Lateral Velocity Fluctuations Research Articles

Experimental and numerical simulation research on the transport process of bubble clusters in aeration system

Improving gas-liquid mass transfer efficiency in aeration systems contributes to energy savings, cost reduction, and enhanced efficiency in wastewater treatment. However, due to the complex nonlinear interactions among bubbles in turbulence, understanding the transport mechanisms of non-uniform bubble clusters in turbulence remains unclear. This study employs a combined approach of experimental research and numerical simulations to investigate the shape, diameter distribution, trajectory, and velocity of bubbles under different aeration port sizes and flow rates. The diameter distribution of bubble clusters exhibits a bimodal distribution. Bubble trajectories during ascent mainly exhibit two types of motion patterns: “Z” shaped and linear. Increasing aeration port size and flow rate both lead to an increase in the maximum bubble diameter. Higher initial flow rates and smaller port sizes induce greater lateral velocity fluctuations in bubbles. The proposed numerical simulation method serves as a reference for simulating the transport of non-uniform bubble clusters.

Operational mechanism of valved-pulsejet engines

The study investigates twelve configurations of the self-aspirated valved pulsejet, focusing on its operational mechanism. It begins by characterizing the engine's acoustic field with varied boundary conditions, revealing an extended effective acoustic length beyond the combustion chamber and tailpipe sections when the valve is open. Radial and lateral velocity fluctuations are hypothesized as causes for vortex structure generation. Analysis of reacting pulsejets using gasoline and ethanol fuels shows that the operating frequencies are unaffected by fuel type, with geometric arrangement as the primary frequency factor. Fluctuations in dynamic pressure, microphone readings, and thrust characterize engine stability, impacted by low-frequency modes and higher harmonics. Tailpipe length emerges as the key geometric factor for performance enhancement with the medium size being the optimal option. Pressure field analysis demonstrates shock wave generation during ignition, serving as an excitation mechanism. Notably, different configurations with varied operating frequencies can yield equivalent thrust, indicating that thrust production is not solely dependent on pressure rise during combustion, despite similar pressure rise observed across most cases.

Investigation on Flow Characteristics and Its Effect on Heat Transfer Enhancement in a Wide Channel With Staggered Diamond-S Pin Fins

Abstract Flow statistic in the mid-plane of a rectangular channel with diamond-s pin fins was obtained by means of particle imaging velocimetry at Re = 10,000. Large-scale and small-scale fluctuations were separated using proper orthogonal decomposition. The flow characteristics were compared to the Nusselt number distribution on the endwall acquired by thermochromic liquid crystal to reveal the flow mechanism driving heat transfer enhancement. Results indicate that local vorticity plays an important role in strengthening Nu on both sides of leading point (Zone 1). Downstream of the two sharp edges on both sides (Zone 2), small size disturbances from shear layer eddies drive local heat transfer. The flow characteristics and heat transfer distribution downstream of the first row (Zone 3) present alternated feature along Y direction due to the interaction between shear layers of neighboring pin fins. Lateral velocity fluctuation induced by large vortex shedding drives the heat transfer augmentation in Zone 3 where there is violent large vortex shedding. Meanwhile, small size disturbances of the shear layer drive local heat transfer enhancement in Zone 3 downstream of pin fins where large vortex shedding is suppressed. For the second and third rows, there is no difference in the flow characteristics downstream of neighboring pin fins. Small-size fluctuations distributed uniformly downstream of large vortex shedding (Zone 4) resulting in a uniformly distributed Nu.

