Turbulence Intensity Distributions Research Articles

Outer turbulent boundary layer similarities for different 2D surface roughnesses at matched Reynolds number

The outer layer similarity in zero pressure gradient (ZPG) boundary layers developing over different geometries of 2D roughness elements is assessed, using single hotwire anemometry. Two types of 2D roughness are used: circular rods and sinewave surfaces with two different heights, k = 1.6, and 2.4 mm, and three different streamwise spacings, i.e. sx=8k,12k, and 16k. These roughnesses cover a range of ratios of the boundary layer thickness (δ) to the roughness height (k) from δ/k = 21 to 45. As expected, all roughnesses caused a downward shift on the wall-unit normalised streamwise mean velocity profiles compared with smooth wall profiles, with a maximum shift observed for rods with a spacing of sx=8k, while the minimum shift is noticed for a sinewave surface with a spacing of sx=16k. The defect velocity profiles collapse entirely for all smooth and rough wall flows when normalised by the friction velocity. It was found that the shape factor, H is a suitable scaling parameter for improving the collapse of the data when using the diagnostic plot. The inner peak of the turbulence intensity profiles for the sinewave roughness is reduced gradually with increasing Reynolds number while the turbulent boundary layer (TBL) develops from a transitionally to a fully rough regime. Meanwhile, this inner peak disappears completely for all rod roughnesses, as the TBL is always in a fully rough regime, and the profiles exhibit only an outer peak, located at a wall-normal location y/δ≈0.06. The results suggest that Townsends similarity hypothesis for 2D surface roughness is relatively well approximated in the outer region of the flow as reflected by the collapse of the distributions of velocity defect, turbulence intensity, skewness, and flatness when scaled with δ.

Variation of Indoor Airflow Patterns under Dynamic Outdoor Wind Conditions in Large Space Naturally Ventilated Buildings

Understanding Indoor Airflow Patterns (IAPs) helps control air contaminants in large naturally ventilated buildings (NVBs). However, the effect of random and unpredictable changes in outdoor wind conditions (OWC) is a major contributor to the variation in IAPs in the NVBs, making the IAP uncontrollable. This study presents the results of field measurements and numerical simulation in a NBV to study the IAP variation characteristic under the dynamical OWC. The OWC data monitored in real time was processed with Kalman Filtering (KF) and the gradient method to decompose the data prior to being entered into the CFD solver. The trend was similar between the simulated and measured data of a full size NVB. In addition, the distribution of internal turbulence intensity varied widely depending on the spatial locations and time intervals. The variation in speeds in the vicinity of windbreaks greatly affected the variation in IAP on a certain frequency scale. The results not only prove that CFD simulation to be an efficient tool for the prediction of time-averaged IAP, but also initialize efficient measures for the control of IAQ in dynamic OWC of large space NVBs.

Attenuation characteristics of Karman vortex passing through the wire mesh

In this study, the attenuation and frequency characteristics of the Karman vortex passing through the wire mesh was investigated. This experiment was conducted by generating the Karman vortex from a cylinder in an acrylic duct of the blowdown type wind tunnel and passing through the wire mesh at the duct exit. The cylinder with a diameter of 6mm was used to generate the Karman vortex, and the Reynolds number was 3000 based on the flow velocity toward the cylinder and the cylinder diameter. The wake of the Karman vortex passing through the wire mesh was measured by hot-wire anemometer, and was visualized by smoke wire method. As a result, it was found that the smaller the wire diameter and open area ratio of the wire mesh, the velocity and turbulence intensity distributions ware more uniform. And, the dominant frequency of the Karman vortex was confirmed that even in the wire mesh that showed uniformity and in the wire mesh that appeared to have collapsed in the photograph by frequency analysis.

LES study on the impact of airway deformation on the airflow structures in the idealized mouth–throat model

To investigate the impacts of upper airway deformation on the airflow structures, the airflow fields in the trachea are simulated using three geometrical models considering three different levels of airway deformations. Structured grids are used to create the high-quality grids. Large eddy simulation with the Smagorinsky sub-grid model is adopted to solve the three-dimensional in-compressible Navier–Stokes equations using the solver pisoFoam in the open-source CFD software OpenFOAM. The numerical results demonstrate that the airway deformation influences the main airflow structures depending on the deformation level. Particularly, it slightly impacts on the laryngeal jet such as the profile and the strength of laryngeal jet. The strength of the laryngeal jet increases slightly for the heavy deformation. In contrast, it impacts on the recirculation zone, secondary vortices, and turbulent kinetic energy more obviously. The increasing airway deformation will produce stronger secondary flow, smaller recirculation zone, and weaker turbulent kinetic energy. The turbulence intensity distribution varies as well. The obviously impacted flow region is mainly within the region of one to six tracheal diameters downstream the glottis.

