Eddy Separation Research Articles

Large-eddy simulation of wind pressures on elevated low-rise buildings

Residential buildings in coastal communities are often elevated to mitigate flooding and wave-surge impacts. However, the elevations change the local wind field characteristics and may increase the aerodynamic pressure on buildings. Post-event reconnaissance showed that elevated buildings experienced severe structural damage to the roofs, walls, and floors during major hurricanes. Currently, our understanding of aerodynamic pressure on elevated houses is limited. In this paper, a large eddy simulation (LES)-based method is developed to better understand the wind effects on elevated buildings. The inflow generation method, discretizing and synthesizing random flow generation, is adopted to generate inflow boundary conditions satisfying the target spectrum. Two sub-grid scale models, the Smagorinsky and wall-adapted local eddy-viscosity models, are employed to represent the unresolved small-scale eddies. It is found that the eddy structure sizes can strongly affect the pressure fluctuations at the eddy separation zones. The present study advances the understanding of aerodynamics on elevated buildings and provides a reference for future LES-based research on wind effect modeling.

A numerical study of rectification driven by a tidal jet

The barotropic tide in fjords is often assumed to play a negligible role in driving the circulation over timescales greater than the tide itself, the residual circulation. However, this is not necessarily true when the tide interacts with the fjords’ topography. Of particular interest are narrow channels, where the tide can excite non-linear flow features such as flow separation eddies and establish a tidal jet. The jet and associated eddies are known to influence transport through the channel and dominate local dispersion. We investigate how the jet can influence the residual circulation in Kvænangen, a Northern Norwegian fjord. We model the flow dynamics using an unstructured grid ocean model, solving the shallow water equations. The model results show that the jet can drive residual currents around the fjords basin if the jet forces a vorticity flux across closed bathymetric contours. We demonstrate that the rectification is sensitive to how the channel is represented in the ocean model. With a wide channel, the fjord-scale residual circulation diminishes. Our work thus highlights the importance of accurately resolving the actual geometry of the coastline, since small-scale flow phenomena generated in narrow channels can force a far-field circulation response and consequently influence the drift of tracers in the fjord.

Flow-Induced Vibration of Cantilever Type Elastic Material in Straight Tricylinder

The hydrodynamic performance and wake evaluation results of a cantilever flexible cylindrical triangular array at Re = 9000 and with L/D spacing ratios ranging from 3 to 7 are presented in this paper. The improved delayed separation eddy current simulation (IDDES) SST k-ω turbulence model and tetrahedral mesh are employed to solve the three-dimensional instantaneous flow field. The objective is to find the L/D ratio that the upstream cylinder has the most pronounced effect on the downstream cylinders, to quantify whether a flexible upstream cylinder is more favorable than the rigid upstream cylinder, and to know the flexible cylinder wake distribution. The findings indicate that, at Re = 9000, when the spacing ratio is 3~7 and the spacing ratio L/D is 6, the amplitude of the downstream cylinder is most affected by the wake of the upstream cylinder; the amplitude of downstream cylinders of the rigid upstream cylinder exerted a 1% or 2% greater effect than the flexible cylinder, with vortices predominantly concentrated in the middle and upper parts of the flexible cylinder rather than at its top.

Stationarity and separation of the Sevastopol eddies in the Black Sea: The role of eddy-topographic interaction and submesoscale dynamics

