Pressure-based Algorithm Research Articles

Accuracy and Reliability of Onset Detection Algorithms in Gait Initiation for Healthy Controls and Participants With Parkinson's Disease.

Accurate and reliable detection of the onset of gait initiation is essential for the correct assessment of gait. Thus, this study was aimed at evaluation of the reliability and accuracy of 3 different center of pressure-based gait onset detection algorithms: A displacement baseline-based algorithm (method 1), a velocity baseline-based algorithm (method 2), and a velocity extrema-based algorithm (method 3). The center of pressure signal was obtained during 10 gait initiation trials from 16 healthy participants and 3 participants with Parkinson's disease. Intrasession and absolute reliability of each algorithm was assessed using the intraclass correlation coefficient and the coefficient of variation of center of pressure displacement during the postural phase of gait initiation. The accuracy was evaluated using the time error of the detected onset by each algorithm relative to that of visual inspection. The authors' results revealed that although all 3 algorithms had high to very high intrasession reliabilities in both healthy subjects and subjects with Parkinson's disease, methods 2 and 3 showed significantly better absolute reliability than method 1 in healthy controls (P = .001). Furthermore, method 2 outperformed the other 2 algorithms in both healthy subjects and subjects with Parkinson's disease with an overall accuracy of 0.80. Based on these results, the authors recommend using method 2 for accurate and reliable gait onset detection.

Numerical modelling of shock-bubble interactions using a pressure-based algorithm without Riemann solvers

The interaction of a shock wave with a bubble features in many engineering and emerging technological applications, and has been used widely to test new numerical methods for compressible interfacial flows. Recently, density-based algorithms with pressure-correction methods as well as fully-coupled pressure-based algorithms have been established as promising alternatives to classical density-based algorithms based on Riemann solvers. The current paper investigates the predictive accuracy of fully-coupled pressure-based algorithms without Riemann solvers in modelling the interaction of shock waves with one-dimensional and two-dimensional bubbles in gas-gas and liquid-gas flows. For a gas bubble suspended in another gas, the mesh resolution and the applied advection schemes are found to only have a minor influence on the bubble shape and position, as well as the behaviour of the dominant shock waves and rarefaction fans. For a gas bubble suspended in a liquid, however, the mesh resolution has a critical influence on the shape, the position and the post-shock evolution of the bubble, as well as the pressure and temperature distribution.

An adaptive inner iterative pressure-based algorithm for steady and unsteady incompressible flows

PurposePressure-based methods have been demonstrated to be powerful for solving many practical problems in engineering. In many pressure-based methods, inner iterative processes are proposed to get efficient solutions. However, the number of inner iterations is set empirically and kept fixed during the whole computation for different problems, which is overestimated in some computations but underestimated in other computations. This paper aims to develop an algorithm with adaptive inner iteration processes for steady and unsteady incompressible flows.Design/methodology/approachIn this work, with the use of two different criteria in two inner iterative processes, a mechanism is proposed to control inner iteration processes to make the number of inner iterations vary during computing according to different problems. By doing so, adaptive inner iteration processes can be achieved.FindingsThe adaptive inner iterative algorithm is verified to be valid by solving classic steady and unsteady incompressible problems. Results show that the adaptive inner iteration algorithm works more efficient than the fixed inner iteration one.Originality/valueThe algorithm with adaptive inner iteration processes is first proposed in this paper. As the mechanism for controlling inner iteration processes is based on physical meaning and the feature of iterative calculations, it can be used in any methods where there exist inner iteration processes. It is not limited for incompressible flows. The performance of the adaptive inner iteration processes in compressible flows is conducted in a further study.

A pressure-based algorithm for internal compressible turbulent flows through a geometrical singularity

Compressible turbulent flow through the abrupt enlargement in pipes is studied numerically by means of Advection Upstream Splitting Method (AUSM+-up). In low Mach numbers, a pressure correction equation of elliptic type is derived. This equation is compatible with the nature of governing equations and retrieves hyperbolic characteristic at higher Mach numbers. It is shown that the proposed numerical algorithm is computationally more efficient than the preconditioned density-based methods. The flow parameters such as reattachment length, pressure loss coefficient and wall shear stress are predicted. It is found that the loss coefficient of the compressible flow rises drastically with increasing Reynolds number while it is constant for incompressible flows. Furthermore, the total-pressure ratio drops with increasing Reynolds number and expansion ratio where it approaches an asymptotic curve. In compressible flow, the pressure is minimum and constant after the enlargement section up to the axial position of the recirculation center.

