Near-wall Flow Structures Research Articles

Experimental study on low-speed streaks in a turbulent boundary layer at low Reynolds number

Abstract

Some observations concerning "laminarization" in heated vertical tubes

For low "turbulent" Reynolds numbers in strongly-heated vertical gas flow, fundamental results of existing direct numerical simulation (DNS) databases have been examined to deduce differences between cases which "revert" to turbulent and those that yield integral parameters which correspond to laminar flow (henceforth laminarizing or laminarized). Objectives are (1) to examine the streamwise evolution of near-wall flow structures and (2) to determine which, if any, proposed laminarization parameters could be used to discriminate between turbulent and laminarizing flows resulting. Views of streamlines in r-θ cross sections showed evidence of a ring of irregular-shaped streamwise vortices near the wall at all locations. The transverse advection of these vortices is hypothesized to provide a path of least resistance to the transport of streamwise momentum in the wall-normal direction. It is demonstrated that a steady streamwise laminar vortex can provide apparent turbulent momentum transport (as a consequence of the Reynolds decomposition) such that a reasonable mean velocity profile is predicted near the wall. For the present cases, the values of the non-dimensional radius (yc,w+ = Reτ,w) and of the modified wall Reynolds number did discriminate whether turbulent or laminarizing flow resulted.

Near-wall topological patterns and flow structures over a simplified Danaus plexippus model

Studies on the high-lift mechanisms of butterfly gliding flights shed light on the design of the micro air vehicles. The flow field around a simplified Danaus plexippus model is investigated using the hydrogen bubble visualization and the Particle Image Velocimetry (PIV) techniques. There are three near-wall topological patterns with different Angles of Attack (AoAs): the separation bubble, the Leading-Edge Vortex (LEV) and the high AoAs flow. For the separation bubble pattern, two saddles and two foci form in the middle of the model. The features of the LEV pattern are the leading-edge separation lines. The topological characteristics of the separation lines are changed by the interaction between the LEV and the Wing-Tip Vortex (WTV). For the high AoAs flow pattern, four unstable foci are found at the forewing and the hindwing respectively. The angle between the trajectory of the WTV and the model increases with increasing AoA even though the slope of the WTV angle versus AoA curve declines at the moderate AoAs.

Similarity Parameter for Synthetic Jet Vortex Rings Impinging onto Porous Walls

Previous studies have shown that vortex rings impinging onto porous walls will pass through the wall more easily as the vortex ring strength, wall porosity, and pore size increase and the wall thickness decreases. Thus, there should exist a similarity parameter formed from these factors that will allow determination of the dynamics of the vortex rings/porous wall interaction. To find this similarity parameter, the effects of the Reynolds number (309–1238) and pore size (0.07–0.20) on the vortex rings/porous wall interaction are investigated by laser-induced fluorescence and particle image velocimetry (PIV) techniques. By dimensional analysis and quantitative investigation of PIV data, a similarity parameter is proposed to effectively characterize the losses in jet dynamical parameters, such as momentum flux, kinetic energy, and vortex ring circulation, due to impingement onto a porous wall. Physically, reflects the relative importance of the forces due to the momentum relative to the viscous forces, and the drag forces associated with the porous wall geometry. Finally, the results of the flow visualization validate that the near-wall flow structure during the interaction is qualitatively similar at the closest matching .

Analysis of a drag reduced flat plate turbulent boundary layer via uniform momentum zones

