Turbulent Kinetic Energy Profiles Research Articles

Modulating local winds and turbulence around a single building obstacle with the obstruction of tall vegetation

Strategic vegetation placement can significantly alter airflow patterns and turbulence, fostering desired wind environments. By comparing scenarios where vegetation is placed upstream, downstream or absent (treeless) relative to a single building using large-eddy simulation, this study provides detailed insights into the sensitivity of flow dynamics to the positioning of the vegetation. Upstream vegetation more significantly disrupts the flow patterns around the building obstacle, altering vertical wind profiles and modifying wake circulations, compared to downstream vegetation. A small shear layer developed at the plant top for upstream vegetation markedly influences turbulent kinetic energy (TKE) on both the leeward and windward sides of the building, shifting the inflection point in vertical TKE profiles by up to 0.13H. By contrast, smaller tree-building separations lead to an effective merging of their aerodynamic profiles, whereas larger separations confine the streamwise breadth of turbulent fluxes, amplifying flux exchanges in the spanwise direction. Spectral analyses reveal that upstream vegetation consistently results in higher power spectral densities of the streamwise turbulence in the residential area than downstream vegetation. While small-scale spanwise velocity fluctuations are found to be comparably energetic at the building's windward side for upstream vegetation, the power becomes substantially concentrated on large-scale eddies in the building wake region, providing specific insights into modulating turbulent eddy motions within the residential zone.

Numerical analysis of a parallel triple-jet of liquid-sodium in a turbulent forced convection regime

In the present study, we have applied a combined wall-resolving dynamic Large-Eddy Simulation (LES) (for the velocity field) and Direct Numerical Simulation (DNS) (for the temperature field) approach for mixing of parallel triple-jets with different temperatures of liquid sodium in a turbulent forced convection regime. Because of the high thermal conductivity of sodium (a low-Prandtl fluid), we adopted the dynamic Smagorinsky subgrid closure for the unresolved velocity scales, while the thermal scales are fully resolved. Furthermore, the Time-dependent Reynolds-Averaged Navier-Stokes (T-RANS) approach with the high-Reynolds number variant (i.e. with the wall functions as boundary conditions along solid boundaries) of the four-equation eddy viscosity model (k−ε−kθ−εθ) was applied. The fine-mesh LES/DNS provided a close agreement with the experimental data for both velocity and temperature fields (for both first- and second-moments). In contrast, the coarse-mesh LES/DNS overestimated the turbulent kinetic energy profiles at different distances from the inlet plane. The T-RANS results confirmed a good agreement with the mean streamwise velocity and turbulent kinetic energy, as well as the mean temperature profiles. Finally, the analysis of power spectral density distributions of the temperature signal revealed that all simulation techniques captured a dominant flow frequency originating from the induced Kelvin-Helmholtz instabilities between the side and central jets. The presented combined dynamic LES/DNS approach is recommended for future simulations of the turbulent forced convection flows of low Prandtl fluids, especially if thermal fatigue effects need to be predicted correctly.

Improvements for the DDES model with consideration of grid aspect ratio and coherent vortex structures

The delayed detached eddy simulation (DDES) model has achieved great success in simulations of massive separations, but has been found to significantly delay the Kelvin-Helmholtz (K-H) instability in weak separations due to the “grey area” problem. A new DDES model was proposed by considering the grid aspect ratio and local coherent vortex structures, named the DDES with an adaptive grid-scale and adaptive coefficient (DDES-AGAC). The DDES-AGAC model was examined in detail by computing four typical types of turbulent flows: fully attached turbulent boundary layer, massive separation, weakly unstable flow with a fixed separation point, and weak separation over a smooth surface due to pressure gradient. The results confirm the advantages of the new DDES-AGAC model over the standard DDES. It exhibits high efficiency and insensitivity to spanwise or circumferential grid spacing in flow simulations of quasi-three-dimensional flows. For the weakly unsteady separation, it helps to accelerate the rapid switch from Reynolds-averaged Navier-Stokes equations to large eddy simulation, unlock the K-H instability in the shear layer, and hence accurately resolve the turbulent structures, profiles of velocity and turbulent kinetic energy, and spectra in the separation regions. Furthermore, it does not result in a noticeable drag or skin friction reduction in the fully attached flow simulations. The DDES-AGAC model is expected to be widely used in simulations of engineering flows with separations.