Numerical characterization of a novel test bench featuring secondary reactions of methane

Recently, a novel test bench was developed at the Bundeswehr University of Munich in order to study secondary near-wall reactions caused by cold injection of methane into an oxygen-rich hot crossflow. The available measurements of the experiment, namely Laser Induced Fluorescence (LIF) of OH allowed to locate the reaction zone and to estimate the autoignition length, while the heat load prediction at the wall was calculated from the thermocouple readings using the inverse heat transfer method. A numerical investigation of this test bench is presented in this work. In the first part, non-reactive Large Eddy Simulations (LES) are used to evaluate the turbulent boundary conditions of the reactive test rig. The use of LES provides an insight in the distribution of velocity, turbulence intensity, temperature and species within the hot flow generator (burner), the fuel injection nozzles and the inlet of the reactive test rig. In the second part, reactive LES computations are used for comparison of the autoignition length and the wall heat fluxes with the experimental data, based on the boundary conditions provided by the non-reactive LES. The results show that in this configuration the autoignition length is more strongly affected by the hot gas inlet temperature than by the turbulent fluctuations of the crossflow. The qualitative comparison with the autoignition length assessed from the experiment is satisfactory and a wall heat flux distribution similar to the experimental distribution is obtained downstream the fuel injection nozzles.

The Influence of Deterministic Surface Roughness and Freestream Turbulence on Transitional Boundary Layers: Heat Transfer Distributions and a New Transition Onset Correlation

AbstractHeat transfer measurements in transitional flat plate boundary layers subjected to surface roughness, strong pressure gradients, and freestream turbulence are presented. The surfaces considered consist of a smooth reference and 26 deterministic surface topographies that vary in roughness element aspect ratio, height, and density. They are designed to cover the full range of roughness regimes from smooth over transitionally rough to fully rough. For each surface, two pressure distributions, characteristic for a suction and a pressure side turbine vane, are investigated. Inlet Reynolds numbers range from 3.0 × 105 to 6.0 × 105 and inlet turbulence intensity is varied between 1% and 8%. Furthermore, different turbulence Reynolds numbers, i.e., turbulence length scales, are realized while the incident turbulence intensity is kept constant. Additionally, the turbulence intensity and Reynolds stress distributions in the freestream along the flat plate are measured using x-wire probes. Results show a strong influence of roughness and turbulence intensity on the onset of transition. The new data set is used to develop an improved correlation considering the roughness height, density, and shape as well as the turbulence intensity and turbulent length scales.

Simplified Steady-State Representation of Slot Synthetic Jet Actuators to Enable Numerical Optimization With Steady RANS Simulations

To address the growing energy use of data centers, waste heat recuperation offers a solution to better integrate these facilities into the broader energy system, thus facilitating a transition to the decarbonization of the energy system. The use of liquid coolants for full immersion cooling or local heat extraction from high power density components is considered for this purpose. However, heat can also be extracted by novel air cooling approaches, perhaps in combination with localized liquid cooling. To optimize heat extraction from air-cooled systems and maximize the heat grade, synthetic jets (SJs) can be used for targeted adaptive cooling in conjunction with air ducting to facilitate maximum heat recuperation potential in rack or server-mounted air-to-liquid heat exchangers. Internal server layouts can be optimized numerically, e.g., using a multiple objective genetic algorithm approach based on minimization of entropy generation rates. However, since SJs are inherently transient flow phenomena, this would require transient flow simulations, which forms a bottleneck in a numerical optimization loop. This research aims to develop a simplified steady-state representation of a synthetic jet actuator (SJA) with slot orifice using a localized body force to generate a similar time-averaged flow field to a real SJA, suitable for steady Reynolds-averaged Navier–Stokes simulations within a numerical optimization loop. Both flow fields are compared in terms of the mean flow field, jet spreading rate, and turbulence intensity distributions.

