Turbulence Intensity Distributions Research Articles

Three-Dimensional Turbulence Intensity in a Compound Channel

AbstractExperimental research was undertaken to investigate the changes in spatial turbulence intensity in a compound channel and the influence of rigid, emergent floodplain vegetation on turbulence intensity. Five tests for two various roughness values of floodplains were realized. In the first series of experiments (three tests), the surface of the main channel bed was smooth and made of concrete, whereas the floodplains and sloping banks were covered by cement mortar composed of terrazzo. In the second set of experiments (two tests), emergent vegetation (trees) on the floodplains, modeled by aluminum pipes, was added. Instantaneous velocities were measured with the use of a three-component acoustic Doppler velocity meter. The influence of floodplain trees, on the distributions of relative turbulence intensity (u′/U, v′/U, w′/U) in the main channel and on the floodplains was presented. It was found that the longitudinal (u′/U) and transverse (v′/U) turbulence values decreased from the bottom upward to t...

0510 高レイノルズ数乱流境界層の対数領域における確率密度関数の自己相似性(OS5-3 壁乱流:統計,構造,動力学の理解,OS5 壁乱流:統計,構造,動力学の理解)

Recently, a relation between the mean velocity profile (MVP) and the turbulence intensity distribution have been reported as these have logarithmic portion in the almost same range of distance from the wall. We focus on the relation between MVP and probability density function (PDF) of measured velocity. We found the two characteristic regions in the Kullback-Leibler divergence distribution in the high Reynolds number turbulent boundary layer at Re_τ =7100. These regions have self-similarity of PDF, but each similarity is not equal. So, these regions have different statistic characteristics each other. We found that these regions are connected to two characteristic regions of quasi logarithmic region and classical logarithmic region in MVP.

Study on Drag Reduction of a Circular Cylinder with Grooves

In the flow around a circular cylinder, a sudden decrease in the drag force occurs at a high Reynolds number, but the same phenomenon occurs at a lower Reynolds number in the case where there exist grooves or roughness on the circular cylinder surface. In previous study of authors, in order to investigate the effect of the depth and number of grooves on a circular cylinder for both arc and triangular grooves, the drag coefficient, pressure, turbulence intensity and velocity distribution were measured. Moreover, the flow around circular cylinders was analyzed by applying the RNG k-ε turbulent model. From these studies, it was found that in order to reduce a drag in a wide range of Reynolds number, the depth and number of grooves is increased and the circular cylinder surface is made to change to a turbulent boundary layer in a low Reynolds number. Furthermore, it was clarified that in order to prevent generating of a separating bubble, capacity of groove is made small and separation is retarded. In this paper, new shape of grooves was developed on the basis of the factor of above-mentioned drag reduction. As a result, the separation point of the circular cylinder with grooves of curved sectional shape shifts to the most downstream side and the drag coefficient becomes the smallest.

Experimental study of the thermal separation in a vortex tube

A vortex tube, a simple mechanical device capable of generating separated cold and hot fluid streams from a single injection, has been used in many applications, such as heating, cooling, and mixture separation. To explain its working principle, both experimental and numerical investigations have been undertaken and several explanations for the temperature separation in have been proposed. However, due to the complexity of the physical process in the vortex tube, these explanations do not agree with each other well and there has not been a consensus.This paper presents an experimental study of the flow properties in a vortex tube focusing on the thermal separation and energy transfer inside the tube. A better understanding of the flow structure inside the tube was achieved, based on the observed three-dimensional velocity, turbulence intensity, temperature and pressure distributions. The gradual transformation of a forced vortex near the inlet to a free vortex at the hot end is reported in this work. The calculated exergy distribution inside the vortex tube indicates that kinetic energy transformation outwards from the central flow contributes to the temperature separation. Experimental results found in this research show a direct relationship between the formation of hot and cold streams and the vortex transformation along the tube.

선박용 SCR 시스템에서 혼합기 구조에 따른 난류유동과 우레아 수용액의 혼합특성

선박용 SCR 시스템에서 유동 및 혼합특성을 개선하기 위해 상하유도 및 스월 형상의 혼합기가 고려되었다. 본 연구의 목적은 혼합기 구조에 따른 난류강도 및 균일지수(Uniformity Index)를 상세히 분석하여 SCR 성능을 개선하는 것이다. 그 결과, 촉매부 전단에서의 농도균일도는 혼합기를 사용하였을 경우 약 5% 개선됨을 알 수 있었다. 상하유도형 혼합기의 사용에 따라 주위에서 높은 RMS 수치 및 상대난류강도를 보이지만, 하류로 진행하면서 두 값은 감소하는 경향을 보였다. 스월형 혼합기의 경우 유동의 진행에 대해 RMS 수치와 상대난류강도의 감소가 비교적 적고 상대난류강도의 경우 상대적으로 균일한 분포를 보였다. 두 혼합기에서 발생하는 유동특성에 의해 혼합효과 및 혼합거리가 달랐음을 알 수 있었다.

