Turbulence Intensity Distributions Research Articles

Flow uniformity assessment in a multi-chamber cabinet of a hybrid solar dryer

Solar drying is an important industrial technology with expanding segment in the industrial solar market. The main drawback in commercializing solar dryers is the non-appropriate design of the drying chamber to perform uniform drying and hence producing a nonhomogeneous product. Uniformity of airflow inside the drying chamber has a significant role in the drying process. The current article presents an experimental and computational analysis of a hybrid solar-thermal dryer with a multi-chamber drying cabinet. A prototype was designed and fabricated to including three separate chambers, and each has one angular-movable tray for experimental measurements. The computational model was developed in ANSYS Fluent software for CFD simulation and validated by comparing the experimental results. A statistical analysis of the simulation results has been performed. The focus was made on the effect of tray inclination angle, 0°, 10°, 20°, 30° and 35°, air velocity and turbulence intensity distribution inside the drying cabinet at various operational conditions. Experimental and numerical analysis results indicate that the best uniformity is achieved in the tray with 30° inclination. Statistical analysis results revealed that around 93% of velocity frequency is obtained at 0.7–0.8 m/s air velocity level for 30° tray angle.

Numerical investigation on splitter plate jet assisted mixing in supersonic flow

Efficient mixing is of great importance for engineering applications of hypersonic propulsion systems. With supersonic air flowing through the interior of combustor in milliseconds, rapid mixing of airstream and fuel becomes one of the great challenges, and it is imperative to employ mixing enhancement strategies to improve the system performance. In present work the splitter plate jet assisted mixing (SPJAM) strategy is proposed and direct numerical simulations are conducted to reveal enhanced-mixing mechanisms. Both instantaneous fine structures and turbulent intensity distributions are obtained to analyze the flow dynamical behaviors. Mixing layer thickness growth rate and structure topology properties affected by SPJAM are researched as well. The results indicate that the introduction of SPJAM can significantly promote mixing by means of the newly found T-shaped structures and its dynamics. The interaction of T-shaped structure and inherent Kelvin-Helmholtz vortex leads to the breakdown of flow structures, which can obviously increase the interface of upper and lower streams. Turbulent intensity analysis suggests that with the effect of SPJAM, the region where turbulent activity exists is notably extended, and the peak values of the fluctuations are much larger. The evolution of transversely-integrated turbulent kinetic energy (IK) indicates that rapid increase of IK appear in the near field of the flow. Spatial correlation analysis results indicate that vortex size is notably increased with the introduction of SPJAM, especially for that with the strategy set at the front of the splitter plate. Through exploration of the enhanced-mixing mechanisms, present preliminary work indicates that the proposed strategy is a good candidate for efficient mixing, and in the future more detailed work including three-dimensional simulations concerning the strategy optimization should be conducted.

CFD-based fluid calculation model selection and field analysis for small reactors

To improve the accuracy of reactor design, this article uses k-ε, DES, LES, and SST four methods to compare the simulation results of the velocity field in the blade area, turbulence intensity, and velocity distribution of the fluid in the reactor, and predict the appropriate field model of the reactor calculation. Therefore, the calculation process is more suitable for the SST model of small reactors.

Analytical Models of Velocity, Reynolds Stress and Turbulence Intensity in Ice-Covered Channels

Ice cover in an open channel can influence the flow structure, such as the flow velocity, Reynolds stress and turbulence intensity. This study analyzes the vertical distributions of velocity, Reynolds stress and turbulence intensity in fully and partially ice-covered channels by theoretical methods and laboratory experiments. According to the experimental data, the vertical profile of longitudinal velocities follows an approximately symmetry form. Different from the open channel flow, the maximum value of longitudinal velocity occurs near the middle of the water depth, which is close to the channel bed with a smoother boundary roughness compared to the ice cover. The measured Reynolds stress has a linear distribution along the vertical axis, and the vertical distribution of measured turbulence intensity follows an exponential law. Theoretically, a two-power-law function is presented to obtain the analytical formula of the longitudinal velocity. In addition, the vertical profile of Reynolds stress is obtained by the simplified momentum equation and the vertical profile of turbulence intensity is investigated by an improved exponential model. The predicted data from the analytical models agree well with the experimental ones, thereby confirming that the analytical models are feasible to predict the vertical distribution of velocity, Reynolds stress and turbulence intensity in ice-covered channels. The proposed models can offer an important theoretical reference for future study about the sediment transport and contaminant dispersion in ice-covered channels.

