Search

This paper provides an experimental investigation on the internal shear layers and the edges of the uniform momentum zones (UMZs) in a turbulent pipe flow. The time-resolved stereoscopic particle image velocimetry data are acquired in the cross-section of the pipe, and span the range of Reynolds number Reτ=340--1259. In the first part of the study, internal shear layers are detected using a three-dimensional detection method, and both their geometry as well as their fingerprint in the flow statistics are examined. Three-dimensional conditional mean flow analysis revealed a strong low-speed region beneath the average shear layers. This low-speed region is associated with positive wall-normal fluctuations, and it is accompanied by two swirling motions having opposite signs on either side in the azimuthal direction. Moreover, the shear layers are stretched by the two opposite azimuthal motions. In the second part of the study, the shear layers are treated as the continuous edges of the UMZs, which are detected using the histogram method following Adrian et al. (J. Fluid Mech., vol. 422, 2000, pp. 1–54) and de Silva et al. (J. Fluid Mech., vol. 786, 2016, pp. 309–331). For this part, two different orientation of the planes are used, i.e. the wall-normal–streamwise plane and the wall-normal–spanwise plane (cross-section of the pipe). Comparison of the detected structures shows that the shear layers mostly overlap with a UMZ edge (in either plane)., Fluid Mechanics

We use Faraday waves to measure interfacial tension σ between two immiscible fluids, with an interest in (ultra)low values of σ. The waves are excited by vertically oscillating the container in which the fluids reside. Using linear stability theory, we map out the accessible range of interfacial tensions. The smallest value (σmin ≈ 5 × 10-4 N/m) is limited by the joint influence of gravity and viscous dissipation. A further limitation is posed by the greatest accelerations that can be realized in a laboratory. We perform experiments on a water-dodecane interface with an increasing concentration of a surfactant in the water layer that decreases the interfacial tension into the ultralow domain [σ = [Formula: see text](10-6 N/m)]. Surprisingly, the smallest measured wavelength is larger by a factor of 2 than that predicted for vanishing σ. We hypothesize the effect of transport of the surfactant in the fluid flow associated with the waves., Fluid Mechanics

Abstract: In microscopic particle image velocimetry (micro-PIV), correlation averaging over multiple frames is often required, leading to a loss in temporal resolution, therefore limiting the measurement accuracy for unsteady flows. Here, we present a new PIV method suitable to study steady and unsteady laminar flows between parallel plates (i.e., Hele-Shaw flow), which is a common flow configuration in microfluidic applications. Our method reduces the effective seeding density and yields similar if not higher signal-to-noise ratio (SNR) compared to conventional micro-PIV. We call this algorithm Ψ -PIV. Ψ -PIV requires a much smaller number of frames to reach the same SNR compared to the widely used correlation averaging method. This leads to a significant improvement of the temporal resolution. The Ψ -PIV algorithm is used in an experimental investigation of steady and unsteady flows in a Hele-Shaw cell. Our experiment shows that Ψ -PIV reduces the number of required frames by 8 times and 30 times compared to the frames required by conventional PIV for steady and unsteady laminar flow, respectively. In this study, PIV and Ψ -PIV use a single-pass cross-correlation to present the underlying difference between the two approaches. Graphic abstract: [Figure not available: see fulltext.]., With Correction: https://doi.org/10.1007/s00348-020-2913-0, Fluid Mechanics

In this work, we extend a planar laser-induced fluorescence method for free surface measurements to a three-dimensional domain using a stereo-camera system, a scanning light sheet, and a modified self-calibration procedure. The stereo-camera set-up enables a versatile measurement domain with self-calibration, improved accuracy, and redundancy (e.g., possibility to overcome occlusions). Fluid properties are not significantly altered by the fluorescent dye, which results in a non-intrusive measurement technique. The technique is validated by determining the free surface of a hydraulic flow over an obstacle and circular waves generated after droplet impact. Free surface waves can be accurately determined over a height of L= 100 mm in a large two-dimensional domain (y(x, z) = 120 × 62 mm2), with sufficient accuracy to determine small amplitude variations (η≈ 0.2 mm). The temporal resolution (Δt= 19 ms) is only limited by the available scanning equipment (f= 1 kHz rate). For other applications, this domain can be scaled as needed. Graphic abstract: [Figure not available: see fulltext.]., Multi Phase Systems, Fluid Mechanics

