Stereoscopic particle image velocimetry (PIV) was used to measure all three instantaneous components of the velocity field in streamwise–spanwise planes a turbulent boundary layer at Re τ =1060 ( θ =2500). Datasets were obtained logarithmic and beyond. The vector fields log z + =92 150) revealed signatures vortex packets similar those proposed by Adrian co-workers their PIV experiments. Groups legs hairpin vortices appeared be coherently arranged streamwise direction. These regions also...

The relationship between the upstream boundary layer and low-frequency, large-scale unsteadiness of separated flow in a Mach 2 compression ramp interaction is investigated by performing wide-field particle image velocimetry (PIV) planar laser scattering (PLS) measurements streamwise–spanwise planes. Planar indicate presence spanwise strips elongated regions uniform momentum with lengths greater than 40δ. These long coherent structures have been observed supersonic (Ganapathisubramani,...

Stereoscopic particle image velocimetry (PIV) measurements are made in streamwise–spanwise and inclined cross-stream planes (inclined at plane, which is consistent with the presence of hairpin packets a low-speed region lifting away from wall.

Wide-field particle image velocimetry measurements were performed in a Mach 2 turbulent boundary layer to study the characteristics of large-scale coherence at two wall-normal locations (, which indicates an increasing trend streamwise length scale with location. The spanwise these uniform-velocity strips increases distance as found subsonic layers. observed is consistent very motion (VLSM) model proposed by Kim & Adrian (Phys. Fluids, vol. 11, 1999, p. 417) for incompressible

Abstract In this study we examine the impact of strength large-scale motions on amplitude and frequency small scales in high-Reynolds-number turbulent boundary layers. Time series hot-wire data are decomposed into large- small-scale components, large scale is considered. The modulation effect examined by conditionally averaging intensity ( ${ u}_{S}^{2} $ ) for various values fluctuation ${u}_{L} ). It shown that increases with increasing value near-wall region, whereas, farther away from...

An array of surface hot-film shear-stress sensors together with a traversing hot-wire probe is used to identify the conditional structure associated large-scale skin-friction event in high-Reynolds-number turbulent boundary layer. It found that events convect at velocity much faster than local mean near-wall region (the convection for fluctuations be close midpoint logarithmic region). Instantaneous data indicate presence structures wall are comparable scale and arrangement superstructure...

Abstract A new method, herein referred to as optimal mode decomposition (OMD), of finding a linear model describe the evolution fluid flow is presented. The method estimates dynamics high-dimensional system which first projected onto subspace user-defined fixed rank. An iterative procedure used find combination and that minimizes residual error. OMD shown be generalization dynamic (DMD), in not optimized but rather proper orthogonal (POD) modes. Furthermore, provide an approximation Koopman...

This paper presents the results of a detailed experimental investigation into effectiveness sinusoidal leading edge serrations on aerofoils for reduction noise generated by interaction with turbulent flow. A parametric study is performed to investigate sensitivity reductions serration amplitude and wavelength. The primarily flat plates in an idealized flow, which we demonstrate captures same behaviour as when identical are introduced onto three-dimensional aerofoils. influence turbulence...

Large-scale secondary flows can sometimes appear in turbulent boundary layers formed over rough surfaces, creating low- and high-momentum pathways along the surface (Barros & Christensen, J. Fluid Mech. , vol. 748, 2014, R1). We investigate experimentally dependence of these on surface/flow conditions by measuring streamwise strips roughness with systematically varied spanwise spacing. find that large-scale are accentuated when spacing elements is roughly proportional to layer thickness...

Abstract An experimental investigation involving space-filling fractal square grids is presented. The flow documented using particle image velocimetry (PIV) in a water tunnel as opposed to previous experiments which mostly used hot-wire anemometry wind tunnels. facility has non-negligible incoming free-stream turbulence (with 2.8 % and 4.4 the streamwise ( ${u}^{\ensuremath{\prime} } / U$ ) spanwise ${v}^{\ensuremath{\prime} directions, respectively) presents challenge terms of comparison...

One-dimensional energy spectra in flat plate zero pressure gradient boundary layers and pipe flows are examined over a wide range of Reynolds numbers ( $2600\leqslant \mathit{Re}_{{\it\tau}}\leqslant 72\,500$ ). The show excellent collapse with Kolmogorov scaling at high wavenumbers for both all numbers. peaks associated the large-scale motions (LSMs) superstructures (SS) behave as they do flows, some minor differences. location outer spectral peak, SS or very (VLSMs) turbulent wall region,...

The recently discovered nonequilibrium turbulence dissipation law implies the existence of axisymmetric turbulent wake regions where mean flow velocity deficit decays as inverse distance from wake-generating body and width grows square root that distance. This behavior is different any documented boundary-free shear to date. Its confirmed in wind tunnel experiments wakes generated by plates with irregular edges placed normal an incoming free stream. characteristics bodies such buildings,...

