Contribution Of Turbulence Research Articles

Interaction Between Waves and Turbulence Within the Nocturnal Boundary Layer

The presence of waves is proven to be ubiquitous within nocturnal stable boundary layers over complex terrain, where turbulence is in a continuous, although weak, state of activity. The typical approach based on Reynolds decomposition is unable to disaggregate waves from turbulence contributions, thus hiding any information about the production/destruction of turbulence energy injected/subtracted by the wave motion. We adopt a triple-decomposition approach to disaggregate the mean, wave, and turbulence contributions within near-surface boundary-layer flows, with the aim of unveiling the role of wave motion as a source and/or sink of turbulence kinetic and potential energies in the respective explicit budgets. By exploring the balance between buoyancy (driving waves) and shear (driving turbulence), a simple interpretation paradigm is introduced to distinguish two layers, namely the near-ground and far-ground sublayer, estimating where the turbulence kinetic energy can significantly feed or be fed by the wave. To prove this paradigm, a nocturnal valley flow is used as a case study to detail the role of wave motions on the kinetic and potential energy budgets within the two sublayers. From this dataset, the explicit kinetic and potential energy budgets are calculated, relying on a variance–covariance analysis to further comprehend the balance of energy production/destruction in each sublayer. With this investigation, we propose a simple interpretation scheme to capture and interpret the extent of the complex interaction between waves and turbulence in nocturnal stable boundary layers.

Impact of building façade geometrical details on pollutant dispersion in street canyons

The present study investigates the impact of building façade geometrical details on the pollutant transport mechanism in long street canyons. Large-eddy simulations (LES), extensively validated with experiments, are performed for four cases: (i) street canyon without façade balconies, (ii) street canyon with balconies at both windward and leeward façades, (ii) street canyon with balconies only at the windward façade and (iv) street canyon with balconies only at the leeward façade. The results show that the building balconies can strongly affect the wind flow field and pollutant dispersion in long street canyons. The most significant impact is observed for the two street canyon cases with balconies at the windward façade, which strongly obstruct the airflow from penetrating deep into the bottom of the canyon. The presence of balconies only at the windward façade and at both façades can increase the area-weighted mean pollutant concentration in the vertical center plane inside the canyon by 80% and 106%, respectively, and reduce the mean pollutant exchange velocity (Ue) by 46% and 54%, respectively. The analysis of the vertical mean convective and turbulent mass fluxes indicates that the presence of balconies mainly decreases the convective contribution to Ue, while the impact on the turbulent contribution is smaller.

Numerical Investigation of Turbulence Production Under Two-Phase Flow in a Simulation of Electrical Submersible Pump Performance

Abstract Investigating the characteristics of gas–liquid two-phase flow in a centrifugal pump is critical for evaluating pump performance. In this study, a numerical simulation was performed to understand the effects of gas–liquid turbulence formation in the impeller channels on the performance of a single-stage centrifugal pump. The focus was on the inlet gas volume fraction (IGVF) at constant flowrate and constant impeller speed on the flow structure in the impeller and the pressure surging characteristics. The correlation between Reynolds shear stress and turbulent kinetic energy on pump performance is shown by examining pressure, velocity, turbulent kinetic energy, and gas volume fraction (GVF). The model verification is verified using the theoretical pump pressure head determined by the manufacturer, which demonstrated adequate uncertainty (μ<5%) for a flowrate of 250 L/min. The results show a 9.41% decrease in wall shear stresses when the in-situ GVF varies between 1% and 10%. Wall shear stresses combined with circulation resulted in a 3.76% decrease in turbulent kinetic energy, while the contribution of turbulence to pump performance degradation is minimal. Other factors such as gas entrapment, impeller inlet blockage due to bubble coalescence, pressure gradient, and bubble size had a much greater impact on performance. The performance decreased dramatically by 26.53% when the GVF increased from 1% to 10%. The proposed flow structure can be further investigated together with the wall shear stress and circulation phenomena, such as vorticity and shear layers in the context of two-phase flow.

