Publications
77 results found
Kankanwadi KS, Buxton ORH, 2019, Turbulent entrainment from a turbulent background
Simultaneous particle image velocimetry (PIV) and planar laser induced fluorescence (PLIF) measurements were conducted in order to investigate the effects of background turbulence on the entrainment process as well as on the behaviour of the wake interface. Previous studies have highlighted the importance of length scale as well as turbulence intensity in the background flow. This paper reports on a parametric study examining entrainment into the wake of a circular cylinder by independently varying background turbulence parameters through the use of turbulence generating grids. Despite the availability of turbulent rotational fluid on both sides of the interface, the classical turbulent/non-turbulent interface result of an enstrophy jump is reproduced, even in the harshest incoming free-stream turbulence conditions. Examining the tortuosity, reveals that both length scale and turbulence intensity in the background turbulence act to increase the interface surface area. Furthermore, the entrainment process is found to be greatly sensitive to the turbulence intensity of the subjected free-stream turbulence. However, despite an increase in surface area, a net reduction in mean entrainment mass flux is observed with increased intensity in the background turbulence. Examining the mass flux PDFs, reveals that this behaviour is a result of substantial, yet infrequent detrainment events.
Breda M, Buxton O, 2018, Effects of multiscale geometry on the large-scale coherent structures of an axisymmetric turbulent jet, Journal of Visualization, Vol: 21, Pages: 525-532, ISSN: 1343-8875
In this study, the effect of multiscale geometry on the near-field structure of an axisymmetric turbulent jet is examined at a global Reynolds number of ReG=10,000. With the aid of tomographic particle image velocimetry, the suppression of the coherent structures due to this fractal geometry is analysed and the changes to the near-field vorticity are evaluated. This particular geometry leads to the breakup of the azimuthal vortex rings present for round jets and to the formation of radial and streamwise opposite-signed patches of vorticity. The latter are found to be responsible for the axis switching of the jet, a phenomenon observed for some noncircular jets where the major axis shrinks and the minor one expands in the near field, effectively switching the two axes of the jet. This was the first time, to the knowledge of the authors, that axis switching has been observed for a jet where the coherent structures have been suppressed. Following the significant differences found in the near field, the far field is examined. There, the integral lengthscale of the large scale eddies Lur and the size of the jet evaluated in terms of the jet half-width r1/2 are found to evolve in a similar fashion, whilst the ratio Lur/r1/2 is found to be higher for the fractal jet than for the round jet, for which the near-field structures have not been suppressed.
Breda M, Buxton O, 2018, Influence of coherent structures on the evolution of an axisymmetric turbulent jet, Physics of Fluids, Vol: 30, Pages: 035109-1-035109-24, ISSN: 1070-6631
The role of initial conditions in affecting the evolution toward self-similarity of an axisymmetric turbulent jet is examined. The jet’s near-field coherence was manipulated by non-circular exit geometries of identical open area, D2e, including a square and a fractal exit, for comparison with a classical round orifice jet. Hot-wire anemometry and 2D-planar particle image velocimetry experiments were performed between the exit and a location 26De downstream, where the Reynolds stress profiles are self-similar. This study shows that a fractal geometry significantly changes the near-field structure of the jet, breaking up the large-scale coherent structures, thereby affecting the entrainment rate of the background fluid into the jet stream. It is found that many of the jet’s turbulent characteristics scale with the number of eddy turnover times rather than simply the streamwise coordinate, with the entrainment rate (amongst others) found to be comparable across the different jets after approximately 3-4 eddies have been overturned. The study is concluded by investigating the jet’s evolution toward a self-similar state. No differences are found for the large-scale spreading rate of the jets in the weakly self-similar region, so defined as the region for which some, but not all of the terms of the mean turbulent kinetic energy equation are self-similar. However, the dissipation rate of the turbulent kinetic energy was found to vary more gradually in x than predicted according to the classical equilibrium theories of Kolmogorov. Instead, the dissipation was found to vary in a non-equilibrium fashion for all three jets tested.
