Load balancing for chemically reacting flows

Purpose - A common observation made when computing chemically reacting flows is how central processing unit (CPU)-intensive these are in comparison to cold flow cases. The update of tens or hundreds of species with hundreds or thousands of reactions can easily consume more than 95% of the total CPU time. In many cases, the region where reactions (combustion) are actually taking place comprises only a very small percentage of the volume. Typical examples are flame fronts propagating through a domain. In such cases, only a small fraction of points/cells needs a full chemistry update. This leads to extreme load imbalances on parallel machines. The purpose of the present work is to develop a methodology to balance the work in an optimal way. Design/methodology/approach - Points that require a full chemistry update are identified, gathered and distributed across the network, so that work is evenly distributed. Once the chemistry has been updated, the unknowns are gathered back. Findings - The procedure has been found to work extremely well, leading to optimal load balance with insignificant communication overheads. Research limitations/implications - In many production runs, the procedure leads to a reduction in CPU requirements of more than an order of magnitude. This allows much larger and longer runs, improving accuracy and statistics. Practical implications - The procedure has allowed the calculation of chemically reacting flow cases that were hitherto not possible. Originality/value - To the authors' knowledge, this type of load balancing has not been published before.