On turbulence and interfacial momentum transfer in dispersed gas-liquid flows: Contribution of bubbly flow experiments under microgravity conditions

The major focus of the present work is to develop an adequate closure model for the turbulent contribution of the interfacial momentum transfer term in turbulent bubbly flow. For this purpose, numerical simulations have been conducted on bubbly pipe flows under microgravity conditions. With no buoyancy effect, the average interfacial forces are negligible due to the small interphase relative motion. In these conditions, the distribution and the dynamics of bubbles are mainly controlled by the turbulence effects. Based on a phenomenological approach, a new closure for the turbulent interfacial momentum transfer is developed within Eulerian two-fluid modelling based on the averaging of the added mass force. The turbulent terms thus obtained comprises a non-linear term and a convective acceleration term associated with the drift velocity. The two-fluid model with the proposed interfacial momentum transfer closure and an original three-equation turbulence model for bubbly flows has been implemented in a 3D CFD code and applied to pipe bubbly flow under microgravity conditions. When compared to conventional two-fluid closures, the turbulent contribution of the added mass force proved to be a relevant closure that provides a substantial improvement in predicting the void fraction distribution and the bubbles’ dynamics.

Heat and Mass Transfer Equations for Turbulent Flow with Wide Ranges of Prandtl and Schmidt Numbers

Turbulent statistics near the wall region were obtained by DNS (direct numerical simulation) method in recent years. Numerical models for the turbulent diffusivity and turbulent Prandtl number were improved based on these data. New logarithmic laws for the scalar distribution in the fully turbulent region were proposed by regression analysis of DNS results. Different values for the coefficients of these logarithmic law were proposed. The functions for Prandtl number effects on temperature distribution were also different in the literature. In the current investigation, an exponential function for the ratio of turbulent to molecular diffusivity is presented. Molecular and turbulent contributions to the diffusion process are compared. The critical points separating the linear and logarithmic regions are determined with a fixed criterion. New logarithmic law is derived for the scalar distribution, which shows good prediction of experimental data and DNS results. Using the derived equation for scalar distribution, new Nusselt and Sherwood number equations are obtained. The derived equations agree well with experimental data over wide ranges of Prandtl and Schmidt numbers. Compared with conventional correlations, the derived equations show better prediction of experimental data at high Schmidt numbers (larger than 930).

Turbulent transport of impurities in 3D devices

The evidence of a large diffusive turbulent contribution to the radial impurity transport in Wendelstein 7-X (W7-X) plasmas has been experimentally inferred during the first campaigns and numerically confirmed by means of gyrokinetic simulations with the code stella. In general, the absence of strong impurity accumulation during the initial W7-X campaigns is attributed to this diffusive term. Given the large variety of possible stellarator configurations, in the present work the diffusive contribution is also calculated in other stellarator plasmas. In particular, a numerical cross-device comparison is presented, where the diffusion (D) and convection (V) coefficients of carbon and iron impurities produced by ion-temperature-gradient (ITG) turbulence are obtained. The simulations have been performed for the helias W7-X, the heliotron LHD, the heliac TJ-II and the quasi-axisymmetric stellarator NCSX at the radial position r/a = 0.75. The results show that, although the size of D and V can differ across the four devices, inward convection is found for all of them. For W7-X, TJ-II and NCSX the two coefficients are comparable and the turbulent peaking factor is surprisingly similar. In LHD, appreciably weaker diffusive and convective impurity transport and significantly larger turbulent peaking factor, in comparison with the other three stellarators, are predicted. All this suggests that ITG turbulence, although not strongly, would lead to negative impurity density gradients in stellarators. Then, considering mixed ITG/trapped electron mode (TEM) turbulence for the specific case of W7-X, it has been quantitatively assessed to what degree pellet fueled reduced turbulence scenarios feature reduced turbulent transport of impurities as well. The results for trace iron impurities show that, although their turbulent transport is not entirely suppressed, a significant reduction of the convection and a stronger decrease of the diffusion term are found. Although the diffusion is still above neoclassical levels, the neoclassical convection would gain under such conditions a greater specific weight on the dynamics of impurities in comparison with standard ECRH scenarios without pellet fueling.