Buxton O, Ewenz Rocher M, Rodriguez-Lopez E, 2018, Influence of strong perturbations on wall-bounded flows, Physical Review Fluids, Vol: 3, Pages: 014605-1-014605-18, ISSN: 2469-990X
Single-point hot-wire measurements are made downstream of a series of spanwise repeating obstacles that are used to generate an artificially thick turbulent boundary layer. The measurements are made in the near field, in which the turbulent boundary layer is beginning to develop from the wall-bounded wakes of the obstacles. The recent paper of Rodríguez-López et al. [E. Rodríguez-López et al., Phys. Rev. Fluids 1, 074401 (2016)] broadly categorized the mechanisms by which canonical turbulent boundary layers eventually develop from wall-bounded wakes into two distinct mechanisms, the wall-driven and wake-driven mechanisms. In the present work we attempt to identify the geometric parameters of tripping arrays that trigger these two mechanisms by examining the spectra of the streamwise velocity fluctuations and the intermittent outer region of the flow. Using a definition reliant upon the magnitude of the velocity fluctuations, an intermittency function is devised that can discriminate between turbulent and nonturbulent flow. These results are presented along with the spectra in order to try to ascertain which aspects of a trip's geometry are more likely to favor the wall-driven or wake-driven mechanism. The geometrical aspects of the trips tested are the aspect ratio, the total blockage, and the blockage at the wall. The results indicate that the presence, or not, of perforations is the most significant factor in affecting the flow downstream. The bleed of fluid through the perforations reenergizes the mean recirculation and leads to a narrower intermittent region with a more regular turbulent-nonturbulent interface. The near-wall turbulent motions are found to recover quickly downstream of all of the trips with a wall blockage of 50%, but a clear influence of the outer fluctuations, generated by the tip vortices of the trips, is observed in the near-wall region for the high total blockage trips. The trip with 100% wall blockage is found to
Baj P, Buxton O, 2017, Interscale energy transfer in the merger of wakes of multi-scale arrays of rectangular cylinders, Physical Review Fluids, Vol: 2, ISSN: 2469-990X
The near wake of a flow past a multiscale array of bars is studied by means of particle image velocimetry (PIV). The aim of this research is to understand the nature of multiscale flows, where multiple coherent motions of nonuniform sizes and characteristic frequencies (i.e., sheddings of particular bars in our considered case) interact with each other. The velocity fields acquired from the experiments are triple decomposed into their mean, a number of coherent fluctuations, and their stochastic part according to a triple decomposition technique introduced recently by Baj et al., Phys. Fluids 27, 075104 (2015). This nonstandard approach allows us to monitor the interactions between different coherent fluctuations representative of sheddings of the particular bars. Further, additional equations governing the kinetic energy of the recognized velocity components are derived to provide better insight into the dynamics of these interactions. Interestingly, apart from the coherent fluctuations associated with sheddings, some additional, secondary coherent fluctuations are also recognized. These seem to appear as a result of nonlinear triadic interactions between the primary shedding modes when the two shedding structures of different characteristic frequencies are in close proximity to one another. The secondary coherent motions are almost exclusively supplied with energy by the primary coherent motions, whereas the latter are driven by the mean flow. It is also found that the coherent fluctuations play an important role in exciting the stochastic fluctuations, as the energy is not fed to the stochastic fluctuations directly from the mean flow but rather through the coherent modes.
Steiros K, Bruce PJK, Buxton ORH, et al., 2017, Effect of blade modifications on the torque and flow field of radial impellers in stirred tanks, PHYSICAL REVIEW FLUIDS, Vol: 2, ISSN: 2469-990X
We perform both high- and low-speed particle image velocimetry and torque measurements to characterize eight radial impeller types in an unbaffled stirred tank. The blade types consist of a set of regular flat blades, used as a baseline, regular blades of increased thickness, perforated blades, and fractal blades. We find a qualitative correlation between the blades' torque coefficient and both vortex coherence and turbulent kinetic energy, possibly explaining the torque differences of the tested impellers. Furthermore, we find that the proposed modifications increase the bulk turbulence levels and mass flow rates while at the same time reducing the shaft torque, showing promise for applications. Finally, we attempt a comparison between fractal and perforated geometries using data from this study and the literature.
