Three-dimensional Velocity Field Research Articles

Experimental and computational investigation of confined laser-induced breakdown spectroscopy

This paper presents an experimental and computational study on laser-induced breakdown spectroscopy (LIBS) for both unconfined flat surface and confined cavity cases. An integrated LIBS system is employed to acquire the shockwave and plasma plume images. The computational model consists of the mass, momentum, and energy conservation equations, which are necessary to describe shockwave behaviors. The numerical predictions are validated against shadowgraphic images in terms of shockwave expansion and reflection. The three-dimensional (3D) shockwave morphology and velocity fields are displayed and discussed.

Combined effects of wave and current in free surface turbulent flow

The present work is focused on the evaluation of combined wave-current interaction through an experimental study carried out in a laboratory flume. The three-dimensional velocity field was measured by an acoustic Doppler velocimeter (ADV). The results highlight the modifications in the mean velocities, turbulence intensities, and Reynolds shear stress due to the superposition of waves of different frequencies on current-only flow. This study also investigates the dominant turbulent bursting event that contributes to the Reynolds shear stress. The results show that at regions near as well as far from the bed, the contributions to the total shear stress from ejection and sweep are dominant. The mean time interval of the occurrence of bursting events is obtained from the measurements of the fractional contributions to the total shear stress. As the frequency of the surface wave in unidirectional current is changed, change in turbulence statistics is recorded which may affect the local sediment mobility in the coastal region.

Three-dimensional inspiratory flow in a double bifurcation airway model

The flow in an idealized airway model is investigated for the steady inhalation case. The geometry consists of a symmetric planar double bifurcation that reflects the anatomical proportions of the human bronchial tree, and a wide range of physiologically relevant Reynolds numbers (Re = 100–5000) is considered. Using magnetic resonance velocimetry, we analyze the three-dimensional fields of velocity and vorticity, along with flow descriptors that characterize the longitudinal and lateral dispersion. In agreement with previous studies, the symmetry of the flow partitioning is broken even at the lower Reynolds numbers, and at the second bifurcation, the fluid favors the medial branches over the lateral ones. This trend reaches a plateau around Re = 2000, above which the turbulent inflow results in smoothed mean velocity gradients. This also reduces the streamwise momentum flux, which is a measure of the longitudinal dispersion by the mean flow. The classic Dean-type counter-rotating vortices are observed in the first-generation daughter branches as a result of the local curvature. In the granddaughter branches, however, the secondary flows are determined by the local curvature only for the lower flow regimes (Re ≤ 250), in which case the classic Dean mechanism prevails. At higher flow regimes, the field is instead dominated by streamwise vortices extending from the daughter into the medial granddaughter branches, where they rotate in the opposite direction with respect to Dean vortices. Circulation and secondary flow intensity show a similar trend as the momentum flux, increasing with Reynolds number up to Re = 2000 and then dropping due to turbulent dissipation of vorticity. The streamwise vortices interact both with each other and with the airway walls, and for Re > 500 they can become stronger in the medial granddaughter than in the upstream daughter branches. With respect to realistic airway models, the idealized geometry produces weaker secondary flows, suggesting that realistic anatomical features may generate more lateral dispersion than canonical symmetric models.

The effect of floodplain grass on the flow characteristics of meandering compound channels

Laboratory experiments were conducted in a large-scale meandering compound channel to investigate the effect of floodplain grass on the main flow field in the channel. Three-dimensional velocity fields, turbulences, and Reynolds shear stresses were measured along half a meander. The experiments revealed that flexible artificial grass planted on a floodplain can significantly reduce the conveyance capability of the entire channel. Two parallel stage-discharge curves increased with increasing flow depth. The additional resistance of the floodplain grass increased the streamwise velocity and conveyance in the main channel along a meander. An analysis of the generation mechanism of secondary flows in the main channel indicated that the secondary current consisted of an enhanced original secondary cell that was strengthened by the centrifugal force and a component of the upstream floodplain flow. The relative dominance of these two components in the secondary flows was primarily determined by the angle between the floodplain flow and the main channel ridge, and also the floodplain roughness. At the same flow depth, the secondary flow in cases with grass on the floodplain was generally stronger than that in the case of a smooth meander bend, although it was weaker near the middle cross-over section. Floodplain grass enhanced the intensity of the lateral turbulence above the bankfull level and significantly modified the turbulence structure, although it had a negligible effect on the vertical turbulence except at the bend entrance. Floodplain grass also affected the Reynolds shear stresses in the main channel, generating stronger lateral shear stresses at a low flow depth. In contrast, at a high flow depth, the distribution of the interface shear stresses changed entirely while its magnitude remained the same. When the floodplains were grassed, the vertical shear stress that was induced by secondary flows was greater at the apexes but reduced at the cross-over sections. These observations highlight the important effects of floodplain grass on the main channel flow.

