Fluid motion for car undertrays in ground effect

Air motion representative of some of the flows past a moving car is studied, particularly in the gap between the car underbody (undertray, front flap or forewing) and the ground, using theory and computation. The ground-affected flows encountered are two- or three-dimensional, laminar, transitional or turbulent, and attached or separated. Given Reynolds numbers in the approximate range 1–10 million, emphasis here is placed first on key physical flow mechanisms: viscous-inviscid interaction filling either much or part of the gap; the generation of strong upstream influence; an abrupt pressure jump at the leading edge; the moving-ground condition; substantial diffuser flow reversals and wake effects; in three dimensions the distinguishing between inflow and outflow edges; and turbulent flow modelling. Second, for various underbody shapes, predictions are presented of the surface pressures and shear stresses, the lift or downforce, and the velocity profiles. Extensions of these to include edge effects, three-dimensionality and turbulence modelling are examined, along with optimization for certain shapes concerned with front-flap design and comparisons with recent experiments.     

Flow Control Through the Use of Topography

In this work, optimal shaft shapes for flow in the annular space between a rotating shaft with axially-periodic radius and a fixed coaxial outer circular cylinder, are investigated. Axisymmetric steady flows in this geometry are determined by solving the full Navier-Stokes equations in the actual domain. A measure of the flow field, a weighted convex combination of the volume averaged square of the L₂-norm of the velocity and vorticity vectors, is employed. It has been demonstrated that boundary shape can be used to influence the characteristics of the flow field, such as its velocity component distribution, kinetic energy, or even vorticity. This ability to influence flow fields through boundary shape may be employed to improve microfluidic mixing or, possibly, to minimize shear in biological applications.     

Haemodynamics in the mouse aortic arch computed from MRI-derived velocities at the aortic root

Mice are widely used to investigate atherogenesis, which is known to be influenced by stresses related to blood flow. However, numerical characterization of the haemodynamic environment in the commonly studied aortic arch has hitherto been based on idealizations of inflow into the aorta. Our purpose in this work was to numerically characterize the haemodynamic environment in the mouse aortic arch using measured inflow velocities, and to relate the resulting shear stress patterns to known locations of high- and low-lesion prevalence. Blood flow velocities were measured in the aortic root of C57/BL6 mice using phase-contrast MRI. Arterial geometries were obtained by micro-CT of corrosion casts. These data were used to compute blood flow and wall shear stress (WSS) patterns in the arch. WSS profiles computed using realistic and idealized aortic root velocities differed significantly. An unexpected finding was that average WSS in the high-lesion-probability region on the inner wall was actually higher than the WSS in the low-probability region on the outer wall. Future studies of mouse aortic arch haemodynamics should avoid the use of idealized inflow velocity profiles. Lesion formation does not seem to uniquely associate with low or oscillating WSS in this segment, suggesting that other factors may also play a role in lesion localization.     

Effect of the aerodynamic structure of a flow on the hydraulic characteristics of the diffuser of a flat-flame injector torch

It is experimentally established that the admission of a twisted flow into a diffuser makes it possible to increase the expansion angle of the flow and reduce the dimensions of the torch.Notation n twist parameter of gas-air flow - P increment in total pressure in diffuser - dif impact efficiency - w₁ flow velocity at diffuser inlet - w₂ flow velocity at diffuser outlet - diffuser divergence angle - drag coefficient Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 42, No. 1, pp. 40–42, January, 1982.     

Exploring the influence of particle shape and air velocity on the flowability in the respiratory tract: a computational fluid dynamics approach

Dry powder inhalers (DPIs) are considered a main drug delivery system through pulmonary route. The main objective of this work is to study the flow of differently shaped microparticles in order to find the optimum shape of drug particles that will demonstrate the best flow to the deep lung. The flowability of particles in air or any fluid depends particularly on the drag force which is defined as the resistance of the fluid molecules to the particle flow. One of the most important parameters that affect the drag force is the particles’ shape. Computational simulations using COMSOL Multi Physics 5.2 software were performed for investigating the particles flow in the air pathways of lung, and the drag force was calculated for different particles shapes. This was accomplished by screening a set of 17 possible shapes that are expected to be synthesized easily in the micro-scale. In addition, the macro-scale behavior of the investigated shapes was also simulated so as to compare the behavior of the flowing particles in both cases. A very big difference was found between the behavior of particles’ flow in the micro and macro scales, but a similar behavior can be obtained if the flow velocity of the microparticles is very high. It was also found that the micro-triangle with aspect ratio 2:1 has the least drag force in both deep and upper lung; so, it should be the shape of choice during the process of particle synthesis for pulmonary drug delivery.     

