Methods for parallel computation of complex flow problems

This paper is an overview of some of the methods developed by the Team for Advanced Flow Simulation and Modeling (TAFSM) [http://www.mems.rice.edu/TAFSM/] to support flow simulation and modeling in a number of “Targeted Challenges”. The “Targeted Challenges” include unsteady flows with interfaces, fluid–object and fluid–structure interactions, airdrop systems, and air circulation and contaminant dispersion. The methods developed include special numerical stabilization methods for compressible and incompressible flows, methods for moving boundaries and interfaces, advanced mesh management methods, and multi-domain computational methods. We include in this paper a number of numerical examples from the simulation of complex flow problems.     

Conceptual design of aeroelastic structures by topology optimization

A topology optimization methodology is presented for the conceptual design of aeroelastic structures accounting for the fluid–structure interaction. The geometrical layout of the internal structure, such as the layout of stiffeners in a wing, is optimized by material topology optimization. The topology of the wet surface, that is, the fluid–structure interface, is not varied. The key components of the proposed methodology are a Sequential Augmented Lagrangian method for solving the resulting large-scale parameter optimization problem, a staggered procedure for computing the steady-state solution of the underlying nonlinear aeroelastic analysis problem, and an analytical adjoint method for evaluating the coupled aeroelastic sensitivities. The fluid–structure interaction problem is modeled by a three-field formulation that couples the structural displacements, the flow field, and the motion of the fluid mesh. The structural response is simulated by a three-dimensional finite element method, and the aerodynamic loads are predicted by a three-dimensional finite volume discretization of a nonlinear Euler flow. The proposed methodology is illustrated by the conceptual design of wing structures. The optimization results show the significant influence of the design dependency of the loads on the optimal layout of flexible structures when compared with results that assume a constant aerodynamic load.     

The stability of an oceanic structure with T–S fuzzy models

In this study we construct and derive analytical solutions for a mathematical model of an oceanic environment in which wave-induced flow fields cause structural surge motion after which a fuzzy control technique is developed to alleviate structural vibration. Specifically the Takagi–Sugeno (T–S) fuzzy model is employed to approximate the oceanic structure and a parallel-distributed-compensation (PDC) scheme is utilized in a control procedure designed to reduce the structural response. All local state feedback controllers are integrated to construct a global fuzzy logic controller. The Lyapunov method is used to achieve structural stability. The interaction between the wave motion and the structural response is explained using the separation of variables method. The surge motion is related to the characteristics of the wave and the structure. A parametric approach is utilized to show these effects. Other parameters remain constant. In an oceanic structural system, platform migration is often caused by the wave force. The stability of an oceanic structure can be proven theoretically based on stability analysis. The decay of the displacement and velocity due to the use of the proposed fuzzy controllers is demonstrated by a numerical simulation.     

Simulation of spilling breaking waves using a two phase flow CFD model

A two phase flow CFD model has been developed for 2D spilling breaking wave simulations. A mass conservative level set method similar to Olsson and Kreiss [Olsson E, Kreiss G. A conservative level set method for two phase flow. J Comput Phys 2005;210(1):225–46] is implemented for capturing the air–water interface. The solver is discretised using a finite volume method based on a curvilinear coordinate system. A fully implicit fractional step method is used to advance simulations in time. The solver has been tested and validated by repeating benchmark results of dam breaking simulation and travelling solitary wave simulation. Finally, we employ this solver to simulate spilling breaking waves in the surf zone. Our results show that surface elevations, the location of the breaking point and undertow profiles can generally be well captured. We have also found that temporal and spatial schemes may have significant impacts on computational results.     

Numerical simulation of ground water mounding and its verification by Hele–Shaw model

Ground water mounding is the rise of the water table above its regional level in a local area of an aquifer in order to provide sufficient head to distribute the water supplied by a localized source to that area. The shape and height of the mound depend on many factors including recharge rate and distribution, geology, hydraulic conductivity, flow/head control locations, saturated thickness and regional flow in the aquifer in that area. In this work, an accurate and efficient numerical model for calculating ground water mounding was developed. Numerical calculations were done on a uniform rectangular grid, obtained by a transformation of the physical domain. Grid for computation were generated by a grid generation code, EagleView, which is developed by the Mississippi State University. Model predictions were verified with tests in a Hele–Shaw model for situations with and without a regional flow, with and without heterogeneity, and for two recharge rates. SAE#50 oil was used as the fluid in the Hele–Shaw. A peristaltic pump was used to supply the constant (and adjustable) recharge rate from the reservoir below the Hele–Shaw model. The results of experiments of estimating mounds and the numerical mounding model are in good agreement. However, mound height of the region below recharge of Hele–Shaw model can not be observed because the flow of this region combines vertical flow from recharge and the rising of the free surface (horizontal flow). Hence, an emulated perched aquifer was used so that mound height of the recharge region can be observed.     