Oscillatory flow in the human airways from the mouth through several bronchial generations

The time-varying flow is studied experimentally in an anatomically accurate model of the human airways from the mouth through several generations of bronchial branching. The airway geometry is obtained from the CT scan of a healthy adult male of normal height and build. The three-component, three-dimensional mean velocity field is obtained throughout the entire model using phase-locked Magnetic Resonance Velocimetry. A pulsatile pump drives a sinusoidal waveform (inhalation and exhalation) with frequency and stroke-length such that the mean trachea Reynolds number at peak inspiration is 4200 and the Womersley number is 7. Integral parameters are defined to quantify the degree of velocity profile non-uniformity (related to axial dispersion) and secondary flow strength (lateral dispersion). It is found that the extrathoracic airways significantly modify the tracheal flow and that the flow at the first bifurcation is highly asymmetric. The effect of flow oscillation is to produce time dependent flow features which are asymmetric with respect to the acceleration and deceleration periods surrounding peak inhalation and exhalation. This is most pronounced in regions of separation and on the secondary flow structure, which are sensitive to local attributes of the real anatomy. This is reflected in the integral parameters, which behave non-monotonically between successive bronchial generations. In general, the measured oscillatory flow in a realistic anatomy confirms many trends derived from idealized models but also possesses qualitatively different large scale flow structures as compared to idealized representations of the upper airways.

Modal energy flow analysis of a highly modulated wake behind a wall-mounted pyramid

We experimentally investigate the highly modulated turbulent wake behind a wall-mounted square-base pyramid protruding through the boundary layer. We present the first modal energy flow analysis of a time-resolved three-dimensional velocity field from experimental particle image velocimetry data. The underlying low-order representation is optimized for resolving the base-flow variation as well as the first and second harmonics associated with vortex shedding – generalizing the triple decomposition of Reynolds & Hussain (J. Fluid Mech., vol. 54, 1972, pp. 263–288). This analysis comprises not only a detailed modal balance of turbulent kinetic energy as pioneered by Rempfer & Fasel (J. Fluid Mech., vol. 275, 1994, pp. 257–283) for proper orthogonal decomposition (POD) models, but also the companion energy balance of the mean flow. The experimental results vividly demonstrate how constitutive elements of mean-field theory (Stuart, J. Fluid Mech., vol. 4, 1958, pp. 1–21) near laminar Hopf bifurcations remain strongly pronounced in a turbulent wake characterized by highly modulated, quasi-periodic shedding. The study emphasizes, for instance, the stabilizing role of mean-field manifolds, as explored in the pioneering POD model of Aubry et al. (J. Fluid Mech., vol. 192, 1988, pp. 115–173). The presented low-order representation of the flow and modal energy flow analyses may provide important insights and reference data for computational turbulence modelling, e.g. unsteady Reynolds-averaged Navier–Stokes simulations.