Flows of inelastic non-Newtonian fluids through arrays of aligned cylinders. Part 1. Creeping flows

Numerical simulations are presented for flows of inelastic non-Newtonian fluids through periodic arrays of aligned cylinders, for cases in which fluid inertia can be neglected. The truncated power-law fluid model is used to define the relationship between the viscous stress and the rate-of-strain tensor. The flow is shown to be dominated by shear effects, not extension. Numerical simulation results are presented for the drag coefficient of the cylinders and the velocity variance components, and are compared against asymptotically valid analytical results. Square and hexagonal arrays are considered, both for crossflow in the plane perpendicular to the alignment vector of the cylinders (flows along the axes of the array as well as off-axis flows), and for flow along the cylinders. It is shown that the observed strong dependence of the drag coefficient on the power-law index (through which the stress tensor is related to the rate-of-strain tensor) can be described at all solid area fractions by scaling the drag on a cylinder with appropriate velocity and length scales. The velocity variance components show only a weak dependence on the power-law index. The results for steady-state drag and velocity variances are used in an approximate theory for flows accelerated from rest. The numerical simulation data for unsteady flows agree very well with the approximate theory.     

Effect of rotor cage's outer and inner radii on the inner flow field of the turbo air classifier

The effects of rotor cage's outer and inner radii on flow field of the turbo air classifier are comparatively analyzed by numerical simulation using ANSYS-FLUENT. The results of quantitative analysis show when the rotor cage's outer and inner radii are increased, the tangential velocity, radial velocity and upward axial velocity decrease in the annular region and near the entrance of the rotor cage. However, when the rotor cage's outer and inner radii are too large or too small, the tangential velocity and radial velocity will be fluctuated greatly. Moreover, the rotor cage's outer and inner radii directly influence the radial velocity distribution in the rotor cage channel. The rotor cage's outer and inner radii should not be too large or too small. Therefore, in the seven contrast rotor cage models, model 100–70 and 90–60 are selected to carry out the calcium carbonate classification experiments due to their small tangential velocity and radial velocity fluctuations and well-distribution in the rotor cage channel. The experimental results reflect the characteristics of the numerically simulated flow field in the classifier.     

Slip flow on a body of revolution

This work investigates the boundary layer development and resulting net frictional drag along a general blunt-nosed body of revolution in uniform slip flow. The curvilinear boundary layer equations are subjected to a Navier slip condition near the surface. For motion near the nose, a similarity-type solution is obtained as a double series, comprising a small velocity slip parameter, from which the non-dimensional drag coefficient is obtained. In addition, the displacement effect of the boundary layer on the outer inviscid flow is determined and interpreted by means of a dimensionless parameter. The general analysis is then applied to the special case of the Rankine half-body. Considering laminar flow along the nose and downstream regions, results are presented for prescribed slip parameters and Reynolds numbers with particular emphasis on the influence of slip on the flow.     

Numerical Simulation of Particle Saltation Process

A numerical model for simulation of particle single-step saltation was developed. The model includes drag force, shear lift force, rotational lift force, buoyancy force, added mass force, and torque. The governing equations were solved using the fourth-order Runge-Kutta scheme. The model was calibrated and verified using the experimental data. The computational results include particle trajectory, longitudinal velocity of particle and flow, relative velocity of particle and flow, dimensionless drag, and lift forces along the trajectory. Sensivity analysis was performed to determine the influence of various parameters. Saltation characteristics were also calculated for various Reynolds numbers in the range of 2.5 to 7.7. It was found that very close to the bed, drag force decreases as Reynolds number increases. An increase of about three times the Reynolds number has a decreasing effect of three times and two times on the drag and lift force, respectively. The influence of Reynolds number increase on the falling phase was less than that on the rising phase.     