Examining relative impacts of atmospheric and oceanic factors on offshore wind farms

Accurate understanding and prediction of how ocean waves affect offshore wind farms are critical to their siting, design, and operation. This study presents a computational framework for simulating finite offshore wind farms using Large Eddy Simulation (LES) and a Dynamic Wave Spectrum Model (Dyn-WaSp). Implementation of the Dyn-WaSp with and without a correction for swell modes is compared to a static roughness (wave phase-averaged) model, which has a similar computational cost. Impacts of the different wave models on the wind’s mean velocity and turbulent kinetic energy profiles in the finite offshore wind farm are examined, and ideal available power at hub height is compared. The dynamic wave spectrum model is shown to predict lower mean velocities in comparison to the phase-averaged approach and predicts higher shear and turbulent kinetic energy, suggesting that loading on turbines is greater than would be estimated by a static roughness model.

Profiling the molecular destruction rates of temperature and humidity as well as the turbulent kinetic energy dissipation in the convective boundary layer

Abstract. A simultaneous deployment of Doppler, temperature, and water-vapor lidars is able to provide profiles of molecular destruction rates and turbulent kinetic energy (TKE) dissipation in the convective boundary layer (CBL). Horizontal wind profiles and profiles of vertical wind, temperature, and moisture fluctuations are combined, and transversal temporal autocovariance functions (ACFs) are determined for deriving the dissipation and molecular destruction rates. These are fundamental loss terms in the TKE as well as the potential temperature and mixing ratio variance equations. These ACFs are fitted to their theoretical shapes and coefficients in the inertial subrange. Error bars are estimated by a propagation of noise errors. Sophisticated analyses of the ACFs are performed in order to choose the correct range of lags of the fits for fitting their theoretical shapes in the inertial subrange as well as for minimizing systematic errors due to temporal and spatial averaging and micro- and mesoscale circulations. We demonstrate that we achieve very consistent results of the derived profiles of turbulent variables regardless of whether 1 or 10 s time resolutions are used. We also show that the temporal and spatial length scales of the fluctuations in vertical wind, moisture, and potential temperature are similar with a spatial integral scale of ≈160 m at least in the mixed layer (ML). The profiles of the molecular destruction rates show a maximum in the interfacial layer (IL) and reach values of ϵm≃7×10-4 g2 kg−2 s−1 for mixing ratio and ϵθ≃1.6×10-3 K2 s−1 for potential temperature. In contrast, the maximum of the TKE dissipation is reached in the ML and amounts to ≃10-2 m2 s−3. We also demonstrate that the vertical wind ACF coefficient kw∝w′2‾ and the TKE dissipation ϵ∝w′2‾3/2. For the molecular destruction rates, we show that ϵm∝m′2‾w′2‾1/2 and ϵθ∝θ′2‾w′2‾1/2. These equations can be used for parameterizations of ϵ, ϵm, and ϵθ. All noise error bars are derived by error propagation and are small enough to compare the results with previous observations and large-eddy simulations. The results agree well with previous observations but show more detailed structures in the IL. Consequently, the synergy resulting from this new combination of active remote sensors enables the profiling of turbulent variables such as integral scales, variances, TKE dissipation, and the molecular destruction rates as well as deriving relationships between them. The results can be used for the parameterization of turbulent variables, TKE budget analyses, and the verification of large-eddy simulations.

