Werner J. A. Dahm

Measurements of entrainment and mixing in turbulent jets

An experimental investigation of entrainment and mixing in the self-similar far field of an axisymmetric free turbulent jet in water is presented. Length and time scales for the flame length fluctuations of reacting jets are shown to be approximately equal to the local characteristic large scale length and time of the flow. It is also shown that instantaneous radial profiles of concentration across the jet do not resemble the mean concentration profile, indicating that the mean profile is a poor representation of the mixed fluid states within the jet. These instantaneous profiles also show that unmixed ambient fluid is transported throughout the entire extent of the jet, and that the mixed fluid composition within the jet can be fairly uniform in regions extending across a large part of the local jet diameter. Lastly, the amount of unmixed ambient fluid on the jet centerline is found to vary roughly periodically with a period approximately equal to the local characteristic large scale time of the flow. These results suggest that large scale transport mechanisms, displaying a characteristic organization, play an important role in entrainment and mixing in the far filed of turbulent jets.

Experimental study of the fine-scale structure of conserved scalar mixing in turbulent shear flows. Part 2. Sc 1

We present results from an experimental investigation into the fine-scale structure associated with the mixing of a dynamically passive conserved scalar quantity on the inner scales of turbulent shear flows. The present study was based on highly resolved two- and three-dimensional spatio-temporal imaging measurements. For the conditions studied, the Schmidt number ( Sc ≡ v/D ) was approximately 2000 and the local outerscale Reynolds number ( Re σ ≡ uσ/v ) ranged from 2000 to 10000. The resolution and signal quality allow direct differentiation of the measured scalar field ζ( x, t ) to give the instantaneous scalar energy dissipation rate field ( Re Sc ) −1 ∇ζċ∇ζ(x, t). The results show that the fine-scale structure of the scalar dissipation field, when viewed on the inner-flow scales for Sc ≡ 1, consists entirely of thin strained laminar sheet-like diffusion layers. The internal structure of these scalar dissipation sheets agrees with the one-dimensional self-similar solution for the local strain–diffusion competition in the presence of a spatially uniform but time-varying strain rate field. This similarity solution also shows that line-like structures in the scalar dissipation field decay exponentially in time, while in the vorticity field both line-like and sheet-like structures can be sustained. This sheet-like structure produces a high level of intermittency in the scalar dissipation field – at these conditions approximately 4% of the flow volume accounts for nearly 25% of the total mixing achieved. The scalar gradient vector field ∇ζ( x, t ) for large Sc is found to be nearly isotropic, with a weak tendency for the dissipation sheets to align with the principal axes of the mean flow strain rate tensor. Joint probability densities of the conserved scalar and scalar dissipation rate have a shape consistent with this canonical layer-like fine-scale structure. Statistics of the conserved scalar and scalar dissipation rate fields are found to demonstrate similarity on inner-scale variables even at the relatively low Reynolds numbers investigated.

Direct, high resolution, four-dimensional measurements of the fine scale structure of Sc≫ 1 molecular mixing in turbulent flows

Results from highly resolved, four‐dimensional measurements of the fine structure of the fully space‐ and time‐varying Sc≫1 conserved scalar field and the associated scalar energy dissipation rate field in a turbulent flow are presented. The resolution achieved in all three spatial dimensions and in time reaches down to the local strain‐limited molecular diffusion scale in the flow, allowing all three components of the instantaneous scalar gradient vector field ∇ζ(x,t) and their time evolution at every point in the data space to be directly evaluated. Results are presented in the form of fine structure maps of the instantaneous dissipation field loge ∇ζ⋅∇ζ(x,t) in several spatially adjacent data planes within an individual three‐dimensional spatial data volume, as well as in several temporally successive data planes from a sequence of such three‐dimensional data volumes. The degree of anisotopy in the underlying scalar gradient field is characterized in terms of the joint distribution β(ϑ,φ) of spherical ...

Blowout of turbulent diffusion flames

A simple description has been formulated of the mechanisms governing the stability of turbulent diffusion flames. It is based upon the nature of the large scale motions that have been observed in turbulent jets and includes a process for maintaining a stable flame. An analysis, based on the proposed model, leads to a single parameter which determines the blowout velocity of pure fuels and of fuels diluted with air and with CO 2 . The parameter is the ratio of two times: a characteristic chemical reaction time and a time associated with the mixing of reentrained hot products into fresh reactants. The agreement with a set of experimental observations for both pure and diluted gases is good.