On the mechanism of a helical motion of fluids in regions of sharp path bending

The pattern of flow in the inner and outer streams of a liquid or gas in regions of a sharp path bending at a right angle is considered. It is shown that a high hydraulic drag makes it energetically favorable for the fluid to perform a twisted helical motion. Such a regime is spontaneously established under real conditions. An example of this hydraulic factor is offered by an atmospheric cyclone.     

Non-uniform slot injection into a laminar boundary layer

Summary Slot injection into a laminar boundary layer in both supersonic and subsonic flow is considered. The blowing rates are sufficiently large to provoke an interaction between the boundary layer and outer inviscid flow, and this interaction is accounted for by triple-deck theory. The non-uniform nature of the blowing velocity models the channel flow from which the injection takes place.     

Influence of decelerating flow on incipient motion of a gravel-bed stream

An experimental study on incipient motion of gravel-bed streams under steady-decelerating flow is presented. Experiments were carried out in a flume with two median grain sizes, d ₅₀ = 16.7 mm for a fixed-bed case and d ₅₀ = 8 mm for a mobile bed case. In addition, an effort is made to determine a simplified method for the estimation of bed shear stress in decelerating flow over fixed and mobile beds for use in field situations. From the observation of eleven fixed-bed and nine mobile-bed velocity profiles, it is revealed that the parabolic law method (PLM) and the Reynolds stress method are comparable for estimation of shear velocity in general. Also, the results show that the shear stress distribution adopts a convex form over fixed and mobile beds. Due to this form the critical Shields parameter value for decelerating flow is less than the reported values in literature. This paper supports Buffington & Montgomery (1997) statement that less emphasis should be given on choosing a universal shields parameter, and more emphasis should be given on choosing defendable values based on flow structure.     

A Method of Body Shape Optimization for Decreasing the Aerodynamic Drag Force in Gas Flow

Aerodynamic flow past bodies of various geometrical shapes was studied, and the aerodynamic drag force was reduced through optimization of the body shape using a specially proposed method. The resulting drag force was compared to that for bodies formed by revolution of the profiles of well-known standard series. The study was performed using the Ansys Fluent software for isothermal laminar steady-state flows of incompressible fluid with constant density in a velocity range of 0–10 m/s. It is shown that the aerodynamic drag force for a body with the optimized shape is lower than analogous values for the bodies of revolution with Su-26 and NASA-0006 reference profiles. In comparison to the aerodynamic-drag-force level of 100% for the body of revolution with NASA-0006 profile, the drag force for Su-26 profile at airflow velocity of 10 m/s is 89.4%, while that for the proposed optimized body shape is 89.2%.     

Hydrodynamics and heat transfer in pulsating turbulent flow of gas in a heated pipe

The paper deals with the investigation of the effect produced by the dependence of the physical properties on temperature and flow rate fluctuations on heat transfer and drag under conditions of turbulent pipe flow of gas. The method of finite differences is used to solve numerically a set of equations of motion, continuity, and energy written in a narrow channel approximation. A model of turbulence is used which takes into account the effect of the variability of the properties and of the nonstationarity of flow on turbulent transfer. In the particular case of steady-state flow of gas being heated, the calculation results fit well the available experimental data. It is found that the heat transfer depends on the heating rate more significantly than the friction drag. In the case of pulsating flow, the part of hydraulic drag is estimated which is spent for the variation of longitudinal velocity along the pipe and is due to the thermal acceleration of gas. It is demonstrated that the main features of pulsating flow, which were previously investigated for a liquid of constant properties and for a dropping liquid of variable viscosity, are retained for the gas being heated as well. Comparison is made for the gas and dropping liquid of the effect made by various process parameters such as the Reynolds, Stokes, and Prandtl numbers, the heating rate, and the form of thermal condition on the wall on the period average Nusselt number and coefficient of friction drag.     