We present an experimental study on the variation in wave impact location and present a mechanism for the development of free surface instabilities on the wave crest for repeatable plunging wave impacts on a vertical wall. The existence of free surface instabilities on an impacting wave is well known, but their characteristics and formation mechanism are relatively unknown. The development of the global wave shape is measured using a visualization camera, whereas the local wave shape is measured with an accurate stereo-planar laser-induced fluorescence technique. A repeatable wave is generated with negligible system variability. The global wave behavior resembles that of a plunging breaker, with a gas pocket cross-sectional area defined by an ellipse of constant aspect ratio. The variability of the local wave profile increases significantly as it approaches the wall. The impact location varies by ∼0.5% of the wave height or more than a typical pressure sensor diameter. Additionally, the wave tip accelerates to a velocity of 1.5√gh0 compared to the global wave velocity of 1.2√gh0. The difference in impact location and velocity can result in a pressure variation of ∼25%. A mechanism for instability development is observed as the wave tip becomes thinner and elongates when it approaches the wall. A flapping liquid sheet develops that accelerates the wave tip locally and this triggers a spanwise Rayleigh–Taylor instability., Green Open Access added to TU Delft Institutional Repository 'You share, we take care!' - Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public., Multi Phase Systems, Hydraulic Structures and Flood Risk, Fluid Mechanics

We report on the experimental investigation of the large-scale instantaneous flow structures in turbulent Taylor–Couette flow using tomographic particle image velocimetry. The results indicate three distinct regimes for counter-rotating flow within a shear Reynolds number range of 11000

Severe vibrations and sound production can occur in dry gas flow through corrugated pipes. The addition of very small amounts of liquid to the dry gas flow potentially mitigates these flow-induced vibrations (FIVs) and noise. The different mechanisms behind this whistling mitigation are studied in this work, where acoustic measurements are combined with flow visualization and droplet sizing. Different corrugation geometries are studied. It is shown that noise mitigation mainly occurs through a geometric alteration of the cavity mouth, resulting in a reduced acoustic source strength. Additional acoustic damping as a consequence of the presence of droplets has a very limited contribution to the mitigation of FIVs. A non-axisymmetric filling of the cavities of a corrugated pipe with liquid is more effective in reducing the acoustic output, compared to an axisymmetric filling. The liquid viscosity has a minor effect on the achieved noise mitigation. To predict the acoustic source strength for a particular cavity geometry a numerical method is developed, based on URANS simulations combined with Howe's energy corollary. An energy balance method is applied to obtain the acoustic source strength from experiments. The whistling frequencies are accurately predicted with the simulations, but the acoustic source strength is over-predicted by a factor 2. Trends in the source strength obtained from simulations, however, closely resemble the experimentally obtained results. The developed method provides an intuitive understanding of sound production by vortical flow structures and shows potential for the prediction of self-sustained oscillations in corrugated pipes., Fluid Mechanics, Multi Phase Systems

Abstract: This paper presents a µPIV measurement of the 3D2C velocity distribution of Taylor droplets moving in a square horizontal microchannel at Cac= 0.005 and Rec= 0.051. We reconstruct the third velocity component and present an accuracy assessment of the reconstruction based on a volume flow balance of the 3D3C velocity field. The velocity field allows the investigation of the 3D flow features such as stagnation regions and the shear rate distribution. The maximum shear rate is located at the entrances and exits of the wall films and at the corner flow (gutter) bypassing the Taylor droplet. The regions of high strain correspond to the cap positions of the Taylor droplets. An experimental data set allows visualization of the streamlines of the velocity distribution on the interface of a Taylor droplet and to directly relate it to the main and secondary vortices of the droplet phase velocity field. The measurement data are provided as a digital resource for the validation of numerical simulation or further assessment. Graphic abstract: [Figure not available: see fulltext.], Fluid Mechanics

Abstract: Obtaining accurate experimental data from Lagrangian tracking and tomographic velocimetry requires an accurate camera calibration consistent over multiple views. Established calibration procedures are often challenging to implement when the length scale of the measurement volume exceeds that of a typical laboratory experiment. Here, we combine tools developed in computer vision and non-linear camera mappings used in experimental fluid mechanics, to successfully calibrate a four-camera setup that is imaging inside a large tank of dimensions ∼10×25×6m3. The calibration procedure uses a planar checkerboard that is arbitrarily positioned at unknown locations and orientations. The method can be applied to any number of cameras. The parameters of the calibration yields direct estimates of the positions and orientations of the four cameras as well as the focal lengths of the lenses. These parameters are used to assess the quality of the calibration. The calibration allows us to perform accurate and consistent linear ray-tracing, which we use to triangulate and track fish inside the large tank. An open-source implementation of the calibration in Matlab is available. Graphic abstract: [Figure not available: see fulltext.]., Fluid Mechanics

