Streamwise Turbulence Intensity Research Articles

Physically Realistic Roughness Closure Scheme to Simulate Turbulent Channel Flow over Rough Beds within the Framework of LES

A physically realistic roughness closure method for the simulation of turbulent open-channel flow over natural beds within the framework of large-eddy simulation (LES) is proposed. The description of bed roughness in LES is accomplished through a roughness geometry function together with forcing terms in the momentum equations. The major benefit of this method is that the roughness is generated from one physically measurable parameter, i.e., the mean grain diameter of the bed material. A series of flows over rough beds, for which mean flow and turbulence statistics are available from experiments, is simulated. Measured and computed values are compared to validate the proposed roughness closure approach. It is found that predicted streamwise velocity profiles, turbulence intensities, and turbulent shear stress profiles match the measured values fairly well. Furthermore, the effect of roughness on the overall flow resistance is predicted in reasonable agreement with experimental values.

An experimental study of the turbulence quantities in the boundary layer and near-wake of an airfoil placed at upstream of a 90° bend

This paper presents the results of an experimental study of the near-wake of a NACA 0012 airfoil placed upstream of a 90° bend section using hot-wire anemometry. The bend has a cross-section of 0.457 m × 0.457 m with a mean radius-to-height ratio of 1.17, and the airfoil is of chord length 0.15 m. Among the features studied are the effects of changes in the orientation of the airfoil with respect to the oncoming flow and the effects of the freestream velocity, namely, 10 m/s, 15 m/s and 20 m/s corresponding to chord Reynolds numbers of 1, 1.5 and 2 × 10 5, respectively. Emphasis is placed on the boundary layer on the upper surface of the airfoil and the wake up to one chord length downstream. The parameters of interest are the mean and turbulence quantities and their variations in the streamwise, normal and spanwise directions. Results from the investigations indicated the sensitivity of the wake characteristics to the boundary layers on the airfoil. Measurements of mean velocity and streamwise turbulence intensity taken in the spanwise direction within the wake revealed wave-like variations which are linked to the presence of vortical structures in the wake.

Use of direct numerical simulation (DNS) data to investigate spatial resolution issues in measurements of wall-bounded turbulence

The effect of limited spatial resolution for hot-wire anemometry (HWA) is investigated by analysing the two-dimensional energy spectra from direct numerical simulation (DNS) of turbulent channel flow at Reτ ≈ 950. Various spanwise filter lengths are applied to the streamwise velocity components in order to mimic the limited spatial resolution of a single-normal hot-wire experiment. Clear attenuation of the small-scale DNS energy is observed as the filter length is increased and good agreement is noted between the missing energy from filtered DNS and that from hot-wire experiments over a range of sensing lengths. The missing energy in the near-wall region is shown to be highly anisotropic in nature, thus bringing into question existing correction schemes that rely on small-scale isotropic flow assumptions. An empirical model of the missing streamwise component energy spectra is formulated, as a function of wire length, and is shown to be useful as a new correction function for the missing energy and streamwise turbulence intensity at the near-wall energetic peak.

