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At least 631 records · Page 35Linked to original sources
arXiv2014

A fluid dynamics multidimensional model of biofilm growth: stability, influence of environment and sensitivity

In this article, we study in details the fluid dynamics system proposed in Clarelli et al (2013) to model the formation of cyanobacteria biofilms. After analyzing the linear stability of the unique non trivial equilibrium of the system, we introduce in the model the influence of light and temperature, which are two important factors for the development of cyanobacteria biofilm. Since the values of the coefficients we use for our simulations are estimated through information found in the literature, some sensitivity and robustness analyses on these parameters are performed. All these elements enable us to control and to validate the model we have already derived and to present some numerical simulations in the 2D and the 3D cases.

physics.flu-dyn↗
arXiv2014

Formation of Large-Amplitude Low-Frequency Waves in Capillary Turbulence on Superfluid He-II

The results of experimental and theoretical studies of the parametric decay instability of capillary waves on the surface of superfluid helium He-II are reported. It is demonstrated that in a system of turbulent capillary waves low-frequency waves are generated along with the direct Kolmogorov-Zakharov cascade of capillary turbulence. The effects of low-frequency damping and the discreteness of the wave spectrum are discussed.

physics.flu-dyn↗
arXiv2014

An improved helical subgrid-scale model and large-eddy simulation methods in helical turbulence

For helical isotropic turbulence, an improved two-term helical subgrid-scale (SGS) model is proposed and four types of dynamic methods are given to do large-eddy simulation (LES), which include the standard dynamic procedure, the least quatratic sum dynamic procedure, the dynamic procedure with single con- straint of helicity dissipation and the dynamic one with dual constraints of energy and helicity dissipation. Tested a priori and a posteriori in both steady and decaying helical isotropic turbulence, the four types of dynamic helical models and the dynamic Smogorinsky model are compared with results of direct numerical simulations (DNS) together. Numerical results demonstrate that the three new types of dynamic helical models predict energy and helicity evolution better than the standard dynamic helical model, and the two constrained helical models predict the energy and helicity dissipation rates better than other models. Fur- thermore, the constrained helical models have higher correlation to the real SGS stress and have more similar probability density functions (PDF) to the DNS results. In general, the two constrained helical models show some more attractive features than other models.

physics.flu-dyn↗
arXiv2014

On a time-discrete approach to solving Navier-Stokes systems

We present a new scheme for solving Navier-Stokes systems. This is inspired by material differentiation and combined with discrete Morse semi-flow. The solution has the first energy inequality, we set some assumption though. This result crucially does not provide any new result, but I believe that by making my scheme is more sophisticated, we can develop the study of Navier-Stokes systems.

math.AP↗
arXiv2014

Closed-loop control of an experimental mixing layer using machine learning control

A novel framework for closed-loop control of turbulent flows is tested in an experimental mixing layer flow. This framework, called Machine Learning Control (MLC), provides a model-free method of searching for the best function, to be used as a control law in closed-loop flow control. MLC is based on genetic programming, a function optimization method of machine learning. In this article, MLC is benchmarked against classical open-loop actuation of the mixing layer. Results show that this method is capable of producing sensor-based control laws which can rival or surpass the best open-loop forcing, and be robust to changing flow conditions. Additionally, MLC can detect non-linear mechanisms present in the controlled plant, and exploit them to find a better type of actuation than the best periodic forcing.

physics.flu-dyn↗
arXiv2014

Triple Point Collision and Origin of Unburned Gas Pockets in Irregular Detonations

The turbulent structure of an irregular detonation is studied through very high resolution numerical simulations of 600 points per half reaction length. The aim is to explore the nature of the transverse waves during the collision and reflection processes of the triple point with the channel walls. Consequently the formation and consumption mechanism of unreacted gas pockets is studied. Results show that as the triple point collides with the wall, the transverse shock interacts with the unreacted pocket. After reflection of the triple point off the wall, the transverse wave interacts with the wall. The structure found to be of a double Mach configuration and does not change before and after reflection. In the second half of the detonation cell the triple point and the transverse wave collide simultaneously with the wall. The strong transverse wave switches from a primary triple point before collision to a new one after reflection. After some time a weak triple point reflects off the wall and hence the structure exhibits more like a single Mach configuration. Due to simultaneous interaction of the triple point and the transverse wave with the wall in the second half of the detonation cell, a larger high pressurized region appears on the wall. During the reflection the reaction zone detaches from the shock front and produces a pocket of unburned gas. Three mechanisms found to be of significance in the re initiation mechanism of detonation at the end of the detonation cell; i: energy resealed via consumption of unburned pockets by turbulent mixing ii: compression waves arise due to collision of the triple point on the wall which helps the shock to jump abruptly to an overdriven detonation iii: drastic growth of the Richtmyer Meshkov instability causing a part of the front to accelerate with respect to the neighboring portions.

