Abstract
Infrared thermography has been successfully adopted in the field of flow diagnostics over the last decades. Detecting the laminar–turbulent boundary layer transition through variations in the convective heat transfer is one of the primary applications due to its impact on the aerodynamic performance. Recent developments in fast–response infrared cameras allow unsteady measurement of fast–moving surfaces and moving transition positions, which must consider the thermal responsiveness of the surface material. Experimental results on moving boundary layer transition positions are highly valuable in the design and optimization of airfoils or rotor blades in unsteady applications, for example regarding helicopter main rotors in forward flight. This review article summarizes recent developments in steady and unsteady infrared thermography, particularly focusing on the development of differential infrared thermography (DIT). The new methods have also led to advances in the analysis of unmoving boundary layer transition for static airfoil test cases which were previously difficult to analyze using single–image methods.
Highlights
Introduction and scopeThe laminar–turbulent transition of the boundary layer (BL) is one of the key factors when optimizing the aerodynamic performance of vehicles, referring to large drag penalties due to different skin friction coefficients, or referring to the impact of the BL parameters on the stall resistance
Experimental results on moving boundary layer transition positions are highly valuable in the design and optimization of airfoils or rotor blades in unsteady applications, for example regarding helicopter main rotors in forward flight
The measurement, prediction, and manipulation of the BL was tackled in numerous studies concentrating on steady aerodynamics with a stationary transition position, as found on fixed–wing aircraft, road vehicles, trains, etc Unsteady inflow conditions result in complex aerodynamics and a moving transition location, as for example seen on helicopter main rotor blades in forward flight
Summary
The laminar–turbulent transition of the boundary layer (BL) is one of the key factors when optimizing the aerodynamic performance of vehicles, referring to large drag penalties due to different skin friction coefficients, or referring to the impact of the BL parameters on the stall resistance. Superposition of rotational and freestream velocities, and the consequent cyclic swashplate input to achieve moment trim, produce periodic variations of both the inflow magnitude and inflow direction at a given radial cross–section It is known from simulations in steady hovering conditions that modeling of the BL transition is crucial for a correct prediction of the rotor power requirement, for example see Egolf et al [1]. It can be expected that these unsteady effects become increasingly important in the future, given that the laminar flow length is a crucial parameter of wind turbine airfoil design [20] Another recent publication by Thiessen and Schülein [21] concentrated on moving transition positions due to the effect of forward flight velocity on a fixed–pitch propeller for unmanned aerial vehicles. Detailed considerations of infrared radiation physics or camera technologies, see [25, 33], or infrared applications on other aerodynamic topics, such as heat flux measurements for super- and hypersonic applications [34], are beyond the current scope
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