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Natural Flight
Published in Malcolm S. Gordon, Reinhard Blickhan, John O. Dabiri, John J. Videler, Animal Locomotion, 2017
After take-off, the pressure difference between the under and upper side of the wing creates a vortex at the wing tip where air rotates upward and inward. Wing tip vortices emerge as soon as the starting vortex is released and the pressure difference is established. In fact the wing tip vortices are continuous with the starting vortex and form part of an extremely long rectangular vortex “ring.” (Figure 4.15). According to fluid dynamic theory, a vortex cannot end in the fluid in the direction along its center (Helmholtz’s theorem). It either ends at the boundary of the fluid or forms a closed loop with itself or another vortex. The starting vortex of an aircraft forms a closed loop with the wing tip or trailing vortices and the bound vortex on the lifting wings. The bound vortex around the wings becomes visible by subtracting the average flow velocity from the local velocities around the wing. The picture of the flow around the fulmar wing can be used to verify this statement. The bound vortex is shed during landing when the speed of the aircraft drops below the value needed to maintain the pressure differences (the Wagner effect reversed).
Effect of coolant pulsation on film cooling performance on turbine vane
Published in Numerical Heat Transfer, Part A: Applications, 2023
Chunhua Wang, Zhuo Zhang, Yunan Wang, Jingzhou Zhang
LES was applied for investigating square-wave pulsating film cooling on turbine vane at two time-averaged blowing ratios of 0.5 and 1.5. The time-averaged and instantaneous features for pulsating film cooling were analyzed, and compared with continuous cases in detail. Some useful conclusions are as follows.On suction surface, the introduction of coolant pulsation causes the decrease of film cooling effectiveness. However, on pressure surface, the cooling performance can be improved by coolant pulsation at high blowing ratio. Compared with pulsating amplitude, the effect of pulsation frequency on film cooling is much weaker.Kidney vortex pair dominates the time-averaged flow fields in film cooling. Coolant pulsation changes the instantaneous velocity of jet, which affects KVP more or less. Besides of this, the frequent start and shut of coolant results in generation of a starting vortex which also affects the KVP scale. On pressure surface, the scale of KVP increases notably as the jet pulsation is introduced. However, on suction surface, KVP does not change obviously as the jet pulsation is applied.Starting vortex is the typical vortex structure in pulsating film cooling. LES results show that the starting vortex is generated in the leading edge of film cooling hole, and the interaction between starting vortex and mainstream results in formation of coherent structures including jet shear layer vortex and hairpin vortex downstream of the hole. However, on suction surface, starting vortex is shaded by plenty of small-scale vortices because that the mainstream has become fully-turbulent flow before reaching the coolant exit.The statistic characteristics and power spectrum density (PSD) of velocity fluctuation were analyzed. On pressure surface, the introduction of coolant pulsation leads to the increase of TKE and PSD value. However, on suction surface, TKE and PSD does not change obviously as the pulsation is introduced. It illustrates that the disturbance of coolant jet on suction-side boundary layer is weaker.