Numerical investigation of Gurney flap influences on aerodynamic performance of a pitching airfoil in low Reynolds number flow

The main purpose of present work is to investigate the aerodynamic performance of a pitching NACA 0012 airfoil equipped with a Gurney flap in flow with low Reynolds number. The aerodynamic influences of flap location, mounting angle, and height are numerically studied. In this regard, a Lagrangian–Eulerian pressure-based numerical algorithm is developed on hybrid grids attached to a pitching solid boundary. A finite volume-based finite element method is used to discretize the governing equations. As reported in previously related studies, this unified algorithm could be used to solve the unsteady incompressible flow in domains with moving mesh and/or moving boundary with sufficient robustness and accuracy. The other advantage of this algorithm is that it does not need any type of dissipation term and/or damping function. Using this unified algorithm, the numerical experiments indicate that the Gurney flap increases the lift and drag coefficients and enhances the aerodynamic efficiency. The best aerodynamic performance is predicted for the case in which the flap is located at trailing edge with the mounting angle of 90°. The flap height is predicted to have different and most impacts on aerodynamic efficiency during upstroke and downstroke. The numerical results show that the airfoils equipped by flaps with height between 6% and 12% of the airfoil chord are the most aerodynamically efficient airfoils. Changing of lift and drag coefficients are due to increase of effective camber and thickness in all cases.

Fully-coupled pressure-based algorithm for compressible flows: Linearisation and iterative solution strategies

The impact of different linearisation and iterative solution strategies for fully-coupled pressure-based algorithms for compressible flows at all speeds is studied, with the aim of elucidating their impact on the performance of the numerical algorithm. A fixed-coefficient linearisation and a Newton linearisation of the transient and advection terms of the governing nonlinear equations are compared, focusing on test-cases that feature acoustic, shock and expansion waves. The linearisation and iterative solution strategy applied to discretise and solve the nonlinear governing equations is found to have a significant influence on the performance and stability of the numerical algorithm. The Newton linearisation of the transient terms of the momentum and energy equations is shown to yield a significantly improved convergence of the iterative solution algorithm compared to a fixed-coefficient linearisation, while the linearisation of the advection terms leads to substantial differences in performance and stability at large Mach numbers and large Courant numbers. It is further shown that the consistent Newton linearisation of all transient and advection terms of the governing equations allows, in the context of coupled pressure-based algorithms, to eliminate all forms of underrelaxation and provides a clear performance benefit for flows in all Mach number regimes.

Improved prediction of 3-D water entry with a CIP method and parallel computing

This paper presents the further development of a constrained interpolation profile (CIP) based method to solve highly non-linear 3-D slamming problems with a parallel computing algorithm and an improved interface capturing scheme. The water entry problems, governed by the Navier-Stokes (N-S) equations, were treated as multi-phase problems (solid, water and air) and solved by the fractional step method, in which the 3-D CIP method was employed for the advection phase and a pressure-based algorithm was applied to the non-advection phase. The interfaces between different phases were captured by using density functions. The advection of the water phase was solved by applying the THINC/WLIC scheme, i.e., tangent of hyperbola for interface capturing with weighted line interface calculation. A six-degree-of-freedom (6DOF) motion solver was developed to simulate the free fall water entry. A Message Passing Interface (MPI) parallel computing scheme was implemented by decomposing the computational domain. The speed-up performance was investigated. Validation studies were carried out for the water entry of a 3-D wedge with prescribed velocities. The free fall water entry of a modified Wigley hull and an inclined circular cylinder was also studied. The computed slamming forces and motions were compared with experimental results and other published numerical results.

Pressure-based algorithm for compressible interfacial flows with acoustically-conservative interface discretisation

