Time-averaged Streamwise Velocity Research Articles

Characteristics of the turbulent flow within short canopy gaps

The turbulent flow within short canopy gaps with lengths $L/h$ = 0.5, 1, 2, 3, and 4, where $h$ denotes the canopy height, is experimentally investigated using planar particle image velocimetry. A thorough examination of the time-averaged streamwise and wall-normal velocities, shear layer growth, turbulent kinetic energy, Reynolds shear stress, and two-point correlations suggests a classification of the flow into two regimes: a skimming flow regime and a shear layer growth regime. In contrast with the skimming flow regime, the shear layer and enhanced turbulence exhibit deep penetration within the gaps and promote mixing in the shear layer growth regime.

On the feasibility of using open source solvers for the simulation of a turbulent air flow in a dairy barn

Two transient open source solvers, OpenFOAM and ParMooN, and the commercial solver Ansys Fluent are assessed with respect to the simulation of the turbulent air flow inside and around a dairy barn. For this purpose, data were obtained in an experimental campaign at a 1:100 scaled wind tunnel model. All solvers used different meshes, discretization schemes, and turbulence models. The experimental data and numerical results agree well for time-averaged stream-wise and vertical-wise velocities. In particular, the air exchange was predicted with high accuracy by both open source solvers with relative differences less than 4% and by the commercial solver with a relative difference of 9% compared to the experimental results. With respect to the turbulent quantities, good agreements at the second (downwind) half of the barn inside and especially outside the barn could be achieved, where all codes accurately predicted the flow separation and, in many cases, the root-mean-square velocities. Deviations between simulations and experimental results regarding turbulent quantities could be observed in the first part of the barn. These deviations can be attributed to the utilization of roughness elements between inlet and barn in the experiment that were not modeled in the numerical simulations. Both open source solvers proved to be promising tools for the accurate prediction of time-dependent phenomena in an agricultural context, e.g., like the transport of particulate matter or pathogen-laden aerosols in and around agricultural buildings.

Modified mixing coefficient for the crossflow between sub-channels in a 5 × 5 rod bundle geometry

We performed experiments to measure a single-phase upward flow in a 5 × 5 rod bundle with spacer grids using a particle image velocimetry, focusing on the crossflow. The Reynolds number based on the hydraulic diameter and the bulk velocity is 10,000. The ratio of pitch between rods and rod diameter is 1.4 and spacer grid is installed periodically. The turbulence in the rod bundle results from the combination of a forced mixing and natural mixing. The forced mixing by the spacer grid persists up to 10Dh from the spacer grid, while the natural mixing is attributed to the crossflow between adjacent sub-channels. The combined effects contribute to a sinusoidal distribution of the time-averaged streamwise velocity along the lateral direction, which is relatively weak right behind the spacer grid as well as in the gap. The streamwise and lateral turbulence intensities are stronger right behind the spacer grid and in the gap. Based on these findings, we newly defined a modified mixing coefficient as the ratio of the lateral turbulence intensity to the time-averaged streamwise velocity, which shows a spatial variation. Finally, we compared the developed model with the measured data, which shows a good agreement with each other.

Self-preserving characteristics in wall-wake flow downstream of an isolated bedform

The results of an experimental study on the turbulence characteristics in flow over and downstream of an isolated bedform are presented. The vertical profiles of time-averaged streamwise velocity, Reynolds stresses and turbulent kinetic energy (TKE) fluxes at different horizontal locations over and downstream of a bedform are shown to demonstrate their variations. The experimental results signify a flow acceleration, separation and deceleration over the stoss-side, at the crest and in the leeside of a bedform. In addition, in the wall-wake region (far downstream of the bedform), the defects (uninterrupted upstream value minus downstream value at a given vertical distance) in time-averaged streamwise velocity, Reynolds shear stress (RSS), turbulence intensities and TKE fluxes demonstrate the self-preserving characteristics in their individual vertical profiles when they are scaled by their individual peak defects. For scaling the vertical distance to achieve the self-preserving characteristics, the defect profiles of velocity were scaled by the half-width of peak velocity defect, those of RSS and turbulence intensities by the half-width of peak RSS defect and those of TKE fluxes by the half-width of individual peak TKE flux defects. The peak defects of all the parameters lessen, as one moves toward the downstream, recovering their uninterrupted upstream profiles.

