Steady Reynolds-averaged Navier-Stokes Research Articles

Numerical and experimental investigations of diffusion-induced boundary layer separation on aero-engine nacelles

Aero-engine nacelles have to fulfill design requirements at both cruise and off-design conditions. Under engine windmilling conditions the ingested streamtube massflow is relatively low. A key off-design condition is take-off, which, in conjunction with an engine windmilling scenario, results in the stagnation point of the ingested streamtube being located significantly inside the intake. The combination of high angle of attack and low engine massflow rates leads to a strong flow acceleration and subsequent diffusion of the boundary layer on the upper quadrants of the external nacelle cowl, which can terminate with subsonic separation from the leading-edge. Under this condition, Reynolds number effects can play a dominant role on the separation onset and characteristics and 3D-annular wind tunnel tests cannot always achieve Reynolds’ number equivalent to full scale. A novel quasi-2D rig configuration representative of the aerodynamics of a full-size aero-engine nacelle under windmilling end of runway conditions examined in detail the characteristics of the boundary layer on the external cowl of a nacelle prior to diffusion-induced separation. Separation of the boundary layer was independently promoted through changes to represent different engine massflow rates and freestream Mach number on the rig to determine the limits of steady Reynolds Averaged Navier Stokes (RANS) methods to discern the onset of boundary layer separation. For the conditions and geometry investigated, the combined experimental and computational results showed that there was laminar to turbulent transition of the boundary layer ahead of the subsonic diffusion. The work showed that steady RANS can predict the onset of boundary layer separation with an uncertainty of approximately 10% on notional engine massflow rate and 0.05 on freestream Mach number relative to a nominal operating freestream Mach number of 0.25. This provides guidance for the industrial design and optimization of future windmilling-tolerant nacelles for large ultra-high bypass ratio turbofan engines.

Fuel-air ratio effect on hydrogen-methane flames in a high pressure burner for gas turbines

This numerical simulation studied the effect of H2-CH4 flame equivalence ratio on turbulent premixed combustion at 50%-50% concentration (by volume). The equivalence ratio was varied from 0.45 to 1.0 in 0.5 increments for 126 kW operating power, matching a 3.3 bar inlet reactant pressure. Tests utilized a gas turbine combustor. The thermal field, flow field, and pollutant emissions (NOx and CO) underwent rigorous analysis. The modelling framework applied steady Reynolds-Averaged Navier-Stokes (RANS) equations coupled to a probability density function (PDF) approach for turbulence-chemistry interactions and a NOx formation model. Results showed increasing equivalence ratio from 0.45 to 1.0 elevated temperature approximately 900 K, significantly promoting NOx up to 1600 ppm and CO beyond 1900 ppm. However, equivalence ratio changes minimally impacted the overall flow field, maintaining stabilized flames. These findings provide new insight on thermochemical effects and flame stability in gas turbine (G.T) combustors across a range of equivalence ratios relevant for clean, high-pressure H2-CH4 combustion. The combined RANS-PDF methodology enables predictive simulation of turbulence, kinetics, emissions, and flame stability to guide optimal fuel-air ratio selection and low-emission combustor design.

A two-stage CFD-GNN approach for efficient steady-state prediction of urban airflow and airborne contaminant dispersion

Urban design requires accurate prediction of wind, gusts, and contaminant dispersion to create sustainable cities. While computational fluid dynamics (CFD) models like large-eddy simulation (LES) provide accurate predictions, they are computationally expensive. Steady Reynolds-averaged Navier-Stokes (SRANS) is fast but inaccurate due to inherent limitations. This study proposes a two-stage CFD-graph neural network (GNN) framework for efficient steady-state prediction. A CFD solver provides the initial state, and then a GNN directly corrects the CFD outputs towards a high-fidelity target in a single inference step, bypassing SRANS flaws. LES results for parametrically generated urban cases provided training data. The proposed framework is thus distinguished from previous GNN applications, which were unsteady RANS surrogates that are still constrained by RANS inaccuracy. SRANS with the k-ε model provided an informative initial state for GNN (termed KE-GNN). Less detailed initializations increased prediction difficulty. Uneven sampling of building density and heights introduced dataset bias, improving specialization but reducing generalizability. For an actual urban case, KE-GNN employing a coarse mesh achieved considerable agreement with the fine LES result and was thousands of times faster. On the same coarse mesh, KE-GNN was seven times faster than a tightly converged SRANS and 280 times faster than an under-resolved LES.

