Production Of Kinetic Energy Research Articles

Sensitivity of idealised baroclinic waves to mean atmospheric temperature and meridional temperature gradient changes

The sensitivity of idealised baroclinic waves to different atmospheric temperature changes is studied. The temperature changes are based on those which are expected to occur in the Northern Hemisphere with climate change: (1) uniform temperature increase, (2) decrease of the lower level meridional temperature gradient, and (3) increase of the upper level temperature gradient. Three sets of experiments are performed, first without atmospheric moisture, thus seeking to identify the underlying adiabatic mechanisms which drive the response of extra-tropical storms to changes in the environmental temperature. Then, similar experiments are performed in a more realistic, moist environment, using fixed initial relative humidity distribution. Warming the atmosphere uniformly tends to decrease the kinetic energy of the cyclone, which is linked both to a weaker capability of the storm to exploit the available potential energy of the zonal mean flow, and less efficient production of eddy kinetic energy in the wave. Unsurprisingly, the decrease of the lower level temperature gradient weakens the resulting cyclone regardless of the presence of moisture. The increase of the temperature gradient in the upper troposphere has a more complicated influence on the storm dynamics: in the dry atmosphere the maximum eddy kinetic energy decreases, whereas in the moist case it increases. Our analysis suggests that the slightly unexpected decrease of eddy kinetic energy in the dry case with an increased upper tropospheric temperature gradient originates from the weakening of the meridional heat flux by the eddy. However, in the more realistic moist case, the diabatic heating enhances the interaction between upper- and low-level potential vorticity anomalies and hence helps the surface cyclone to exploit the increased upper level baroclinicity.

A Modeling Study of Interannual Variability of Bay of Bengal Mixing and Barrier Layer Formation

AbstractThe year to year variability of surface mixing in the Bay of Bengal (BoB) is examined with the help of an Ocean Dynamic‐Thermodynamic Model (ODTM) and observational data. The model embeds a conventional nonlinear primitive equation based reduced gravity model (RGM) with “N” active layers overlying a 1/2 quiescent layer. The model is coupled to a high‐resolution mixed layer model (MLM) to capture the physics of the mixed layer. The MLM incorporates a level‐2 turbulence closure mixing scheme based on Mellor and Yamada (1982). As a result of the coupling, our model calculates the net hydrostatic pressure as a sum of those due to the thickness of RGM layers with constant densities and due to the dynamic topography of MLM levels with variable densities. At each time step the coupling between RGM and MLM is achieved as follows: (i) a layer‐averaged tendency of momentum and tracers (i.e., temperature and salinity) of the MLM is updated to the RGM, (ii) momentum and tracers of the MLM get advected by large‐scale horizontal dynamics of the RGM, and (iii) horizontal layer divergence of the RGM aides for the vertical advection of the MLM. By virtue of this coupling, the model faithfully reproduces the seasonal cycle of temperature, salinity, sea surface height, thermocline, mixed and barrier layer thickness of the tropical Indian Ocean. A simulation with Coordinated Ocean‐Ice Reference Experiment (CORE) forcing for 15 years (from 1995 to 2009) is used as a test case to study the interannual variability (IAV) of the mixed layer depth and barrier layer thickness (BL) in BoB. The dominant modes of IAV in the BoB mixing are governed by the correspondingly varying surface momentum, heat, and fresh water fluxes with very little contribution from entrainment of heat and/or salt at the base of the mixed layer. Further, these fluxes are controlled by El Niño‐Southern Oscillation (ENSO) variability with very little influence from Indian Ocean Dipole‐Zonal Mode (IODZM). The BL IAV is predominantly controlled by precipitation forcing from ENSO. A stability analysis revealed that the turbulent kinetic energy (TKE) and the stability function (SH) are negatively correlated when ENSO is the dominant forcing. Such a correlation between TKE and SH is expected since unstable stratification conditions exist during positive ENSO, where the kinetic energy production is enhanced by the unstable buoyancy forcing leading to an increased TKE. During the negative phase of ENSO, stably stratified conditions exist, where the kinetic energy production is offset by the stable buoyancy force, reducing the TKE. This is indicative of high (low) turbulent kinetic energy production, low (high) flux Richardson number based stability function (SH), and low (high) dominance of buoyancy‐driven mixing during positive (negative) phases of ENSO. The results highlight that the counteracting influence of TKE and SH is a plausible reason for relatively weaker amplitude of IAV of BoB mixing compared to its normal seasonal cycle.

