RANS CFD Simulations Research Articles

Physical parametric study of bacterial biofilm disruption and removal by jet impingement: A CFD investigation

Bacterial biofilm formation is one of the most critical challenges in industrial and health systems. It is the leading cause of nearly 80 % of clinical infections. Biofilms are usually evaluated from the physiological point of view as microbial cells enclosed in an extracellular polymer and represent a complex, dynamic bacterial community. Cohesive mechanical forces hold the bacteria within the biofilm together. These forces contribute to the biofilm's mechanical and structural stability, resulting in its viscoelastic properties. Studying the biofilm's mechanical properties as a viscoelastic matter can provide a better understanding of the biofilm's characteristics and the survival strategies of bacteria in the biofilm state of life. In the present study, a 2D-axisymmetric RANS-CFD simulation model using the Volume Of Fluid (VOF) method was developed to track changes in biofilm surface and ripple formation across six biofilm thicknesses ranging from 15 μm to 115 μm, two substrate geometries including flat and curved, and two substrate roughnesses of 0.4 μm and 0.9 μm height. A modified Herschel-Bulkley model was employed to capture the biofilm's non-Newtonian shear-thinning behavior under high-velocity gas jet impingement. The results revealed the importance of biofilm thickness, substrate geometries, and properties like roughness in biofilm's mechanical behavior. Higher thickness of biofilm resulted in smaller jet-impingement region and more significant ripples formation. Biofilm on curved substrates decreased in thickness more rapidly than those on flat substrates. The substrate's geometry impacted biofilm folding patterns, removal, and mechanical behavior. Selected substrate roughnesses did not create any resistance against the mechanical removal of biofilms. Here, we successfully captured biofilms' non-Newtonian and shear-thinning behavior with numerical simulations consistent with previously reported experimental results. This could benefit industrial and health systems by tackling biofilm-related challenges and developing more accurate and detailed models of biofilms to optimize tools for drug testing or screening and enhance the models for in-vitro and in-vivo analysis.

Behavior and mechanisms of Doppler wind lidar error in complex terrain: stable flow case study at Perdigão

A numerical experiment is carried out investigating the magnitude of biases in ground-based lidar measurements in complex flow conditions. Biases assessed include those arising from flow curvature and from the interaction of turbulence with the wind field reconstruction (WFR) algorithms used by a WindCube lidars and anemometers. RANS-CFD and WRF-LES simulations were performed for the Perdig˜ao Field Experiment site for a range of atmospheric conditions. Virtual anemometer and lidar data were generated for four locations: two near exposed ridge tops and two in low-speed regions in the valley. The LES data at these four locations show that the scalar inflation terms (the relation between scalar and vector averaged wind speed) for virtual lidar and virtual cups agree very well with predictions using perturbation theory. While the lidar errors vary greatly with location and height, the contribution from the flow curvature tends to be larger than the differences arising from scalar inflation. For one lidar/mast pair near the ridge top, comparisons between simulations and measurements are carried out for a resonant mountain wave event on June 14th, 2017, and for the whole duration of the Perdig˜ao campaign for winds perpendicular to the ridges. The lidar error during the mountain wave, a period of strong stability and low inversion height, is significantly larger than the campaign average. The sensitivity of the lidar error to atmospheric stability is confirmed by the RANS simulations, which suggests strong sensitivity of flow curvature error to stability conditions and to the shape of the wind speed profile near the top of the boundary layer.

Numerical modeling of air ejectors covering supersonic, subsonic and closed-port operations

The physics and modeling of air ejectors in on- and off-design conditions have been extensively addressed in the past, reaching a good level of maturity. However, to achieve a robust model at system scale integrating an ejector, there is a need for developing 0D models suitable to tackle abnormal functioning modes, such as cases where the ejector works in subsonic conditions, reverse flows, or closed ports. Based on state-of-the-art models, a new 0D model has been built in the current work, where proper subroutine implementations allow covering normal and abnormal operations of convergent nozzle ejectors. The current research also focuses on understanding the physical behavior of air ejectors for aeronautical applications. In this regard, CFD-RANS simulations are used to perform model verification and calibration as well as to gain knowledge of the whole operational envelope. The regimes reproduced by the 0D model, and validated by CFD, cover the normal operating mode with different choking regimes, the occurrence of a subsonic primary flow, and the closed secondary port case. Additional cases are run to assess other features of the ejector behavior, such as the influence of geometrical properties on the choking mechanism and the impact of thermal effects.

