Role of Actuation Frequency in Controlled Flow Reattachment over a Stalled Airfoil

AIAA Journal ◽  
2002 ◽  
Vol 40 (2) ◽  
pp. 209-216 ◽  
Author(s):  
Michael Amitay ◽  
Ari Glezer
Keyword(s):  
Actuation Frequency ◽  
Controlled Flow ◽  
Flow Reattachment

Haemorheological Effects on Thrombocyte Adhesion and Aggregation Found Under Controlled Flow Conditions

1979 ◽  
Author(s):  
P.D. Richardson
Keyword(s):  
Rate Of Change ◽  
Continuous Observation ◽  
Flow Conditions ◽  
Wide Range ◽  
Stream Migration ◽  
Local Shear ◽  
Videotape Recording ◽  
Controlled Flow ◽  
A Site

Thrombocyte adhesion and aggregation in a vessel or on a chamber wall can be measured most readily if the flow is controlled and steady, and continuous observation is used. Videotape recording is very helpful for subsequent quantification of the dynamics. The adhesion of each thrombocyte can occur for a finite time interval:this interval has been observed to have a wide range. Platelets which escape often leave open a site which attracts other platelets preferentially. The rate of change of adhesion density (platelets/mm2) is affected by the local shear rate and the shear history upstream. Aggregation is affected similarly, and also proceeds with some platelet turnover. The role of erythrocytes in facilitating cross-stream migration of thrombocytes (which can enhance the growth rate of large thrombi) appears due in part to convective flow fields induced by the motion of erythrocytes in a shear flow, which can be demonstrated theoretically and experimentally. Observations of the phenomenlogy of adhesion and aggregation under controlled flow conditions and comparison with fLu id-dynamically based theory allows representation in terras of a small number of parameters with prospects of prediction of behaviour over a wide range of haemodynamic conditions; biochemical changes lead to changes in values of the parameters, so that activating agents and inhibiting agents modify values in different directions.

Analysis and Modelling of a Fluidic Actuator

2008 ◽  
Author(s):  
Caroline Braud ◽  
Arthur Dyment ◽  
Jim Kostas ◽  
Jean Marc Foucaut ◽  
Michel Stanislas
Keyword(s):  
Vortex Generator ◽  
Transient Behavior ◽  
Physical Parameters ◽  
Pulsed Jet ◽  
Transient Period ◽  
Fluidic Actuators ◽  
Precise Quantification ◽  
Controlled Flow ◽  
Layer Control

This paper deals with the fluidic actuators in the pulsed mode used in turbulent boundary layer control, in particular as vortex generator (VG) in order to delay separation. Recently the study by Kostas et al (2007) has shown the importance of the transient phase of the VG actuators. In particular, an enhancement of the vortex-generation mechanism has been observed during the transient period, that is responsible for an increase of turbulence stress levels up to 200% relatively to the non actuated case. A large dependency of the exit velocity on the physical characteristic of the feed tube has been detected. This dependency suggests that a precise quantification of the pulsed jet dynamic during the transient period is necessary. In this work the transient behavior of the fluidic actuators used by Kostas et al (2007) is analyzed and experimented. A model is developed to explain the dynamics of the flow inside the actuator. On the whole, experiments show that the role of all physical parameters is consistent with the foreseen properties. The results obtained help to separate the input dynamic of the controller from the controlled flow. Another perspective of this work is to provide a guide for the design of fluidic actuators.

scholarly journals Tad pili play a dynamic role in Caulobacter crescentus surface colonization

2019 ◽  
Author(s):  
Matteo Sangermani ◽  
Isabelle Hug ◽  
Nora Sauter ◽  
Thomas Pfohl ◽  
Urs Jenal
Keyword(s):  
Caulobacter Crescentus ◽  
Optical Trap ◽  
Natural Environments ◽  
Surface Attachment ◽  
Surface Colonization ◽  
Bacterial Surface ◽  
Controlled Flow ◽  
Mechanical Sensing

ABSTRACTBacterial surface attachment is mediated by rotary flagella and filamentous appendages called pili. Here, we describe the role of Tad pili during surface colonization of Caulobacter crescentus. Using an optical trap and microfluidic controlled flow conditions as a mimic of natural environments, we demonstrate that Tad pili undergo repeated cycles of extension and retraction. Within seconds after establishing surface contact, pili reorient cells into an upright position promoting walking-like movements against the medium flow. Pili-mediated positioning of the flagellated pole close to the surface facilitates motor-mediated mechanical sensing and promotes anchoring of the holdfast, an adhesive substance that affords long-term attachment. We present evidence that the second messenger c-di-GMP regulates pili dynamics during surface encounter in distinct ways, promoting increased activity at intermediate levels and retraction of pili at peak concentrations. We propose a model, in which flagellum and Tad pili functionally interact and together impose a ratchet-like mechanism that progressively drives C. crescentus cells towards permanent surface attachment.

