High Turbulence Intensity Research Articles

Theoretical and experimental study on the rheological properties of WIS grout and the dispersion and sealing mechanism

Recently a new grout material called water inflow sealing (WIS) was invented for sealing water inflow in tunneling and underground constructions. In this study, a special experimental method called intubated counter grouting (ICG) was proposed to investigate the influence of water dispersion on the rheological properties of the grout during the grouting process, and to testify the sealing performance of the grout, such as instant gelling ability (IGA) and anti-dispersion ability (ADA). In the experiment, dispersion was restricted in the downstream of the channel with a high turbulence intensity. The influences of ADA and IGA were therefore decoupled and evaluated separately. Experimental results revealed two distinctive sealing mechanisms of WIS. For a low initial velocity of water, WIS turned the shear flow of water into an overall movement of a plug by absorbing water into the particles. For a high initial velocity and the situation that the particles reached the outlet before sufficiently expanding, WIS modified the rheology of the water in the channel and reduced its velocity till the static state. The distinctive feature of WIS brings a reformation on the sealing mechanism and provides an effective way to control water inflow with high pressure and velocity.

Structur Structural Dynamics of A al Dynamics of AWT-27 Wind T -27 Wind Turbine Blade urbine Blade

Wind energy is one of the world’s current leading renewable energy resources. One of the major aspects of studying wind turbines is the structural dynamics for the turbine structure including blades and support structure. In the current work, the blades of the Advanced Wind Turbine (AWT-27) are investigated in a dynamic approach. Different wind fields have been generated for the study to provide different Design Load Conditions (DLCs). Three laminar wind velocities of 5 m/s, 12 m/s, and 17 m/s were simulated. Turbulent wind flow fields have also been generated at the three standard classes A, B and C of high, medium, and low turbulence intensities respectively. The generated wind fields were used as inputs to calculate the aerodynamic loads for each wind condition using the Blade Element Momentum (BEM) theory. Aerodynamic loads have been calculated, namely, the shear force on five different locations along the blade length. Results of the simulation are summarized such that the shear forces at the blade root, 30%, 50%, 70% of the blade length are known for each wind condition. The summary serves as a guide for further optimization of the blade structural design.

Freestream effects on the interaction between near-wall vortex and film ejected from a fan-shaped hole

A flat plate film cooling experiment was conducted to investigate the effects of near-wall vortex on the film cooling performance of a fan-shaped hole. A vane-shaped vortex generator was installed upstream of a fan-shaped hole to simulate a near-wall vortex. Infrared thermography was used to measure the cooling effectiveness and heat transfer coefficient for the blowing ratios from 0.5 to 2.5. Inlet flow properties of turbulence intensity and boundary layer thickness were varied to set up different freestream conditions. Results show that the vortex affects the film cooling to varying degree under different freestream conditions. In case of low inlet turbulence, upstream boundary layer thickness is a significant factor on the film cooling performance. With a thin boundary layer, vortex intensifies the mixing of freestream and coolant jet. The decrease in lateral-average film cooling effectiveness ranges from 0.02 to 0.06 depending on the blowing ratios and relative positions of the vortex. The heat flux through the test plate is significantly increased by 20%. Thickened boundary layer promotes coolant jet to penetrate the freestream flow, which degrades the film cooling performance. However, thick boundary layer restrains the vortex disturbance on the freestream flow, and the heat flux of the test plate is only increased by 5.6%. In the case of high turbulence intensity, boundary layer plays an insignificant role. The near-wall vortex dissipates quickly and exerts little impact on the film cooling effectiveness and surface heat transfer coefficient.

Fluid-Structure Interaction Simulations of Repaired Type A Aortic Dissection: a Comprehensive Comparison With Rigid Wall Models.