Major surface melting over the Ross Ice Shelf part II: Surface energy balance

AbstractThe West Antarctic climate is under the combined impact of synoptic and regional drivers. Regional factors have contributed to more frequent surface melting with a similar pattern recently, which accelerates ice loss and favors global sea‐level rise. Part I of this research identified and quantified the two leading drivers of Ross Ice Shelf (RIS) melting, viz. foehn effect and direct marine air advection, based on Polar WRF (PWRF) simulations. In this article (Part II), the impact of clouds and the pattern of surface energy balance (SEB) during melting are analyzed, as well as the relationship among these three factors. In general, net shortwave radiation dominates the surface melting with a daily mean value above 100 W·m−2. Foehn clearance and decreasing surface albedo respectively increase the downward shortwave radiation and increase the absorbed shortwave radiation, significantly contributing to surface melting in areas such as western Marie Byrd Land. Also, extensive downward longwave radiation caused by low‐level liquid cloud favors the melting expansion over the middle and coastal RIS. With significant moisture transport occurring over more than 40% of the time during the melting period, the impact from net radiation can be amplified. Moreover, frequent foehn cases can enhance the turbulent mixing on the leeside. With a Froude number (Fr) around 1 or slightly larger, fast downdrafts or reversed wind flows can let the warm foehn air penetrate down to the surface with up to 20 W·m−2 in sensible heat flux transfer to the ground. However, when the Froude number is close to infinity with breaking waves on the leeside, the contribution of turbulence to the surface warming is reduced. With better understanding of the regional factors for the surface melting, prediction of the future stability of West Antarctic ice shelves can be improved.

Mean thermal energy balance analysis in differentially heated vertical channel flows

Direct numerical simulations of differentially heated vertical channel (DHVC) flows were performed for Ra=105–109 to investigate the characteristics of the streamwise mean momentum and mean thermal energy equations. The log law for mean temperature was observed for Ra≥108 at y+>50, where y+ is the wall-normal distance normalized by the viscous wall unit. From the mean momentum equation, negligible viscous force and logarithmically increasing Reynolds shear stress were observed in the region where the log law for mean temperature occurred. The streamwise mean velocity did not exhibit a linear relationship with y+ close to the wall and did not show logarithmic development far from the wall due to the buoyancy force. In the mean thermal energy equation, a constant heat flux layer was observed, and the turbulent heat flux contribution was scaled by the inverse of wall-normal distance to satisfy the log law of mean temperature. For a high Rayleigh number (Ra=109), the turbulent heat flux spectra contained scale-separated inner and outer sites with linearly growing energetic structures along the wall-normal distance, which was not observed for a low Rayleigh number (Ra=106). The flow structures of turbulent heat flux originated from the upward wall-normal velocity fluctuations that triggered the non-directional structures of the temperature. These results suggest that the scale separation between the viscous and outer length scales with the wall-attached energetic structures resulted in the log law for mean temperature. These findings could serve as the basis of scaling formulations for the mean velocity and temperature in DHVC flows.

Morpho-kinematics of the Molecular Gas in a Quasar Host Galaxy at Redshift Z=0.654

e present a new study of archival ALMA observations of the CO(2-1) line emission ofthe host galaxy of quasar RX J1131. The quasar, at redshift z S ∼0.654, is lensed by a foreground galaxy at z L ∼0.30. Particular attention is paid to the mechanism of gravitational lensing. A simple lens model, shown to well reproduce the optical images obtained by the Hubble Space Telescope, is applied to the ALMA CO(2-1) images, providing a tool to understand the uncertainties attached to the evaluation of the source brightness and kinematics. Uncertainties attached to the process of data reduction are carefully evaluated. Evidence for the robustness of previously published results is obtained. A system of polar coordinates is introduced to better match the specificity ofthe imaging process. It provides particularly clear evidence for rotation of the gas contained in the galaxy. A simple rotating disc model is shown to give an excellent overall description of the morpho-kinematics of the source. It gives evidence for a hot spot of emission located near the quasar, overlapping the caustic and corresponding to an enhancement of emission by a factor ∼2.5. The possible presence of a companion galaxy suggested by some previous authors is not confirmed. The rotation curve is studied with reference to the predictions of the disc model. De-tailed comparison between model and observations gives evidence for a more complex dynamics than implied by the model. Doppler velocity dispersion within the beam size in the image plane is found to contribute 60±10 km s −1 to the line width. It accounts for the observed line width when a2- σ cut is applied to the data. However, when using a less severe cut, a significant amount of turbulence may be accommodated, preventing a reliable evaluation of the contribution of turbulence to the line width.