CFD microscale modelling of flow behavior in different parts of a rotating packed bed

Process intensification (PI) is playing a key role in alleviating the challenge of reducing carbon footprint of many chemical processes and bringing down their development costs. Over the years, many PI technologies have been investigated with rotating packed bed (RPB) technology receiving much of the attention for its potential of significant intensification in terms of capital expenditure, operating costs, and hardware size. In this study, microscale CFD simulations of a rotating packed bed were conducted, and the results were validated with experimental data. The results show the strong relation between the reverse flow at the packing outer periphery and the gas maldistribution factor. The latter is mainly caused by the accelerating flow in the outer cavity. Inside the wire mesh packing, the gas flow is found to be almost fully uniform for nearly half of the total packing depth. Also, turbulent kinetic energy (TKE) levels at the packing outer edge are strongly linked to the slip tangential velocity component, while at its inner edge, they depend mainly on the radial packing velocity. The so-called gas end effect zone is detected by observing the TKE profiles near the packing outer edge. The latter accounts for less than 10% of the total packing depth. The validity of the widely used porous media model in RPBs’ packing for both radial and tangential directions is confirmed by the obtained results, but this excludes the packing inner and outer edges. In the inner cavity region, gas exhibits two distinctive behaviors and transits from free vortex flow to swirling flow as the flow becomes close to the vortex core. As a result of this transition, the increase in shear stress accelerates the decrease in the gas tangential velocity in the vortex core and help speed up the favorable pressure gradient and flow establishment beyond the vortex core.

Design of parabolic conic gas cyclones for coarse particle classification: A CFD study with Response Surface Methodology

Gas cyclones have traditionally been used as particle separators to remove dust from gas streams, with the goal of achieving a dust-free gas flow at the exit pipe and recovering particles to the dust outlet. Typically, the particle size for removal to the dust outlet is less than 1 μm. However, gas cyclones can also be targeted for particle classification rather than particle removal. In this study, the effect of the shape of the curved conical wall, specifically the parabolic concave and convex conic designs, on particle classification in a 31 mm diameter gas cyclone was evaluated using Computational Fluid Dynamics (CFD). The CFD model predictions for standard small-scale gas cyclones were validated using experimental data available in the literature for various inlet flow rates. The curvature of the conical wall varied from convex to concave, resulting in the observation of sharper and coarser cut sizes for the parabolic conic designs. The cut size increases to 4.91 μm for the concave design at a flow rate of 30 liters per minute, compared to 3.38 μm and 2.65 μm for the convex and standard design cyclones, respectively. The axial, tangential, pressure, and turbulent kinetic energy profiles were used to provide an explanation for the observed results. The interaction effects between the parameters were explored using the Response Surface Methodology. Further, Monte Carlo simulations are performed to observe the trends in predictions by exploring the design space.

THE NUMERICAL ANALYSIS OF MIXING DEPTH AND THE THICKNESS OF BBL CONSIDERING THE SUBMERGED AQUATIC VEGETATION AND WIND STRESS

Mangrove forests, salt marshes, and seagrass meadows have a vital role in mitigating global warming (Duarte et al.,2013). They contribute about 50 percent of carbon burial in ocean sediments (Duarte et al., 2013). On the other hand, submerged aquatic vegetations (SAVs) significantly affect the mixing depth (the thickness of the upper layer), resulting in a change in carbon flux with the atmosphere. According to the numerical simulations in the previous studies, mixing depth becomes larger with increasing SAV's density and decreasing SAV’s height (Herb et al., 2005, Vilas et al., 2017). On the other hand, Coates et al. (2009) indicated that wind stress was not crucial for mixing depth in SAV’s meadow since it did not change the vertical turbulent kinetic energy (TKE) profile, except in the thin surface layer, thus mixing depth became similar even though wind speed varied under the effect of SAVs (Coates et al., 2009). In addition, it is crucial to estimate the thickness of the benthic boundary layer (BBL) because dissolved inorganic carbon elucidates from bottom sediments, affecting carbon flux. Therefore, we aim to clarify how wind stress impacts the mixing depth and thickness of BBL with SAVs.

