-cell captures time-resolved forces and moment while time-resolved stereo digital image correlation (DIC) measurements are performed to capture membrane motions. The lift and drag coefficients of the rigid wing in ground-effect follow well-established trends while the membrane wing appears to exhibit improved coefficients and efficiency (compared to the rigid wing) when in ground-effect. Proper orthogonal decomposition (POD) is applied to study the spatiotemporal structure of membrane vibrations. With increasing angles-of-attack and/or decreasing heights above ground, mode shapes of membrane deformation are dominated by large-scale fluctuations that have a smaller number of local extrema along the chord. Ground-effect induces modifications to the membrane deformation, which appear to be similar to the modifications induced by increasing angles-of-attack in free-flight. At high angles-of-attack in free-flight or at moderate angles in ground-effect, two POD modes of membrane fluctuations are found to be sufficient to capture 90% of all membrane deformations. Under these conditions, a membrane deformation with maximum camber near the trailing edge of the membrane wing is found to correlate with high lift, low drag and a nose down pitching moment. The extrema in membrane deformations and lift and drag forces occur simultaneously, while there is a time-lag between the deformation and the pitching moment