Morpho-kinematics of the circumstellar envelope of the AGB star R Dor: a global view

ABSTRACT We analyse new ALMA observations of the 29SiO (ν = 0, J = 8–7) and SO2 (ν = 0, 343,31−342,32) line emissions of the circumstellar envelope (CSE) of the oxygen-rich asymptotic giant branch (AGB) star, R Dor. With a spatial resolution of ∼2.3 au, the observations cover distances below ∼30 au from the star providing a link between earlier observations and clarifying some open issues. The main conclusions are the following. (i) Rotation is confined below ∼15 au from the star, with velocity reaching a maximum below 10 au and morphology showing no significant disc-like flattening. (ii) In the south-eastern quadrant, a large Doppler velocity gas stream is studied in more detail than previously possible and its possible association with an evaporating planetary companion is questioned. (iii) A crude evaluation of the respective contributions of rotation, expansion and turbulence to the morpho-kinematics is presented. Significant line broadening occurs below ∼12 au from the star and causes the presence of high Doppler velocity components near the line of sight pointing to the centre of the star. (iv) Strong absorption of the continuum emission of the stellar disc and its immediate dusty environment is observed to extend beyond the disc in the form of self-absorption. The presence of a cold SiO layer extending up to some 60 au from the star is shown to be the cause. (v) Line emissions from SO, 28SiO, CO and HCN molecules are used to probe the CSE up to some 100 au from the star, revealing the presence of two broad back-to-back outflows, the morphology of which is studied in finer detail than in earlier work.

Does the Magnetic Field Suppress Fragmentation in Massive Dense Cores?

Abstract Theoretical and numerical works indicate that a strong magnetic field should suppress fragmentation in dense cores. However, this has never been tested observationally in a relatively large sample of fragmenting massive dense cores. Here, we use the polarization data obtained in the Submillimeter Array Legacy Survey of Zhang et al. to build a sample of 18 massive dense cores where both fragmentation and magnetic field properties are studied in a uniform way. We measured the fragmentation level, N mm, within the field of view common to all regions of ∼0.15 pc, with a mass sensitivity of ∼0.5 M ☉, and a spatial resolution of ∼1000 au. In order to obtain the magnetic field strength using the Davis–Chandrasekhar–Fermi method, we estimated the dispersion of the polarization position angles, the velocity dispersion of the H13CO+(4–3) gas, and the density of each core, all averaged within 0.15 pc. A strong correlation is found between N mm and the average density of the parental core, although with significant scatter. When large-scale systematic motions are separated from the velocity dispersion and only the small-scale (turbulent) contribution is taken into account, a tentative correlation is found between N mm and the mass-to-flux ratio, as suggested by numerical and theoretical works.

Direct numerical simulation analysis of heat transfer deterioration of supercritical fluids in a vertical tube at a high ratio of heat flux to mass flowrate

The heat transfer deterioration (HTD) of supercritical water in heated vertical tubes at high heat flux to mass flow rate ratios is investigated using direct numerical simulations at an inlet Reynolds number of Reb0=5400 based on the inlet bulk velocity and tube diameter. The heated tube has a length of 75 times the tube diameter. Both forced and mixed convections (upward and downward flows) are simulated. The results show that primary and secondary HTDs occur in all flows considered herein. The causes of the HTD are comprehensively analyzed using the Fukagata–Iwamoto–Kasagi identity, turbulent heat flux, turbulence production, and turbulent kinetic energy. The FIK decomposition shows that the turbulent contribution Nu2 is the dominant part of the total Nusselt number NuFIK. The turbulence reduction caused by flow acceleration is the main reason for the decrease in Nu2 and the occurrence of the primary HTD. Furthermore, buoyancy first damps the turbulence, exacerbating the HTD, and then forms an M-shaped velocity profile, which enhances the heat transfer. The secondary HTD, which is less pronounced than the primary one, comes from the decrease in the mean enthalpy gradient and enthalpy fluctuation caused by the position variation of the maximum specific heat.