Rodríguez-López E, Brizzi LE, Valeau V, et al., 2017, Aeroacoustic Characterization of Single- and Multiscale Porous Fences, AIAA Journal, Vol: 56, Pages: 264-278, ISSN: 0001-1452
An experimental characterization of the acoustic behavior and the flowfield past several wall-mounted porousfences is presented. Particle image velocimetry is conducted in the near field of the fences. It shows a spanwiseperiodicity (associated with the periodic geometry of the grid) of regions where well-defined wakes appear as opposedto regions with a lower mass flow rate influenced by strong recirculations. The acoustic behavior is characterizedusing a microphone array enabling the spatial locationof noise sourcesto bedetermined. Broadband noise is observedfor all grids except for the fractal square grid, where a clear peak appears. The intensity of noise spectra displaysscalabilitywith MachnumberM6(dipoles), and the preferentialfrequencyfor the fractal square case is shown to scalewith thefreestreamvelocity(suggestingasheddingmechanism).For certaingrids, themeanandinstantaneous spatiallocations of the noise sources also present a spanwise preferential arrangement coincident with regions where the flowpresents well-defined wakes. Regarding the intermittency of the unsteady sources, they present smaller temporalwidth, separation, and spatial extent for increasing freestream velocity. However, the ratio between these quantitiesseems to be constant to within 6% for different Mach numbers.
Prigent SL, Buxton ORH, Bruce PJK, 2017, Coherent structures shed by multiscale cut-in trailing edge serrations on lifting wings, Physics of Fluids, Vol: 29, ISSN: 1070-6631
This experimental study presents the effect of multiscale cut-in trailing edge serrations on the coherentstructures shed into the wake of a lifting wing. Two-probe span-wise hot-wire traverses are performedto study spectra, coherence, and phase shift. In addition, planar particle image velocimetry is used tostudy the spatio-temporal structure of the vortices shed by the airfoils. Compared with a single tonesinusoidal serration, the multiscale ones reduce the vortex shedding energy as well as the span-wisecoherence. Results indicate that the vortex shedding is locked into an arch-shaped cell structure. Thisstructure is weakened by the multiscale patterns, which explains the reduction in both shedding energyand coherence.
Breda M, Buxton O, 2017, Near and far-field analysis of an axisymmetric fractal-forced turbulent jet, Progress in Turbulence VII Proceedings of the iTi Conference in Turbulence 2016, Editors: Orlu, Talamelli, Oberlack, Peinke, Publisher: Springer, Pages: 211-217, ISBN: 9783319579344
This volume collects the edited and reviewed contribution presented in the 7th iTi Conference in Bertinoro, covering fundamental and applied aspects in turbulence.
Rodriguez-Lopez E, Bruce PJK, Buxton ORH, 2017, Flow characteristics and scaling past highly porous wall-mounted fences, Physics of Fluids, Vol: 29, ISSN: 1070-6631
An extensive characterization of the flow past wall-mounted highly porous fences based on single-and multi-scale geometries has been performed using hot-wire anemometry in a low-speed windtunnel. Whilst drag properties (estimated from the time-averaged momentum equation) seem to bemostly dependent on the grids’ blockage ratio; wakes of different size and orientation bars seem togenerate distinct behaviours regarding turbulence properties. Far from the near-grid region, the flowis dominated by the presence of two well-differentiated layers: one close to the wall dominated bythe near-wall behaviour and another one corresponding to the grid’s wake and shear layer, originatingfrom between this and the freestream. It is proposed that the effective thickness of the wall layer canbe inferred from the wall-normal profile of root-mean-square streamwise velocity or, alternatively,from the wall-normal profile of streamwise velocity correlation. Using these definitions of wall-layerthickness enables us to collapse different trends of the turbulence behaviour inside this layer. Inparticular, the root-mean-square level of the wall shear stress fluctuations, longitudinal integral lengthscale, and spanwise turbulent structure is shown to display a satisfactory scaling with this thicknessrather than with the whole thickness of the grid’s wake. Moreover, it is shown that certain gridsdestroy the spanwise arrangement of large turbulence structures in the logarithmic region, which arethen re-formed after a particular streamwise extent. It is finally shown that for fences subject to aboundary layer of thickness comparable to their height, the effective thickness of the wall layer scaleswith the incoming boundary layer thickness. Analogously, it is hypothesized that the growth rate ofthe internal layer is also partly dependent on the incoming boundary layer thickness.