Hot-wire spatial resolution issues in wall-bounded turbulence

Careful reassessment of new and pre-existing data shows that recorded scatter in the hot-wire-measured near-wall peak in viscous-scaled streamwise turbulence intensity is due in large part to the simultaneous competing effects of the Reynolds number and viscous-scaled wire length l+. An empirical expression is given to account for these effects. These competing factors can explain much of the disparity in existing literature, in particular explaining how previous studies have incorrectly concluded that the inner-scaled near-wall peak is independent of the Reynolds number. We also investigate the appearance of the so-called outer peak in the broadband streamwise intensity, found by some researchers to occur within the log region of high-Reynolds-number boundary layers. We show that the ‘outer peak’ is consistent with the attenuation of small scales due to large l+. For turbulent boundary layers, in the absence of spatial resolution problems, there is no outer peak up to the Reynolds numbers investigated here (Reτ = 18830). Beyond these Reynolds numbers – and for internal geometries – the existence of such peaks remains open to debate. Fully mapped energy spectra, obtained with a range of l+, are used to demonstrate this phenomenon. We also establish the basis for a ‘maximum flow frequency’, a minimum time scale that the full experimental system must be capable of resolving, in order to ensure that the energetic scales are not attenuated. It is shown that where this criterion is not met (in this instance due to insufficient anemometer/probe response), an outer peak can be reproduced in the streamwise intensity even in the absence of spatial resolution problems. It is also shown that attenuation due to wire length can erode the region of the streamwise energy spectra in which we would normally expect to see kx−1 scaling. In doing so, we are able to rationalize much of the disparity in pre-existing literature over the kx−1 region of self-similarity. Not surprisingly, the attenuated spectra also indicate that Kolmogorov-scaled spectra are subject to substantial errors due to wire spatial resolution issues. These errors persist to wavelengths far beyond those which we might otherwise assume from simple isotropic assumptions of small-scale motions. The effects of hot-wire length-to-diameter ratio (l/d) are also briefly investigated. For the moderate wire Reynolds numbers investigated here, reducing l/d from 200 to 100 has a detrimental effect on measured turbulent fluctuations at a wide range of energetic scales, affecting both the broadband intensity and the energy spectra.

거대 화상용 PIV 시스템을 이용한 실차 내부 공기벨트 토출흐름의 속도장 측정 연구

Most vehicles have a heating, ventilating and air conditioning (HVAC) device to control the thermal condition and to make comfortable environment in the passenger compartment. The improvement of ventilation flow inside the passenger compartment is crucial for providing comfortable environment. For this, better understanding on the variation of flow characteristics of ventilation air inside the passenger compartment with respect to various ventilation modes is strongly required. Most previous studies on the ventilation flow in a car cabin were carried out using computational fluid dynamics (CFD) analysis or scale-down water-model experiments. In this study, whole ventilation flow discharged from the air vent of a real passenger car was measured using a special PIV (particle image velocimetry) system for large-size FOV (field of view). Under real recirculation ventilation condition, the spatial distributions of stream-wise turbulence intensity and mean velocity were measured in the vortical panel-duct center plane under the panel ventilation mode. These experimental data would be useful for understanding the detailed flow structure of real ventilation flow and validating numerical predictions.

Turbulence statistics and structures of drag-reducing turbulent boundary layer in homogeneous aqueous surfactant solutions

In our earlier work [Itoh et al., Phys. Fluids 17, 075107 (2005)], the additional maximum of the streamwise turbulence intensity near the center of the drag-reducing turbulent boundary layer was found in the homogeneous dilute aqueous surfactant solution which was a mixture of cetyltrimethyl ammonium chloride with sodium salicylate as counterion. In this work, we systematically investigated the influence of the drag-reducing surfactant on the velocity fields of the turbulent boundary layer at various Reynolds numbers Reθ from 301 to 1437 and the drag reduction ratio DR from 8% to 74% under different streamwise locations and concentration and temperature of solutions using a two-component laser-Doppler velocimetry (LDV) system. It was revealed that all data on DR versus the wall-shear rate obtained here were collapsed on a single curve. We verified the existence of the additional maximum of the streamwise turbulence intensity near the center of the boundary layer which appeared at relatively large drag reduction ratios and small Reynolds numbers. It was found that the additional maximum of streamwise turbulence intensity and its wall-normal location were independent of the streamwise location, wall-shear rate, Reynolds number, and drag reduction ratio. The additional maximum could be explained by the bilayered structure model proposed, in which the flow in the near-wall region is in shear-induced structure (SIS) and viscoelastic, whereas the flow in the region away from the wall is in non-SIS and nonviscoelastic. This model was based on measurements of the shear viscosity. We also performed particle image velocimetry measurements, which revealed that the fluctuating velocity vector fields showed two situations, with low and high activity. In low activity, the velocity fluctuations were attenuated largely across the turbulent boundary layer. In high activity, fluctuating velocity vectors were almost parallel to the wall and relatively large in both regions near the wall and the center of the boundary layer, which seemed to be a bilayered structure and supported the bilayered structure model.

