Cross-stream Plane Research Articles

Influence of Coherent Vortex Rolls on Particle Dynamics in Unstably Stratified Turbulent Channel Flows

This work investigates the dynamics of heavy particles dispersed in turbulent channel flows under unstable thermal stratification conditions using point-particle direct numerical simulations (PP-DNS), to quantify the influence of large-scale coherent vortex rolls, arising from the combined effects of shear and buoyancy, on the spatial distribution and preferential sampling behavior of inertial particles. We examined three particle Stokes numbers (St+=0.6,60,120) and two friction Richardson numbers, Riτ=0.272 and Riτ=27.2, which exemplify the regimes below and above the critical condition for vortex roll formation, respectively. The results indicate that the flow reorganization into large-scale longitudinal vortices significantly alters the topological features of small scales in the near-wall region impinged by the thermal plumes, resulting in a prevalence of vorticity-dominated topologies. The interplay between this phenomenon and the tendency of particles to preferentially sample strain-dominated topologies leads to a distinctive asymmetric particle distribution in the near-wall planes. Inertial particles markedly accumulate in the strain-dominated regions where the coherent thermal plumes emerge from the walls, while avoiding the vorticity-dominated impingement zones. This peculiar particle response to the vortex rolls is most pronounced when the particle response time matches the characteristic timescale of the large-scale coherent motions in the cross-stream planes.

Experimental study of the impact of blade-tip mounted rotors on the X-Rotor vertical-axis wind turbine

The Horizon 2020 European Commission-funded project - X-ROTOR - proposes a radical rethink of the traditional vertical-axis wind turbine geometry. The X-Rotor vertical axis wind turbine relies on blade-tip mounted rotors, referred to as secondary rotors, for power generation and takeoff. This study examines the aerodynamic effects of secondary rotors on a scaled X-Rotor model’s loading in an open-jet wind tunnel. Particle image velocimetry measurements are taken at two cross-stream planes within the volume of rotation of a scaled turbine model at two phase-locked positions. The measurements are compared with cases without secondary rotors present to understand the local impact of the blade-tip mounted devices on the wake and vortex strengths. The results indicate an accelerated turbulent diffusion of the trailing tip-vortex of the X-Rotor, and the subsequent local in-plane velocity gradients induced by the trailing tip-vortex are diminished. These insights and experimental database contribute to the development and validation of numerical models of the X-Rotor with blade-tip mounted rotors.

Turbulent/non-turbulent interface of the turbulent boundary-layer over a spanwise-heterogeneous converging/diverging riblets rough wall

Turbulent boundary layer (TBL) flows over various spanwise-heterogeneous rough walls exhibit spanwise variations in turbulence statistics, implying modifications of the turbulent/non-turbulent interface (T/NTI). In this work, based on the datasets of TBL flows over a spanwise-alternating converging/diverging riblets wall, we investigate behavior of T/NTI, properties of ‘bubble’ and ‘drop’ that are associated with T/NTI, as well as connections between T/NTI and large-scale structures within the boundary layer. The flow datasets were obtained through stereoscopic particle image velocimetry measurements on the cross-stream plane of the TBL flow at a Reynolds number Reθ = 1.3 × 104. The T/NTI and associated ‘bubble’ and ‘drop’ are identified using a kinetic energy criterion, while the large-scale structures by a streak detection algorithm. It is observed that the T/NTI height and the occurrence of ‘bubble’ and ‘drop’ display a spanwise-heterogeneous feature, with lower T/NTI height and higher occurrence probability of ‘bubble’ over the diverging section of riblets than over the converging section of riblets. Results of conditional average show that the ‘bubble’ is associated with positive streamwise fluctuations, common-down flow and a pair of counter-rotating streamwise vortices, while the ‘drop’ with negative streamwise fluctuation, common-up flow and a pair of counter-rotating streamwise vortices. Probability density functions of the ‘bubble’ size and fractal dimensions of the T/NTI are found similar for both the riblets and smooth wall flows. Furthermore, it is observed that the large-scale low-speed structures occurring preferentially over the converging section of riblets play a role in elevating T/NTI while the large-scale high-speed ones over the diverging section of riblets lowering down T/NTI.

