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Accelerating a C++ CFD code with OpenACC

Computational Fluid Dynamics (CFD) is a valuable tool to study the behavior of fluids. Today, many areas of engineering use CFD. For example, the automotive industry uses CFD to study airflow around cars, and to optimize the car body shapes to reduce drag and improve fuel efficiency. To get accurate results in fluid simulation it is necessary to capture complex phenomena such as turbulence, which requires very accurate models. These complex models result in very long computing times. In this post I describe how I used OpenACC to accelerate the ZFS C++ CFD solver with NVIDIA Tesla GPUs.

The ZFS flow solver

Engine
Figure 1: Using ZFS to study fluid flow within an internal combustion engine with moving pistons and valves.

The C++ flow solver ZFS (Zonal Flow Solver) is developed at the Institute of Aerodynamics at RWTH Aachen, Germany. ZFS solves the unsteady Navier-Stokes equations for compressible flows on automatically generated hierarchical Cartesian grids with a fully-conservative second-order-accurate finite-volume method [1, 2, 3]. To integrate the flow equations in time ZFS uses a 5-step Runge-Kutta method with dual time stepping [2]. It imposes boundary conditions using a ghost-cell method [4] that can handle multiple ghost cells [5, 6]. ZFS supports complex moving boundaries which are sharply discretized using a cut-cell type immersed-boundary method [1, 2, 7].

Among other topics, scientists have used ZFS to study the flow within an internal combustion engine with moving pistons and valves, as Figure 1 shows. Figure 2 shows how the Lattice-Boltzmann solver in ZFS was used to better understand airflow within the human nasal cavity.
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