Wave-like motion and secondary currents in arrays of emergent cylinders induced by large scale eddying motion

Free-surface flows with riparian corridors are known to develop large eddies resulting from the instability associated to the inflectional profile of the longitudinal velocity in the spanwise direction. They periodically generate strong momentum exchanges inside the vegetation corridor, triggering a wave-like motion, detectable as free-surface oscillations and out-of-phase velocity components. We propose a characterization of the flow inside the vegetation corridor, focusing on the wave-like motion and its influence on secondary currents. We conditionally sample the fluid motion to highlight the structure of the phase-averaged coherent structure. Quadrant analysis shows that there is a strong variation of Reynolds stress anisotropy in the spanwise direction, which is one of the key generation mechanisms of secondary currents. Spectrograms of longitudinal and lateral velocity fluctuations reveal that the oscillatory motion is imposed on the whole of the vegetated layer, because of continuity. The analysis of the phase-averaged 2D vertical-longitudinal flow reveals that there is a complex 3D pattern of mass fluxes associated to each large eddy. In particular there is an anti-symmetric net mass imbalance which, by mass conservation, generates a mass flux directed outwards, to the main channel, near the bottom of the channel. The Eulerian expression of this motion is obtained as the spatial average of the flow over the length of the large eddy, resulting in the pattern of the secondary current in the vertical-spanwise plane. It is shown that the secondary motion is a necessary feature of free-surface turbulent flows that develop large-scale inflectional instabilities.

A scheme to correct the influence of calibration misalignment for cross-wire probes in turbulent shear flows

Velocity fluctuations, measured via multi-wire probes, are very sensitive to misalignment between the calibration coordinate system and that of the wind tunnel. The present study proposes a scheme to correct the erroneous velocity fluctuations processed from a misaligned calibration while investigating a wall-bounded turbulent shear flow. The scheme is based on the premise that the viscous-scaled spectral energy distribution in the small-scales is invariant with Reynolds number and solely depends on the viscous-scaled spatial resolution of the sensor. Energy spectra processed from the misaligned calibration, in this small-scale range, are compared with the ‘expected’ spectra obtained via synthetic experiments on a direct numerical simulation data set. The erroneous lateral velocity spectra is found to be either relatively amplified or attenuated, by almost the same factor, at all wall-normal distances across the shear flow. A unique gain, defined to be the correction ratio, is thus obtained by forcing the erroneous spectra onto the reference spectra in this scale range. This ratio is further used to rectify the time series of the lateral velocity fluctuations, acquired across the shear flow, via Fourier analysis. The scheme is shown to be effective for experiments conducted across a decade of Reynolds number and using probes of varying spatial resolution.

PIV measurement and numerical investigation on flow characteristics of simulated fast reactor fuel subassembly

Abstract The flow characteristics of reactor fuel assembly always intrigue the designers and the experimentalists among the myriad phenomena that occur simultaneously in a nuclear core. In this work, the visual experimental method has been developed on the basis of refraction index matching (RIM) and particle image velocimetry (PIV) techniques to investigate the detailed flow characteristics in China fast reactor fuel subassembly. A 7-rod bundle of simulated fuel subassembly was fabricated for fine examination of flow characteristics in different subchannels. The experiments were performed at condition of Re = 6500 (axial bulk velocity 1.6 m/s) and the fluid medium was maintained at 30 °C and 1.0 bar during operation. As for results, axial and lateral flow features were observed. It is shown that the spiral wire has an inhibitory effect on axial flow and significant intensity of lateral flow mixing effect is induced by the wire. The root mean square (RMS) of lateral velocity fluctuation was acquired after data processing, which indicates the strong turbulence characteristics in different flow subchannels.