High spatial resolution particle-image velocimetry (PIV) and micro-particle tracking velocimetry (μ-PTV) measurements have been performed to analyze the flow structures in a turbulent flat plate boundary layer undergoing spanwise traveling transversal surface waves. The scope of this investigation is to examine the connection between friction drag reduction and the structural properties of the instantaneous flow fields manipulated by the surface wave. The experiments were conducted for a momentum based Reynolds number of Reθ=2250 at dimensionless wave amplitudes of A+=6, 7, 8, and 11, a wave period of T+=94, and a wave length of λ+=3738. Local drag reduction of up to 4.9% at 0.087 boundary layer thicknesses downstream of the moving surface was measured. The Reynolds shear stress in the logarithmic region (50≤y+≤250) is suppressed due to the surface waves, whereas the wall-normal velocity fluctuations are enhanced in the outer layer. The turbulent contribution term in the Fukagata, Iwamoto, and Kasagi (FIK) friction drag decomposition [18] is reduced by 2.7% due to the dampened Reynolds shear stress above the wave crest. The analysis of the large-scale uniform momentum zones (UMZs) indicates that the population of these zones above the wave trough is nearly identical compared to the non-actuated turbulent boundary layer, while above the wave crest the number of UMZs is decreased by 3.5%. The conditional statistics of the modal velocity based on the near-wall velocity fluctuation evidence that there are strong links between the near-wall flow events and the UMZs. The near-wall flow structures of high fluctuating intensity are less correlated to the large-scale UMZs due to the spanwise surface wave motion, which diminishes the momentum transfer from the outer large-scale energetic structures to the near-wall flow, and thus, leads to the friction drag reduction.

Interaction of coherent flow structures in adverse pressure gradient turbulent boundary layers

With the aim to characterize the near-wall flow structures and their interaction with large-scale motions in the log-law region, time-resolved planar and volumetric flow field measurements were performed in the near-wall and log-law region of an adverse pressure gradient turbulent boundary layer following a zero pressure gradient turbulent boundary layer at a friction Reynolds number $Re_{\unicode[STIX]{x1D70F}}=5000$. Due to the high spatial and temporal resolution of the measurements, it was possible to resolve and identify uniform-momentum zones in the region $z/\unicode[STIX]{x1D6FF}<0.15$ or $z^{+}<350$ and to relate them with well known coherent flow motions near the wall. The space–time results confirm that the turbulent superstructures have a strong impact even on the very near-wall flow motion and also their alternating appearance in time and intensity could be quantified over long time sequences. Using the time record of the velocity field, rare localized separation events appearing in the viscous sublayer were also analysed. By means of volumetric particle tracking velocimetry their three-dimensional topology and dynamics could be resolved. Based on the results, a conceptual model was deduced that explains their rare occurrence, topology and dynamics by means of a complex interaction process between low-momentum turbulent superstructures, near-wall low-speed streaks and tilted longitudinal and spanwise vortices located in the near-wall region.

Experimental investigation on the structure of turbulence in the bottom wave-current boundary layers

This paper presents insights into the structure of turbulence in combined wave-current boundary layers, based on experiments performed in flumes of different scale using Particle Image Velocimetry and hydrogen bubble visualisation. Flow conditions covered a range of wave frequencies, wave amplitudes and mean flow conditions. Results show that the spacing between turbulent streaks varies periodically with the passage of each wave, increasing when the flow accelerates and decreasing when the flow decelerates. A new formula has been put forward, relating the streak spacing variation and the wave-induced orbital displacements. The near-wall flow structure suggests a rhythmic pattern in terms of the velocity gradients across the flume. Waves with higher frequencies and larger amplitudes lead to a greater reduction of mean streak spacing, together with a greater increase of the maximum Reynolds shear stress induced by ejections. These results can be useful for better predictions of the hydrodynamics and sediment transport in combined wave-current flows.

Experimental Study of the Influence of the Shape of the Gap between the Rib and Flat Plate on the Near-Wall Flow Structure and Heat Transfer

The article presents an analysis of the results of an experimental study of the dynamic and thermal characteristics of the turbulent boundary layer of the air near a heated plate at qw = const with rectangular ribs having slit channels of different geometry: confusor, diffuser, and plane-parallel. The slit channel is located between the plate and the lower rib wall. The results are compared with similar data for a solid rib without the slit channel. A Pitot–Prandtl microprobe with a microthermocouple and the Dantec Dynamics hot-wire anemometer were used, thus making it possible to study the laminar sublayer, the transition domain, and the outer part of the boundary layer. The influence of the slit profile on the average and the pulsation characteristics of the turbulent dynamic and thermal boundary layers in the median section of the plate with the slit rib is revealed. It is found that the separated zone disappears in the flow behind the ribs with the confusor slit.

