We investigate dilute suspensions of Taylor-microscale-sized particles in homogeneous isotropic turbulence. In particular, we focus on the effect of particle shape on particle–fluid interaction. We conduct laboratory experiments using a novel experimental technique to simultaneously measure the kinematics of fluid and particle phases. This uses transparent particles having the same refractive index as water, whose motion we track via embedded optical tracers. We compare the turbulent statistics of a single-phase flow to the turbulent statistics of the fluid phase in a particle–laden suspension. Two suspensions are compared, one in which the particles are spheres and the other in which they are prolate ellipsoids with aspect ratio 2. We find that spherical particles at volume fraction reduce the turbulent kinetic energy (TKE) by 15 % relative to the single-phase flow. At the same volume fraction (and slightly smaller total surface area), ellipsoidal particles have a much smaller effect: they reduce the TKE by 3 % relative to the single-phase flow. Spectral analysis shows the details of TKE reduction and redistribution across spatial scales: spherical particles remove energy from large scales and reinsert it at small scales, while ellipsoids remove relatively less TKE from large scales and reinsert relatively more at small scales. Shape effects are far less evident in the statistics of particle rotation, which are very similar for ellipsoids and spheres. Comparing these with fluid enstrophy statistics, we find that particle rotation is dominated by velocity gradients on scales much larger than the particle characteristic length scales.

Shape effects on turbulent modulation by large nearly neutrally buoyant particles

The titania-coated silica nanomaterials with three different shapes (nanosphere, nanorod, nanotube) are fabricated to examine the influence of particle geometry on ER fluid in nanometer-size region. The effect of particle geometry on ER activity is studied by varying the geometric aspect ratio of dispersing materials. The ER activities of titania-coated silica nanomaterials exhibit a dependence on their aspect ratio. Interestingly, the shear stress of titania-coated silica nanomaterials based ER fluids increases with increasing the aspect ratio. Geometrical study is performed to give deep insights into the primary factors that determine the ER activity. It is believed that the geometrical effect originated from high aspect ratio played a dominant role in enhancing the performance of ER fluid. Furthermore, the dielectric property analysis based on dielectric loss model clarifies that an increase in aspect ratio has been coupled with the larger achievable polarizability and short relaxation time of interfacial polarization. Consequently, the increment in aspect ratio has strong influence on ER activity, provides outstanding enhancement in shear stress value of titania-coated silica nanotube based ER fluid.

Geometrical study of electrorheological activity with shape-controlled titania-coated silica nanomaterials.

In this study, the influence of particle geometry on electrorheological (ER) activity is examined by varying the aspect ratio of graphene oxide (GO)-wrapped silica materials. The GO-wrapped silica material-based ER fluid exhibits a high shear stress as the aspect ratio of the particle increases; this is attributed to the flow resistance and mechanical stability of the fluid. Additionally, the dielectric loss model is used to investigate the dielectric properties of ER fluids, which have been shown to be associated with the enhancement of ER activity. The GO-wrapped silica material with a higher aspect ratio exhibits a higher dielectric constant and shorter relaxation times for interfacial polarization, due to greater polarizability. Thus, the aspect ratio of GO-wrapped silica materials plays a prominent role in the enhancement of ER performance.

Enhanced electrorheological performance of a graphene oxide-wrapped silica rod with a high aspect ratio

Investigation of flow characteristics around a sphere placed in a boundary layer over a flat plate

Mesoporous silica (mSiO2) particle-based electrorheological (ER) fluids were examined to determine the effect of the particles’ aspect ratio on ER performance. Multigram-scale mSiO2 particles were fabricated with different aspect ratios using the sol–gel method. The ER performance of various mSiO2 particle-based ER fluids improved with an increasing aspect ratio due to the better flow resistance and mechanical stability. Moreover, an incremental increase in the aspect ratio enhanced the interfacial polarization of the material. Thus, mSiO2 materials with a high aspect ratio exhibited the highest ER performance due to combined contributions from geometrical effects and their dielectric properties. In addition, various mSiO2-based ER fluids showed potential for large-scale production and a wide applicable range of electric field strengths (up to 10.0 kV mm−1).

Electrorheological performance of multigram-scale mesoporous silica particles with different aspect ratios

The effects of freestream turbulence on the drag coefficient of a sphere

Spherical object in wind is a common scenario in daily life and engineering practice. The main challenge in understanding the aerodynamics in turbulent wind lies in the multi-aspect of turbulence. This paper presents a wind tunnel study, which focuses on the role of turbulence integral length scale on the drag of a sphere. Particular turbulent flow conditions were achieved via the proper combination of wind speed, orifice perforated plate, sphere diameter (D) and distance downstream from the plate. The drag was measured in turbulent flow with , , and turbulence intensity Tu up to 6.3%. Our results confirmed the general trends of decreasing drag coefficient and critical Reynolds number with increasing turbulence intensity. More interestingly, the unique role of the relative integral length scale has been revealed. Over the range of conditions studied, an integral length of approximately 65% the sphere diameter is most effective in reducing the drag.

Niloofar Moradian

Papers

The effects of freestream turbulence on the drag coefficient of a sphere

Advancing drag crisis of a sphere via the manipulation of integral length scale