Experimental study of a passive control of airfoil lift using bioinspired feather flap

Birds are known for their extraordinary agility, maneuverability, flexibility and endurance during their flight, even under some adverse flying conditions. Bird wings have been the most inspirational element, attracting the attention of researchers to reveal the underlying physical mechanism of lift production as well as to apply the results into the artificial flying vehicles. This paper presents a systematic experimental investigation on a passive flow control of a NACA0012 airfoil using real feather flap which is installed on the suction or pressure surface. The focus of the present study is to determine the major role of a real feather flap in the aerodynamic performance of a NACA0012 airfoil at small attack angles (α). The feather flap width w and its installation position xin are varied from 0.27c to 0.8c and from 0.0 to 0.2c, respectively, where xin is measured from the leading edge of the airfoil, and c is the chord length of the airfoil. Detailed particle image velocimetry (PIV) measurements are conducted to understand the origin of the aerodynamic benefits introduced by the feather flap. The flap mounted on the suction side may have a positive impact only at large α, beyond the stall. On the other hand, when mounted on the pressure side, the feather flap is proved to be beneficial to improve the aerodynamic performance of the airfoil at small α (= −4° to 8°). The lift CL and lift-to-drag ratio CL/CD are enhanced by 186% and 72%, respectively, for w = 0.53c, xin = 0.2c at α = 2°. Time-averaged and instantaneous vorticities, time-averaged streamwise velocity, and lateral velocity around the flapped airfoil weaken, decrease and increase, respectively, compared with those around the plain airfoil, which are attributed to the increased CL and CL/CD.

Smooth Open Channel with Increasing Aspect Ratio: Influence on Secondary Flow

A high-resolution particle image velocitmetry system is used to investigate the relationship between secondary flow and aspect ratio in a straight channel. Considering the symmetry of open channel flow, the flow parameters in half of the flume are measured. Since the variation of the aspect ratio has a direct impact on the intensity and structure of secondary flows, this study was conducted in a smooth open channel to study the influence of aspect ratio on the structure and strength of secondary flows with aspect ratio change from 3 to 7.5 under supercritical flow condition. Profiles and contour-maps of time-averaged stream-wise and vertical velocities were acquired using precise measuring instruments. The results show that there are several secondary flow cells in the cross section, and their structure affects the velocity distribution and energy distribution, which makes the velocity distribution deviate from the traditional logarithmic distribution, and the maximum velocity occur below the surface. The flow intensity of secondary flows is different under different aspect ratios. Results show great agreement with classical theory.

Near-bed turbulence structures in water-worked and screeded gravel-bed flows

Coherent structures and their impact on the near-bed time-averaged flow structure in a water-worked gravel-bed (WGB) and a screeded gravel-bed (SGB) are analyzed. Instantaneous velocities were measured using a particle image velocimetry system in the WGB and SGB flows in a flume with rectangular cross section. To ascertain the response of the WGB with respect to the SGB to the coherent structures, the time- and double-averaged flow, and the spatially averaged (SA) turbulence parameters, the experimental flow conditions for both the beds were kept identical. The surface gravels in the WGB were spatially organized owing to the water action. By contrast, the surface gravels in the SGB were randomly poised. These result in a higher roughness height in the WGB than in the SGB. Time series analysis for the instantaneous velocity and vorticity on a central vertical plane along the streamwise direction proves that the coherent structures in the near-bed flow zone are constituted by rapidly and slowly moving fluid streaks. Besides, the time-averaged streamwise velocity, vorticity, turbulence level, third-order correlations, and turbulent kinetic energy (TKE) budget are analyzed in the WGB and SGB. Their contours are plotted on the central vertical plane to study their spatial distributions. In addition, the SA higher-order correlations and TKE budget in the WGB and SGB are examined. A comparative study infers that the higher roughness in the WGB than in the SGB causes both the time-averaged and SA turbulence parameters in the former to be greater than those in the latter.