Parametric study of a frequency-modulated pulse jet by measurements of flow characteristics

Recently, pulsed jet actuators have been discovered as an efficient and promising technique for flow separation control. In this paper, a systematic experimental investigation is carried out using a flow control approach, in order to study the flow field characteristics of a baseline single pulsed jet actuator, under different excitation conditions. The findings are used to provide a better understanding of pulsed blowing actuator operation and the associated flow physics. In these experiments, two types of signal waveforms have been implemented to produce the pulsed excitation of jet; a simple square-wave excitation signal for the first case and a burst modulated excitation signal for the second case. All measurements were conducted using a single hot-wire system to an axial distance x/D ≤ 30 along the centerline. The effects of electrical parameters on properties of a single pulsed jet, including the excitation frequency from 10 up to 220 Hz and the duty cycle from 15% up to 80% for the first case, and different combinations of the carrier frequency and the modulating frequency for the second case have been studied. These measurements revealed a significant dependence of mean centerline velocity and turbulence intensity to excitation signal, frequency, and duty cycle. The statistical flow properties, such as mean centerline velocity decay and turbulence intensity distribution of pulsed jets are quantified and compared together over a wide range of pulsing parameters. This investigation aids in designing and optimizing flow control actuators in aerodynamics and combustion chambers.

Hydraulic characteristics of countercurrent jets on adverse-sloped beds

Abstract The counter-current jet (CCJ) acts like a reverse surface jet layer covering the free jump surface and has potential applications in the energy dissipation of hydraulic engineering. The present study investigated the hydraulic characteristics of CCJs on adverse-sloped beds. The results showed that, compared to the horizontal bed, the slope didn't increase the energy dissipation rate of CCJ but reduced the return flow length and upstream depth. The velocity distribution along the depth was divided into the boundary layer region, mixing region, and reverse surface jet region. The velocity distribution in the boundary layer region and the mixing region was similar to the classical wall jet. The jet Froude number and the bed slope had no significant effect on the turbulence intensity distribution and turbulence kinetic energy (TKE) distribution of CCJ. The distribution of TKE was similar to that of a submerged jump. The maximum absolute turbulence intensity appeared at exactly half of the maximum velocity. The maximum TKE appeared at the mixing region. Besides, empirical formulas for estimating length scales and maximum velocity attenuation are proposed. The results could provide a reference for the potential application of CCJ in energy dissipation in hydraulic engineering.

The Application of Flame Image Velocimetry to After-injection Effects on Flow Fields in a Small-Bore Diesel Engine

This study implements Flame Image Velocimetry (FIV), a diagnostic technique based on post-processing of high-speed soot luminosity images, to show the in-flame flow field development impacted by after-injection in a single-cylinder, small-bore optical diesel engine. Two after-injection cases with different dwell times between the main injection and after-injection, namely, close-coupled and long-dwell, as well as a main-injection-only case are compared regarding flow fields, flow vector magnitude, and turbulence intensity distribution. For each case, high-speed soot luminosity movies from 100 individual combustion cycles are recorded at a high frame rate of 45 kHz for FIV processing. The Reynolds decomposition using a spatial filtering method is applied to the obtained flow vectors so that bulk flow structures and turbulence intensity distributions can be discussed. The results show significant after-injection-induced flow structures as a group of vectors travelling back toward the center of the combustion chamber upon the jet impingement on the piston bowl wall and then jet-to-jet collision. Despite higher cyclic variations caused by the after-injection, increased bulk flow magnitude and turbulence intensity upon the after-injection event suggests locally enhanced mixing. Compared to the long-dwell after-injection, the close-coupled after-injection shows more significant turbulence enhancements.

Non-axisymmetric complete flow conditioning gauzes

This paper presents a novel methodology for the design of a gauze that produces distributions of stagnation pressure, swirl angle, pitch angle and turbulence intensity, tailored in both the radial and circumferential directions. A distortion gauze is made from a large number of small-scale circumferential and radial blades with tailored thickness and camber distributions. By controlling the blade design independently in both the radial and circumferential directions, the target inflow pattern can be achieved. 1D correlations are used to initialise the blades and they are refined using full 3D CFD simulations. The final design is additively manufactured for use in rotating rigs. In this paper, the method has been used to reproduce four target inflow patterns with large variations in stagnation pressure and flow angularity. Two examples model the inlet flow distortion seen at the aerodynamic interface plane of an aft-mounted boundary layer ingesting fan. The final two examples model the inlet distortion at inlet to an axial compressor spool caused by upstream structural struts in a swan neck duct. The gauzes are shown to replicate the structures of the target flow in an experimental test. These kind of flow structures would be extremely difficult or impossible to replicate in an experiment in any other way.Graphical abstract