Wake Characteristics of Tall Buildings in a Realistic Urban Canopy

The presence of tall buildings in cities affects momentum and scalar exchange within and above the urban canopy. As wake effects can be important over large distances, they are crucial for urban-flow modelling on and across different spatial scales. We explore the aerodynamic effects of tall buildings on the microscale to local scales with a focus on the interaction between the wake structure, canopy and roughness sublayer flow of the surroundings in a realistic urban setting in central London. Flow experiments in a boundary-layer wind tunnel use a 1:200 scale model with two tall buildings (81 m and 134.3 m) for two wind directions. Large changes in mean flow, turbulence statistics and instantaneous flow structure of the wake are evident when tall buildings are part of the complex urban canopy rather than isolated. In the near-wake, the presence of lower buildings displaces the core of the recirculation zone upwards, thereby reducing the vertical depth over which flow reversal occurs. This amplifies vertical shear at the rooftop and enhances turbulent momentum exchange. In the near part of the main wake, lateral velocity fluctuations and hence turbulence kinetic energy are reduced compared to the isolated building case as eddies generated in the urban canopy and roughness sublayer distribute energy down to smaller scales that dissipate more rapidly. Evaluation of a wake model for flow past isolated buildings suggests model refinements are needed to account for such flow-structure changes in tall-building canopies.

Settling of finite-size particles in turbulence at different volume fractions

We study the settling of finite-size rigid spheres in quiescent fluid and in sustained homogeneous isotropic turbulence (HIT) by direct numerical simulations using an immersed boundary method to account for the dispersed solid phase. We consider semi-dilute and dense suspensions of rigid spheres with solid volume fractions $$\phi =0.5{-}10\%$$ , solid-to-fluid density ratio $$R=1.02$$ , and Galileo number (i.e., the ratio between buoyancy and viscous forces) $$Ga=145$$ . In HIT, the nominal Reynolds number based on the Taylor microscale is $$Re_{\lambda } \simeq 90$$ , and the ratio between the particle diameter and the nominal Kolmogorov scale is $$(2a)/\eta \simeq 12$$ (being a the particle radius). We find that in HIT the mean settling speed is less than that in quiescent fluid for all $$\phi $$ . For $$\phi =0.5\%$$ , the mean settling speed in HIT is $$8\%$$ less than in quiescent fluid. However, by increasing the volume fraction the difference in the mean settling speed between quiescent fluid and HIT cases reduces, being only $$1.7\%$$ for $$\phi =10\%$$ . Indeed, while at low $$\phi $$ the settling speed is strongly altered by the interaction with turbulence, at large $$\phi $$ this is mainly determined by the (strong) hindering effect. This is similar in quiescent fluid and in HIT, leading to similar mean settling speeds. On the contrary, particle angular velocities are always found to increase with $$\phi $$ . These are enhanced by the interaction with turbulence, especially at low $$\phi $$ . In HIT, the correlations of particle lateral velocity fluctuations oscillate around zero before decorrelating completely. The time period of the oscillation seems proportional to the ratio between the integral lengthscale of turbulence and the particle characteristic terminal velocity. Regarding the mean square particle displacement, we find that it is strongly enhanced by turbulence in the direction perpendicular to gravity, even at the largest $$\phi $$ . Finally, we investigate the collision statistics for all cases and find the interesting result that the collision frequency is larger in quiescent fluid than in HIT for $$\phi =0.5{-}1\%$$ . This is due to frequent drafting–kissing–tumbling events in quiescent fluid. The collision frequency becomes instead larger in HIT than in still fluid for $$\phi =5{-}10\%$$ , due to the larger relative approaching velocities in HIT, and to the less intense drafting–kissing–tumbling events in quiescent fluid. The collision frequency also appears to be almost proportional to the estimate for small inertial particles uniformly distributed in space, though much smaller. Concerning the turbulence modulation, we find that the mean energy dissipation increases almost linearly with $$\phi $$ , leading to a large reduction of $$Re_{\lambda }$$ .

Interaction between the atmospheric boundary layer and a standalone wind turbine in Gansu—Part I: Field measurement

