| 1. |
- Biancofiore, Luca
(författare)
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Crossover between two- and three-dimensional turbulence in spatial mixing layers
- 2014
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Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press (CUP). - 0022-1120 .- 1469-7645. ; 745, s. 164-179
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Tidskriftsartikel (refereegranskat)abstract
- We investigate how the domain depth affects the turbulent behaviour in spatially developing mixing layers by means of large-eddy simulations based on a spectral vanishing viscosity technique. Analyses of spectra of the vertical velocity, of Lumley's diagrams, of the turbulent kinetic energy and of the vortex stretching show that a two-dimensional behaviour of the turbulence is promoted in spatial mixing layers by constricting the fluid motion in one direction. This finding is in agreement with previous works on turbulent systems constrained by a geometric anisotropy, pioneered by Smith, Chasnov & Waleffe (Phys. Rev. Lett., vol. 77, 1996, pp. 2467-2470). We observe that the growth of the momentum thickness along the streamwise direction is damped in a confined domain. An almost fully two-dimensional turbulent behaviour is observed when the momentum thickness is of the same order of magnitude as the confining scale.
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| 2. |
- Biancofiore, L., et al.
(författare)
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Streak instability in viscoelastic Couette flow
- 2017
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Ingår i: PHYSICAL REVIEW FLUIDS. - : American Physical Society. - 2469-990X. ; 2:4
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Tidskriftsartikel (refereegranskat)abstract
- The secondary instability of nonlinear streaks and transition to turbulence in viscoelastic Couette flow are studied using direct numerical simulations. Viscoelasticity is modeled using the FENE-P constitutive equations. Both the polymer concentration beta and Weissenberg number Wi are varied in order to assess their effects on transition at moderate Reynolds number. The base streaks are obtained from nonlinear simulations of the Couette flow response to a streamwise vortex. We select the initial amplitude of the vortex which yields a desired maximum amplitude of the nonlinear streaks during their temporal evolution. The development of streaks in both Newtonian and non-Newtonian flows is primarily due to the action of streamwise vorticity onto the mean shear. In the viscoelastic case, it is also affected by the polymer torque, which opposes the vorticity and becomes more pronounced at large Weissenberg number. Streaks with the same maximum streamwise velocity perturbation can therefore have different total kinetic energy at higher Weissenberg number. At every streak amplitude of interest, harmonic forcing is introduced along the transverse direction to trigger the secondary instability and breakdown to turbulence. We demonstrate that the critical amplitude of the forcing, A(d), increases at large Weissenberg number. The degree of stabilization due to elasticity depends on the initial streak intensity, A(s),(in). For weak streaks the critical amplitude for secondary instability is more sensitive to Wi than for strong ones. This is explained by the existence of two different mechanisms that can trigger transition to turbulence. The perturbation to weak streaks is initially stabilized by the polymer torque which acts to oppose the amplification of wall-normal vorticity and, as a result, delays breakdown to turbulence. The secondary instability of strong streaks, on the other hand, is more immune to this stabilizing influence of the polymer.
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| 3. |
- Bronte Ciriza, David, et al.
(författare)
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Optically Driven Janus Microengine with Full Orbital Motion Control
- 2023
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Ingår i: ACS PHOTONICS. - 2330-4022. ; 10:9, s. 3223-3232
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Tidskriftsartikel (refereegranskat)abstract
- Microengines have shown promise for a variety of applications in nanotechnology, microfluidics, and nanomedicine, including targeted drug delivery, microscale pumping, and environmental remediation. However, achieving precise control over their dynamics remains a significant challenge. In this study, we introduce a microengine that exploits both optical and thermal effects to achieve a high degree of controllability. We find that in the presence of a strongly focused light beam, a gold-silica Janus particle becomes confined at the stationary point where the optical and thermal forces balance. By using circularly polarized light, we can transfer angular momentum to the particle, breaking the symmetry between the two forces and resulting in a tangential force that drives directed orbital motion. We can simultaneously control the velocity and direction of rotation of the particle changing the ellipticity of the incoming light beam while tuning the radius of the orbit with laser power. Our experimental results are validated using a geometrical optics phenomenological model that considers the optical force, the absorption of optical power, and the resulting heating of the particle. The demonstrated enhanced flexibility in the control of microengines opens up new possibilities for their utilization in a wide range of applications, including microscale transport, sensing, and actuation.
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