Manipulation of the flow induced by afterbody vortices using sweeping jets

In this study, the impact of oscillatory fluids on the flow induced by afterbody vortices, resembling those of a high-speed train, was investigated in wind tunnel experiments. The afterbody vortices were generated by a cylindrical model with a slanted base at Reynolds numbers ranging from 87 000 to 200 000. A fluidic oscillator was utilized to produce sweeping jets (SWJ) acting on the vortical flow, which is similar to those observed over the rear of a typical high-speed train. Our particle image velocimetry measurements on cross-stream planes reveal that the oscillatory flow produced by the sweeping jet significantly reduces the magnitude of induced flow, resulting in a 17.7% and 15.9% decrease in the total velocity and horizontal slipstream, respectively, at a velocity ratio of 6.4. This is attributed to the injection of the kinetic energy of sweeping jets into the core of the afterbody vortices, which leads to a higher dispersion of vorticity and incoherent energy distributions. The iso-surfaces of resultant velocities reveal that the fluid injection by the sweeping jet increased local velocities, resulting in a spanwise shrinkage of the induced velocity structure downstream. The study demonstrates the capability of sweeping jets in decreasing induced flow by manipulating afterbody vortices and their potential for reducing the slipstream produced by high-speed trains at the tracksides.

Review of research on streamwise corner boundary layer

This paper is a brief review of recent research on the streamwise corner boundary layer as it affects the component performance evaluation of both the theoretical and practical aircraft design. Typical examples include aircraft wing–body junction, rectangular air intakes, and turbine-hub flow. The paper addresses the questions of what we know and do not know about the streamwise corner boundary layer. Streamwise corner flows are characterized by the presence of secondary flows in the cross-stream planes, which are driven by the normal and secondary-shear components of the Reynolds stress tensor. Extensive studies of analysis for Prandtl's second kind of secondary flows have promoted the understanding of characteristics and formation of streamwise corner boundary layer. However, compared to the flat plate boundary layer, the research on the streamwise corner boundary layer is still far from enough, especially in the similarity solution, the instability, and transition mechanism. In recent years, a significant progress has been achieved in the study on the streamwise corner boundary layer in turbulent flow through direct numerical simulation and stress−ω Reynolds stress model.

Supersonic Flow Separation in Front of a Forward-Facing Step

A detailed experimental study has been done to understand the Shock Wave-Boundary Layer Interaction (SWBLI) over a Forward-Facing Step (FFS) of height (h) in a Mach 2.5 flow using Particle Image Velocimetry (PIV). In addition to PIV, unsteady wall pressure measurements, high-speed schlieren and surface oil flow visualization have been studied. Flow separation using surface oil flow visualization was observed around 4.1 step heights upstream of the step face. Shock motions extracted from the schlieren image sequence shows a peak frequency of approximately 1000 Hz, which corresponds to the dominant (low) frequency at which the shock oscillates; this being two orders in magnitude smaller than the characteristics frequency of the incoming boundary layer. Also, spectra of the unsteady wall pressure data shows that the shock oscillates with a peak frequency of approximately 1000 Hz. The effect of shock location (xs) from instantaneous velocity PIV fields in the cross-stream plane due to an upstream influence parameter (line-averaged boundary layer streamwise velocity, ul) and a downstream parameter (separation bubble or recirculation region area, A) is studied. Instantaneous cross-stream PIV measurements show significant variations of the separation shock location, incoming boundary layer velocity fluctuations and recirculation region. Besides, conditionally averaged velocity field based on incoming boundary layer streamwise velocity fluctuations and separation bubble area in the cross-stream plane shows that the shock location (xs) is correlated to the size of the separation bubble and not to boundary layer velocity fluctuations.

Secondary flows in turbulent boundary layers developing over truncated cone surfaces

The evolution of a turbulent boundary layer over truncated cones in staggered and random configurations is investigated using direct numerical simulations. Three random planform densities and one staggered, which closely match the configuration in the recent experiment by Womack et al. (J. Fluid Mech., vol. 933, 2022, A38), are considered. The experiment is also used to validate the present computations which are found to reproduce the flow measurements very accurately. Spanwise heterogeneity in the mean streamwise velocity in the form of high and low momentum pathways was detected in both the staggered and random arrangements, indicating the presence of secondary flows in the cross-stream plane. The momentum pathways found in the staggered arrangements are much weaker and confined near the roughness crest, while in the random arrangements they approach the edge of the boundary layer. We found a clear correlation of the momentum pathways with the leading edge of the roughness, while the correlation to the local topography was very weak for all the cases studied. Vorticity transport analysis is also employed to explore the nature of the secondary flows formed, highlighting the strong presence of vortex stretching near the leading edge of the roughness. The impact of the momentum pathways on the flow statistics, especially in the random arrangements, is demonstrated by the breakdown of outer-layer similarity.