Fluid-to-fluid similarity and CFD predictions of surrogate fluid as a replacement for R11 refrigerant used in a subcooled flow boiling analysis: Validation against reference experimental data

In this study, we present the possibility of substituting the primary fluid Freon 11 (CCl3F, R11) used in the experimental thermohydraulic test facility which offers the advantage to obtain phase change at relatively low heat fluxes and pressure if compared to water working fluid. Nevertheless, R11 is a CFC (Chlorofluorocarbures) part of the gases that contribute to the degradation of the ozone layer.For this purpose, four working fluids (non-CFC) are judiciously selected to replace the R11, namely the R123, R134a, R143a and CO2, and this through multidimensional Computational Fluid Dynamics (CFD) modeling of steady state flow boiling phenomenon. The calculation model covers a wide range of mass fluxes (435 – 1443 kg/m2s) and heat fluxes (1.2– 36 W/cm2); it predicts the heat transfer rates and transitions points for each flow regime. In this research, the experiment's data were compared to the outcomes of Ansys-CFX computations, using a steady Reynolds-Averaged Navier-Stokes (Steady-RANS) model, to investigate the best replacement refrigerant for R11. The turbulence effects on the mean flow and near wall are taken into consideration using the Shear Stress Transport (SST k-ω) turbulence model. The results from the four fluids were transposed to their water equivalent and compared to those deduced from the experimentation and correlations available in the literature to select the appropriate substitution fluid. The results qualitative and quantitative for vertical and horizontal subcooled boiling flow at the CHF location show that, the most effective surrogate fluid is R123 (HCFC), is still widely used in the refrigeration and air-conditioning industry, then R134a (HFC), then R143a (HFC).

Validation of helicity-corrected Spalart-Allmaras model for corner separation prediction in incompressible flow with OpenFOAM

Steady Reynolds-averaged Navier-Stokes (RANS) computations save significant computational resources compared to unsteady RANS and large eddy simulation (LES). However, the ability of most RANS models to accurately predict flow separation in compressor/fan blade rows is limited. Recent research focused on reducing the computational cost of predicting compressor/fan stall points with steady computations has shown that the helicity-corrected Spalart-Allmaras (SA) turbulence model is able to avoid over-predicting corner separations and thus lead to converged RANS up to the actual stall point. To date, this model has mostly been implemented in in-house codes or in commercial codes as a user add-on, where the source code is not available. In a recent paper, the authors implemented the helicity-corrected SA model in OpenFOAM, an open-source CFD package. In this paper, the differences in the flow field for RANS solutions with the original SA model, the helicity-corrected SA model, and Menter’s shear stress transport (SST) model are highlighted for a linear cascade with incompressible flow. A NACA 65-1810 cascade is used, and computational results are compared to experimental data.

Effects of arcade design on wind and thermal environment inside an idealized 2D urban canyon with realistic solar heating

The arcade design has been widely used in the architecture design since it can provide space for commercial retails and improve local thermal comfort. However, the influences of different arcade design parameters on wind and thermal environment under realistic solar heating conditions are not fully understood, which hinders further application of the arcade design. This paper aims to investigate the effects of different arcade design parameters on wind and thermal environment under different solar heating conditions. Numerical wind flow, solar radiation and heat transfer simulations of a two-dimensional deep street canyon with aspect ratio of three are performed. The steady Reynolds-Averaged Navier-Stokes model with standard k−ε turbulence model is employed in this investigation, while considering solar radiation model. Moreover, the realistic solar heating simulations are achieved by setting different local solar times, i.e., LST0900 (leeward wall heating), LST1200 (street ground heating) and LST1500, (windward wall heating). Under realistic solar heating conditions, the building wall heating and the street ground heating are shown to evidently influence the strength and location of the predominant recirculation inside the street canyon, especially for the windward wall heating. Moreover, analysis results have shown that the arcade width has larger influences on wind and thermal environment than that of the arcade height. Particularly, the wind velocity increases 2.1 to 3.8 times, the air temperature decreases 0.42–2.24 °C and the Physiological Equivalent Temperature value drops between 2.2 °C and 7.0 °C when the arcade width increases to half of building width.

