Steady-state Computational Fluid Dynamics Research Articles

Computational fluid dynamics analysis of a twisted three-bladed H-Darrieus rotor

A steady-state two-dimensional computational fluid dynamics analysis was performed using FLUENT 6.2 software to analyze the performance of a twisted three-bladed H-Darrieus rotor. The flow over the rotor was simulated by using unstructured-mesh finite volume method coupled with moving mesh technique to solve mass and momentum conservation equations. The standard k-ε turbulence model was chosen. Second-order upwind discretization scheme was adopted for pressure-velocity coupling of the flow. The aerodynamic coefficients, such as lift coefficient, drag coefficient, and lift-to-drag coefficient, were evaluated with respect to angle of attack for two chord Reynolds numbers. The power coefficient of the rotor was also evaluated. The results were validated by using experimental values for the twisted three-bladed H-Darrieus rotor. The experiments were earlier conducted in a subsonic wind tunnel available in the department. The results showed good matching between the two approaches. The effect of twist angle at the chord ends on the performance of the rotor was also evaluated.

Preliminary design of a university laboratory hybrid rocket engine

This paper gives an overview of the preliminary design of a small hybrid rocket engine (HRE) that is being built in support of propulsion research activities at Ryerson University. The initial design work was undertaken primarily with a paraffin and gaseous oxygen (GOx) propellant combination in mind, but the system is readily adaptable to other propellant combinations for future investigations. When a reusable, non-eroding nozzle is necessary for accurate engine performance measurements, a water-cooled copper design is to be employed. This approach offers the added benefit of allowing the cooling water temperature rise to be measured, and thus the amount of heat being handled by the nozzle can be quantified. The preliminary nozzle design is supported by a steady-state computational fluid dynamics (CFD) analysis of the nozzle flows and associated heat fluxes. The overall engine design is further evaluated by examination of internal ballistic simulation results with respect to such factors as expected performance (chamber pressure, thrust, specific impulse, and total impulse) for a given oxidizer mass flow rate and nozzle throat size.

Harmonic Balance Analysis of Blade Row Interactions in a Transonic Compressor

In this paper we apply the harmonic balance technique to analyze an inlet guide vane and rotor interaction problem, and compare the computed flow solutions to existing experimental data. The computed results, which compare well with the experimental data, demonstrate that the technique can accurately and efficiently model strongly nonlinear periodic flows, including shock/vane interaction and unsteady shock motion. Using the harmonic balance approach, each blade row is modeled using a computational grid spanning just a single blade passage regardless of the actual blade counts. For each blade row, several subtime level solutions that span a single time period are stored. These subtime level solutions are related to each other through the time derivative term in the Euler (or Navier―Stokes) equations, which is approximated by a pseudo-spectral operator, by complex periodicity conditions along the periodic boundary of each blade row's computational domain, and by the interface boundary conditions between the vane and rotor. Casting the governing equations in harmonic balance form removes the explicit dependence on time. Mathematically, the equations to be solved are similar in form to the steady Euler (or Navier―Stokes) equations with an additional source term proportional to the fundamental frequency of the unsteadiness. Thus, conventional steady-state computational fluid dynamics techniques, including local time stepping and multigrid acceleration, are used to accelerate convergence, resulting in a very efficient unsteady flow solver.

Swimming Propulsion Forces Are Enhanced by a Small Finger Spread

The main aim of this study was to investigate the effect of finger spread on the propulsive force production in swimming using computational fluid dynamics. Computer tomography scans of an Olympic swimmer hand were conducted. This procedure involved three models of the hand with differing finger spreads: fingers closed together (no spread), fingers with a small (0.32 cm) spread, and fingers with large (0.64 cm) spread. Steady-state computational fluid dynamics analyses were performed using the Fluent code. The measured forces on the hand models were decomposed into drag and lift coefficients. For hand models, angles of attack of 0 degrees, 15 degrees, 30 degrees, 45 degrees, 60 degrees, 75 degrees, and 90 degrees, with a sweep back angle of 0 degrees, were used for the calculations. The results showed that the model with a small spread between fingers presented higher values of drag coefficient than did the models with fingers closed and fingers with a large spread. One can note that the drag coefficient presented the highest values for an attack angle of 90 degrees in the three hand models. The lift coefficient resembled a sinusoidal curve across the attack angle. The values for the lift coefficient presented few differences among the three models, for a given attack angle. These results suggested that fingers slightly spread could allow the hand to create more propulsive force during swimming.

