On Modelling The Stability Of Fluid Flows In Compliant Pipes Applied To Hemodynamic Problems

2015 ◽  
Vol 10 (4) ◽  
pp. 29-42
Author(s):  
Andrey Boiko
Keyword(s):  
Hydrodynamic Instability ◽  
Fluid Flows ◽  
Physical Models ◽  
Compliant Walls ◽  
The Stability

The study is directed to formulate physical models adequate to describe mechanisms of interactions of compliant walls of pipes and hydrodynamic disturbances aiming to reveal the conditions responsible for hydrodynamic instability in hemodynamic problems.

scholarly journals Data-Driven Stability Assessment of Multilayer Long Short-Term Memory Networks

2021 ◽  
Vol 11 (4) ◽  
pp. 1829
Author(s):  
Davide Grande ◽  
Catherine A. Harris ◽  
Giles Thomas ◽  
Enrico Anderlini
Keyword(s):  
Neural Network ◽  
Network Architecture ◽  
Short Term Memory ◽  
Moving Average ◽  
Machine Learning Algorithms ◽  
Physical Models ◽  
Data Driven ◽  
The Stability ◽  
And Control ◽  
Nonlinear Autoregressive

Recurrent Neural Networks (RNNs) are increasingly being used for model identification, forecasting and control. When identifying physical models with unknown mathematical knowledge of the system, Nonlinear AutoRegressive models with eXogenous inputs (NARX) or Nonlinear AutoRegressive Moving-Average models with eXogenous inputs (NARMAX) methods are typically used. In the context of data-driven control, machine learning algorithms are proven to have comparable performances to advanced control techniques, but lack the properties of the traditional stability theory. This paper illustrates a method to prove a posteriori the stability of a generic neural network, showing its application to the state-of-the-art RNN architecture. The presented method relies on identifying the poles associated with the network designed starting from the input/output data. Providing a framework to guarantee the stability of any neural network architecture combined with the generalisability properties and applicability to different fields can significantly broaden their use in dynamic systems modelling and control.

scholarly journals Soft channel formation and symmetry breaking in exotic active emulsions

2020 ◽  
Vol 10 (1) ◽  
Author(s):  
L. N. Carenza ◽  
G. Gonnella ◽  
A. Lamura ◽  
D. Marenduzzo ◽  
G. Negro ◽  
...  
Keyword(s):  
Shear Rate ◽  
Rheological Properties ◽  
Self Assembly ◽  
Contractile Activity ◽  
Active Material ◽  
Fluid Flows ◽  
Shear Rates ◽  
Two Phases ◽  
The Stability ◽  
Isotropic Fluid

Abstract We use computer simulations to study the morphology and rheological properties of a bidimensional emulsion resulting from a mixture of a passive isotropic fluid and an active contractile polar gel, in the presence of a surfactant that favours the emulsification of the two phases. By varying the intensity of the contractile activity and of an externally imposed shear flow, we find three possible morphologies. For low shear rates, a simple lamellar state is obtained. For intermediate activity and shear rate, an asymmetric state emerges, which is characterized by shear and concentration banding at the polar/isotropic interface. A further increment in the active forcing leads to the self-assembly of a soft channel where an isotropic fluid flows between two layers of active material. We characterize the stability of this state by performing a dynamical test varying the intensity of the active forcing and shear rate. Finally, we address the rheological properties of the system by measuring the effective shear viscosity, finding that this increases as active forcing is increased—so that the fluid thickens with activity.

The stability of boundary-layer flow over single-and multi-layer viscoelastic walls

1988 ◽  
Vol 196 ◽  
pp. 359-408 ◽  
Author(s):  
K. S. Yeo
Keyword(s):  
Boundary Layer ◽  
Boundary Layer Flow ◽  
Single Layer ◽  
Viscous Sublayer ◽  
Rigid Base ◽  
Theoretical Treatment ◽  
Dynamical Condition ◽  
Compliant Walls ◽  
Layer Flow ◽  
The Stability

In this paper, we are concerned with the linear stability of zero pressure-gradient laminar boundary-layer flow over compliant walls which are composed of one or more layers of isotropic viscoelastic materials and backed by a rigid base. Wall compliance supports a whole host of new instabilities in addition to the Tollmien-Schlichting mode of instability, which originally exists even when the wall is rigid. The perturbations in the flow and the compliant wall are coupled at their common interface through the kinematic condition of velocity continuity and the dynamical condition of stress continuity. The disturbance modes in the flow are governed by the Orr-Sommerfeld equation using the locally-parallel flow assumption, and the response of the compliant layers is described using a displacement-stress formalism. The theoretical treatment provides a unified formulation of the stability eigenvalue problem that is applicable to compliant walls having any finite number of uniform layers; inclusive of viscous sublayer. The formulation is well suited to systematic numerical implementation. Results for single- and multi-layer walls are presented. Analyses of the eigenfunctions give an insight into some of the physics involved. Multi-layering gives a measure of control over the stability characteristics of compliant walls not available to single-layer walls. The present study provides evidence which suggests that substantial suppression of disturbance growth may be possible for suitably tailored compliant walls.