Heat transfer enhancement of supercritical carbon dioxide in eccentrical helical tubes

A type of eccentrical helical tube (EHT) was proposed for more efficient and compact supercritical carbon dioxide (SCO2) heat exchangers in refrigeration systems. The forced convective heat transfer of SCO2 flowing in three designed eccentrical helical tubes with pitches of 25, 50 mm and eccentricities of 0.9, 1.5 mm was simulated by using the three-dimensional steady Reynolds-averaged Navier-Stokes equations, shear stress transport turbulence model and energy equation in ANSYS CFX 2019 R2 at mass fluxes of 200, 400 kg/m2s, inlet pressures of 8, 9 MPa, outwards wall heat fluxes of 12, 24 kW/m2. The flow and heat transfer models were validated in the plain tube with experimental mean heat transfer coefficient and empirical Darcy friction factor. Influences of pitch, eccentricity and operational conditions on heat transfer enhancement were identified. Flow pattern, heat transfer mechanism and vortex kinematics were clarified. Heat transfer enhancement of SCO2 in the EHT was compared with water and air in the EHT, twisted elliptical tube and conical tube, respectively. A pitch of 25 mm and eccentricity of 1.5 mm at 6 mm tube diameter can achieve a better thermal-hydraulic performance. A higher SCO2 mass flux raises Nusselt number and reduces friction factor, a higher inlet pressure and a larger wall heat flux reduces Nusselt number but increases friction factor. The ratio f/f0 is ranged in 2.52–1.68 and drops off with increasing inlet pressure and nominal Reynolds number, the ratios Nu/Nu0, ψ and η curves show a concave shape near the pseudocritical point ranged in 1.68–1.06 and 1.06–1.55, 1.23–0.83 and 0.83–1.30, 0.67–0.50 and 0.50–0.92 at the pitch of 25 mm and eccentricity of 1.5 mm, inlet pressure of 8 MPa, mass flux of 400 kg/m2s, and wall heat flux of 12 kW/m2. A helical flow pattern with a core flow occurs in the EHT, the local heat transfer coefficient can be correlated to absolute helicity. The high-wall shear stress on the ridge of the EHT is responsible for the heat transfer enhancement. The f/f0 value of the EHT is in between the twisted elliptical tube and the conical tube, but the Nu/Nu0 and ψ values are comparable.

Prediction of urban airflow fields around isolated high-rise buildings using data-driven non-linear correction models

When it comes to predicting urban airflow, steady Reynolds-averaged Navier-Stokes (SRANS) models that rely on Reynolds stress often face a challenge called the closure problem. This problem involves unresolved structural flaws and uncertainties in the closure coefficients used in the models. Previous attempts to recalibrate coefficients for specific urban flows without breaking the linear constitutive relation have resulted in simulation results constrained by the baseline turbulence model. Therefore, this study aims to enhance the performance of SRANS models by addressing these structural flaws. To achieve this, a novel data-driven framework is proposed. It leverages the deterministic symbolic regression algorithm to discover explicit algebraic expressions for a non-linear Reynolds stress correction model. The robustness of the correction model is ensured by maintaining the linear eddy viscosity model for iterative calculations while keeping the non-linear component frozen. The proposed framework is evaluated using three isolated building cases with varying geometric configurations and inflow boundary conditions. Findings demonstrate that computational fluid dynamics (CFD) predictions incorporating the data-driven non-linear correction model consistently align closer to wind tunnel experimental results compared to both standard and non-linear versions of the k-ε turbulence model. This improvement is reflected in reduced reattachment lengths and more accurate mean velocity distributions in the wake of buildings. However, it should be noted that there is a possibility of overpredicting wind velocity in the windward area. This study introduces valuable insights and additional strategies to enhance the prediction accuracy of SRANS models in urban airflow simulations.