Exchange Flow Variability between Hypersaline Shark Bay and the Ocean

In Shark Bay, a large hypersaline bay in Western Australia, longitudinal density gradients force gravitational circulation that is important for Bay-ocean exchange. First-time observations of vertical stratification and velocity are presented, confirming the presence of a steady, near-bed dense water outflow from Shark Bay’s northern Geographe Channel that persisted through all stages of the tide. Outflow velocities were 2–3 times stronger than the outflows recorded previously in Naturaliste Channel (in the west), and were more resistant to breakdown by tidal mixing. Estimates of turbulent kinetic energy production derived from the variance method showed a more complex structure in the Geographe Channel, due to shear between surface and bottom layers. Turbulence varied between flood and ebb tide, with peak levels of turbulence occurring during reversal of tidal flows. For both channels, the main source of turbulence was tidal flow along the seabed, with the bottom current speed cubed, |Ub3|, providing a reasonable proxy for tidal mixing and prediction of dense water outflows from Shark Bay majority of the time. Orientation and deeper water of the Geographe Channel along the main axis of the longitudinal density gradient provided an explanation for the predominant outflow from the Bay’s northern entrance. These density-driven currents could potentially influence recruitment of commercially fished scallops and prawns through the dispersal and flushing of larvae.

Nonlinear optimal large-scale structures in turbulent channel flow

Coherent structures in turbulent shear flows take the form of packets of hairpin vortices reaching the outer region of the boundary layer along with streaks of different size, going from the near-wall to the outer region. The latter can be explained by the linear transient growth of the perturbations of the mean turbulent profile. Whereas, the former are recently found to be optimally-growing only in the presence of nonlinear effects, as ascertained for a turbulent channel flow at a low friction Reynolds number. The present work aims at investigating whether large-scale streaks can be optimally-growing in a nonlinear framework for a turbulent channel flow. Changing the friction Reynolds number from 180 to 590, the nonlinear optimal perturbation tends towards more robust large-scale streaks and vortical structures of smaller size. These streaks are generated by a coherent large-scale lift-up mechanism, acting as a source term in the energy balance, inducing a positive turbulent kinetic energy production at the outer scale. This indicates that the outer energy production peak arising between the two considered Reynolds numbers can be associated with the growth of optimal large-scale streaks, which represent a robust feature of turbulent channel flows.

Direct numerical simulation of turbulent flow and heat transfer in a spatially developing turbulent boundary layer laden with particles

Direct numerical simulations of particle-laden flows in a spatially developing turbulent thermal boundary layer over an isothermally heated wall have been performed with realistic fully developed turbulent inflow boundary conditions. To the authors’ best knowledge, this is the first time the effects of inertial solid particles on turbulent flow and heat transfer in a flat-plate turbulent boundary layer have been investigated, using a two-way coupled Eulerian–Lagrangian method. Results indicate that the presence of particles increases the mean streamwise velocity and temperature gradients of the fluid in the near-wall region. As a result, the skin-friction drag and heat transfer are significantly enhanced in the particle-laden flows with respect to the single-phase flow. The near-wall sweep and ejection motions are suppressed by the particles and hence the Reynolds shear stress and wall-normal turbulent heat flux are attenuated, which leads to reductions in the production of the turbulent kinetic energy and temperature fluctuations. In addition, the coherence and spacing of the near-wall velocity and temperature streaky structures are distinctly increased, while the turbulent vortical structures appear to be disorganized under the effect of the particles. Moreover, the intensity of the streamwise vortices decreases monotonically with increasing particle inertia.