Flow analysis of a ribbed helix lip seal with consideration of fluid–structure interaction

Ribbed helix lip seals for rotating shafts have been widely used to retain oil and exclude contaminants in many applications throughout the industry. The objective of this study is to better understand the basic flow behavior associated with the pumping process of a ribbed helix lip seal. The theoretical model consists of a flow analysis of the lubricating film of the hydraulic fluid in conjunction with a stress analysis of the lip seal distortion. The complicated mechanical interaction between the oil flow and rubber deformation was simulated using a coupled fluid–structure approach implemented in a commercial computational fluid dynamics (CFD) code ESI-CFD, ACE+®. The flow characteristics and rubber deformation around a ribbed helix lip seal were fully resolved in a pumping-rate test environment, where both air and oil sides were filled with oil initially. The three-dimensional pressure field solved by the model via the coupled flow-stress analysis was compared with the predictions obtained from the model via the nondeformable rubber assumption to elucidate the significant effect of the fluid–structure interaction on accurate simulation of the oil pumping behavior. In the rotating speed ranging from 1000 to 6000 rpm, both measured and calculated pumping rates increase with the shaft speed for a ribbed helix lip seal. As compared to the baseline case, calculations with considering the fluid–structure interaction at higher rotary speeds can result in thicker oil films, and in turn produce greater pumping rates.     

Numerical simulation of patient-specific left ventricular model with both mitral and aortic valves by FSI approach

Intraventricular flow is important in understanding left ventricular function; however, relevant numerical simulations are limited, especially when heart valve function is taken into account. In this study, intraventricular flow in a patient-specific left ventricle has been modelled in two-dimension (2D) with both mitral and aortic valves integrated. The arbitrary Lagrangian–Eulerian (ALE) approach was employed to handle the large mesh deformation induced by the beating ventricular wall and moving leaflets. Ventricular wall deformation was predefined based on MRI data, while leaflet dynamics were predicted numerically by fluid–structure interaction (FSI). Comparisons of simulation results with in vitro and in vivo measurements reported in the literature demonstrated that numerical method in combination with MRI was able to predict qualitatively the patient-specific intraventricular flow. To the best of our knowledge, we are the first to simulate patient-specific ventricular flow taking into account both mitral and aortic valves.     

Fully Lagrangian and Lattice Boltzmann Methods for the Advection-Diffusion Equation

In the last decade or so, the Lattice–Boltzmann method (LBM) has achieved great success in computational fluid dynamics. The Fully–Lagrangian method (FLM) is the generalization of LBM for conservation systems. LBM can also be developed from FLM. In this paper a FL model and a LB model are developed for D-dimensional advection-diffusion equation. The LB model can be viewed as an improved version of the FL model. Numerical results of simulation of 1-dimensional advection-diffusion equation are presented. The numerical results are found to be in good agreement with the analytic solution.     

A comparison of analytical–numerical models for nonlinear vibrations of cylindrical shells

The nonlinear flexural vibration behaviour of cylindrical shells has received considerable attention to date. It is pointed out that, although in a well-known reference case there seems to be a reasonable agreement, there are unresolved discrepancies between the results obtained by different authors. In the present paper, the problem is studied using various analytical–numerical models with different levels of accuracy and complexity. The frequency–amplitude curves from the different analysis models developed are compared both for isotropic shells and for an orthotropic composite shell. Secondary modes can play an important role. In more complicated cases modal interactions may significantly influence the nonlinear vibration behaviour, and the results obtained strongly depend on the analysis model chosen.     

Inter-spike-intervals analysis of AER Poisson-like generator hardware

Address–Event–Representation (AER) is a communication protocol for transferring images between chips, originally developed for bio-inspired image-processing systems. Such systems may consist of a complicated hierarchical structure with many chips that transmit images among them in real time, while performing some processing (for example, convolutions). In developing AER-based systems it is very convenient to have available some means of generating AER streams from on-computer stored images. Rank order coding (ROC) and Poisson rate coding are the extremes of spikes coding. In this paper, we present a pseudo-random hardware method for generating AER streams in real time from a sequence of images stored in a computer's memory. The Kolmogorov–Smirnov test has been applied to quantify that this method follows a Poisson distribution of the spikes. A USB–AER board, developed by our RTCAR group, have been used for the measurements. An example scenario of use under the EU CAVIAR project is presented.