Validation of Hydrodynamic Stability of Supercritical Once-Through Boiler

2008 ◽  
Author(s):  
Bing Wei ◽  
Dong Zhou
Keyword(s):  
Flow Instability ◽  
Hydrodynamic Instability ◽  
Pressure Head ◽  
Operating Conditions ◽  
Hydrodynamic Characteristics ◽  
Boiler Operation ◽  
Calculation Results ◽  
Low Mass ◽  
The Stability ◽  
Heating Loads

Operating safety is one of the most important things to supercritical once-through boilers. To study the hydrodynamic characteristics of fluid in water walls of supercritical once-through boilers and to find out the instable factors will be of great significance to boiler operation. In this paper the mathematical models for hydrodynamic characteristics of fluid in water walls are established. With an example of 600MW boiler, by using the calculation program, the hydrodynamic characteristics curves without and with the throttles at the inlets of the water walls at different operating conditions are presented, the fluid flow instability and the reasons are analyzed. The calculation results show that the boiler operates stably and safely at 100% MCR (Maximum Continuous Rating) condition, the hydrodynamic instability exists at low heating loads of 30% MCR. The method of installing the throttles at the inlets of the water wall pipes will increase the parabola characteristics, help to improve the fluid instability to a certain stable extent, but due to the small curve slopes at low mass flowrates, still need to pay more attention to the low heating loads operation. The existence of gravity pressure head is good to the stability of the vertical upward flow.

Plasma spray process modelling using artificial neural networks: Application to Al2O3-TiO2 (13% by weight) ceramic coating structure

2004 ◽  
Vol 120 ◽  
pp. 363-370
Author(s):  
S. Guessasma ◽  
G. Montavon ◽  
C. Coddet
Keyword(s):  
Thermal Spraying ◽  
Processing Parameters ◽  
Ceramic Coatings ◽  
Physical Models ◽  
Spray Process ◽  
Artificial Neural ◽  
Artificial Neural Network Ann ◽  
The Stability ◽  
Experimental Values ◽  
Physical Phenomena

Thermal spraying is a versatile technique of coating manufacturing implementing large variety of materials and processes. The manufacture control is constrained by the understanding of the physical phenomena occurring during the spraying. It is however penalized by the large number of processing parameters (up to 50), their interdependencies, their correlations with the coating attributes and the stability of the process. Numerous statistical, heuristic or physical models intended to response to these constrains, very often partially because considering some aspects of the process. This work aims at considering a more global approach based on a powerful statistical methodology using artificial intelligence. Following this approach, the physical phenomena are encoded in a structure called Artificial Neural Network (ANN). An application of the ANN methodology is discussed in the case of the APS spray process. Some processing parameters categories are related to some coating properties for alumina-titania (13% by weight) ceramic coatings. ANN optimization is presented and discussed. Predicted results show globally a well agreement with the experimental values. Some conclusions point out the advantages of the ANN on the conventional methods, such as the design of experiments, used usually to recognize the controlling factors in a process.

Static Stability of Floating Units in Operational Conditions: A Physics-Driven Approach

2017 ◽  
Author(s):  
Neil Luxcey ◽  
Øystein Johannessen ◽  
Sébastien Fouques
Keyword(s):  
Static Stability ◽  
Stability Study ◽  
Operational Stability ◽  
Physical Models ◽  
Floating Platform ◽  
Line System ◽  
Operational Conditions ◽  
Axis Of Rotation ◽  
Numerical Tool ◽  
The Stability

When designing a new floating unit concept, static stability computations are performed in order to check stability criteria defined in regulations. Calculations for design conditions generally include the estimation of buoyancy force, gravity force and wind force acting on the floater for a given condition and a desired axis of rotation. However, when studying the stability of a floating platform in operational conditions, all external forces acting on the unit should be comprised in the assessment in order to get a more realistic — and even physically admissible — picture of the platform stability. Those forces include among others wind, current and anchor line system forces. In addition, limiting the study to one axis of rotation may not provide a complete picture of the floater stability, especially when the hull is of a semi-submersible type. Following this physical approach, a numerical tool has been developed based on the SINTEF Ocean’s SIMA software package. The latter package initially includes a time domain simulator of complex multibody systems for marine operations. The developed tool provides accurate physical models for each force component that may have effects on the stability. It opens the possibility to study the operational stability of a floater without restraining the study to one axis of rotation. It also allows the analysis of damaged conditions with large inclination angles. This paper describes the model implemented in this numerical tool. Validation work is presented for simple geometries. Results from an operational stability study of a semi-submersible are discussed. Finally, possible further work is discussed.

scholarly journals Some peculiar features of hydrodynamic instability development

2013 ◽  
Vol 371 (2003) ◽  
pp. 20120288 ◽  
Author(s):  
E. Meshkov
Keyword(s):  
Hydrodynamic Instability ◽  
Liquid Interface ◽  
Necessary Condition ◽  
Density Jump ◽  
Interface Instability ◽  
Heavy Medium ◽  
Instability Development ◽  
Rayleigh Taylor Instability ◽  
Heavy Gas ◽  
The Stability