Assessment of Computational Fluid Dynamics for Supersonic Shock Containing Jets

This paper describes a combined experimental and computational investigation of supersonic jets operating off design. Axisymmetric steady-state computational fluid dynamics simulations were performed with a Reynolds-averaged Navier―Stokes solver using a two-equation turbulence model. These results were compared directly with experimental results for two differen t axisymmetric nozzles operating at over- and underexpanded conditions. Pitot and static pressure probes were used to measure the Mach number and local static pressure at various downstream locations. A comparison of both experimental and numerically predicted schlieren visualizations were performed for a qualitative assessment of the accuracy of the flowfield predictions. Quantitative comparisons of pitot and static pressures were made between the experiments and simulations. This provides an assessment of the quality of the numerical simulations as well as an indication of the errors associated with any interference between the pressure probes and the flow in the experiments.

Swept-Leading-Edge Pylon Effects on a Scramjet Pylon-Cavity Flameholder Flowfield

This study explores the effect of adding a pylon to the leading edge of a cavity flameholder in a scramjet combustor. Data were obtained through a combination of wind-tunnel experimentation and steady-state computational fluid dynamics. Wind-tunnel data were collected using surface pressure taps, static and total probe data, shadowgraph flow visualization, and particle image velocimetry. Computational fluid dynamics models were solved using the commercial FLUENT software. The addition of an intrusive device to the otherwise low-drag cavity flamebolder offers a potential means of improving combustor performance by enabling combustion products to propagate into the main combustor flow via the low-pressure region behind the pylon. This study characterized the flowfield effects of adding the pylon as well as the effect of changing Reynolds numbers over the range of approximately 33 x 10 6 to 55 × 10 6 m ―1 at a Mach number of 2. The addition of the pylon resulted in approximately 3 times the mass flow passing through the cavity compared with the cavity with no pylon installed. Reynolds number effects were weak. The addition of the pylon led to the cavity fluid traveling up to the top of the pylon wake and significantly increasing the exposure and exchange of cavity fluid with the main combustor flow.

Compartmental Models for Continuous Flow Reactors Derived from CFD Simulations

Reactor modeling is of major interest in environmental technology. In this context, new contaminants with higher degradation requirements increase the importance of reactor hydraulics. CFD (Computational Fluid Dynamics) may meet this challenge but is expensive for everyday use. In this paper, we provide research and practice with a methodology designed to automatically reduce the complexity of such a high-dimensional flow model to a compartmental model. The derivation is based on the concentration field of a reacting species which is included in the steady state CFD simulation. While still capturing the most important flow features, the compartmental model is fast, easy to use, and open for process modeling with yet unknown compounds. The inherent overestimation of diffusion by compartmental models has been corrected by locally adjusting turbulent fluxes. We successfully applied the methodology to the ozonation process and experimentally verified it with tracer experiments. The loss of information was quantified as a deviation from CFD performance prediction for different reactions. With increasing discretisation of the compartmental model, these deviations diminish. General advice on the necessary discretisation is given.

Investigation of Heat Transfer and Pressure Drop of an Impinging Jet in a Cross-Flow for Cooling of a Heated Cube

The objective of this study is to investigate the thermal performance and the cost measured in pressure drops of a targeted cooling system with use of an impinging jet in combination with a low-velocity channel flow on a heated wall-mounted cube. The effects of the Reynolds numbers of the impinging jet and the cross-flow, as well as the distance between the top and bottom plates, are investigated. A steady-state 3D computational fluid dynamics model was developed with use of a Reynolds stress model as turbulence model. The geometrical case is a channel with a heated cube in the middle of the base plate and two inlets, one horizontal channel flow and one vertical impinging jet. The numerical model was validated against experimental data with a similar geometrical setup. The velocity field was measured by particle image velocimetry and the surface temperature was measured by an infrared imaging system. This case results in a very complex flow structure where several flow-related phenomena influence the heat transfer rate and the pressure drops. The average heat transfer coefficients on each side of the cube and the pressure loss coefficients are presented; correlations for the average heat transfer coefficient on the cube and the pressure loss coefficients are created.