Optimisation of a Multi-Element Airfoil for a Fixed-Wing Airborne Wind Energy System

Airborne wind energy systems benefit from high-lift airfoils to increase power output. This paper proposes an optimisation approach for a multi-element airfoil of a fixed-wing system operated in pumping cycles to drive a drum-generator module on the ground. The approach accounts for the different design objectives of the tethered kite’s alternating production and return phases. The airfoil shape is first optimised for the production phase and then adapted for the requirements of the return phase by modifying the flap setting. The optimisation uses the multi-objective genetic algorithm NSGA-II in combination with the fast aerodynamic solver MSES. Once the optimal shape is determined, the aerodynamic performance is verified through CFD RANS simulations with OpenFOAM. The resulting airfoil achieves satisfactory performance for the production and return phases of the pumping cycles, and the CFD verification shows a fairly good agreement in terms of the lift coefficient. However, MSES significantly underpredicts the airfoil drag.

Investigation of reacting fuel jets in hot vitiated crossflow

Near-wall reacting jets can occur in different engineering applications, such as rocket combustion chambers and in gas turbine combustors and turbines. This work presents results of experimental investigations of reacting gaseous fuel jets (hydrogen, methane, propane) injected normally into a hot oxygen-rich cross-flow using OH laser-induced fluorescence and OH* chemiluminescence diagnostics. RANS CFD simulations with EDC combustion model are performed for comparison. To determine the surface heat fluxes, the inverse heat conduction method was applied using measured wall temperature data as input. It was observed, that hydrogen jets ignite directly at the injection location and caused a higher surface heat flux than the hydrocarbon jets, where jet-like diffusion flames emerged. With the latter fuels, a significant ignition delay length was observed. Operation with methane fuel showed the least stability, least reactivity and the lowest surface heat flux. Increasing the jet momentum ratio led to a reduced ignition delay length due to enhanced jet-crossflow interaction, and to a reduction of surface heat flux as the reacting jet detached more from the surface. CFD results using the Eddy-Dissipation Concept (EDC) combustion model predicted ignition delay lengths and reaction zone shapes in qualitative agreement with the experiment but with a trend of overestimating combustion temperatures.

Low-Order Reactor-Network-Based Prediction of Pollutant Emissions Applied to FLOX® Combustion

Prediction of pollutant emissions is a key aspect of modern combustor design in energy conversion systems. In the presented work, a simple and robust model based on low-order reaction networks is applied to a FLOX® laboratory combustor at atmospheric conditions. The applied approach is computationally cheap and therefore highly suited for design studies. Steady-state CFD RANS simulations are carried out, serving as a basis for the network generation algorithm. CFD results are validated with experimental data for flow field and combustion. Different degrees of fidelity of reactor network models are taken into consideration and findings are opposed to measurements, evaluating the quality of the low-fidelity models. Validation of CO and NOx emission results of reactor network modeling provides accurate qualitative and quantitative reproduction of experimental findings, depending on the degree of heat loss applied on the combustion system. The introduced approach is therefore readily applicable to large-scale, industrial, and gas turbine combustion.

Ship staff wind comfort and wind safety in the walkways of containerships and case study for a feeder containership

ABSTRACT This paper presents the investigation of wind speeds on the walkways of a feeder container ship with 3D steady RANS CFD simulations and examining them in terms of wind comfort and safety. Wind velocity profiles on walkways are studied by simulating a steady ship speed of 17 knots (8.75 m/s) with different atmospheric wind directions and atmospheric wind speeds. Wind velocities up to 21.8 m/s were measured by numerically simulating the walkways on the ship. Especially winds that coming from bow directions may create higher effective wind velocity values on the walkways even if the atmospheric wind velocities on the ship deck are low, the effective wind velocity which is combined with the airflow due to the speed of the ship at some wind directions can exceed 10 m/s. Compared to a flat bare deck, structures around walkways can reduce the effective wind speed by up to 70%.