Turbulence reduction by screens

1988 ◽  
Vol 197 ◽  
pp. 139-155 ◽  
Author(s):  
Johan Groth ◽  
Arne V. Johansson
Keyword(s):  
Wind Tunnel ◽  
Reynolds Numbers ◽  
Wind Tunnels ◽  
Small Scale ◽  
Flow Conditions ◽  
Upstream Side ◽  
Turbulence Suppression ◽  
Controlled Flow ◽  
Reduction Effectiveness

Turbulence suppression by use of screens was studied in a small wind tunnel especially designed and built for the purpose. Wide ranges of mesh sizes and wire-diameter Reynolds numbers were covered in the present investigation, enabling the study of sub- and super-critical screens under the same, well-controlled, flow conditions. For the latter type small-scale fluctuations, produced by the screen itself, interact with the incoming turbulence. In the immediate vicinity of the screen the turbulence was found to be highly anisotropic and the intensities were higher than on the upstream side. Downstream of a short initial decay region, where the intensities decrease rapidly, the return to isotropy was found to be much slower than for the unmanipulated turbulence. The latter was generated by a square rod grid, and was shown to become practically isotropic beyond a distance of roughly 20 mesh widths from the grid. The role of the turbulence scales for the overall reduction effectiveness, and for the optimization of screen combinations for application in low-turbulence wind tunnels was studied.

Venoarterial CO2 difference during regional ischemic or hypoxic hypoxia

2000 ◽  
Vol 89 (4) ◽  
pp. 1317-1321 ◽  
Author(s):  
Benoit Vallet ◽  
Jean-Louis Teboul ◽  
Stephen Cain ◽  
Scott Curtis
Keyword(s):  
Blood Flow ◽  
Tissue Hypoxia ◽  
Hypoxic Hypoxia ◽  
Systemic Hemodynamics ◽  
Membrane Oxygenator ◽  
Normal Range ◽  
Ischemic Hypoxia ◽  
Controlled Flow

To test the role of blood flow in tissue hypoxia-related increased veno-arterial Pco 2difference (ΔPco 2), we decreased O2 delivery (D˙o 2) by either decreasing flow [ischemic hypoxia (IH)] or arterial Po 2 [hypoxic hypoxia (HH)] in an in situ, vascularly isolated, innervated dog hindlimb perfused with a pump-membrane oxygenator system. Twelve anesthetized and ventilated dogs were studied, with systemic hemodynamics maintained within normal range. In the IH group ( n = 6), hindlimbD˙o 2 was progressively lowered every 15 min by decreasing pump-controlled flow from 60 to 10 ml · kg−1 · min−1, with arterial Po 2 constant at 100 Torr. In the HH group ( n = 6), hindlimbD˙o 2 was progressively lowered every 15 min by decreasing Po 2 from 100 to 15 Torr, when flow was constant at 60 ml · kg−1 · min−1. LimbD˙o 2, O2 uptake (V˙o 2), and ΔPco 2 were obtained every 15 min. Below the criticalD˙o 2,V˙o 2 decreased, indicating dysoxia, and O2 extraction ratio (V˙o 2/D˙o 2) rose continuously and similarly in both groups, reaching a maximal value of ∼90%. ΔPco 2 significantly increased in IH but never differed from baseline in HH. We conclude that absence of increased ΔPco 2 does not preclude the presence of tissue dysoxia and that decreased flow is a major determinant in increased ΔPco 2.

Role of inertia and thixotropy in start-up flows of aging soft materials: Transient dynamics and shear banding in a rate-controlled flow field

2018 ◽  
Vol 62 (4) ◽  
pp. 1001-1016 ◽  
Author(s):  
Anika Jain ◽  
Ramanish Singh ◽  
Lakshmi Kushwaha ◽  
V. Shankar ◽  
Yogesh M. Joshi
Keyword(s):  
Flow Field ◽  
Shear Banding ◽  
Soft Materials ◽  
Transient Dynamics ◽  
Start Up ◽  
Rate Controlled ◽  
Controlled Flow

scholarly journals Dispersive effects in Rossby-wave hydraulics

1999 ◽  
Vol 401 ◽  
pp. 27-54 ◽  
Author(s):  
E. R. JOHNSON ◽  
S. R. CLARKE
Keyword(s):  
Potential Vorticity ◽  
Rossby Waves ◽  
Soliton Solutions ◽  
Finite Amplitude ◽  
Large Dispersion ◽  
Hydraulic Theory ◽  
Steady Solutions ◽  
Controlled Flow ◽  
Rotating Channel

This paper considers the role of long finite-amplitude Rossby waves in determining the evolution of flow along a rapidly rotating channel with an uneven floor. The Rossby waves travel on a potential vorticity interface in a channel with a cross-channel step change in depth, where step position varies slowly along the channel. A nonlinear wave equation is derived describing the evolution of the potential vorticity interface. To leading order this is the hydraulic equation derived by Haynes, Johnson & Hurst (1993). Dispersion appears at the next order. Various solution regimes are identified. As well as slowly varying hydraulic solutions, two further types of steady solutions appear: approach-controlled flows and twin supercritical leaps. Both these solutions are characterized by leaps between supercritical branches of the hydraulic function. It is shown how the position and size of these ‘supercritical leaps’ can be determined within the context of hydraulic theory. To fully resolve the internal structure of these leaps dispersive effects must be included and leaps are shown to correspond to kink soliton solutions of the steady unforced problem. It is also shown that increasing dispersion (decreasing topographic length scale) causes the loss of the subcritical solution branch in some subcritical flows. The only candidate for a steady solution in these regimes is then an approach-controlled flow. Integrations of initial value problems show that in general flows evolve towards the dispersive form of the solution predicted by hydraulic theory, at least near the topographic perturbation. However, in those subcritical flows where sufficiently large dispersion causes the subcritical branch to disappear, unsteady integrations evolve to approach-controlled flows even when the dispersion is sufficiently small that the subcritical branch still exists.

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