Large Eddy Simulation of Turbulent Flow Through Submerged Vegetation

Large Eddy Simulations (LES) are performed for an open channel flow through idealized submerged vegetation with a water depth (h) to plant height (hp) ratio of h/hp = 1.5 according to the experimental configuration of Liu et al. (J Geophys Res Earth Sci, 2008). They used a 1D laser Doppler velocimeter (LDV) to measure longitudinal and vertical velocities as well as turbulence intensities along several verticals in the flow and the data are used for the validation of the present simulations. The code MGLET is used to solve the filtered Navier–Stokes equations on a Cartesian non-uniform grid. In order to represent solid objects in the flow, the immersed boundary method is employed. The computational domain is idealized with a box containing 16 submerged circular cylinders and periodic boundary conditions are applied in both longitudinal and transverse directions. The predicted streamwise as well as vertical mean velocities are in good agreement with the LDV measurements. Furthermore, fairly good agreement is found between calculated and measured streamwise and vertical turbulence intensities. Large-scale flow structures of different shapes are present in the form of vortex rolls above the vegetation tops as well as locally generated trailing and von- Karman-type vortices due to flow separation at the free end and the sides of the cylinders. In this paper, the flow field is analyzed statistically and evidence is provided for the existence of these structures based on the LES.

Turbulence characteristics of flow in a spiral corrugated culvert fitted with baffles and implications for fish passage

Baffles are often used to retrofit culverts to aid in fish passage. The objective of this experimental investigation was to compare the turbulent flow structure inside a full-scale spiral corrugated culvert fitted with sloped- and slotted-weir baffles to available turbulence descriptions for non-baffled culverts. In addition, the turbulent flow structure inside a full-scale culvert was compared to fish passage preferences. Velocity measurements were taken in a 1.83 m-diameter, 12.2 m-long corrugated metal culvert fitted with sloped- and slotted-weir baffles using a Sontek Micro-Acoustic Doppler Velocimeter for flow rates of 0.043, 0.085, 0.113, and 0.198 m 3/s and a culvert slope of 1.14%. Results showed there were only minor differences in the turbulent flow structure created by each baffle type. The most significant differences included higher lateral turbulent intensities on the edges of the jet created by the slotted-weir baffles, and higher turbulent kinetic energies on the left side of the culvert (looking downstream) caused by the sloped-weir baffles. Downstream from a slotted-weir baffle, a reduced velocity and streamwise turbulent intensity zone was found near the left edge of the flow that was similar to that found in the bare culvert. The sloped-weir baffles produced a less pronounced zone near the right edge of the flow. No significant relationships could be found between the turbulence results of this study and biological fish passage tests performed at the same experimental site due to the lack of substantial differences in streamwise and lateral turbulence intensity distributions downstream from the tested baffles.

PIV measurements of hull wake behind a container ship model with varying loading condition

Flow characteristics of the hull wake behind a container ship model were investigated under different loading conditions (design and ballast loadings) by employing the particle image velocimetry (PIV) technique. Measurements were made at four transverse locations and two longitudinal planes for three Reynolds numbers (Re) (= U 0 Lpp/ ν, where U 0 is the freestream velocity, Lpp is the length between two perpendiculars of the ship model and ν is the kinematic viscosity) of 5.08×10 5, 7.60×10 5, and 1.01×10 6. It was observed that symmetric, large-scale, longitudinal counter-rotating vortices (with respect to centerline) of nearly the same strength were formed in the near wake. For the ballast-loading condition, the vortices appear at propeller plane below the propeller-boss. The vortex center exhibits a significant upward shift near the propeller-boss as the Reynolds number increase, and as the flow moves downstream. Under the design-loading condition, the vortices first appear at a further downstream location than that for the ballast-loading condition above the propeller-boss. This difference in the flow structure can significantly change the inflow conditions to the propeller blades, such as the streamwise mean velocity profiles and turbulence intensity distributions at the propeller plane. In particular, under the ballast-loading condition, asymmetric inflow may weaken the propulsion and cavitation performance of the marine propeller.