One of the eddy “hotspots” in the Black Sea is the area of stationary Sevastopol anticyclone. Here this eddy can reside for >3–6 months significantly affecting the cross-shelf exchange and nutrient fluxes in the Black Sea. Detaching from its stationary location, the eddy moves westward, where it finally dissipates. In this article, we use altimetry measurements and a high-resolution (1 km) Nemo numerical model to investigate in detail the physical processes responsible for the observed evolution of the Sevastopol eddies. The main reason for the stationarity of such eddies is the sharp turn of the continental slope, which plays the role of the meridional wall blocking the vortex propagation in the cyclonic direction. Such a wall is situated at depths of 200–500 m and acts only on sufficiently intense eddies with large vertical extensions, while weak eddies with a small thickness do not feel it. The eddy trajectory near the obstacle represents a cyclonic spiral, which displaces to the southeast before the eddy separation. The process of vortex separation from the wall can prolong for several months and can be divided into 4 stages: 1) Movement to the north after the collision with the meridional wall due to the action of the reactive forces; 2) The formation of an attached submesoscale cyclonic lens-like eddy (SCE) at 200–400 m caused by the rise of horizontal shear due to the interaction of currents with the local uplift of the bathymetry near the continental slope; 3) The movement of the formed vortex dipole to the southwest due to self-propelling force; 4) Displacement of the dipole to the south beyond the meridional wall and further movement of the Sevastopol eddy to the west.During the separation process, Sevastopol AE loses a large amount of energy due to leakage of the mass during its interaction with the wall and transfer of a large part of its energy to the attached SCE. This process explains the observed decrease in the intensity of the Black Sea eddies to the west of the places of their stationarity.

Predicting the Loop Current dynamics combining altimetry and deep flow measurements through the Yucatan Channel

The Loop Current is the main mesoscale feature of the Gulf of Mexico oceanic circulation. With peak velocities above 1.5 m s–1, the Loop Current and its mesoscale eddies are of interest to fisheries, hurricane prediction and of special concern for the security of oil rig operations in the Gulf of Mexico, and therefore understanding their predictability is not only of scientific interest but also a major environmental security issue. Combining altimetric data and an eddy detection algorithm with 8 years of deep flow measurements through the Yucatan Channel, we developed a predictive model for the Loop Current extension in the following month that explains 74% of its variability. We also show that 4 clusters of velocity anomalies in the Yucatan Channel represent the Loop Current dynamics. A dipole with positive and negative anomalies towards the western side of the Channel represents the growing and retracted phases respectively, and two tripole shape clusters represent the transition phases, the one with negative anomalies in the center associated with 50% of the eddy separation events. The transition between these clusters is not equally probable, therefore adding predictability. Finally, we show that eddy separation probability begins when the Loop Current extends over 1800 km (~27.2°N), and over 2200 km of extension, eddy detachment and reattachment is more frequent than separation. These results represent a step forward towards having the best possible operational Loop Current forecast in the near future, incorporating near real-time data transmission of deep flow measurements and high resolution altimetric data.

Upper–Lower Layer Coupling of Recurrent Circulation Patterns in the Gulf of Mexico

Abstract The study of the relationship between the upper and lower layers in the Gulf of Mexico (GoM) has experienced a lot of progress in recent years. Nevertheless, an examination of their coupling for the entire GoM in a statistically consistent manner is still needed. Layer thickness data from a GoM 21-yr free-running simulation are used to examine the coupling between the upper (<250 m) and lower (>1000 m) layers, focusing on the dominant modes of variability through a Hilbert empirical orthogonal function (HEOF) analysis. The results show that the three leading modes are associated with the coupling between both layers during the life cycle of the Loop Current (LC) and the LC eddy (LCE) separation process, consistent with previous observational studies. These modes are cyclical, with periodicities in agreement with the mean LCE separation period, indicating recurrence of the circulation patterns. The fourth mode of the upper layer is associated with the translation of LCEs and their dissipation in the northwestern GoM, while in the lower layer it captures variability related to the strengthening of the circulation along the Sigsbee Gyre western branch. This mode does not show cyclicity, suggesting persistence of the associated circulation patterns with a dominant time scale of 14 months. Evidence and corroboration of recently observed lower-layer circulation features are provided. The application of the HEOF technique used here can complement the three-dimensional oceanic assimilation methods by projecting surface information to depth in a statistically consistent manner. Significance Statement The purpose of this study is to better understand the relationship between the upper and lower ocean layers in the Gulf of Mexico. While much is known about their variability separately, an examination of the coupling through the whole region and in a long time period is still needed. We use a free numerical simulation of the circulation in the Gulf to accomplish this goal. The dominant circulation patterns in the lower layer are tied to the upper ones and are governed by the same temporal scales. Our findings point to a way to better understand the response of deep circulation to upper circulation, and may contribute to a better prediction of ocean dynamics.