Combined effect of wave and current in rough bed free surface flow

In this paper, detailed experimental results are reported to study the effect of the surface wave of different frequencies on turbulent characteristics of flow over a regular array of cubical roughness. The velocity time series of current-only and combined wave-current flow is analyzed to obtain the distribution of first and the second order turbulence statistics, turbulent kinetic energy production, turbulent dissipation and kinetic energy fluxes at a fixed Reynolds number. To highlight the changes due to combined wave-current flow, a comparative study was made between the velocity profiles for current-only flow and those measured for combined wave-current flow. As expected, the rough profiles are shifted downwards with the amount equivalent to the roughness function (ΔU+), which was obtained directly from mean velocity profiles. Further, the thickness of turbulent boundary layer increases with the superposition of surface wave compared to current-only flow. Particular attention is also given to understand the behavior of kinetic energy flux to explicitly account for turbulent momentum transport characteristics over roughness under wave-current interactions.

Reynolds stress structures in a self-similar adverse pressure gradient turbulent boundary layer at the verge of separation.

Mean Reynolds stress profiles and instantaneous Reynolds stress structures are investigated in a self-similar adverse pressure gradient turbulent boundary layer (APG-TBL) at the verge of separation using data from direct numerical simulations. The use of a self-similar APG-TBL provides a flow domain in which the flow gradually approaches a constant non-dimensional pressure gradient, resulting in a flow in which the relative contribution of each term in the governing equations is independent of streamwise position over a domain larger than two boundary layer thickness. This allows the flow structures to undergo a development that is less dependent on the upstream flow history when compared to more rapidly decelerated boundary layers. This APG-TBL maintains an almost constant shape factor of H = 2.3 to 2.35 over a momentum thickness based Reynolds number range of Reδ2 = 8420 to 12400. In the APG-TBL the production of turbulent kinetic energy is still mostly due to the correlation of streamwise and wall-normal fluctuations, 〈uv〉, however the contribution form the other components of the Reynolds stress tensor are no longer negligible. Statistical properties associated with the scale and location of sweeps and ejections in this APG-TBL are compared with those of a zero pressure gradient turbulent boundary layer developing from the same inlet profile, resulting in momentum thickness based range of Reδ2 = 3400 to 3770. In the APG-TBL the peak in both the mean Reynolds stress and the production of turbulent kinetic energy move from the near wall region out to a point consistent with the displacement thickness height. This is associated with a narrower distribution of the Reynolds stress and a 1.6 times higher relative number of wall-detached negative uv structures. These structures occupy 5 times less of the boundary layer volume and show a similar reduction in their streamwise extent with respect to the boundary layer thickness. A significantly lower percentage of wall-attached structures is observed in the present case when compared with a similar investigation of a rapidly decelerating APG-TBL, suggesting that these wall-attached features could be the remanent from the lower pressure gradient domain upstream.

A Kármán–Howarth–Monin equation for variable-density turbulence

We present a generalisation of the Kármán–Howarth–Monin (K–H–M) equation to include variable-density (VD) effects. The derived equation (i) reduces to the original K–H–M equation when density is a constant and (ii) leads to a VD analogue of the $4/5$-law with the same value of constant ($=4/5$) appearing as the prefactor of the dissipation rate. The equation is employed to understand negative turbulent kinetic energy production in a $\text{SF}_{6}$ turbulent round jet with an initial density ratio of 4.2. From a Reynolds-averaged Navier–Stokes (RANS) perspective, negative production means that the mean flow is strengthened at the expense of the energy of turbulent fluctuations. We show that the associated energy transfer is accomplished by the deformation of smaller turbulent eddies into large ones in the development region of the jet and is captured by the linear scale-by-scale energy transfer term in the VD K–H–M equation. The nonlinear transfer term of the VD K–H–M equation depicts a conventional forward cascade for all eddies having a size less than the Eulerian integral length scale, regardless of their orientation. The net effect is a retarded energy cascade in the non-Boussinesq jet that has not been accounted for by existing turbulence theories. Implications of this observation for turbulence modelling are discussed.

