Recent theoretical, numerical, and experimental investigations performed at the Department of Mechanics, KTH Stockholm, and the Department of Mechanical Engineering, Eindhoven University of Technology, are reviewed, and new material is presented to clarify the role of the boundary-layer streaks and their instability with respect to turbulent breakdown in bypass transition in a boundary layer subject to free-stream turbulence. The importance of the streak secondary-instability process for the generation of turbulent spots is clearly shown. The secondary instability manifests itself as a growing wave packet located on the low-speed streak, increasing in amplitude as it is dispersing in the streamwise direction. In particular, qualitative and quantitative data pertaining to temporal sinuous secondary instability of a steady streak, impulse responses both on a parallel and a spatially developing streak, a model problem of bypass transition, and full simulations and experiments of bypass transition itself are collected and compared. In all the flow cases considered, similar characteristics in terms of not only growth rates, group velocity, and wavelengths but also three-dimensional visualizations of the streak breakdown have been found. The wavelength of the instability is about an order of magnitude larger than the local boundary-layer displacement thickness ␦ ‫ء‬ , the group velocity about 0.8 of the free-stream velocity U ϱ , and the growth rate on the order of a few percent of U ϱ / ␦ ‫ء‬ . The characteristic structures at the breakdown are quasistreamwise vortices, located on the flanks of the low-speed region arranged in a staggered pattern.

We present direct numerical simulations of subcritical transition to turbulence in a particle-laden channel flow, with particles assumed rigid, spherical, and heavier than the fluid. The equations describing the fluid flow are solved with an Eulerian mesh, whereas those describing the particle dynamics are solved by Lagrangian tracking. Two-way coupling between fluid and particles is modeled with Stokes drag. The numerical code is first validated against previous results from linear stability: the nonmodal growth of streamwise vortices resulting in streamwise streaks is still the most efficient mechanism for linear disturbance amplification at subcritical conditions as for the case of a single phase fluid. To analyze the full nonlinear transition, we examine two scenarios well studied in the literature: (1) transition initiated by streamwise independent counter-rotating streamwise vortices and one three-dimensional mode and (2) oblique transition, initiated by the nonlinear interaction of two symmetric oblique waves. The threshold energy for transition is computed, and it is demonstrated that for both scenarios the transition may be facilitated by the presence of particles at low number density. This is due to the fact that particles may introduce in the system detrimental disturbances of length scales not initially present. At higher concentrations, conversely, we note an increase of the disturbance energy needed for transition. The threshold energy for the oblique scenario shows a more significant increase in the presence of particles, by a factor about four. Interestingly, for the streamwise-vortex scenario the time at which transition occurs increases with the particle volume fraction when considering disturbances of equal initial energy. These results are explained by considering the reduced amplification of oblique modes in the two-phase flow. The results from these two classical scenarios indicate that, although linear stability analysis shows hardly any effect on optimal growth, particles do influence secondary instabilities and streak breakdown. These effects can be responsible of the reduced drag observed in turbulent channel flow laden with heavy particles.

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Microfluidic devices and micro-pumps are increasingly necessitated in many fields ranging from untethered soft robots, to pharmaceutical and biomedical technology. While realization of such devices is limited by miniaturization constraints of conventional actuators, these restrictions can be resolved by using smart material transducers instead. This paper proposes and investigates the first ionic polymer metal composite (IPMC) actuator-driven linear peristaltic pump. With the aim of designing a monolithic device, our concept is based on a single IPMC actuator that is etched on both sides and cut with kirigami-inspired slits by laser ablation. Our pump has a planar configuration, operates with low activation voltages (< 5 V) and is simple to manufacture and thus miniaturize. We build proofof-principle prototypes of an open and closed design of our proposed pump concept, model the closed design, and evaluate both configurations experimentally. Results show the feasibility of the proposed IPMC-driven pump. Without any optimization, the open pump achieved pumping rates of 669 pL • s −1 , while the closed pump configuration attained a 4.57 Pa pressure buildup and 9.18 nL • s −1 pumping rate. These results indicate feasibility of the concept and future work will focus on design optimization.

Film cooling is studied on a jet in a cross-flow. The influence of a hole production imperfection on the jet-cross flow interaction is investigated experimentally by means of particle image velocimetry and liquid crystal thermography. To simulate the imperfection a half torus was placed inside the hole. The experiments were conducted with and without the production imperfection and the velocity ratio was varied. If the imperfection is absent, vortices are generated by means of the Kelvin-Helmholtz instability and separation on the hole trailing edge. The imperfection produces additional vortical structures and the flow field starts to oscillate. The deteriorated flow field changes the heat transfer. Surface temperature measurements show that the production imperfection reduces the film cooling effectiveness. The influence of the production imperfection on the film cooling effectiveness decreases with the enhanced velocity ratio.