This study aimed to evaluate the effect of aortic wall compliance on intraluminal hemodynamics within surgically repaired type A aortic dissection (TAAD). Fully coupled two-way fluid-structure interaction (FSI) simulations were performed on two patient-specific post-surgery TAAD models reconstructed from computed tomography angiography images. Our FSI model incorporated prestress and different material properties for the aorta and graft. Computational results, including velocity, wall shear stress (WSS) and pressure difference between the true and false lumen, were compared between the FSI and rigid wall simulations. It was found that the FSI model predicted lower blood velocities and WSS along the dissected aorta. In particular, the area exposed to low time-averaged WSS () was increased from 21 cm2 (rigid) to 38 cm2 (FSI) in patient 1 and from 35 cm2 (rigid) to 144 cm2 (FSI) in patient 2. FSI models also produced more disturbed flow where much larger regions presented with higher turbulence intensity as compared to the rigid wall models. The effect of wall compliance on pressure difference between the true and false lumen was insignificant, with the maximum difference between FSI and rigid models being less than 0.25 mmHg for the two patient-specific models. Comparisons of simulation results for models with different Young’s moduli revealed that a more compliant wall resulted in further reduction in velocity and WSS magnitudes because of increased displacements. This study demonstrated the importance of FSI simulation for accurate prediction of low WSS regions in surgically repaired TAAD, but a rigid wall computational fluid dynamics simulation would be sufficient for prediction of luminal pressure difference.

Turbulent Intensity of Blood Flow in the Healthy Aorta Increases With Dobutamine Stress and is Related to Cardiac Output.

Introduction: The blood flow in the normal cardiovascular system is predominately laminar but operates close to the threshold to turbulence. Morphological distortions such as vascular and valvular stenosis can cause transition into turbulent blood flow, which in turn may cause damage to tissues in the cardiovascular system. A growing number of studies have used magnetic resonance imaging (MRI) to estimate the extent and degree of turbulent flow in different cardiovascular diseases. However, the way in which heart rate and inotropy affect turbulent flow has not been investigated. In this study we hypothesized that dobutamine stress would result in higher turbulence intensity in the healthy thoracic aorta. Method: 4D flow MRI data were acquired in twelve healthy subjects at rest and with dobutamine, which was infused until the heart rate increased by 60% when compared to rest. A semi-automatic segmentation method was used to segment the thoracic aorta in the 4D flow MR images. Subsequently, flow velocity and several turbulent kinetic energy (TKE) parameters were calculated in the ascending aorta, aortic arch, descending aorta and whole thoracic aorta. Results: With dobutamine infusion there was an increase in heart rate (66 ± 9 vs. 108 ± 13 bpm, p < 0.001) and stroke volume (88 ± 13 vs. 102 ± 25 ml, p < 0.01). Additionally, there was an increase in Peak Average velocity (0.7 ± 0.1 vs. 1.2 ± 0.2 m/s, p < 0.001, Peak Max velocity (1.3 ± 0.1 vs. 2.0 ± 0.2 m/s, p < 0.001), Peak Total TKE (2.9 ± 0.7 vs. 8.0 ± 2.2 mJ, p < 0.001), Peak Median TKE (36 ± 7 vs. 93 ± 24 J/m3, p = 0.002) and Peak Max TKE (176 ± 33 vs. 334 ± 69 J/m3, p < 0.001). The relation between cardiac output and Peak Total TKE in the whole thoracic aorta was very strong (R2 = 0.90, p < 0.001). Conclusion: TKE of blood flow in the healthy thoracic aorta increases with dobutamine stress and is strongly related to cardiac output. Quantification of such turbulence intensity parameters with cardiac stress may serve as a risk assessment of aortic disease development.

Intensification of coal slime separation in fluidized bed flotation column: change of turbulence

ABSTRACT Flotation cell and flotation column are widely used as the separation equipment for coal slime. In order to obtain better flotation results for different particle sizes, different turbulence intensity of flow field need to be created, but the turbulence intensity is not easy to adjust in conventional flotation equipment. In this paper, a new fluidized bed flotation column was designed to control turbulence intensity by the liquid velocity and static bed height. The effects of liquid velocities (0.198 m/s,0.226 m/s,0.254 m/s) and static bed heights (0.317 m,0.343 m,0.380 m) on coal slime flotation with different particle sizes were studied, which were evaluated by the combustible recovery, ash content, and the yield of clean coal. The research results showed that the flotation performance of coarse particles is better with low turbulence caused by low liquid velocity and great static bed height than that in high turbulence. Under the condition of high liquid velocity and low bed height, a high turbulence intensity environment is created, and the fine particle separation efficiency is better than that in low turbulence.