physics.flu-dyn↗
arXiv2014

On the flow of Oldroyd-B fluids with fractional derivatives over a plate that applies shear to the fluid

The motion of incompressible fractional Oldroyd-B fluids between two parallel walls perpendicular to a plate that applies time-dependent shear stresses to the fluid is studied by means of integral transforms. In the special cases of Newtonian and second grade fluids, these shear stresses reduces to fH(t)sin(wt) or fH(t)cos(wt). General solutions for velocity are presented as a sum of Newtonian solutions and the corresponding non-Newtonian contributions. They reduce to the similar solutions corresponding to the motion over an infinite plate if the distance between walls tend to infinity and can be easy particularized to give the similar solutions for ordinary and fractional Maxwell or second grade fluids performing the same motions. As a check of general results some known solutions from the literature are recovered as limiting cases. Finally, the influence of fractional parameters on the fluid motion and the distance between walls for which the measured value of the velocity in the middle of the channel is unaffected by their presence (more exactly, it is equal to the velocity corresponding to the motion over an infinite plate) are graphically determined.

physics.flu-dyn↗
arXiv2014

Inflow turbulence generation for eddy-resolving simulations of turbomachinery flows

A simple variant of recycling and rescaling method to generate inflow turbulence using unstructured grid CFD codes is presented. The method has been validated on large eddy simulation of spatially developing flat plate turbulent boundary layer. The proposed rescaling algorithm is based on the momentum thickness which is more robust and essentially obviates the need of finding the edge of the turbulent boundary layer in unstructured grid codes. Extension of this algorithm to hybrid RANS/LES type of approaches and for wall-bounded turbomachinery flows is also discussed. Results from annular diffuser with different inflow boundary layer characteristics is presented as an example application to show the utility of such an algorithm.

physics.flu-dyn↗
arXiv2014

Adaptive Inflow Control System

This article presents the idea and realization for the unique Adaptive Inflow Control System being a part of well completion, able to adjust to the changing in time production conditions. This system allows to limit the flow rate from each interval at a certain level, which solves the problem of water and gas breakthroughs. We present the results of laboratory tests and numerical calculations obtaining the characteristics of the experimental setup with dual-in-position valves as parts of adaptive inflow control system, depending on the operating conditions. The flow distribution in the system was also studied with the help of three-dimensional computer model. The control ranges dependences are determined, an influence of the individual elements on the entire system is revealed.

physics.flu-dyn↗
arXiv2014

Continuous data assimilation for the three-dimensional Navier-Stokes-$α$

Motivated by the presence of a finite number of determining parameters (degrees of freedom) such as modes, nodes and local spatial averages for dissipative dynamical systems, we present a continuous data assimilation algorithm for the three-dimensional Navier-Stokes-$α$ model. This algorithm consists of introducing a nudging process through general type of approximation interpolation operator (that is constructed from observational measurements) that synchronizes the large spatial scales of the approximate solutions with those of the unknown solutions the Navier-Stokes-$α$ equations that corresponds to these measurements. Our main result provides conditions on the finite-dimensional spatial resolution of the collected data, sufficient to guarantee that the approximating solution, that is obtained from these collected data, converges to the unkown reference solution (physical reality) over time. These conditions are given in terms of some physical parameters, such as kinematic viscosity, the size of the domain and the forcing term.

math.AP↗
arXiv2014

Anomalous dispersion of Lagrangian particles in local regions of turbulent flows revealed by convex hull analysis

Local regions of anomalous particle dispersion, and intermittent events that occur in turbulent flows can greatly influence the global statistical description of the flow. These local behaviors can be identified and analyzed by comparing the growth of neighboring convex hulls of Lagrangian tracer particles. Although in our simulations of homogeneous turbulence the convex hulls generally grow in size, after the Lagrangian particles that define the convex hulls begin to disperse, our analysis reveals short periods when the convex hulls of the Lagrangian particles shrink, evidence that particles are not dispersing simply. Shrinkage can be associated with anisotropic flows, since it occurs most frequently in the presence of a mean magnetic field or thermal convection. We compare dispersion between a wide range of statistically homogeneous and stationary turbulent flows ranging from homogeneous isotropic Navier-Stokes turbulence over different configurations of magnetohydrodynamic turbulence and Boussinesq convection.

physics.flu-dyn↗
arXiv2014

Comments on the compositional boundary condition for diffuse interface model of contact line

