“…Previous work on pliant membrane wings (39,41,45,(47)(48)(49)53) shows a correlation between an increasing coefficient of lift and a decrease of pre-strain, for similar test conditions. This correlation is due to the relative increase of camber of the membrane wing when decreasing the membrane pre-strain.…”
Section: Resultsmentioning
confidence: 62%
“…The membrane was stretched, and another set of images was recorded. Both sets of images were used for Digital Image Correlation (DIC) in VIC3D in order to determine the state of strain of the membrane (47,48) . Figure 4. The pre-strain fixture used for adjusting membrane tension.…”
Section: Wind Tunnel and Testing Apparatusmentioning
Flow separation followed by aerodynamic stall limits the operation of aircraft. Expanding the flight envelope of aircraft has been a goal of aerodynamicists for decades. This work presents findings from tests in the Oregon State University wind tunnel investigating the effectiveness of a passively actuated suction-surface flap on membrane wings. Experiments were conducted on a rigid plate and membrane wings with and without a pop-up flap. All wings had an aspect ratio of 2, while membrane pre-strain and Reynolds number were varied. An increase in lift at stall was observed for all testing conditions with flap deployment. The observed average increase in maximum lift varied from 5% to 15% for different test conditions. The variation in flap effectiveness is compared to membrane pre-strain, Reynolds number, and wing camber. A quadratic relationship between modelled camber and flap effectiveness is observed, and an optimal level of membrane camber is found to maximise flap effectiveness.
“…Previous work on pliant membrane wings (39,41,45,(47)(48)(49)53) shows a correlation between an increasing coefficient of lift and a decrease of pre-strain, for similar test conditions. This correlation is due to the relative increase of camber of the membrane wing when decreasing the membrane pre-strain.…”
Section: Resultsmentioning
confidence: 62%
“…The membrane was stretched, and another set of images was recorded. Both sets of images were used for Digital Image Correlation (DIC) in VIC3D in order to determine the state of strain of the membrane (47,48) . Figure 4. The pre-strain fixture used for adjusting membrane tension.…”
Section: Wind Tunnel and Testing Apparatusmentioning
Flow separation followed by aerodynamic stall limits the operation of aircraft. Expanding the flight envelope of aircraft has been a goal of aerodynamicists for decades. This work presents findings from tests in the Oregon State University wind tunnel investigating the effectiveness of a passively actuated suction-surface flap on membrane wings. Experiments were conducted on a rigid plate and membrane wings with and without a pop-up flap. All wings had an aspect ratio of 2, while membrane pre-strain and Reynolds number were varied. An increase in lift at stall was observed for all testing conditions with flap deployment. The observed average increase in maximum lift varied from 5% to 15% for different test conditions. The variation in flap effectiveness is compared to membrane pre-strain, Reynolds number, and wing camber. A quadratic relationship between modelled camber and flap effectiveness is observed, and an optimal level of membrane camber is found to maximise flap effectiveness.
“…Furthermore, an increase in reduced frequency led to an increase in maximum C L . Membrane wings have a higher slope of lift and postponed stall [87]. Additionally, the membrane kinematics was closely relevant to membrane tension and free-stream velocity [88].…”
Prevailing utilization of airfoils in the design of micro air vehicles and wind turbines causes to gain attention in terms of determination of flow characterization on these flight vehicles operating at low Reynolds numbers. Thus, these vehicles require flow control techniques to reduce flow phenomena such as boundary layer separation or laminar separation bubble (LSB) affecting aerodynamic performance negatively. This chapter presents a detailed review of traditional passive control techniques for flight vehicle applications operating at low Reynolds numbers. In addition to the traditional methods, a new concept of the pre-stall controller by means of roughness material, flexibility and partial flexibility is highlighted with experimental and numerical results. Results indicate that passive flow control methods can dramatically increase the aerodynamic performance of the aforementioned vehicles by controlling the LSB occurring in the pre-stall region. The control of the LSB with new concept pre-stall control techniques provides lift increment and drag reduction by utilizing significantly less matter consumption and low energy. In particular, new types of these methods presented for the first time by the chapter's authors have enormously influenced the progress of separation and LSB, resulting in postponing of the stall and enhancing the aerodynamic performance of wind turbine applications.
“…Furthermore, an increase in reduced frequency led to an increase in maximum C L . Membrane wings having a higher slope of lift and postponed stall (Osterberg, 2016). Additionally, the membrane kinematics was closely relevant to membrane tension and freestream velocity (Song et al , 2008).…”
Purpose
The purpose of this exhaustive experimental study is to investigate the fluid-structure interaction in the flexible membrane wings over a range of angles of attack for various Reynolds numbers.
Design/methodology/approach
In this paper, an experimental study on fluid-structure interaction of flexible membrane wings was presented at Reynolds numbers of 2.5 × 104, 5 × 104 and 7.5 × 104. In the experimental studies, flow visualization, velocity and deformation measurements for flexible membrane wings were performed by the smoke-wire technique, multichannel constant temperature anemometer and digital image correlation system, respectively. All experimental results were combined and fluid-structure interaction was discussed.
Findings
In the flexible wings with the higher aspect ratio, higher vibration modes were noticed because the leading-edge separation was dominant at lower angles of attack. As both Reynolds number and the aspect ratio increased, the maximum membrane deformations increased and the vibrations became visible, secondary vibration modes were observed with growing the leading-edge vortices at moderate angles of attack. Moreover, in the graphs of the spectral analysis of the membrane displacement and the velocity; the dominant frequencies coincided because of the interaction of the flow over the wings and the membrane deformations.
Originality/value
Unlike available literature, obtained results were presented comparatively using the sketches of the smoke-wire photographs with deformation measurement or turbulence statistics from the velocity measurements. In this study, fluid-structure interaction and leading-edge vortices of membrane wings were investigated in detail with increasing both Reynolds number and the aspect ratio.
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