Flow past a circular cylinder executing sinusoidal rotary oscillations about its own axis is studied experimentally. The experiments are carried out at a Reynolds number of 185, oscillation amplitudes varying from π/8 to π, and at non-dimensional forcing frequencies (ratio of the cylinder oscillation frequency to the vortex-shedding frequency from a stationary cylinder) varying from 0 to 5. The diagnostic is performed by extensive flow visualization using the hydrogen bubble technique, hotwire anemometry and particle-image velocimetry. The wake structures are related to the velocity spectra at various forcing parameters and downstream distances. It is found that the phenomenon of lock-on occurs in a forcing frequency range which depends not only on the amplitude of oscillation but also the downstream location from the cylinder. The experimentally measured lock-on diagram in the forcing amplitude and frequency plane at various downstream locations ranging from 2 to 23 diameters is presented. The far-field wake decouples, after the lock-on at higher forcing frequencies and behaves more like a regular Bénard-von Kármán vortex street from a stationary cylinder with vortex-shedding frequency mostly lower than that from a stationary cylinder. The dependence of circulation values of the shed vortices on the forcing frequency reveals a decay character independent of forcing amplitude beyond forcing frequency of ∼1.0 and a scaling behaviour with forcing amplitude at forcing frequencies 1.0. The flow visualizations reveal that the far-field wake becomes two-dimensional (planar) near the forcing frequencies where the circulation of the shed vortices becomes maximum and strong three-dimensional flow is generated as mode shape changes in certain forcing parameter conditions. It is also found from flow visualizations that even at higher Reynolds number of 400, forcing the cylinder at forcing amplitudes of π/4 and π/2 can make the flow field two-dimensional at forcing frequencies greater than ∼2.5.

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The main objective is to improve the most commonly addressed weakness of the laminated composites (i.e. delamination due to poor interlaminar strength) using carbon nanotubes (CNTs) as reinforcement between the laminae and in the transverse direction. In this work, a chemical vapor deposition technique has been used to grow dense vertically aligned arrays of CNTs over the surface of chemically treated two-dimensionally woven cloth and fiber tows. The nanoforest-like fabrics can be used to fabricate three-dimensionally reinforced laminated nanocomposites. The presence of CNTs aligned normal to the layers and in-between the layers of laminated composites is expected to considerably enhance the properties of the laminates. To demonstrate the effectiveness of our approach, composite single lap-joint specimens were fabricated for interlaminar shear strength testing. It was observed that the single lap-joints with through-the-thickness CNT reinforcement can carry considerably higher shear stresses and strains. Close examination of the test specimens showed that the failure of samples with CNT nanoforests was completely cohesive, while the samples without CNT reinforcement failed adhesively. This concludes that the adhesion of adjacent carbon fabric layers can be considerably improved owing to the presence of vertically aligned arrays of CNT nanoforests.

The unique mechanical, physical, and chemical properties of carbon nanotubes and graphene project them as seemless and stand alone materials with performances much better than other metallic and nonmetallic materials in wide variety of applications. Due to their perfect hexagonal lattice structural configuration, electrons can be transmitted without resistance, heat can be conducted even better than in diamond, and loads are carried out and transferred more efficient than in high strength steels. However, carbon nanotubes and graphene sheets are not free of structural defects, which are basically induced during their processing and purification stages. Owing to the fact that defects exist in different configurations and they can considerably influence the performance and properties of carbon nanotubes and graphene, they are not always undesirable. Therefore, with increased interests in functionalization of carbon nanotubes and graphene, detailed studies and investigations regarding the effects of such vacancy defects, due to the absence of carbon atoms from the lattice structure, on their properties warrants a detailed investigation. Nevertheless, the advantages and disadvantages of the presence of vacancy defects strongly depend on their applications. In this work, an attempt is made to numerically model and analyze the effects of vacancy defects of carbon nanotubes and graphene sheets on their effective mechanical properties. The effects of these defects in different configurations of carbon nanotubes and graphene sheets are (i.e., armchair, chiral, and zig-zag, depending on the direction of applied load) reported. A detailed finite element analysis has been performed to predict their axial mechanical properties.