Laminar similarities between accelerating and decelerating turbulent flows

An experimental study of a pipe flow ramping monotonically between two turbulent states has been undertaken. Ensemble-averaged mean and turbulent flow quantities obtained from two-component particle image velocimetry and hot-film anemometry measurements have been presented. It is shown that the time-developments of the mean and turbulence quantities in accelerating and decelerating flows are similar during an initial stage following the transients. Specifically, the mean perturbation velocity (defined as the surplus/deficit from the initial value) can be described using self-similar expressions. The duration of this initial stage is shown to be a decreasing function of the dimensionless parameter δ=ν/uτ02(1/Ub0)dUb/dt. Data from studies of linearly accelerating and decelerating flows as well as impulsively accelerating and decelerating flows have been used to validate the results, covering four orders of magnitude of δ. The highest initial Reynolds number investigated in this study (35,700) is, however, relatively low thus requiring further studies at high Reynolds numbers to assure the universality of the results. We have also shown that the time-developments of the mean and turbulent quantities between an accelerating and a decelerating flow loose their similarity as the transient proceeds beyond the initial stage. The departure was explained by the time-evolvement of the production of turbulence kinetic energy, which exhibit differences between the two types of transients.

Turbulence Measurement of Vertical Dense Jets in Crossflow

Turbulence measurement of a dense jet perpendicularly issued into a crossflow is investigated experimentally. The flow-velocity components were extensively measured with a high frequency Acoustic Doppler Velocimeter (ADV) system, whereas, a Micro Scale Conductivity Temperature instrument was used to measure the jet salinity. Special attention is given to understand the jet flow-structures in the flow symmetry plane. The flow velocity-fields, the jet trajectory, the turbulence intensities, the turbulent kinetic energy, the turbulent length scales, and the dispersion coefficients have been analyzed. The flow velocity-fields show that the dense jet is characterized by two distinct regions: an ascending region, of jet-like mixing, and a descending region of plume-like mixing. In this study, a new scaling approach of the jet trajectories, based on the jet characteristic length scales, is proposed, leading to an empirical closed-form expression to predict the jet trajectory. The turbulence analysis shows that the jet is accompanied by high levels of flow-turbulence intensities and large kinetic energy production. The results of the turbulent length scales indicate that the ambient flow-field, without jet effect, is an isotropic process. However, in the jet flow-field, a significant spatial-variation of the turbulent length scales was observed, indicating an anisotropic process. The trends of the dispersion coefficients follow those of the turbulent length scales. In comparison with the ambient flow, the jet flow-field shows a decrease of the longitudinal dispersion coefficient and an increase of the vertical one, leading to the increase of the jet width.

Scaling and Similarity of the Anisotropic Coherent Eddies in Near-Surface Atmospheric Turbulence

Abstract The low-wavenumber regime of the spectrum of turbulence commensurate with Townsend’s “attached” eddies is investigated here for the near-neutral atmospheric surface layer (ASL) and the roughness sublayer (RSL) above vegetation canopies. The central thesis corroborates the significance of the imbalance between local production and dissipation of turbulence kinetic energy (TKE) and canopy shear in challenging the classical distance-from-the-wall scaling of canonical turbulent boundary layers. Using five experimental datasets (two vegetation canopy RSL flows, two ASL flows, and one open-channel experiment), this paper explores (i) the existence of a low-wavenumber k−1 scaling law in the (wind) velocity spectra or, equivalently, a logarithmic scaling ln(r) in the velocity structure functions; (ii) phenomenological aspects of these anisotropic scales as a departure from homogeneous and isotropic scales; and (iii) the collapse of experimental data when plotted with different similarity coordinates. The results show that the extent of the k−1 and/or ln(r) scaling for the longitudinal velocity is shorter in the RSL above canopies than in the ASL because of smaller scale separation in the former. Conversely, these scaling laws are absent in the vertical velocity spectra except at large distances from the wall. The analysis reveals that the statistics of the velocity differences Δu and Δw approach a Gaussian-like behavior at large scales and that these eddies are responsible for momentum/energy production corroborated by large positive (negative) excursions in Δu accompanied by negative (positive) ones in Δw. A length scale based on TKE dissipation collapses the velocity structure functions at different heights better than the inertial length scale.