Effect of turbulence intensity on aerodynamic characteristics of extended trailing edge airfoil

PurposeThis paper aims to study the extended trailing edge airfoil for a range of angle of attack at different intensities of turbulence.Design/methodology/approachIn this paper, an experimental study on NACA 0020 airfoil with thin extended trailing edge modification of amplitude of h = 0.1c, 0.2c and 0.3c at the Reynolds number of 2.14 × 105 are tested. The research was carried out for an angle of attack ranging from 0° = α = 35° for the turbulence intensity of 0.3%, 3%, 5%, 7% and 12%. From the experimental readings, the surface pressures are scanned using a Scanivalve (MPS2464) pressure scanner for a sampling frequency of 700 Hz. The scanned pressures are converted to aerodynamic force coefficient and the results are combined and discussed.FindingsThe airfoil with the extended trailing edge will convert the adverse pressure gradient to a plateau pressure zone, indicating the delayed flow separation. The CL value at higher turbulence intensity (TI = 12%) for the extended trailing edge over perform the base airfoil at the post-stall region. The maintenance of flow stability is observed from the spectral graph.Practical implicationsA thin elongated trailing edge attached to the conventional airfoil serves as a flow control device by delaying the stall and improving the lift characteristics. Additionally, extending the airfoil's trailing edge helps to manage the performance of the airfoil even at a high level of turbulence.Originality/valueDistinct from existing studies, the presented results reveals how the extended trailing edge attached to the airfoil performs in the turbulence zone ranging from 0.3% to 12% of TI. The displayed pressure distribution explains the need for increasing trailing edge amplitude (h) and its impact on flow behaviour. The observation is that extended trailing edge airfoil bears to maintain the performance even at higher turbulence region.

Effect of free-stream turbulence properties on different transition routes for a zero-pressure gradient boundary layer

The natural and bypass routes to boundary-layer transition to turbulence are traditionally investigated independently in fluid mechanics applications. Nevertheless, in certain flow regimes both mechanisms could coexist and interact. In this work, large-eddy simulations (LES) were performed for a zero-pressure gradient boundary layer developing over a flat plate to investigate the transition mechanism for variable free-stream turbulent properties. Four different combinations of turbulence intensity and integral length scale were analyzed, and two main transition mechanisms were observed. High free-stream turbulence intensity instigates pure bypass transition through the amplification of a continuous Orr–Sommerfeld (O–S) mode that breaks down after secondary instability. Instead, at low free-stream turbulence intensity, discrete and continuous O–S modes interact and are both involved in the transition process. Visual inspection of the LES snapshots provides a detailed insight in Tollmien–Schlichting (TS) waves–streaks mutual interaction and clearly identifies two main mechanisms involved in turbulence breakdown. On one hand, TS waves trigger varicose instability of streaky structures. On the other hand, streaks cause secondary instability of TS waves with emerging Λ-structure formation. Then, dynamic mode decomposition (DMD) is applied to extract the main stability properties for both types of transition route and to highlight coherent structure dynamics, which is hardly observable in the literature. Specifically, for low-medium free-stream turbulence levels, DMD extracts unstable modes clearly related to streaks–TS waves interaction and leading to the formation of Λ structures. Therefore, the streaks–TS waves interaction is proved to be destabilizing and to trigger secondary instability leading to turbulence breakdown.

Effect of turbulence on the performance of a pair of vertical-axis wind turbines

The focus of this study is to investigate under high turbulence intensity the performance and wake characteristics of a pair of closely spaced counter-rotating vertical axis wind turbines (VAWTs) in wind tunnel-controlled conditions. To achieve this, wind tunnel experiments for an isolated VAWT and dual VAWTs are conducted. A squared-mesh wooden grid is placed upstream of the wind turbines to increase the level of free stream turbulence. The wind tunnel tests confirm the increase in power coefficient of paired VAWTs in turbulent flows compared to smooth flows. Additionally, this study analyses the near-wake downstream of the paired turbines in both smooth and turbulent flows to better understand the flow physics.