We investigate the fluid motion generated by a moving rowing blade. The blade follows a complex path with rather strong acceleration and subsequent deceleration. The blade path is mimicked at a 1:2 scale in a large open-top water tank using a robot system. The tank is transparent, thus enabling full optical access for performing large-field particle image velocimetry (PIV). The robot system allows us to precisely repeat subsequent rowing blade motions. PIV measurements in the same plane show that the fluid motion is highly repeatable, except for the small-scale turbulent fluid motions. When combined with direct measurements of the forces on the rowing blade (Grift et al. 2019a) the PIV data provide insight in the variation of the hydrodynamic forces acting on the blade during motion. This makes it possible to improve the efficiency and effectiveness of the propulsion which is of great relevance to competitive rowing., Fluid Mechanics

Abstract: The feasibility of particle image velocimetry (PIV) in a thermally convective supercritical fluid was investigated. Hereto a Rayleigh–Bénard convection flow was studied at pressure and temperature above their critical values. The working fluid chosen was trifluoromethane because of its experimentally accessible critical point. The experiments were characterized by strong differences in the fluid density from the bottom to the top of the cell, where the maximum relative density difference was between 17 and 42%. These strong density changes required a careful selection of tracer particles and introduced optical distortions associated with strong refractive index changes. A preliminary background oriented schlieren (BOS) study confirmed that the tracer particles remained visible despite significant local blurring. BOS also allowed estimating the velocity error associated with optical distortions in the PIV measurements. Then, the instantaneous velocity and time-averaged velocity distributions were measured in the mid plane of the cubical cell. Main difficulties were due to blurring and optical distortions in the boundary layer and thermal plumes regions. An a posteriori estimation of the PIV measurement uncertainty was done with the statistical correlation method proposed by Wieneke (Measure Sci Technol 26:074002, 2015). It allowed to conclude that the velocity values were reliably measured in about 75% of the domain. Graphic abstract: [Figure not available: see fulltext.]., Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public., RST/Reactor Physics and Nuclear Materials, Fluid Mechanics, Executive board

The vestibular system in the inner ear senses angular head manoeuvres by endolymph fluid which deforms a gelatinous sensory structure (the cupula). We constructed computer models that include both the endolymph flow (using CFD modelling), the cupula deformation (using FEM modelling), and the interaction between both (using fluid–structure interaction modelling). In the wide utricle, we observe an endolymph vortex. In the initial time steps, both the displacement of the cupula and its restorative forces are still small. As a result, the endolymph vortex causes the cupula to deform asymmetrically in an S-shape. The asymmetric deflection increases the cupula strain near the crista and, as a result, enhances the sensitivity of the vestibular system. Throughout the head manoeuvre, the maximal cupula strain is located at the centre of the crista. The hair cells at the crista centre supply irregularly spiking afferents, which are more sensitive than the afferents from the periphery. Hence, the location of the maximal strain at the crista may also increase the sensitivity of the semicircular canal, but this remains to be tested. The cupula overshoots its relaxed position in a simulation of the Dix-Hallpike head manoeuvre (3 s in total). A much faster head manoeuvre of 0.222 s showed to be too short to cause substantial cupula overshoot, because the cupula time scale of both models (estimated to be 3.3 s) is an order of magnitude larger than the duration of this manoeuvre., Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public., Fluid Mechanics, Multi Phase Systems