The diffuse interface model of Cahn-Hilliard-van der Waals is often used to study various aspects of multi-phase flows such as droplets coalescence and contact line dynamics. The original model of Cahn-Hilliard-van der Waals uses an approximation which neglects some surface contributions to the free energy of the system and is justified by the large distance between the region of observation and the outer surface of the system. This is not a priori accurate when considering the dynamics of the contact line on a solid surface. A modification of the so called natural compositional boundary conditions obtained by minimization of the surface contribution to the free energy of the system is proposed for the vapor-liquid interface of a single component and for a quasi-incompressible binary mixture of partially miscible liquids. The results are obtained for a stagnant fluid in thermodynamic equilibrium but should be valid in a flow if local thermodynamic equilibrium can be assumed.

physics.flu-dyn↗
arXiv2014

In vivo recording of aerodynamic force with an aerodynamic force platform

Flapping wings enable flying animals and biomimetic robots to generate elevated aerodynamic forces. Measurements that demonstrate this capability are based on tethered experiments with robots and animals, and indirect force calculations based on measured kinematics or airflow during free flight. Remarkably, there exists no method to measure these forces directly during free flight. Such in vivo recordings in freely behaving animals are essential to better understand the precise aerodynamic function of their flapping wings, in particular during the downstroke versus upstroke. Here we demonstrate a new aerodynamic force platform (AFP) for nonintrusive aerodynamic force measurement in freely flying animals and robots. The platform encloses the animal or object that generates fluid force with a physical control surface, which mechanically integrates the net aerodynamic force that is transferred to the earth. Using a straightforward analytical solution of the Navier-Stokes equation, we verified that the method is accurate. We subsequently validated the method with a quadcopter that is suspended in the AFP and generates unsteady thrust profiles. These independent measurements confirm that the AFP is indeed accurate. We demonstrate the effectiveness of the AFP by studying aerodynamic weight support of a freely flying bird in vivo, which demonstrates that its upstroke is inactive.

physics.flu-dyn↗
arXiv2014

Heterogeneous Computing on Mixed Unstructured Grids with PyFR

PyFR is an open-source high-order accurate computational fluid dynamics solver for mixed unstructured grids that can target a range of hardware platforms from a single codebase. In this paper we demonstrate the ability of PyFR to perform high-order accurate unsteady simulations of flow on mixed unstructured grids using heterogeneous multi-node hardware. Specifically, after benchmarking single-node performance for various platforms, PyFR v0.2.2 is used to undertake simulations of unsteady flow over a circular cylinder at Reynolds number 3 900 using a mixed unstructured grid of prismatic and tetrahedral elements on a desktop workstation containing an Intel Xeon E5-2697 v2 CPU, an NVIDIA Tesla K40c GPU, and an AMD FirePro W9100 GPU. Both the performance and accuracy of PyFR are assessed. PyFR v0.2.2 is freely available under a 3-Clause New Style BSD license (see www.pyfr.org).

physics.flu-dyn↗
arXiv2014

Asymptotic dynamics of inertial particles with memory

Recent experimental and numerical observations have shown the significance of the Basset--Boussinesq memory term on the dynamics of small spherical rigid particles (or inertial particles) suspended in an ambient fluid flow. These observations suggest an algebraic decay to an asymptotic state, as opposed to the exponential convergence in the absence of the memory term. Here, we prove that the observed algebraic decay is a universal property of the Maxey--Riley equation. Specifically, the particle velocity decays algebraically in time to a limit that is $\mathcal O(ε)$-close to the fluid velocity, where $0<ε\ll 1$ is proportional to the square of the ratio of the particle radius to the fluid characteristic length-scale. These results follows from a sharp analytic upper bound that we derive for the particle velocity. For completeness, we also present a first proof of existence and uniqueness of global solutions to the Maxey--Riley equation, a nonlinear system of fractional-order differential equations.

math-ph↗
arXiv2014

A Numerical Approach to Solving Nonlinear Differential Equations on a Grid with Potential Applicability to Computational Fluid Dynamics

A finite element method for solving nonlinear differential equations on a grid, with potential applicability to computational fluid dynamics (CFD), is developed and tested. The current method facilitates the computation of solutions of a high polynomial degree on a grid. A high polynomial degree is achieved by interpolating both the value, and the value of the derivatives up to a given order, of continuously distributed unknown variables. The two-dimensional lid-driven cavity, a common benchmark problem for CFD methods, is used as a test case. It is shown that increasing the polynomial degree has some advantages, compared to increasing the number of grid-points, when solving the given benchmark problem using the current method. The current method yields results which agree well with previously published results for this test case.

physics.comp-ph↗
arXiv2014

Theoretical conditions of pipe flow transition: new approach to the old problem

For Hagen-Poiseuille flow the mechanism of laminar-to-turbulent transition originated from the deceleration of disturbed streams on the pipe wall is analyzed. An expression for the critical Reynolds number and the disturbance in the form of ring obstacle is derived. The good qualitative agreement between the theoretical results and published experimental data is demonstrated

physics.flu-dyn↗