Traditional and New Types of Passive Flow Control Techniques to Pave the Way for High Maneuverability and Low Structural Weight for UAVs and MAVs

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

Numerical and Experimental Investigation of Thickness Effect on the Cambered Airfoils at Low Reynolds Numbers

In this study, experimentaland numerical study for the cambered airfoils was conducted at Re = 1.5x105and Re = 2.5x105and different angles of attack. In the experimental analysis, oil-flow visualization and force measurement techniques were utilized. For numerical analysis, the k-w SSTtransition model was used to predict the flow over the cambered airfoils. The time-dependent aerodynamic force coefficients of the cambered NACA2412, NACA2415 and NACA2418 airfoils pointed out the force fluctuations formations due to unsteady flow on the airfoils. Whereas the force coefficient increased as the airfoil thickness increased, a decrease in the lift coefficient was observed due to adverse pressure gradients. Moreover, as the airfoil thickness increased, the separation occurred earlier due to the effect of adverse pressure gradients, so it got closer to the leading edge and became shorter. However, the prediction of the separation point was delayed in numerical analysis and the prediction of the reattachment points was more consistent

Experimental investigation on effect of partial flexibility at low aspect ratio airfoil – Part I: Installation on suction surface

Effects of flexible membrane mounted over suction surface of NACA 4412 airfoil were experimentally investigated at Reynolds number of 5x104 and low aspect ratio (AR=1) in this paper. The smoke-wire visualization method has been performed for flow visualization to demonstrate flow phenomena as laminar separation bubble (LSB), leading edge separation at z/c=0.4 and tip vortices at z/c=0.1. Values of velocity, Reynolds stress and turbulence statistics were measured by means of a constant temperature anemometer (CTA) system. Results of smoke-wire experiment revealed that size and height of LSB formed along z/c=0.4 at lower angles of attack such as α=8° was mitigated. Moreover, stall phenomenon as a result of boundary layer separation was apparently postponed at higher angles of attack. Velocity value was increased, whereas values of Reynold stress and turbulent kinetic energy was decreased with reduction of amount of fluctuations in flow. Consequently, using flexible membrane over suction surface of airfoil allowed the LSB to be mitigated or extinguished, resulting in exhibition of more stable flow characteristics

Effect of partial flexibility over both upper and lower surfaces to flow over wind turbine airfoil

Fluid-structure interaction phenomena on NACA 4412 airfoil having membrane material which was partially mounted on both its suction and pressure surfaces were experimentally investigated in wind tunnel measurements. Different experiments including Digital Image Correlation, smoke-wire, force measurement, and hot-wire systems were conducted at varying angles of attack, α (0° to 24°), and different Reynolds numbers (Re = 2.5 × 104, 5 × 104&nbsp;and 7.5 × 104). The controlled case gave benefits of up to 2 times in lift coefficient at lower angles of attack (α = 0° − 10°), at the same time drag coefficient decreased. Moreover, the partially flexible airfoil presented high power efficiencies at the pre-stall angles of attack (α = 0° − 10°). Using the flexibility caused the shear layer to approach, laminar separation bubble to suppress, and the wake region to shrunk, indicating the high lift force and less drag force production. The membrane vibrated and deformed due to the flow over the airfoil. Then, the membrane vibration caused the flow to trigger, which caused fluid–structure interaction to perform and the laminar separation bubble to shrink. The transition to turbulence formed earlier and these two flow phenomena moved towards upstream, resulting in having fewer flow fluctuations and increasing the flow stability. This ensures important advantages such as enhancement of aerodynamic performance, power efficiency, and decreasing vibration and noise for wind turbine blades.</p

Effect of partial flexibility over both upper and lower surfaces to flow over wind turbine airfoil

© 2020 Elsevier LtdFluid-structure interaction phenomena on NACA 4412 airfoil having membrane material which was partially mounted on both its suction and pressure surfaces were experimentally investigated in wind tunnel measurements. Different experiments including Digital Image Correlation, smoke-wire, force measurement, and hot-wire systems were conducted at varying angles of attack, α (0° to 24°), and different Reynolds numbers (Re = 2.5 × 104, 5 × 104 and 7.5 × 104). The controlled case gave benefits of up to 2 times in lift coefficient at lower angles of attack (α = 0° − 10°), at the same time drag coefficient decreased. Moreover, the partially flexible airfoil presented high power efficiencies at the pre-stall angles of attack (α = 0° − 10°). Using the flexibility caused the shear layer to approach, laminar separation bubble to suppress, and the wake region to shrunk, indicating the high lift force and less drag force production. The membrane vibrated and deformed due to the flow over the airfoil. Then, the membrane vibration caused the flow to trigger, which caused fluid–structure interaction to perform and the laminar separation bubble to shrink. The transition to turbulence formed earlier and these two flow phenomena moved towards upstream, resulting in having fewer flow fluctuations and increasing the flow stability. This ensures important advantages such as enhancement of aerodynamic performance, power efficiency, and decreasing vibration and noise for wind turbine blades