Turbine blade boundary layer separation suppression via synthetic jet: An experimental and numerical study

The present paper focuses on the analysis of a synthetic jet device (with a zero net massflow rate) on a separated boundary layer. Separation has been obtained on a flat plate installed within a converging-diverging test section specifically designed to attain a local velocity distribution typical of a high-lift LPT blade. Both experimental and numerical investigations have been carried out. Unsteady RANS results have been compared with experiments in terms of time-averaged velocity and turbulence intensity distributions. Two different Reynolds number cases have been investigated, namely Re = 200, 000 and Re = 70, 000, which characterize low-pressure turbine operating conditions during take-off/landing and cruise. A range of synthetic jet aerodynamic parameters (Strouhal number and blowing ratio) has been tested in order to analyze the features of control — separated boundary layer interaction for the aforementioned Reynolds numbers.

Turbulence measurements in spatially-varied flow with increasing discharge

This experimental study explores the distribution of Reynolds shear stresses and turbulence intensities for a flume receiving lateral inflow via a set of vertical nozzles. The data set allows for developing turbulence models to describe a wider range of spatially-varied flow conditions. The turbulent flow structures are detailed both in the near-field close to where the flow enters the flume, and in the far-field. It was found that a lateral inflow increases the turbulence level, but it decreases in the transverse direction towards the side walls. As the lateral inflow rate increases, the turbulence parameters increase only slightly. The turbulence parameters also increase from the upstream end of the lateral inflow zone towards the downstream end. The turbulent kinetic energy (TKE) production was estimated. Small regions of negative TKE production exist, which are typically near the lateral inflow addition.

Effects of accelerating and decelerating flows in a channel with vegetated banks and gravel bed

Experiments were conducted in an 8 m long, 40 cm wide, and 60 cm deep re-circulating flume with vegetated banks and gravel bed to study the effects of accelerating and decelerating flows on the flow structure. Significant dip phenomenon was observed in velocity profiles under decelerating flow and the velocity defect law was not suitable due to the effect of vegetated banks. Near the vegetated banks, the turbulence intensities illustrated convex distribution for both accelerating and decelerating flows. Application of quadrant analysis revealed that the effect of vegetated banks is important on the flow structure and the Reynolds stress distribution. Accelerating and decelerating flows did not show a significant difference on percentage of time occupied by each kind of turbulent event within bursting cycle, but affected the shape of joint probability distributions of turbulent intensities in the horizontal and vertical directions.

Boundary‐layer turbulence characteristics during aeolian saltation

A great deal of effort has been expended in measuring turbulence phenomena in clean air flows. However, no previous measurements have been successfully made of the vertical distributions of turbulence intensity and Reynolds stress in a fully adjusted boundary‐layer flow saturated with saltating particles. The present wind tunnel study addresses this knowledge gap using a custom designed laser‐Doppler anemometer (LDA). The amount of turbulence is found to increase with the introduction of saltating particles to the airflow. Over the lowest 15% of boundary layer, vertical profiles of the streamwise wind speed provide friction velocities that lie well within the narrow range of those derived from direct measurement of the Reynolds stress. Relative to clean air, aeolian saltation is demonstrated to increase the magnitude but not the frequency of burst‐sweep events that primarily contribute to the total fluid stress. Within several millimeters above the bed surface, all vertical profiles of wind speed converge upon a focal point, as the local fluid stress declines toward the mobile bed.

Analysis of the 3D zone of flow establishment from a ship’s propeller

In the present study an experimental investigation of the time-averaged velocity and turbulence intensity distributions from a ship’s propeller, in “bollard pull” condition (zero speed of advance), is reported. Previous studies have focused mainly on the velocity profile of not a rotating ship propeller but a plain jet. The velocity profile of a propeller is investigated experimentally in this study. The velocity measurements were performed in laboratory by using a Laser Doppler Anemometry (LDA). The measurements demonstrated two-peaked ridges velocity profile with a low velocity core at the centre within the near wake. The two-peaked ridges combined to be one-peaked ridge at 3.68 diameters downstream indicating the end of the zone of flow establishment. The study provides useful information from a rotating ship’s propeller rather than a simplified plain jet to researchers investigating flow velocity generated from a propeller and probably resulting local scouring.