Characterization of a Fast Response Probe for Inflow Turbulence Measurements on Wind Turbine Blades

The performance of a wind turbine, its aerodynamic and acoustic predictions, and expected power output are affected by atmospheric turbulence. Previous studies showed that the installation of high-frequency measurement devices on blades is needed in order to analyze the effect of the inflow turbulence on boundary layer transition and on noise emissions. Therefore, the accuracy of a novel 5-hole high-frequency pitot tube designed to measure high-frequency velocity fluctuations is assessed by testing its capability to determine flow angles and velocity components in a well-defined wind tunnel setup. Moreover, the vortex shedding frequency and magnitude for the flow behind a cylinder and wire are analyzed to evaluate its performance for various turbulent flow scenarios. It is seen that the flow angles and the velocity components can accurately be calculated. The peaks observed in the spectra sampled at high frequencies that are relevant for boundary layer analysis demonstrate the characteristics of the flow with high turbulence intensity values. As the probe is validated for inflow turbulence measurements, it will be used on a wind turbine blade to identify the relevant frequencies that are induced by the atmospheric turbulence into the boundary layer and to improve wind turbine noise emission models.

On the Mechanism Responsible for Extreme Turbulence Intensity Generation in the Hi-Pilot Burner

In this study, we apply particle image velocimetry (PIV), hot-wire anemometry (HWA), and large-eddy simulation (LES) to identify and characterize a key mechanism by which high-intensity turbulence measured in the “Hi-Pilot” burner is generated. Large-scale oscillation of the high-velocity jet core about its own mean axial centerline is identified as a dominant feature of the turbulent flow field produced by this piloted Bunsen burner. This oscillation is linked to unsteady flow separation along the expanding section of the reactant nozzle and appears stochastic in nature. It occurs over a range of frequencies (100–300 Hz) well below where the turbulent kinetic energy (TKE) spectrum begins to follow a – 5/3 power law and results in a flow with significant scale separation in the TKE spectrum. Although scale separation and intermittency are not unusual in turbulent flows, this insight should inform analysis and interpretation of previous, and future studies of this unique test case.

Effects of turbulence intensity and n-pentanol concentration on droplet evaporation and auto-ignition

Evaporation and auto-ignition characteristics of an n-pentanol-diesel droplet are investigated under a high-temperature (900 K) and turbulent environment. Turbulence intensity and n-pentanol mass fraction are varied between 0 and 0.527 m/s and 0–50%, respectively. Droplet evaporation is controlled by the gas temperature, which is affected by turbulent transport and chemical reactions. Diesel vapor accumulates around the droplet at all turbulence intensities, whereas strong turbulence facilitates the transport of n-pentanol vapor to far areas. Turbulence intensity has little impact on the droplet temperature and evaporation rate of pure diesel. The addition of n-pentanol reduces both the evaporation rate and droplet temperature at a high turbulence intensity of 0.527 m/s, but it has little influence on droplet temperature in a static environment. The vapor distribution determines the chemical activity of gas phase around the droplet, and consequently the auto-ignition characteristics. With the increase of n-pentanol concentration, the auto-ignition delay firstly decreases and then increases under low turbulence intensities (0–0.264 m/s), while it monotonicallyincreases under a high turbulence intensity (0.395 and 0.527 m/s). The auto-ignition delay firstly decreases and then increases with the increase of turbulence intensity, regardless of n-pentanol concentration.

Hydrodynamics of Stirred Tank and Bubble Breakup Behavior Induced by Rushton Turbine

The hydrodynamics of stirred tanks and bubble breakup are crucial in gas-liquid flows, yet this system has not been well characterized for different operating conditions. In this work, the numerical method was used to investigate the hydrodynamics of six- flat blades impeller (Rushton turbine) and the results were employed to understand the bubble breakup behavior in the stirred tank. Simulation results of predicted flow pattern, power number, and the distribution of turbulence energy generated were performed with COMSOL Multiphysics. Numerical results showed good agreement with the experimental literature. The effect of rotational speed on bubble breakup behavior, such as breakage probability, the average number of daughter bubbles, and the breakage time was investigated using the high-speed imaging method. The main finding is that the breakage process occurs in the high energy area of high turbulence intensity, which is located within a distance equal to the blade width of a radius of (15-35 mm). The breakage probability (Bp) was found to be increased by 12.61 percent for a mother bubble of 4 mm at 340 rpm, with an average fragmentation of up to 22 fragments. Furthermore, the bubble breakage time was found to decrease with increasing impeller rotational speed, with an average value of 19.8 ms.