To analyze on-water rowing performance, a valid determination of the power loss due to the generation of propulsion is required. This power los can be calculated as the dot product of the net water force vector ( ~ F w;o ) and the time derivative of the position vector of the point at the blade where ~ F w;o is applied (~r PoA = w ). In this article we presented a method that allows for accurate determination of both parameters using a closed system of three rotational equations of motion for three different locations at the oar. Additionally, the output of the method has been validated. An oar was instrumented with three pairs of strain gauges measuring local strain. Force was applied at different locations of the blade, while the oar was fixed at the oarlock and the end of the handle. Using a force transducer and kinematic registration, the force vector at the blade and the deflection of the oar were measured. These data were considered to be accurate and used to calibrate the measured strain for bending moments, the deflection of the oar and the angle of the blade relative to its unloaded position. Additionally, those data were used to validate the output values of the presented method plus the associated instantaneous power output. Good correspondence was found between the estimated perpendicular blade force and its reference (ICC = .999), while the parallel blade force could not be obtained (ICC = .000). The position of the PoA relative to the blade could be accurately obtained when the perpendicular force was 5.3 N (ICC = .927). Instantaneous power output values associated with the perpendicular force could be obtained with reasonable accuracy (ICC = .747). These results suggest that the power loss due to the perpendicular water force component can be accurately obtained, while an additional method is required to obtain the power losses due to the parallel force., Fluid Mechanics

We present results on the drag on, and the flow field around, a submerged rectangular normal flat plate, which is uniformly accelerated to a constant target velocity along a straight path. The plate aspect ratio is chosen to be to resemble an oar blade in (competitive) rowing, the sport which inspired this study. The plate depth, i.e. the distance from the top of the plate to the air-water interface, the plate acceleration and the plate target velocity are varied, resulting in a plate width based Reynolds number of . In our analysis we distinguish three phases; (i) the acceleration phase during which the plate drag is enhanced, (ii) the transition phase during which the plate drag decreases to a constant steady value upon which (iii) the steady phase is reached. The plate drag force is measured as function of time which showed that the steady-phase plate drag at a depth of plate height (20 mm depth for a plate height of 100 mm) increased by 45 % compared to the plate top at the surface (0 mm). Also, it is shown that the drag force during acceleration of the plate increases over time and is not captured by a single added mass coefficient for prolonged accelerations. Instead, an entrainment rate is defined that captures this behaviour. The formation of starting vortices and the wake development during the time of acceleration and transition towards a steady wake are studied using hydrogen bubble flow visualisations and particle image velocimetry. The formation time, as proposed by Gharib et al. (J. Fluid Mech., vol. 360, 1998, pp. 121-140), appears to be a universal time scale for the vortex formation during the transition phase., Fluid Mechanics

We investigate the deformation of a linear viscoelastic compliant coating in a turbulent flow for a wide range of coating parameters. A one-way coupling model is proposed in which the turbulent surface stresses are expressed as a sum of streamwise-travelling waves with amplitudes determined from the stress spectra of the corresponding flow over a rigid wall. The analytically calculated coating deformation is analysed in terms of the root-mean-square (r.m.s.) surface displacement and the corresponding point frequency spectra. The present study systematically investigates the influence of five coating properties namely density, stiffness, thickness, viscoelasticity and compressibility. The surface displacements increase linearly with the fluid/solid density ratio. They are linearly proportional to the coating thickness for thin coatings, while they become independent of the thickness for thick coatings. Very soft coatings show resonant behaviour, but the displacement for stiffer coatings is proportional to the inverse of the shear modulus. The viscoelastic loss angle has only a significant influence when resonances occur in the coating response, while Poisson's ratio has a minor effect for most cases. The modelled surface displacement is qualitatively compared with recent measurements on the deformation of three different coatings in a turbulent boundary-layer flow. The model predicts the order of magnitude of the surface displacement, and it captures the increase of the coating displacement with the Reynolds number and the coating softness. Finally, we propose a scaling that collapses all the experimental data for the r.m.s. of the vertical surface displacement onto a single curve., Fluid Mechanics, Support Process and Energy, Multi Phase Systems

The challenge presented in this research is to determine the local volumetric bubble concentration (or void fraction) in the center of a microbubble cloud with limited optical access and without disturbing the flow. By applying defocused volumetric shadowgraphy to an aerated water column we were able to measure the characteristics of single microbubbles in the control volume for a void fraction of 0.078 percent. The time-averaged local bubble concentration in the center of the water column was measured over four periods (800 seconds each) to investigate the repeatability. Two reference methods, based on differential pressure (ΦV = 0.081 percent ± 0.011) and direct observation of the bubble's in-depth z-position, both validate the results independently., Multi Phase Systems, Fluid Mechanics