On the role of transverse motion in pseudo-steady gravity currents

Flow in the body of gravity currents is typically assumed to be statistically two-dimensional, and cross-stream flow is often neglected (Simpson 1997; Meiburg et al. 2015). Here, we assess the validity of such assumptions using Shake-the-Box particle tracking velocimetry measurements of experimental gravity current flows. The resulting instantaneous, volumetric, whole-field velocity measurements indicate that cross-stream and vertical velocities (and velocity fluctuations) are equivalent in magnitude and thus are key to energy distribution and dissipation within the flow. Further, the presented data highlight the limitations of basing conclusions regarding body structure on a single cross-stream plane (particularly if that plane is central). Spectral analysis and dynamic mode decomposition of the fully three-dimensional, volumetric velocity data suggests internal waves within the current body that are associated with coherent three-dimensional motions in higher Reynolds number flows. Additionally, a potential critical layer at the height of the downstream velocity maximum is identified.

Unsupervised deep learning of spatial organizations of coherent structures in a turbulent channel flow

We examined the capability of an unsupervised deep learning network to capture the spatial organizations of large-scale structures in a cross-stream plane of a fully developed turbulent channel flow at Reτ≈180. For this purpose, a generative adversarial network (GAN) is trained using the instantaneous flow fields in the cross-stream plane obtained by a direct numerical simulation (DNS) to generate similar flow fields. Then, these flow fields are examined by focusing on the turbulent statistics and the spatial organizations of coherent structures. We extracted the intense regions of the streamwise velocity fluctuations (u) and the vortical structures in the cross-stream plane. Comparing the DNS and GAN flow fields, it is revealed that the network not only presents the one-point and two-point statistics quite accurately but also successfully predicts the structural characteristics hidden in the training dataset. We further explored the meandering motions of large-scale u structures by measuring their waviness in the cross-stream plane. It is shown that as the size of the u structures increases, they exhibit more aggressive waviness behavior which in turn increases the average number of vortical structures surrounding the low-momentum structures. The success of GAN in this study suggests its potential to predict similar information at a high Reynolds number and, thus, be utilized as an inflow turbulence generator to provide instantaneous boundary conditions for more complicated problems, such as turbulent boundary layers. This has the potential to greatly reduce the computational costs of DNS related to a required large computational domain at high Reynolds numbers.

The effects of flame orientation and mean shear on flame stretch in highly turbulent premixed flames using DNS

In the present work, direct numerical simulation data of a laboratory-scale high Karlovitz number (Ka) jet flame with a strong mean shear and a high Ka freely propagating planar flame without a mean shear were analyzed to explore the influence of flame orientation and mean shear on flame stretch in highly turbulent flames. Three flame fronts, i.e. the leading, in-plane and trailing front, were identified for the jet flame. The leading and trailing front is defined as the flame normal being facing the downstream and upstream regions, respectively, while the flame normal of the in-plane front is near the cross-stream plane. It was found that the flame stretch is positive in the leading front, and is negative in the trailing front of the jet flame. As for the planar flame, the statistics of the flame stretch are similar in various flame fronts. The components of the flame stretch are analyzed. The normal vector of the leading front is aligned with the most compressive strain rate in the jet flame, resulting in a large tangential strain rate, while the tangential strain rate of the trailing front is significantly lower. In contrast, the alignment characteristics and the strain rates of the planar flame are not sensitive to the flame orientation. The value of the stretch due to curved propagating flame front is large and negative in the trailing front of the jet flame due to the negative correlation between the displacement velocity and curvature, which is weak in the leading and in-plane front. The correlation between the displacement velocity and curvature is, however, similar in various fronts of the planar flame. Overall, it is suggested that the effects of flame orientation and mean shear should be accounted for in understanding turbulence-flame interactions in practical combustion devices.

Numerical Study of Different Models for Turbulent Flow in 90° Pipe Bend

The investigation of the turbulence statistics for single-phase turbulent flow around pipe bends have mainly been experimental. Considering the cost-effectiveness of the numerical computational fluid dynamics (CFD) compared to experimental measurement, this work aims to study the accuracies of different CFD models, and establish their functionality and limitations. This paper investigates the capabilities of different numerical turbulence models and spatial discretization schemes for CFD analysis of a pipe bend. The pipe has a curvature radius which is seven times the inside diameter and a Reynolds number of 34132. The numerical modelling was developed on the commercial CFD software, ANSYS Fluent, evaluating the following viscous models:,, Spalart-Allmaras and Reynolds Stress Models. The streamwise velocities of the flow at cross-stream planes along the bend at 45º and 75º were computed for each of the turbulence models under different discretization schemes. The numerical results were compared against the existing experimental data for streamwise velocity, in order to investigate the accuracies of the different models. The results showed that the realizable K-E and Spalart-Allmaras models exhibited the best agreements with the experimental measurements, with respective average errors of 3.83% and 3.27% under the first-order spatial discretization scheme. It was also observed that the velocity profiles obtained through the K-W models (Standard and BSL), Transition SST and Reynolds Stress models exhibited better correlation with the experimental velocity profiles within the velocity transition region.