Study of the Propulsive Potential of the Hand and Forearm in Swimming

Computational Fluid Dynamics (CFD) has been used in biomechanics studies applied to medicine and sport. Concerning sports, the main results suggested that a three dimensional CFD analysis of a human form could provide useful information about swimming. PURPOSE: To analyze the propulsive force in a swimmer hand/forearm 3-D segment using CFD. METHODS: A 3-D domain was created to simulate the fluid flow around a model of a swimmer hand and forearm. Models were created by computerized tomography scans of a male swimmer. Steady-state computational fluid dynamics analyses were performed. Flow velocities were chosen to be within or near the range of typical hand velocities during the swimming underwater path: from 0.5 m/s to 4.0 m/s. Angles of attack of hand/forearm models of 0°, 45° and 90°, with sweepback angles of 0°, 90°, 180° and 270° were used for the calculations. The combined hand and forearm forces were decomposed into drag (CD) and lift (CL) coefficients. RESULTS: In table 1 the values of CD and CL obtained for the different angles of attack and sweepback angles are presented. Both CD and CL remained constant throughout the flow velocities tested.Table 1: CD and CL values for the different angles of attack and sweepback angles (V=2.0 m/s).CONCLUSIONS: The CD was the main responsible for the hand/forearm propulsion, with a maximum value at an angle of attack of 90°. The CL seems to play an important role at an angle of attack of 45°, especially when the little finger leads the motion (sweepback angle = 180°). These data confirm recent studies reporting supremacy of drag component and an important contribution of the lift force to the overall propulsive force generation by the hand/forearm in swimming phases, when the angle of attack nears 45°. Supported by FCT (SFRH/BD/25241/2005; POCTI/10/58872/2004).

Numerical evaluation of pollutant dispersion in the built environment: Comparisons between models and experiments

Steady-state RANS Computational Fluid Dynamics (CFD) simulations of pollutant dispersion in the neutrally stable atmospheric boundary layer are made with the commercial code Fluent 6.1 for three case studies: plume dispersion from an isolated stack, low-momentum exhaust from a rooftop vent on an isolated cubic building model and high-momentum exhaust from a rooftop stack on a low-rise rectangular building with several rooftop structures. The results are compared with the Gaussian model, the semi-empirical ASHRAE model and wind tunnel and full-scale measurements. It is shown that in all three cases and with all turbulence models tested, the lateral plume spread is significantly underestimated. It is suggested that transient simulations might be required to achieve more accurate results. The numerical results are quite sensitive to the value of the turbulent Schmidt number. The comparisons however cannot clearly indicate which Schmidt number is most suitable for which type of flow due to the large number of other error sources in the simulations, including steady-state RANS modelling, turbulence modelling, near-wall treatment limitations and unintended streamwise gradients in the turbulent kinetic energy profiles.

Numerical investigation of flow field configuration and contact resistance for PEM fuel cell performance

A steady-state three-dimensional non-isothermal computational fluid dynamics (CFD) model of a proton exchange membrane fuel cell is presented. Conservation of mass, momentum, species, energy, and charge, as well as electrochemical kinetics are considered. In this model, the effect of interfacial contact resistance is also included. The numerical solution is based on a finite-volume method. In this study the effects of flow channel dimensions on the cell performance are investigated. Simulation results indicate that increasing the channel width will improve the limiting current density. However, it is observed that an optimum shoulder size of the flow channels exists for which the cell performance is the highest. Polarization curves are obtained for different operating conditions which, in general, compare favorably with the corresponding experimental data. Such a CFD model can be used as a tool in the development and optimization of PEM fuel cells.

