On the role of surface permeability for the control of flow around a circular cylinder

The circular cylinder with porous materials coating (PMC) is studied in detail to reveal the sensitivity of surface permeability to the flow control and noise reduction. Two-dimensional simulations were firstly used to identify the critical values of permeability and thickness. Parametric study results show that, there is a critical permeability value which produces the minimum force fluctuation and maximum noise reduction. Additionally, the porous coating can work more efficient for noise reduction with larger thickness. The further three-dimensional simulation is employed to understand the underlying physical mechanisms of flow control. The results show that the spanwise vorticity is modified more than that of other directions and behaves more synergistically. The pressure field adjacent to the cylinder surface indicates that the adverse pressure gradient is changed to the favorable pressure gradient around the porous surface which contributes partly to the vortex shedding suppression

Interfacial Interaction Enhanced Rheological Behavior in PAM/CTAC/Salt Aqueous Solution—A Coarse-Grained Molecular Dynamics Study

Interfacial interactions within a multi-phase polymer solution play critical roles in processing control and mass transportation in chemical engineering. However, the understandings of these roles remain unexplored due to the complexity of the system. In this study, we used an efficient analytical method—a nonequilibrium molecular dynamics (NEMD) simulation—to unveil the molecular interactions and rheology of a multiphase solution containing cetyltrimethyl ammonium chloride (CTAC), polyacrylamide (PAM), and sodium salicylate (NaSal). The associated macroscopic rheological characteristics and shear viscosity of the polymer/surfactant solution were investigated, where the computational results agreed well with the experimental data. The relation between the characteristic time and shear rate was consistent with the power law. By simulating the shear viscosity of the polymer/surfactant solution, we found that the phase transition of micelles within the mixture led to a non-monotonic increase in the viscosity of the mixed solution with the increase in concentration of CTAC or PAM. We expect this optimized molecular dynamic approach to advance the current understanding on chemical–physical interactions within polymer/surfactant mixtures at the molecular level and enable emerging engineering solutions

A Mechanism on Liquid-Phase Turbulence Modulation by Microbubbles

Although bubbly flows have been extensively applied to many industrial fields, efficiency improvements of bubbly equipments are greatly limited due to the absence of accurate understandings of liquid turbulence modulation by bubbles. In order to obtain an accurate mechanism explanation, an Euler-Lagrange two-way method was used to investigate and analyze deeply the liquid turbulence modulation by microbubbles in this paper. The present investigation shows that for the bubbly flow laden with microbubbles the liquid turbulence modulation is mainly caused by the drag force, and the turbulence modulation level is related to the microbubble diameter. A detailed mechanism explanation on the physical phenomenon was presented here

Numerical Simulation of 3D Solid-Liquid Turbulent Flow in a Low Specific Speed Centrifugal Pump: Performance Comparison of Four Geometric Models

For numerically simulating 3D solid-liquid turbulent flow in low specific speed centrifugal pumps, there exist several problems including how to design geometrical shape of the calculation model to represent the real pump and how to predict pump performance accurately to guide the design of pump. To solve these problems, four kinds of geometric models were designed. The performance of a low specific speed solid-liquid centrifugal pump was predicted, and the results showed that the improved prediction methods are more accurate than the traditional method. Moreover, the simulation results of the entire flow field of the geometric model including balance holes and the lateral clearances of impeller in which liquid rotates with half speed of impeller are closer to the real situation

Mixed convection heat transfer from a particle in supercritical water

This paper aims to study a steady laminar convection flow over a spherical particle in supercritical water in pseudo-critical zone. Forced convection, free convection, assisting convection, and opposing convection from a spherical particle in supercritical water (SCW) were studied based on a numerical model fully accounting for variations of thermo-physical properties in pseudo-critical zone. Variation of specific heat plays a main role in determination of heat transfer coefficient. The gravity direction has a remarkable effect on heat transfer. Quantitative relationship between variable properties and Nusselt number has been established based on the simulation results

Time-Dependent Shear-Induced Nonlinear Viscosity Effects in Dilute CTAC/NaSal Solutions: Mechanism Analyses

The time-dependent shear-induced nonlinear viscosity effects of dilute surfactant solutions (CTAC/NaSal) at constant shear rate were tested by using the rheometer Couette cell. The apparent viscosity evolution curve can be divided into five stages: weak shear-thickening (Stage I), weak shear-thinning and plateau (Stage II), sharp shear-thickening (Stage III), oscillating adjustment (Stage IV), and rough plateau (Stage V). In Stage I, the stretching effects of shear flow lead to the weak increase in apparent viscosity at the inception of shearing. The apparent viscosity curve firstly decreases in Stage II and then levels off. The apparent viscosity plateau is caused by the forming and slipping of micellar lumps at the inner cylinder wall surface. Once the volume of lump exceeds a certain degree, the nucleation process of forming SIS is triggered, which is the beginning of Stage III and then the apparent viscosity increases sharply. The variations of apparent viscosity in adjusting period are rather complicated in Stage IV, and the variations mainly depend on the situation of SISs network. In Stage V, coupled with obvious oscillations, the apparent viscosity maintains a basically constant plateau value, indicating that the SISs network is fully developed and saturated at the corresponding shear rate