Experiments and numerical simulations of the wake field behind a horizontal-axis wind turbine are carried out to investigate the interaction between the atmospheric boundary layer and a stand-alone wind turbine. The tested wind turbine (33 kW) has a rotor diameter of 14.8 m and hub height of 15.4 m. An anti-icing digital Sonic wind meter, an atmospheric pressure sensor, and a temperature and humidity sensor are installed in the upstream wind measurement mast. Wake velocity is measured by three US CSAT3 ultrasonic anemometers. To reflect the characteristics of the whole flow field, numerical simulations are performed through large eddy simulation (LES) and with the actuator line model. The experimental results show that the axial velocity deficit rate ranges from 32.18% to 63.22% at the three measuring points. Meanwhile, the time-frequency characteristics of the axial velocities at the left and right measuring points are different. Moreover, the average axial and lateral velocity deficit of the right measuring point is greater than that of the left measuring point. The turbulent kinetic energy (TKE) at the middle and right measuring points exhibit a periodic variation, and the vortex sheet-pass frequency is mostly similar to the rotational frequency of the rotor. However, this feature is not obvious for the left measuring point. Meanwhile, the power spectra of the vertical velocity fluctuation show the slope of −1, and those of lateral and axial velocity fluctuations show slopes of −1 and −5/3, respectively. However, the inertial subranges of axial velocity fluctuation at the left, middle, and right measuring points occur at 4, 7, and 7 Hz, respectively. The above conclusion fully illustrates the asymmetry of the left and right measuring points. The experimental data and numerical simulation results collectively indicate that the wake is deflected to the right under the influence of lateral force. Therefore, wake asymmetry can be mainly attributed to the lateral force exerted by the wind turbine on the fluid.

Flow structure and heat transfer of a sweeping jet impinging on a flat wall

While an impinging jet has been widely investigated because of its remarkable convective heat transfer performance, the impingement of a sweeping jet which undergoes periodic oscillation has drawn little interest. We experimentally examined the heat transfer of a sweeping jet impinging on a flat wall for several Reynolds number and nozzle-to-plate spacings and discovered unsteady flow structure to characterize heat transfer capability. Local Nusselt number on the wall was evaluated by measuring temperature with thermocouples, and a flow was visualized quantitatively using particle image velocimetry. The distribution of the Nusselt number is different from that of a round jet, exhibiting two distinct regions. Near the center of a sweeping jet, a high Nusselt number zone is formed without a noticeable peak commonly observed in a round jet. Away from the central region, the Nusselt number decreases monotonically. The trends of the Nusselt number at the two regions are correlated with the first mode of the flow structure obtained by proper orthogonal decomposition (POD). The boundary of the two regions is a local minimum of the first POD mode near the wall, and the magnitude of the first POD mode is large in the central region of high Nusselt number. It was also found that the distributions of mean lateral velocity and lateral velocity fluctuation were clearly different between the two regions, which implies that both quantities should be considered for the analysis of heat transfer performance.

A numerical investigation of the asymmetric wake mode of a squareback Ahmed body – effect of a base cavity

Numerical simulations of the turbulent flow over the flat backed Ahmed model at Reynolds number $Re\simeq 4\times 10^{5}$ are conducted using a lattice Boltzmann solver to clarify the mean topology of the static symmetry-breaking mode of the wake. It is shown that the recirculation region is occupied by a skewed low pressure torus, whose part closest to the body is responsible for an extra low pressure imprint on the base. Shedding of one-sided vortex loops is also reported, indicating global quasi-periodic dynamics in conformity with the seminal work of Grandemange et al. (J. Fluid Mech., vol. 722, 2013, pp. 51–84). Despite the limited low frequency resolution of the simulation, power spectra of the lateral velocity fluctuations at different locations corroborate the presence of this quasi-periodic mode at a Strouhal number of $St=0.16\pm 0.03$. A shallow base cavity of $5\,\%$ of the body height reduces the drag coefficient by $3\,\%$ but keeps the recirculating torus and its interaction with the base mostly unchanged. The drag reduction lies in a global constant positive shift of the base pressure distribution. For a deep base cavity of $33\,\%$ of the body height, a drag reduction of $9.5\,\%$ is obtained. It is accompanied by a large elongation of the recirculation inside the cavity that considerably attenuates the low pressure sources therein together with a symmetrization of the low pressure torus. The global quasi-periodic mode is found to be inhibited by the cavity.