We present results on the analysis of measurements on the dispersion of a contaminant in a turbulent boundary layer. The contaminant is a fluorescent dye (Rh.-B) that was introduced in a turbulent boundary layer (푅푅푒푒# = 3050) in a water channel. The velocity field was measured by means of tomographic PIV. Simultaneously, the concentration field was measured by means of laser-induced fluorescence (LIF). The LIF data clearly reveal uniform concentration zones with well-defined boundaries. These zones are extracted from the probability density function of the local concentration. The key question is what flow structures make up their boundaries, and how these uniform concentration zones are related to uniform momentum zones. We evaluate the Lyapunov exponents of the rate of separation of two close points while they are advected by the flow. Local maxima of the Lyapunov exponents are known to create temporary barriers for transport in two-dimensional flows. Alternatively, the boundaries of uniform concentration zones may also be formed by shear layers, which are local maxima of the shear vorticity. There is a striking concentration of both the Lyapunov exponent and the shear vorticity on real boundaries. The correlation with the Lyapunov field is the weakest of the two. However, the Lyapunov exponent is a Lagrangian average, extending over episodes where these boundaries were formed, whereas the shear vorticity is based on a planar cross section of the instantaneous velocity field., Fluid Mechanics

Two-phase flow in ribbed or corrugated pipes is of interest in many industrial applications. Experiments are performed to assess the flow regime characteristics in upward annular flow through vertical smooth and corrugated pipes. From high speed recordings, the flow regime and temporal film characteristics are obtained. A novel implementation of a planar laser-induced fluorescence (PLIF) method is used to measure the film thickness, preventing strong reflections from deteriorating the measurements. Liquid accumulation between the ribs of the corrugated pipe is also measured using a PLIF technique. Furthermore, droplet sizing is performed combining shadowgraphic and interferometric techniques to capture a large droplet size range. The measurements show that the presence of pronounced corrugations at the pipe wall causes a strong increase in entrainment of liquid into the gas flow. The entrainment is correlated to the filling of the corrugations with liquid; it is significantly reduced (from 90% entrainment to 50%) when the corrugations are entirely filled with liquid. The amount of liquid filling of the corrugations is related to the superficial liquid film flow velocity. The liquid filling fraction (α) scales with the Weber and liquid Reynolds number, and the obtained scaling also holds when the experiments are repeated with a different liquid (mono-ethylene glycol) and with a larger corrugation geometry. Droplets occurring in corrugated pipe flow are 30–50% larger compared to the smooth pipe, as a consequence of the locally (at the locations of the cavities) increased film thickness., Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public., Fluid Mechanics, Multi Phase Systems

In micro-PIV, reaching the optimum image density has always been difficult due to high displacement gradient, coagulation of particles at the inlet and due to particles adhering to the surface to name a few. The most widely used method is to take ensemble correlation average of multiple PIV images to extract the velocity field. However, this leads to low temporal resolution. Hence, a method with high temporal resolution is pivotal to study unsteady laminar flow in microfluidic application. This work aims at developing a new PIV algorithm which reduces the effective seeding density and at the same time yield similar if not higher SNR compared to conventional PIV. We call this algorithm Ψ-PIV. This method is suitable for steady and unsteady laminar flows which are generally found in microfluidic applications. The reliability and the precision of the new method as a function of particle image pairs is investigated by synthetic image analysis. The main advantage of Ψ-PIV is its ability to achieve higher temporal resolution from the typical micro-PIV raw images. The experimental investigation for the flow around a 2D cylinder in a Hele-Shaw cell showed that the Ψ-PIV results in a reduction by approximately a factor of 25 in the number of frames required compared to conventional PIV., Fluid Mechanics

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

Simultaneous particle-image velocimetry and laser-induced fluorescence combined with large-eddy simulations are used to investigate the flow and pollutant dispersion behaviour in a rural-to-urban roughness transition. The urban roughness is characterized by an array of cubical obstacles in an aligned arrangement. A plane fence is added one obstacle height h upstream of the urban roughness elements, with three different fence heights considered. A smooth-wall turbulent boundary layer with a depth of 10h is used as the approaching flow, and a passive tracer is released from a uniform line source 1h upstream of the fence. A shear layer is formed at the top of the fence, which increases in strength for the higher fence cases, resulting in a deeper internal boundary layer (IBL). It is found that the mean flow for the rural-to-urban transition can be described by means of a mixing-length model provided that the transitional effects are accounted for. The mixing-length formulation for sparse urban canopies, as found in the literature, is extended to take into account the blockage effect in dense canopies. Additionally, the average mean concentration field is found to scale with the IBL depth and the bulk velocity in the IBL., Fluid Mechanics, Atmospheric Physics, Atmospheric Remote Sensing