The influence of turbulent transport in reactive processes: A combined numerical and experimental investigation in a Taylor-Couette reactor

Turbulent reactive flows are ubiquitous in industrial processes. Decoupling transport effects from intrinsic chemical reactions requires an in-depth understanding of fluid flow physics; computational fluid dynamics (CFD) methods have been widely used for this purpose. Most CFD simulations of reactive liquid-phase flows, where the Schmidt numbers are large, rely on isotropic eddy viscosity models. However, the assumption of turbulent isotropy in most stirred reactors and wall-bounded flows is fundamentally incorrect and leads to erroneous results. Here, we apply a systematic CFD approach to simulate liquid-phase diffusive and convective transport phenomena that occur in a Taylor-Couette (TC) reactor. We resolve the turbulent flow by extracting statistics from large eddy simulation which is used to tune the anisotropic Reynolds stress model. In addition, we conducted a series of turbulent precipitation and mixing studies in a TC reactor that was designed and fabricated in-house. The numerical model is successfully validated against a published torque correlation and it is found to accurately describe the advection and diffusion of chemical species. The validated model is then used to demonstrate key flow properties in the reactor. We define new local turbulent Peclét numbers to characterize the relative increase in diffusivity from turbulent advection and observe a 29% increase in the turbulent contribution as Reynolds number is doubled. Both reactive simulations and experiments show an increase in overall reaction rates with increased turbulence. The results from reactive simulations provide a deeper understanding of flow-kinetics interactions at turbulent conditions.

Analysis of Turbulence Effects in a Patient-Specific Aorta with Aortic Valve Stenosis

Blood flow in the aorta is often assumed laminar, however aortic valve pathologies may induce transition to turbulence and our understanding of turbulence effects is incomplete. The aim of the study was to provide a detailed analysis of turbulence effects in aortic valve stenosis (AVS).MethodsLarge-eddy simulation (LES) of flow through a patient-specific aorta with AVS was conducted. Magnetic resonance imaging (MRI) was performed and used for geometric reconstruction and patient-specific boundary conditions. Computed velocity field was compared with 4D flow MRI to check qualitative and quantitative consistency. The effect of turbulence was evaluated in terms of fluctuating kinetic energy, turbulence-related wall shear stress (WSS) and energy loss.ResultsOur analysis suggested that turbulence was induced by a combination of a high velocity jet impinging on the arterial wall and a dilated ascending aorta which provided sufficient space for turbulence to develop. Turbulent WSS contributed to 40% of the total WSS in the ascending aorta and 38% in the entire aorta. Viscous and turbulent irreversible energy losses accounted for 3.9 and 2.7% of the total stroke work, respectively.ConclusionsThis study demonstrates the importance of turbulence in assessing aortic haemodynamics in a patient with AVS. Neglecting the turbulent contribution to WSS could potentially result in a significant underestimation of the total WSS. Further work is warranted to extend the analysis to more AVS cases and patients with other aortic valve diseases.

