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Efficient thrust enhancement by modified pitching motion
- Zaka Muhammad, Md. Mahbub Alam, Bernd R. Noack
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- Journal:
- Journal of Fluid Mechanics / Volume 933 / 25 February 2022
- Published online by Cambridge University Press:
- 21 December 2021, A13
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- Article
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Thrust and/or efficiency of a pitching foil (mimicking a tail of swimming fish) can be enhanced by tweaking the pitching waveform. The literature, however, show that non-sinusoidal pitching waveforms can enhance either thrust or efficiency but not both simultaneously. With the knowledge and inspiration from nature, we devised and implemented a novel asymmetrical sinusoidal pitching motion that is a combination of two sinusoidal motions having periods T1 and T2 for the forward and retract strokes, respectively. The motion is represented by period ratio $\mathrm{\mathbb{T}} = {T_1}/T$, where T = (T1 + T2)/2, with $\mathrm{\mathbb{T}} > 1.00$ giving the forward strokes (from equilibrium to extreme position) slower than the retract strokes (from extreme to equilibrium position) and vice versa. The novel pitching motion enhances both thrust and efficiency for $\mathrm{\mathbb{T}} > 1.00$. The enhancement results from the resonance between the shear-layer roll up and the increased speed of the foil. Four swimming regimes, namely normal swimming, undesirable, floating and ideal are discussed, based on instantaneous thrust and power. The results from the novel pitching motion display similarities with those from fish locomotion (e.g. fast start, steady swimming and braking). The $\mathrm{\mathbb{T}} > 1.00$ motion in the faster stroke has the same characteristics and results as the fast start of prey to escape from a predator while $\mathrm{\mathbb{T}} < 1.00$ imitates braking locomotion. While $\mathrm{\mathbb{T}} < 1.00$ enhances the wake deflection at high amplitude-based Strouhal numbers (StA = fA/U∞, where f and A are the frequency and peak-to-peak amplitude of the pitching, respectively, and U∞ is the freestream velocity), $\mathrm{\mathbb{T}} > 1.00$ improves the wake symmetry, suppressing the wake deflection. The wake characteristics including wake width, jet velocity and vortex structures are presented and connected with $S{t_d}( = fd/{U_\infty })$, ${A^{\ast}}( = A/d)$ and $\mathrm{\mathbb{T}}$, where d is the maximum thickness of the foil.
Chapter 5 - From Figures of Speech to Fists of Fury: Unchecked Incitements to Violence
- Bilal Zahoor
- With Raza Rumi
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- Book:
- Rethinking Pakistan
- Published by:
- Anthem Press
- Published online:
- 20 January 2022
- Print publication:
- 23 September 2020, pp 59-66
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Summary
Unquiet Classrooms
Universities are emblems of a nation's aspirations and the capstones of any education system. In the life of the next generation, a university degree is the first real transition into adulthood. But in the years leading up to graduation, university life is supposed to be a safe space: a place to learn about the world while being protected from it. This, perhaps, is the reason why every incidence of campus violence leaves us anguished and transfixed; more so when this violence is emanating not from terrorist outsiders but from student groups within, simply for holding different views. If this 4 per cent – the percentage of the population going to a university – can be flipped, what does this mean for the rest?
Pakistan was gripped with fervent attention when fellow students killed one of their own after finding him in his hostel room. Most people reading this line will probably have the name Mashal Khan flash before their eyes in an act of recognition. But the student we refer to here is Adnan Abdul Qadir, murdered in 2010 in the University of Engineering and Technology (UET), Peshawar, by eight students of the Islami Jamiat Talaba (IJT), for listening to music in his hostel room. Just as in Mashal Khan's case, Adnan's death infuriated people and spurred questions about how far extremist tendencies had pervaded our social fabric. But Adnan's story fading from the memory is an indictment of how much things have worsened and, in some ways, normalised, as only those incidents that are truly blood-curdling or new in some fashion manage to capture our collective imagination.
Atomised Anger
In this essay we would argue that unchecked hate speech is at the very root of this issue. New ideas, aimed at supplanting society's norms and justifying violent means to achieve them, have gained credence over time with some of them, as we shall see in a while, enjoying the tacit support of the state. With the advent of digital communication technologies, the world, and not just Pakistan, has seen an explosion in the availability of pulpits for the preachers of hate. Deleterious microphones have been atomised, and the means of controlling broadcasts have yet to be conclusively decided.