A scalar emanating from a point source in a turbulent boundary layer does not mix homogeneously, but is organized in large regions with little variation of the concentration: uniform concentration zones. We measure scalar concentration using laser-induced fluorescence and, simultaneously, the three-dimensional velocity field using tomographic particle image velocimetry in a water tunnel boundary layer. We identify uniform concentration zones using both a simple histogram technique, and more advanced cluster analysis. From the complete information on the turbulent velocity field, we compute two candidate velocity structures that may form the boundaries between two uniform concentration zones. One of these structures is related to the rate of point separation along Lagrangian trajectories and the other one involves the magnitude of strong shear in snapshots of the velocity field. Therefore, the first method allows for the history of the flow field to be monitored, while the second method only looks at a snapshot. The separation of fluid parcels in time was measured in two ways: the exponential growth of the separation as time progresses (related to finite-time Lyapunov exponents and unstable manifolds in the theory of dynamical systems), and the exponential growth as time moves backward (stable manifolds). Of these two, a correlation with the edges of uniform concentration zones was found for the past Lyapunov field but not with the time-forward future field. The magnitude of the correlation is comparable to that of the regions of strong shear in the instantaneous velocity field.

Multiphase Flow Phenomena and Applications, pp. 67-81 (2018) No AccessExperimental Investigation of the Interaction Between a Stationary Rigid Sphere and a Turbulent Boundary LayerRené van Hout, Jerke Eisma, Edwin Overmars, Gerrit E. Elsinga, and Jerry WesterweelRené van HoutTechnion — Israel Institute of Technology, Technion City, Haifa, Israel, Jerke EismaTU-Delft, 3ME Aero- and Hydrodynamics Laboratory Leeghwaterstraat 21, 2628 CA, Delft, The Netherlands, Edwin OvermarsTU-Delft, 3ME Aero- and Hydrodynamics Laboratory Leeghwaterstraat 21, 2628 CA, Delft, The Netherlands, Gerrit E. ElsingaTU-Delft, 3ME Aero- and Hydrodynamics Laboratory Leeghwaterstraat 21, 2628 CA, Delft, The Netherlands, and Jerry WesterweelTU-Delft, 3ME Aero- and Hydrodynamics Laboratory Leeghwaterstraat 21, 2628 CA, Delft, The Netherlandshttps://doi.org/10.1142/9789813227392_0004Cited by:0 PreviousNext AboutSectionsPDF/EPUB ToolsAdd to favoritesDownload CitationsTrack CitationsRecommend to Library ShareShare onFacebookTwitterLinked InRedditEmail Abstract: Time-resolved tomographic particle image velocimetry (PIV) measurements (acquisition rate 250 Hz) were performed in a turbulent boundary layer on the side wall of an open channel, water flow facility (cross section 60 × 60 cm2, W × H), 3.5m downstream of the inlet at a bulk flow velocity of Ub = 0.17m/s (Reb = UbH/ν = 97, 679, δ0.99 = 45.0 mm, Reθ = 752). The measurement volume was a horizontal slab (60 × 15 × 60mm3, l × w × h) extending from the wall, 30 cm above the bottom. The setup comprised four high-speed ImagerPro HS cameras (2016×2016 pixels), a high-speed laser (Nd:YLF, Darwin Duo 80M, Quantronix), optics/prisms, and data acquisition/processing software (LaVision, DaVis 8.2). Data were acquired with and without a stationary held sphere that had a diameter, D = 6mm (D+ = 51, "+" denotes inner wall scaling), and was positioned at x3 = 5.4 and 37.6mm (x3+ = 43 and 306) from the wall (measured from the sphere's center). Sphere Reynolds numbers based on D and the average streamwise velocity at the sphere's center were 692 and 959, respectively. The mean streamwise velocity profiles of the undisturbed boundary layer clearly exhibit a canonical shape. Introducing the sphere strongly affected log layer and buffer layer mean velocity and Reynolds stress profiles. Recovery to the undisturbed boundary layer characteristics is faster with the sphere positioned closest to the wall. When positioned at h+ = 306, near-wall, uplifted, coherent vortical structures extend from the wall up to the sphere's wake with which they interact. Keywords: Particle-laden flowsphereturbulent boundary layer FiguresReferencesRelatedDetails Multiphase Flow Phenomena and ApplicationsMetrics History KeywordsParticle-laden flowsphereturbulent boundary layerPDF download