Analysis of characteristic wake flow modes on a generic transonic backward-facing step configuration

To gain insight into characteristic wake flow modes, which among others are responsible for asymmetrical loads on the engine extensions of space launchers at transonic speeds, combined experimental and numerical investigations of a turbulent wake flow are performed at Ma∞=0.8 and Reh=1.8⋅105. The experiments are conducted at the Bundeswehr University Munich using planar PIV and wall pressure measurements, while the numerical investigations are performed by the Institute of Aerodynamics at RWTH Aachen University using a zonal RANS–LES approach and dynamic mode decomposition (DMD). The analysis is done on a planar space launcher configuration that consists of a backward-facing step with a long shock-free forebody avoiding undesired shock-boundary-layer interactions upstream of the analyzed wake flow region. The investigated wake flow is characterized by a highly unsteady behavior of the shear layer shedding from the forebody and subsequently reattaching onto the splitter plate. The strong variation of the reattachment positions in the spanwise and the streamwise direction leads to pronounced wall pressure oscillations and consequently, structural loads. By means of a classical statistical analysis, i.e., spatial and temporal Fourier transforms and two-point correlations of experimental and numerical data, one spatial coherent scale with a spanwise wavelength of λz≈2h and two characteristic frequencies of Srh≈0.01 and Srh≈0.07 have been obtained for the investigated wake flow problem. To clarify the coherent fluid motion dominating the detected spatio-temporal behavior, a DMD algorithm is applied to the time-resolved three-dimensional streamwise velocity field. The frequencies of the first two extracted stable sparsity DMD modes closely coincide with the characteristic peaks in the wall pressure spectra. The analysis of the three-dimensional shape of the extracted DMD modes reveals that the detected wake flow behavior is caused by a pronounced low-frequency cross-pumping motion of the recirculation region and a high-frequency cross-flapping motion of the shear layer, respectively. Both wake flow modes feature a pronounced nearly-periodical variation in the spanwise direction with an approximate spanwise wavelength of two step heights. Thus, the extracted underlying wake modes clearly explain the occurrence of corresponding peaks in the wall pressure spectra, the periodical formation of wedge-shaped reattachment regions on the splitter plate, and the detection of finger-like structures in the PIV experiments.

Vertical and horizontal displacements of Los Angeles from InSAR and GPS time series analysis: Resolving tectonic and anthropogenic motions

We resolve the complex ground deformation associated with tectonic and anthropogenic activities in Los Angeles, California by combining Interferometric Synthetic Aperture Radar (InSAR) and Global Positioning System (GPS) measurements acquired from 2003 to 2007. The three-dimensional (3-D) cumulative displacement velocity field is first derived based on Weighted Least Squares (WLS) InSAR analysis of both ascending and descending ENVISAT ASAR data aided by GPS measurements at 54 Southern California Integrated GPS Network (SCIGN) stations. Clear subsidence can be observed from the results in the Pomona and San Gabriel areas, respectively, while uplift in the Santa Fe Springs. The displacements are mainly due to the extraction and injection of groundwater or oil. The northwestern horizontal displacement detected also suggests that the area had experienced interseismic strain accumulation related to the San Andreas Fault. The varying northeastern displacement magnitudes indicate increasing displacement accumulation away from the San Andreas Fault. The results are validated with GPS measurements at 15 independent stations, with the root mean squares (RMS) residuals of the discrepancies being 2.4, 1.5 and 0.9mm/yr in vertical, northern and eastern directions, respectively. The seasonal displacement in Santa Ana Basin due to anthropogenic activities is examined with eight pairs of ascending and descending InSAR LOS displacement estimations selected from the WLS InSAR solution. It is clear that almost the entire Santa Ana Basin experiences significant seasonal vertical displacement with the maximal displacement reaching 43mm. The Newport-Inglewood Fault forms a sharp boundary of the seasonal vertical displacement field. In addition, seasonal oscillations of 10–20mm in east direction are detected near the margin of the Santa Ana Basin, across the Newport-Inglewood Fault. Strong correlations are found between the groundwater levels and the seasonal displacements at three wells, yielding an average elastic skeletal storage coefficients of 5.9×10−4. This indicates that the Santa Ana basin is dominated by the elastic deformation.