A test coupon coated with light calcareous tubeworm fouling was scanned, scaled and reproduced for wind-tunnel testing to determine the equivalent sand grain roughness ks. It found that this surface had a ks = 0.325 mm, substantially less than previously reported values fouling. This result used predict drag on fouled full scale ship. To achieve this, modified method predicting total of spatially developing turbulent boundary layer (TBL), such as hull ship, is presented. The numerically...

The wake of an isolated model-scale wind turbine is analysed in a set inflow conditions having freestream turbulence intensity between 3 % and 12 %, integral time scales the range 0.1 to 10 times convective timescale based on diameter. It observed that generated by evolves more rapidly, with onset evolution being closer turbine, for high low scale flows, accordance literature, while flows at higher result slow evolution, akin low-turbulence despite highly-turbulent ambient condition. delayed...

Cinematographic stereoscopic particle image velocimetry measurements were performed to resolve small and intermediate scales in the far field of an axisymmetric co-flowing jet. Measurements a plane normal axis jet time-resolved measurement was converted quasi-instantaneous three-dimensional data by using Taylor's hypothesis. The enabled computation all nine components velocity gradient tensor over volume. results based on statistical analysis data, including joint p.d.f.s conditional...

Abstract The turbulent flow over a forward-facing step is studied using two-dimensional time-resolved particle image velocimetry. structure and behaviour of the separation region in front investigated conditional averages based on area reverse present. relation between position upstream shape presented. It shown that when ‘closed’ form, can become unstable resulting ejection fluid corner step. to grow simultaneously both wall-normal streamwise directions, point where maximum extent limited...

The propulsive performance of a pair tandem flapping foils is sensitively dependent on the spacing and phasing between them. Large increases in thrust efficiency hind foil are possible, but mechanisms governing these enhancements remain largely unresolved. Two-dimensional numerical simulations single oscillating heave pitch at Reynolds number 7000 performed over broad dense parameter space, allowing effects inter-foil ( $S$ ) $\unicode[STIX]{x1D711}$ to be investigated range non-dimensional...

The influence of turbulence on the flow around a wall-mounted cube immersed in turbulent boundary layer is investigated experimentally with particle image velocimetry and hot-wire anemometry. Free-stream used to generate profiles where normalised shear at height fixed, but intensity adjustable. free-stream generated an active grid formed artificial floor wind tunnel. development Reynolds number ( $Re_{x}$ ) ratio $h$ thickness $\unicode[STIX]{x1D6FF}$ are held constant $Re_{x}=1.8\times...

An experimental investigation of a turbulent boundary-layer flow over heterogeneous surface is carried out to examine the mean and turbulence characteristics, document variation skin friction that might affect applicability traditional scaling similarity laws. The heterogeneity imposed along spanwise direction consists streamwise-aligned smooth raised strips whose spacing $S$ comparable thickness ( $S/\unicode[STIX]{x1D6FF}=O(1)$ ). Single-point velocity measurements alongside direct...

Secondary flows can develop in turbulent boundary layers that grow over surfaces with spanwise inhomogeneities. In this article, we demonstrate the formation of secondary both experimental and numerical tests dissect instantaneous structure topology these motions. We show is not very sensitive to Reynolds number range investigated, direct simulations experiments produce similar results mean flow as well dispersive stress distributions. The methods capture time-resolved features provide...

Experiments were conducted in the fully rough regime on surfaces with large relative roughness height ( $h/{\it\delta}\approx 0.1$ , where $h$ is and ${\it\delta}$ boundary layer thickness). The generated by distributed LEGO ® bricks of uniform height, arranged different configurations. Measurements made both floating-element drag balance high-resolution particle image velocimetry six configurations frontal solidities, ${\it\lambda}_{F}$ at fixed plan solidity, ${\it\lambda}_{P}$ vice versa...

We perform particle image velocimetry (PIV) measurements of various terms the non-homogeneous Kármán–Howarth–Monin equation in most inhomogeneous and anisotropic region grid-generated turbulence, production which lies between grid peak turbulence intensity. use a well-documented fractal is known to magnify streamwise extent abate its activity. On centreline around centre that two-point advection transport are dominant significant too. It therefore impossible apply usual Kolmogorov arguments...

Dual-plane particle image velocimetry experiments were performed in a turbulent boundary layer with Reτ=1160 to obtain all components of the velocity gradient tensor. Wall-normal locations logarithmic and wake region examined. The availability complete tensor facilitates improved identification vortex cores determination their orientation size. Inclination angles computed using statistical tools such as two-point correlations joint probability density functions. Also, technique was employed...