arXiv subjects

Xuerao He

Publications and source records attributed to Xuerao He.

3 recordsLinked to original sources

arXiv2025

Linear and nonlinear instabilities in highly shear thinning fluid flow through a pipe

Shear-thinning fluids flowing through pipes are crucial in many practical applications, yet many unresolved problems remain regarding their turbulent transition. Using highly robust numerical tools for the Carreau-Yasuda model, we discovered that linear instability can arise when the power-law index falls below 0.35. This inelastic non-axisymmetric instability can universally arise in generalised Newtonian fluids that extend the power-law model. The viscosity ratio from infinite to zero shear rate can significantly impact instability, even if it is small. Two branches of finite-amplitude travelling wave solutions bifurcate subcritically from the linear critical point. The solutions exhibit sublaminar drag reduction, a phenomenon not possible in the Newtonian case.

physics.flu-dyn↗

arXiv2023

Numerical analyses of wire-plate electrohydrodynamic flows

We present numerical analyses of 2-D electrohydrodynamic (EHD) flows of a dielectric liquid between a wire electrode and two plate electrodes with a Poiseuille flow, using direct numerical simulation and global stability analysis. Both conduction and injection mechanisms for charge generation are considered. In this work, we focused on the intensity of the cross-flow and studied the EHD flows without a cross-flow, with a weak cross-flow and with a strong cross-flow. (1)In the case without a cross-flow, we investigated its nonlinear flow structures and linear dynamics. We found that the flow in the conduction regime is steady, whereas the flow in the injection regime is oscillatory, which can be explained by a global stability analysis. (2)The EHD flow with a weak cross-flow is closely related to the flow phenomena in electrostatic precipitator (ESP). Our analyses indicate that increasing the cross-flow intensity or the electric Reynolds number leads to a less stable flow. Based on these results, we infer that one should adopt a relatively low voltage and weak cross-flow in the wire-plate EHD flow to avoid flow instability, which may hold practical implications for ESP. (3)The case of strong cross-flow is examined to study the EHD effect on the wake flow. By comparing the conventional cylindrical wake with the EHD wake in linear and nonlinear regimes, we found that the EHD effect brings forward the vortex shedding in wake flows. Besides, the EHD effect reduces the drag coefficient when the cross-flow is weak, but increases it when it is strong.

physics.flu-dyn↗

arXiv2022

Moffatt eddies in electrohydrodynamic flows: numerical simulations and analyses

We study numerically a sequence of eddies in two-dimensional electrohydrodynamic (EHD) flows of a dielectric liquid, driven by an electric potential difference between a hyperbolic blade electrode and a flat plate electrode (or the blade-plate configuration). The electrically-driven flow impinges on the plate to generate vortices, which resemble Moffatt eddies (Moffatt, $\textit{J. Fluid Mech.}$ vol. 18, 1-18, 1964). Such a phenomenon in EHD was first reported in the experimental work of Perri $\textit{et al., J. Fluid Mech.}$ vol. 900, A12, 2020. We conduct direct numerical simulations of the EHD flow with three Moffatt-type eddies in a large computational domain at moderate electric Rayleigh numbers ($ T $, quantifying the strength of the electric field). The ratios of size and intensity of the adjacent eddies are examined and they can be compared favourably to the theoretical prediction of Moffatt; interestingly, the quantitative comparison is remarkably accurate for the two eddies in the farfield. Our investigation also shows that a larger $T$ strengthens the vortex intensity, and a stronger charge diffusion effect enlarges the vortex size. A sufficiently large $T$ can further result in an oscillating flow, consistent with the experimental observation. In addition, a global stability analysis of the steady blade-plate EHD flow is conducted. The global mode is detailedly characterised at different values of $T$. When $T$ is large, the confinement effect of the geometry in the center region may lead to an increased oscillation frequency. This work contributes to the quantitative characterisation of the Moffatt-type eddies in electrohydrodynamic flows.

physics.flu-dyn↗