COMPUTATIONAL FLUID DYNAMICS METHOD FOR THE ANALYSIS OF HYDRODYNAMICS PERFORMANCE OF MANGROVE ROOT MODELS

Mangroves play a prominent role in obstructing water currents in riverbanks, shorelines, and coastal areas. Mangrove roots have a significant contribution to the resiliency of the vegetation structure. In this work, the flow characteristics past a cluster of circular cylinders (patch), which represents the mangrove roots, are investigated. Numerical simulations were conducted for five patch porosities (φ = 86% to 96%) for Reynolds number ranging from 2000 to 10,000. The complex two-dimensional flow structure of the cylinder wake is reasonably captured and time-averaged streamwise velocity, vorticity, and turbulence intensity are presented. A comparison is made between flow around one cylinder and a patch of cylinders with the same diameter to determine the impact of the patch porosity. Based on the results, unlike for a canonical cylinder, the patch porosity gives a rise in the Strouhal number but drops drag coefficient. We found that blockage parameter for a patch decreases with porosity for all Reynolds numbers. A von Karman vortex sheet is predicted behind the patch with a periodicity which agrees well with experiment. In addition, the vorticity field for the least porous patch reveals a delay in the formation of von Karman vortex street due to the small vortices in the near wake, whereas the vortices evolve in the far wake and produce swirling flow.

OpenFOAM based LES of slot jet impingement heat transfer at low nozzle to plate spacing using four SGS models

The objective of the present investigation is to assess the effectiveness of large eddy simulation (LES) in turbulent slot jet impingement heat transfer at low nozzle to plate spacing. Four different sub-grid stress (SGS) models, namely, Smagorinsky, WALE (wall adapting local eddy-viscosity), k-equation and dynamic k-equation, were considered for Reynolds number of 20,000. Computations were performed using OpenFOAM, an open source finite-volume based CFD code. Time and span-wise averaged mean streamwise velocity and root mean square (r.m.s.) velocity fluctuations in the stagnation and wall jet regions are presented. Nusselt number distributions on the impingement wall are also presented. The computed LES results are compared with the reported experimental data. A secondary peak in the Nusselt number was observed using the four SGS models as in the experimental data. LES of slot jet impingement heat transfer using four SGS models, including WALE, has been investigated for the first time in the present paper. It is observed that the WALE and dynamic k-equation SGS models perform well in complex flow regions of turbulent slot jet impingement heat transfer.

Large eddy simulation of flow past a square cylinder with rounded leading corners: A comparison of 2D and 3D approaches

The unsteady flow past a square cylinder with rounded leading corners is predicted at Re = 2600 by large eddy simulations (LES). The corner radius r is fixed at 3 mm and the cylinder width D is 12.7 mm, resulting in r /D = 0.236. Detailed comparisons are made between the two-dimensional (2D) and the three-dimensional (3D) results with previous experimental measurements [32, 33]. The results show that the Strouhal number agrees well with previous measurement [33], indicating that both 2D and 3D LES are capable of capturing the shedding of large-scale vortical strutures. The statistical quantities (e.g., the drag and lift coefficients, the time-averaged streamwise velocity, the root-mean-square of streamwise velocity and the Reynolds stress) were calculated and validated against experimental results [32, 33]. The 3D LES results show reasonable agreement with the experimental measurement [32, 33], while the results predicted by 2D LES deviate far from the experimental measurement [32, 33]. It is further demonstrated that the recirculation length and the vortex formation length are consistent with experimental results [33], while it is greatly shortened in 2D LES.