A pressure-based algorithm for the simulation of compressible interfacial flows is presented. The algorithm is based on a fully-coupled finite-volume framework for unstructured meshes with collocated variable arrangement, in which the governing conservation laws are discretised in conservative form and solved in a single linear system of equations for velocity, pressure and specific total enthalpy, with the density evaluated by an equation of state. The bulk phases are distinguished using the Volume-of-Fluid (VOF) method and the motion of the fluid interface is captured by a state-of-the-art compressive VOF method. A new interface discretisation method is proposed, derived from an analogy with a contact discontinuity, that performs local changes to the discrete values of density and total enthalpy based on the assumption of thermodynamic equilibrium, and does not require a Riemann solver. This interface discretisation method yields a consistent definition of the fluid properties in the interface region, including a unique definition of the speed of sound and the Rankine–Hugoniot relations, and conserves the acoustic features of the flow, i.e. compression and expansion waves. A variety of representative test cases of gas–gas and gas–liquid flows, ranging from acoustic waves and shock tubes to shock-interface interactions in one-, two- and three-dimensional domains, is used to demonstrate the capabilities and versatility of the presented algorithm in all Mach number regimes. The propagation, reflection and transmission of acoustic waves, shock waves and rarefaction fans in interfacial flows are predicted accurately, even for difficult cases that feature fluids with shock impedance matching, transonic shock tubes or strong shocks in gas–liquid flows, as well as on unstructured meshes.

A pressure-based Mach-uniform method for viscous fluid flows

ABSTRACTA pressure-based, Mach-uniform method is developed by combining the SLAU2 numerical scheme and the higher temporal order pressure-based algorithm. This hybrid combination compensates the limitation of the SLAU2 numerical scheme in the low-Mach number regime and deficiencies of the pressure-based method in the high-Mach number regime. A momentum interpolation method is proposed to replace the Rhie-Chow interpolation for accurate shock-capturing and to alleviate the carbuncle phenomena. The momentum interpolation method is consistent in addition to preserving pressure–velocity coupling in the incompressible limit . The postulated pressure equation allows the algorithm to compute the subsonic flows without empirical scaling of numerical dissipation at low-Mach number computation. Several test cases involving a broad range of Mach number regimes are presented. The numerical results demonstrate that the present algorithm is remarkable for the calculation of viscous fluid flows at arbitrary Mach number including shock wave/laminar boundary layer interaction and aerodynamics heating problem.

DEVELOPMENT AND VALIDATION OF AN IN-HOUSE CFD CODE FOR ENVIRONMENTAL FLOW SIMULATION USING MEASUREMENTS IN ATMOSPHERIC BOUNDARY LAYER WIND TUNNEL

This paper deals with the important issue of the gaseous pollutants dispersion in the environmental flow within urban atmospheres including the products of on-ground agriculture fires. The paper presents an accurate and efficient semi-implicit pressure-based algorithm developed for solving numerically the conservation equations governing weakly compressible turbulent environmental flow of multicomponent fluid. An in-house CFD code has been developed to implement the numerical solution of the present algorithm. The code is an integrated one consisting of three main elements which are the pre-processor, the solver and the post-processor. This in-house CFD code undergoes a standard validation process, using six generic test cases simulating a steady-state environmental flow associated with gas flow from a ground point source under different flow conditions. A prerequisite verification of the code has been considered for successful validation. The detailed standard validation process was performed according to AIAA Guide by comparing the predicted results with measurements especially conducted for the present work in the test section of the Atmospheric Boundary Layer Wind Tunnel facility of Assiut University. The results showed good qualitative and quantitative agreements for the present in-house CFD code with the corresponding flow measurements. These agreements together with the satisfaction of the prerequisite verification insure a satisfactory validation of the present in-house CFD code.

Numerical Simulation of One-Dimensional Two-Phase Flow Using a Pressure-Based Algorithm

Two-phase flow is studied numerically one-dimensionally using a two-fluid equation system. A hyperbolic type of governing equation is applied in our simulation. The segregated solution method is promoted and adopted in this article. By establishing mass conservation-based pressure correction, the SIMPLE algorithm is extended to two-phase flow simulations. Numerical results for well-known two-fluid air/water flow benchmark problems are presented and discussed.

A Parallel Multigrid Finite-Volume Solver on a Collocated Grid for Incompressible Navier-Stokes Equations

Multigrid techniques are widely used to accelerate the convergence of iterative solvers. Serial multigrid solvers have been efficiently applied to a broad class of problems, including fluid flows governed by incompressible Navier-Stokes equations. With the recent advances in high-performance computing (HPC), there is an ever-increasing need for using multiple processors to solve computationally demanding problems. Thus, it is imperative that new algorithms be developed to run the multigrid solvers on parallel machines. In this work, we have developed a parallel finite-volume multigrid solver to simulate incompressible viscous flows in a collocated grid. The coarse-grid equations are derived from a pressure-based algorithm (SIMPLE). A domain decomposition technique is applied to parallelize the solver using a Message Passing Interface (MPI) library. The multigrid performance of the parallel solver has been tested on a lid-driven cavity flow. The scalability of the parallel code on both single- and multigrid solvers was tested and the characteristics were analyzed. A high-fidelity benchmark solution for lid-driven cavity flow problem in a 1,024 × 1,024 grid is presented for a range of Reynolds numbers. Parallel multigrid speedup as high as three orders of magnitude is achieved for low-Reynolds-number flows. The optimal multigrid efficiency is validated, i.e., the computational cost is shown to increase proportionally with the problem size.