Numerical Analysis of Plume Structures of Fluids on a Horizontal Heated Plate

Numerical investigations have been carried out to predict the near-wall dynamics in indirect natural convection for air (Pr = 0.7) and water (Pr = 5.2). Near-wall flow structures appear to be line plumes. Three-dimensional laminar, steady-state model was used to model the problem. Density was formulated using the Boussinesq approximation. Flux scaling, plume spacing and plume lengths obtained numerically are found to have the same trend with the results available in the literature. Plume length and Nusselt number, Nu exhibits an increasing trend with an increase in Rayleigh number, RaH for both Pr fluids. The plume spacing is found to have an inverse relationship with RaH. The cube root of Rayleigh number based on plume spacing, Raλ1/3 is found to have a slight dependence on the dimensionless plume spacing, λ/H. Nu scales as Nu∼CRaHn, n = 0.26 for air and n = 0.3 for water. Heat transfer is thus found to be dominated by near-wall phenomenon. Nu shows a nonlinear relationship with LpH/A and is found to be an accurate representation of heat transfer.

The instantaneous structure of secondary flows in turbulent boundary layers

Secondary flows can develop in turbulent boundary layers that grow over surfaces with spanwise inhomogeneities. In this article, we demonstrate the formation of secondary flows in both experimental and numerical tests and dissect the instantaneous structure and topology of these secondary motions. We show that the formation of secondary flows is not very sensitive to the Reynolds number range investigated, and direct numerical simulations and experiments produce similar results in the mean flow as well as the dispersive and turbulent stress distributions. The numerical methods capture time-resolved features of the instantaneous flow and provide insight into the near-wall flow structures, that were previously obscured in the experimental measurements. Proper orthogonal decomposition was shown to capture the essence of the secondary flows in relatively few modes and to be useful as a filter to analyse the instantaneous flow patterns. The secondary flows are found to create extended regions of high Reynolds stress away from the wall that comprise predominantly sweeps similar to what one would expect to see near the wall and which are comparable in magnitude to the near-wall stress. Analysis of the instantaneous flow patterns reveals that the secondary flows are the result of a non-homogeneous distribution of mid-size vortices.

Large-eddy simulation of flow over an axisymmetric body of revolution

Wall-resolved large-eddy simulation (LES) is used to simulate flow over an axisymmetric body of revolution at a Reynolds number,$Re=1.1\times 10^{6}$, based on the free-stream velocity and the length of the body. The geometry used in the present work is an idealized submarine hull (DARPA SUBOFF without appendages) at zero angle of pitch and yaw. The computational domain is chosen to avoid confinement effects and capture the wake up to fifteen diameters downstream of the body. The unstructured computational grid is designed to capture the fine near-wall flow structures as well as the wake evolution. LES results show good agreement with the available experimental data. The axisymmetric turbulent boundary layer has higher skin friction and higher radial decay of turbulence away from the wall, compared to a planar turbulent boundary layer under similar conditions. The mean streamwise velocity exhibits self-similarity, but the turbulent intensities are not self-similar over the length of the simulated wake, consistent with previous studies reported in the literature. The axisymmetric wake shifts from high-$Re$to low-$Re$equilibrium self-similar solutions, which were only observed for axisymmetric wakes of bluff bodies in the past.

An In-depth Study on Drag Reduction of Turbulent Channel Flow Induced by Spatially Oscillating Lorentz Force

To investigate the in-depth mechanism of turbulence control and drag reduction in a channel flow by using a spatially oscillating Lorentz force, the pseudo-spectral method based on standard Fourier–Chebyshev spectral method is used to solve N-S equations. The characteristics of controlled flow fields and vortex structures are described as well as the mechanism of the flow field control. The result shows that the near-wall flow structure is altered substantially and the turbulence drag decreases sharply under the action of the distributed Lorentz force with proper control parameters, the boundary-layer fluid are reciprocating periodic moving, and the vortex generation is impacted, and that only periodically well-organized streamwise vortexes are observed in the near-wall region of the turbulent channel flow.