Effects of anthropogenic heat due to air-conditioning systems on an extreme high temperature event in Hong Kong

Anthropogenic heat flux is the heat generated by human activities in the urban canopy layer, which is considered the main contributor to the urban heat island (UHI). The UHI can in turn increase the use and energy consumption of air-conditioning systems. In this study, two effective methods for water-cooling air-conditioning systems in non-domestic areas, including the direct cooling system and central piped cooling towers (CPCTs), are physically based, parameterized, and implemented in a weather research and forecasting model at the city scale of Hong Kong. An extreme high temperature event (June 23–28, 2016) in the urban areas was examined, and we assessed the effects on the surface thermal environment, the interaction of sea–land breeze circulation and urban heat island circulation, boundary layer dynamics, and a possible reduction of energy consumption. The results showed that both water-cooled air-conditioning systems could reduce the 2 m air temperature by around 0.5 °C–0.8 °C during the daytime, and around 1.5 °C around 7:00–8:00 pm when the planetary boundary layer (PBL) height was confined to a few hundred meters. The CPCT contributed around 80%–90% latent heat flux and significantly increased the water vapor mixing ratio in the atmosphere by around 0.29 g kg−1 on average. The implementation of the two alternative air-conditioning systems could modify the heat and momentum of turbulence, which inhibited the evolution of the PBL height (a reduction of 100–150 m), reduced the vertical mixing, presented lower horizontal wind speed and buoyant production of turbulent kinetic energy, and reduced the strength of sea breeze and UHI circulation, which in turn affected the removal of air pollutants. Moreover, the two alternative air-conditioning systems could significantly reduce the energy consumption by around 30% during extreme high temperature events. The results of this study suggest potential UHI mitigation strategies and can be extended to other megacities to enable them to be more resilient to UHI effects.

Identification and quantification of losses in a LPT cascade by POD applied to LES data

A POD based procedure has been developed to identify and account for the different contributions to the entropy production rate caused by the unsteady aerodynamics of a low-pressure (LP) turbine blade. LES data of the extensively studied T106A cascade have been used to clearly highlight the capability of POD to identify deterministic incoming wake related modes, stochastic fine-scale structures embedded within the bulk of the wake carried during migration, and coherent structures originating in the boundary layer as a consequence of the wake-boundary layer interaction process. The POD modes computed by a kinematic kernel generate a full and complete basis, where both the velocity and enthalpy fields have been projected through an extended POD procedure to determine the relative coefficients. This allows to separately compute orthogonal sets of contributions to turbulent kinetic energy production, enthalpy-velocity correlation and turbulent dissipation of resolved structures, thus clearly identifying the dominating modes (i.e. phenomena) responsible for the overall entropy production rate. Moreover, low-order truncation of these different contributions have been grouped into three different parts: those arising from the deterministic incoming wake, those due to the turbulence carried by the wakes and its interaction with the boundary layer, and those related to boundary layer events. The spatial integration of these low-order truncations restricted to the time-mean boundary layer, wake mixing and the potential flow regions of the blade passage allows gathering further information on the unsteady loss generation mechanisms, and where they mainly act. Particularly, results show that the procedure is able to decompose losses into the dominant contributions, thus providing a new tool for a rapid and clear identification of the different sources of losses in complex unsteady flow fields.

Large-eddy-simulation approach in understanding flow structures of 2D turbulent density currents over sloping surfaces

A numerical investigation was performed to understand the flow dynamics of 2D density currents over sloping surfaces. Large eddy simulation was conducted for lock-exchange (L-E) release currents and overflows. 2D Navier–Stokes equations were solved using the Boussinesq approximation. The effects of the lock aspect-ratio (height/length of lock), slope, and Reynolds number on the flow structures and turbulence mixing have been analyzed. Results have confirmed buoyancy within the head of the two-dimensional currents is not conserved which contradicts the classical thermal theory. The lock aspect-ratio dictates the fraction of initial buoyancy which is carried by the head of the current at the beginning of the slumping (horizontal) and accelerating phase (over a slope), which has important implications on turbulence kinetic energy production, and hence mixing in the current. For L-E flows over a slope, increasing slope angle enhances the turbulence production. Increasing slope results in shear reversal within the density current resulting in shear-instabilities. Differences in turbulence production mechanisms and flow structures exist between the L-E and constant-flux release currents resulting in significant differences in the flow characteristics between different releases.