Equation discovery of dynamized coefficients in the k-ε model for urban airflow and airborne contaminant dispersion

Urban airflow and pollution modeling using steady Reynolds-averaged Navier-Stokes (SRANS) with two-equation models has faced accuracy challenges, limiting its reliability for sustainable city design. The low accuracy has been attributed to SRANS ill-conditioning, the linear eddy viscosity hypothesis, and uncertainty contributed by empirical formulations and coefficients. Many studies have attempted to modify the two-equation models specifically for urban problems by correcting the model formulations and calibrating the coefficients. However, these corrections often improve accuracy for targeted regions or quantities while diminishing it elsewhere. This stems from inflexibility in the model form. To mitigate the trade-offs and improve generalizability, this study introduces multiple dynamic correctors to the k-ε coefficients for expanded design space. In particular, the robustness and redistributability of the corrected model were considered by retaining the theoretical interpretation of turbulence and employing symbolic correctors. The gene expression programming technique was employed for equation discovery. The increased model flexibility mitigated the trade-offs in accuracies for different quantities in the training case. The generalizability of a corrected model was evaluated for airflow and dispersion around a single building, a building array, and a group of complex buildings. The corrected model performed consistently for the three flow types and exhibited generalizability.

Inter-room pollutant transmission routes in naturally ventilated street canyon buildings

Airborne transmission of pollutants is a significant concern in urban areas, especially since the recent pandemic. While previous studies have explored inter-room pollutant crossflow in highrise buildings and isolated scenarios, the ventilation characteristics within street canyon configurations have not received enough attention. Understanding ventilation characteristics in this layout will help limit the spread of pollutants along the street. This study investigates the wind-driven transmission characteristics of pollutants between the indoor rooms situated adjacent to a street canyon, subjected to perpendicular approach flow. Numerical simulations were performed using the 3D steady Reynolds-Averaged Navier-Stokes approach and validated against surface pressure measurements inside a wind tunnel. The impacts of seven building ventilation strategies were compared, each representing a window opening combination on the two buildings. Based on the set environmental conditions, the results indicate that the inter-room transmission is dependent on both the ventilation efficiency and the specific location of the room. Increasing the ventilation rate decreases the cross transmission between the rooms, while rooms at the centre of the canyon have an increased susceptibility of being contaminated. For instance, switching from cross to single-sided ventilation decreased the average air change rate from 42.3h-1 to 1.4h-1, coinciding with a rise in pollutant concentration by 114-fold and 12-fold at the canyon's centre and side, respectively. Rooms at the edge of the canyon were more contagious and could contaminate other rooms located further inside the street, albeit in small doses. Our findings present new insights within the domain of indoor-outdoor air interaction, shedding light on the interplay between room location and potential inter-unit pollutant transmission. In addition, the pressure measurement results from the wind tunnel experiment provide a valuable resource for other researchers aiming to validate numerical simulations for building engineering applications.

Design, analysis and test of conventional and winglet propellers under highly loaded conditions

In this paper, some conventional and winglet inboard propellers are designed, analyzed and tested under highly loaded conditions. The design objective is to achieve the highest efficiency at a given propeller RPM and target power. A non-linear optimization solver is coupled with the panel method to design the conventional propellers. A robust parameterized geometry definition is given for the winglet propellers, with the flexibility to change the shapes of the winglet, including its location and direction, and its sections’ pitch and camber. The winglet propellers are designed using the SHERPA (Simultaneous Hybrid Exploration that is Robust, Progressive, and Adaptive) algorithm and the performance of each geometry in the optimization process is predicted by the steady RANS (Reynolds-Averaged Navier–Stokes) method with periodic boundary conditions. Both the conventional propellers and winglet propellers are designed under the open water condition and multiple designs are further analyzed by the unsteady RANS method, together with the hull, the rudder, the free surface and an inclined shaft angle. It is found that (i) in the conventional propeller designs, the unsteadiness in propeller-induced forces on the hull reduces with increased distance from the propeller blade tip to the bottom of the hull; (ii) the efficiency of optimized conventional propellers generally decreases with the decrease of propeller diameter; (iii) under the specified design conditions, the blade tips of the optimized winglet propellers exhibit a tendency to bend towards the pressure side of the blade; (iv) when appropriately designed, winglet propellers can attain enhanced efficiency and reduced unsteadiness in propeller-induced forces acting on the hull’s underside, in comparison to conventional propellers of the same overall diameter. Consequently, winglet propellers are good design choices for highly loaded conditions with strict constraints on diameter. Finally, one conventional propeller design and one winglet propeller design are selected to make full-scale prototypes and tested on the water, where the fuel economy of these prototypes is measured and compared with the numerical predictions. The results demonstrate that the winglet design surpasses the optimal conventional propeller design, yielding a 13.2% improvement in fuel economy under light conditions and a 9.3% improvement under heavy conditions. These findings substantiate the advantages of utilizing winglet designs for propellers operating under highly loaded conditions.