이중제트에서 노즐과 노즐사이의 각도 변화에 따른 유동 특성

The characteristics of flow on unventilated dual jets was experimentally investigated. The two nozzles each with an aspect ratio of 20 were separated by 6 nozzle widths. Reynolds number based on nozzle width was set to 5,000 by nozzle exit velocity. All measurements were made over a range of nozzle-to-nozzle angles from 0˚ to 25˚. The particle image velocimetry and pressure transducer were employed to measure turbulent velocity components and mean static pressure, respectively. It was shown that a recirculation zone with sub-atmospheric static pressure was bounded by the inner shear layers of the individual jets and the nozzles plated. As nozzle-to-nozzle inclined angles were decreased, it was found that the spanwise turbulent intensity is greater than the streamwise turbulent intensity in the merging region. In the combined region, the velocity of dual jets agree well with that of single jet, but the turbulence intensity of dual jets not agree with that of single jet.

PIV Study of Adverse and Favorable Pressure Gradient Turbulent Flows Over Transverse Ribs

This paper reports an experimental study of the combined effects of rib roughness and pressure gradient on turbulent flows produced in asymmetric converging and diverging channels. Transverse square ribs with pitch-to-height ratio of 4 were attached to the bottom wall of the channel to produce the rib roughness. A particle image velocimetry technique was used to conduct measurements at several streamwise-transverse planes located upstream, within, and downstream of the converging and diverging sections of the channel. From these measurements, the mean velocities and turbulent statistics at the top plane of the ribs and across the channel were obtained. The data revealed non-negligible wall-normal motion and interaction between the cavities and overlying boundary layers. The different drag characteristics of the rough bottom wall and the smooth top wall produced asymmetric distributions of mean velocity and turbulent statistics across the channel. The asymmetry of these profiles is most extreme in the presence of adverse pressure gradient. Because of the manner in which pressure gradient modifies the mean flow and turbulence production, it was found that the streamwise turbulence intensity and Reynolds shear stress in the vicinity of the ribs are lower in the adverse pressure gradient than in the favorable pressure gradient channel. The results show also that the combined effects of rib roughness and adverse pressure gradient on the turbulent intensity statistics are significantly higher than when roughness and adverse pressure gradient are applied in isolation.

The impact of single natural vegetation elements on flow characteristics

AbstractIn this experimental study, measurements were conducted to explore the impacts of different forms of individual natural vegetative elements within the flow domain on velocity and turbulence characteristics. All the experiments were performed in a flume measuring 26 m in length, 0·98 m in width and 0·85 m in depth, and real tree saplings were utilized to represent the vegetation element. In order to analyse this commonly observed nature phenomenon in floodplains, trees with wide trunks were classified into three groups on the basis of their volume versus height relation. Throughout the velocity measurements three acoustic Doppler velocimeters were employed. Time‐averaged velocity, streamwise and vertical turbulence intensities and turbulent kinetic energy parameters were examined. Additionally, a formulation that gives the velocity profile at a certain distance downstream of vegetation was introduced and the validity of the proposed formulation was checked with experimental data. It is seen that despite their porous structures, the presence of vegetation considerably disturbs the flow field and dissipates a remarkable amount of energy by turbulence. Copyright © 2008 John Wiley & Sons, Ltd.

Two- and Four-Way Coupled Euler–Lagrangian Large-Eddy Simulation of Turbulent Particle-Laden Channel Flow

Large-eddy simulations (LES) of a vertical turbulent channel flow laden with a very large number of solid particles are performed. The motivation for this research is to get insight into fundamental aspects of co-current turbulent gas-particle flows, as encountered in riser reactors. The particle volume fraction equals about 1.3%, which is relatively high in the context of modern LES of two-phase flows. The channel flow simulations are based on large-eddy approximations of the compressible Navier–Stokes equations in a porous medium. The Euler–Lagrangian method is adopted, which means that for each individual particle an equation of motion is solved. The method incorporates four-way coupling, i.e., both the particle-fluid and particle–particle interactions are taken into account. The results are compared to single-phase channel flow in order to investigate the effect of the particles on turbulent statistics. The present results show that due to particle–fluid interactions the mean fluid profile is flattened and the boundary layer is thinner. Compared to single-phase turbulent flow, the streamwise turbulence intensity of the gas phase is increased, while the normal and spanwise turbulence intensities are reduced. This finding is generally consistent with existing experimental data. The four-way coupled simulations are also compared with two-way coupled simulations, in which the inelastic collisions between particles are neglected. The latter comparison clearly demonstrates that the collisions have a large influence on the main statistics of both phases. In addition, the four-way coupled simulations contain stronger coherent particle structures. It is thus essential to include the particle–particle interactions in numerical simulations of two-phase flow with volume fractions around one percent.

