2 results
Rapidly pitching plates in decelerating motion near the ground
- Dibya R. Adhikari, Samik Bhattacharya
-
- Journal:
- Journal of Fluid Mechanics / Volume 986 / 10 May 2024
- Published online by Cambridge University Press:
- 06 May 2024, A22
-
- Article
-
- You have access Access
- Open access
- HTML
- Export citation
-
Birds employ rapid pitch-up motions close to the ground for different purposes: perching birds use this motion to decelerate and come to a complete stop while hunting birds, such as bald eagles, employ it to catch prey and swiftly fly away. Motivated by these observations, our study investigates how natural flyers accomplish diverse flying objectives by rapidly pitching their wings while decelerating near ground. We conducted experimental and analytical investigations focusing on rapidly pitching plates during deceleration in close proximity to the ground to explore the impact of ground proximity on the unsteady dynamics. Initially, we executed synchronous pitch-up motion, where both pitching and deceleration have the same motion duration, at different ground heights. Experimental results demonstrate that as the pitching wing approaches the ground, the instantaneous lift increases by approximately $38\,\%$ compared with a far-from-ground case, while the initial peak drag force remains relatively unchanged. Our analytical model conforms to this trend, predicting an increase in lift force as the wing approaches the ground, indicating enhanced added mass and circulatory lift force due to the ground effect. Next, we examined asynchronous pitch-up motion cases, where rapid pitching motions were initiated at different stages of deceleration. The results reveal that initiating the wing pitch early in the deceleration leads to the formation of larger counter-rotating vortices at the early stage of the manoeuvre. These vortices generate stronger dipole jets that orient backward in the later stages of the manoeuvre after impinging with the ground surface, which hunting birds utilize to accelerate after catching prey. Conversely, when the wing pitch is delayed, smaller vortices form, but their growth is postponed until late in the manoeuvre. This delayed vortex growth produces lift and drag force at the end phase of the manoeuvre that facilitates a smooth landing or perching. Thus, through strategic tuning of a rapid pitch-up motion with deceleration, natural flyers, such as birds, achieve diverse flying objectives.
The unsteady force response of an accelerating flat plate with controlled spanwise bending
- Kamlesh Joshi, Samik Bhattacharya
-
- Journal:
- Journal of Fluid Mechanics / Volume 933 / 25 February 2022
- Published online by Cambridge University Press:
- 06 January 2022, A56
-
- Article
-
- You have access Access
- Open access
- HTML
- Export citation
-
The unsteady force response of an accelerating flat plate, subjected to controlled spanwise bending, is investigated experimentally. The flat plate was held normal to the flow (at an angle of attack of $90^{\circ }$), and it was dynamically bent along the spanwise direction with the help of internal actuation. Two bending directions were tested. In one case, part of the plate (denoted by flexion ratio) was bent into the incoming flow (the bend-down configuration). In another case, the plate was bent away from the flow (the bend-up configuration). We used two different aspect ratio ($AR$) plates, namely $AR = 2$ and 3. Three acceleration numbers, namely $A_c = 0.57$, 1.6 and 3.2 (corresponding to dimensional acceleration of 0.036, 0.1 and 0.2 m s$^{-2}$, respectively) were tested with a fixed terminal Reynolds number (Re) of 18 000. For each acceleration number, three bending durations, namely 1.2, 2.4 and 3.6 s were implemented. The results indicate that the highest impulse was imparted by the highest bending rate (duration 1.2 s) during all three accelerations tested. We show that controlled spanwise bending can significantly change the unsteady force response by manipulating the inertial forces during a start-up manoeuvre. The unsteady forces depend on the vector sum of the forward acceleration and the bending acceleration of the plate. The unsteady drag was augmented when the plate was bent towards the incoming flow. The initial force peaks were significantly reduced when the bending direction was reversed. The development of the edge vortices from the flat plate was measured with the help of particle image velocimetry (PIV) at the 70 % and the 90 % span locations. The PIV measurements were also carried out at the midchord plane closer to the tip region to capture the growth of the tip vortex. The vorticity field calculated from these PIV measurements revealed that controlled bending contributed to a variation in the circulation growth of the edge vortices. During the bend-down case, the circulation growth was faster and the tip vortices stayed closer to the plate. This resulted in increased interaction with the edge vortex at the 90 % span. This interaction was more severe for $AR = 2$. During the bend-up case, the growth of the edge vortex was delayed, but the vortex grew for a longer time compared with the bend-down case. Finally, a mathematical model is presented which correctly captured the trend of the force histories measured experimentally during both the bend-up and bend-down cases.