Buxton O, Breda M, Chen X, 2017, Invariants of the velocity-gradient tensor in a spatially developing inhomogeneous turbulent flow, Tenth International Symposium on Turbulence and Shear Flow Phenomena
Breda M, Buxton O, 2017, Role of initial conditions in the evolution of an axisymmetric turbulent jet due to geometrical effects on the near-field coherence, Tenth International Symposium on Turbulence and Shear Flow Phenomena
Buxton O, Breda M, Chen X, 2017, Invariants of the velocity gradient tensor in a spatially developing inhomogeneous turbulent flow, Journal of Fluid Mechanics, Vol: 817, Pages: 1-20, ISSN: 1469-7645
Tomographic PIV experiments were performed in the near-field of the turbulent flow pasta square cylinder. A classical Reynolds decomposition was performed on the resultingvelocity fields into a time invariant mean flow and a fluctuating velocity field. This fluc-tuating velocity field was then further decomposed into coherent and residual/stochasticfluctuations. The statistical distributions of the second and third invariants of the ve-locity gradient tensor were then computed at various streamwise locations, along thecentre line of the flow and within the shear layers. These invariants were calculated fromboth the Reynolds-decomposed fluctuating velocity fields and the coherent and stochas-tic fluctuating velocity fields. The range of spatial locations probed incorporates regionsof contrasting flow physics, including a mean recirculationregion and separated shearlayers, both upstream and downstream of the location of peakturbulence intensity alongthe centre line. These different flow physics are also reflected in the velocity gradientsthemselves with different topologies, as characterised by the statistical distributions ofthe constituent enstrophy and strain-rate invariants, forthe three different fluctuatingvelocity fields. Despite these differing flow physics the ubiquitous self-similar “tear drop”-shaped joint probability density function between the second and third invariants of thevelocity gradient tensor is observed along the centre line and shear layer when calcu-lated from both the Reynolds decomposed and the stochastic velocity fluctuations. These“tear drop”-shaped joint probability density functions are not, however, observed whencalculated from the coherent velocity fluctuations. This “tear drop” shape is classicallyassociated to the statistical distribution of the velocitygradient tensor invariants in fullydeveloped turbulent flows in which there are no coherent dynamics present, and hencespectral peaks at low waven
Prigent SL, Buxton ORH, Bruce PJK, 2017, Experimental investigation of the wake of a lifting wing with cut-in sinusoidal trailing edges, AIAA Journal, Vol: 55, Pages: 1590-1601, ISSN: 1533-385X
The wake behind a NACA0012 wing at incidence with cut-in sinusoidal trailing edges is experimentally investigated. A wing model with interchangeable trailing edges is used to study their impact on the wake properties. Both vertical and spanwise traverses of hot wires are done at different downstream positions to obtain the downstream evolution of statistical properties and to perform spectral analysis. Stereoscopic particle image velocimetry is used to study the flow structure in a spanwise/cross-stream plane. Spanwise inhomogeneity of the velocity deficit and of the wake width is observed and explained by the presence of a spanwise/cross-stream flow induced by the cut-in modifications. Spectral analysis shows a decrease of shedding intensity with a shorter trailing-edge wavelength, with a reduction of up to 57% when compared to a straight blunt wing. Blunt sinusoidal trailing edges exhibit a reduction of spanwise correlation compared to a blunt straight one. A sharp cut-in design is also studied, which exhibits a more broadband shedding spike at a lower frequency.
