# FLOW Gallery

## Jet in Crossflow

This animation is based on data from Direct Numerical Simulation (DNS) of a jet in crossflow at several low values of the velocity inflow ratio R. We show that, as the velocity ratio R increases, the flow evolves from simple periodic vortex shedding (a limit cycle) to more complicated quasi-periodic behavior, before finally exhibiting asymmetric chaotic motion. We also perform a stability analysis just above the first bifurcation, where R is the bifurcation parameter. Using the overlap of the direct and the adjoint eigenmodes, we confirm that the first instability arises in the shear layer downstream of the jet orifice on the boundary of the backflow region just behind the jet. This video won a Milton-van-Dyke award (Gallery of Fluid Motion - Physics of Fluids) at the APS DFD 2010.

References: Bagheri et al., J. Fluid Mech. (2009)

Contact: Milos Ilak

## Turbulent Boundary Layer

The second half of the animation requires red-cyan glasses for the 3D effects. Submission to APS Gallery of Fluid Motion (2011). mp4

Submission to APS Gallery of Fluid Motion (2010). mpg

Visualisation of transient LES data (2011). mp4

Visualisation of DNS data (2011). mp4

Time-dependent visualisations of large-scale direct and large-eddy simulations (DNS and LES) of a turbulent boundary layer reaching up to $Re_\theta=4300$ are presented. The focus of the present fluid dynamics video is on analysing the coherent vortical structures in the boundary layer: It is clearly shown that hairpin vortices are indeed present in the simulation data, however they are characteristic remainders of the laminar-turbulent transition at lower Reynolds numbers. At higher $Re$ (say $Re_\theta>2000$), these structures are no longer seen as being dominant; the coherence is clearly lost, both in the near-wall region as well as in the outer layer of the boundary layer. Note, however, that large-scale streaks in the streamwise velocity, which have their peak energy at about half the boundary-layer thickness, are unambiguously observed. Visualisation from 2010.

Related links: DNS data, Poster APS DFD 2009

Projects: Wall Turbulence

Contact: Philipp Schlatter

## Turbulence in a Three-Dimensional Diffuser

Pseudocolors of streamwise velocity, red contours indicating separated zones (u=0). Gray isosurfaces show instantaneous pressure. Simulation with nek5000 (spectral-element, 11th order space, 3rd order time with a total of 220 752 000 grid points). Visualisation from 2011.

Related links: DNS data

References: Ohlsson et al., J. Fluid Mech. (2010)

Projects: Diffuser

Contact: Johan Malm

## Transition close to criticality in Couette Flow

Visualisations of the flow in a two-dimensional (streamwise/spanwise) plane in the centre of the domain for Couette flow at Reynolds numbers which are very close to the critical Reynolds number of Re about 323. The flow is initiated via random noise (or a localised disturbance in one case), and very clear large-scale laminar-turbulent patters appear. For Re greater than 323 turbulent spots tend to grow, whereas for lower Re they decay.

Contact: Philipp Schlatter

References: Duguet et al., Journal of Fluid Mechanics, vol. 650, pp. 119-129, 2010.

Projects: Transition in Couette flow

## Transition in Boundary Layers

Visualization of a boundary layer undergoing transition due to ambient free-stream turbulence. This scenario is commonly called "bypass transition", and is particularly relevant in turbomachinery applications. Visualisation from 2006.

References: Brandt et al., J. Fluid Mech. (2004)

Contact: Philipp Schlatter

2017-07-04