A Pressure-Based Algorithm for Gaseous Hydrogen/Liquid Oxygen Jet Flame at Supercritical Pressure

Computational tools of turbulent combustion have practical applications for various fields including liquid rocket engines, but some numerical issues are still presented for solving supercritical combustion. In the present study, several of these numerical issues are studied and discussed. Turbulent flow and thermal fields of gaseous hydrogen/cryogenic liquid oxygen flame at supercritical pressure are simulated by a turbulence model. To realize real-fluid combustions, the modified Soave-Redlich-Kwong (SRK) equations of state (EOS) are implemented into the flamelet model with a look-up table as functions of mean and variance of mixture fraction, scalar dissipation rate, enthalpy, and pressure. For supercritical combustion flows, modified forms of the pressure implicit with splitting of operator (PISO) algorithm for solving the pressure-velocity linked equation are introduced. From a comparison of instantaneous temperature distributions for gaseous hydrogen/cryogenic liquid oxygen flame at supercritical pressure, the capability of each method based on the different solution sequence is examined and the effective sequence is explored. The results show that the updated mixture fraction reflected in the pressure correction loop is a critical factor for numerical stability. Also, the relative performance of six convection schemes for supercritical combustion is discussed.

Fluid structures of flapping airfoil with elliptical motion trajectory

In the present study, the effect of elliptical motion trajectory on the aerodynamic characteristics and propulsive performance of a flapping airfoil is evaluated. A periodic horizontal motion (forward/backward) is combined with vertical motion (upward/downward) of the airfoil to introduce a new kinematic parameter, and of course, an elliptical motion trajectory for flapping airfoil. Similar kinematics is also observed in the flying of birds and swimming of the penguins or turtles. For this modeling, the Navier–Stokes equations are used to simulate the unsteady flow field over a two dimensional NACA0012 airfoil. The Navier–Stokes equations are discretized based on the finite volume method and are solved with a pressure-based algorithm. The flow is assumed to be laminar and incompressible and transient terms are conducted using a second order Euler implicit scheme. It is shown that the combination of horizontal and vertical motions for the flapping airfoil changes the kinematics, motion trajectory and hence the effective angle of attack profile during the flapping cycle. The elliptical motion trajectory will also influence on the fluid structures and change the vortex shedding pattern and wake zone behind the airfoil. Additionally, the introduced kinematics may influence significantly on the aerodynamics and propulsive performance of either pure plunging or pitching/plunging airfoil.

A Coupled Pressure-Based Finite-Volume Solver for Incompressible Two-Phase Flow

This article reports on the formulation and testing of a coupled pressure-based algorithm for the solution of steady incompressible disperse two-phase-flow problems. The method is formulated within a Eulerian-Eulerian framework in the context of a collocated finite-volume scheme. An equation for pressure is derived from overall mass conservation following the segregated mass conservation–based algorithm (MCBA) approach and using an extended two-phase flow form of the Rhie-Chow interpolation technique. The newly developed pressure-based coupled solver differs from pressure-based segregated solvers in that it accounts implicitly for the pressure–velocity and the interphase drag couplings that are present in disperse multiphase flows to yield a system of coupled equations linking the velocity and pressure fields. The performance and accuracy of the coupled multiphase algorithm are assessed by solving eight one-dimensional two-phase flow problems spanning the spectrum from dilute bubbly to dense gas–solid flows. Each problem is solved over three grid systems with sizes of 10,000, 30,000, and 50,000 control volumes, respectively. Results are compared in terms of iterations and CPU time with similar ones generated using the segregated MCBA-SIMPLE algorithm. The newly developed coupled solver is shown to yield substantial decrease in the required number of iterations and CPU time, with the rate of solution acceleration varying between 1.3 and 4.6.

