Numerical investigation of unsteady phenomena in wall jets

Abstract

Direct numerical simulations (DNS) based on the complete Navier-Stokes equations are employed to investigate transitional wall jets. To allow for realistic comparison with laboratory experiments, the spatial model is used for the simulations. Our simulations have shown that when the wall jet is forced with large amplitude disturbances, nonlinear mechanisms can cause the ejection of vortex pairs from the wall layer that then can repeatedly interact with the wall jet layer and thus strongly influence the breakdown process. The vortex ejection is preceded by apparent 'mergings' of disturbance vortices in a subharmonic cascade. From our results it can be conjectured that this merging process is due to a 2D secondary instability mechanism. (Author) Abstract Direct numerical simulations (DNS) based on the complete Navier-Stokes equations are employed to investigate transitional wall jets. To allow for realistic comparison with laboratory experiments, the spatial model is used for the simulations. Our simulations have shown that when the wall jet is forced with large amplitude disturbances, nonlinear mechanisms can cause the ejection of vortex pairs from the wall layer that then can repeatedly interact with the wall jet layer and thus strongly influence the breakdown process. The vortex ejection is preceeded by apparent "mergings" of disturbance vortices in a subharmonic cascade. From our results it can be conjectured that this merging process is due to a two-dimensional secondary instability mechanism.

Numerical Investigation of the Control of Separation from Curved and Blunt Trailing Edges Using DNS and LES

“…The structure at x = 45 is somewhat enhanced and in the process of lifting off from the wall. This lift-off of dipolar vortices by mutual induction is a very common phenomenon in laminar wall jets [22]. In the turbulent flow, however, the near wall vortex is usually too weak to propel the pair away from the wall jet core.…”

Control of Separation Using Pulsed Wall Jets: Numerical Investigations Using DNS and LES

“…In a DNS~precursor calculation, a 2-D base flow is computed (Fig. 1) which closely matches the simi-larity solution that was used in [5] as a base flow. To ensure that the DNS base flow is independent of the computational domain height, special care must be taken to recover the induced flow field of the wall jet far away from the wall, where the similarity solution is invalid.…”

“…DNS is limited to relatively low Reynolds number flows and is, in the foreseeable future, impractical for high Reynolds number flows which are of greatest engineering interest. This leaves Reynolds-averaged Navier-Stokes (RAXS) calculations and Large-Eddy Simulation (LES) as the only realistic alternatives to compute turbulent flows [5]. However, traditional RANS methods are limited to time-steady solutions, an assumption unsuitable for complex turbulent flows where unsteady large structures play a dominant role in the transfer of heat and momentum between different regions of the flow.…”

“…This ap-1 American Insitute of Aeronautics and Astronautics proach has been developed in collaboration with C. Speziale [5] because of the limitations in traditional computational strategies such as Direct Numerical Simulation (DNS), Reynolds-averaged NavierStokes (RANS) computation, and Large Eddy Simulation (LES). DNS is limited to relatively low Reynolds number flows and is, in the foreseeable future, impractical for high Reynolds number flows which are of greatest engineering interest.…”

“…The model used in this investigation is our new FSM. Following Speziale [5], the turbulent stress tensor components are written as…”