Hull form optimization for reduced calm-water resistance and improved vertical motion performance in irregular head waves

A high-efficiency potential-flow based ship hydrodynamic optimization tool is presented in this paper to optimize hull forms for reduced calm-water resistance and improved vertical motion performance in irregular head waves. Besides calm-water ship resistance, the significant amplitudes of heave and pitch motion response spectra in irregular head waves are determined as objective functions. To obtain a more reliable result of ship motion in waves, the three-dimensional potential flow solver by using translating and pulsating line segment source Green's function is employed to solve ship radiation and diffraction problems and further to evaluate ship vertical motion. Several deformation methods are utilized to move the control points of NURBS hull surfaces to achieve the hull form deformation.For the purpose of illustration, this optimization tool is applied to the optimization design of the prototype of DTMB 5512. The resistance and vertical motion performance of four optimal hulls in Case 2 are investigated and further verified by the RANS CFD simulations. The optimization results in two cases indicate that this optimization tool can yield optimal ships with reduced calm-water resistance and improved vertical motion performance, and a reasonable determination of feasible region can increase the possibility of obtaining such target ship.

RANS CFD simulations can be successfully used for simulating snowdrift on roofs in a long period of snowstorm

RANS CFD simulations can be successfully used for simulating snowdrift on roofs in a long period of snowstorm

Multi-Degree of Freedom Propeller Force Models Based on a Neural Network and Regression

Accurate and efficient prediction of the forces on a propeller is critical for analyzing a maneuvering vessel with numerical methods. CFD methods like RANS, LES, or DES can accurately predict the propeller forces, but are computationally expensive due to the need for added mesh discretization around the propeller as well as the requisite small time-step size. One way of mitigating the expense of modeling a maneuvering vessel with CFD is to apply the propeller force as a body force term in the Navier–Stokes equations and to apply the force to the equations of motion. The applied propeller force should be determined with minimal expense and good accuracy. This paper examines and compares nonlinear regression and neural network predictions of the thrust, torque, and side force of a propeller both in open water and in the behind condition. The methods are trained and tested with RANS CFD simulations. The neural network approach is shown to be more accurate and requires less training data than the regression technique.

Mean pressure coefficient distributions over hyperbolic paraboloid roof and canopy structures with different shape parameters in a uniform flow with very small turbulence

The hyperbolic paraboloid with two high and low corners is the most basic anticlastic double curved shape of tensile surface structures. However, this shape is currently not considered in the existing wind load standards and only very few investigations are documented in literature. Therefore, this study aims to map the mean aerodynamics over hyperbolic paraboloid roofs and extremely thin canopies to help engineers understand the general aerodynamics of these structures. In this study, scaled wind tunnel testing is performed on rigid models of a roof with enclosing walls and a canopy without enclosing walls to draft mean Cp-distributions and standard deviations for various wind orientations. These mean Cp-distributions are validated with the limited studies in literature and used to calibrate and benchmark a numerical wind tunnel, using CFD RANS simulations, that allows to further reduce the thickness of the canopy compared to the wind tunnel models. The numerical wind tunnel is used to study the mean wind load distributions for hyperbolic paraboloid roofs and canopies with different surface curvature for the most important wind orientations. The results in this paper show good agreement between wind tunnel experiments and CFD simulations, with similar variations as established by different studies of the Silsoe cube. First mean Cp-distributions obtained with scaled wind tunnel testing are presented for hyperbolic paraboloid canopies of only 5 mm thick, with similar mean net Cp and only slightly larger suction over the upper and lower face compared to the numerical results for a zero-thickness canopy. The numerical study of different surface curvatures indicates different geometrical patterns in the mean Cp-distributions when the flow starts to separate for highly curved hypars. In general, suction and pressure increase for higher surface curvature, with similar geometrical patterns in the mean Cp-distributions, as long as the flow remains attached. If the flow separates for highly curved hypars, the geometrical pattern differs and the mean Cp drops due to the different behavior of the flow. Finally, the presented mean Cp-distributions give insight in the aerodynamics of hypar roofs and canopies with different surface curvature and could form a basis for future analysis towards peak loads for the design of these structures.