DEVELOPMENT, VERIFICATION AND VALIDATION OF AN IN-HOUSE CFD CODE FOR WEAKLY COMPRESSIBLE FLOW

The work of this paper represents the first stage of a whole modeling and computational solution procedure for an on-going research on agricultural fires at the laboratory of environmental research, Assiut University. The paper presents an accurate and efficient semi-implicit pressure-based algorithm developed for solving numerically the conservation equations governing weakly compressible single-fluid flows. The algorithm uses a finite-volume technique applied to structured non-orthogonal multi-block curvilinear meshes with co-located grid arrangement. An in-house CFD code has been developed to implement the numerical solution of the present algorithm. The code is an integrated one consisting of the main elements, the pre-processor, the solver and the post-processor. This in-house CFD code undergoes a standard verification and validation process using a variety of standard test cases covering broad range of CFD applications. The results of the present code for all test cases are in good agreement with those of the well-established reference CFD codes. This insures the present code verification and validation.

A Fully Coupled Navier-Stokes Solver for Fluid Flow at All Speeds

This article deals with the formulation and testing of a newly developed, fully coupled, pressure-based algorithm for the solution of fluid flow at all speeds. The new algorithm is an extension into compressible flows of a fully coupled algorithm developed by the authors for laminar incompressible flows. The implicit velocity–pressure–density coupling is resolved by deriving a pressure equation following a procedure similar to a segregated SIMPLE algorithm using the Rhie-Chow interpolation technique. The coefficients of the momentum and continuity equations are assembled into one matrix and solved simultaneously, with their convergence accelerated via an algebraic multigrid method. The performance of the coupled solver is assessed by solving a number of two-dimensional problems in the subsonic, transsonic, supersonic, and hypersonic regimes over several grid systems of increasing sizes. For a desired level of convergence, results for each problem are presented in the form of convergence history plots, tabulated values of the maximum number of required iterations, the total CPU time, and the CPU time per control volume.

A Finite-Volume-Based Approach for Dynamic Fluid-Structure Interaction

It is common to adopt finite-element methods to solve solid mechanics problems and finite-volume methods for fluid dynamics computations. The use of different methods causes complication of the solution procedure for problems involving both fluids and solids. In this study, a partitioned approach based on the finite-volume method for dynamic fluid–structure interaction is presented. The method is formulated in a way suitable for an unstructured mesh with arbitrary grid geometry. The variables for the fluid are stored at the centroids of grid cells, whereas those for the solid at the grid nodes. The latter arrangement makes it more suitable for large structure deformation. After spatial discretization for the solid using the finite-volume approach, the resulting system of ordinary differential equations is solved implicitly using the dual-time-stepping scheme. As for the fluid calculation, a pressure-based algorithm is employed and the time step is advanced in a prediction-correction manner. The finite-volume method for the solid is assessed by calculating the deformation and dynamics of a cantilever under various loads. Good agreement with analytical solutions is obtained. Then, the solution procedure is applied to two cases with coupled fluid flow and structure dynamics. One is the flow over a vertical plate with one end fixed on the floor in a channel. The other is the flow over a cylinder with a plate attached to it on the lee side.

Computations of partial and super cavitating flows using implicit pressure-based algorithm (IPA)

An implicit pressure-based algorithm (IPA) has been developed for the computations of the two-phase cavitating flows. A pressure correction equation is derived to meet the mass conservation law and the pressure–velocity coupled method is used to solve for the governing equations of the flows. Both partial and super cavitating flows are simulated in this work to validate the IPA approach and to investigate the unsteady features of the cavitating flows. It is found that strong re-entrant jets are present for all the simulations of the cavitating flows over the flat-nose cylindrical objects. For partial cavitating flows, the re-entrant jets appear periodically and become the main reason of cavitation shedding. For super cavitating flows, the re-entrant jets mainly interact with the wake flows of the object.

Transonic Flow Simulation Around the Pitching Airfoil with Accurate Pressure-Based Algorithm

A new pressure based implicit procedure to solve the Euler and Navier-Stokes equations is developed to predict transonic viscous and inviscid flowsaround the pitching airfoil with high resolution scheme. In this process, nonorthogonal and non moving mesh with collocated finite volume formulation areused. In order to simulate pitching airfoil, oscillation of flow boundary condition is applied. The boundedness criteria for this procedure are determined from Normalized Variable Diagram (NVD) scheme. The procedure incorporates the k - e eddy-viscosity turbulence model. In the new algorithm, the computation time is considerably reduced. This process is tested for inviscid and turbulent transonic aerodynamic flows around pitching airfoil.The results are compared with other existing numerical solutions and with experiment data. The comparisons show that the resolution quality of the developed algorithm is considerable.