Numerical Simulation of Iced Wing Using Separating Shear Layer Fixed Turbulence Models

Aerodynamic prediction of glaze-ice accretion on airfoils and wings is studied using the Reynolds-averaged Navier–Stokes method. Two separation fixed turbulence models are developed by considering the nonequilibrium characteristics of turbulence. The key ad hoc fix is a term of the local ratio of turbulent production to dissipation, which is used to amplify the destruction term of the equation to increase the eddy viscosity in a separating shear layer of the fully turbulent region. A shear stress limiter is adopted to appropriately simulate the beginning process of the shear layer transition when the turbulence is under development. The proposed separation fixed terms can be easily implemented into current solvers. The case of flat-plate, periodic hill, and two iced airfoils and a three-dimensional swept wing with ice accretions are numerically tested using the modified models. The results indicate that the separating shear layer fixes improve the ability of the models in predicting the stall behavior at large angles of attack. The simulated averaged flowfield and turbulence intensity distribution are consistent with experimental data.

PIV Study on Grid-Generated Turbulence in a Free Surface Flow

To investigate the feature of turbulence developing behind the filter device in a current flow, the flow fields at intermediate downstream distance of an immersed grid in an open water channel are recorded using a two-dimensional (2D) Particle Image Velocimetry (PIV) system. The measurements on a series of vertical and horizontal sections are conducted to reveal the stream-wise evolution and depth diversity of grid turbulence in the free surface flow. Unlike the previous experiments by Laser Doppler Velocimetry (LDV) and Hot-Wire Anemometry (HWA), the integral scales and space-time correlations are estimated without using the Taylor hypothesis in this paper. The distributions of mean velocity, turbulence intensity and integral scale show the transition behavior of grid-generated flow from perturbations to fully merged homogenous turbulence. The distributions of velocity and turbulence intensity become more uniform with increasing distance. While the spatial divergence of integral scale becomes more pronounced as the flow structures develop downstream. The vertical distributions of flow parameters reveal the diversity of flow characteristics in the water depth direction influenced by free surface and the outer part of turbulence boundary layer (TBL) from the channel bottom. The applicability of the newly proposed two-order elliptic approximation model for the space-time correlations of the decaying grid turbulence in channel flow is verified at different positions. The calculated convection velocity for large-scale motion and sweep velocity for small-scale motion based on this model bring a new insight into the dynamic pattern of this type of flow.

Measurement and Comparison of Melt-Blowing Airflow Fields: Nozzle Modifications to Reduce Turbulence and Fibre Whipping.

In the melt-blowing process, micro/nanofibrous nonwovens are attenuated and formed through aerodynamic force in a turbulent airflow field. In this work, two types of airflow-directors were added under a common melt-blowing slot-die nozzle to obtain modified airflow fields. The effect of airflow-directors on time-averaged characteristics, turbulence intensity, and temperature fluctuation intensity are achieved through the simultaneous measurement of fluctuating velocity and fluctuating temperature using a two-wire probe hot-wire anemometer. Moreover, the influence of airflow-directors on fibre oscillations are also investigated through high-speed photography. The distribution of turbulence intensity and temperature fluctuation intensity reveals the characteristics of fluctuating airflow fields formed by different melt-blowing slot-die nozzles. Through the analyses of airflow characteristics and fibre oscillations, we can find that the arrangement of airflow-directors has a great impact on both turbulence distribution and fibre oscillation.