Rodríguez-López E, Bruce PJK, Buxton ORH, 2017, Experimental measurement of wall shear stress in strongly disrupted flows, Journal of Turbulence, Vol: 18, Pages: 271-290, ISSN: 1468-5248
Mean and fluctuating wall shear stress is measured in strongly disrupted cases generated by various low-porosity wall-mounted single- and multi-scale fences. These grids generate a highly turbulent wake which interacts with the wall-bounded flow modifying the wall shear stress properties. Measurement methods are validated first against a naturally growing zero pressure gradient turbulent boundary layer showing accuracies of 1% and 4% for extrapolation and direct measurement of the mean shear stress respectively. Uncertainty associated with the root mean square level of the fluctuations is better than 2% making it possible to measure small variations originating from the different fences. Additionally, probability density functions and spectra are also measured providing further insight into the flow physics. Measurement of shear stress in the disrupted cases (grid+TBL) suggest that the flow characteristics and turbulence mechanisms remain unaltered far from the grid even in the most disrupted cases. However, a different root mean square level of the fluctuations is found for different grids. Study of the probability density functions seem to imply that there are different degrees of interaction between the inner and outer regions of the flow.
Steiros K, Bruce PJK, Buxton ORH, et al., 2017, Power consumption and form drag of regular and fractal-shaped turbines in a stirred tank, AIChE Journal, Vol: 63, Pages: 843-843, ISSN: 0001-1541
Previous wind-tunnel measurements have shown that fractal-shaped plates have increased drag compared to square plates of the same area. In this study, the power consumption and drag of turbines with fractal and rectangular blades in a stirred tank are measured. Power number decreases from rectangular to fractal impellers by over 10%, increasingly so with fractal iteration number. Our results suggest that this decrease is not caused by the wake interaction of the blades, nor solely by the wake interaction with the walls either. Pressure measurements on the blades’ surface show that fractal blades have lower drag than the rectangular ones, opposite to the wind tunnel experiment results. All tested blades’ center of pressure radius increases with Re, while their drag coefficient decreases, a possible effect of the solid body rotation expansion with Re. Spectral analysis of the pressure signal reveals two peaks possibly connected to the blades’ roll vortices.
Rodriguez-Lopez E, Bruce P, Buxton O, 2016, Near field development of artificially generated high Reynolds number turbulent boundary layers, Physical Review Fluids, Vol: 1, Pages: 1-22, ISSN: 2469-990X
Particle image velocimetry is conducted in the near field of two distinct wall-mounted trips for the artificial generation of a high Reynolds number turbulent boundary layer. The first of these trips consists of high aspect ratio obstacles, which are supposed to minimize the influence of their wakes on the near-wall region, contrasting with low aspect ratio trips, which would enhance this influence. A comprehensive study involving flow description, turbulent-nonturbulent interface detection, a low-order model description of the flow and an exploration of the influence of the wake in the near-wall region is conducted and two different mechanisms are clearly identified and described. First, high aspect ratio trips generate a wall-driven mechanism whose characteristics are a thinner, sharper, and less tortuous turbulent-nonturbulent interface and a reduced influence of the trips' wake in the near-wall region. Second, low aspect ratio trips generate a wake-driven mechanisms in which their turbulent-nonturbulent interface is thicker, less sharply defined, and with a higher tortuosity and the detached wake of the obstacles presents a significant influence on the near-wall region. Study of the low-order modeling of the flow field suggests that these two mechanisms may not be exclusive to the particular geometries tested in the present study but, on the contrary, can be explained based on the predominant flow features. In particular, the distinction of these two mechanisms can explain some of the trends that have appeared in the literature in the past decades.