Comparison between passive scalar and velocity fields in a turbulent cylinder wake

This work compares the enstrophy with the scalar dissipation rate, as well as the passive scalar variance with the turbulent kinetic energy, in the presence of coherent Kármán vortices in the intermediate wake of a circular cylinder. Measurements are made at$x/d=10$, 20 and 40, where$x$is the streamwise distance from the cylinder axis and$d$is the cylinder diameter, with a Reynolds number of$2.5\times 10^{3}$based on the cylinder diameter and the free-stream velocity. A probe consisting of eight hot wires (four X-wires) and four cold wires is used to measure simultaneously the three components of the fluctuating velocity and vorticity vectors, as well as the fluctuating temperature gradient vector at nominally the same point in the plane of the mean shear. It is found that the enstrophy and scalar dissipation spectra collapse approximately at all wavenumbers except around the Kármán vortex street wavenumber for$x/d\geqslant 20$. The spectral similarity between the streamwise velocity fluctuation$u$and the passive scalar$\unicode[STIX]{x1D703}$is better than that between the velocity fluctuation vector$\boldsymbol{q}$and$\unicode[STIX]{x1D703}$. This is closely related to the highly organized lateral velocity fluctuation$v$in this flow. The present observations are fully consistent with the expectation that small scales of the velocity and temperature fields are more likely to exhibit a close relationship than scales associated with the bulk of the turbulent energy or scalar variance. The variation across the wake of the time scale ratio between scalar and velocity fields is significantly smaller than that of the turbulent Prandtl number.

Near-ground turbulence of the Bora wind in summertime

Bora is a strong, temporally and spatially variable downslope wind that blows along the Eastern Adriatic Coast and many other dynamically similar places around the world. Bora mean velocity rarely exceeds 20m/s, while its strong gusts create significant problems for engineering structures, traffic and agriculture. Previous meteorological and geophysical studies laid foundations on Bora large- and mesoscale motions, but further research is necessary to fully understand and resolve Bora turbulence in a form usable for engineers. In this study, unique high-frequency measurements, carried out simultaneously in three heights on a meteorological tower in the hinterland of the city of Split, Croatia, are analyzed for a single summertime Bora episode. Time histories and vertical profiles are studied for turbulence intensity, Reynolds shear stress and turbulence length scales in comparison with values recommended in major international wind engineering standards. Turbulence intensity and Reynolds shear stress proved to be not that sensitive to meandering of the mean wind velocity, as the observed values remain within the same range during the time record. This trend applies on mean wind velocities larger than 5m/s, as for smaller velocities the spread of values increases considerably. Turbulence length scales are observed to increase with increasing mean wind velocity and vice versa. With increasing height from the ground, turbulence intensity and absolute Reynolds shear stress decrease, turbulence length scales increase, all in agreement with the atmospheric physics. However, while turbulence intensity, Reynolds shear stress and longitudinal turbulence length scales generally agree well with the values recommended in international standards for the respective terrain type, turbulence length scales related to lateral and vertical velocity fluctuations are much larger than the standard values.

Comparison between velocity- and vorticity-based POD methods in a turbulent wake

In this paper, the velocity-based POD and the vorticity-based POD have been systematically compared in three characteristic regions of the flow downstream of a two-dimensional circular cylinder, namely the near, intermediate and far wakes. The two-point space correlation function is used to determine which of the two methods is better suited for extracting the large-scale flow structures based on the repartition of energy among the different POD modes. It is found that the POD, based on the lateral velocity fluctuation v, leads to the most optimum extraction in all three flow regions, while the vorticity-based POD is only effective in the near and intermediate wakes. Based on two-point space correlation functions, a scenario is proposed for the application of POD to the present two-dimensional wake .