Wind field characteristics over hilly and complex terrain in turbulent boundary layers

This study aims to provide a turbulent inlet generation method for Large Eddy Simulation with the consideration of cross-correlation modification, which can overcome the difficulties in completely meeting the specified turbulent characteristics, spectrum and time-correlation and integrate conveniently with the Spectral Element Method and Finite Volume Method. The proposed method is utilized for simulating turbulent boundary layer flow over two types of three-dimensional hills with different slopes, and then is validated by comparing with the wind tunnel tests. From the results, the proposed turbulent inlet generation method for Large Eddy Simulation can accurately predict the distribution of mean wind velocity and turbulence intensity over hilly terrain, and the Large Eddy Simulation with Spectral Element Method appears more efficient in competition than that with Finite Volume Method. Finally, the proposed method is employed to simulate turbulent boundary layer flow over real terrain in Changsha, China. The wind field characteristics of the target complex terrain are analyzed and discussed, which can be used for the fields of wind farm micro-siting over complex terrain in engineering practice.

Hysteresis effect of propeller jet flows in viscoelastic fluids: Transformation process and turbulence intensity

This work aimed to investigate the flow pattern transformation process resolved in time and to propose a physical explanation for the hysteresis effect of a propeller jet in a viscoelastic fluid. The propeller jet flow in xanthan gum solution was measured using PIV (Particle Image Velocimetry) and UVP (Ultrasound Velocity Profiler). Based on the distribution of turbulence intensity, the semi-radial and the axial flow patterns generated at the same rotational speed were classified as laminar and turbulent flow, respectively. Therefore, the Reynolds number was considered inappropriate to characterize the flow fields in the hysteresis range, since both laminar and turbulent flow may occur at the same Reynolds number. The distribution of normal stresses derived from the flow induced structure was considered as the determining factor for the flow field formation.

Flow Redistribution at Bridge Contractions in Compound Channel for Extreme Hydrological Events and Implications for Sediment Scour

This study investigates the flood flow characteristics in a compound channel subject to both lateral contraction (caused by bridge abutment/embankment) and vertical contraction (caused by bridge deck submergence) at bridge sites. Three abutment setback distances and three pressure flow types [free surface (FS), submerged orifice (SO) and overtopping (OT)] are tested. The results show that turbulence structures in the approach channel remained the same irrespective of downstream obstruction. However, lesser abutment setback from the main channel led to greater flow acceleration as the flow approached the bridge section, where SO flow had the highest flow intensity and FS flow had the lowest. At a bridge contraction, the distributions of flow velocity, turbulence intensity, and Reynolds shear stress are significantly affected by the type of pressure flow and the extent of contraction. Enclosed counterclockwise secondary circulating flows in the main channel may occur for a combination of long or medium setback abutments and a FS flow, while other conditions usually feature open-ended upward flows due to flow relief. The strong downslope flow component detected at the main channel bank has a noticeable bank erosion capability. The bed shear stress, which is an indicator of sediment scour, is highlighted by apparent peak zones at the main channel bank and abutment toe. Finally, clear relationships between turbulence intensity, unit discharge ratio, and lateral contraction length are found. In general, normalized turbulence intensities on the floodplain and in the main channel can be used to assess the contribution of macroturbulence to the final bed topography after scour.