Fiscaletti D, Elsinga GE, Attili A, et al., 2016, Scale dependence of the alignment between strain rate and rotation in turbulent shear flow, Physical Review Fluids, Vol: 1, Pages: 1-14, ISSN: 2469-990X
The scale dependence of the statistical alignment tendencies of the eigenvectors of the strain-rate tensor ei, with the vorticity vector ω, is examined in the self-preserving region of a planar turbulent mixing layer. Data from a direct numerical simulation are filtered at various length scales and the probability density functions of the magnitude of the alignment cosines between the two unit vectors |ei⋅ωˆ| are examined. It is observed that the alignment tendencies are insensitive to the concurrent large-scale velocity fluctuations, but are quantitatively affected by the nature of the concurrent large-scale velocity-gradient fluctuations. It is confirmed that the small-scale (local) vorticity vector is preferentially aligned in parallel with the large-scale (background) extensive strain-rate eigenvector e1, in contrast to the global tendency for ω to be aligned in parallel with the intermediate strain-rate eigenvector [Hamlington et al., Phys. Fluids 20, 111703 (2008)]. When only data from regions of the flow that exhibit strong swirling are included, the so-called high-enstrophy worms, the alignment tendencies are exaggerated with respect to the global picture. These findings support the notion that the production of enstrophy, responsible for a net cascade of turbulent kinetic energy from large scales to small scales, is driven by vorticity stretching due to the preferential parallel alignment between ω and nonlocal e1 and that the strongly swirling worms are kinematically significant to this process.
Park O, Burns RA, Buxton O, et al., 2016, Mixture fraction, soot volume fraction, and velocity imaging in the soot-inception region of a turbulent non-premixed jet flame, Proceedings of the Combustion Institute, Vol: 36, Pages: 899-907, ISSN: 1873-2704
Steiros K, Bruce, Buxton O, et al., 2016, Flow Field Characteristics and Energy Injection in a Tank Stirred by Regular and Fractal Blade Impellers, Proceedings of the 5th International Conference on Jets, Wakes and Separated Flows (ICJWSF2015), Editors: Segalini, Publisher: Springer, Pages: 363-369, ISBN: 978-3-319-30602-5
Baj P, Bruce PJK, Buxton ORH, 2016, On a PLIF quantification methodology in a nonlinear dye response regime, Experiments in Fluids, Vol: 57, ISSN: 1432-1114
A new technique of planar laser-induced fluorescence calibration is presented in this work. It accounts for a nonlinear dye response at high concentrations, an illumination light attenuation and a secondary fluorescence’s influence in particular. An analytical approximation of a generic solution of the Beer–Lambert law is provided and utilized for effective concentration evaluation. These features make the technique particularly well suited for high concentration measurements, or those with a large range of concentration values, c, present (i.e. a high dynamic range of c). The method is applied to data gathered in a water flume experiment where a stream of a fluorescent dye (rhodamine 6G) was released into a grid-generated turbulent flow. Based on these results, it is shown that the illumination attenuation and the secondary fluorescence introduce a significant error into the data quantification (up to 15 and 80 %, respectively, for the case considered in this work) unless properly accounted for.
Rodríguez-López E, Bruce PJK, Buxton ORH, 2016, On the Formation Mechanisms of Artificially Generated High Reynolds Number Turbulent Boundary Layers, Boundary-Layer Meteorology, Vol: 160, Pages: 201-224, ISSN: 0006-8314
We investigate the evolution of an artificially thick turbulent boundary layer generatedby two families of small obstacles (divided into uniform and non-uniform wall normaldistributions of blockage). One- and two-point velocity measurements using constant temperatureanemometry show that the canonical behaviour of a boundary layer is recovered afteran adaptation region downstream of the trips presenting 150 % higher momentum thickness(or equivalently, Reynolds number) than the natural case for the same downstream distance(x ≈ 3 m). The effect of the degree of immersion of the trips for h/δ 1 is shown to play asecondary role. The one-point diagnostic quantities used to assess the degree of recovery ofthe canonical properties are the friction coefficient (representative of the inner motions), theshape factor and wake parameter (representative of the wake regions); they provide a severetest to be applied to artificially generated boundary layers. Simultaneous two-point velocitymeasurements of both spanwise and wall-normal correlations and the modulation of innervelocity by the outer structures show that there are two different formation mechanisms forthe boundary layer. The trips with high aspect ratio and uniform distributed blockage leavethe inner motions of the boundary layer relatively undisturbed, which subsequently drivethe mixing of the obstacles’ wake with the wall-bounded flow (wall-driven). In contrast, thelow aspect-ratio trips with non-uniform blockage destroy the inner structures, which are thenre-formed further downstream under the influence of the wake of the trips (wake-driven).