Magnetic resonance velocity imaging of turbulent gas flow in a packed bed of catalyst support pellets

Compressed sensing magnetic resonance methods have been used to image the time-averaged velocity and turbulent kinetic energy in 3D for turbulent gas flowing through a bed of porous, hollow cylindrical catalyst support pellets. Velocity and turbulent kinetic energy images were acquired at a spatial resolution of 0.70 mm (x) × 0.70 mm (y) × 1.0 mm (z) for particle Reynolds numbers, Rep, of 500, 2500 and 6500 in a bed with a tube-to-particle diameter ratio of 4.7. These data represent the first full-field measurements of turbulent gas flow in packed beds of non-spherical pellets. The resulting images reveal several interesting features of the hydrodynamics in this system. A large degree of flow heterogeneity is observed in the bed, with regions of high-speed fluid observed near the walls and in large voids, and regions of backflow observed in the wake of pellets, between pellets, and within the pellet holes. For increasing Rep, the normalized axial velocity at the wall is found to increase, and the normalized turbulent kinetic energy becomes more homogeneous throughout the bed. The correlation between the turbulent kinetic energy and time-averaged velocity shows that the highest turbulent kinetic energy occurs in regions of intermediate time-averaged velocity. Further, the turbulent kinetic energy profile at the pellet-scale is substantially different from the case of simple channel flow for Rep≥ 2500. Overall, these measurements clearly demonstrate the ability of magnetic resonance methods for acquiring full-field flow data in packed bed systems using commercially-relevant pellets and flow conditions.

Assessment of turbulence correction for actuator-line model of a horizontal-axis tidal stream turbine in near-wake region

Near-wake characteristics of a turbine play an important role in turbine–wake interactions and performance assessment in turbine arrays. In this study, the near-wake flow behind a laboratory-scale tidal stream turbine is simulated and compared with experimental data. An actuator-line model is used to represent the rotating blades. Further, two physics-based turbulence corrections are proposed for unsteady Reynolds-averaged Navier–Stokes (URANS) modeling of the tidal stream turbine. These corrections seek to model additional turbulent production near the blade boundary layer and in the tip shear region. The data analysis for the fluctuating velocity spectrum in the experiment highlights that the turbulence kinetic energy (TKE) can be decomposed into three parts: the background turbulence from the onset flow, rotor-scale turbulence, and blade-induced turbulence. The blade-induced turbulence dominates near the rotor plane and decreases sharply beyond one diameter downstream. Near-wake profiles of velocity and TKE are compared with experimental data. The results show that turbulence corrections in the URANS model enhance the production of turbulence induced by the blade rotation. It can improve turbulence prediction in the near-wake region with a minor impact on the loads and wake velocity.

Model and method to predict the turbulent kinetic energy induced by tidal currents, application to the wave-induced turbulence

A prediction model for the turbulent kinetic energy (TKE) induced by tidal-currents is proposed as a function of the barotropic velocity only, along with a robust method evaluating the different parameters involved using Acoustic Doppler Current Profiler (ADCP) measurements from Alderney Race. We find that the model is able to reproduce correctly the TKE profiles with coefficients of correlation on average higher than 0.90 and normalised root-mean-square errors (NRMSE) less than 14%. Different profiles are also tested for the mean velocity, no satisfactory prediction model is found but we are able to have decent estimates of the velocity shear and friction velocity. Two applications are then carried out. First the turbulent budget terms are estimated and discussed. We identify the turbulent production and dissipation of TKE as the most important mechanisms, then we discuss the validity of several theoretical results derived for isotropic turbulence for this application. A strong departure for the estimation of the turbulent dissipation is notably found and explained by the turbulent anisotropy. At last the prediction model for the TKE is used to infer the wave-induced TKE. We show the importance of removing the tidal component, waves can have a strong influence down to mid-depth.

Numerical Study of Flow Characteristics Inside Serrated Dodger Channels Using the Openfoam Program

The dodger building is one part of the dam component that serves to divert the flow of the river during construction work. One of the requirements for evasive buildings is to be able to drain river water well, so that the characteristics of the flow in the dodger channel need to be known to see how the flow conditions that occur in the dodge channel. This study aims to examine the flow characteristics of the Tapin Dam circumvention channel due to the design of the channel tunnel using a tooth shape, by modeling the relationship between the flow characteristics (velocity profile, pressure drop profile, and turbulent kinetic energy profile) of the jagged channel dimension shape using numerical computational analysis with the help of CFD (Computational Fluid Dynamics) software. From the modeling results, it will be known the influence of channel dimensions on flow characteristics in the dodger channel.