Influences of Gap Flow on Air Resistance Acting on a Large Container Ship

In terms of speed lost and fuel consumed, wind loads are considered one of the main factors for large ship design, especially for container ships. Alongside water resistance, air resistance in strong wind conditions has a significant impact on the fuel efficiency and performance of container ships with large box-type bodies. This paper reports the effects of wind loads acting on a 20,000 TEU container ship carrying large numbers of deck containers using a commercial CFD software program (ANSYS Fluent V14.5 with RANS equation). A 1/255.3 scale model was used in this study to reveal the air resistance on the container ship configuration. The aerodynamic formations of the complex vortices, pressure, velocity contours, and streamlines, as well as the air forces acting on the container ship, are presented and discussed. The pressure distributions show that the gap air flows increase the stagnation pressure at the face side and decrease the pressure on the backside of each container gap through separate eddies. The difference in pressures created in the gaps contribute to the air resistance acting on the ship. It is confirmed that the use of side covers of deck containers to close the gap flows between container blocks can significantly reduce the air resistance for wind directions in the range of 30 to 60 degrees.

Generation of high-frequency topographic Rossby waves in the Gulf of Mexico

The Loop Current Eddy (LCE) separation cycle energizes deep circulation in the eastern Gulf of Mexico, transferring energy from the surface intensified Loop Current (LC) to the typically quiescent lower layers. To document the generation and radiation of deep energy during this cycle, an array of 24 current and pressure recording inverted echo sounders (CPIES) is deployed in the region 89°W to 86°W, 25°N to 27.5°N with the intent to capture circulation near bathymetric features thought to be important for current-topographic interactions: Campeche Bank, Mississippi Fan, and West Florida Shelf. During the nearly two-year deployment, June 2019 to May 2021, three LCE separation events are observed, during which energy injected into the deep Gulf organizes into two distinct frequency bands (1/100 – 1/20 days–1 and 1/20 – 1/10 days–1). High-frequency variability dominates the array’s northwest corner in the vicinity of the Mississippi Fan. Wave properties are consistent with topographic Rossby Waves (TRWs) with wavelengths of 150 – 300 km. Their generation coincides with each LCE separation and is attributed to an upper-lower layer resonant coupling between surface meanders and the sloping topography of the Mississippi Fan. TRWs captured by the CPIES array will likely intensify as wavelengths shorten in steeper topography along propagation pathways towards the Sigsbee Escarpment, generating hazardous currents with the potential to disrupt oil and gas operations in the region.

Slope and Shelf Flow Anomalies Off Oregon Influenced by the El Niño Remote Oceanic Mechanism in 2014–2016

AbstractOutputs of the regional ocean circulation model are analyzed to demonstrate the measurable impact of the El Niño remote oceanic forcing mechanism along the US West Coast during the major heat wave period of 2014–2016. The 2‐km horizontal resolution model, based on the Regional Ocean Modeling System (ROMS), was run for the period of 2009–2018. Though the model does not assimilate observations, it performs well by comparison with time series data explaining observed variability on temporal scales from several days to seasonal and interannual. The El Niño‐related oceanic anomalies provided by a global state estimate are introduced in the regional model at the southern boundary at 24N. These propagate alongshore with coastally trapped waves (CTWs) and influence the variability off Oregon (41°–46°N). In particular, CTWs are evident in the subsurface along‐slope current, vs, and in the depth of the 26.5 kg m−3 isopycnal surface over the slope, z26.5. In summer 2014 and 2015, vs anomalies are positive (northward) and z26.5 anomalies are negative (deeper) along the US West Coast. In addition to the CTW patterns, z26.5 anomalies also exhibit slow‐moving features associated with undercurrent widening, separation, and subsurface eddy variability. Over the Oregon shelf, El Niño conditions contributed to the sharp weakening of the southward alongshore current throughout the water column in July 2014 and 2015, despite the near‐average southward, upwelling‐favorable winds.

