Scalar imaging velocimetry measurements of the velocity gradient tensor field in turbulent flows. II. Experimental results

Su, Lester K.; Dahm, Werner J. A.

doi:10.1063/1.868970

Cited by 66 publications

(50 citation statements)

References 16 publications

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“…The agreement between the results in Fig. 8 is satisfactory, where PDFs of the production terms show clearly positively skewed distributions, as shown also in Lu¨thi et al (2005) and Su and Dahm (1996b).…”

Section: Resultssupporting

confidence: 69%

3D scanning particle tracking velocimetry

Hoyer¹,

Holzner²,

Lüthi

et al. 2005

Exp Fluids

111

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In this article, we present an experimental setup and data processing schemes for 3D scanning particle tracking velocimetry (SPTV), which expands on the classical 3D particle tracking velocimetry (PTV) through changes in the illumination, image acquisition and analysis. 3D PTV is a flexible flow measurement technique based on the processing of stereoscopic images of flow tracer particles. The technique allows obtaining Lagrangian flow information directly from measured 3D trajectories of individual particles. While for a classical PTV the entire region of interest is simultaneously illuminated and recorded, in SPTV the flow field is recorded by sequential tomographic high-speed imaging of the region of interest. The advantage of the presented method is a considerable increase in maximum feasible seeding density. Results are shown for an experiment in homogenous turbulence and compared with PTV. SPTV yielded an average 3,500 tracked particles per time step, which implies a significant enhancement of the spatial resolution for Lagrangian flow measurements.

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Section: Resultssupporting

confidence: 69%

3D scanning particle tracking velocimetry

Hoyer¹,

Holzner²,

Lüthi

et al. 2005

Exp Fluids

111

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“…This was observed in numerical simulations in different flows (Ruetsch & Maxey 1991, 1992Tsinober 2001;Tsinober & Galanti 2001;Vedula et al 2001;Brethouwer et al 2003 and references therein) at rather small Taylor microscale Reynolds numbers Re λ ∼ 100. No similar results have been obtained so far in the laboratory (with the exception of Su & Dahm (1996), who were able to obtain the alignments between the vector, G, and the eigenframe, λ k , of the rate-of-strain tensor, s ij , in the far field of a turbulent jet flow at Re λ ∼ 50), especially for high-Reynolds-number flows. Instead, a surrogate quantity, the mixed skewness,…”

Section: Introductionmentioning

confidence: 78%

Velocity and temperature derivatives in high- Reynolds-number turbulent flows in the atmospheric surface layer. Part 3. Temperature and joint statistics of temperature and velocity derivatives

et al. 2007

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This is part 3 of our work describing experiments in which explicit information was obtained on all the derivatives, i.e. spatial derivatives, ∂/∂x j , and temporal derivatives, ∂/∂t, of velocity and temperature fields (and all the components of velocity fluctuations and temperature) at the Reynolds number Re λ ∼ 10 4 .This part is devoted to the issues concerning temperature with the emphasis on joint statistics of temperature and velocity derivatives, based on preliminary results from a jet facility and the main results from a field experiment. Apart from a number of conventional results, these contain a variety of results concerning production of temperature gradients, such as role of vorticity and strain, eigencontributions, geometrical statistics such as alignments of the temperature gradient and the eigenframe of the rate-of-strain tensor, tilting of the temperature gradient, comparison of the true production of the temperature gradient with its surrogate. Among the specific results of importance is the essential difference in the behaviour of the production of temperature gradients in regions dominated by vorticity and strain. Namely, the production of temperature gradients is much more intensive in regions dominated by strain, whereas production of temperature gradients is practically independent of the magnitude of vorticity. In contrast, vorticity and strain are contributing equally to the tilting of the vector of temperature gradients.The production of temperature gradients is mainly due to the fluctuative strain, the terms associated with mean fields are unimportant. It was checked directly (by looking at corresponding eigen-contributions and alignments), that the production of the temperature gradients is due to predominant compressing of fluid elements rather than stretching, which is true of other processes in turbulent flows, e.g. turbulent energy production in shear flows. Though the production of the temperature gradient and its surrogate possess similar univariate PDFs (which indicates the tendency to isotropy in small scales by this particular criterion), their joint PDF is not close to a bisector. This means that the true production of the temperature gradient is far from being fully represented by its surrogate. The main technical achievement is demonstrating the possibility of obtaining experimentally joint statistics of velocity and temperature gradients.

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“…The final result in (20) is an operator that extracts the nonlocal background strain rate tensor S B ij at any point x from the total strain rate tensor S ij . For the Taylor expansion in (10), this operator involves Laplacians of the total strain rate field S ij (x).…”

Section: A Evaluating the Background Strain Rate Tensormentioning

confidence: 99%

“…An understanding of how vortical structures are stretched by the strain rate field S ij (x) is thus essential to any description of the energetics of such flows. Over the last two decades, direct numerical simulations (DNS) [5,6,7] and experimental studies [8,9,10,11,12] of the fine-scale structure of turbulence have revealed a preferred alignment of the vorticity with the intermediate eigenvector of the strain rate tensor. This result has been widely regarded as surprising.…”

Section: Introductionmentioning

confidence: 99%

Local and nonlocal strain rate fields and vorticity alignment in turbulent flows

2008

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Local and nonlocal contributions to the total strain rate tensor Sij at any point x in a flow are formulated from an expansion of the vorticity field in a local spherical neighborhood of radius R centered on x. The resulting exact expression allows the nonlocal (background) strain rate tensor S B ij (x) to be obtained from Sij (x). In turbulent flows, where the vorticity naturally concentrates into relatively compact structures, this allows the local alignment of vorticity with the most extensional principal axis of the background strain rate tensor to be evaluated. In the vicinity of any vortical structure, the required radius R and corresponding order n to which the expansion must be carried are determined by the viscous lengthscale λν. We demonstrate the convergence to the background strain rate field with increasing R and n for an equilibrium Burgers vortex, and show that this resolves the anomalous alignment of vorticity with the intermediate eigenvector of the total strain rate tensor. We then evaluate the background strain field S B ij (x) in DNS of homogeneous isotropic turbulence where, even for the limited R and n corresponding to the truncated series expansion, the results show an increase in the expected equilibrium alignment of vorticity with the most extensional principal axis of the background strain rate tensor.

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Scalar imaging velocimetry measurements of the velocity gradient tensor field in turbulent flows. II. Experimental results

Cited by 66 publications

References 16 publications

3D scanning particle tracking velocimetry

3D scanning particle tracking velocimetry

Velocity and temperature derivatives in high- Reynolds-number turbulent flows in the atmospheric surface layer. Part 3. Temperature and joint statistics of temperature and velocity derivatives

Local and nonlocal strain rate fields and vorticity alignment in turbulent flows

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