Experimental Investigation of Flow Structures around a Torpedo-like Geometry Placed in a Boundary Layer Flow

Defense applications for both under oceans and seas, particularly underwater vehicles have been considered in this research. With this aim, flow characteristics around a torpedo-like geometry under the effect of the boundary layer flow over a smooth flat plate have been experimentally examined by using PIV technique. All of the experiments have been done for Re=20000 and Re=40000 based on the length (L) of the geometry as a characteristic length. As a result, time-averaged streamwise velocity components , velocity vectors , streamline topologies and Reynolds stress correlations in the wake region of the torpedo-like geometry have been acquired in the range of 0 ≤ G/D ≤ 1.5. Here, G is the space between the bottom point of the geometry and flat plate surface; D stands for the diameter of the geometry. It is found that at the smallest value of G/D=0.25, jet-like flow occurs between the plate and the model which causes a powerful scouring. As the gap ratio is increased to G/D=0.5 and G/D=1.0, the jet-like flow diminishes slightly and then the flow structure in the wake region becomes similar to the uniform incoming flow condition for G/D=1.50. Due to the effect of the jet-like flow and boundary layer flow, time-averaged flow patterns present asymmetrical distributions which are clearly shown a bigger size focus close to the plate in streamline topology. Reynolds stress patterns form more powerful viscous forces in the boundary layer flow due to the occurrence of eddy vortices and viscosity effect. It is observed from the aforementioned flow patterns that interaction between the flow structure, the model and boundary layer flow yields very complex structure. In order to decrease the energetic flow in this condition, passive or active flow control method can be integrated on the torpedo-like geometry.

Segregation dynamics of a binary-size mixture in a three-dimensional rotating drum

The knowledge of the granular segregation phenomenon induced by size or density differences in the ubiquitous polydisperse systems is important, hence numerous studies have focused on this for the simple yet practical rotating drum. However, in view of the distinctly different characteristics of the two regions (namely, active and passive) for a drum operated in the rolling regime, an understanding of the segregation dynamics in each region is warranted, but remains a gap to date. Accordingly, this study aimed at numerically studying the segregation dynamics of the solid phase in a three-dimensional rotating drum consisting of a binary-size mixture via the Discrete Element Method (DEM). The results demonstrate that the total kinetic energy, the angle of repose, the time-averaged streamwise (i.e., parallel to the bed surface) velocity and the position of the active-passive interface are global parameters, which are not influenced by size-segregation and thereby provides a critical basis for comparing different systems of various polydispersities. Also, the quick onset of the rapid radial segregation leads to sharp initial changes of the variables associated with the two particle types, after which the slow axial segregation leads to a gradual change of these variables over time. Furthermore, size-segregation leads to the redistribution of the particle number of the two particle types in the active and passive regions. Relative to the monodisperse system, the collision forces exerted on the small and large particles are slightly higher and lower, respectively. The results here provide important insights on the dynamics associated with the inevitable segregation phenomenon, which contributes to better operation and predictive capability of the rotating drum.

Turbulent flow over an array of boulders placed on a rough, permeable bed

Large eddy simulations (LES) of flow over wall-mounted boulders placed on a rough, permeable bed, composed of one layer of densely-packed spheres, are performed. This configuration is based on the experimental configuration reported in Papanicolaou et al. (2012) [14] the data of which are used to validate the LES. A fairly good agreement between calculated and measured time-averaged streamwise velocity and turbulence intensity profiles is found confirming the accuracy of the simulation. Details of the time-averaged and instantaneous flow are revealed using various visualisation methods. The time-averaged flow is characterised by a symmetric pair of funnel vortices comprising the recirculation zone in the lee of the boulder. The funnel vortex is bounded by significant shear layers caused by high-momentum fluid filling the low momentum wake zone behind the boulder. Bed roughness and permeability appears to play an important role in this flow, it affects the near-bed velocity profiles and subsurface fluid is entrained into the low pressure zone behind the boulder, specifically into the funnel vortex, moreover surface-subsurface flow exchange in particular in the vicinity of the boulders disrupts the secondary flow in the wake. The instantaneous flow features various turbulence structures which are educed by means of isosurfaces of the Q-criterion. Shear layer roll-up leads to high-frequency shedding of roller vortices, which are subsequently stretched by significant velocity gradients and deformed into a hairpin or horseshoe form. Vertical and horizontal shear layer flapping is revealed and quantified via vorticity contour plots and velocity spectra.