Impact of eave and roof pitch on cross ventilation for an isolated building with sawtooth roof

An eave refers to an extension attached to the building roof to protect the interior space from direct solar radiation and improve the performance on cross ventilation. In this study, the impact of eave inclination angle and roof pitch of an isolated sawtooth roof building on cross ventilation were investigated. The eave configurations at either windward or leeward openings were included. 3D steady Reynolds-Averaged Navier-Stokes (RANS) equation in combination with the Shear-Stress Transport model (SST k-ω model) was used for the Computational Fluid Dynamics (CFD) simulations. Grid sensitivity study was carried out and the performance of cross ventilation was evaluated based on the non-dimensional velocity magnitude, spatial distribution of pressure coefficient as well as the ventilation rate of the building. For the simulation model with 55° roof pitch, it is observed that a region with high velocity magnitude formed on top of the leeward eave due to the higher roof pitch and presence of the leeward eave. Results also indicated that the building model with 90° leeward eave and 55° roof pitch has the highest increment in ventilation rate which is 7.16%. On the other hand, the building model with 90° windward eave has the highest pressure coefficient because more blockage of airflow is caused by a steeper roof as the roof pitch of the building increases. Furthermore, the building model with 90° leeward eave shows a larger region with negative pressure at the leeward façade indicating higher airflow leaving the leeward opening. Therefore, the airflow behavior and characteristic are both dependent on the roof pitch and eave inclination angle for a naturally ventilated building.

Parametric optimization of vortex generator configuration for flow control in an intake duct for waterjet propulsion

Vortex generators (VG) are simple miniature devices that suppress flow separation in aeronautical elements. In this study, VG device operation in a submerged flush waterjet inlet was numerically investigated to improve the flow quality entering the jet pump at low Inlet Velocity Ratio (IVR). Steady Reynolds-averaged Navier-Stokes equations are solved using the SST k-ω turbulence model and used to simulate the flow field, with the effects of the orientation, angle of attack, height and installation position of VGs thoroughly analyzed. The hydrodynamic performance of the intake duct was assessed over the operational range. The results show that VGs arranged in a divergent configuration better suppress flow separation. The zonal magnitude of the high turbulent kinetic energy on the ramp side was reduced, with the non-uniformity in the pump face plane reduced from 0.41 to 0.26. Excessive angles of attack and VG heights have negative effects. Through parametric studies, VGs were found to have best performance in the target duct with an angle of attack of 40⁰ and a height measuring 20% of the bottom boundary layer thickness. In addition, VGs installed along the keel of a ship can effectively suppress the flow distortion and improve the overall efficiency of the intake duct.

Convective heat transfer in a thermal chimney for freshwater production in geothermal total flow systems

The convective heat transfer of air in a laboratory-scale thermal chimney with rectangular cross-section of constant area and two row electrical heaters simulating two heat exchangers was studied experimentally and numerically at 60–200 ℃ nominal temperatures of the top row heaters, 100 ℃ of the low row heaters and 20 ℃ ambient temperature to verify our design concept on freshwater production in geothermal total flow systems. Computational fluid dynamics simulations of air convective heat transfer were performed in ANSYS 2019R CFX based on the three-dimensional, steady Reynolds-averaged Navier-Stokes equations, Boussinesq buoyancy model, k-ω turbulence model, and energy equation. The thermal radiation between heater surfaces and chimney walls was considered. The overall thermal and heat transfer characteristics, temperature and flow fields in the chimney were obtained. Effects of boundary condition of heater surface and thermal radiation between two row heaters on heat transfer were discussed. The thermal characteristics of the chimney with two row heaters are better than that with single row heaters. The predicted thermal power and convective Nusselt number agree with the experimental data, and the convective Nusselt number of the low row heaters is enhanced by (11.6–29.8) % compared with the single row heaters. The optimal operating nominal temperature of top row heaters should be higher than 140 ℃, and the optimal centre-to-centre row gap ratio is 5. Multiple jets in the gaps among the heaters and temperature jump crossing each row were observed. The maximum velocity and temperature jump rise with increasing heater nominal temperature.