Development of a curled wake of a yawed wind turbine under turbulent and sheared inflow

Abstract. Redirecting the wake from an upstream wind turbine by yawing its rotor can reduce the negative impact of the wake on a downstream turbine. The present research investigated wind turbine wake behaviour for three yaw angles [-30,0,30∘] at different inflow turbulence levels and shear profiles under controlled conditions. Experiments were conducted using a model wind turbine with 0.6 m diameter (D) in a large wind tunnel. A short-range lidar WindScanner was used to map the wake with high spatial and temporal resolution in vertical, cross-stream planes at different downstream locations and in a horizontal plane at hub height. The lidar WindScanner enabled fast measurements at multiple locations in comparison to the standard hot-wire measurements. The flow structures and the energy dissipation rate of the wake were measured from 1 up to 10 D, and for one inflow case up to 16 D, downstream of the turbine rotor. A strong dependency of the wake characteristics on both the yaw angle and the inflow conditions was observed. In addition, the curled wake that develops under yaw misalignment due to the counter-rotating vortex pair was more pronounced with a boundary layer (sheared) inflow condition than for uniform inflow with different turbulence levels. Furthermore, the lidar velocity data and calculated quantities such as the energy dissipation rate compared favourably with hot-wire data from previous experiments with a similar inflow condition and wind turbine model in the same facility, lending credibility to the measurement technique and methodology used here. The results of this measurement campaign provided a deeper understanding of the development of the wake for different yaw angles and inflow conditions which can help improve wake models.

Pressure-informed velocity estimation in a subsonic jet

This work aims to estimate the time-resolved velocity field that is directly associated with pressure fluctuations in a subsonic round jet. Synchronous measurements of the velocity field and in-flow pressure fluctuations were performed on streamwise and cross-stream measurement planes. The spectral linear stochastic estimation and neural network architectures were implemented to estimate time-resolved velocity fields. In all cases, pressure fluctuations inside the flow are found to be connected to the streamwise convection of large-scale coherent structures in the flow, and the unique advantage of the bidirectional long-short-time-memory (LSTM) method is also reported in this work.

Direct numerical simulation of turbulent duct flow with inclined or V-shaped ribs mounted on one wall

In this research, highly disturbed turbulent flow of distinct three-dimensional characteristics in a square duct with inclined or V-shaped ribs mounted on one wall is investigated using direct numerical simulation. The turbulence field is highly sensitive to not only the rib geometry but also the boundary layers developed over the side and top walls. In a cross-stream plane secondary flows appear as large longitudinal vortices in both inclined and V-shaped rib cases due to the confinement of four sidewalls of the square duct. However, owing to the difference in the pattern of cross-stream secondary flow motions, the flow physics is significantly different in these two ribbed duct cases. It is observed that the mean flow structures in the cross-stream directions are asymmetrical in the inclined rib case but symmetrical in the V-shaped rib case, causing substantial differences in the momentum transfer across the spanwise direction. The impacts of rib geometry on near-wall turbulence structures are investigated using vortex identifiers, joint probability density functions between the streamwise and vertical velocity fluctuations, statistical moments of different orders, spatial two-point autocorrelations and velocity spectra. It is found that near the leeward and windward rib faces, the mean inclination angle of turbulence structures in the V-shaped rib case is greater than that of the inclined rib case, which subsequently enhances momentum transport between the ribbed bottom wall and the smooth top wall.

Direct numerical simulation of turbulent elliptical pipe flow under system rotation about the major axis

The effect of Coriolis forces on the turbulent flow in an elliptical pipe subjected to spanwise rotation has been studied using direct numerical simulations (DNS). In response to the system rotation, large-scale secondary flows appear in the cross-stream plane as a pair of counter-rotating vortices, which significantly impact the turbulence statistics and structures of the flow. The characteristics of the turbulence field is investigated in both physical and spectral spaces through analyses of the first- and second-order statistical moments, as well as the budget balance of the Reynolds stress transport equation and coherent flow structures.