Database of Near-Wall Turbulent Flow Properties of a Jet Impinging on a Solid Surface under Different Inclination Angles

In the present paper, direct numerical simulation (DNS) and particle image velocimetry (PIV) have been applied complementarily in order to generate a database of near-wall turbulence properties of a highly turbulent jet impinging on a solid surface under different inclination angles. Thereby, the main focus is placed on an impingement angle of 45 ∘ , since it represents a good generic benchmark test case for a wide range of technical fluid flow applications. This specific configuration features very complex flow properties including the presence of a stagnation point, development of the shear boundary layer and strong streamline curvature. In particular, this database includes near-wall turbulence statistics along with mean and rms velocities, budget terms in the turbulent kinetic energy equation, anisotropy invariant maps, turbulent length/time scales and near-wall shear stresses. These properties are useful for the validation of near-wall modeling approaches in the context of Reynolds-averaged Navier–Stokes (RANS) and large-eddy simulations (LES). From this study, in which further impingement angles ( 0 ∘ , 90 ∘ ) have been considered in the experiments only, it turns out that (1) the production of turbulent kinetic energy appears negative at the stagnation point for an impingement angle other than 0 ∘ and is balanced predominantly by pressure-related diffusion, (2) quasi-coherent thin streaks with large characteristic time scales appear at the stagnation region, while the organization of the flow is predominantly toroidal further downstream, and (3) near-wall shear stresses are low at the stagnation region and intense in regions where the direction of the flow changes suddenly.

Comparison of K-E Turbulence Model Wall Functions Applied on a T-Junction Channel Flow

The flow acting in a T-junction channel is present in several industrial applications, such as air conditioning systems, water cooling circuits, gas exhaust systems and others. In order to numerically simulate this case, the Average Reynolds Navier-Stokes (RANS) equation is used for a two-dimensional stationary flow using the k-e model together with wall functions such as standard wall function, Enhanced and Menter-Lechner wall treatments. The moment ratio used is = 2 and the Reynolds number at the inlet of the flow parallel to the channel is Re = 15,000. The results were compared with the literature data using Large Scale Simulation (LES). The results obtained for k-e model Enhanced and Menter-Lechner wall treatment were satisfactory and close to that found by the LES simulation, however, results obtained from k-e model standard wall function presented large deviation to literature, mainly in the boundary layer and K production profiles. In general, the results presented small distortions for the profiles of turbulent kinetic energy production near walls, however, they illustrate in an analogous manner to the literature the production of turbulent kinetic energy K concentrated in the shear layers between flows. The main results analyzed in this paper are the length of the recirculation bubble, boundary layer profile, mean velocity magnitude and kinetic energy production k.

Simulations of vehicle-induced mixing and near-road aerosol microphysics using computational fluid dynamics

Understanding the fate of ultrafine particles (UFP), especially from combustion sources, is essential to assess their impact on health and climate. Here, we present simulations of the behavior of UFP in the near-roadway environment (up to 300 m downwind) based on a model with coupled computational fluid dynamics (CFD) and aerosol microphysics. It is found that vehicle-induced mixing (VIM) caused by the combined effect of vehicle wake formation and production of turbulent kinetic energy plays an important role in downwind dilution of pollutants. Various methodologies for simulating VIM are explored, and a computationally efficient approach based on an effective roughness length for vehicle-induced mixing is proposed. Whereas the coagulation behavior of ultrafine particles is relatively well understood, condensation and/or evaporation can have equally large or larger impacts on the number and sizes of particles downwind of a roadway. Through a set of sensitivity simulations, we show that the particle losses are potentially significant via evaporation but depend strongly on several parameters or processes that are poorly understood and difficult to fully constrain for on-road traffic using measurements: the volatility distribution of organic species and gas-phase concentrations of semi-volatile organics.

