ALBERT

Beschreibung: In a rectangular cross-section wind tunnel, a separated oblique shock reflection is set to interact with the turbulent boundary layer (oblique shock wave/turbulent boundary layer interaction (SBLI)) both on the bottom wall and in the corners formed by the intersection of the floor with the sidewalls. To examine how corner separations can affect the 'quasi-two-dimensional' main interaction and by what mechanisms this is achieved, an experimental investigation has been conducted. This examines how modifications to the corner separation affect an oblique shock reflection. The nature of the flow field is studied using flow visualisation, pressure-sensitive paint and laser Doppler anemometry. The results show that the size and shape of central separation vary considerably when the onset and magnitude of corner separation changes. The primary mechanism explaining the coupling between these separated regions appears to be the generation of compression waves and expansion fans as a result of the displacement effect of the corner separation. This is shown to modify the three-dimensional shock structure and alter the adverse pressure gradient experienced by the tunnel floor boundary layer. It is suggested that a typical oblique SBLI in rectangular channels features several zones depending on the relative position of the corner waves and the main interaction domain. In particular, it has been shown that the position of the corner 'shock' crossing point, found by approximating the corner compression waves by a straight line, is a critical factor determining the main separation size and shape. Thus, corner effects can substantially modify the central separation. This can cause significant growth or contraction of the separation length measured along the symmetry line from the nominally two-dimensional baseline value, giving a fivefold increase from the smallest to the largest observed value. Moreover, the shape and flow topology of the centreline separation bubble is also considerably changed by varying corner effects. © 2019 Cambridge University Press.

Beschreibung: In a stable background density gradient, initially turbulent flows eventually evolve into a state dominated by low-Froude-number dynamics and frequently also contain persistent pattern information. Much empirical evidence has been gathered on these latter stages, but less on how they first got that way, and how information on the turbulence generator may potentially be encoded into the flow in a robust and long-lasting fashion. Here an experiment is described that examines the initial stages of evolution in the vertical plane of a turbulent grid-generated wake in a stratified ambient. Refractive-index-matched fluids allow optically based measurement of early (Nt 〈 2) stages of the flow, even when there are strong variations in the local density gradient field. Suitably averaged flow measures show the interplay between internal wave motions and Kelvin-Helmholtz-generated vortical modes. The vertical shear is dominant at the wake edge, and the decay of horizontal vorticity is observed to be independent of Fr. Stratified turbulence, originating from Kelvin-Helmholtz instabilities, develops up to non-dimensional time Nt ≈ 10, and the scale separation between Ozmidov and Kolmogorov scales is independent of Fr at higher Nt. The detailed measurements in the near wake, with independent variation of both Reynolds and Froude numbers, while limited to one particular case, are sufficient to show that the initial turbulence in a stratified fluid is neither three-dimensional nor universal. The search for appropriately generalizable initial conditions may be more involved than hoped for. © 2015 Cambridge University Press.