Observations on flow and local scour around submerged flexible vegetation

This study experimentally investigated the effects of submerged vegetation on the characteristics of flow and the formation of a scour hole. The submerged vegetations were modeled by bundled plastic fibers. Experiments were performed for various spacings of the plastic fibers, resulting in vegetation density ranging from 0.21 to 0.65. The vegetation models were aligned with the approaching flow in a rectangular channel. Vertical distributions of time-averaged velocity and turbulence intensity at various streamwise distances were evaluated using an acoustic Doppler velocimeter (ADV). The characteristic lengths of the scour hole, including scour depth, dune height, the length of the scour hole, the horizontal distance of the maximal scour depth, and the horizontal distance of the dune crest, were determined from the bed profiles. Velocity and turbulence intensity plots indicated that the deflected flow over the vegetation was promoted with the decrease in spanwise spacing. Downstream of the vegetation zone, two shear layers developed from the vegetation zone. As the spanwise plant spacing increased, the turbulence intensity of the upper shear layer decreased because of the strong flow through the vegetation. Vegetation densities were used to examine the effects of vegetation on the characteristic lengths of the scour hole. Scour depth, dune height, length of the scour hole, horizontal distance of the maximal scour depth, and horizontal distance of the dune crest increased linearly with a decrease in vegetation density.

A new formulation for the streamwise turbulence intensity distribution in wall-bounded turbulent flows

The distribution of the streamwise velocity turbulence intensity has recently been discussed in several papers both from the viewpoint of new experimental results as well as attempts to model its behavior. In the present paper numerical and experimental data from zero pressure-gradient turbulent boundary layers, channel and pipe flows over smooth walls have been analyzed by means of the so called diagnostic plot introduced by Alfredsson & Örlü [P.H. Alfredsson, R. Örlü, The diagnostic plot–a litmus test for wall bounded turbulence data, Eur. J. Mech. B Fluids 29 (2010) 403–406]. In the diagnostic plot the local turbulence intensity is plotted as function of the local mean velocity normalized with a reference velocity scale. Alfredsson et al. [P.H. Alfredsson, A. Segalini, R. Örlü, A new scaling for the streamwise turbulence intensity in wall-bounded turbulent flows and what it tells us about the outer peak, Phys. Fluids 23 (2011) 041702] observed that in the outer region of the boundary layer a universal linear decay of the turbulence intensity independent of the Reynolds number exists. This approach has been generalized for channel and pipe flows as well, and it has been found that the deviation from the previously established linear region appears at a given wall distance in viscous units (around 120) for all three canonical flows. Based on these results, new empirical fits for the streamwise velocity turbulence intensity distribution of each canonical flow are proposed. Coupled with a mean streamwise velocity profile description the model provides a composite profile for the streamwise variance profile that agrees nicely with existing numerical and experimental data. Extrapolation of the proposed scaling to high Reynolds numbers predicts the emergence of a second peak of the streamwise variance profile that at even higher Reynolds numbers overtakes the inner one.

Natural ventilation performance of single room building with fluctuating wind speed and thermal mass

Natural ventilation is driven by either buoyancy forces or wind pressure forces or their combinations that inherit stochastic variation into ventilation rates. Since the ventilation rate is a nonlinear function of multiple variable factors including wind speed, wind direction, internal heat source and building structural thermal mass, the conventional methods for quantifying ventilation rate simply using dominant wind direction and average wind speed may not accurately describe the characteristic performance of natural ventilation. From a new point of view, the natural ventilation performance of a single room building under fluctuating wind speed condition using the Monte-Carlo simulation approach was investigated by incorporating building facade thermal mass effect. Given a same hourly turbulence intensity distribution, the wind speeds with 1 min frequency fluctuations were generated using a stochastic model, the modified GARCH model. Comparisons of natural ventilation profiles, effective ventilation rates, and air conditioning electricity use for a three-month period show statistically significant differences (for 80% confidence interval) between the new calculations and the traditional methods based on hourly average wind speed.

Simulation of turbulent boiling flow in a vertical rectangular channel with one heated wall

The main objective of this work is to critically assess some dominant mechanisms for velocity field development and turbulence intensity distribution in the near wall region through the comparison of the experimental measurements and numerical results. The new set of boiling flow experiments in rectangular vertical channel with one heated wall, performed at the Texas A&M University (Estrada-Perez and Hassan, 2010), was used to validate the prediction capability of the CFD codes. The refrigerant HFE-301 was used as a working fluid. Liquid velocity and turbulent kinetic energy profiles close to the heated wall were compared. Simulation results successfully predict the main experimental tendencies associated with the heat flux and Reynolds number variation. However, a disagreement in velocity and turbulence magnitude can still be observed in the boiling region at lower Reynolds numbers. To investigate the modelling deficiencies, the sensitivity of the velocity and turbulence profiles to the wall roughness parameter was explored and discussed.