Bruce PJK, Buxton ORH, Rodriguez-Lopez E, 2016, Downstream evolution of perturbations in a zero pressure gradient turbulent boundary layer, Progress in Turbulence VI: Proceedings of the iTi Conference on Turbulence 2014, Vol: 165, Pages: 133-137, ISSN: 0930-8989
This paper examines the evolution of perturbations generated by various trips in a zero pressure gradient turbulent boundary layer. Measurements taken using hot-wire anemometry show that the evolution of the boundary layer towards the natural state is strongly dependent on the trip geometry. In particular the mechanisms creating the boundary layer appear to depend primarily on the wall normal distribution of blockage ratio, recovering the natural properties more rapidly for a uniform distribution of blockage (wall normal cylinders) than for non-uniform blockage (sawtooth fence). The relative size of the trip with respect to the boundary layer is shown to be a second order effect. Standard behaviour (characterized by the skin friction coefficient, CfCf, the wake component, ΠΠ, and the shape factor, H) is recovered successfully 500D∼75h500D∼75h downstream, presenting 175%175% higher momentum thickness, θθ, than the natural case for the same downstream distance.
Rodriguez-Lopez E, Bruce PJK, Buxton O, 2016, Downstream Evolution of Perturbations in a Zero Pressure Gradient Turbulent Boundary Layer, Progress in Turbulence VI, Editors: Peinke, Kampers, Oberlack, Waclawczyk, Talamelli, Publisher: Springer, ISBN: 978-3-319-29129-1
Buxton O, Ganapathisubramani B, 2016, Concurrent Scale Interactions in the Far-Field of a Turbulent Mixing Layer, Progress in Turbulence VI, Editors: Peinke, Kampers, Oberlack, Waclawczyk, Talamelli, Publisher: Springer, ISBN: 978-3-319-29129-1
Prigent SL, Buxton O, Bruce PJK, 2016, Experimental investigation of the wake of a lifting wing with cut-in sinusoidal trailing edges, 54th AIAA Aerospace Sciences Meeting, Publisher: American Institute of Aeronautics and Astronautics
The wake behind a NACA0012 wing at incidence with cut-in sinusoidal trailing edges(TE) is experimentally investigated. A wing model with interchangeable trailing edges isused to study their impact on the wake properties. Both vertical and span-wise traversesof hot-wires are done at different downstream positions, to obtain the downstream evolutionof statistical properties and to perform spectral analysis. Stereoscopic particle imagevelocimetry is used to study the flow structure in a span-wise/cross-stream plane.Span-wise inhomogeneity of the velocity deficit and of the wake width is observed andexplained by the presence of a span-wise/cross-stream flow induced by the cut-in modifications.Spectral analysis shows a decrease of shedding intensity with shorter TE wavelength,with up to 57% reduction when compared to a straight blunt wing. Blunt sinusoidal TEsoffer a reduction of span-wise correlation compared to the blunt straight one, which ismitigated when compared to an unmodified wing. A sharp cut-in design is also studied,that exhibits a more broad-band shedding spike at a lower frequency.
Buxton ORH, 2015, Modulation of the velocity gradient tensor by concurrent large-scale velocity fluctuations in a turbulent mixing layer, Journal of Fluid Mechanics, Vol: 777, ISSN: 1469-7645
The modulation of small-scale velocity and velocity gradient quantities by concurrent large-scale velocity fluctuations is observed by consideration of the Kullback–Leibler divergence. This is a measure that quantifies the loss of information in modelling a statistical distribution of small-scale quantities conditioned on concurrent positive large-scale fluctuations by that conditioned on negative large-scale fluctuations. It is observed that the small-scale turbulence is appreciably ‘rougher’ when the concurrent large-scale fluctuation is positive in the low-speed side of a fully developed turbulent mixing layer, which gives further evidence to the convective scale modulation argument of Buxton & Ganapathisubramani (Phys. Fluids, vol. 26, 2014, 125106, 1–19). The definition of the small scales is varied, and regardless of whether the small-scale fluctuations are dominated by dissipation or have the characteristic features of inertial range turbulence they are shown to be modulated by the concurrent large-scale fluctuations. The modulation is observed to persist even when there is a large gap in wavenumber space between the small and large scales, although local maxima are observed at intermediate length scales that are significantly larger than the predefined small scales. Finally, it is observed that the modulation of small-scale dissipation is greater than that for enstrophy with the modulation of the vortex stretching term, indicative of the interaction between strain rate and rotation, being intermediate between the two.
