Abstract
Numerical analysis of a multibody mechanism moving in the air is a complicated problem in computational fluid dynamics (CFD). Analyzing the motion of a multibody mechanism in a commercial CFD software, i.e., ANSYS Fluent®, is a challenging issue. This is because the components of a mechanism have to be constrained next to each other during the movement in the air to have a reliable numerical aerodynamics simulation. However, such constraints cannot be numerically modeled in a commercial CFD software, and needs to be separately incorporated into models through the programming environment, such as user-defined functions (UDF). This study proposes a nonlinear-incremental dynamic CFD/multibody method to simulate constrained multibody mechanisms in the air using UDF of ANSYS Fluent®. To testify the accuracy of the proposed method, Newton–Euler dynamic equations for a two-link mechanism are solved using Matlab® ordinary differential equations (ODEs), and the numerical results for the constrained mechanisms are compared. The UDF results of ANSYS Fluent® shows good agreement with Matlab®, and can be applied to constrained multibody mechanisms moving in the air. The proposed UDF of ANSYS Fluent® calculates the aerodynamic forces of a flying multibody mechanism in the air for a low simulation cost than the constraint force equation (CFE) method. The results could have implications in designing and analyzing flying robots to help human rescue teams, and nonlinear dynamic analyses of the aerodynamic forces applying on a moving object in the air, such as airplanes, birds, flies, etc.
Highlights
Analyzing the equations of motion for a multibody mechanism is an intricate issue in the field of dynamic mechanics, since several dynamic equations with high-order nonlinearities are essentially required to be solved [1,2]
When it comes to the constrained mechanisms, the most important point is that two bodies kept their inner distance until the end of the simulation time. Another point is that the role of the spatial location of body 2 here does not become dominant in changing the location of body 1. It implies that in a constrained system, the bodies show more realistic moves under the aerodynamic forces, which is preferable in computational fluid dynamics (CFD) simulations
This study proposed a new user-defined functions (UDF) coding to analyze the dynamics of a multibody mechanism with different degree-of-freedom in ANSYS Fluent® software
Summary
Analyzing the equations of motion for a multibody mechanism is an intricate issue in the field of dynamic mechanics, since several dynamic equations with high-order nonlinearities are essentially required to be solved [1,2]. If a multibody mechanism enlarges with some active and passive joints moving in the air, the difficulties of analysis of the system will be amplified due to the initial and boundary conditions of the model [3]. Such mechanisms can drift away from their joint connections during their movement in the air owing to the set of complicated aerodynamic forces, such as drag, shear, and normal [4]. Constraint force equation (CFE) has been widely used for dynamic analysis of multibody mechanisms [5,6,7,8,9,10,11]. CFE is in wide using for various types of multibody mechanisms under different initial and boundary conditions, the main disadvantage of that is the external forces applying on a mechanism should be determined during the procedure as a known function and cannot be dependent on the spatial location of each part/component of a multibody mechanism
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