Skin-Friction and Surface-Pressure Structures in Near-Wall Flows

Skin-friction and surface-pressure in a viscous flow constitute a pair of the intrinsically coupled physical quantities denoted by the notation (, ), which determines the near-wall flow structures. The exact (, ) relation derived from the Navier–Stokes (NS) equations provides a rational foundation for reconstruction of the (, ) structures in complex flows. Based on this coupling relation, a field can be obtained from the perturbed field for the given boundary enstrophy flux field of a base flow as an inverse problem in the first-order approximation. Then, the corresponding near-wall velocity field is reconstructed by using the Taylor-series-expansion solution of the NS equations. Some elemental and complex (, ) structures in three-dimensional flow separations are reconstructed to demonstrate the applicability of the proposed method.

Particle segregation in turbulent Couette–Poiseuille flow with vanishing wall shear

Inertial particles dispersed in wall-bounded flows in pipes and channels are known to accumulate close to the walls. The segregation ability depends greatly on the inertia-selection effects of the near-wall quasi-coherent turbulent structures, which are formed near both walls where shear stresses are high. Here, however, we investigated if and how particles segregate in the vicinity of walls in absence of mean shear. A tailor-made turbulent Couette–Poiseuille flow was designed such that the mean shear vanished at the moving wall, thereby resulting in an asymmetric flow with conventional near-wall turbulent structures only at one wall. In addition, Large-Scale Structures (LSSs) were observed in the flow, which greatly influenced the distribution of the inertial particles. Particles of five different inertia groups were embedded in the directly simulated turbulence field and examined. It was found that particles of high inertia segregated near the stationary wall where mean shear prevailed, but also near the moving wall where mean shear was absent. However, due to the qualitatively distinct near-wall flow structures, the inertia effects on the actual segregation were different at the two walls. Mechanisms causing the asymmetric wall-normal segregation were explored with the focus on the moving-wall region, where the quasi-coherent turbulent structures were absent, and the local fluid structures were dominated by imprints of the LSSs.

Numerical Investigation of Vortex Ring Ground Plane Interactions

The interaction between multiple laminar thin vortex rings and solid surfaces was studied numerically so as to investigate flow patterns associated with near-wall flow structures. In this study, the vortex–wall interaction was used to investigate the tendency of the flow toward recirculatory behavior and to assess the near-wall flow conditions. The numerical model shows very good agreement with previous studies of single vortex rings for the case of orthogonal impact (angle of incidence, θ = 0 deg) and oblique impact (θ = 20 deg). The study was conducted at Reynolds numbers 585 and 1170, based on the vortex ring radius and convection velocity. The case of two vortex rings was also investigated, with particular focus on the interaction of vortex structures postimpact. Compared to the impact of a single ring with the wall, the interaction between two vortex rings and a solid surface resulted in a more highly energized boundary layer at the wall and merging of vortex structures. The azimuthal variation in the vortical structures yielded flow conditions at the wall likely to promote agitation of ground based particles.

Direct simulation of conjugate heat transfer of jet in channel crossflow

We present a DNS study of a hot, low momentum laminar water jet discharged into a cold turbulent channel stream through a circular orifice in one of the steel channel walls. The channel wall has a finite thickness and its outer side is cooled under Robin type thermal boundary conditions for a realistic external environment, leading to a conjugate heat transfer system. Nusselt number and r.m.s temperature fluctuations on the wall are compared with our earlier DNS results for the simpler iso-thermal and adiabatic conditions at the channel inner surface. Temperature fluctuations inside the channel wall are resolved to provide data for a conjugate heat transfer (CHT) thermal fatigue test case related to the ageing of pipe walls and welds studies, as found, for example, in power plant piping T-junctions. The crossflow Reynolds number is Re=3333, jet-to-crossflow velocity ratio is R=1/6 and fluid-to-solid conductivity ratio is 1/64.The near-wall mean flow structures, a horseshoe vortex ahead and on the sides of the jet orifice, a shallow recirculation behind the discharge and a counter-rotating vortex pair drawing in a blanket of cooler cross-flow, lead to a complex convective and turbulent wall heat transfer pattern around the orifice. The main findings are:(i)Wall maps of Nusselt number and r.m.s temperature, θr.m.s, for conjugate heat transfer are only qualitatively similar to the iso-thermal and adiabatic wall cases.(ii)Inside the solid θr.m.s and its dissipation, analysed from RANS modelling perspective, show that predicted thermal spot length scales are discontinuous on the interface, at variance with the 2-point spectrum-derived scales.(iii)At the high wavenumber range, the spanwise temperature spectra decrease according to exponential-decay spectral models for the fluid turbulence in the Kolmogorov range, but with large exponential coefficients increasing with depth inside the solid.