The effects of the return channel geometry on the aerodynamic performance of a centrifugal compressor: A numerical study

In this paper, steady RANS (Reynolds-Averaged Navier-Stokes) numerical modeling of the return system of a centrifugal compressor is carried out to investigate the effects of various geometrical parameters on the aerodynamic performance of the return system. The effects of using splitter vanes in the return-channel, the width ratio of the return channel and the diverging angle of the walls are examined. Numerical results revealed that using splitter vanes can enhance the stage averaged pressure ratio and polytropic efficiency by 0.74 and 3.21%, respectively. Results showed that increasing the width ratio can enhance the efficiency and pressure ratio near the choke region, however, it has marginal effects on the performance at low flow coefficients. Results also showed that, due to the negligible effects of the diverging angle on the stage performance, using diverging hub wall can reduce the total axial length of the multistage compressor. Finally, a new return system is designed based on the combination of the best geometrical parameters found in the current study. Simulation results demonstrated that the average pressure ratio and efficiency of the compressor have been improved by 0.99% and 4%, respectively, by the newly developed return system compared to the original design.

Investigation of convective heat transfer at the facade with balconies for a multi-story building

As a common building appendage, balconies can significantly alter the flow pattern near the building façade, thus greatly affecting the convective heat transfer coefficient (CHTC) of façades. In the present study, computational fluid dynamics (CFD) simulations are performed with the 3D steady Reynolds-averaged Navier-Stokes (RANS) SST k-ω model to evaluate the convective heat transfer at the building façade with balconies. This model is systematically validated by a reduced-scale wind tunnel experiment and then utilized to conduct simulations with high-resolution grids. The effects of the balcony height (height of balcony parapet walls), depth, and length on the forced convective heat exchange at the façade and balcony surfaces are analyzed in detail. The results show that with the presence of balconies, the surface-averaged CHTC (CHTCavg) is reduced by about 17.5% at the windward facade, while it is reduced by 35.2% at the leeward façade. Furthermore, when the height of the balcony (Hp) varies from 0.5 to 1.5 m, CHTCavg decreases by up to 39%, 48.8%, and 50% on the leeward facade, the inner surfaces of windward and leeward balconies, respectively. However, balcony depth and length have relatively non-significant effects on the CHTCavg of building facades and balconies' surfaces. Finally, new correlations are established to describe the average CHTC along the façade and balconies’ surfaces. The difference in CHTCavg obtained from the correlations and the simulations is less than 6%. The findings of the present study would facilitate the calculation of the cooling and heating loads of buildings with balconies.

Impingement Heat Transfer Enhancement in Crossflow by V-Shaped Protrusion Vortex Generator

This paper numerically studies the effect of a V-shaped protrusion vortex generator on flow and heat transfer of impingement cooling in crossflow. The simulations are performed by solving three-dimensional steady Reynolds-Averaged Navier-Stokes equations with shear stress transport k-Omga turbulence model. Four crossflow ratios, five locations and heights of the V-shaped protrusion vortex generator are discussed. Results indicate that (1) the V-shaped protrusion vortex generator effectively improves the peak value of Nusselt number and widens the high heat transfer range at the crossflow ratios of 0.3 and 0.4 due to the decrease of actual crossflow ratio and the generation of an additional counter-rotating vortex pair, while it has little effect at the crossflow ratios of 0.1 and 0.2; (2) as the distance between vortex generator and jet hole increases from 2.5 to 4.5 times of diameter, the Nusselt number increases and the jet total pressure drop decreases, thus the thermal performance factor increases with a maximum increasement of 22.57%; and (3) the Nusselt number increases with the increase of vortex generator height, and when the crossflow ratio is large enough, the impingement heat transfer can be improved even that a shallow V-shaped protruding vortex generator is used.