Hydraulic characteristics of open-channel flow with partially-placed double layer rigid vegetation

Vegetation in watercourses can influence different aspects of flow structure, subsequently affecting many processes of flow, such as pollutant transportation, sediment deposition and hydrophyte habitat distribution. Vegetation often occurs on one side of a channel, which requires understanding the effects of partial vegetation on the flow. Although many studies have been done on flows through uniform vegetation, this type of flow is unrealistic, as in natural floodplains, the vegetation in riparian zones is usually non-uniform. There is little study on the hydraulic characteristics of the flow with the co-existence of short and tall vegetation under either emergent or submerged conditions. In this paper, a novel experiment with rigid vegetation of two heights in one side of a channel was conducted to understand flow characteristics such as velocity profile, turbulence intensity, Reynolds stress, and discharge distribution. Experimental results revealed that the velocity is almost constant over the short vegetation height and increases sharply with the depth just above the short vegetation. Similarly, the Reynolds stress had little variation in the short vegetation layer, but started to increase rapidly from the top of the short vegetation to the water surface, which indicates the presence of strong mixing layer near the top of the short vegetation. Additionally, a strong shear layer existed between non-vegetated and vegetated zones, indicating the reduction effect of vegetation on the flow velocity. Furthermore, modifications are needed to properly calculate the hydraulic radius and Manning's coefficient for the flow with double-layered vegetation.

Wind farm power maximization through wake steering with a new multiple wake model for prediction of turbulence intensity

A new multiple wake model is developed for wind farm power prediction and wind farm control. First, numerical simulations are conducted for two wind turbines under different layout sets, and the characteristics of mean velocity and turbulence intensity in multiple wakes are systematically investigated. A new multiple wake model considering the local effective turbulence on the rotor and the wake interaction effects is proposed. The proposed model can favorably predict the mean velocity and turbulence intensity distributions in multiple wake regions, as well as the power production in wind farm comparing with numerical simulations and field measurements. Finally, the new proposed multiple wake model is applied to wind farm modelling and optimization framework, which enables the maximization of wind farm power production by wake steering control. The wind sector width of 2° with the wind speed bin of 0.5 m/s is proposed for the lookup-table-based wake steering optimization. The proposed values reduce the prediction error of annual energy production gain from 34.5% to 3.2% comparing with the conventional values of 5° and 1 m/s. In addition, the yaw offset limit of ±15° is recommended to satisfy both the maximization of power production and the safety requirement of International Electrotechnical Commission (IEC) standard.

Effects of Secondary Currents on Turbulence Characteristics of Supercritical Open Channel Flows at Low Aspect Ratios

In this paper, we present secondary current effects on the turbulence characteristics of supercritical narrow open channel flows over a smooth fixed bed. The main hydraulic parameters are low channel width to flow depth ratios varying between 1 and 2, and Froude numbers (F) ranging from 2 to 4. Detailed profiling of instantaneous streamwise and vertical flow velocities was conducted in a laboratory flume using a 2D laser Doppler anemometry. The cross-sectional distributions of mean flow velocities, turbulence intensities, Reynolds, and bed shear stresses were obtained from the measurements. The mean streamwise and vertical flow velocity distributions reveal that four pairs of secondary current cells are formed: a pair of well-developed free-surface vortices near the water surface, a pair of bottom vortices near the bed, and two pairs of mid-vortices between the free-surface and bottom vortices. These secondary currents cause bulging of the contour lines of the streamwise velocities with respect to the water surface and the bottom corner bisectors resulting in an undulated pattern of the mean velocity distribution across the cross-section. Furthermore, they cause the velocity dip phenomenon, i.e., the maximum flow velocity occurs well below the surface, and redistribute the Reynolds and bed shear stresses in transverse direction. The results demonstrate that decreasing the aspect ratio increases the strength of the secondary currents causing a significant change in flow patterns with larger free-surface vortices compared to the bottom vortices. Compared to the aspect ratio effect, the Froude number only slightly impacts the flow characteristics as a result of flow non-uniformity. For all investigated aspect ratios and Froude numbers, bed shear stresses are concentrated at the flume center, and on average 5 to 10% higher than their mean values. The modified wake-log-law holds both in the inner and outer regions, matching well with the experimental data for all test conditions. The present findings are discussed with literature data, and their impact on engineering applications is demonstrated.