Experimental Study of the Far Field of Incompressible Swirling Jets

The far field of an incompressible swirling jet has been studied using two-component laser Doppler anemometry. Three pairs of symmetric injectors were used to produce weak-to-moderate swirling jets. Velocity profiles of the mean and fluctuating streamwise and azimuthal velocity components were measured in jets with two swirl numbers (S = 0.15 and 0.25) at axial locations up to 50 jet exit diameters. The velocity and turbulence intensity profiles, centerline decay, and growth rates for the various swirling jets have been compared with those obtained in the same facility without swirl (S = 0). Like the previous observations for the near jet, there was no observable effect on the properly scaled far jet for the S = 0.15 case. The results were virtually identical to the non-swirling jet. For the S = 0.25 case, the only statistically significant effect was a shift in the virtual origin (from x/D* = 0.75 to -2.9). The recent predictions of equilibrium similarity theory were found to be in excellent agreement with the experimental results. In particular, the mean azimuthal component of velocity falls off as the inverse square of the downstream distance. By contrast, the mean stream-wise velocity and turbulence intensities fall off with the inverse of the downstream distance. As a consequence, the mean azimuthal equation uncouples from the rest, and so the asymptotic swirling jet behaves like the non-swirling jet.

Double-averaged velocity and stress distributions for hydraulically-smooth and transitionally-rough turbulent flows

We analyse experimental measurements of turbulent open-channel flow over hydraulically-smooth and transitionally-rough beds using the double-averaging methodology. Oil with a viscosity of 15×10−6 m2/s is used instead of water so that transitional-range roughness Reynolds numbers can be achieved with large (11.1 mm) roughness elements, allowing spatial variations in the mean velocity field to more easily be measured. Distributions of double-averaged velocities, turbulence intensities, form-induced intensities, and viscous, Reynolds, form-induced and total shear stresses are studied with comparisons made between distributions for hydraulically-smooth, transitionally-rough, and fully-rough boundaries. Measured streamwise turbulence intensities for all experiments peaked at a constant distance from the bed (z++d+ = 15) when elevation scale is adjusted using the zero-plane displacement d for the logarithmic velocity distribution. This collapse suggests that turbulence intensity distributions may be useful in assessing appropriate values of d for transitionally-rough and fully-rough boundaries. Form-induced normal and shear stresses above the roughness tops were found to collapse towards a common curve independent of roughness Reynolds number.

Comparison of turbulent flow over solid and porous structures mounted on the bottom of a rectangular channel

This paper presents the findings of an experimental study on the turbulent flow field in the vicinity of structures mounted on the bottom of a rectangular open channel. Three model structures used in this study have geometric porosities ( ε ) of 0%, 34.9% and 47.5%. Detailed measurements of the time-averaged velocity components, turbulent intensity components, turbulent kinetic energy and Reynolds stress were taken by the acoustic Doppler velocimeter (ADV) at different locations around the structures. Measurements obtained with the electro-magnetic micro-velocimeter were used to observe the velocity distributions within the porous structures. As the structure porosity increases, the turbulence intensity, turbulent kinetic energy and Reynolds shear stress over the structure all decrease. Among the structures used in this study, the porous structures show the elongating of the recirculation region in the downstream direction due to bleed flow passing through the gaps of the structures. Significant maximum streamwise turbulent intensity of the porous structure with ε = 47.5 % decreases by about 37% compared to that of the solid structure ( ε = 0 % ). The data presented in this paper would be useful to develop and verify mathematical models for flow both over and within a porous structure.