Temporal and spatial intermittencies within channel flow turbulence near transition

Spatiotemporal intermittency in transitional channel flow turbulence in an extended domain is related to temporal intermittency in the minimal channel. Near-wall flow structures and mean profiles in regions of low drag closely resemble those found in low drag temporal intervals in the minimal channel.

Panoramic diagnostics of shear stresses on the channel wall with a step using the liquid crystals

Measurements results on the shear stresses of surface friction by means of thin-film coatings based on cholesteric liquid crystals and specialized software for digital processing of experimental video are presented in the paper. The calibration dependencies of shear stress relative to the hue and azimuth angle as well as shear stress spatial distribution at subsonic turbulent flow (V ∝ = 84 m/s) around a step, trapezoidal in plane (Reynolds number calculated for step height h, Re h = 2.57•104), with a base angle of 46° were derived for two geometries of experiment. The experiments demonstrated high sensitivity of liquid crystals to rearrangement of the near-wall flow structure and possibility to obtain quantitative data about mean shear stress levels.

On the correspondence between flow structures and convective heat transfer augmentation for multiple jet impingement

The correspondence between local fluid flow structures and convective heat transfer is a fundamental aspect that is not yet fully understood for multiple jet impingement. Therefore, flow field and heat transfer experiments are separately performed investigating mutual–jet interactions exposed in a self-gained crossflow. The measurements are taken in two narrow impingement channels with different cross-sectional areas and a single exit design. Hence, a gradually increased crossflow momentum is developed from the spent air of the upstream jets. Particle image velocimetry (PIV) and liquid crystal thermography (LCT) are used in order to investigate the aerothermal characteristics of the channel with high spatial resolution. The PIV measurements are taken at planes normal to the target wall and along the centreline of the jets, providing quantitative flow visualisation of jet and crossflow interactions. Spatially resolved heat transfer coefficient distributions on the target plate are evaluated with transient techniques and a multi-layer of thermochromic liquid crystals. The results are analysed aiming to provide a better understanding about the impact of near-wall flow structures on the convective heat transfer augmentation for these complex flow phenomena.

Experimental and Numerical Investigation of an Axial Rotary Blood Pump.

Left ventricular assist devices (LVADs) have become a standard therapy for patients with severe heart failure. As low blood trauma in LVADs is important for a good clinical outcome, the assessment of the fluid loads inside the pump is critical. More specifically, the flow features on the surfaces where the interaction between blood and artificial material happens is of great importance. Therefore, experimental data for the near-wall flows in an axial rotary blood pump were collected and directly compared to computational fluid dynamic results. For this, the flow fields based on unsteady Reynolds-averaged Navier-Stokes simulations-computational fluid dynamics (URANS-CFD) of an axial rotary blood pump were calculated and compared with experimental flow data at one typical state of operation in an enlarged model of the pump. The focus was set on the assessment of wall shear stresses (WSS) at the housing wall and rotor gap region by means of the wall-particle image velocimetry technique, and the visualization of near-wall flow structures on the inner pump surfaces by a paint erosion method. Additionally, maximum WSS and tip leakage volume flows were measured for 13 different states of operation. Good agreement between CFD and experimental data was found, which includes the location, magnitude, and direction of the maximum and minimum WSS and the presence of recirculation zones on the pump stators. The maximum WSS increased linearly with pressure head. They occurred at the upstream third of the impeller blades and exceeded the critical values with respect to hemolysis. Regions of very high shear stresses and recirculation zones could be identified and were in good agreement with simulations. URANS-CFD, which is often used for pump performance and blood damage prediction, seems to be, therefore, a valid tool for the assessment of flow fields in axial rotary blood pumps. The magnitude of maximum WSS could be confirmed and were in the order of several hundred Pascal.