3 results
Reducing flow separation of an inclined plate via travelling waves
- A. M. Akbarzadeh, I. Borazjani
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- Journal:
- Journal of Fluid Mechanics / Volume 880 / 10 December 2019
- Published online by Cambridge University Press:
- 18 October 2019, pp. 831-863
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Many aquatic animals propel themselves by generating backward traveling waves over their body, which is thought to reattach the flow when the wave speed ($C=\unicode[STIX]{x1D706}f$, where $\unicode[STIX]{x1D706}$ is wavelength and $f$ is frequency) is larger than the swimming speed ($U$). This has inspired the use of travelling waves, which have recently been generated at low amplitudes using smart materials, to reduce flow separation on an inclined plate. To see if low-amplitude travelling waves (amplitude approximately 0.01 of chord length $L$) can reduce the separation on an inclined plate, large-eddy simulations are performed. The simulations are carried out for Reynolds number ($Re$) 20 000 and an angle of attack of $10^{\circ }$ with different wavelengths and frequencies. The travelling waves at a low reduced frequency ($f^{\ast }=fL/U=6$ and $\unicode[STIX]{x1D706}^{\ast }=\unicode[STIX]{x1D706}/L=0.2$, where $U$ is free-stream velocity) do not affect the flow separation and aerodynamic performance compared to the flat inclined plate. Nevertheless, increasing the wave speed by increasing the reduced frequency to 20 and 30 reduces flow separation. However, increasing the wave speed by increasing the wavelength, in contrast to the common belief, does not monotonically reduce the flow separation. In fact, increasing the wave speed by increasing the wavelength from 0.15 to 0.5 at constant frequency $f^{\ast }=20$ increases the separation, but increasing from 0.5 to 1.0 and 2.0, interestingly, reduces flow separation. These observations indicate that the wave speed is not the only parameter for flow reattachment, but both wavelength and frequency individually impact flow separation by affecting two competing but interconnected mechanisms: the axial momentum, imparted onto the fluid by the undulations, tends to reattach the flow but the lateral velocity tends to detach it. In fact, increasing $f^{\ast }$ and $\unicode[STIX]{x1D706}^{\ast }$ increases both the axial momentum and the lateral velocity, which are competing to attach and detach the flow, respectively.
On the scaling of propagation of periodically generated vortex rings
- H. Asadi, H. Asgharzadeh, I. Borazjani
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- Journal:
- Journal of Fluid Mechanics / Volume 853 / 25 October 2018
- Published online by Cambridge University Press:
- 22 August 2018, pp. 150-170
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The propagation of periodically generated vortex rings (period $T$) is numerically investigated by imposing pulsed jets of velocity $U_{jet}$ and duration $T_{s}$ (no flow between pulses) at the inlet of a cylinder of diameter $D$ exiting into a tank. Because of the step-like nature of pulsed jet waveforms, the average jet velocity during a cycle is $U_{ave}=U_{jet}T_{s}/T$. By using $U_{ave}$ in the definition of the Reynolds number ($Re=U_{ave}D/\unicode[STIX]{x1D708}$, $\unicode[STIX]{x1D708}$: kinematic viscosity of fluid) and non-dimensional period ($T^{\ast }=TU_{ave}/D=T_{s}U_{jet}/D$, i.e. equivalent to formation time), then based on the results, the vortex ring velocity $U_{v}/U_{jet}$ becomes approximately independent of the stroke ratio $T_{s}/T$. The results also show that $U_{v}/U_{jet}$ increases by reducing $Re$ or increasing $T^{\ast }$ (more sensitive to $T^{\ast }$) according to a power law of the form $U_{v}/U_{jet}=0.27T^{\ast 1.31Re^{-0.2}}$. An empirical relation, therefore, for the location of vortex ring core centres ($S$) over time ($t$) is proposed ($S/D=0.27T^{\ast 1+1.31Re^{-0.2}}t/T_{s}$), which collapses (scales) not only our results but also the results of experiments for non-periodic rings. This might be due to the fact that the quasi-steady vortex ring velocity was found to have a maximum of 15 % difference with the initial (isolated) one. Visualizing the rings during the periodic state shows that at low $T^{\ast }\leqslant 2$ and high $Re\geqslant 1400$ here, the stopping vortices become unstable and form hairpin vortices around the leading ones. However, by increasing $T^{\ast }$ or decreasing $Re$ the stopping vortices remain circular. Furthermore, rings with short $T^{\ast }=1$ show vortex pairing after approximately one period in the downstream, but higher $T^{\ast }\geqslant 2$ generates a train of vortices in the quasi-steady state.
Hydrodynamics of swimming in stingrays: numerical simulations and the role of the leading-edge vortex
- R. G. Bottom II, I. Borazjani, E. L. Blevins, G. V. Lauder
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- Journal:
- Journal of Fluid Mechanics / Volume 788 / 10 February 2016
- Published online by Cambridge University Press:
- 05 January 2016, pp. 407-443
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Stingrays, in contrast with many other aquatic animals, have flattened disk-shaped bodies with expanded pectoral ‘wings’, which are used for locomotion in water. To discover the key features of stingray locomotion, large-eddy simulations of a self-propelled stingray, modelled closely after the freshwater stingray, Potamotrygon orbignyi, are performed. The stingray’s body motion was prescribed based on three-dimensional experimental measurement of wing and body kinematics in live stingrays at two different swimming speeds of 1.5 and $2.5L~\text{s}^{-1}$ ($L$ is the disk length of the stingray). The swimming speeds predicted by the self-propelled simulations were within 12 % of the nominal swimming speeds in the experiments. It was found that the fast-swimming stingray (Reynolds number $Re=23\,000$ and Strouhal number $St=0.27$) is approximately 12 % more efficient than the slow-swimming one ($Re=13\,500$, $St=0.34$). This is related to the wake of the fast- and slow-swimming stingrays, which was visualized along with the pressure on the stingray’s body. A horseshoe vortex was discovered to be present at the anterior margin of the stingray, creating a low-pressure region that enhances thrust for both fast and slow swimming speeds. Furthermore, it was found that a leading-edge vortex (LEV) on the pectoral disk of swimming stingrays generates a low-pressure region in the fast-swimming stingray, whereas the low- and high-pressure regions in the slow-swimming one are in the back half of the wing and not close to any vortical structures. The undulatory motion creates thrust by accelerating the adjacent fluid (the added-mass mechanism), which is maximized in the back of the wing because of higher undulations and velocities in the back. However, the thrust enhancement by the LEV occurs in the front portion of the wing. By computing the forces on the front half and the back half of the wing, it was found that the contribution of the back half of the wing to thrust in a slow-swimming stingray is several-fold higher than in the fast-swimming one. This indicates that the LEV enhances thrust in fast-swimming stingrays and improves the efficiency of swimming.