An Approach to Parallel Computing in an Eulerian-Lagrangian Two-Phase Flow Model

An approach to parallel solution of an Eulerian-Lagrangian model of dilute gas-solid flows is presented. Using Lagrangian treatments for the dispersed phase, one of the principal computational challenges arises in models in which inter-particle interactions are taken into account. Deterministic treatment of particle-particle collisions in the present work pose the most computationally intensive aspect of the simulation. Simple searches lead to algorithms whose cost is O(N2p) where Np is the particle population. The approach developed in the current effort is based on localizing collision detection neighborhoods using a cell-index method and spatially distributing those neighborhoods for parallel solution. The method is evaluated using simulations of the gas-solid turbulent flow in a vertical channel. The instantaneous position and the velocity of any particle is obtained by solving the equation of motion for a small rigid sphere assuming that the resulting force induced by the fluid reduces to the drag contribution. Binary particle collisions without energy dissipation or inter-particle friction are considered. The carrier flow is computed using Large Eddy Simulation of the incompressible Navier-Stokes equations. The entire dispersed-phase population is partitioned via static spatial decomposition of the domain to maximize parallel efficiency. Simulations on small numbers of distributed memory processors show linear speedup in processing of the collision detection step and nearly optimal reductions in simulation time for the entire solution.

Computational Aerodynamics of a Winston-Cup-Series Race Car

Since the goal of racing is to win and since drag is a force that the vehicle must overcome, a thorough understanding of the drag generating airflow around and through a race car is greatly desired. The external airflow contributes to most of the drag that a car experiences and most of the downforce the vehicle produces. Therefore, an estimate of the vehicle’s performance may be evaluated using a computational fluid dynamics model. First, a computer model of the race car was created from the measurements of the car obtained by using a laser triangulation system. After a computer-aided drafting model of the actual car was developed, the model was simplified by removing the tires, roof strakes, and modification of the spoiler. A mesh refinement study was performed by exploring five cases with different mesh densities. By monitoring the convergence of the computed drag coefficient, the case with 2 million elements was selected as being the baseline case. Results included flow visualization of the pressure and velocity fields and the wake in the form of streamlines and vector plots. Quantitative results included lift and drag, and the body surface pressure distribution to determine the centerline pressure coefficient. When compared with the experimental results, the computed drag forces were comparable but expectedly lower than the experimental data mainly attributable to the differences between the present model and the actual car.

A Remote Area Power Supply Using Wind Power and Cold Thermal Storage

A remote area power supply using cold thermal storage and wind as the energy source is proposed. The primary objective is to provide a renewable energy remote area power supply with cheaper and more robust storage than lead-acid batteries. The proposal amalgamates a vapour compression refrigeration system with a Rankine cycle engine, both using the same working fluid. A tank of freezing brine acts as the condenser in the Rankine cycle and as the evaporator in the refrigeration cycle but also provides the “energy storage”. Analysis of the system indicates that it is practical and that its performance is comparable with existing battery based systems.

Stochastic Estimation and Non-Linear Wall-Pressure Sources in a Separating/Reattaching Flow

Simultaneous wall-pressure and PIV measurements are used to study the conditional flow field associated with surface-pressure generation in a separating/reattaching flow established over a fence-with-splitter-plate geometry. The conditional flow field is captured using linear and quadratic stochastic estimation based on the occurrence of positive and negative pressure events in the vicinity of the mean reattachment location. The results shed light on the dominant flow structures associated with significant wall-pressure generation. Furthermore, analysis based on the individual terms in the stochastic estimation expansion shows that both the linear and non-linear flow sources of the coherent (conditional) velocity field are equally important contributors to the generation of the conditional surface pressure.