Theoretical analysis of unsteady buoyant turbulent heat and mass transport from a vertical plate using LRN k-ϵ model

The contemporary analysis has been developed to investigate the transient natural convective combined heat and mass transport of a turbulent flow adjacent to the plate by utilizing the low Reynolds number (LRN) k-ϵ model. The research problem on turbulent flow is assumed to be two dimensional and viscous incompressible. The governing turbulent equations such as continuity, momentum, energy, concentration, turbulent kinetic energy (TKE) and dissipation rate of TKE are considered as per the flow geometry. Due to the non-availability of analytical or direct numerical techniques, the corresponding highly nonlinear PDE’s are solved by adopting the standard implicit type of finite difference method namely Crank–Nicolson scheme. Also, this scheme is unconditionally stable and helps to keep the governing turbulent flow equations in discretized form and are solved via tridiagonal matrix algorithm. The simulated results for several control parameters including turbulent Reynolds number are graphically displayed and also analyzed the flow profiles. To understand the physical phenomenon of turbulent flow, the authors shown the turbulent behavior of average momentum, energy and mass transfer rates. It has been noted that, the average temperature and concentration fields boosted with rising values of . Also, TKE and dissipation rate of TKE profiles increase with enhancing turbulent values. In addition, the simulated turbulent flow results from the LRN k-ϵ model are compared with usual laminar flows as a special case and found to be in respectable agreement.

Numerical study of transient convective turbulent boundary layer flow along a vertical plate: analysis of kinetic energy and its dissipation rate

ABSTRACT The present article numerically investigates the turbulent buoyancy-driven (natural convection) flow along a vertical plate with a low Reynolds turbulence two-equation k-ϵ model. The deployed turbulence model is appropriate for low Reynolds number (LRN) turbulent flow adjacent to a solid boundary, and the turbulence kinetic energy (TKE) and dissipation rate of TKE are estimated using the momentum equations and are solved simultaneously with the mean flow conservation equations. Two-dimensional time-dependent viscous incompressible turbulent flow is simulated. This flow domain is governed by a highly non-linear group of partial differential equations, namely the time-averaged continuity, momentum, and energy, and also the flow property is determined through TKE, and dissipation rate of TKE equations. Since these equations are not solvable using analytical methods, an implicit second-order finite difference method is employed to solve the governing turbulent flow equations numerically. The simulated time-averaged velocity, temperature, TKE, and dissipation rate of TKE profiles along with friction factor and heat transfer rate are computed for different values of turbulent Reynolds and Prandtl numbers. Average velocity, temperature, turbulence energy, and dissipation rate under both transient and steady-state conditions are decreased with increment in . There is a decrement in average transient velocity and turbulence energy rates with increasing values, whereas the average temperature profile increases. Average steady temperature and turbulence energy profiles are also enhanced with rising values of . The majority of researchers focus on laminar flow and thus far turbulent flow has mainly been addressed through experiments only. To better elucidate the flow in this condition, the novelty of the present study is to utilize a numerical FDM procedure to probe more deeply into the turbulence characteristics in buoyancy-driven (natural convection) flow along a vertical plate with a low Reynolds turbulence two-equation k-ϵ model, as this topic is fundamental to many applications in science and engineering. Furthermore, the results of the turbulent flow simulations obtained from the current model are corroborated with existing results for the special case of laminar flow and a good correlation is achieved.

Evaluation of spacer-induced hydrodynamic mixing using particle image velocimetry: Impact on membrane distillation performance