A general Lagrangian tracking methodology for riverine flow and transport

AbstractThis article presents a new public domain tool for generalized Lagrangian particle tracking in rivers. The approach can be applied with a variety of two‐ and three‐dimensional flow solvers. Particle advection by the flow is incorporated using flow fields from the chosen solver assuming particles follow the Reynolds‐averaged flow, although some other simple passive and active particle behaviors are also treated. Turbulence effects are treated using a random walk algorithm with spatial step lengths randomly chosen from Gaussian distributions characterized by the diffusivity from the flow solver. Our work extends this concept to a general framework that is solver and coordinate system independent to allow easy comparisons between differing flow treatments. To better treat problems where detailed information is required in specific regions, the approach includes novel cloning and colligation algorithms which enhance local resolution at modest computational expense. We also provide tools for computing local concentrations and total exposure over a user‐specified time interval. Several examples of predictions are provided to illustrate applications of the technique, including examination of the role of curvature‐driven secondary flows, storage in lateral separation eddies, treatment of larval drift, treatment of fuel spill dispersion, river‐floodplain connections, and sedimentation in floodplain ponds by tie channel connections. We also demonstrate that the model can reproduce analytically derived concentration profiles for simple diffusivities. These examples show that the Lagrangian particle tracking approach and the extensions proposed here are broadly applicable and viable for treating difficult river problems with multiple temporal and spatial scales. The examples also illustrate the utility of the cloning/colligation extensions and show how these can decrease the computational effort required on problems where high local resolution is required. Enhancement of the tools and even broader applicability can be achieved through the inclusion of multiple particle populations and particle–particle interactions.

Turbulence-induced ventilation of an isolated building: Ventilation route identification using spectral proper orthogonal decomposition

Turbulence dominates the natural ventilation of a building when openings are parallel to the external wind, and multiple factors associated with turbulence simultaneously affect natural ventilation. This study applies spectral proper orthogonal decomposition (SPOD) to quantitatively identify the main ventilation routes produced by turbulence in an isolated building with two symmetric openings. The turbulence data are obtained from large-eddy simulations of various wind directions and window casements. Two major turbulence-induced ventilation mechanisms are observed when the wind is parallel to the openings: The first is in the low-frequency region and is characterized by a slight change in the wind direction of the external flow, which contributes to cross ventilation. The second is in the high-frequency region and is characterized by a flow separation on the side of the building, which contributes to single-sided ventilation. The ventilation routes are visualized using SPOD modes and animations. In addition, the impact of window casements on turbulence-induced ventilation is investigated. The installation location of the window casements can either enhance or weaken the cross ventilation depending on whether the turbulence is efficiently utilized in the ventilation and can significantly affect the indoor cross ventilation route. However, regardless of the side that they are installed on the window, the casements may slightly improve single-sided ventilation by limiting the occurrence of separated eddies near the openings. • Visualization of turbulence-induced ventilation in an isolated two-opening building. • Quantitative analysis of the contributors to natural ventilation. • Cross ventilation induced by external wind direction changing. • Single-sided ventilation induced by flow separation. • Impact of window casements on turbulence-induced ventilation.

Experimental and numerical investigation of flow over spiral grooved cylinders

This study experimentally and numerically investigates the performance of a circular cylinder with a spiral grooved surface in terms of reducing wind drag. Its application in the overhead high-power conductor plays a vital role, especially in typhoon conditions. Wind tunnel tests have shown that at the critical Reynolds number (Re), the coefficients of wind drag decrease to a greater extent in a spiral grooved cylinder than in a smooth circular one. Moreover, a cylinder with a shallow groove and a small number of spirals could reduce the coefficient of drag in typhoon conditions. To gain an insight into the underlying fluid mechanism, a large-eddy simulation of turbulent flow from a critical to a super-critical Re has been carried out to approximate the flow separation and turbulent eddies over the spiral grooved cylinder. The results of the wind tunnel test have been used as a benchmark for the numerical results. The flow characteristics have been established about the near-wall flow separation and far wake flow, the pressure coefficient, the skin-friction coefficient, drag coefficient, and Q-criterion field.