Predicting wake meandering in real-time through instantaneous measurements of wind turbine load fluctuations

The present study deals with the real-time estimation of the wind turbine wake meandering and the determination of the best indicators to measure in order to build a real-time wake meandering model. Results are obtained through experiments performed in an atmospheric boundary layer wind tunnel, where a specific set-up enables to measure simultaneously the incoming lateral velocity fluctuations, the lateral force fluctuations applied to the wind turbine model and to track in real-time the lateral position of its wake. It is an extension of a previous work about the determination of good candidates to build some real-time predictors of the wake meandering. The strong correlations between the incoming transverse velocity fluctuations, the lateral force fluctuations and the lateral position of the wake farther downstream are quantified, confirming that the large-scale turbulent eddies impact directly the wake meandering. Three different versions of a wake meandering predictor model are compared. It leads to the conclusion that the monitoring of the global force fluctuations applied to a wind turbine could be used to predict in real-time the meandering of the generated wake.

Laser Doppler measurement and CFD validation in 3 × 3 bundle flow

The five-beam three-component laser Doppler system is operated in the three-beam two-component mode to measure the 3×3 bundle flow with simple grid spacer. Experiment has been conducted at Re=15,200 and 29,900. According to the experiment result, the root mean square (RMS) of axial velocity fluctuation shows large value in the gap and the near-wall region of the edge sub-channel which is induced by the axial velocity gradient. Significant intensity of lateral velocity fluctuation is observed which indicates the strong lateral mixing in a 3×3 rod bundle. Through the correlation analysis coherent structures have been observed in the gap region. The spectral analysis shows that the LDV measurement complies to the Komogorov spectrum law, f−5/3, well. The low-frequency peak spectral density of the axial velocity fluctuation has been observed in the gap region connecting sub-channels with velocity difference. The performance of the SSG model and the baseline Reynolds stress model are investigated based on the experiment result. The models predict higher axial velocity in the interior sub-channel and lower in the edge and corner ones than the experiment result. Large discrepancy between the calculated and measured axial flow velocity is resulted from failure in calculating the strong negative u′w′¯ in the gap region connecting different sub-channels.

GS43 単純形状モデルを用いた鉄道車両床下流れの数値シミュレーション

Large-eddy simulation of flow around a train was conducted to investigate unsteady under body flows. The results of this study showed that large lateral velocity fluctuations were generated by two vortex streets along the train and that they propagated along the streamwise direction under the train.

Effect of Mesh Grids on the Turbulent Mixing Layer of an Axisymmetric Jet

This article focuses on the effect that two different mesh grids have on the structure of the mixing layer of an axisymmetric jet. Detailed measurements of mean velocity and turbulent velocity fluctuations are made with an X hot-wire probe in the range 0.5 ≤ x/d ≤ 10, where x is the longitudinal distance from the nozzle exit plane and d is the nozzle diameter. The grids are introduced at two locations—one location just downstream of the nozzle exit plane and the other location upstream of the nozzle exit plane in order to perturb the nozzle exit boundary layer. One mesh completely covers the nozzle (full mesh or FM) and the other mesh covers the central, high-speed zone (disk mesh or DM). With reference to the undisturbed jet, FM yields a significant reduction in the turbulence intensity and width of the shear layer, whereas DM enhances the turbulence intensity and increases the width of the shear layer. Both grids suppress the formation of the Kelvin–Helmholtz instability in the mixing layer. Results are presented, mainly at x/d = 5 and 6 in both the spectral domain and physical space. In the latter context, second- and third-order structure functions associated with u (the longitudinal velocity fluctuation) and v (the lateral or radial velocity fluctuation) are presented only for the flow perturbed by placing the mesh outside the nozzle. All mesh geometries have a more significant effect on the second-order structure function of u than on that of v. The third-order energy transfer term is affected in such a way that, relative to the undisturbed jet, its peak location is shifted to a smaller scale when FM is used and to a larger scale with DM. This is consistent with our observations that FM reduces the turbulence in the shear layer while DM enhances it. It is suggested that the large-scale vortices that are formed at the edge of the grids play a significant role in the transfer of energy.

