This letter reports on the pronounced turbulence modulations and the accompanying drag reduction observed in a two-way coupled simulation of particle-laden channel flow. The present results support the view that drag reduction can be achieved not only by means of polymeric or fiber additives but also with spherical particles. © 2010 American Institute of Physics. ͓doi:10.1063/1.3478308͔The inevitable pressure loss associated with transport of fluids in pipelines determines the pumping requirements. The pressure loss arises in order to overcome the skin-friction drag. Several investigations of turbulent drag reduction have thus been motivated by the economical benefits of pressureloss reduction. Drag reduction can be achieved in many different ways, for instance, by means of additives suspended into an otherwise Newtonian fluid ͑see, e.g., the reviews by Lumley 1 and Gyr and Bewersdorff 2 ͒. A notable example is the substantial drag reduction observed when flexible polymers are dissolved in the carrier fluid as reported in Refs. 3 and 4 ͑see also the recent reviews 5,6 ͒. However, the findings with respect to drag reduction are not as clear-cut for additives other than polymers and chemicals. Pressure drop measurements reported from a variety of solid-fluid systems were scrutinized by Radin et al. 7 While drag reduction was obtained with fibrous additives ͑e.g., nylon or cotton͒, no drag reduction was reported for nonfibrous additives irrespective of the shape of the solid ͑spherical, platelet, or needle-shaped͒. More recently, drag reduction has been reported from computer experiments by Paschkewitz et al. 8 and Gillissen et al.9 also for rigid fibers. Suspensions of spherical particles in turbulent pipe and channel flows have been extensively studied during the years with the view to better understand the particle transport, dispersion, and segregation in wall-turbulence and also the influence of the presence of the solid spheres on the turbulence of the carrier fluid. The current understanding of the complex behavior of inertial spherical particles in turbulent wall flows was summarized in a keynote lecture by Soldati. 10 The survey of experimental studies and two-way coupled computer simulations by Balachandar and Eaton 11 furthermore addressed the turbulence modulation observed in the presence of inertial particles. In dilute suspensions where particle collisions are of negligible importance, the fluid turbulence may be affected by an enhanced dissipation due to the particles, kinetic energy transfer between the fluid and the particles, and wakes formed in the lee of the solid particles. The relative importance of these mechanisms depends on the particle Reynolds number, the particle-to-fluid density ratio, and the particle-to-turbulence length-scale ratio. By adopting the turbulence intensity as a measure of the turbulence activity, Gore and Crowe 12 showed that a particle diameter of 1/10 the size of the most energetic eddies represents a demarcation between increase and decrease of the carrier-phase tur...

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Transfer of mechanical energy between solid spherical particles and a Newtonian carrier fluid has been explored in two-way coupled direct numerical simulations of turbulent channel flow. The inertial particles have been treated as individual point particles in a Lagrangian framework and their feedback on the fluid phase has been incorporated in the Navier-Stokes equations. At sufficiently large particle response times the Reynolds shear stress and the turbulence intensities in the spanwise and wall-normal directions were attenuated whereas the velocity fluctuations were augmented in the streamwise direction. The physical mechanisms involved in the particle-fluid interactions were analysed in detail, and it was observed that the fluid transferred energy to the particles in the core region of the channel whereas the fluid received kinetic energy from the particles in the wall region. A local imbalance in the work performed by the particles on the fluid and the work exerted by the fluid on the particles was observed. This imbalance gave rise to a particleinduced energy dissipation which represents a loss of mechanical energy from the fluid-particle suspension. An independent examination of the work associated with the different directional components of the Stokes force revealed that the dominating energy transfer was associated with the streamwise component. Both the mean and fluctuating parts of the Stokes force promoted streamwise fluctuations in the near-wall region. The kinetic energy associated with the cross-sectional velocity components was damped due to work done by the particles, and the energy was dissipated rather than recovered as particle kinetic energy. Componentwise scatter plots of the instantaneous velocity versus the instantaneous slip-velocity provided further insight into the energy transfer mechanisms, and the observed modulations of the flow field could thereby be explained.

The translational and rotational dynamics of oblate spheroidal particles suspended in a directly simulated turbulent channel flow have been examined. Inertial disk-like particles exhibited a significant preferential orientation in the plane of the mean shear. The rotational inertia about the symmetry axis of the disk-like particles hampered the spin-up of the flattest particles to match the mean flow vorticity. The influence of the particle shape on the orientation and rotation diminished as the translational inertia increased from Stokes number 1 to 30. An isotropization of both orientation and rotation could be observed in the core region of the channel. The translational motion of the oblate spheroids had a weak dependence on the aspect ratio. We therefore concluded that inertial particles sample nearly the same flow field irrespective of shape. Nevertheless, the orientation and rotation of disk-like particles turned out to be qualitatively different from the dynamics of fibre-like particles.

The interaction between gas flow and liquid flow, governed by fluid dynamic principles, is of substantial importance in both fundamental science and practical applications. For instance, a precisely designed gas shearing on liquid solution may lead to efficacious production of advanced nanomaterials. Here, we devised a needleless Kármán vortex solution blow spinning system that uses a roll-to-roll nylon thread to deliver spinning solution, coupled with vertically blowing airflow to draw high-quality nanofibers with large throughput. A wide variety of nanofibers including polymers, carbon, ceramics, and composites with tunable diameters were fabricated at ultrahigh rates. The system can be further upgraded from single thread to multiple parallel threads and to the meshes, boosting the production of nanofibers to kilogram scale without compromising their quality.