Pedestrian Wind Analysis for Indian Sub-Continent

Urbanization in India is causing local wind and climate changes near high-rise buildings, affecting the urban wind climate pedestrian-level winds to dangerous and uncomfortable levels. Hence, today's architectural planning Wind comfort has to not only consider the wind load and internal conditions, but also must consider the outdoor wind building design environment. This study reviewed the methods of evaluating pedestrian-level wind weather, different Wind tunnel test standards of wind comfort, and various techniques for evaluating pedestrian-level wind speed. In the CFD Numerical following section, a brief overview of the current research on this topic and the way forward in India is Modeling related to the design and configuration of windward buildings at the pedestrian level. After analysing the previous documents, the authors recommend it is very necessary to homogenize the various wind standards, as it might have different consequences for planers. Among the various wind tunnel experimental technologies, compared to the hot wire wind speed measurement method, the Irwin probe’s accuracy and simplicity is justified, and can be installed in multiple positions to measure wind speed horizontal of pedestrians at the same time. For the numerical simulation, several researchers have used the Renault medium Navier stokes-based technique, although the precision of this technique is not as good as the simulation of large eddies and the simulation of separate eddies. However, this technology is not only precise but also costs less due to its requirement of less computing resources.

Formation of a subsurface anticyclonic eddy from the California Undercurrent off northwestern Baja California, Mexico

Hydrographic observations off northwestern Baja California in March 2017 revealed the presence of a subsurface anticyclonic eddy in a cross-shelf transect at ∼ 32 ° N. This eddy’s width was roughly 30 km and it was located about 15 km from the shelf, at depths between 100 m and 400 m. The eddy’s core had hydrographic characteristics consistent with those found in the California Undercurrent (CUC), which suggests that the eddy was originated in the CUC. Numerical-modeling simulations were used to elucidate the origin and evolution of the observed eddy. These simulations also showed a subsurface eddy which crossed the cross-shelf transect’s location with size, depth, and hydrographic characteristics similar to those of the observed eddy (but formed in November 2007, in a simulation for the 2004–2011 period). Thus, this numerical eddy is considered as analogous to the observed one, hence the life cycle is assumed to be the same in both cases. The eddy formation occurred roughly 40 km southeast of the cross-shelf transect, in a zone characterized by a submarine point, where the isobaths change their orientation abruptly and where an enhanced eddy-energy production occurs, which shows that the irregular bottom-topography is a factor that triggered such an eddy formation. The eddy formed in a period of weak along-shelf velocities, 4-10 days after an event of the most intense velocities of the year, associated with the passage of a wind-driven coastal trapped wave generated in remote locations south of the study area. The flow disturbances forced by this wave could be an additional physical factor which predisposes the water column to instability. However, at least in the cases herein analyzed, the eddy separation required an additional momentum/vorticity input by an adjacent mesoscale cyclonic gyre. Comparison with numerical and laboratory–experimental results support the notion that a baroclinic instability process is involved in the eddy generation. The path followed by the subsurface eddy described herein shows a link of the hydrographic characteristics between northwestern Baja California and the Southern California Bight. • Hydrographic observations show a subsurface anticyclonic eddy off Baja California. • An eddy detected in numerical simulations shows consistency with the observations. • Numerical results elucidate the formation and evolution of the observed eddy. • The eddy formed in a zone where isobaths change their direction abruptly. • The formation was linked to weak along-shelf flows and an adjacent cyclonic gyre.