On the flow organization of a chevron synthetic jet

In the present study, the flow fields generated by two synthetic jets with a chevron and a conventional circular nozzle exits are studied and compared. For both configurations, the devices are operated at the same input electrical power, thus leading to Reynolds and Strouhal numbers equal to 5600 and 0.115 (for the circular exit) and 6000 and 0.106 (for the chevron exit). Phase-locked stereoscopic particle image velocimetry measurements are used to reconstruct the three-dimensional coherent vortex structures. Time-averaged and phase-averaged mean and turbulent statistics are analysed and discussed. The flow field strongly depends on the exit geometry. In presence of the chevron exit, the conventional vortex ring issued through the circular nozzle exit, is replaced by a non-circular vortex ring with additional streamwise vortices. The mutual interaction between these structures prevents the axis-switching of the non-circular vortex ring during its convection. These streamwise vortices disappear convecting downstream and the vortex ring assumes a circular shape. Comparing the two configurations, the chevron exit generates a larger time-averaged streamwise velocity along the centreline but with lower turbulent kinetic energy intensity. Differences are also present between the notch and the apex planes of the chevron exit. In the notch plane, both the time-averaged axial velocity component profile in the spanwise direction and the shearlayer width are wider than in the apex plane. Furthermore, the presence of the streamwise vortices causes a flow motion towards the jet axis in the apex plane and an opposite motion in the notch plane.

Time-averaged Turbulent Flow Characteristics over a Highly Spatially Heterogeneous Gravel-Bed

The present study focuses on the time-averaged turbulence characteristics over a highly spatially-heterogeneous gravel-bed. The timeaveraged streamwise velocity, Reynolds shear and normal stresses, turbulent kinetic energy, higher-order moments of velocity fluctuations, length scales, and the turbulent bursting were measured over a gravel-bed with an array of larger gravels. It was observed that the turbulence characteristics do not vary significantly above the crest level of the array as compared to those below the array. The nondimensional streamwise velocity decreases considerably with a decrease in depth below the array. Below the array, the Reynolds shear stress (RSS) deviates from the gravity- law of RSS distributions. Turbulence intensities reduce below the crest level of the gravel-bed. The third-order moments of velocity fluctuations increase below the crest level of the gravel-bed and give a clear indication of sweeps as the predominating event which were further verified with the quadrant analysis plots. The turbulent length scales values change significantly below the crest level of the gravel-bed.

Porous media approach for RANS simulation of compound open-channel flows with submerged vegetated floodplains

The main goal of this study is the 3D numerical simulation of river flows with submerged vegetated floodplains. Since, vegetation layers are usually dense and present a large spatial heterogeneity they are here represented as a porous media. Standard semi-empirical relations drawn for porous beds packed with non-spherical particles are used to estimate the porous media parameters based on the averaged geometry of the vegetation elements. Thus, eliminating the uncertainty arising from a bulk drag coefficient approach and allowing the use of a coarser mesh. The free flow is described by Reynolds-averaged Navier–Stokes (RANS) equations, whereas the porous media flow is described by the volumetric-average of RANS equations. The volume-of-fluid method and an anisotropic explicit algebraic Reynolds stress model are used for free-surface and turbulence closure, respectively. The simulation approach is validated against results by other authors featuring vegetated flows in horizontal and rectangular open-channel. The computed results show that the time-averaged streamwise velocity and Reynolds shear stress vertical profiles are properly simulated. The validated approach was applied to simulate compound open-channel flows with submerged vegetated floodplains and compared with data obtained in an experimental facility. The results show that the proposed porous media approach is adequate to simulate flows with submerged vegetation on the floodplains.

自律制御ボート型ロボットの開発と河川流速計測の試み

The present study developed autonomous-mobile floating-robot using one-board microcomputer Arduino which could measure automatically mean velocity in an open-channel flow. Camera tracking system and PID control method could make the robot remain the position against main stream, and then time-averaged streamwise velocity was evaluated reasonably by a duty-ratio of screw propeller motor. Reliable laboratory experiments with electromagnetic velocimetry provided the calibration curve that connects the duty-ratio and mean current velocity. Furthermore, the present floating could be found to move successfully in not only the laboratory flume but also in natural creek.