Two decades of long-term observations of polar mesospheric echoes at 69°N

Since 1999 radar continuous observations of polar mesospheric echoes have been conducted on the Norwegian island of Andøya (69.30°N, 16.04°E), with the ALWIN radar (1999–2008) and MAARSY (since 2011). Traditionally these observations have been named after their seasonal occurrence, i.e., Polar Mesosphere Summer Echoes (PMSE) and Polar Mesosphere Winter Echoes (PMWE). PMSE are much stronger than PMWE and are known to be due to contributions of charged-dust (ice) particles and turbulence. On the other hand, most PMWE are mainly due to turbulence. Both echoes depend on electron density and its gradients. The use of MAARSY, more 17 dB sensitive than the ALWIN radar, makes it possible to observe turbulence-dominated echoes, usually below 80 km, all year round with a mean seasonal occurrence frequency of about 14%. Their minimum occurrence rate is 2% in July and August, while there are two maxima, one in March/April (22%) and the second one in October (26%). On average the dust-dominated summer echoes starts on 14 May and ends on 26 August (i.e. 105 days) with an average occurrence in June/July of 95%. These summer echoes occur mainly between 80 and 90 km, and present a maximum daily occurrence around 13:00 LT. On the other hand, the turbulence-dominated winter echoes occur mainly between 55 and 80 km with maximum daily occurrence around 11:30 LT. Besides the seasonal and daily occurrence of these echoes, we present the variability of occurrence frequency rates for the summer layers since 1994. After eliminating the effects of geomagnetic and/or solar activity the occurrence of the summer dust-dominated echoes show a positive trend of about 0.32%̇/yr over the last twenty seven years which might be related to the observed negative mesospheric temperature trends at polar latitudes.

Thermal and Turbulent Properties of the Warm Neutral Medium in the Solar Neighborhood

Abstract The transition from the diffuse warm neutral medium (WNM) to the dense cold neutral medium (CNM) is what set the initial conditions to the formation of molecular clouds. The properties of the turbulent cascade in the WNM, essential to describe this radiative condensation process, have remained elusive in part due to the difficulty in mapping out the structure and kinematics of each H i thermal phase. Here we present an analysis of a 21 cm hyper-spectral data cube from the GHIGLS H i survey where the contribution of the WNM is extracted using ROHSA, a Gaussian decomposition tool that includes spatial regularization. The distance and volume of the WNM emission is estimated using 3D dust extinction map information. The thermal and turbulent contributions to the Doppler line width of the WNM were disentangled using two techniques, one based on the statistical properties of the column density and centroid velocity fields, and the other on the relative motions of CNM structures as a probe of turbulent motions. We found that the volume of WNM sampled here (5.2 × 105 pc3), located at the outer edge of the Local Bubble, shows thermal properties in accordance with expected values for heating and cooling processes typical of the solar neighborhood: P th/k B = (4.4 ± 2.6) × 103 K cm−3, n = 0.74 ± 0.41 cm−3, and T k = (6.0 ± 1.3) × 103 K. The WNM has the properties of sub/trans-sonic turbulence, with a turbulent Mach number at the largest scale probed here (l = 130 pc) of , a density contrast of , and velocity and density power spectra compatible with k −11/3. The low Mach number of the WNM provides dynamical conditions that allow the condensation mode of thermal instability to grow freely and form CNM structures, as predicted by theory.

Finite control volume and scalability effects in velocimetry for application to aeroacoustics

The ability to scale the field of view for velocimetry methods is particularly attractive for aeroacoustics studies, where turbulence contributions in long-lived, low wavenumber structures account for the most important sources of radiated noise. Thanks to its core operating principles, Doppler global velocimetry (DGV) offers interesting opportunities for large-scale flow measurements. As is well known, a larger field-of-view (FOV) in the measurement will change the spatial resolution, meaning that each measurement is integrated across a larger control volume. This finite size will affect the velocity and turbulence measurements, as well as the observable wavenumbers in the measurement. The present study confirms the viability of DGV for studying low wavenumbers while examining the influence on higher wavenumbers of the less coherent turbulent structures also present in high-speed flows. In the flows examined by the current work, mean velocity bias error due to integration over the measurement region was shown to be small, less than 0.005%, for control volume heights up to 4 mm. Although significant energy attenuation occurs for high wavenumbers, the low wavenumber turbulent structures which dominate far-field noise are found to be unaffected by the size of the control volume required for large measurement of both laboratory- and full-scale supersonic jet flows. The results indicate a large-scale velocimetry system such as DGV can be a valuable tool for researchers to study aeroacoustics in high-speed flows.

