769 research outputs found
High-accuracy DNS of supersonic base flows and control of the near wake
No full textLarge-scale numerical simulations of axisymmetric, supersonicbase flows were conducted at various Reynolds numbers. Direct Numerical Simulations (DNS) were employed to investigate the hydrodynamic stability behaviorof the near-wake region. As a consequence of physical flow instabilities, large coherent structures evolve that have a significant impact on the mean flow and are responsible for a considerable amount of base-drag. It is demonstrated that the deliberate exclusion or reinforcement of certain helical modes can lead to a rise in base-pressure and thus decreasing the drag of a blunt body at supersonic speed. For these investigations, a high-order accurate compressible Navier-Stokes solver in cylindrical coordinates with high parallel efficiency was developed and employed on the SGI Origin3900 shared memory complex at the ERDC MSRC. In addition to providing vital insight into the physical mechanisms in supersonic base flows, the DNS results are intended for use as benchmark data for the development of a Flow Simulation Methodology (FSM) for high Reynolds number turbulent flow
Direct numerical simulations of transitional supersonic base flows
No full textTransitional supersonic base flows at M=2.46 are investigated
using Direct Numerical Simulations. Results are presented for Reynolds numbers based on the cylinder diameter ReD=30,000-100,000. As a consequence of flow instabilities, coherent structures develop that have a profound impact on the global flow behavior. Simulations with various circumferential domain sizes are conducted to investigate the effect of coherent structures associated with different azimuthal modes on the mean flow, in particular on the base pressure which determines the base drag.Temporal spectra reveal that frequencies found in the axisymmetric mode can be related to dominant higher modes present in the flow. It is shown that azimuthal modes with low wavenumbers cause a flat base pressure distribution and that the mean base pressure value increases when the most dominant modes are deliberately eliminated. Visualizations of instantaneous flow quantities and turbulence statistics at ReD=100,000 show good agreement with experiments at a significantly higher Reynolds number. For these investigations, a high-order accurate compressible Navier-Stokes solver in cylindrical coordinates developed specifically for this research was used
Identification of large coherent structures in supersonic axisymmetric wakes
No full textDirect numerical simulation data of supersonic axisymmetric wakes are analysed for the existence of
large coherent structures. Wakes at Ma ¼ 2:46 are considered with results being presented for cases at
Reynolds numbers ReD ¼ 30; 000 and 100,000. Criteria for identification of coherent structures in freeshear
flows found in the literature are compiled and discussed, and the role of compressibility is
addressed. In particular, the ability and reliability of visualisation techniques intended for incompressible
shear-flows to educe meaningful structures in supersonic wakes is scrutinised. It is shown that some of
these methods retain their usefulness for identification of vortical structures as long as the swirling rate is
larger than the local compression and expansion rates in the flow field. As a measure for the validity of
this condition in a given flow the ‘vortex compressibility parameter’ is proposed which is derived here.
Best ‘visibility’ of coherent structures is achieved by employing visualisation techniques and proper
orthogonal decomposition in combination with the introduction of artificial perturbations (forcing of
the wake). The existence of both helical and longitudinal structures in the shear layer and of hairpin-like
structures in the developing wake is demonstrated. In addition, elongated tubes of streamwise vorticity
are observed to emanate from the region of recirculating flo
High accuracy DNS and LES of high Reynolds number, supersonic base flows and passive control of the near wake
No full textSupersonic axisymmetric base flows are prototypical for flows behind projectiles and missiles. For these flows, drag reduction can be achieved by means of passive control of the near wake. Thereby, large (turbulent) coherent structures play a dominant role. The objective of the present investigationis to elucidate if and how successful passive flow control techniques modify these structures. To this end, first Direct Numerical Simulations (DNS) for a Reynolds number of ReD = 100,000 and Mach number of Ma = 2.46 were performed using a high-order accurate and highly parallelized research code which was developed at the University of Arizona. Thereby, roughly 52 million grid points were employed. The DNS data serve to visualize typical structures of the unsteady flow field and to verify that the use of less computational costly RANS/LES methods is applicable for this flow. Two of these methods, the Flow Simulation Methodology (FSM) and Detached Eddy Simulations (DES), were then employed to investigate the supersonic base flow at ReD = 3.3 106 and Ma = 2.46 using between 460,000 and seven million grid points. For the DES, the commercial CFD-code Cobalt was employed. This unstructured grid solver allowed then to perform simulations with boat-tailing. The obtained mean flow data is compared to available experimental result
Numerical investigation of flow control mechanisms for drag reduction in supersonic base-flows
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