Numerical prediction of the effect of vent arrangements on the ventilation and energy transfer in a multi-span glasshouse using a bi-band radiation model

The distributed climate inside a greenhouse mainly results from mass, momentum and energy transfers occurring through the openings or through the cover. These transfers can be assessed using computational fluid dynamics (CFD) techniques. However, to date very few studies considered properly the radiative exchanges between the surfaces inside the calculation domain by solving simultaneously the radiative transfer equation (RTE) and the convective ones. The object of this work is to implement this equation in order to get an overview of the resulting climate inside the greenhouse. The analysis focuses on the heterogeneity of the climatic parameters in the canopy vicinity as well as on the ventilation rate. For the purpose of the study, a two-dimensional steady-state CFD model was developed. It is based on the resolution of the Navier–Stokes equations with the Boussinesq assumption and a k–ε closure. Solar and atmospheric radiations were included by solving the RTE and distinguishing short [0–3 μm] and long wavelength [3–100 μm] contributions. The virtual tracer gas technique was implemented to assess the ventilation rate. The model was first successfully validated against experimental data considering a glasshouse divided into two compartments separated by a plastic partition and comparing simulated and recorded temperatures inside the shelter and along the walls (roof and sides). Simulations were then carried out for a 2500 m2 four-span sloped-roof glasshouse covered with a 4-mm-thick horticulture glass and equipped with continuous roof vents and one side-opening. Results show that roof-opening configurations combined with side-vent location strongly affect inside ventilation and microclimate parameters. Thus, for hot summer conditions with relatively low wind, computed ventilation rates varied from 2 to 40 air changes per hour whereas temperature differences varied from 3 to 13 °C. This study also showed that other characteristics such as climate heterogeneity must be investigated in order to define the best ventilation configuration and that strong velocity and temperature heterogeneity can occur at the plant level. Simulations, however, suggest that the symmetric roof-vent configuration seems to be a good compromise between cooling and homogeneity performances.

Performance prediction of proton exchange membrane fuel cells using a three-dimensional model

In this study a steady-state three-dimensional computational fluid dynamics (CFD) model of a proton exchange membrane fuel cell is developed and presented for a single cell. A complete set of conservation equations of mass, momentum, species, energy transport, and charge is considered with proper account of electrochemical kinetics based on Butler–Volmer equation. The catalyst layer structure is considered to be agglomerate. This model enables us to investigate the flow field, current distribution, and cell voltage over the fuel cell which includes the anode and cathode collector plates, gas channels, catalyst layers, gas diffusion layers, and the membrane. The numerical solution is based on a finite-volume method in a single solution domain. In this investigation a CFD code was used as the core solver for the transport equations, while mathematical models for the main physical and electrochemical phenomena were devised into the solver using user-developed subroutines. Three-dimensional results of the flow structure, species concentrations and current distribution are presented for bipolar plates with square cross section of straight flow channels. A polarization curve is obtained for the fuel cell under consideration. A comparison between the polarization curves obtained from the current study and the corresponding available experimental data is presented and a reasonable agreement is obtained. Such CFD model can be used as a tool in the development and optimization of PEM fuel cells.

Flame interactions and smoke containment by downward displacement ventilation

Smoke contamination even from small fires of sensitive components in telephone central offices (TCO) and semiconductor clean rooms has caused disastrous disruptions in services and excessive financial repercussions. Containment and removal of smoke can be achieved by downward displacement ventilation, also used for other purposes in large floor area semiconductor facilities. Downward ventilation reverses the upward flow from the fire and removes the smoke and other combustion products through the floor. Small-scale model experiments under turbulent fire plume conditions investigate the physical parameters such as downward velocity, ceiling clearance, and size of the fire that control the extent of the smoke containment around the fire. The extent of smoke contamination is mapped by measuring the temperatures in several locations on the vertical axis and in the space around the fire. The downward velocities cause oscillation of the fire plume and the flames around the vertical axis. By combining the experimental data with similarity analysis, a characteristic length scale and nondimensional relations are deduced for the extent of the smoke containment and the onset of plume oscillations. These relations allow design of displacement ventilation effective to contain the smoke but not cause fire spread owing to the leaning of the flames. Previously reported discrepancies in the apparent width and behavior of the plume are resolved by examining the experimental data and assessing the suitability of steady state computational fluid dynamics to model the present flow.

