Disclinations are topological singularities of molecular arrangement in liquid crystals, which typically occur when the average orientation of molecules makes aπrotation along a fictitious closed loop taken inside the liquid crystal. Depending on the sense of molecular rotation, the disclination lines are either of 1/2 or −1/2 strength. When two disclination lines with the opposite strength meet, they are annihilated without trace. It is hence generally considered difficult in the nematic phase to stabilize a condensed array of free-standing disclination lines without the aid of topological objects like colloidal inclusions. Here we show that a free-standing web of 1/2-strength twist disclination lines can be stably formed in thin liquid crystal cells by means of a judicious combination of orientationally patterned confining surfaces fabricated by the micropatterned photoalignment technique. Theoretical model indicates that disclination lines are held apart at the intersection by a repulsive force generated by the Frank elasticity.

In a cavity flow with acoustic radiation, self-sustained oscillations are controlled by a continuously or intermittently driven plasma actuator with elongated electrodes in the streamwise direction, which induces spanwise non-uniformity of the incoming boundary layer. To evaluate the control effects, wind tunnel experiments and compressible flow simulations were conducted. As indicated by sound pressure measurements at the fundamental frequency, the sound reduction level afforded by the intermittent control may be higher than that provided by the continuous control, although both have the same power consumption. In particular, the sound reduction was highest at a certain intermittent frequency (i.e., effective frequency) under a constant driving voltage and duty ratio. To clarify the mechanism for the influence of the intermittent frequency on the control effects, the time variation of the incoming boundary layer and cavity flow during the intermittent period was investigated. The oscillations remained weak under the control of the effective intermittent frequency, while the attenuation and amplification of the oscillations were repeated in the actuator-on and actuator-off durations, respectively, under the control of intermittent frequencies lower than the effective frequency. The amplification rate of the oscillations was found to be correlated with the quality factor of the radiated sound without control. The relationship between the effective frequency and characteristic time for this amplification is also discussed in this paper.

In order to better understand the effects of acoustic resonance on flows around a cascade of flat plates, fluid structures and acoustic fields were examined by aeroacoustic direct simulations and wind tunnel experiments. Computations and experiments were performed for the flows around five parallel plates with and without the acoustic resonance changing the freestream velocity. The aspect ratio of the plates, C/b, is 15.0, and the separation-to-thickness ratio, s/b, is 6.0. For the resonant condition of a freestream velocity of 44 m/s, the Reynolds number based on the plate thickness, b, and the freestream velocity is 5.8 × 10³. The computational results revealed that large-scale vortices composed of fine-scale vortices are shed in the wake of the plates. Both experimental and computational results indicated that the shedding of the large-scale vortices is more synchronized in the spanwise direction for the resonant condition. Moreover, the shedding of the large-scale vortices from one plate and those from the neighboring plates become more synchronized in the resonant condition. The mode of this synchronization was found to be an anti-phase mode, in which the vortex is shed from the upper or lower face of one plate when the vortex is shed from the lower or upper faces of the neighboring plates. Computation of the flow around a single plate was also performed, and the radiation of the acoustic waves from the downstream edge due to vortex shedding from the plate was indicated. When acoustic resonance occurs in the flows around the cascade of flat plates, vortex shedding in the above-mentioned mode contributes to the intensification of the standing waves between the plates. Moreover, standing waves were demonstrated to induce new vortices around the upstream edges of the plates in synchronization.

The fluid-acoustic interactions in a flow over a two-dimensional rectangular cavity are investigated by directly solving the compressible Navier–Stokes equations. The upstream boundary layer is turbulent. The depth-to-length ratio of the cavity is 0.5. Phase-averaged flow fields reveal the mechanism for the acoustic radiation. Large-scale vortices form in the shear layer that separates from the upstream edge of the cavity. When a large-scale vortex collides with the downstream wall, the low-pressure fluid in the vortex spreads along the downstream wall. As a result, a local downward velocity is induced by the local pressure gradient, causing the upstream fluid to expand. Finally, an expansion wave propagates to the outside of the cavity. The large-scale vortices originate from the convective disturbances that develop in the shear layer. The disturbances grow due to the Kelvin–Helmholtz instability, similar to the growth of those in a laminar cavity flow. To clarify the mechanism for the generation of the initial convective disturbances, computations for backward-facing step flows with an artificial acoustic source are also performed. As the artificial acoustic waves become more intense, the initial convective disturbances in the shear layer become more intense while the spatial growth rate of these disturbances does not change. This means that the initial convective disturbances in the shear layer are induced by the acoustic waves.

To clarify fluid–acoustic interactions in an actual recorder with opened and closed tone holes, flow and acoustic fields were directly numerically simulated on the basis of the compressible Navier–Stokes equations. To validate the simulation accuracy, the flow field around the windway and sound pressure above the window were measured. The predicted acoustic fields clarify changes of the positions of pressure nodes and anti-nodes in accordance with the state of the tone holes and the Mach number of the jet velocity. The fundamental mechanism of the self-sustained oscillations in a three-dimensional actual recorder is visualized by the predicted acoustic and flow fields. This result is also consistent with the relationship between the jet behaviors and pressure fluctuations based on the jet-drive model. Moreover, the effects of the fine vortices in the jet, which appear at the high Mach number of jet velocity of 0.099, on the sound are discussed. The time difference between the generation of the disturbances and the most intense deflection of the jet is identified and compared with the time delay of acoustic reflection around the window.

We investigate how the interaction of particles mediated by an elastic deformation of a nematic liquid crystal is influenced by the initial deformation of the director field. To this end, we calculate the interaction energy between particles in a nematic cell with hybrid boundary conditions, homeotropic on the surface of one confining plate and planar on the other. We find an analytic form of the interaction energy in the case of weak anchoring on the surface of the particle. This interaction energy sensitively depends on the position of the two particles as well as the interparticle distance and can be nonmonotonic with a minimum in its landscape. This nontrivial energy landscape might lead to a chainlike superstructure of particles.