Computational Fluid Simulation Research Articles

State-of-the-art and needs for jet instability and direct contact condensation model improvements

There is a common understanding among thermal-hydraulic experts that the system analysis codes have currently reached an acceptable degree of maturity. Reliable application, however, is still limited to the validated domain. There is a growing need for qualified codes in assessing the safety of the existing reactors and for developing advanced reactor systems. Under conditions involving multi-phase flow simulations, the use of classical methods, mainly based upon the one dimensional approach, is not appropriate at all. The use of new computational models, such as the direct numerical simulation, large-eddy simulation or other advanced computational fluid dynamics methods, seems to be more suitable for more complex events. For this purpose, the European Commission financed NURESIM Integrated Project (as a part of the FP6 programme), was adopted to provide the initial step towards a Common European Standard Software Platform for Modeling, recording and recovering computer data for nuclear reactor simulations. Some of the studies carried out at the University of Pisa within the framework of the NURESIM project are presented in this paper. They mainly concern the investigation of two critical phenomena connected with jet instabilities and direct contact condensation that occur during emergency core cooling. Through these examples, the state-of-the-art and the need for model improvements and validation against new experimental data for the sake of getting a better understanding and more accurate predictions are discussed.

Numerical and Experimental Analysis of an Axial Flow Left Ventricular Assist Device: The Influence of the Diffuser on Overall Pump Performance

Thousands of adult cardiac failure patients may benefit from the availability of an effective, long-term ventricular assist device (VAD). We have developed a fully implantable, axial flow VAD (LEV-VAD) with a magnetically levitated impeller as a viable option for these patients. This pump's streamlined and unobstructed blood flow path provides its unique design and facilitates continuous washing of all surfaces contacting blood. One internal fluid contacting region, the diffuser, is extremely important to the pump's ability to produce adequate pressure but is challenging to manufacture, depending on the complex blade geometries. This study examines the influence of the diffuser on the overall LEV-VAD performance. A combination of theoretical analyses, computational fluid (CFD) simulations, and experimental testing was performed for three different diffuser models: six-bladed, three-bladed, and no-blade configuration. The diffuser configurations were computationally and experimentally investigated for flow rates of 2-10 L/min at rotational speeds of 5000-8000 rpm. For these operating conditions, CFD simulations predicted the LEV-VAD to deliver physiologic pressures with hydraulic efficiencies of 15-32%. These numerical performance results generally agreed within 10% of the experimental measurements over the entire range of rotational speeds tested. Maximum scalar stress levels were estimated to be 450 Pa for 6 L/min at 8000 rpm along the blade tip surface of the impeller. Streakline analysis demonstrated maximum fluid residence times of 200 ms with a majority of particles exiting the pump in 80 ms. Axial fluid forces remained well within counter force generation capabilities of the magnetic suspension design. The no-bladed configuration generated an unacceptable hydraulic performance. The six-diffuser-blade model produced a flow rate of 6 L/min against 100 mm Hg for 6000 rpm rotational speed, while the three-diffuser-blade model produced the same flow rate and pressure rise for a rotational speed of 6500 rpm. The three-bladed diffuser configuration was selected over the six-bladed, requiring only an incremental adjustment in revolution per minute to compensate for and ease manufacturing constraints. The acceptable results of the computational simulations and experimental testing encourage final prototype manufacturing for acute and chronic animal studies.

Modeling ventilation and dispersion for covered roadways

A series of computational fluid simulations was conducted to assess the relative impacts of several forced ventilation configurations on the distribution of atmospheric pollution concentrations from vehicles operating within a covered roadway. These conditions are frequently encountered in urban highways, airports, parking structures and so forth. Ventilation configurations tested included ceiling curtains and jets. A simple 2D geometry without artificial ventilation was used as a base case together with a worst-case flow scenario that assumed no transverse ambient wind flow. The base-case simulations were compared to simulations for various auxiliary ventilation alternatives. The simulations demonstrate that ventilation is necessary to effectively minimize peak ambient concentrations in covered roadways. Auxiliary overhead ventilation ports are found to result in the most dramatic improvement in pollutant concentrations. The simulations also indicate that regions of low velocity and vortex-type flow patterns with flow reversals are characteristic of situations with very high values of peak pollutant concentrations.