An Experimental Study of the Rotational Effects on Separated Turbulent Flow During Stall Delay

Three-dimensional velocity fields were measured using tomographic particle image velocimetry (Tomo-PIV) on a model of the blade of a small-scale horizontal axis wind turbine (HAWT) to study the effects of rotation on separated turbulent flows during stall delay at a global tip speed ratio (TSR) of 3 and a Reynolds number of 4800. The flow fields on a static airfoil were also measured at a similar angle-of-attack (AOA) and Reynolds number for comparison. It was observed that the blade’s rotation in the streamwise direction significantly affected both the mean flow and the turbulence statistics over the suction surface. The mean velocity fields revealed that, different from the airfoil flow at large AOA, the recirculation region with reversed flow did not exist on the suction surface of the blade and the flow was rather attached. Mean spanwise flow from blade’s root to its tip was also generated by the rotation. The mean vorticity vector of the blade flow was found to be tilted in the rotational direction of the blade, as well as in the wall-normal direction. Of particular effects of the rotation on Reynolds stresses were the enhancement of 〈w 2〉 and the creation of strong 〈v w〉. The production of Reynolds stresses was also affected by blade’s rotation directly through the rotational production terms and indirectly by dramatically changing the fluctuating velocity fields. The distribution of enstrophy was observed to be modified by rotation, too.

Tomographic PIV behind a prosthetic heart valve

The instantaneous three-dimensional velocity field past a bioprosthetic heart valve was measured using tomographic particle image velocimetry. Two digital cameras were used together with a mirror setup to record PIV images from four different angles. Measurements were conducted in a transparent silicone phantom with a simplified geometry of the aortic root. The refraction indices of the silicone phantom and the working fluid were matched to minimize optical distortion from the flow field to the cameras. The silicone phantom of the aorta was integrated in a flow loop driven by a piston pump. Measurements were conducted for steady and pulsatile flow conditions. Results of the instantaneous, ensemble and phase-averaged flow field are presented. The three-dimensional velocity field reveals a flow topology, which can be related to features of the aortic valve prosthesis.

Spatially-averaged turbulent flow over cubical roughness in wave-current co-existing environment

The paper describes an experimental study carried out in a laboratory flume to investigate the interaction of surface-wave with unidirectional current over cube mounted rough-bed (3D flow). The cubes were made of wood and were positioned at the channel-bed with different spacing. The spacing was chosen to represent different roughness types under different spacing conditions as: isolated roughness flow, wake interference flow and skimming flow. The three-dimensional velocity field was measured by an acoustic Doppler velocimetre (ADV). The study particularly focused on the changes induced in the spatially-averaged velocity profiles, turbulence intensities and Reynolds shear stress due to the superposition of waves of different frequencies on current-induced flow. Spatially-averaged velocity profiles show two distinct distributions: linear or exponential distribution below the roughness crest and logarithmic distribution above the roughness crest. It is found that the shift of the velocity profile depends strongly on the spacing of roughness elements. The profile of stream-wise turbulence intensity changes significantly below the roughness crest due to presence of surface wave, whereas bottom-normal spatially-averaged turbulence intensity does not change much with increase in wave frequency. Within the roughness crest near the bottom, the form-induced shear stress changes significantly from negative to positive value with change in roughness spacing, but is not affected much due to the presence of surface-wave.

Present-Day 3D Velocity Field of Eastern North America Based on Continuous GPS Observations

The Saint Lawrence River valley in eastern Canada was studied using observations of continuously operating GPS (CGPS) stations. The area is one of the most seismically active regions in eastern North America characterized by many earthquakes, which is also subject to an ongoing glacial isostatic adjustment. We present the current three-dimensional velocity field of eastern North America obtained from more than 14 years (9 years on average) of data at 112 CGPS stations. Bernese GNSS and GITSA software were used for CGPS data processing and position time series analysis, respectively. The results show the counterclockwise rotation of the North American plate in the No-Net-Rotation model with the average of 16.8 ± 0.7 mm/year constrained to ITRF 2008. We also present an ongoing uplift model for the study region based on the present-day CGPS observations. The model shows uplift all over eastern Canada with the maximum rate of 13.7 ± 1.2 mm/year and subsidence to the south mainly over northern USA with a typical rate of −1 to −2 mm/year and the minimum value of −2.7 ± 1.4 mm/year. We compared our model with the rate of radial displacements from the ICE-5G model. Both models agree within 0.02 mm/year at the best stations; however, our model shows a systematic spatial tilt compared to ICE-5G. The misfits between two models amount to the maximum relative subsidence of −6.1 ± 1.1 mm/year to the east and maximum relative uplift of 5.9 ± 2.7 mm/year to the west. The intraplate horizontal velocities are radially outward from the centers of maximum uplift and are inward to the centers of maximum subsidence with the typical velocity of 1–1.6 ± 0.4 mm/year that is in agreement with the ICE-5G model to the first order.