Gramian-Aware Control Approach for Atmospheric Gust Harvesting

Micro-aerial vehicles (MAVs) lack the endurance times demanded by typical mission applications, and previous work to minimize flight path disturbances has also quantified their high atmospheric sensitivities. This study introduces an approach to modulate the sensitivity through real-time parameter variation that translates into a net energy gain of the vehicle. By applying a control-theoretic disturbance sensitivity framework and observability gramian via a “gust capture metric,” the formulation modulates vehicle gust response to sensed favorable (or unfavorable) vertical gusts. The approach is implemented in a nonlinear simulation and an experimental test environment for a representative 21 g MAV. These results include the development of a system-identified MAV flight dynamics model and the development of an experimental facility to provide automated, repeatable indoor flight tests over a gust field. Nonlinear model simulation indicates that the control law provides altitude gain over an idealized gust field, consistent with theoretical analysis. The experimental flight facility was equipped with a gust field generation capability, and its velocity and turbulence intensity distribution was quantified. The experimental flight tests evaluating altitude gain under vehicle controller modification show agreement with theoretical and simulation results and indicate the importance of turbulence distribution in atmospheric gust harvesting experiments.

Wind Environment Simulation Accuracy in Traditional Villages with Complex Layouts Based on CFD.

Using wind speed, wind direction, and turbulence intensity values as evaluation indicators, the ventilation performance of villages with complex building layouts was studied. We used the SKE, RNG, and RKE solvers in CFD-3D steady-state Reynolds-averaged Navier–Stokes (RANS) to simulate the wind environment of a village. The findings show that for the simulation of rural wind environments with complex building layouts, steady-state simulation solvers need to be evaluated in detail to verify their accuracy. In this study, a village with a complex architectural layout in Southern Shaanxi, China, was taken as the research object, and three steady-state simulation solvers were used to evaluate the ventilation performance of the village. The simulated data were compared with the measured data to find the most suitable solver for this kind of village wind environment simulation. The results show that for the simulation of the village wind environment with a complex building layout, the RNG simulation results have the lowest reliability among the three steady-state solvers. The reliability of wind speed distribution and turbulence intensity distribution are 0.7881 and 0.2473, respectively. However, the wind speed and turbulence intensity values obtained by the SKE solver are the closest to the measured values, which are 0.8625 and 0.9088, respectively. Therefore, for villages with complex building layouts, the SKE solver should be the first choice for simulating wind environment distribution. When using the RNG solver, the overall turbulence intensity value obtained is higher than the measured value. The average deviation between the simulated data and SKE and RKE at a height of 1.7 m is 42.61%. The main reason for this is that RNG overestimates the vortices and underestimates the airflow rate in the building intervals.

Study of Two-Phase Flow Behavior around Chute Aerator Using OpenFOAM

Aeration devices are installed on chute spillways to prevent cavitation damage in high-velocity flows. Large quantities of air-entrained characterize bottom aerators along with the jet interfaces and a strong de-aeration process near the impact of the water jet with the spillway bottom. Appropriate prediction of the air entrainment process, flow characteristics, and two-phase flow pattern at the aerator would contribute to reliable spillway operation. The mathematical formulation of two-phase flow at an aerator remains a challenging issue for spillway design due to its complexities. In the present study, 2D numerical simulations are performed to predict the distribution of air concentrations, flow velocity, turbulent intensity, and water surface profile along the chute aerator using open-source OpenFOAM software and RNG k-ε turbulent model. The correlation coefficients obtained between the numerical and experimental results indicate that a proper agreement exists between the relative cavity length and velocity profile results. The research results show that the turbulent intensity of flow passing over the aerator ramp is significantly increases. It means that the ramp acts as a turbulent generator on the spillway chute. The maximum value of turbulence intensity occurs at the 6-8% of flow depth(h) from the tip of the ramp, and it is raised as the Froude number increases. It is concluded that the intake air flow rate and air-entrainment coefficient (𝛽) are linearly increased concerning the Froude number.

Computational Modeling of Flow Characteristics in Three Products Hydrocyclone Screen

Three products hydrocyclone screen (TPHS) can be considered as the combination of a conventional hydrocyclone and a cylindrical screen. In this device, particles are separated based on size under the centrifugal classification coupling screening effect. The objective of this work is to explore the characteristics of fluid flow in TPHS using the computational fluid dynamics (CFD) simulation. The 2 million grid scheme, volume fraction model, and linear pressure–strain Reynolds stress model were utilized to generate the economical grid-independence solution. The pressure profile reveals that the distribution of static pressure was axisymmetric, and its value was reduced with the increasing axial depth. The maximum and minimum were located near the tangential inflection point of the feed inlet and the outlets, respectively. However, local asymmetry was created by the left tangential inlet and the right screen underflow outlet. Furthermore, at the same axial height, the static pressure gradually decreased along the wall to the center. Near the cylindrical screen, the pressure difference between the inside and the outside cylindrical screen dropped from positive to negative as the axial depth increased from −35 to −185 mm. Besides, TPHS shows similar distributions of turbulence intensity I, turbulence kinetic energy k, and turbulence dissipation rate ε; i.e., the values fell with the decrease in axial height. Meanwhile, from high to low, the pressure values are distributed in the feed chamber, the cylindrical screen, and conical vessel; the value inside the screen was higher than the outer value.

