Connecting the time evolution of the turbulence interface to coherent structures

Unified description of turbulent entrainment

“…Neamtu-Halic et al. 2020), although one should keep in mind that the choice of local or global entrainment description can isolate different aspects of entrainment (engulfing or nibbling, for example) which one can then link to each other via local–global approximate balances such as the one discussed in § 4.4. It is hoped that the current description will be able to assist in connecting macro-scale to micro-scale entrainment events and processes.…”

Role of vortical structures for enstrophy and scalar transport in flows with and without stable stratification

“…For a more detailed discussion on the DNSs concept and numerical configuration, we refer to van Reeuwijk et al [39,40]. Following Neamtu-Halic et al [41], we simulated three different flow cases, namely, a wall-jet (β = 0) and two different gravity currents (β > 0). The flow cases differ in the initial Richardson number Ri 0 = B 0 cos(α)/u 2 0 , where B 0 = βgc 0 h 0 is a conserved quantity in the simulations, whereas the initial bulk Reynolds number Re 0 = u 0 h 0 /ν is kept constant.…”

“…To contain the computational costs, we extract 2D OECSs from vertical planes of 3D data. We use a criterion defined in Neamtu-Halic et al [41] in which a maximum of the IVD field is selected only if the ratio between the two eigenvalues of the Hessian of IVD at the location of IVD maxima is below a threshold. The rationale behind this criterion is based on the fact that a 2D OECS in a slice results from the intersection of the tubular structures with the plane itself.…”

“…Recently, [34] used experimental data of a gravity current to show that OECSs modulate the area of the TNTI and that their modulating capacity diminishes with increasing stratification. Moreover, OECSs have been observed to organise the flow field on the TNTI proximity thereby imposing the local entrainment velocity [34] and setting the mechanism that produces/destroys the surface area of the nearby TNTI [41]. It remains to be understood how OECSs affect the process of the entrainment and how do they adapt at different levels of stratification.…”

A metamorphosis of three-dimensional wave structure in transitional and turbulent boundary layers

“…However, due to the non-uniformity of background vorticity in boundary layers, the value of LAVD may rely on the selected initial fluid volume. Recently, the determination of isosurfaces of the enstrophy field has been applied to stratified flows to identify turbulent/non-turbulent interfaces (known as TNTI) (Neamtu-Halic et al 2020). The evolution of enstrophy is closely related to vortex stretching effects, which is a critical kinematic mechanism in a shearing process.…”