雷诺数对沟槽减阻特性影响的数值分析

采用雷诺平均N—S方程和RNG κ-ε湍流模型计算V型沟槽面的湍流边界层流动和黏性阻力，通过改变来流速度大小和沟槽面布置位置，研究了雷诺数对沟槽减阻特性的影响规律。计算结果表明，来流速度对沟槽减阻率的影响很大，对于一种尺度的V型沟槽，存在着一个具有较好减阻效果的来流速度范围，最大减阻率可迭8．6％；沟槽面在沿来流方向上的布置位置对其减阻效果的影响则非常小。     

Improvement of the efficiency of diffuser flow by means of regular vortices

The effect of the positive action of localized vortex formations on the flow in separation diffusers is considered. The analysis uses the mathematical model of an elementary vortex diffuser based on the Navier-Stokes equations for an incompressible, viscous liquid with effective turbulent viscosity. The finitedifference method of fractional steps, amounting to trial runs, is used to obtain the solution. Calculations of the flow in a flat channel with a step widening, where ribs of various length are mounted in the direction across the flow, are performed. The drag and recovery coefficients as functions of the rib length and location are determined.Deceased.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 58, No. 5, pp. 737–742, May, 1990.     

Approximate simulation of storm water runoff over pervious pavement

The approximate simulation of storm water run-off over pervious pavement is carried out using both experimental method and numerical simulation. The slope and flow rate changeable flume and uniform porous media are used to approximately simulate the pervious pavement. A 3D computational fluid dynamics – discrete element method is used for numerical simulation of interaction between pervious pavement and fluid. The effects of variation of parameters, including inflow rate, infiltration outflow rate and slope on surface run-off are analysed. The average flow velocity within the surface run-off region and shear velocity increases with the increasing permeability of pervious pavement. The turbulent kinetic energy distribution along depth in the free-flow region is more uniform than empirical relationship for flow over impermeable surfaces. Equations for flow depth and velocity over pervious pavement have been deduced. The results of this study are helpful for the hydraulic design of pervious concrete pavement.     

小麦颗粒弯管流动特性数值模拟研究

目的 为研究小麦颗粒在弯管处的气力输送的特性。方法 以欧拉-欧拉双流体模型为基础,结合壁面碰撞摩擦模型、颗粒动理学的固体应力和Gidaspow曳力模型构建出小麦颗粒在弯管处的气力输送模型,采用FLUENT对弯管处小麦颗粒气力输送过程进行数值模拟,分析小麦颗粒在流经弯管过程中及弯管后直管中的小麦颗粒密度分布、气固两相速度、小麦颗粒与壁面剪切力和颗粒相湍动能。结果 经过仿真分析和实验验证,小麦颗粒在流经弯管过程中,其颗粒相体积分数、气固两相速度、颗粒和壁面剪切力以及颗粒相湍动能4个方面随着流入弯管的角度变化而改变;由于颗粒-颗粒、颗粒-管壁之间的碰撞摩擦,小麦颗粒在流出弯管后随着输送距离的增大其各项参数逐渐减缓。结论 采用FLUENT软件进行仿真得到了弯管内小麦颗粒的流动特性,并通过实验验证了仿真的可靠性。此次研究结合气固两相理论,为弯管气力输送设计的研发和优化提供了理论基础。     

Novel ejector model for performance evaluation on both dry and wet vapors ejectors

A novel ejector model is proposed for the performance evaluation on ejectors with both dry and wet vapor working fluids at critical operating mode. A simple linear function is defined in order to approach the real velocity distribution inside the ejector. Mass flow rates of the primary flow and secondary flow are derived by integrating the velocity function at the inlet section of the mixing chamber. By considering the flow characteristics of the critical-mode operating ejector, the developed model contains only one energy conservation equation and is independent of the flow in the mixing chamber and the diffuser. Experimental data from different ejector geometries and various operation conditions reported earlier are used to verify the effectiveness of the new model. Results show that the model has a good performance in predicting the mass flow rates and the entrainment ratio for both dry and wet vapor ejectors.