Influence of inlet boundary conditions on 3D steady RANS simulations of non-isothermal mechanical ventilation in a generic closure

Inlet boundary conditions at the supply opening play an important role in the accuracy and reliability of computational fluid dynamics (CFD) simulations for indoor airflow. A non-exhaustive overview of past CFD simulations of ventilation flow in generic enclosures indicates that uniform hypotheses of inlet boundary conditions are commonly used. However, due to thermal effects and a complex air supply system geometry, constant values of inlet airflow quantities can be insufficient for an accurate simulation of non-isothermal ventilation flow. In addition, this can lead to biased conclusions on the performance of turbulence models and other computational settings. To assess this issue, a well-documented experiment on non-isothermal mechanical ventilation with detailed information of inlet conditions from the literature is used in this study. Different turbulence models and inlet boundary conditions are validated, including seven steady Reynolds-averaged Navier-Stokes (RANS) models, six methods of imposing inlet air velocity, three methods of specifying air temperature, and a wide range of turbulence quantities. The parametric study demonstrates that in this particular case, i.e., cold air supply from a single round nozzle diffuser close to the ceiling, ω-based turbulence models perform better than the ε-based turbulence models. The impacts of different methods to specify inlet air temperature profile and turbulence quantities on the simulation results are not significant. However, if the inlet airflow direction is not taken into account appropriately, significant deviations are observed between CFD simulations and experimental data.

CFD assessment of wind energy potential for generic high-rise buildings in close proximity: Impact of building arrangement and height

High-rise building complexes are of great importance for enabling sustainable urban development in large parts of the world. Earlier studies have indicated that high wind speed regions can be present along the passage between two high-rise buildings as well as above the roofs. At such locations, urban wind energy could be harvested by installing wind turbines between and/or above the roof of the buildings. However, the available wind energy potential around an array of generic high-rise buildings in close proximity has not yet been assessed for different building configurations. This paper conducts a detailed evaluation of the impacts of the building arrangement and height for a 2 × 2 array with a building height-to-street width ratio of 30 on the mean wind velocity and the wind energy potential along the passages between both upstream and downstream buildings as well as on their roofs. The following parameters are analyzed: (i) the passage width between the two upstream buildings (w), (ii) the streamwise distance between the upstream and downstream buildings (d), and (iii) the height difference between the upstream and downstream buildings (ΔH). The 3D steady Reynolds-averaged Navier-Stokes (RANS) equations are solved using the Reynolds stress model (RSM) turbulence model for closure. The CFD results are validated using wind-tunnel measurements of mean wind speed and turbulence intensity performed for the same building array. The results show elevated wind power density along the upstream passages for small w (=0.15B), high d (=0.6B), and equal building height (ΔH = 0). In contrast, comparatively high values of w, small d, and ΔH < 0 yield high wind power densities between the downstream buildings. Among the different wind turbine types considered, horizontally-mounted vertical axis wind turbines seem the most promising option for wind energy harvesting between the buildings.

Buffet boundary prediction using RANS-based criteria and adjoint methods

In this work a procedure is proposed for a fast estimate of the transonic buffet boundary on aerofoils and wings. This algorithm aims to be an alternative to unsteady computational fluid dynamics simulations that are prohibitively expensive for complex flow cases. The algorithm combines steady Reynolds-averaged Navier-Stokes simulations with an adjoint method. Reynolds-averaged Navier-Stokes simulations were used to determine the buffet onset by means of suitable criteria, while the adjoint method was used to compute the sensitivities of the buffet parameters to the flight conditions, and draw the buffet boundary. The procedure was tested for two-dimensional configurations with available experimental data and for two different criteria. Particular attention was paid to the efficiency of the method and its ability to reduce computational cost with respect to purely RANS criteria for buffet boundary estimation. Possible variations to the procedure were also discussed in the results section together with the effect of some key parameters of the proposed algorithm.