Experimental and numerical investigation of turbulent wake flow around wall-mounted square cylinder of aspect ratio 2

The turbulent wake velocity fields around a short wall-mounted square cylinder of aspect ratio 2 fully immersing into a thick turbulent boundary layer are obtained experimentally with PIV and numerically with LES. Comparison between the two sets of data is made on the time-averaged mean velocities, turbulent fluctuating velocities, and the detailed characteristics of energetic large-scale coherent structures. It is noted that a good comparison of the low-order statistics of velocity fields does not guarantee a satisfactory reproduction of the coherent structures. These flow structures show great similarities in the spatial patterns and dynamic performance between PIV and LES, but apparent differences in turbulent kinetic energy contribution. On the horizontal plane parallel to the wall, the dominant antisymmetric spatial mode pair illustrates the evolution process of Karman vortex shedding with a clear spectral peak around the characteristic frequency while three extra frequency peaks are found in the symmetric spatial modes. Further investigation on the velocity field on the central vertical plane reveals the connection between the unsteadiness of shear layer flow at the free end and the spanwise symmetric vortex shedding activities. It is confirmed that the flapping motion and vortex convection mechanisms observed for the present cylinder could modulate the symmetric wake dynamics. The three-dimensional extreme wake patterns, available from LES results, are related to the most energetic Karman vortex shedding demonstrate upward flow in the vortex weakening side. Meanwhile, the downwash flow at the extreme time instants is found to become stronger always for this kind of short cylinder with a dipole wake pattern.

Investigation of the wake energy recovery of cross-flow turbines in paired configuration by means of 3d-CFD and analysis of the streamwise momentum budget

Literature suggests that cross-flow turbines (CFTs) could be suitable for off-shore wind farms because of the high power-density achievable by shortening the distance between arrays as allowed by a fast wake energy recovery experimentally detectable. By means of 3d-CFD, we analysed the effect of the rotation verse of CFTs in counter-rotating paired configuration on the wake behaviour. Then, we applied the momentum-budget approach to identify the fluid dynamic mechanisms which are more effective in supporting the streamwise momentum recovery. The following results are found. (A) The counter-rotating vortices occurring in the near-wake as a consequence of the vortex shedding at the blade tip are responsible for the vertical advection that enters high momentum flow inside the wake. (B) The turbulent transport contribution is less important, yet it becomes significant starting from the medium wake. (C) For the inner-downwind layout the wake shape appears similar to that of a single turbine, whereas for the inner-upwind layout it is greatly contracted in horizontal direction and enlarged in vertical direction (D). The momentum recovery appears slightly more delayed than in case of a single turbine, yet the velocity deficit appears less extensive for the inner upwind layout that, especially thanks to the wake lateral contraction, could be the preferable in farms consisting in staggered arrays, allowing to shorten the lateral distance between adjacent pairs.

Assessment of pressure field and performance parameter reconstruction from velocity data for ramjet inlet

The current flow measurement devices for ramjet inlet still have many limitations in the supersonic internal flow, such as the qualitative feature of schlieren and the low resolution of pressure tap/probe. In this paper, a non-intrusive measurement scheme for the internal pressure field and performance parameters of ramjet inlet based on particle image velocimetry (PIV) data is established and quantitatively evaluated by using a typical model. This scheme is combined with two important PIV-based pressure reconstruction approaches, namely, the spatial integration in conservative form and the MacCormack method, which are especially suitable for the supersonic flows with shock waves. The feasibility of this scheme is firstly confirmed by using a numerical velocity field, and the turbulence contribution is concluded to be negligible. Then, a simulated PIV experiment is conducted to achieve a synthetic velocity field with various theoretical measurement errors for the comprehensive analyses of the accuracy and robustness of this measurement method. Compared with the traditional equipment, the PIV-based method can obtain the high-resolution velocity and pressure distributions of whole flow field without interference, and the performance parameters are also available along the flow path. The good reconstruction results show the great advantage and attraction of this non-intrusive method in the future wind tunnel test for ramjet inlet.