Global-local nonlinear model reduction for flows in heterogeneous porous media

In this paper, we combine discrete empirical interpolation techniques, global mode decomposition methods, and local multiscale methods, such as the Generalized Multiscale Finite Element Method (GMsFEM), to reduce the computational complexity associated with nonlinear flows in highly-heterogeneous porous media. To solve the nonlinear governing equations, we employ the GMsFEM to represent the solution on a coarse grid with multiscale basis functions and apply proper orthogonal decomposition on a coarse grid. Computing the GMsFEM solution involves calculating the residual and the Jacobian on a fine grid. As such, we use local and global empirical interpolation concepts to circumvent performing these computations on the fine grid. The resulting reduced-order approach significantly reduces the flow problem size while accurately capturing the behavior of fully-resolved solutions. We consider several numerical examples of nonlinear multiscale partial differential equations that are numerically integrated using fully-implicit time marching schemes to demonstrate the capability of the proposed model reduction approach to speed up simulations of nonlinear flows in high-contrast porous media

PyFly: A fast, portable aerodynamics simulator

We present a fast, user-friendly implementation of a potential flow solver based on the unsteady vortex lattice method (UVLM), namely PyFly. UVLM computes the aerodynamic loads applied on lifting surfaces while capturing the unsteady effects such as the added mass forces, the growth of bound circulation, and the wake while assuming that the flow separation location is known a priori. This method is based on discretizing the body surface into a lattice of vortex rings and relies on the Biot–Savart law to construct the velocity field at every point in the simulated domain. We introduce the pointwise approximation approach to simulate the interactions of the far-field vortices to overcome the computational burden associated with the classical implementation of UVLM. The computational framework uses the Python programming language to provide an easy to handle user interface while the computational kernels are written in Fortran. The mixed language approach enables high performance regarding solution time and great flexibility concerning easiness of code adaptation to different system configurations and applications. The computational tool predicts the unsteady aerodynamic behavior of multiple moving bodies (e.g., flapping wings, rotating blades, suspension bridges) subject to incoming air. The aerodynamic simulator can also deal with enclosure effects, multi-body interactions, and B-spline representation of body shapes. We simulate different aerodynamic problems to illustrate the usefulness and effectiveness of PyFly

A thermosensitive electromechanical model for detecting biological particles

Miniature electromechanical systems form a class of bioMEMS that can provide appropriate sensitivity. In this research, a thermo-electro-mechanical model is presented to detect biological particles in the microscale. Identification in the model is based on analyzing pull-in instability parameters and frequency shifts. Here, governing equations are derived via the extended Hamilton’s principle. The coupled effects of system parameters such as surface layer energy, electric field correction, and material properties are incorporated in this thermosensitive model. Afterward, the accuracy of the present model and obtained results are validated with experimental, analytical, and numerical data for several cases. Performing a parametric study reveals that mechanical properties of biosensors can significantly affect the detection sensitivity of actuated ultra-small detectors and should be taken into account. Furthermore, it is shown that the number or dimension of deposited particles on the sensing zone can be estimated by investigating the changes in the threshold voltage, electrode deflection, and frequency shifts. The present analysis is likely to provide pertinent guidelines to design thermal switches and miniature detectors with the desired performance. The developed biosensor is more appropriate to detect and characterize viruses in samples with different temperatures

Flapping wings in line formation flight: A computational analysis

The current understanding of the aerodynamics of birds in formation flights is mostly based on field observations. The interpretation of these observations is usually made using simplified aerodynamic models. Here, we investigate the aerodynamic aspects of formation flights. We use a potential flow solver based on the unsteady vortex lattice method (UVLM) to simulate the flow over flapping wings flying in grouping arrangements and in proximity of each other. UVLM has the capability to capture unsteady effects associated with the wake. We demonstrate the importance of properly capturing these effects to assess aerodynamic performance of flapping wings in formation flight. Simulations show that flying in line formation at adequate spacing enables significant increase in the lift and thrust and reduces power consumption. This is mainly due to the interaction between the trailing birds and the previously-shed wake vorticity from the leading bird. Moreover, enlarging the group of birds flying in formation further improves the aerodynamic performance for each bird in the flock. Therefore, birds get significant benefit of such organised patterns to minimise power consumption while traveling over long distances without stop and feeding. This justifies formation flight as being beneficial for bird evolution without regard to potential social benefits, such as, visual and communication factors for group protection and predator evasion. © 2014 Royal Aeronautical Society

Sensitivity analysis of the performance of olympic rowing boats

A multidisciplinary approach is implemented to model and analyze the performance of Olympic rowing boats. A reduced-order model that couples rowers motions with the hull, oars and hydrodynamic and hydrostatic forces is detailed. This model is complemented with a sensitivity analysis carried out by means of a non intrusive polynomial chaos expansion. Different strategies for the evaluation of the polynomial chaos expansion coefficients are implemented and tested. Sensitivity analysis results for a lightweight single scull are presented and discussed. This analysis contrasts the effects of varying forces exerted by the rowers, weights of rowers and cadence of motions on the boat performance. \ua9 2010 by A. Mola, M. Ghommem and M. R. Hajj

Multi-physics modelling and sensitivity analysis of olympic rowing boat dynamics

A multidisciplinary approach is implemented to model and analyse the performance of Olympic rowing boats. A reduced-order model that couples rowers motions with the hull, oars and hydrodynamic and hydrostatic forces is detailed. This model is complemented with a sensitivity analysis carried out by means of a non intrusive polynomial chaos expansion. Sensitivity analysis results for two different boat classes, namely, a lightweight single scull and a coxless four are presented and discussed. This analysis contrasts, for both classes, the effects of varying forces exerted by the rowers, weights of rowers and cadence of motions on the boat performance

Control of Limit Cycle Oscillations of a Two-Dimensional Aeroelastic System

Linear and nonlinear static feedback controls are implemented on a nonlinear aeroelastic system that consists of a rigid airfoil supported by nonlinear springs in the pitch and plunge directions and subjected to nonlinear aerodynamic loads. The normal form is used to investigate the Hopf bifurcation that occurs as the freestream velocity is increased and to analytically predict the amplitude and frequency of the ensuing limit cycle oscillations (LCO). It is shown that linear control can be used to delay the flutter onset and reduce the LCO amplitude. Yet, its required gains remain a function of the speed. On the other hand, nonlinear control can be effciently implemented to convert any subcritical Hopf bifurcation into a supercritical one and to significantly reduce the LCO amplitude

Shape optimisation and performance analysis of flapping wings

In this paper, we consider the shape optimization of flapping wings in forward flight. This analysis is performed by combining a gradient-based optimiser with the unsteady vortex lattice method. The objective is to identify a set of optimised shapes that maximise the propulsive efficiency under lift, thrust, and area constraints. The geometry of the wings is modelled using B-splines. The flow simulations using the optimal wing shapes indicate that changes in the shape have significant effects on averaged quantities. The optimal shape configuration substantially increases the time averaged thrust while, at the same time, it acquires a larger input of aerodynamic power. Increasing the number of variables (i.e., providing the wing shape with a greater degree of spatial freedom) enables increasingly superior designs. This study should provide better guidance for shape design of engineered flying systems. © Civil-Comp Press, 2012