Energetic structures in the turbulent boundary layer over a spanwise-heterogeneous converging/diverging riblets wall

Time-invariant (or mean) secondary flows, in terms of large-scale counter-rotating roll modes filling boundary layer, have been observed in turbulent boundary layer (TBL) flows over various spanwise-heterogeneous rough walls. Recent studies show that these mean secondary flows are inherently connected with instantaneous large-scale structures in TBL flows over such spanwise-heterogeneous rough walls. In this work, the technique of proper orthogonal decomposition (POD) is used to extract dominant energetic (and thus large-scale) structures from TBL flows over the surface with a spanwise-periodic converging/diverging riblets pattern, one of the spanwise-heterogeneous rough walls adopted previously. POD analyses are conducted on the three fluctuating components of velocities, measured by stereoscopic particle image velocimetry at Reθ = 13 000, in a cross-stream plane of the TBL flows over a spanwise section of the converging riblets pattern, where low-momentum eruptions consistently occur. It is found that first two POD modes with large temporal coefficients are linked to large-scale structures oscillating vigorously in the transverse direction within one section of the converging riblets. Superposition of these instantaneous large-scale structures reveals the observed pattern of mean secondary flows. Furthermore, these large-scale structures are associated with enhanced streamwise vortices and spanwise gradients of the streamwise velocity, compared with that in the smooth wall flow, thus rendering profound effects on the spatial correlations of velocities as well as the distributions of Reynolds stresses.

Energetic motions in turbulent partially filled pipe flow

Turbulent partially filled pipe flow was investigated using stereoscopic particle imaging velocimetry in the cross-stream plane for a range of flow depths at a nominally constant Reynolds number of 30 000 (based on the bulk velocity and hydraulic diameter). Unlike full pipe flow, which is axisymmetric, the turbulent kinetic energy exhibits significant azimuthal (and radial) variation. Proper orthogonal decomposition (POD) of the fluctuating velocity field indicates that the leading-order POD modes occupy the “corners” where the free surface meets the pipe wall and that these modes, which are closely linked to the instantaneous cellular structure, contribute nearly a quarter of the overall turbulent kinetic energy. Spatial distributions of the large- and very-large-scale motions (LSMs/VLSMs) estimated from pseudo-instantaneous three-dimensional velocity fields reveal a preference for the sides (in close proximity to the free surface) and bottom quadrant of the pipe. That the LSMs and VLSMs are shown to populate a region spanning the width of the free surface, as well as the corners, strongly suggests that there is a dynamical connection between LSMs/VLSMs and the instantaneous cellular structures in turbulent partially filled pipe flow, which can explain the spatial redistribution of the turbulent kinetic energy.

Investigation of The Decay of a Single vortex with PIV Technique

The formation and development of a single vortex is investigated in a wind tunnel. The wake from a lifting rectangular wing based on CLARK-Y profile of 11.7% is studied up to 80 chords downstream. Digital particle image velocimetry is used to measure the velocity field in the cross stream plane from which is the vortex is measured. Two different experimental test were carried out in two different conditions of the tunnel, without and with grid. From the first condition, the vortex is observed to be quite stable and long-lived. A correlation for the peak tangential velocity u θ , max and the vortex core r c has been compared with the Lamb-Oseen model, and it gives a good agreement. The circulation Γ is observed to be proportional to the free-stream U ∞ (range 8 − 20 m/s) and the angle of attack α (range 1−12 deg). The vorticity ω, however, diffuses over time within the growth of the vortex core r c . This growth can be approached by the Squire model. Apparently, the vortex characteristics become weaker when the experiments are performed under grid turbulence condition. Due to the influence of the grid turbulence, some experiments give inconsistent result. This makes difficult to draw conclusions. However, still some interesting information is found in this case.

Self-excited primary and secondary instability of laminar separation bubbles

Abstract

Stereoscopic PIV measurements of flow over a riblet surface at high Reynolds number

The effect of drag reducing riblets on the flow structure was examined experimentally for a turbulent boundary layer at Reθ = 9890 and riblet spacing s+ = 13.4. Trapezoidal riblets were used, which were attached to the water tunnel wall as a coating. Force measurements were performed to quantify the amount of drag reduction. Then, the mechanism underlying this reduction was investigated by stereo-PIV measurements in the cross-stream plane. To determine the effect of the drag reducing riblets, the results were compared with the smooth flat plate. Time-averaged turbulent statistics such as turbulent kinetic energy and Reynolds shear stress were found to be lower over the riblets compared to the flat surface. Two-point correlations of the fluctuating velocity components were calculated to elucidate the average flow structure size and strength, where riblets significantly suppressed the turbulent structures. Quadrant analysis of the Reynolds shear stress was performed to assess the change in ejection and sweep events and the results were found to be in correspondence with previous works.