In many applications, finite-sized particles are immersed in a turbulent boundary layer (TBL) and it is of interest to study wall effects on the instantaneous shedding of turbulence structures and associated mean velocity and Reynolds stress distributions. Here, 3D flow field dynamics in the wake of a prototypical, small sphere (D+ = 50, 692 < Re-D < 959) placed in the TBL's outer, logarithmic, and buffer layer, were measured using time-resolved tomo-PIV. Increasing wall proximity increasingly tilted the mean recirculating wake away from the wall implying a negative lift force. Mean velocity deficit recovery scaled with the mean wake length with minor effects of wall proximity. Farthest from the wall, streamwise Reynolds normal stresses encircled the mean wake as an axisymmetric tubular "shell," while transverse and wall-normal stresses extended off its tip as axisymmetric tapered cones. Wall proximity removed axisymmetry and attenuated values near the wall. Reynolds shear stresses were distributed as antisymmetric lobes extending off the mean wake displaying increasing values with reducing sphere-wall gap. Instantaneous snapshots revealed a wake densely populated by "archlike" vortices with shedding frequencies lower than for a sphere in uniform flow except in the buffer layer. Tilting of the wake away from the wall resulted from self-induced motion of shed hairpinlike vortices whose symmetry plane was increasingly wall-normal oriented with reduced sphere-wall gap.

New experimental research is presented on the characteristics of interfaces and internal shear layers that are present in a turbulent boundary layer (TBL). The turbulent/non-turbulent (T/NT) interface at the outer boundary of the TBL shows the presence of a finite jump in streamwise velocity and is characterised by a thin shear layer. It appears that similar layers of high shear occur also within the TBL which separate regions of almost uniform momentum. It turns out that they exhibit similar characteristics as the external T/NT interface. Furthermore, the spatial growth rate of the TBL, that is derived from theoretical analysis, can be correctly predicted from a momentum balance near the external T/NT interface. Similarly, the entrainment velocities for the average internal layers have been determined. Results indicate that internal layers move slower in the vicinity of the wall, whereas they move faster than the large scale boundary layer growth rate in the outer region of the TBL. It is believed that shear layers bound large scale flow regions of approximately uniform momentum. Hence, the entrainment velocities of these internal layers may be interpreted as growth rates of the large scale motions in a TBL.

Experimental research is presented on the characteristics of interfaces and internal layers that are present in a turbulent boundary layer (TBL). Both the turbulent non-turbulent interface (T/NT) and internal shear layers are detected in snapshots of the stereo-PIV data. It turns out that the internal layers exhibit similar characteristics compared to the T/NT interface. A theoretical approximation of the large scale boundary layer growth indicates that the correct boundary layer growth can be obtained by employing a modified first order jump model on the conditional statistics. Employing the same framework to the internal shear layers indicates that shear layers tend to move slower in close proximity to the wall, whereas they accelerate when moving away from the wall. Based on previous research it is believed that these internal layers separate large regions of approximately uniform momentum. Hence, boundary entrainment velocities may be interpreted as growth rates of large scale motions in a TBL.

Simulation, PIV data, and local models show characteristics and conditional statistics of turbulence either side and within interfacial layers [ I ] depending on the mean profile and the presence of resistive/porous walls. We consider well-developed inhomogeneous horizontal turbulent shear flows in the x direction lying beneath very thin outside interfacial shear layers [ I O ] located at z = z I, that separate the region [ S ] with strong turbulence from an outer region [ O ] region of weak turbulence and low shear ([2,6,9,13]). There are also gradients of mean and fluctuating concentration C in these regions and within the layer. Where the [ S ] region is a turbulent boundary layer above a rigid, impervious surface at z =0 or a porous rigid surface lying below z =0, internal interfacial shear layers [ I S ] form near z =0, and in some cases also within [ S ]. In the shear region S below the interface, the mean velocity is ⟨ u 1 ⟩ with significant mean shear Ω =d ⟨ u 1 ⟩ ~ Δ U L / L below the interface, which is comparable to the large scale strain in the turbulence, Σ u ’/ L , where U L is the characteristic change of the mean velocity across the scale L of the shear layer and u ’ is the rms turbulence. Note that Δ U L u ’~( τ S ) (1/2) , where S is the Reynolds stress.

Time-resolved velocity field measurements in the wake of the flapping wings of the DelFly II Micro Aerial Vehicle (MAV) in forward flight configuration were obtained by Stereoscopic Particle Image Velocimetry (Stereo-PIV). The PIV measurements were performed at several spanwise planes in the wake of the flapping wings and at a high framing rate to allow a reconstruction of the temporal development of the three dimensional wake structures throughout the flapping cycle. The wake reconstruction was performed by interpolating between the measurement planes through a Kriging interpolation procedure. First, the general wake topology of the DelFly II model is described in conjunction with the behavior of the distinctive flow structures, in particular, tip vortex, trailing edge vortex, and root vortex. Second, the effect of reduced frequency is investigated by changing the flapping frequency. Comparison of the three dimensional wake structures for different cases of reduced frequency reveals major differences in both formation and interaction of vortical structures.