Probabilistic calibration of fatigue safety factors for offshore wind turbine concrete structures

Current fatigue design rules for offshore concrete structures were adopted from the oil and gas industry. When better models or more information are available, partial safety factors can be re-calibrated according to target reliability levels for offshore wind turbines. This paper describes a framework for reliability-based calibration of fatigue partial safety factors for offshore wind turbine concrete structures. Offshore wind turbine loads accounting for the statistical distribution of turbulence intensity are estimated using a fully-integrated aeroelastic model. Based on available experimental fatigue tests, a fatigue reliability model for concrete is formulated and applied in two numerical examples. Results indicate that the recommended material partial safety factor in the DNV standard for Offshore Concrete Structures can be lowered without compromising structural safety. The proposed modification can potentially contribute to structural design optimization and further cost reduction in offshore wind energy.

Effects of turbulence modelling in AD/RANS simulations of single wind & tidal turbine wakes and double wake interactions

A comparative analysis of the RANS-based generalized actuator disc method (AD/RANS) with different turbulence models for simulating single wind & tidal turbine wakes and double wake interactions is performed. The tested turbulence models include the Realizable k-ε, SST k-ω, Linear Reynolds-stress (RSM) and two proposed RSM models with modified closure constants. The evaluation is based on comparing their results against five sets of published experimental data for various types of turbines operating under a wide range of inflow conditions. Special emphasis is put on both horizontal and vertical distributions of velocity deficit and turbulence intensity throughout the whole wake region. The results identify that the wake flow prediction is particularly sensitive to the turbulence model implemented, with a maximum difference reaching up to 40%. Overall, the proposed Mod2 RSM model (along with a new derivation of turbulence intensity) is recommended as a robust choice for capturing wake behaviors, in both near and far field from the turbine. Additionally, the weaknesses and strengths of other models are discussed, aiming to give a guideline for choosing the right model for the right job as well as to provide a reference study about the accuracy improvement of wake predictions.

Effect of annular flow pulsation on flow and mixing characteristics of double concentric jets at low central jet Reynolds number

This study investigated the effects of annulus pulsation on the flow characteristics and mixing properties of double-concentric jets at a low central jet Reynolds number. The central jet Reynolds number was fixed to Rec = 313. Annulus pulsation was induced by employing a solenoid valve. Streak pictures of instantaneous smoke flow patterns and long-exposure images of jet flows were observed using the laser-light-sheet-assisted smoke flow visualization method. A binary edge-detection technique was used to compute the jet spread widths. Particle image velocimetry (PIV) was utilized to examine the time-averaged velocity vectors, streamline patterns, velocity properties, turbulence intensity distributions, and vorticity contours. A radial dispersion improvement index was defined to quantitatively estimate the mixing property using the tracer-gas concentration detection technique. When the annular flow was pulsated with high amplitudes, it merged toward the central axis owing to an increase in radial momentum. Stagnation points were intermittently witnessed on the central axis. Large jet spread width exhibited, especially at low annulus Reynolds numbers. Turbulence intensities increased drastically in the flow field by the axial elongation of the recirculation region due to the annulus pulsations. The axial elongation of the recirculation region and the intermittent stagnation points led to effective dispersion of the central jet into the annular flow. The radial dispersion improvement index revealed that the mixing capability between the jets could be enhanced up to 80% in the recirculation region by applying annulus pulsations.

Electrochemical degradation of herbicide diuron on flow-through electrochemical reactor and CFD hydrodynamics simulation