Baj P, Bruce PJK, Buxton ORH, 2015, The triple decomposition of a fluctuating velocity field in a multiscale flow, Physics of Fluids, Vol: 27, Pages: 075104-1-075104-24, ISSN: 1070-6631
A new method for the triple decomposition of a multiscale flow, which is based on the novel optimal mode decomposition (OMD) technique, is presented. OMD provides low order linear dynamics, which fits a given data set in an optimal way and is used to distinguish between a coherent (periodic) part of a flow and a stochastic fluctuation. The method needs no external phase indication since this information, separate for coherent structures associated with each length scale introduced into the flow, appears as the output. The proposed technique is compared against two traditional methods of the triple decomposition, i.e., bin averaging and proper orthogonal decomposition. This is done with particle image velocimetry data documenting the near wake of a multiscale bar array. It is shown that both traditional methods are unable to provide a reliable estimation for the coherent fluctuation while the proposed technique performs very well. The crucial result is that the coherence peaks are not observed within the spectral properties of the stochastic fluctuation derived with the proposed method; however, these properties remain unaltered at the residual frequencies. This proves the method’s capability of making a distinction between both types of fluctuations. The sensitivity to some prescribed parameters is checked revealing the technique’s robustness. Additionally, an example of the method application for analysis of a multiscale flow is given, i.e., the phase conditioned transverse integral length is investigated in the near wake region of the multiscale object array.
Rodriguez-Lopez E, Bruce PJK, Buxton ORH, 2015, A robust post-processing method to determine skin friction in turbulent boundary layers from the velocity profile, Experiments in Fluids: experimental methods and their applications to fluid flow, Vol: 56, Pages: 1-16, ISSN: 0723-4864
The present paper describes a method to extrapolate the mean wall shear stress, 𝜏𝑤𝑎𝑙𝑙, and the accurate relative position of a velocity probe with respect to the wall, Δ𝑦, from an experimentally measured mean velocity profile in a turbulent boundary layer. Validation is made between experimental and direct numerical simulation data of turbulent boundary layer flows with independent measurement of the shear stress. The set of parameters which minimize the residual error with respect to the canonical description of the boundary layer profile is taken as the solution. Several methods are compared, testing different descriptions of the canonical mean velocity profile (with and without overshoot over the logarithmic law) and different definitions of the residual function of the optimization. The von Kármán constant is used as a parameter of the fitting process in order to avoid any hypothesis regarding its value that may be affected by different initial or boundary conditions of the flow. Results show that the best method provides an accuracy of Δ𝑢𝜏≤0.6% for the estimation of the friction velocity and Δ𝑦+≤0.3 for the position of the wall. The robustness of the method is tested including unconverged near-wall measurements, pressure gradient, and reduced number of points; the importance of the location of the first point is also tested, and it is shown that the method presents a high robustness even in highly distorted flows, keeping the aforementioned accuracies if one acquires at least one data point in 𝑦+<10. The wake component and the thickness of the boundary layer are also simultaneously extrapolated from the mean velocity profile. This results in the first study, to the knowledge of the authors, where a five-parameter fitting is carried out without any assumption on the von Kármán constant and the limits of the logarithmic layer further from its existence.
Rabey PK, Wynn A, Buxton ORH, 2015, The kinematics of the reduced velocity gradient tensor in a fully developed turbulent free shear flow, JOURNAL OF FLUID MECHANICS, Vol: 767, Pages: 627-658, ISSN: 0022-1120
- Author Web Link
- Cite
- Citations: 9
This data is extracted from the Web of Science and reproduced under a licence from Thomson Reuters. You may not copy or re-distribute this data in whole or in part without the written consent of the Science business of Thomson Reuters.