A semi-analytical noise prediction model for airfoils with serrated trailing edges

Trailing edge serrations are a widely used passive technique for the suppression of aerodynamic noise from wind turbines. Despite their popularity, no reliable engineering prediction tool has yet been developed to estimate the noise reduction for different serrations. This paper concerns the development of an engineering noise prediction tool, based on a recently developed mathematical model. Results show that the new model has several advantages over Howe's model, as it can take both destructive and constructive sound interference effects into account. Two surface pressure wavenumber-frequency models are implemented, namely Chase and TNO models, to demonstrate the sensitivity of the model to boundary layer characteristics. The boundary layer parameters needed in the wavenumber-frequency models are obtained using RANS CFD simulations. Far-field noise comparisons are provided between the proposed prediction tool and experimental data for a NACA0018 airfoil. A parametric study regarding the boundary layer changes of serrated airfoils signifies the need for more reliable wavenumber-frequency models. The results presented in the paper show that the proposed engineering tool can provide a fairly accurate estimate of the noise reduction performance of serrated airfoils, but its accuracy relies heavily on the availability of reliable near-field boundary layer information.

CFD modelling of air quality in Pamplona City (Spain): Assessment, stations spatial representativeness and health impacts valuation

A methodology based on CFD-RANS simulations (WA CFD-RANS, Weighted Averaged Computational Fluid Dynamic-Reynolds–Averaged Navier–Stokes simulations) which includes appropriate modifications, has been applied to compute the annual, seasonal, and hourly average concentration of NO2 and NOX throughout the city of Pamplona (Spain) at pedestrian level during 2016. The results have been evaluated using measurements provided both by the city's network of air quality monitoring stations and by a network of mobile microsensors carried around by cyclists during their daily commutes, obtaining a maximum relative error lower than 30% when computing NO2 annual average concentrations.The model has taken into account the actual city layout in three dimensions, as well as the traffic emissions. The resulting air pollution maps provided information critical for studying the traffic-related health effects of NO2 and their associated external costs in the city of Pamplona and the spatial representativeness of the current network of air quality monitoring stations (it has not been carried out for an entire city to date). The developed methodology can be applied to similar cities, providing useful information for the decision-makers.

An annual energy production estimation methodology for onshore wind farms over complex terrain using a RANS model with actuator discs

This work presents a new methodology for the interpolation and extrapolation of the wind power generated by each wind turbine in a wind farm at different wind speeds for a given wind direction, running only three RANS-CFD simulations, two with and one without wind turbines. The wind turbines are modeled as uniformly loaded actuator discs with an adaptation for wake interaction cases. Three RANS turbulence models are used, standard k-ε, k-ε-fP and the realizable k-ε model modified to account for Coriolis forces and with appropriate coefficients for the simulation of ABL flows. The turbulence models are validated against wake deficit and power output measurements from the wind turbine test site Wieringermeer. Then, the new methodology proposed to predict the wind turbine power is detailed and evaluated with simulation results and measurements for a large onshore wind farm, particularly for a strong wake interaction case. It is observed that the power predictions using the proposed strategy are in good agreement with the obtained from different wind velocity simulations and power measurements. The methodology is a useful tool for wind farm annual energy production estimation.

Transition between free, mixed and forced convection

In this contribution, numerical methods are discussed to predict the heat transfer to liquid metal flowing in rectangular flow channels. A correct representation of the thermo-hydraulic behaviour is necessary, because these numerical methods are used to perform design and safety studies of components with rectangular channels. Hence, it must be proven that simulation results are an adequate representation of the real conditions. Up to now, the majority of simulations are related to forced convection of liquid metals flowing in circular pipes or rod bundle, because these geometries represent most of the components in process engineering (e.g. piping, heat exchanger). Open questions related to liquid metal heat transfer, among others, is the behaviour during the transition of the heat transfer regimes. Therefore, this contribution aims to provide useful information related to the transition from forced to mixed and free convection, with the focus on a rectangular flow channel. The assessment of the thermo-hydraulic behaviour under transitional heat transfer regimes is pursued by means of system code simulations, RANS CFD simulations, LES and DNS, and experimental investigations. Thereby, each of the results will compared to the others. The comparison of external experimental data, DNS data, RANS data and system code simulation results shows that the global heat transfer can be consistently represented for forced convection in rectangular flow channels by these means. Furthermore, LES data is in agreement with RANS CFD results for different Richardson numbers with respect to temperature and velocity distribution. The agreement of the simulation results among each other and the hopefully successful validation by means of experimental data will fosters the confidence in the predicting capabilities of numerical methods, which can be applied to engineering application.

