Dominik Krug

Investigations on the local entrainment velocity in a turbulent jet

We report an experimental analysis of the local entrainment velocity in the self-similar region of a turbulent jet. Particle tracking velocimetry is performed to determine the position of the convoluted, instantaneous turbulent/non-turbulent interface and to compute velocity and velocity derivatives in the proximity of the interface. We find that the local entrainment velocity is mostly governed by a viscous component and that its magnitude depends on the local shape of the interface. It is illustrated that local entrainment is faster for surface elements concave towards the turbulent region. A closer analysis of the plane spanned by mean and Gaussian curvature reveals that depending on the surface shape, different small-scale mechanisms are dominant for the local entrainment process, namely, viscousdiffusion for concave shapes and vortex stretching for convex shapes. Key quantities influencing viscousdiffusion and vortex stretching in the entrainment process are identified. It is illustrated that the viscous advancement of the interface into the non-turbulent region mostly depends on the shape of the enstrophy profile normal to the interface. The inviscid contribution is intimately related to the alignment of vorticity with the eigenvectors of the rate of strain tensor. Finally, the analysis substantiates that the convolution of the instantaneous interface is driven by the advection of the underlying fluid together with a contribution from the local entrainment velocity, with the advection velocity being the governing part.

A combined scanning PTV/LIF technique to simultaneously measure the full velocity gradient tensor and the 3D density field

We present a newly developed combined scanning particle tracking velocimetry (SPTV) and scanning laser-induced fluorescence (SLIF) technique. The new method allows for the first time to measure the full velocity gradient tensor and the three-dimensional density field simultaneously in a refractive index matched environment. The data thus obtained will be valuable in investigating the interaction between turbulence, i.e. the dynamics of vorticity and strain, and the density field. In this study, we describe the implementation of the measurement system in detail. As a showcase, the new technique is applied to a gravity current flow and the measured data are validated by imposing various checks. Further results are presented that illustrate the capability of the SPTV/SLIF technique in the investigation of interface dynamics.

Small-scale entrainment in inclined gravity currents

We investigate the effect of buoyancy on the small-scale aspects of turbulent entrainment by performing direct numerical simulation of a gravity current and a wall jet. In both flows, we detect the turbulent/nonturbulent interface separating turbulent from irrotational ambient flow regions using a range of enstrophy iso-levels spanning many orders of magnitude. Conform to expectation, the relative enstrophy isosurface velocity

Clustering of particles in turbulence due to phoresis.

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Effects of mean shear on the local turbulent entrainment process

vn in the viscous superlayer scales with the Kolmogorov velocity for both flow cases. We connect the integral entrainment coefficient E to the small-scale entrainment and observe excellent agreement between the two estimates throughout the viscous superlayer. The contribution of baroclinic torque to

The turbulent/non-turbulent interface in an inclined dense gravity current

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