Net-type spacers are used to promote hydrodynamic mixing in the feed channel of crossflow membrane filtration. The induced fluid flow mitigates concentration and temperature polarization effects on the membrane surface, leading to the enhancement of heat/mass transfer across the membrane. The objective of this research is to study the effect of spacer geometry on fluid flow and heat/mass transfer in the feed channel. The particle image velocimetry (PIV) technique was used to measure flow velocity in the feed channel with net-type spacers. Flow characteristics at a wide range of Reynolds numbers were described in terms of time-averaged mean velocity and turbulence kinetic energy (TKE). The PIV result revealed distinct profiles of mean velocity and TKE at different channel heights with bi-planar spacers. The spacer with the largest hydrodynamic angle of 120° (Net120) was found to be the most efficient in promoting flow unsteadiness because it led to the largest channel-averaged TKE among the selected spacers. The unsteady flow regime on Net120 was accompanied by the greatest pressure drop over the feed channel. Moreover, the effect of spacer-induced mixing on heat/mass transfer across the membrane was further evaluated via the flux measurement with vacuum membrane distillation experiments. In comparison with other spacers, Net120 resulted in a relatively higher flux because of the improved hydrodynamic mixing. These findings reveal the validity of the experimental approach in identifying the impact of key spacer design parameters on hydrodynamic mixing. Nusselt number was calculated to relate the hydrodynamic flow and thermal flow in the feed channel of membrane distillation, and a modified Nusselt number correlation was proposed to better predict convective heat transfer in the spacer-filled channel in VMD.

Field measurement of the erosion threshold of silty seabed in the intertidal flat of the Yellow River Delta with a newly-developed annular flume

Accurately measuring the critical shear stress is crucial for numerous applications, such as sediment transport modeling, erosion prediction, and the design of sustainable coastal engineering structures. However, developing reliable and precise in-situ measurement devices faces significant challenges due to the harsh and dynamic nature of aquatic environments. Factors like turbulence and waves introduce complexities that must be considered when designing and calibrating these devices. The newly developed Openable Underwater Carousel In-situ Flume (OUC-IF) was used to determine the critical shear stress (τc) and quantify erosion rates. Acoustic Doppler Velocimeter (ADV) was employed to measure 3D near-bottom velocities, which were then used to estimate and pre-calibrate bed shear stress (τ) applied on the seabed in the annular flume. Three computation methods of shear stress were evaluated: turbulent kinetic energy (TKE), direct covariance (COV), and log profile (LP). In-situ erosion experiments were conducted for the first time at two sites in the tidal flat of the Yellow River Delta (site 1 with a water depth of 1.32 m and site 2 with a water depth of 0.75 m). The critical shear stress was found to be 0.10 Pa at site 1 and 0.19 Pa at site 2, and the erosion rates of the sediments were successfully measured. The effect of wave-seabed interactions on erosion resistance was explored by theoretically estimating the wave-induced pore pressure of the seabed based on the observed data. The max liquefaction degree of the seabed at site 1 and site 2 was 0.035 and 0.057, respectively, and the average erosion coefficient Me was 2.63E-05 kg m-2s-1 at site 1 and 3.48E-05 kg m-2s-1 at site 2.

Numerical simulation of an idealised Richtmyer–Meshkov instability shock tube experiment

The effects of initial conditions on the evolution of the Richtmyer–Meshkov instability (RMI) at early to intermediate times are analysed, using numerical simulations of an idealised version of recent shock tube experiments performed at the University of Arizona (Sewell et al., J. Fluid Mech., vol. 917, 2021, A41). The experimental results are bracketed by performing both implicit large-eddy simulations of the high-Reynolds-number limit as well as direct numerical simulations (DNS) at Reynolds numbers lower than those observed in the experiments. Various measures of the mixing layer width $h$ , known to scale as ${\sim }t^\theta$ at late time, based on both the plane-averaged turbulent kinetic energy and volume fraction profiles are used to explore the effects of initial conditions on $\theta$ and are compared with the experimental results. The decay rate $n$ of the total fluctuating kinetic energy is also used to estimate $\theta$ based on a relationship that assumes self-similar growth of the mixing layer. The estimates for $\theta$ range between 0.44 and 0.52 for each of the broadband perturbations considered and are in good agreement with the experimental results. Decomposing the mixing layer width into separate bubble and spike heights $h_b$ and $h_s$ shows that, while the bubbles and spikes initially grow at different rates, their growth rates $\theta _b$ and $\theta _s$ have equalised by the end of the simulations. Overall, the results demonstrate important differences between broadband and narrowband surface perturbations, as well as persistent effects of finite bandwidth on the growth rate of mixing layers evolving from broadband perturbations. Good agreement is obtained with the experiments for the different quantities considered; however, the results also show that care must be taken when using measurements based on the velocity field to infer properties of the concentration field.