Unsteady Flow Structure on a Centrifugal Pump: Experimental and Numerical Approaches

Both experimental and numerical studies of the unsteady pressure field inside a centrifugal pump have been carried out. The unsteady patterns found for the pressure fluctuations are compared and a further and more detailed flow study from the numerical model developed will be presented in this paper. Measurements were carried out with pressure transducers installed on the volute shroud. At the same time, the unsteady pressure field inside the volute of a centrifugal pump has been numerically modelled using a finite volume commercial code and the dynamic variables obtained have been compared with the experimental data available. In particular, the amplitude of the fluctuating pressure field in the shroud side wall of the volute at the blade passing frequency is successfully captured by the model for a wide range of operating flow rates. Once the developed numerical model has shown its capability in describing the unsteady patterns experimentally measured, an explanation for such patterns is searched. Moreover, the possibilities of the numerical model can be extended to other sections (besides the shroud wall of the volute), which can provide plausible explanations for the dynamic interaction effects between the flow at the impeller exit and the volute tongue at different axial positions. The results of the numerical simulation are focused in the blade passing frequency in order to study the relative effect of the two main phenomena occurring at that frequency for a given position: the blade passing in front of the tongue and the wakes of the blades.

Wall Heat Flux Under Persistent Vortices

It is commonly perceived that turbulent flows yield turbulent wall fluxes, while laminar flows yield correspondingly laminar wall fluxes. Experiments support a recent theory that turbulent flows can yield laminar wall fluxes if the flow is “persistent.” Adding strong, stationary vortices to a turbulent boundary layer lowers the wall heat flux to a laminar value.

Periodic Velocity Measurements in a Wide and Large Radius Ratio Automotive Torque Converter at the Pump/Turbine Interface

The unsteady flow field due to blade passing at the pump/turbine interface of a torque converter was studied. The current geometry is wide and has a large outer to inner radius ratio. A laser velocimeter was used to measure the periodic velocity components at four operating conditions determined by the speed ratios between the turbine and pump of 0.065 (near stall), 0.600, 0.800, and 0.875 (coupling point). The flow fields at the pump exit and turbine inlet planes were visualized and are presented. Using instantaneous pump and turbine blade positions with the velocity data, animations (“slow-motion movies”) are generated to effectively visualize and understand the unsteady behavior. The turbine inlet flow was markedly periodic due to the exiting jet/wake from the upstream pump passage; however, the pump exit flow field showed little dependence on the turbine blade positions. The highest unsteadiness was seen for the highest speed ratios. Four “shots” from the sequences of one cycle for all speed ratios and each plane are presented herein. The results are also compared to unsteady results for a previously examined torque converter with a small radius ratio to determine the effect of parametric geometric changes on the flow field. Generally, the unsteady velocity fields show no significant difference for the two geometries — the trends are the same.

A Large-Eddy Simulation of Bubble-Liquid Jets

A Large-Eddy Simulation (LES) with a two-way coupling is used to study bubble-liquid two-phase confined jets in a two-dimensional channel. The results show the large-eddy vortex structures of both liquid flow and bubble motion, the shear-generated and bubble-induced liquid turbulence. For comparison, the second-order moment (SOM) modeling was also carried out for the same case. Both LES and SOM results indicate much stronger bubble fluctuation than the liquid fluctuation, the enhancement of liquid turbulence by bubbles even for the higher velocity case. Both shear production and the production due to bubble-liquid interaction are important for the liquid turbulence generation in the case studied. The LES statistical results and the SOM simulation results are in qualitative agreement with each other.

Meshing Method for Transient Computation With Complex Geometry in CFD

The description of mesh evolution during a transient computation with moving walls and mesh adaptation has to respect many rules. Using a good mesh for each computational time step is important for accuracy of results. The complexity of geometry can make this objective more complex. A method has been developed to obtain a good moving mesh description with complex boundary geometry. It is based on a local observation of boundary movement and can be resumed by two main ideas: • Add cells where the volume of solution domain increases. • Slide the mesh where the boundary has a tangential displacement.

Computation of Axial Flow Compressor to Intake Flow Distortion

The effects of the parameters of inlet distortions on the trend of downstream flow feature in axial compressor are simulated using an integral method. Other than the ratio of drag-to-lift coefficients of the blade and the angle of incidence, the value of distorted inlet velocity is found to be another essential parameter to control the distortion propagation. With this in mind, a distortion propagation line and corresponding distortion propagation factor are proposed to express the effect of the two main inlet parameters: the angle of incidence and the distorted inlet velocity, on the propagation of distortion. From the viewpoint of compressor efficiency, the distortion propagation is further described by a compressor critical performance. The results provide a physical insight of compressor axial behavior and asymptotic behavior of the propagation of inlet distortion, and confirm the active role of compressor in determining the velocity distribution when compressor responds to an intake flow distortion.