An energy-efficient continuous flow-through electrochemical reactor (FT-ER) equipped with PbO2 filter anodes is designed and employed for electrochemical elimination towards herbicide diuron in the aquatic environment. Boosting performances, including substantially higher electrochemical mineralization kinetics and lower energy consumption, could be realized on the FT-ER compared with the traditional flow-by model. The effect of current density and flow rate on the degradation efficiency is also investigated. The glorious degradation kinetics could be ascribed to the large electrochemical surface area, strong generation power of reactive oxidants, and enhanced mass transfer process derived from the filter structure. Furthermore, we establish the computational fluid dynamics simulation to elucidate the dynamic flow behavior of the electrolyte, including fluid velocity and turbulent intensity distribution. The profound influence of FT-ER for the herbicide diuron removal stems from the homogenization of electrolyte flux and turbulence promoter of the filter structure. Finally, the continuous flow-through configuration exhibits excellent elimination performances, uniform fluid dynamics together with good stability. The filter anodes present a promising option for electrochemical treatment towards diverse organic pollutants in the aquatic environment and would be benefit to the evolution of cost-efficient electrochemical treatment techniques and reactors.

Measurement of turbulence properties

The aim of the research is to investigate anisotropic turbulence intensities, id est to investigate the distribution of Reynolds stresses and energy spectra in a square cross-section channel, downstream of a semi-active jet turbulence grid generating anisotropic turbulent airflow. In addition to the semi-active jet turbulence grid, another type of turbulence grid was developed and experimentally investigated. This grid contains vertical, flexible strips of aluminum (in this case, there are no perpendicular (horizontal) grid elements), which vibrate at a frequency depending on the velocity of the main airflow. Besides the investigation of the velocity- and turbulence intensity distributions, another main objective of the research is to measure the von Kármán energy spectrum when the turbulence cannot be considered isotropic. This aspiration of ours is justified by the knowledge gap present in the literature in this specific field. Monin has carried out a theoretical study to extend and generalize the von Kármán – Howarth isotropic principal stress equation to the anisotropic regime. The proposed new experimental work aims to provide a solid experimental background for verifying and validating the physical correctness of the Monin equation, which may result in a new theoretical understanding and perception of the major issues and the nature of anisotropic turbulence. Since the anisotropic energy spectra are expected to exhibit different characteristics from the isotropic Kolmogorov spectra, these new experimental results may contribute to the development of new anisotropic and engineering turbulence models that can be used in industrial applications.

Numerical study on mixing flow behavior in gas-liquid ejector

A multiphase mathematic model based on realizable k-e turbulence model for subsonic flow was presented to investigate the mixing flow behaviors between gas and water in the gas-liquid ejector. The simulation was carried out to predict the pumping performance of the ejector by a commercial computational code ANSYS-FLUENT 15.0. General agreements between the predicted results and experimental data validated the present theoretical model. Using the present approach, the pressure, velocity, and turbulence intensity distribution along center-line and contours of gas and liquid volume fraction profiles were predicted. It is found that the mixing process between gas and water in ejector can be divided into three periods, co-axial flow, mixing shock flow, and bubble flow. The prediction results show that the mixing shock is a dominant position in affecting the mixing flow behavior in gas-liquid ejector and the ejector’s performances.

Prototype Observation of Flow Characteristics in an Inclined-Tube Settling Tank for Fine Sandy Water Treatment

Clarifying the flow characteristics in the settling tank is the foundation for the design optimization and efficiency improvement of sediment removal. However, few research works have been undertaken on the precise observation of the flow movement in the tank. For this purpose, research into an inclined-tube settling tank for the water-sediment separation of the Yellow River was carried out. An acoustic Doppler velocimeter (ADV) was utilized to measure the velocity distribution in the tank. The dimensionless turbulence intensity and dimensionless Reynolds shear stress distribution were obtained subsequently. It was found that the velocity distribution in the stable settlement area (SSA) and the settlement area with inclined tubes (ISA) was uniform. The turbulence intensity and Reynolds shear stress in the tank were beneficial to the formation of the flocs and the subsequent acceleration of the sediment settling. The flow pattern could be significantly improved by the flow adjustment board and the inclined tubes. However, the inlet, the baffle and the overflow weir in the tank had negative effects on the flow pattern. The results enhanced our understanding of the flow characteristics in the inclined-tube settling tank and indicated a direction for optimizing the tank structure and improving the settling efficiency.