Comparison of low-order aerodynamic models and RANS CFD for full scale 3D vertical axis wind turbines

A Double Multiple Streamtube model, a free-wake vortex model (both widely used for vertical axis wind turbine design) and RANS CFD simulations are used in this work to predict the performance of the 17 m Vertical Axis Wind Turbine, field tested by Sandia National Laboratories. The three-dimensional, full scale calculations are compared with the experiments in terms of power coefficient, power and instantaneous turbine torque to assess the validity of each model. Additionally, the two aerodynamic models and RANS CFD are compared to each other in terms of thrust and lateral force. The two models and CFD agree well with the experiments at the turbine optimal tip speed ratio. However, away from the optimal tip speed ratio, the streamtube model significantly deviates from the experimental data and from the other numerical models. RANS CFD gives a good agreement with the experiments, slightly underestimating the power coefficient at every tip speed ratio tested. The vortex model proves to be a useful tool with a better accuracy than the streamtube model and a much lower computational cost compared to RANS CFD.

Modelling complex terrain effects for wind farm layout optimization

The flow over four analytical hill geometries was calculated by CFD RANS simulations. For each hill, the results were converted into numerical models that transform arbitrary undisturbed inflow profiles by rescaling the effect of the obstacle. The predictions of such models are compared to full CFD results, first for atmospheric boundary layer flow, and then for a single turbine wake in the presence of an isolated hill. The implementation of the models into the wind farm modelling software flapFOAM is reported, advancing their inclusion into a fully modular wind farm layout optimization routine.

Automotive Drag Reduction through Distributed Base Roughness Elements

This paper focuses on the effect of base roughness added to the rear of an automotive reference model, the Windsor model. This roughness addition was found to reduce both the drag and the lift of the model. RANS CFD simulations presented here replicate the experimentally observed drag reduction and enable a detailed examination of the mechanisms behind this effect. Investigations into the wake structure of the configurations with base roughness and the baseline case without base roughness showed the main changes to the wake to include a reduction in the overall size of the wake with base roughness present. Furthermore a reduction in the near wall velocities at the rear of the model caused stretching of the upper and lower vortices, a more turbulent near wake and pressure recovery over much of the rear face. This leads to reduce levels of pressure drag on the model.

Numerical Characterization of Aerodynamic Losses of Jet Arrays for Gas Turbine Applications

Jet array is an arrangement typically used to cool several gas turbine parts. Some examples of such applications can be found in the impingement cooled region of gas turbine airfoils or in the turbine blade tip clearances control of large aero-engines. In order to correctly evaluate the impinging jet mass flow rate, the characterization of holes discharge coefficient is a compulsory activity. In this work, an aerodynamic analysis of jet arrays for active clearance control was performed; the aim was the definition of a correlation for the discharge coefficient (Cd) of a generic hole of the array. The data were taken from a set of CFD RANS simulations, in which the behavior of the cooling system was investigated over a wide range of fluid-dynamics conditions. Furthermore, several different holes arrangements were investigated in significant detail, with the aim of evaluating the influence of the hole spacing on the discharge coefficient distribution. Tests were conducted by varying the jet Reynolds number in a wide range of effective engine operative conditions (Re = 2000-12,000, Pressure- Ratio = 1.01-1.6). To point out the reliability of the CFD analysis, some comparisons with experimental data, measured at the Department of Energy Engineering of the University of Florence, were drawn. An in-depth analysis of the numerical data set has underlined the opportunity of an efficient reduction through the mass velocity ratio of hole and feeding pipe: the dependence of the discharge coefficients from this parameter is roughly logarithmic.