Improved Quadrant Analysis for Large-Scale Events Detection in Turbulent Transport

Quadrant analysis has been widely used to investigate the turbulent characteristics in the atmospheric boundary layer (ABL). Although quadrant analysis can identify turbulent structures that contribute significantly to turbulent fluxes, the approach to the hyperbolic hole and its parameter, referred to as hole size, remains uncertain and varies among different studies. This study discusses an improved quadrant analysis with an objective definition of the hole size for the isolation of large coherent structures from small-scale background fluctuations. Eddy covariance data collected 50 m above the grass canopy were used to analyze and evaluate the proposed method. This improved quadrant analysis ensures that the detected large coherent eddies play a dominant role in transporting fluxes, occupying 10% of the total time, with mean flux contributions ranging from 62% to 95% for momentum and 35–104% for scalars. The separated background small-scale eddies are isotropically characterized by a comparable time duration and flux contributions in each quadrant. It is observed that under an unstable atmosphere, large-scale ejections are more active than sweeps, while under stable conditions, they are equally important. Furthermore, mechanical-driven transport under near-neutral conditions only enhances ejection and sweep motions of momentum. In contrast, the buoyancy-driven scenarios under unstable conditions enhance the large-scale activities of sensible heat alone.

Performance Enhancement Analysis of a Horizontal Axis Wind Turbine by Vortex Trapping Cavity

Abstract A wind turbine blade has the particularity of containing twisted and tapered thick airfoils. The challenge with this configuration is the highly separated flow in the region of high twist. This research presents a numerical investigation of the effectiveness of a vortex trapping cavity (VTC) on the aerodynamics of the National Renewable Energy Laboratory phase VI wind turbine. First, simulations are conducted on the S809 profile to study the fluid flow compared to the airfoil with the redesigned VTC. Second, the blade is simulated with and without VTC to assess its effect on the torque and the flow patterns. The results show that for high angles of incidence at Rec = 106, the lift coefficient increases by 10% and the wake region appears smaller for the case with VTC. For wind speeds larger than 10 m/s, the VTC improves the torque by 3.9%. This is due to the separation that takes place in the vicinity of the VTC and leads to trapping early separation eddies inside the cell. These eddies roll up forming a coherent laminar vortex structure, which in turn sheds periodically out of the cell. This phenomenon favorably reshapes excessive flow separation, reenergizes the boundary layer, and globally improves blade torque.

Transient Analysis of Flow Unsteadiness and Noise Characteristics in a Centrifugal Compressor with a Novel Vaned Diffuser

The demands to apply transonic centrifugal compressor have increased in the advanced gas turbine engines. Various techniques are used to increase the aerodynamic performance of the centrifugal compressor. The effects of the inclined leading edges in diffuser vanes of a transonic centrifugal compressor on the flow-field unsteadiness and noise generation are investigated by solving the compressible, three-dimensional, transient Navier–Stokes equations. Diffuser vanes with various inclination angles of the leading edge from shroud-to-hub and hub-to-shroud are numerically modeled. The results show that the hub-to-shroud inclined leading edge improves the compressor performance (2.6%), and the proper inclination angle is effective to increase the stall margin (3.88%). In addition, in this study, the transient pressure variations and radiated noise prediction at the design operating point of the compressor are emphasized. The influences of the inclined leading edges on the pressure waves were captured in time/space domain with different convective velocities. The pressure fluctuation spectra are calculated to investigate the tonal blade passing frequency (BPF) noise, and it is shown that the applied inclination angles in the diffuser blades are effective, not only to improve the aerodynamic performance and stall margin, but also to reduce the BPF noise (7.6 dB sound pressure level reduction). Moreover, it is found that the diffuser vanes with inclination angles could suppress the separation regions and eddy structures inside the passages of the diffuser, which results in reduction of the overall sound pressure level and the broadband noise radiated from the compressor.