On the wake characteristics of a model wind turbine and a porous disc: Effects of freestream turbulence intensity

Porous discs are the experimental counterparts of the numerical actuator disc concept and are frequently used in wind tunnel studies to reproduce the far-wake characteristics of wind turbines. The potential of using porous discs instead of wind turbines comes with the benefits such as operational easiness and design simplicity. This paper presents the results of an experimental study focusing on the wake development characteristics of a single porous disc and a single wind turbine under different freestream turbulence intensities but at similar Reynolds number conditions. The measurements are performed using two-dimensional two-component particle image velocimetry (2D2C PIV) technique up to seven diameters downstream distance in the wake of the models. The wake flows are analyzed in terms of the development of mean wake velocity deficit, wake decay, and wake turbulence characteristics. The results show that the wake recovery occurs faster for both the porous disc and the turbine with increasing freestream turbulence intensity. The wake width increases with the freestream turbulence intensity, indicating faster diffusion of the wake for both models. On the other hand, the wake width of the porous disc and the turbine is similar at the same turbulence intensity conditions, although they operate with slightly different thrust coefficients. Turbulent kinetic energy profiles in the far wake of the porous disc and the turbine, which are significantly different for the low turbulence intensity conditions, become similar for the high turbulence intensity case. The comparison of the turbulent kinetic energy, decay rate and production characteristics points to different turbulence mechanisms in the wake of the porous disc and the turbine. In the near wake, the wake decay rate and turbulent kinetic energy levels are higher in the porous disc case due to enhanced mixing levels associated with the grid-generated turbulence. In the far wake, the decay rate of the turbine wake is higher, most probably due to the breakdown of the tip vortices. When the freestream turbulence is increased, the differences in the turbulent wake flow characteristics diminish in the far wake, particularly after a 4.5-diameter downstream distance.

Numerical Wind Tunnel Simulation for Pollutants Dispersion with Urban 2003

A CFD numerical wind tunnel software was made to simulate the air flow and diffusion of pollutants in urban areas. This softeware is very compact. With the collaboration of the modelling tool AutoCAD and the post-processing software Paraview, a set of standardized numerical simulation process of atmospheric diffusion has been established. The model of the software is based on Open Foam open-source platform, which can perform 3D large-scale parallel operation, and solve 3D Navier Stokes equations on Computer cluster. Based on the observation data of iop9 from the 2003 urban joint field study in Oklahoma City, Oklahoma, the numerical wind tunnel is further evaluated. The wind and concentration field in the near and middle regions, as well as the wind speed, wind direction, friction velocity and turbulent kinetic energy (TKE) profiles in the urban wake region are compared between the numerical wind tunnel and the urban 2003 field experiment. The results show that the numerical wind tunnel can be an economic and effective tool to simulate the diffusion of pollutants in urban areas.

Heat transfer characteristics of jet impingement on a surface mounted with ribs using LES

The present study investigates flow and heat transfer features of jet impingement on a surface fitted with detached ribs at Re = 20,000 using OpenFOAM 4.1 based large eddy simulation (LES). The WALE SGS model is applied. A wall resolved simulation is performed to understand the flow behaviour and heat transfer enhancement in the impingement regions and ribs. The WALE model is observed to be suitable for accurate calculations of complex application-based flow configurations. A comprehensive discussion on flow physics is carried out to understand heat transfer in the rib regions and the effects of flow and turbulence parameters. The investigation reveals that augmentations in the velocity and turbulence are directly linked to the local heat transfer enhancement in the clearance between the ribs and heated plate. A dual peak exists for turbulence kinetic energy profiles. The local heat transfer is observed to be maximum in the clearance region between the second rib and plate. The region of maximum local heat transfer was observed to be related to the peak velocity and turbulence kinetic energy in the same region. The present computation is capable of capturing small-scale dynamics and turbulence structures. The present results may be utilized in testing RANS-based turbulence models and the findings are useful for researchers and engineers working on EV, HPC and related cooling industries where high localized cooling is required.