Theoretical Analysis of CFD-DEM Mathematical Model Solution Change With Varying Computational Cell Size

Successful verification and validation is crucial to build confidence in the application of coupled Computational Fluid Dynamics - Discrete Element Method (CFD-DEM). Model verification includes ensuring a mesh-independent solution, which poses a major difficulty in CFD-DEM due to the complicated solution relationship with computational cell size. In this paper, we investigate the theoretical relationship between the solution and computational cell size by tracing the effects of a change in cell size through the mathematical model. The porosity profile for simulations of fixed-particle beds is determined to be Gaussian, and the average and standard deviation of the representative distribution are reported against cell size. We find the standard deviation of bed porosity increases exponentially as the cell size is reduced, and the drag calculations are very sensitive to changes in the porosity standard deviation, resulting in an exponential change in expected drag when the cell size is small relative to the particle diameter. The divided volume fraction method of porosity calculation is shown to be superior to the centred volume fraction method, as it reduces the porosity standard deviation. The sensitivity of five popular drag laws to changes in the porosity profile is presented, and the Ergun and Beetstra drag laws are shown to be the least sensitive to changes in the cell size.

Aeromechanics of Long Jumps in Spider Crickets: Insights From Experiments and Modeling

Flapping, gliding, running, crawling, and swimming in animals have all been studied extensively in the past and have served as sources of inspiration for engineering designs. In this paper, we describe the aeromechanics of a mode of locomotion that straddles ground and air: jumping. The subject of our study is the spider cricket of the family Rhaphidophoridae, an animal that is among the most proficient of long-jumpers in nature. The focus of the study is to understand the aeromechanics of the aerial portion of the jump of this animal. The research employs high-speed videogrammetry to track the crickets’ posture and appendage orientation throughout their jumps. Experiments demonstrate that these insects employ carefully controlled and coordinated positioning of their limbs during their jumps so as to increase jump distance and stabilize body posture. Simple phenomenological models based on drag laws indicate that the conformation of the limbs during the latter portion of the jump is stable to pitch and enables these animals to land in a controllable manner. Insights from this study could be useful in the design of micro-robots that exploit jumping as a means of locomotion.

Prediction of Solid Recirculation Rate and Solid Volume Fraction in an Internally Circulating Fluidized Bed

This paper presents a numerical study of gas and solid flow in an internally circulating fluidized bed (ICFB). Two-fluid Eulerian model with kinetic theory of granular flow option for solid phase stress closure and various drag laws were used to predict the hydrodynamic behavior of ICFB. 2D and 3D geometries were used to run the simulations. The 2D simulation results by various drag laws show that the Arastoopour and Gibilaro drag models able to predict the fluidization dynamics in terms of flow patterns, void fractions and axial velocity fields close to the experimental data. The effect of superficial gas velocity, presence of draft tube on solid hold-up distribution, solid circulation pattern, and variations in gas bypassing fraction for the 3D ICFB are investigated. The mechanism governing the solid circulation and solids concentration in an ICFB has been explained based on gas and solid dynamics obtained from the simulations. Predicted total granular temperature distributions in the draft tube and annular zones qualitatively agree with experimental data. The total granular temperature tends to increase with increasing solids concentration in the dilute region (ε < 0.1) and decreases with an increase of solids concentration in the dense region (ε > 0.1). In the dense zone, the decreasing trend in the granular temperature is mainly due to the reduction of the mean free path of the solid particles.