Using computational fluid dynamic for hemodialysis air chamber design modification.

Design modification is a main step in developing machines and artificial organ. A new dialysis apparatus air chamber detects clot formation and interruption of blood refining circuit. Due to using enough anticoagulant, thrombosis may occur because of vortex formed by turbulent flow. Turbulent blood flow causes to endothelial injury thus supporting the formation of a thrombus. Computational fluid dynamics can estimate the flow velocity and turbulence distribution and it can be used as applicable tool to design diagnosis and modifying. In this paper CFD simulation used to find the high turbulent intensity region within the chamber and an optimization method is adopted based on the geometry changing and trying the simulation results. The turbulent intensity is chosen as a criterion to achieving to an optimized condition. Finally, a best geometry is derived for the chamber entrance by this process and modified prototype is manufactured. This refined chamber maintains on the dialysis machine and tested for several patients with different blood characteristics. The results show that no more clot formation has been observed in this new designed chamber.

Study of turbulent flame characteristics of water vapor diluted hydrogen-air micro-mixing combustion

Hydrogen has been regarded as an important alternative fuel, especially in gas turbine applications. However, the premixed combustion of pure hydrogen has problems such as easily flashback and high NOx emissions. A more reasonable method of combustion is to use micro-mixing technology combined with water vapor dilution. This work experimentally investigated the influences of dilution rate (D) and equivalence ratio (φ) on the flame characteristics of hydrogen-air micro-mixing combustion diluted with water vapor. Results show that increasing D reduces the flame area by reducing the downstream OH radical signal, which is contrary to the effect of CO2 dilution. The flame local curvature radius shows that the micro-mixing combustion is a high turbulence intensity combustion, and the convex structure is more frequent. Moreover, increasing φ or decreasing D leads to a decrease in the instantaneous average curvature radius (Rave), indicating more small-scale wrinkle structures are generated. D and φ have a similar influence on the fluctuations of Rave and flame area in the time domain, while the fluctuation of the flame area is more evident near the lean-burn limit. When D = 25%, the frequency characteristics of the flame structure and chemical reactions are synchronized.

Effects of Body Forces on Turbulent Kinetic Energy Transport in Premixed Flames

The effects of buoyancy on turbulent premixed flames are expected to be strong due to the large changes in density between the unburned and fully burned gases. The present work utilises three-dimensional direct numerical simulations of statistically planar turbulent premixed flames under decaying turbulence to study the influence of buoyancy on the evolution of turbulent kinetic energy within the flame brush. Four sets of turbulence parameters have been studied, with four different values of Froude number for each set. It is found that for a given set of turbulence parameters, flame wrinkling increases with an increase in body force magnitude in the case of unstable stratification, which is also reflected in the increased values of turbulent burning velocity and flame surface area. An increase in body force magnitude in the case of stable stratification acts to reduce the extent of flame wrinkling. Turbulent kinetic energy and its dissipation rate are found to be affected by both the magnitude and direction of the body force. For low turbulence intensities considered here, turbulent kinetic energy increases from the leading edge of the flame brush before decaying eventually towards the product side of the flame brush. For high turbulence intensities, the turbulent kinetic energy is found to decay across the flame brush, and it is also found that the effect of body force on the evolution of turbulent kinetic energy is marginal in the case of high turbulence intensities. The effects of body force magnitude and direction on the statistical behaviours and closures of the various terms of the turbulent kinetic energy transport equation are analysed, and existing models have been modified to account for Froude number effects, where necessary.

Effect of profiled endwall on heat transfer under different turbulence intensitie

The efficiency of the gas turbine engine has been enhanced by improving aerodynamic performance and reducing heat load through control of secondary vortices according to the first-stage vane endwall configurations. In this study, the heat transfer features of flat and profiled endwalls were investigated under different turbulence intensities. Experiments were conducted under a fixed turbine-vane exit Reynolds number of 300,000 and Mach number of 0.15 on both geometries. The test specimen was scaled up 3.23 times based on the actual gas turbine vane geometry. The profiled endwall configuration was optimized considering the combustor outlet/exit geometry. Five control points were set for the vane geometry, and the minimum aerodynamic loss at the vane exit plane was determined as an objective function. As the turbulence intensity is increased, the heat transfer is increased on both endwall geometries. However, the intensity of the horseshoe vortex was significantly reduced with the profiled endwall compared with the flat endwall. Consequently, the area-averaged Sherwood number on the profiled endwall was about 25.8% (under the low turbulence intensity) and 19% (under the high turbulence intensity) lower than those of a flat endwall case, respectively.