Inverse Transonic Airfoil Design Using Parallel Simulated Annealing and Computational Fluid Dynamics

Inverse Transonic Airfoil Design Using Parallel Simulated Annealing and Computational Fluid Dynamics

Inverse transonic airfoil design using parallel simulated annealing and computational fluid dynamics

Inverse transonic airfoil design using parallel simulated annealing and computational fluid dynamics

Investigation of Large Eddy Simulation Code Scaling Performance and Network Type Influence on a Linux PC Cluster

The parallel performance of a large eddy simulation computational fluid dynamics code was evaluated on a Pentium III PC cluster with 64 processors running under Linux. Scaling capability of the code and comparative code performance under Fast Ethernet, Myrinet, and Gigabit Ethernet were investigated. The code shows very good scalability for a scaled speedup test with wall clock time remaining practically constant for any number of processors. The code does not saturate the Fast Ethernet bandwidth. Whereas Myrinet demonstrates a lower difference between CPU and wall clock time, indicating more efficient interprocessor communication, its overall wall clock time is equal to or higher than for the code compiled for Fast Ethernet. Gigabit Ethernet shows a very slight advantage over Fast Ethernet.

BUOYANT FLOW GENERATED BY THERMAL CONVECTION OF A SIMULATED PIG

The effects of internal occupants and supplement heating make up an essential issue for the prediction andcontrol of fresh ventilating air distribution in an enclosure. The influence from livestock is complex, since they are mobileobstacles, producing heat and contaminants in irregular geometry. As a part of the basic studies of these influences, theinvestigations of air motion in a thermal buoyant flow caused by free convection around a livestock body are reported inthis article. A simulated pig, made of a painted metal tube (1 m long and 0.5 m in diameter) with covered ends and heatelements inside, was used as the heat source in the experiments. The experiments were carried out in a full-scale room, 5m 11 m in floor area, with a 2.4 m side wall height, and sloped ceiling to center (height: 4.8 m). The simulated pig wasplaced near the center of the floor. The vertical temperature difference in the room space was less than 0.3C. Thevelocity and temperature in the thermal plume were measured with six sensors (each has both temperature and velocityelements) placed at 0.2 m horizontal intervals. Data were acquired at 14 levels from 0.2 to 2.4 m above the top surface ofthe simulated pig. The data-sampling period was 30 min in steady state for each measurement.The results show that the plume was quite thin at the beginning (yd 0.6 m) in the central radial plane of the model.Observations showed that the laminar flow at the beginning remained for some distance before it became turbulent andspread. When the distance from the top of the model increased (yd 0.8 m), the temperature and velocity profile of the jetfit Gaussian distributions. The temperature profiles were slightly wider than the velocity profiles. Numerical simulations(Computational Fluid Dynamics) were applied for the same experimental set-up in computing the airflow over the pigsimulator. Transient simulation with fixed time stepping provided similar results to the measurements, indicating the CFDsimulation method used in the study has potential for prediction of buoyant flow generated by this type heat source.

DENISIS — A three-dimensional partially ionized dusty magnetoplasma code

The highly involved macroscopic low-frequency dynamics of partially ionized dusty plasmas can be studied by means of computational fluid simulations. We derived a general set of numerically tractable balance equations taking into account mass, momentum and energy conservation. On the grounds of these equations we developed a quasi-neutral compressible three-dimensional multi-fluid code that integrates the closed set of equations for the electron, ion, dust and neutral components. The integration scheme of this explicit code (DENISIS = Dust Electron Neutral Ion Self-consistent Integration Scheme) is based on the Leapfrog and Dufort-Frankel method modified by a numerically damped treatment of the nonconservative terms in the balance equations. The applicability of the code is illustrated by modelling characteristic plasma phenomena as wave and instability processes.