Prediction of Scattered Broadband Shock-Associated Noise

A mathematical model is developed for the prediction of broadband shock-associated noise in the presence of an airframe body. Model arguments are dependent on the vector Green’s function of the linearized Euler equations, steady Reynolds-averaged Navier–Stokes solutions, and the two-point cross-correlation of the equivalent source. The equivalent source is dependent on steady Reynolds-averaged Navier–Stokes solutions of the jet flow that capture the nozzle and airframe geometry. Contours of the time-averaged streamwise velocity component and turbulent kinetic energy are examined with varying airframe position relative to the nozzle exit. Propagation effects are incorporated by approximating the vector Green’s function of the linearized Euler equations. This approximation involves the use of ray theory and an assumption that broadband shock-associated noise is relatively unaffected by the refraction of the jet shear layer. A nondimensional parameter is proposed that quantifies the changes of the broadband shock-associated noise source with varying jet operating condition and airframe position. Scattered broadband shock-associated noise possesses a second set of broadband lobes that are due to the effect of scattering. Presented predictions from an overexpanded jet demonstrate relatively good agreement compared to a wide variety of measurements.

Numerical study of the rounded corners effect on flow past a square cylinder

Purpose – The purpose of this paper is to numerically investigate the influence of corner radius on flow past a square cylinder at a Reynolds number 500. Design/methodology/approach – Six models were studied, for R/D=0 (square cylinder), 0.1, 0.2, 0.3, 0.4, and 0.5 (circular cylinder), where R is the corner radius and D is the characteristic dimension of the body. The transient two-dimensional (2D) laminar and large eddy simulations (LES) models were employed using finite volume code. The Strouhal number, mean drag coefficient (CD), and root mean square (RMS) value of lift coefficient (CL,RMS), for different R/D values, were computed and compared with experimental and other numerical results. Findings – The computational results showed good agreement with previously published results for a Reynolds number, Re=500. It was found that the corner effect on a square cylinder greatly influences the flow characteristics around the cylinder. Results indicate that, as the corner radius ratio, R/D, increases, the Strouhal number increases rapidly for R/D=0-0.2, and then gradually rises between R/D=0.2 and 0.5. The minimum values of the mean drag coefficient and the RMS value of lift coefficient were found around R/D=0.2, which is verified by the time averaged streamwise velocity deficit profile. Originality/value – On the basis of the numerical results, it is concluded that rounded corners on a square cylinder are useful in reducing the drag and lift forces generated behind a cylinder. Finally, it is suggested that with a rounded corner ratio of around R/D=0.2, the drag and oscillation of the cylinder can be greatly reduced, as compared to circular and square cylinders.

Influence of collective boulder array on the surrounding time-averaged and turbulent flow fields

Arrays of large immobile boulders, which are often encountered in steep mountain streams, affect the timing and magnitude of sediment transport events through their interactions with the approach flow. Despite their importance in the quantification of the bedload rate, the collective influence of a boulder array on the approach time-averaged and turbulent flow field has to date been overlooked. The overarching objective is, thus, to assess the collective effects of a boulder array on the time-averaged and turbulent flow fields surrounding an individual boulder within the array, placing particular emphasis on highlighting the bed shear stress spatial variability. The objective of this study is pursued by resolving and comparing the time-averaged and turbulent flow fields developing around a boulder, with and without an array of isolated boulders being present. The results show that the effects of an individual boulder on the time-averaged streamwise velocity and turbulence intensity were limited to the boulder’s immediate vicinity in the streamwise (x/dc < 2-3) and vertical (z/dc < 1) directions. Outside of the boulder’s immediate vicinity, the time-averaged streamwise velocity was found to be globally decelerated. This global deceleration was attributed to the form drag generated collectively by the boulder array. More importantly, the boulder array reduced the applied shear stress exerted on the individual boulders found within the array, by absorbing a portion of the total applied shear. Furthermore, the array was found to have a “homogenizing” effect on the near-bed turbulence thus significantly reducing the turbulence intensity in the near-bed region. The findings of this study suggest that the collective boulder array bears a portion of the total applied bed shear stress as form drag, hence reducing the available bed shear stress for transporting incoming mobile sediment. Thus, the effects of the boulder array should not be ignored in sediment transport predictions. These effects are encapsulated in this study by Equation (6).