Wind turbine blade trailing edge crack detection based on airfoil aerodynamic noise: An experimental study

In recent years, with the development of the wind power industry and the increase in the number of wind turbines, the condition monitoring of blades and the detection of damage are increasingly important. In this work, a new non-contact damage-detection approach is experimentally investigated based on the measurement of airfoil aerodynamic noise. A NACA 0018 airfoil with chord of 200 mm with different trailing edge crack sizes, 0.2, 0.5, 1.0 and 2.0 mm, is investigated. Experiments are conducted at different mean flow velocities, inflow turbulence intensities and angles of attack. Far-field noise scattered from the airfoil is measured by means of a microphone array. The spectral differences of sound pressure level between the damaged cases and the baseline (without any damage) are compared. As expected, at small angles of attack, with clean or low turbulence intensities (e.g. ∼ 4% in the experiment) flow, by increasing the size of the crack, tonal noise appears at trailing-edge thickness-based Strouhal number, Sth, approximatively equal to 0.1. However, at higher angles of attack (e.g. ± 10° and ± 15°) or under conditions of high turbulence intensity (e.g. ∼ 7%), the amplitude of the tonal peak diminishes suggesting that complementary measurements or longer acquisition time to remove inflow turbulence effects are needed to monitor trailing edge cracks.

THE HYDRODYNAMIC CHARACTERISTICS FOR VEGETATIVE CHANNEL WITH GRAVEL BED DUNES

Aquatic plants are known to provide flow resistance and impact the turbulence intensity and turbulent kinetic energy within the vegetated area. This paper further investigates the impact of both vegetation and dunes in open channels to the hydrodynamic characteristic of flow. Emergent vegetations were built from rigid wooden rod in staggered arrangement with 0.5% vegetations density were applied in the flume. Experiments were conducted with flow rate of 0.0058 m3/s throughout the experiments. Dunes were constructed from gravel of 2 mm size diameter in the shape of standing waves of three different lee slope angles of 3⁰, 6⁰ and 9⁰. Flow velocities are measured by using a velocimeter to get the raw data for the three-dimensional flow velocity in the x, y, and z directions. The velocities data were then analysed to calculate the mean velocity, turbulence intensity and turbulent kinetic energy. Experimental results showed that, for all three lee slope angles presented higher flow velocity in the vegetated channel compared to the non-vegetated channel. It was also found that greater lee slope angle dunes generate higher velocity for both channels with and without vegetation. Higher turbulence intensity can be found near the bed area and greater turbulence intensity also shown in the positive slope of a dunes compared to negative slope area. Higher turbulent kinetic energy values were recorded within the vegetated channel compared to the non-vegetated channels.

Numerical analysis of vane endwall film cooling and heat transfer with different mainstream turbulence intensities and blowing ratios

In present study, effects of mainstream turbulence intensity and blowing ratio on the endwall film cooling and heat transfer are numerically investigated. Upstream double-row streamwise cylindrical holes with inclination angel (θ) of 30 deg is applied. Four blowing ratios (M = 0.5, 1.0 1.5, 2.0) and three turbulence intensities (Tu = 5%, 15%, 25%) are studied. Distributions of film cooling effectiveness and heat transfer coefficient are presented together with vortex structure. Results show that with the increasing of blowing ratio, the strength of horseshoe vortex is significantly weakened and the level of film cooling effectiveness is obviously improved. In addition, the low heat transfer zone downstream the holes and upstream the leading edge is enlarged because of the increased momentum, while the high heat transfer zone at the exit of passage gradually shrinks. With the increasing of turbulence intensity, the variation of film cooling effectiveness is very limited, but the level of heat transfer near the pressure side in the second half of the vane passage obviously decreases. Moreover, a more uniform film coverage and heat transfer distribution is obtained at high turbulence intensities.