2,386 research outputs found
Effect of 3D Roughness Patch on Instability Amplification in a Supersonic Boundary Layer
Surface roughness is known to have a substantial impact on the aerothermodynamic loading of high-speed vehicles, particularly via its influence on the laminar-turbulent transition process within the boundary layer. Numerical simulations are performed to investigate the effects of a distributed region of densely packed, sinusoidal shape roughness elements on a Mach 3.5 flat plate boundary layer for flow conditions corresponding to the planned conditions of an upcoming experiment in the Mach 3.5 Supersonic Low Disturbance Tunnel at the NASA Langley Research Center. Analysis of convective instabilities in the wake of the roughness patch was reported in a previous paper and the current work extends that analysis to instability amplification across the length of the roughness patch. Quasiparallel stability analysis of the modified boundary layer flow over the patch indicates two dominant families of unstable disturbances, namely, a group of high frequency modes that amplify in localized regions along the roughness patch and another group of low frequency modes that have smaller peak amplification rates but amplify steadily both above the roughness patch and in the wake region behind it. The results suggest that the amplification factors associated with the high-frequency modes are sufficiently low, at least for the roughness patches considered in this paper, so that these modes are unlikely to have a major influence on the transition process. The amplification of the low-frequency modes within the region of the roughness patch is further quantified via direct numerical simulations. Results confirm the strongly destabilizing influence of the roughness patch on the first mode instabilities, yielding an N-factor increment of N 3.6 for a roughness patch length of eight wavelengths
Reversing cooling flows with AGN jets: shock waves, rarefaction waves, and trailing outflows
The cooling flow problem is one of the central problems in galaxy clusters,
and active galactic nucleus (AGN) feedback is considered to play a key role in
offsetting cooling. However, how AGN jets heat and suppress cooling flows
remains highly debated. Using an idealized simulation of a cool-core cluster,
we study the development of central cooling catastrophe and how a subsequent
powerful AGN jet event averts cooling flows, with a focus on complex
gasdynamical processes involved. We find that the jet drives a bow shock, which
reverses cooling inflows and overheats inner cool core regions. The shocked gas
moves outward in a rarefaction wave, which rarefies the dense core and
adiabatically transports a significant fraction of heated energy to outer
regions. As the rarefaction wave propagates away, inflows resume in the cluster
core, but a trailing outflow is uplifted by the AGN bubble, preventing gas
accumulation and catastrophic cooling in central regions. Inflows and trailing
outflows constitute meridional circulations in the cluster core. At later
times, trailing outflows fall back to the cluster centre, triggering central
cooling catastrophe and potentially a new generation of AGN feedback. We thus
envisage a picture of cool cluster cores going through cycles of
cooling-induced contraction and AGN-induced expansion. This picture naturally
predicts an anti-correlation between the gas fraction (or X-ray luminosity) of
cool cores and the central gas entropy, which may be tested by X-ray
observations.Comment: Slightly revised version, accepted for publication in MNRAS. 14
pages, 10 figure
Stabilization of a Swept-Wing Boundary Layer by Discrete Roughness Elements at High Reynolds Numbers
Direct numerical simulations (DNS) are performed to study potential stabilizing ef- fect of spanwise periodic discrete roughness elements (DREs) on cross ow instabilities in a spatially developing three-dimensional boundary layer over an in nite-swept natural- laminar- ow wing at a freestream Mach number of 0:75 and a chord Reynolds number of approximately 25 million. In the DNS, both the spanwise periodic DREs and distributed roughness in the leading-edge region are implemented to simulate a typical experimen- tal scenario in which multiple steady cross ow modes including the most unstable mode (i.e., the \target" mode) emerge because of the presence of naturally distributed surface roughness in the leading edge region and spanwise periodic control cylinders of subcritical wavelength are used to force small-wavelength disturbances (i.e., the control mode) for damping the target mode. The DNS results show that the e ectiveness of DRE control is sensitive to roughness diameter, height, and chordwise placement. For the DRE parame- ters considered in this study, the stabilizing e ect on the target mode is small within the computational domain that ended at about 35% of the chord
Two-photon Rabi model: Analytic solutions and spectral collapse
The two-photon quantum Rabi model with quadratic coupling is studied using
extended squeezed states and we derive -functions for Bargmann index
and . The simple singularity structure of the -function allows to draw
conclusions about the distribution of eigenvalues along the real axis. The
previously found picture of the spectral collapse at critical coupling
has to be modified regarding the low lying states, especially
the ground state: We obtain a finite gap between ground state and the continuum
of excited states at the collapse point. For large qubit splitting, also other
low lying states may be separated from the continuum at . We
have carried out a perturbative analysis allowing for explicit and simple
formulae of the eigenstates. Interestingly, a vanishing of the gap between
ground state and excited continuum at is obtained in each
finite order of approximation. This demonstrates cleary the non-pertubative
nature of the excitation gap. We corroborate these findings with a variational
calculation for the ground state.Comment: 13 pages, 4 figure
- …