Numerical simulation of flows in tight-lattice fuel bundles

The flow in tight rod bundles is characterized by long-term, large-scale coherent patterns in the stream-wise direction. In the present work, the issue of simulating these structures through unsteady CFD simulations employing periodic boundary conditions in the stream-wise direction, will be addressed. The validity of the approach is assessed through the comparison of a large eddy simulation (LES) for similar flow conditions inside a simplified geometry and experimental data. A powerful statistical tool (proper orthonormal decomposition) is used to analyze the time varying solution. The flow field has been decomposed into a series of normal modes, identifying the structures responsible for the flow transfer between sub-channels. Additional insights on the physics of these coherent structures are obtained. An unsteady Reynolds averaged Navier–Stokes simulation (URANS) of the flow in a rod bundle has then been carried out. The comparison between numerical results and experimental results [Krauss, T., Meyer, L., 1998. Experimental investigation of turbulent transport of momentum and energy in a heated rod bundle. Nucl. Eng. Design 180, 185–206] proves that accuracy can be achieved for averaged statistics such as stream-wise velocity, turbulent intensity and wall shear stresses.

Numerical Study of the Transition Regime between the Skimming and Wake Interference Flows in a Water Flume by Using the Lattice-Model Approach

A numerical study to describe the transition regime between the skimming and wake interference flows due to the influence of an idealized bed roughness in a water flume was carried out here using the lattice model approach. The model reproduced the skimming, transition, and wake interference regimes for different aspect ratios that determine the bed roughness geometry. The simulated turbulent structures were visualized by drawing the trajectories of a large number of passive tracer particles released in the computational domain, and the results agreed with those reported by the research works. The dimensionless streamwise and vertical turbulent intensities were calculated at five test sections. The results obtained supported the visualized flow patterns permitting us to detect the presence of a shear layer developed at the top of the roughness element, whose strength varied according to the flow regime simulated.

Turbulence Structures in Flow over Two-Dimensional Dunes

This paper presents a large eddy simulation (LES) of turbulent open channel flow over two-dimensional periodic dunes. The Reynolds number R based on the bulk velocity U(bulk) and the maximum flow depth h, is approximately 25,000. The instantaneous flow field is investigated with special emphasis on the occurrence of coherent structures. Instantaneous vortices were visualized and it is shown that separated vortices are formed downstream of the dune crest due to Kelvin–Helmholtz instabilities. Near the point of reattachment the so-called kolk-boil vortex evolves in form of a hairpin vortex. Also present are previously separated vortices, which are convected along the stoss side of the downstream dune and elevated toward the water surface. The existence of near wall streaks which reform shortly after reattachment is also shown. The spacing between two low-speed streaks is very similar to that observed previously over smooth and rough walls. For validation, profiles of the time-averaged velocities, streamwise, and wall-normal turbulent intensities and the Reynolds shear stress calculated by the LES are presented and compared with available laser Doppler velocimetry measurements and overall good agreement is found.

Turbulent Characteristics of a Flowfield over a Heated Riblet Surface

Experimental studies of a turbulent flowfield developing over the heated riblet surface were carried out in a low-speed wind tunnel. The instantaneous velocity fields were measured by using a particle image velocimetry (PIV) in a plane that was parallel to the riblet surface, and compared with those over a heated smooth surface. The reduction ratio of the skin friction coefficient over the heated riblet surface reached up to 8% for the riblet spacing of about 13 viscous wall units. This is almost the same spacing as that obtained for the non-heated condition. The streamwise and wall-normal turbulent intensities against the local time-mean velocity were decreased in their peak values for the heated riblet. The instantaneous fluctuating velocity fields obtained by subtracting the ensemble-averaged velocity from instantaneous velocity vectors revealed that the appearance of a lowspeed streaky structure became more intermittent over the riblet surface. This fact suggested a reduction in turbulence production in the near-wall region. Furthermore, it was also found that a perturbation of the spanwise velocity fluctuation was restricted near the riblet surface.