Mingang Jin

Simulating Natural Ventilation in and Around Buildings by Fast Fluid Dynamics

Natural ventilation is a sustainable technology that can provide a well-built environment and also save energy. The application of natural ventilation to buildings requires a careful approach in the early design phase, and fast, simple design tools are greatly needed. Fast fluid dynamics (FFD) can provide useful airflow information at a speed much faster than CFD so that it is a potential design tool for natural ventilation. This study thus validated FFD with test cases representing different types of natural ventilation. The results showed that FFD was capable of predicting the main air flow feature and ventilation rate with reasonable accuracy for wind-driven or buoyancy-driven natural ventilation. FFD simulation can reflect the influence of wind direction and surrounding buildings on natural ventilation.

Improvements of Fast Fluid Dynamics for Simulating Air Flow in Buildings

Fast fluid dynamics (FFD) can potentially be used for real-time indoor air-flow simulations. This study developed two-dimensional fast fluid dynamics (2-D FFD) into three-dimensional fast fluid dynamics (3-D FFD). The implementation of boundary conditions at the outlet was improved with a local mass conservation method. A near-wall treatment for the semi-Lagrangian scheme was also proposed. This study validated the 3-D FFD with five flows that have features of indoor air flow. The results show that the 3-D FFD can successfully capture the three dimensionality of air-flow and provide reliable and reasonably accurate simulations for indoor air flows with a speed of about 15 times faster than current computational fluid dynamics (CFD) tools.

Reduction of Numerical Diffusion in FFD Model

Abstract Fast flow simulations are needed for some applications in building industry, such as the conceptual design of indoor environment or teaching of Heating Ventilation and Air Conditioning (HVAC) system design in classroom. Instead of pursuing high accuracy, those applications require only conceptual distributions of the flow but within a short computing time. To meet these special needs, a Fast Fluid Dynamics (FFD) method was proposed to provide fast airflow simulation with some compromise in accuracy. This study is to further improve the FFD method by reducing the numerical viscosity that is caused by a linear interpolation in its semi-Lagrangian solver. We propose a hybrid scheme of a linear and a third-order interpolation to reduce the numerical diffusion in low order scheme and the numerical dispersion in high order scheme. The FFD model with both linear and hybrid interpolations are evaluated by simulating four different indoor flows. The results show that the hybrid interpolation can significantly improve the accuracy of the FFD model with a small amount of extra computing time.

Coupling indoor airflow, HVAC, control and building envelope heat transfer in the Modelica Buildings library

This paper describes a coupled dynamic simulation of an indoor environment with heating, ventilation, and air conditioning (HVAC) systems, controls and building envelope heat transfer. The coupled simulation can be used for the design and control of ventilation systems with stratified air distributions. Those systems are commonly used to reduce building energy consumption while improving the indoor environment quality. The indoor environment was simulated using the fast fluid dynamics (FFD) simulation programme. The building fabric heat transfer, HVAC and control system were modelled using the Modelica Buildings library. After presenting the concept, the mathematical algorithm and the implementation of the coupled simulation were introduced. The coupled FFD–Modelica simulation was then evaluated using three examples of room ventilation with complex flow distributions with and without feedback control. Further research and development needs were also discussed.

Optimization of air supply location, size, and parameters in enclosed environments using a computational fluid dynamics-based adjoint method

Optimal design of an indoor environment based on specific design objectives requires a determination of thermo-fluid control methods. The control methods include the air supply location, size, and parameters. This study used a computational fluid dynamics- (CFD) based adjoint method to identify the optimal air supply location, size, and parameters. Through defining the air distribution in a certain area (design domain) as a design objective in a two-dimensional, ventilated cavity, the adjoint method can identify the air supply location, size, and parameters. However, the air supply location, size, and parameters were not unique, which implied multiple solutions. By using any of the air supply location, size, and parameters identified as boundary conditions for forward CFD simulations, the computed air distribution in the design domain was the same as that used as a design objective. Thus, the computing costs did not depend on the number of design variables.

Improvement of fast fluid dynamics with a conservative semi-Lagrangian scheme

Purpose – The purpose of this paper is to develop a simple and efficient conservative semi-Lagrangian scheme (SL) for solving advection equation in fast fluid dynamics (FFD), so FFD can provide fast indoor airflow simulations while preserving conservation for energy and species transport. Design/methodology/approach – This study thus proposed a mass-fixing type conservative SL that redistributes global surplus/deficit on the advected field after performing the standard semi-Lagrangian advection. The redistribution weights were designed to preserve the properties of conservatives and monotonicity. Findings – The effectiveness of the conservative SL was validated with several test cases, and the results show that the proposed scheme is indeed conservative with negligible impact on the accuracy of the standard solutions. The numerical tests show that the proposed scheme was indeed conservative with negligible impact on the accuracy of the flow prediction. Originality/value – The FFD with conservative SL can ...

Accelerating fast fluid dynamics with a coarse-grid projection scheme

Fast fluid dynamics is an intermediate model that can provide fast and informative building airflow simulations. Although reasonably good simulation accuracy is important for fast fluid dynamics, computational efficiency is the primary concern, and it is necessary to further increase fast fluid dynamics speed. Because the most time-consuming part of fast fluid dynamics is solving the stiff pressure equation, this study proposed the application of a coarse-grid projection scheme, which solves the momentum equation on the fine grid level and the pressure equation on the coarse grid level. Therefore, appropriate approaches for mapping velocity and pressure information between different grid levels were investigated in this study. To evaluate the accuracy and computational efficiency of fast fluid dynamics with the coarse-grid projection scheme in simulating building airflows, this study tested it with building airflows of varying complexity. The results showed that the coarse-grid projection scheme would not have a negative impact on the accuracy of fast fluid dynamics in the simulation of building airflows, and it could significantly reduce the fluctuations that occur within the simulations. The coarse-grid projection scheme was able to accelerate fast fluid dynamics by approximately 1.5 times, and thus fast fluid dynamics with the coarse-grid projection scheme achieved a computing speed that was 30 to 50 times faster than computational fluid dynamics models.

Implementation of a fast fluid dynamics model in OpenFOAM for simulating indoor airflow

ABSTRACT This study implemented fast fluid dynamics (FFD) in Open Field Operation and Manipulation and used a local searching method that made the FFD solver applicable to unstructured meshes. Because the split scheme used in FFD is not conservative, this investigation developed a combined scheme that used a split scheme for the continuity and momentum equations and an iterative scheme for scalar equations. The combined scheme ensures conservation of the scalars. This investigation used two two-dimensional cases and one three-dimensional case, with the experimental data, to test the FFD solver. The predicted results were similar with different types of mesh and numerical scheme and agreed in general with the experimental data.