Numerical Simulation and Experimental Study of Jet Distance in Abrasive Waterjet Polishing

In this paper, it introduces the work principle of abrasive waterjet polishing (AWJP) and analyzes the jet distance to the influence of the AWJP under a certain pressure, with a numerical simulation and analysis to the jet distance based on jet mechanics and fluid dynamics. By establishing the physical models of the different jet distance of the AWJP and adopting the Realizable k-ε model and the SIMPLEC algorithm, it gains the jet flow field of the AWJP with different jet distance models and the distribution of turbulence intensity and pressure and velocity on the workpiece surface. The numerical simulation results of the different jet distance were analyzed and compared, according to the AWJP to the characteristic requirement of the jet. By doing the polishing experiment with a abrasive waterjet cutting machine, it verifies that the best polishing distance range of AWJP is from 10 times to 13 times of the nozzle diameter, which provides the theoretical basis for the process research of the AWPJ in the future.

Spatio-Temporal Surface Shear-Stress Variability in Live Plant Canopies and Cube Arrays

This study presents spatiotemporally-resolved measurements of surface shear-stress τs in live plant canopies and rigid wooden cube arrays to identify the sheltering capability against sediment erosion of these different roughness elements. Live plants have highly irregular structures that can be extremely flexible and porous resulting in considerable changes to the drag and flow regimes relative to rigid imitations mainly used in other wind-tunnel studies. Mean velocity and kinematic Reynolds stress profiles show that well-developed natural boundary layers were generated above the 8 m long wind-tunnel test section covered with the roughness elements at four different roughness densities (λ = 0, 0.017, 0.08, 0.18). Speed-up around the cubes caused higher peak surface shear stress than in experiments with plants at all roughness densities, demonstrating the more effective sheltering ability of the plants. The sheltered areas in the lee of the plants are significantly narrower with higher surface shear stress than those found in the lee of the cubes, and are dependent on the wind speed due to the plants ability to streamline with the flow. This streamlining behaviour results in a decreasing sheltering effect at increasing wind speeds and in lower net turbulence production than in experiments with cubes. Turbulence intensity distributions suggest a suppression of horseshoe vortices in the plant case. Comparison of the surface shear-stress measurements with sediment erosion patterns shows that the fraction of time a threshold skin friction velocity is exceeded can be used to assess erosion of, and deposition on, that surface.

Experimental Study on Flow Turbulence Distribution around a Spur Dike with Different Structure

The spur dike is an important building in river guiding and bank protection. In the present paper, the turbulence intensity distribution around spur dikes with different structures under the same flow condition is studied through model experiment and it gives the turbulence contours. Combining with theoretical analysis, it can be calculated that the turbulence intensity in the arc-like spur dike and fan-like are relatively weaker than that of the hook-like spur dike, and the strongest turbulence intensity occurs around the trapezoidal spur dike. The conclusion will provide a scientific basis for a reasonable choice of the structure of a spur dike, which provides the references for a better structure of a spur dike.

B223 ロッドバンドル内液流の乱流特性(OS9 熱・流動)

Validation of turbulence models is not an easy task mainly due to lack of experimental data on turbulent flows in rod bundles. In this study, we therefore measured distributions of velocity and turbulence intensities of water flow in a 2x2 rod bundle by using a stereoscopic PIV system. The diameter and pitch of the rods were 20 and 25 mm, respectively. Area-averaged velocity was 2 m/s. Local time-averaged velocity and turbulent intensities were obtained from 10,000 instantaneous velocities at each measurement point. Experimental results showed that horizontal components of turbulence intensity in the cross-section are anisotropic near the rod gap. Since eddies larger than the gap width cannot exist in the rod gap, the turbulence spectrum takes low values in the rod gap compared with center region of subchannel. To the contrary, difference in the turbulence spectrum of eddies smaller than the gap width is small between the rod gap and the center of subchannel.

An experimental study on laminar-turbulent transition at high free-stream turbulence in boundary layers with pressure gradients

We report here the results of a study on measurements and prediction of laminar-turbulent transition at high free-stream turbulence in boundary layers of the airfoil-like geometries with presence of the external pressure gradient changeover. The experiments are performed for a number of flow cases with different flow Reynolds number, turbulence intensity and pressure gradient distributions. The results were then compared to numerical calculations for same geometries and flow conditions. The experiments and computations are performed for the flow parameters which are typical for turbomachinery applications and the major idea of the current study is the validation of the turbulence model which can be used for such engineering applications.

CFD Application on Food Industry; Energy Saving on the Bread Oven

2 Abstract: In this paper, application of computational fluid dynamics simulation of the cooking process of bread and energy saving inside an oven is presented. A two dimensional CFD model is applied to investigate the hot air distribution and bread cooking conditions inside the stove. Results are shown as velocity contours, turbulence intensity, static temperature and pressure distribution. It was found that the turbulence intensity was the highest at part of the stove, which means which part of bread would cook the fastest as it receives larger quantity of energy compared to other parts. The arrangement of the stove is discussed and some improvement offered to find a desire temperature distribution that leads to a suitable cooking process. In addition, the current results can be useful for next improvements on the existing stove for energy saving.