Cause analysis and preventive measures of pipeline corrosion and leakage accident in alkylation unit

Owing to the corrosive, flammable, and explosive properties of pipeline materials in alkylation units, corrosion failure may result in leakage, fire, and explosion accidents. A bursting accident occurred in the 90° elbow of an alkylation unit pipeline. Failure analysis was performed by macroscopic analysis, metallographic analysis, chemical composition, corrosion form analysis, and corrosion product analysis. Finite element software was used to analyse the flow velocity, turbulence intensity, pressure, and wall stress distribution. The fault tree was combined with the analytic hierarchy process to realise the transformation of the structural importance-judgement factor-judgement matrix and to weight the events that may cause device corrosion failure. The results showed that uniform corrosion and stress corrosion of hydrofluoric acid occur in the pipeline and uniform corrosion is the dominant effect. Hydrofluoric acid corrosion leads to severe thinning of the pipe wall, making its strength drop below the bearing strength and rupture occurs. Quality of internal anticorrosion coating, water content of the medium and working temperature are the key factors to prevent pipeline leakage in alkylation unit.

STATISTICAL CHARACTERISTICS OF PRESSURE FLUCTUATION IN SHOCK WAVE AND TURBULENT BOUNDARY LAYER INTERACTION

Shock wave and turbulent boundary layer interaction widely exists in the internal and external flow of high-speed aircraft. The aerodynamic performance and flight safety of aircraft are seriously affected by the strong pressure fluctuation in the interaction region. To investigate statistical characteristics of fluctuating pressure, the interaction between an incident shock of 33.2° and a spatially developed Mach 2.25 turbulent boundary layer is analyzed by means of direct numerical simulation (DNS). The numerical results have been carefully validated against with previous experiment and DNS at similar flow conditions in terms of mean velocity profile, turbulence intensity and wall pressure distribution. Statistics at the wall and in the outer layer, including fluctuation intensity, power spectral density, two-point correlation and space-time correlation, are quantitatively compared. The differences between them are analyzed in detail. It is found that the effect of the shock interaction on the wall-pressure fluctuation and the fluctuating pressure in the outer layer are utterly different. Based on the analysis of the power spectra density, the fluctuations in the separated region are both characterized by the low-frequency content, but in the reattachment region, the peak frequency of outer pressure fluctuations quickly shifts to higher frequency, with the low-frequency energy of wall-pressure fluctuation still being predominant. It is identified that the two-point correlations of pressure fluctuation at the wall and in the outer layer are both more elongated in the spanwise direction than that in the streamwise direction. The integral scale at the wall is generally increased, while the one in the outer layer increases sharply after passing the shock and then gradually decreases. The analysis of space-time correlation indicates that the iso-correlation contours are similar to the elliptical distribution and the convection velocity deduced by the correlation is dramatically decreased. Downstream of the interaction, the convection velocity in the outer layer is higher than that of wall-pressure fluctuation.

Impacts of urban morphology on improving urban wind energy potential for generic high-rise building arrays

Previous findings have indicated better performance attained by modified urban morphologies for wind energy utilization only in single and pair buildings, or medium-dense low-rise building arrays. Hence, the main purpose of this study is to address the research gaps to complete a fundamental understanding of the influences of urban morphology in compact high-rise urban areas on enhancing urban wind energy harvest for sustainable urban development. A comprehensive parametric study is conducted using the computational fluid dynamics tool to analyze the impacts of urban morphologies on the wind energy potential for a 6 × 6 array of generic high-rise buildings, including (i) urban density altered from compact to sparse urban layouts, (ii) building corner shapes of sharp and rounded corners, (iii) urban layouts of in-line and staggered patterns, and (iv) wind directions of 0° and 45°. This investigation implements the three-dimensional steady Reynolds-averaged Navier-Stokes equations with the Reynolds stress model to explore the distributions of wind speed, power density, and turbulence intensity over the building array. The results indicate that decreasing urban plan area density reduces the unacceptable turbulence areas with relatively higher wind power density on the roof. Besides, round corners can produce elevated power densities up to 201% greater than sharp corners beside the building. Even under the oblique wind direction of 45°, the rounded corner still shows better wind energy potentials than the sharp corner. The in-line urban layout demonstrates more significant areas with higher power densities and low turbulence intensities than the staggered urban layout.

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.