Comparison of 4DVAR and EnKF state estimates and forecasts in the Gulf of Mexico

An experiment is conducted to compare four‐dimensional variational (4DVAR) and ensemble Kalman filter (EnKF) assimilation systems and their predictability in the Gulf of Mexico (GoM) using the Massachusetts Institute of Technology general circulation model (MITgcm). The quality of the ocean‐state estimates, forecasts, and the contribution of ensemble prediction are evaluated. The MITgcm–Estimating the Circulation and Climate of the Ocean (ECCO) 4DVAR (MITgcm–ECCO) and the MITgcm–Data Assimilation Research Testbed (DART) EnKF (MITgcm–DART) systems were used to compute two‐month hindcasts (March–April, 2010) by assimilating satellite‐derived along‐track sea‐surface height (SSH) and gridded sea‐surface temperature (SST) observations. The estimates from both methods at the end of the hindcast period were then used to initialize forecasts for two months (May–June, 2010). This period was selected because a loop current (LC) eddy (Eddy Franklin: Eddy‐F) detachment event occurred at the end of May 2010, immediately after the Deepwater Horizon (DwH) oil spill. Despite some differences between the setups, both systems produce analyses and forecasts of comparable quality and both solutions significantly outperformed model persistence. A reference forecast initialized from the 1/12° Hybrid Coordinate Ocean Model (HYCOM)/NCODA global analysis also performed well. The EnKF experiments for sensitivity to filter parameters showed enhanced predictability when using more ensemble members and stronger covariance localization, but not for larger inflation. The EnKF experiments varying the number of assimilation cycles showed enhanced short‐term (long‐term) predictability with fewer (more) assimilation cycles. Additional hindcast and forecast experiments at other times of significant LC evolution showed mixed performance of both systems, which depends strongly on the background state of the GoM circulation. The present work demonstrates a practical application of both assimilation methods for the GoM and compares them in a limited number of realizations. The overall conclusion showing improved short‐term (long‐term) predictability for EnKF (4DVAR) carries an important caveat that the results from this study are specific to a few 4DVAR and EnKF LC eddy separation experiments in the GoM and cannot be generalized to conclude the relative performance of both methods, especially in other applications. However, some of the concepts and methods should carry over to other applications.

Reynolds Number Dependence of Higher Order Statistics for Round Turbulent Jets Using Large Eddy Simulations

Despite the large repository of experimental and computational studies on the topic of turbulent jets, inconclusive and conflicting estimates prevail in regard to certain terms in the turbulence energy budget and the dependence (or lack of dependence) of these and other flow physics on the jet Reynolds number. No comprehensive study exists which adequately addresses these inconsistencies. The purpose of this study is to resolve these contradictions and ascertain the true dependence of the flow statistics on the jet Reynolds number. This is accomplished through high fidelity Large Eddy Simulations (LES), which are performed for a single isothermal round jet at three different Reynolds numbers, encompassing nearly two orders of magnitude. In each case, results are compared to well-accepted experimental and computational studies, and excellent agreement is found with experimental quantities either directly acquired or computed directly from raw data. A separate discrete eddy simulation of the flow in the nozzle upstream of the jet inlet is performed and is found to be crucial in quantifying the flow physics in the near field (e.g., virtual origin). Results show a definite Reynolds number dependence for nearly all third order terms and this is non-negligible especially for the mean convection and production terms in the turbulent kinetic energy budget.

LES simulation and experimental validation of the unsteady aerodynamics of blunt wind turbine airfoils

In order to investigate the unsteady performance of blunt wind turbine airfoils caused by boundary layer separation and wake eddies, this paper studies the aerodynamic performance by large eddy simulation (LES) and wind tunnel experiment at a Reynolds number of 2.62 × 10ˆ5. The blunt airfoils are obtained by enlarging the trailing edge of the DU 91-W2-250 airfoil to 6% and 10% chords symmetrically on both pressure and suction sides of the airfoil. The simulation was carried out with the incompressible finite-volume Navier-Stokes code EllipSys3D; and, the experiment was done in a wind tunnel with a cross-section of 0.5 m × 0.5 m by measuring the surface pressure and wake velocities using ESP-64HD pressure scanner and TSI hot-wire anemometer. The unsteady wake was captured by hot-wire in the wind tunnel, and LES with EllipSys3D. Both experiment and LES show that the spectrum of aerodynamic forces has a broadband nature which is in coincidence with the wake eddies, implying that the unsteady Kármán vortex sheet is the driving mechanism of the force fluctuation. Moreover, the trailing edge size affects the separation bubbles and transition process in the boundary layer. It shows that the boundary layer near the leading edge is unstable in the spanwise direction, which is characterized by low frequency waves.