We discuss the results of experiments that illustrate some features of a turbulent mixing zone (TMZ) structure at a gas–liquid interface (Rayleigh–Taylor instability) and at a gas–gas interface accelerated by shock waves (Richtmyer–Meshkov instability). The important feature is the existence of a heavier substance concentration (density) jump at the interface between the heavy medium and the TMZ. It is found that the existence of this jump is a generic feature of any developed TMZ and is the necessary condition for its continuous development. In the case of a gas–liquid interface, the stable existence of this jump is connected with the stability of the cupola of gas bubbles penetrating into the liquid in a TMZ. The important feature of the development of interface instability accelerated by an unsteady shock is the decaying ability (up to full suppression) of the interface instability in the case when a decaying wave passes through the interface in the direction from light gas to heavy gas.

scholarly journals MODELING FLUID FLOWS IN DISTENSIBLE TUBES FOR APPLICATIONS IN HEMODYNAMICS

2013 ◽  
Vol 24 (05) ◽  
pp. 1350030 ◽  
Author(s):  
X. DESCOVICH ◽  
G. PONTRELLI ◽  
S. MELCHIONNA ◽  
S. SUCCI ◽  
S. WASSERTHEURER
Keyword(s):  
Fluid Pressure ◽  
Three Dimensional ◽  
Fluid Flows ◽  
Two Dimensional ◽  
Wall Deformation ◽  
Pulsatile Flows ◽  
Compliant Walls ◽  
Flow Pressure ◽  
Wall Perturbation ◽  
Bounce Back

We present a lattice Boltzmann (LB) model for the simulation of hemodynamic flows in the presence of compliant walls. The new scheme is based on the use of a continuous bounce-back boundary condition, as combined with a dynamic constitutive relation between the flow pressure at the wall and the resulting wall deformation. The method is demonstrated for the case of two-dimensional (axisymmetric) pulsatile flows, showing clear evidence of elastic wave propagation of the wall perturbation in response to the fluid pressure. The extension of the present two-dimensional axisymmetric formulation to more general three-dimensional geometries is currently under investigation.

scholarly journals Sprawl Angle in Simplified Models of Vertical Climbing: Implications for Robots and Roaches

2011 ◽  
Vol 8 (3-4) ◽  
pp. 441-452 ◽  
Author(s):  
Goran A. Lynch ◽  
Lawrence Rome ◽  
Daniel E. Koditschek
Keyword(s):  
Open Loop ◽  
Physical Models ◽  
Stride Frequency ◽  
Mass Center ◽  
Electromechanical Actuators ◽  
Lateral Forces ◽  
Potential Benefits ◽  
The Stability ◽  
New Hypothesis ◽  
Operating Regimes

Empirical data taken from fast climbing sprawled posture animals reveals the presence of strong lateral forces with significant pendulous swaying of the mass center trajectory in a manner captured by a recently proposed dynamical template [1, 2]. In this simulation study we explore the potential benefits of pendulous dynamical climbing in animals and in robots by examining the stability and power advantages of variously more and less sprawled limb morphologies when driven by conventional motors in contrast with animal-like muscles. For open loop models of gait generation inspired by the neural-deprived regimes of high stride-frequency animal climbing, our results corroborate earlier hypotheses that sprawled posture may be required for stability. For quadratic-in-velocity power output actuation models typical of commercially available electromechanical actuators, our results suggest the new hypothesis that sprawled posture may confer significant energetic advantage. In notable contrast, muscle-powered climbers do not experience an energetic benefit from sprawled posture due to their sufficiently distinct actuator characteristics and operating regimes. These results suggest that the potentially significant benefits of sprawled posture climbing may be distinctly different depending upon the details of the climbers sensorimotor endowment. They offer a cautionary instance against mere copying of biology by engineers or rote study of physical models by biologists through this reminder of how even simple questions addressed by simple models can yield nuanced answers that only begin to hint at the complexity of biological designs and behaviors.

Analysis of Instability for OTSG by Using Multivariable Frequency Domain Method

2009 ◽  
Author(s):  
Su-xia Hou ◽  
Yun Tai ◽  
Fu-yu Zhao
Keyword(s):  
Frequency Domain ◽  
Hydrodynamic Instability ◽  
Computer Code ◽  
Domain Theory ◽  
Frequency Domain Method ◽  
Stable Boundary ◽  
Numerical Examples ◽  
Parametric Effects ◽  
The Stability ◽  
Good Agreement

A new method for analyzing the problem of the thermal-hydrodynamic instability of OTSG (once-through steam generator) with multiple boiling channels is presented in this paper. The mothod is based on modern frequency domain theory and is more efficient for analyzing the instability of OTSG with coupling interactions and complicated boundary conditions than the usually used single variable method. A mathematical model for a OTSG is derived from the foundamental equations by use of the small perturbation and Laplace-transform techniques. The stable boundary and parametric effects on the stability of the system are evaluated with a computer code. Numerical examples are given in the paper and the predictions of the model are in good agreement with the experimental results.

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