88 research outputs found
Relaminarisation of Re_Ï„=100 channel flow with globally stabilising linear feedback control
The problems of nonlinearity and high dimension have so far prevented a complete solution of the control of turbulent flow. Addressing the problem of nonlinearity, we propose a flow control strategy which ensures that the energy of any perturbation to the target profile decays monotonically. The controller’s estimate of the flow state is similarly guaranteed to converge to the true value. We present a one-time off-line synthesis procedure, which generalises to accommodate more restrictive actuation and sensing arrangements, with conditions for existence for the controller given in this case. The control is tested in turbulent channel flow (Re_τ = 100) using full-domain sensing and actuation on the wall-normal velocity. Concentrated at the point of maximum inflection in the mean profile, the control directly counters the supply of turbulence energy arising from the interaction of the wall-normal perturbations with the flow shear. It is found that the control is only required for the larger-scale motions, specifically those above the scale of the mean streak spacing. Minimal control effort is required once laminar flow is achieved. The response of the near-wall flow is examined in detail, with particular emphasis on the pressure and wall-normal velocity fields, in the context of Landahl’s theory of sheared turbulence
Relaminarisation of Re_{\tau} = 100 channel flow with globally stabilising linear feedback control
The problems of nonlinearity and high dimension have so far prevented a
complete solution of the control of turbulent flow. Addressing the problem of
nonlinearity, we propose a flow control strategy which ensures that the energy
of any perturbation to the target profile decays monotonically. The
controller's estimate of the flow state is similarly guaranteed to converge to
the true value. We present a one-time off-line synthesis procedure, which
generalises to accommodate more restrictive actuation and sensing arrangements,
with conditions for existence for the controller given in this case. The
control is tested in turbulent channel flow () using full-domain
sensing and actuation on the wall-normal velocity. Concentrated at the point of
maximum inflection in the mean profile, the control directly counters the
supply of turbulence energy arising from the interaction of the wall-normal
perturbations with the flow shear. It is found that the control is only
required for the larger-scale motions, specifically those above the scale of
the mean streak spacing. Minimal control effort is required once laminar flow
is achieved. The response of the near-wall flow is examined in detail, with
particular emphasis on the pressure and wall-normal velocity fields, in the
context of Landahl's theory of sheared turbulence
The association between organizational behavior factors and health-related quality of life among college teachers: a cross-sectional study
Die PMN-Elastase als Marker in der Diagnostik und Verlaufskontrolle von Knochen- und Gelenkinfektionen
Die PMN-Elastase als Marker in der Diagnostik und Verlaufskontrolle von Knochen- und Gelenkinfektionen
Actuated transitory metal−ligand bond as tunable electromechanical switch
6 páginas, 4 figuras.Electrically tunable molecules are highly attractive for the construction of molecular devices, such as switches, transistors, or machines. Here, we present a novel nanomechanical element triggered by an electrical bias as external stimulus. We demonstrate that a transitory chemical bond between a copper atom and coordinating organic molecules adsorbed on a metal surface acts as variable frequency switch, which can be actuated and probed by means of low-temperature scanning tunneling microscopy. Whereas below a threshold bias voltage the bond is permanently either formed or broken the bonding state continuously oscillates at higher voltages. The switching rate of the bistable molecular system can be widely tuned from below 1 Hz up to the kilohertz regime. The quantum yield per tunneling electron to trigger a transition between the two states varies spatially and is related to the local density of states of the bonded and nonbonded configuration.Peer reviewe
- …