A simple expression for predicting the inlet roundness of micro-nozzles

The inlet roundness of micro-nozzles can directly influence properties of the liquid jets conducted through them. Obtaining accurate information regarding the inlet roundness of such tiny nozzles, however, is not easy. This is mainly due to the minute dimensions of these nozzles which render most nondestructive examination methods ineffective. In this study, a series of steady-state two-phase computational fluid dynamics simulations is performed to predict the inlet roundness of micro-nozzles used for producing constricted waterjets, i.e., waterjets resulting from a detached nozzle flow. Different micro-nozzles with inlet roundness ranging from r/d = 0 to 0.18 (where r and d are the inlet radius of curvature and the capillary diameter, respectively) were considered to obtain an expression for predicting the nozzle's inlet roundness as a function of its discharge coefficient. It is demonstrated that the discharge coefficient of nozzles conducting a detached flow increases with increasing inlet roundness. The inlet roundness predicted by our expression is in good agreement with the actual roundness determined by sectioning the nozzle and imaging its cross-section. Our expression is believed to be useful for manufacturers and users of capillary micro-nozzles for producing liquid micro-jets.

Predicting drift from field spraying by means of a 3D computational fluid dynamics model

In order to investigate and understand drift from field sprayers, a steady state computational fluid dynamics (CFD) model was developed. The model was developed in 3D in order to increase the understanding of the causes of drift: a deviation in the wind direction cannot be captured by a 2D approach, the wake behind a wind screen is not symmetrical, the effects of a changed nozzle orientation may not be symmetrical. The model's accuracy was validated with field experiments carried out according to the international standard ISO 22866. A field sprayer with a spray boom width of 27 m and 54 nozzles (Hardi ISO F110-03 at 3 bar) was driving at 2.22 m/s over a flat pasture. During the experiments the wind direction was perpendicular to the tractor track. The model explained the variation in drift replicates during each single field experiment through varying boom height (0.3–0.7 m), wind velocity (1.3–2.5 m/s), wind deviation (−18° to +18°) from the direction perpendicular to the tractor track and injection velocity of the droplets (17–27 m/s). Boom movements had the highest impact on the variations in drift values (deviations in drift deposits of 25%), followed by variation in wind velocity (deviations in drift deposits of 3%) and injection velocity of the droplets (deviations in drift deposits of 2.5%). Wind deviation from the direction perpendicular to the tractor track had a reducing effect on the drift values (deviations in drift deposits of 2%). Small variations in driving speed had little influence on drift values. Near drift (<5 m) is predicted well by the model but the increased complexity compromised the predictions at greater distances. The model will be further developed in order to improve far drift prediction. Dynamic simulations will be performed and the model for turbulent dispersion will be optimized. The model did not require calibration.

Validation of Implicit Algorithms for Unsteady FlowsIncluding Moving and Deforming Grids

Implicit subiteration algorithms for time-accurate Navier-Stokes flow solvers for moving body and deforming applications have been investigated. Several example calculations are computed to demonstrate the performance of these algorithms for solving unsteady Navier-Stokes problems. A set of relatively simple two-dimensional validation cases has been identified to assess the performance of unsteady CFD solvers. These cases demonstrate the advantages of second order time derivatives and subiterations for unsteady flow simulations. This investigation also indicates that the same level or a reduced level of numerical error relative to a baseline calculation using a smaller time step can be achieved with large time steps using subiterations when a highly convergent inner algorithm is used. here currently exists a great interest in performing calculations using Computational Fluid Dynamics (CFD) for high Reynolds number unsteady flows. Researchers are beginning to apply the new hybrid RANS/LES class of turbulence models to a host of unsteady high Reynolds flows containing large-scale coherent turbulent structures. Applications involving moving and deforming bodies are also becoming more common. Validation and verification of CFD codes becomes much more difficult for these unsteady flows than for traditional steady state CFD problems. Grid convergence studies do not address all of the relevant dimensions of a problem. Often refining the results in resolution of smaller scale structure in the unsteady flow, and hence a grid resolved solution does not exist except in the Direct Numerical Simulation (DNS) limit. In many cases convergence can only be judged in a statistical sense. The choice of time step can have a tremendous effect on the time-accuracy of a solution. The high frequency spectral regime will be under-resolved if the chosen time step is too large. Large time steps can introduce error in the solution if no means are provided to locally converge (i.e. convergence in both time and space at each time step) the solution. If the time step is too small a tremendous number of iterations will be required in order to adequately resolve the low frequency spectral regime. Hence the selection of a time step for a typical CFD problem is an exercise in the art of compromise and often requires some a priori knowledge of the unsteady nature of the flow. Much of the numerical technology for the solution of the Navier-Stokes equations over the last three decades has been focused on obtaining steady-state solutions. These algorithms generally provide large amounts of numerical dissipation in order to damp out spurious numerical fluctuations rapidly. Excessive numerical dissipation can cause the unsteady structures in the flow to be over-damped. This study focuses on subiteration strategies that allow for large time steps and local convergence at each step. Large time steps are useful in problems that require assembly or remeshing at each time step since it minimizes the number of these operations required for a simulation. This effort outlines the basic implicit numerical algorithms required for unsteady flow applications using large time steps including moving and deforming body applications. Simple two-dimensional example cases are provided that allow numerical algorithms to be assessed for unsteady flow applications. The test cases were chosen based on the availability of analytical solutions to the Navier-Stokes equations and/or a regular periodic behavior of the flow. This allows the user to evaluate the ability of a flow solver to provide a locally converged solution in time and to