Variable-viscosity mixing in the very near field of a round jet

One of the persistent challenges in turbulent mixing is understanding of the phenomenology associated with heterogeneous mixtures of gases. The turbulence in single, homogeneous fluid has drawn much attention. However, heterogeneous mixing of gases is prevalent in real flows, whether nonreactive or reactive. This paper is dedicated to advancing our understanding of the turbulent mixing phenomenology in a round jet flow of propane issuing in an oxidizer. The ratio of the propane/oxidizer kinematic viscosities is 1/5.5, whereas their densities are nearly equal. Therefore, the main physical property that differentiates the two fluids is their viscosity. The focus of this paper is on the role played by the viscosity gradients on the turbulent flow, over the first several diameters. The addressed questions concern a comparison between the present flow, which is a variable-viscosity flow (hereafter, VVF), and an air jet flow issuing in the air (constant-viscosity flow, hereafter CVF). The comparison is made at the same initial conditions, i.e. the same jet momentum per surface unit, M0 = ρ0U20 = 130 and 360 kg m−1 s−2 (U0 is the injection velocity and ρ0 is the fluid density). For doing so, an experiment has been designed and both the velocity and the scalar fields have been measured by using a technique based on hot-wire anemometry and Rayleigh light scattering, as well as laser Doppler velocimetry which provides two velocity components. The comparison criteria between the VVF and CVF are the instantaneous aspect of the turbulent velocity field, the axial mean velocity () and the rms (root mean squares) of the two measured velocity fluctuations. The one-point kinetic energy budget, which involves the mean energy dissipation rate, is developed and the expression of the mean energy dissipation rate is revisited. The latter involves additional terms reflecting the velocity–viscosity gradient correlation. This contribution is positive, thus enhancing the real mean energy dissipation rate in VVF. We show that, in comparison with the CVF, the VVF evolves much faster toward a self-similar regime, the decay of the axial mean velocity is enhanced and the mean energy dissipation rate is more important. We discuss, finally, a possible mechanism for the accelerated turbulence decay observed in VVF. Firstly, an external viscous fluid is brought inwards of the jet core by the jet frontier instabilities (nozzle-wake instabilities, Kelvin–Helmholtz instabilities and instabilities associated with the variable-viscosity interface). Secondly, these viscous-fluid blobs lead to enhanced local velocity gradients, to greater local dissipation and to increased production of lateral velocity fluctuations behind these irregular obstacles. While this scenario is speculative, it is likely to explain the observed statistics of the velocity field.

Evaluation of various non-linear k–ɛ models for predicting wind flow around an isolated high-rise building within the surface boundary layer

This paper evaluates the performance of various non-linear k–ɛ models for predicting wind flow around an isolated high-rise building at a 1:1:2 (length: width: height) scale and within the turbulent boundary layer. Two cubic models, proposed by Ehrhard et al. and Craft et al., and one quadratic model proposed by Shih et al., are examined by comparing their simulation results with the experimental data. In this study, the three non-linear models can reproduce the vortex shedding behind building during unsteady calculations. However, these non-linear models cannot achieve a convergent solution in steady calculations. For the Shih model and the Ehrhard model, the magnitudes of the lateral velocity fluctuations normalized by the inlet wind speed at the height of building are only approximately 0.022, and for the Craft model, this dimensionless magnitude increases to approximately 0.11. These three non-linear models can reproduce the reverse flow on building roof. The Craft model slightly overpredicts the reattachment length on building roof. The other two non-linear models produce large roof re-circulation vortexes, which extend over the entire roof without attachment. Among the RANS models studied here, the Craft model shows the best agreement with the experimental data. The Craft model predicts the shortest reattachment length behind building among the revised k–ɛ models, while the other two non-linear models greatly overestimate the reattachment length behind building. The good performance of the Craft non-linear model in the wake region can be interpreted from the magnitude of the total kinetic energy after considering the solved wind fluctuations.