Vortex structure and dynamics in the near field of a coaxial jet

We present results from an experimental and numerical investigation into the structure of vortex patterns and the dynamics of their interactions for the incompressible flow in the near field of a round coaxial jet issuing into a quiescent ambient fluid. A two-colour planar laser-induced-fluorescence technique is used to document the flow field via still photographs and cine sequences over a limited range of parameters. We examine the effects of varying the velocity ratio as well as the absolute velocities of the two coaxial streams for equal densities and for a single area ratio. Results show that a variety of widely differing near-field vortex patterns can arise, with very different interaction dynamics, which can depend both on the velocity ratio and on the absolute velocities of the two streams. The observed vortex structures and their dynamics are interpreted in terms of the instability of the initially cylindrical and concentric vorticity layers separating each of the fluid streams, and their subsequent rollup to form wake-like or shear-layer-like vortices. Our results show that in addition to the velocity jump across each of these vorticity layers, an accounting of the layer thicknesses and the wake defect within each layer can be essential to understanding the resulting near-field structure that occurs. Ensuing dynamical interactions between the vortices formed from each layer can produce a strong coupling between the development of the two layers. These resulting vortex structures and interaction dynamics are also seen to produce widely differing mixing patterns in the jet near field.

A scalar imaging velocimetry technique for fully resolved four-dimensional vector velocity field measurements in turbulent flows

This paper presents an experimental technique for obtaining fully resolved measurements of the vector velocity field u(x,t) throughout a four‐dimensional spatiotemporal region in a turbulent flow. The method uses fully resolved four‐dimensional scalar field imaging measurements in turbulent flows [Phys. Fluids A 3, 1115 (1991)] to extract the underlying velocity field from the exact conserved scalar transport equation. A procedure for accomplishing this is described, and results from a series of test cases are presented. These involve synthetically generated scalar fields as well as actual measured turbulent flow scalar fields advected numerically by various imposed flow fields. The imposed velocity fields are exactly known, allowing a careful validation of the technique and its potential accuracy. Results obtained from a zeroth iteration of the technique are found to be very close to the exact underlying vector velocity field. Further results show that successive iterations bring the velocity field from the zeroth iteration even closer to the exact result. It is also shown that the comparatively dense velocity field information that this technique provides is well suited for accurate extraction of the more dynamically insightful strain rate and vorticity fields e(x,t) and ω(x,t).

Dual-plane stereo particle image velocimetry measurements of velocity gradient tensor fields in turbulent shear flow. I. Accuracy assessments

Results are presented from quantitative assessments of the accuracy of velocity gradients measured by a dual-plane stereo particle image velocimetry (DSPIV) technique that allows direct, highly resolved, nonintrusive measurements of all nine simultaneous components of the velocity gradient tensor fields ∂ui∕∂xj at the quasi-universal intermediate and small scales of turbulent shear flows. The present results systematically determine the sources of errors in DSPIV measurements and the resulting accuracy of velocity gradients obtained from such measurements. Intrinsic errors resulting from asymmetric stereo imaging are found by synthetic particle imaging to be no larger than 0.8%. True particle imaging in finite-thickness light sheets is found from single-plane imaging tests to produce net errors in measured velocity differences of 6% for in-plane components and 10% for out-of-plane components. Further errors from limits on the accuracy of independent dual light sheet generation and positioning are found fr...

Coflowing turbulent jet diffusion flame blowout

We present results from an experimental and theoretical investigation into the blowout mechanism in turbulent diffusion flames. The blowout stability limits of coflowing turbulent jet diffusion flames are formulated in terms of a recently proposed flame stabilization mechanism based on the large scale organization of entrainment and mixing observed in turbulent shear flows. In contrast to the linear similarity scaling of the more commonly studied simple turbulent jet flames, the nonlinear scaling of coflowing turbulent jets allows an essential element of this stabilization mechanism to be investigated. Results show that when the flame stability criterion is evaluated for the last large structure in the flame as is consistent with the underlying physical picture for this stabilization mechanism, a large reduction in the blowout limit is expected for even a small coflow velocity. This phenomenon is experimentally verified and good quantitative agreement is demonstrated with a set of measurements for the blowout limits of such coflowing turbulent jet flames.

Dual-plane stereo particle image velocimetry measurements of velocity gradient tensor fields in turbulent shear flow. II. Experimental results

Results are presented from highly resolved dual-plane stereo particle image velocimetry (DSPIV) measurements for the structure, statistics, similarity, and scaling of all nine simultaneous components of the velocity gradient tensor fields ∂ui∕∂xj on the quasi-universal intermediate and small scales of turbulent shear flows. Measurements were obtained at three combinations of the outer-scale Reynolds number Reδ and the local mean shear rate S in the fully developed self-similar far field of a turbulent jet, and thus reflect the combined effects of the large-scale structure, spatial inhomogeneities, and anisotropies inherent in such a flow. Conditions addressed in this study correspond to local outer-scale Reynolds numbers Reδ=6,000 and 30,000 and local mean shear values Sδ∕uc=0 and 1.7, corresponding to Taylor-scale Reynolds numbers Reλ≈44 and 113 and shear rates Sk∕e=0 and 2.1. Gradient fields investigated here include the individual velocity gradient component fields, the strain rate component fields and...

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

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