Temporal scales for nearshore hits of current-driven pollution in the Gulf of Finland

Lagrangian trajectories of water parcels reconstructed using the TRACMASS model from three-dimensional velocity fields by the RCO model for 1965–2004 are used to analyse the temporal scales and the probability for the hits to the nearshore by pollution originating from a major fairway in the Gulf of Finland and transported by surface currents. Increasing the simulation length from 10 to 20days induces a linear increase in particle age, but the pattern of nearshore hits remains the same. A reasonable benefit can be reached by relatively small shifts of certain parts of the present fairway in a few locations. The overall probabilities do not reveal any trend for 1965–2004. The largest changes in the nearshore hits are revealed for the proportion of hits to the opposite nearshore areas. This feature probably reflects an abrupt turn of the geostrophic air-flow over the southern Baltic Sea by ~40° since 1987.

A tomographic particle image velocimetry investigation of the flow development over dual step cylinders

This experimental study focuses on the near wake development of a dual step cylinder geometry consisting of a long base cylinder of diameter d to which a larger diameter (D) cylinder of length L is attached coaxially at mid-span. The experiments cover a range of Reynolds numbers, 2000 ≤ ReD ≤ 5000, diameter ratios, 1.33 ≤ D/d ≤ 2.0 and large cylinder aspect ratios, 0.5 ≤ L/D ≤ 5 using Tomographic particle image velocimetry. Distinct changes in wake topology are observed varying the above parameters. Supporting previous experimental studies on the same geometry involving flow visualization and planar measurements, four distinct flow regimes are identified to which a distinct three-dimensional wake topology can be associated. The vortex-dominated wake dynamical behaviour is investigated with Proper Orthogonal Decomposition (POD) and conditional averaging of three-dimensional velocity fields is used to exemplify the different shedding regimes. The conditionally averaged flow fields are shown to quantitatively resolve flow features equivalent to those obtained from a reduced order model consisting of the first ten to twenty POD modes, identifying the dominant vortex shedding cells and their interactions.

Experimental study of vorticity-strain rate interaction in turbulent partially premixed jet flames using tomographic particle image velocimetry

In turbulent flows, the interaction between vorticity, ω, and strain rate, s, is considered a primary mechanism for the transfer of energy from large to small scales through vortex stretching. The ω-s coupling in turbulent jet flames is investigated using tomographic particle image velocimetry (TPIV). TPIV provides a direct measurement of the three-dimensional velocity field from which ω and s are determined. The effects of combustion and mean shear on the ω-s interaction are investigated in turbulent partially premixed methane/air jet flames with high and low probabilities of localized extinction as well as in a non-reacting isothermal air jet with Reynolds number of approximately 13 000. Results show that combustion causes structures of high vorticity and strain rate to agglomerate in highly correlated, elongated layers that span the height of the probe volume. In the non-reacting jet, these structures have a more varied morphology, greater fragmentation, and are not as well correlated. The enhanced spatiotemporal correlation of vorticity and strain rate in the stable flame results in stronger ω-s interaction characterized by increased enstrophy and strain-rate production rates via vortex stretching and straining, respectively. The probability of preferential local alignment between ω and the eigenvector of the intermediate principal strain rate, s2, which is intrinsic to the ω-s coupling in turbulent flows, is larger in the flames and increases with the flame stability. The larger mean shear in the flame imposes a preferential orientation of ω and s2 tangential to the shear layer. The extensive and compressive principal strain rates, s1 and s3, respectively, are preferentially oriented at approximately 45° with respect to the jet axis. The production rates of strain and vorticity tend to be dominated by instances in which ω is parallel to the s1¯-s2¯ plane and orthogonal to s3¯.