Anisotropy stress invariants of thermally stratified wind turbine array boundary layers using large eddy simulations

In the interest of understanding the dynamics and energy transfer between the atmospheric boundary layer and large wind turbine arrays, a description of the turbulence anisotropy in the wake region of wind turbines is necessary. Flux of high momentum flow into the wind turbine array by anisotropic turbulence is a dominant factor of mean kinetic energy resupply for the wind farm. Under thermal stratification, the behavior of the turbulence field, and the energy flux, is significantly altered. Here, a thermally stratified wind turbine array boundary layer is studied in detail using a combination of Lumley and barycentric maps together with the recently introduced spheroid and color maps. The atmospheric flow is modelled using a large eddy simulation driven by a constant geostrophic wind and a time-varying ground surface temperature, obtained from a selected period of the Cooperative Atmosphere-Surface Exchange Study-99 field experiment. The wind farm is modelled using the traditional actuator-disk with rotation and yaw-alignment. The results show that turbulence under unstable stratification tends to be more isotropic than that under neutral and stable stratification. The turbulent mixing attributed to positive buoyancy in unstable regimes determines the energy distribution between flow layers, directly affecting the vertical distribution of anisotropy. Contrarily, in the stably stratified flow, negative buoyancy damps the turbulent fluctuations, hence affecting the evolution of the Reynolds stress, leading to an increase of flow anisotropy. In both thermal stratification regimes, wind turbines alter the structure of the turbulence within the atmospheric boundary layer by creating regions of greater anisotropy and expanding the boundary layer vertically. For the modeling community, it is important to note that the wind turbine wakes display a larger degree of anisotropy that is related well to the production of turbulent kinetic energy and mean kinetic energy entrainment, and hence, it is important to model correctly for accurate power forecasting. Leveraging the new insights into wind turbine/atmospheric boundary layer interaction found here will benefit the formulation of a new generation of efficient low order models for commercial application.

Flow characteristics around a deformable stenosis under pulsatile flow condition

A specific portion of a vulnerable stenosis is deformed periodically under a pulsatile blood flow condition. Detailed analysis of such deformable stenosis is important because stenotic deformation can increase the likelihood of rupture, which may lead to sudden cardiac death or stroke. Various diagnostic indices have been developed for a nondeformable stenosis by using flow characteristics and resultant pressure drop across the stenosis. However, the effects of the stenotic deformation on the flow characteristics remain poorly understood. In this study, the flows around a deformable stenosis model and two different rigid stenosis models were investigated under a pulsatile flow condition. Particle image velocimetry was employed to measure flow structures around the three stenosis models. The deformable stenosis model was deformed to achieve high geometrical slope and height when the flow rate was increased. The deformation of the stenotic shape enhanced jet deflection toward the opposite vessel wall of the stenosis. The jet deflection in the deformable model increased the rate of jet velocity and turbulent kinetic energy (TKE) production as compared with those in the rigid models. The effect of stenotic deformation on the pulsating waveform related with the pressure drop was analyzed using the TKE production rate. The deformable stenosis model exhibited a phase delay of the peak point in the waveform. These results revealed the potential use of pressure drop waveform as a diagnostic index for deformable stenosis.

Simulation of Multistage Compressor at Off-Design Conditions

Computational fluid dynamics (CFD) has been widely used for compressor design, yet the prediction of performance and stage matching for multistage, high-speed machines remains challenging. This paper presents the authors' effort to improve the reliability of CFD in multistage compressor simulations. The endwall features (e.g., blade filet and shape of the platform edge) are meshed with minimal approximations. Turbulence models with linear and nonlinear eddy viscosity models are assessed. The nonlinear eddy viscosity model predicts a higher production of turbulent kinetic energy in the passages, especially close to the endwall region. This results in a more accurate prediction of the choked mass flow and the shape of total pressure profiles close to the hub. The nonlinear viscosity model generally shows an improvement on its linear counterparts based on the comparisons with the rig data. For geometrical details, truncated filet leads to thicker boundary layer on the filet and reduced mass flow and efficiency. Shroud cavities are found to be essential to predict the right blockage and the flow details close to the hub. At the part speed, the computations without the shroud cavities fail to predict the major flow features in the passage, and this leads to inaccurate predictions of mass flow and shapes of the compressor characteristic. The paper demonstrates that an accurate representation of the endwall geometry and an effective turbulence model, together with a good quality and sufficiently refined grid, result in a credible prediction of compressor matching and performance with steady-state mixing planes.