The computational modeling of the ventilation flows within a rapid development drivage

In recent years, the increase in the drivage rates of continuous miners has exacerbated the environmental conditions experienced within the rapid development headings of many deep UK coal mines. This paper presents the conclusions drawn from the analysis of a series of validated computational studies conducted to assess the effectiveness of alternative auxiliary ventilation systems on the mitigation of any adverse environmental conditions experienced within these drivages. A series of steady-state computational fluid dynamics (CFD) models were constructed to replicate the ventilation flow patterns seen at the head end of a drivage during the various stages of a cutting and bolting cycle. The results obtained from these simulations were compared against the data obtained from a series of full scale ventilation experiments conducted within a rapid development drivage of a representative UK deep coal mine. It is concluded that CFD models may be successfully used to identify the ventilation characteristics associated with the various auxiliary ventilation systems during a typical cutting bolting cycle.

Local Wall Shear Stress Variations Predicted by Computational Fluid Dynamics for Hygienic Design

Food producers depend to a great extent on the availability of easy-to-clean processing equipment. This paper presents initial work carried out to improve the application of commercially available computational fluid dynamics (CFD) programs as a tool to improve hygienic design of food processing equipment. In the present paper, previous work on using CFD to improve hygienic design are combined with work that shows the importance of measured local mean wall shear stresses and the fluctuations in these wall shear stresses with respect to cleaning effects of the flow. Results of wall shear stress and the fluctuations acquired by the measuring technique are difficult to apply directly for actual design optimization. Instead, steady-state CFD simulations using STAR-CD were applied to predict the fluctuation of equivalent mean wall shear stresses on the entire surface of pipes with various diameter changes (gradual and sudden expansions or contractions). Measured fluctuation rates of wall shear stress are shown to be analogous to the turbulence intensity in the near-wall region predicted by CFD simulations. The results are promising; hence, an important step has been taken towards setting up a method for evaluating cleanability by steady-state CFD simulations based on evaluation of the mean wall shear stresses and the turbulence intensity.

Flow pattern visualization in a mimic anaerobic digester using CFD

Three-dimensional steady-state computational fluid dynamics (CFD) simulations were performed in mimic anaerobic digesters to visualize their flow pattern and obtain hydrodynamic parameters. The mixing in the digester was provided by sparging gas at three different flow rates. The gas phase was simulated with air and the liquid phase with water. The CFD results were first evaluated using experimental data obtained by computer automated radioactive particle tracking (CARPT). The simulation results in terms of overall flow pattern, location of circulation cells and stagnant regions, trends of liquid velocity profiles, and volume of dead zones agree reasonably well with the experimental data. CFD simulations were also performed on different digester configurations. The effects of changing draft tube size, clearance, and shape of the tank bottoms were calculated to evaluate the effect of digester design on its flow pattern. Changing the draft tube clearance and height had no influence on the flow pattern or dead regions volume. However, increasing the draft tube diameter or incorporating a conical bottom design helped in reducing the volume of the dead zones as compared to a flat-bottom digester. The simulations showed that the gas flow rate sparged by a single point (0.5 cm diameter) sparger does not have an appreciable effect on the flow pattern of the digesters at the range of gas flow rates used.