上流境界層内に設置した噴流の設置間隔がキャビティ音に及ぼす影響

This work presents an experimental investigation for effects of the spanwise spacing of small-jets placed in the approaching boundary layer on cavity tone. The sound and velocity fields of a flow around a cavity were measured with a low noise wind tunnel. The freestream velocity was changed from U0 = 10 - 45 m/s. The depth-to-length ratio of the cavity was D/L = 0.5. The upstream boundary layer is laminar, and the boundary layer thickness is δ/L = 0.065 at U0 = 30 m/s. The effects of the jets from the wall in the upstream boundary layer with various spanwise spacing p/L = 0.1, 0.25, 0.4, 0.5, 0.6 and 1.0 on the cavity tone were investigated. The jet velocity was Uj = 3.0 m/s. The results show the cavity tone can be reduced by control of blowing jets. The control effect on the cavity tone depends on the spanwise spacing and velocity of blowing jets. In this experiment, the most effective spanwise spacing of jets for noise reduction was p/L = 0.4 - 0.6. The three-dimensional spanwise velocity field was induced by blowing jets. As a result, the spanwise coherence of the vortices inducing intense sound became weak, and the radiating sound became weak.

変分原理を用いた強制対流熱伝達の最適化

Using a variational method, the incompressible steady velocity fields are numerically determined so that they can optimize heat transfer in plane Couette flow. The velocity field is supposed to be periodic in the wall-parallel directions. The Prandtl number is set to unity, and the consistent boundary conditions are imposed on the velocity and temperature fields. The excess of the wall heat flux over the total energy dissipation is taken as an objective functional. At high Reynolds number, the optimal states exhibit three-dimensional velocity fields with large heat flux and small energy dissipation in comparison to a turbulent state. In this report, we discuss the roles of the three-dimensional vortex structures in the heat flux enhancement, and show the characteristic hierarchical vortex structure at high Reynolds number.

Turbulence Statistics of Wave-Current Flow over a Submerged Cube

AbstractThis paper describes an experimental study carried out in a laboratory flume to investigate the interaction of a surface wave with a unidirectional current over a submerged cubic obstacle. The three-dimensional velocity field was measured using an acoustic Doppler velocimeter (ADV). The results highlight the changes induced in the mean velocity profile, turbulent intensity, and Reynolds shear stress in a plane of symmetry from the superposition of surface waves of different frequencies. Modifications in the mean velocities, turbulence intensities, and Reynolds shear stresses with respect to the flat surface case, in the vicinity of the cube, are explored. This study also investigates the dominant turbulent bursting event that contributes to the Reynolds shear stress in the near-bed flow influenced by the cube. The results show that near the boundary, the contributions to the total shear stress from ejection and sweep are dominant. However, away from the boundary, the outward and inward interaction...

Influence of the Geometric Parameter on the Regimes of Natural Convection and Thermal Surface Radiation in a Closed Parallelepiped

We have performed a numerical analysis of the stationary regimes of thermogravitational convection and thermal surface radiation in a closed differentially heated parallelepiped. The mathematical model formulated in dimensionless natural velocity–pressure–temperature variables was realized numerically in the control volume approach. Analysis of the radiative heat exchange was carried out on the basis of the surface radiation approach with the use of the balance method in the Polyak variant. We have obtained three-dimensional temperature and velocity fields, as well as dependences for the mean Nusselt number reflecting the influence of the geometric parameter, the Rayleigh number, and the reduced emissive factor of the walls on the flow structure and the heat transfer.

An Experimental Study on the Effect of a Hydraulic Structure on the Three-dimensional Flow in a Meandering Channel

The objective of this study is to examine the three-dimensional turbulent flows occurring in the meandering channel with presence of a groyne. A series of laboratory experiments are carried out in a meandering channel with trapezoidal cross sections. The channel is a 24.4 m long, 1.5 m wide, and the bottom slope in the longitudinal direction is 0.02. Two cases with and without the groyne are considered in the experiment. Three-dimensional velocity fields are measured using an acoustic Doppler velocimetry (ADV) at approximately sixty locations in each cross section. The measured velocity fields are averaged in time, and the time-averaged flow revealed that the mean velocity magnitude along the outer bank of the channel was reduced significantly and the direction of the primary flow was directed toward the center of the channel due to the presence of the groyne.