Flow modeling and flow calculations

One application-field of flow analysis is the analytical modeling and prediction of the flow field around lifting surfaces such as wings and propellers. Their geometry influences the flow. The reduction of drag and increase of the propulsion and lift will lead to a greater efficiency. Flow simulations are mainly done in ANSYS Fluent, on a 36-core calculation cluster.

 

Prediction of thrust and drag in oscillating foils lies at the basis of Huygens Engineers. The founders started their cooperation on a large, highly disciplined and AP Moller-Maersk funded project to investigate the technical and economic feasibility of flapping foil propulsors for container ships. The program comprised of several phases, including an analytical prediction of thrust and efficiency, and culminated on the scientific side in a successful systematic measurement program performed at MARIN. The results of the measurement program have been presented at the ONR conference in Gothenburg and have been published under the name “A Systematic Experimental Study on Powering Performance of Flapping Foil Propulsors”. CFD modelling of the performance of the flapping foils was done after the measurement program. The illustration shows a CFD simulation of a flapping foil, while the video demonstrates the flapping propulsor in practice.

Another application of flow simulation is the steady-state modeling of laminar and turbulent flows. Flow simulations are used to predict specific system characteristics such as the pressure drop and heat flow in a heat exchanger or the temperature of a cooling fin, which would be hard to estimate analytically. A few objects of steady state flow simulation are displayed in the figure below. Taking the case of the flow over a ribbed profile as an example, the goal was to gather gradient information of the flow around/in the cavities, which determines the convection characteristics of the flow.

In the previous examples of flow simulations, the steady-state solutions were sufficient to analyze the situation. However, sometimes these will not suffice, and modeling of unsteady flows is needed. The video shows an unsteady convection around a heating element, as simulated in ANSYS Fluent. Since the heating element is constantly raising the temperature of the environment/the surrounding gases, no steady-state solution can be used to describe the process.

Simulations with moving geometries require a high-quality mesh. Making such a mesh for the complete geometry is challenging, especially in the case of large displacements. A common approach for situations like these is the use of overset meshes, where individual components are meshed separately and can overlap (almost) arbitrarily. An example of the mesh, flow and pressure fields of a 2-lobe-pump can be found in the figure and video, where every component grid is given a different color. The mesh consists of field cells were the governing equations (transport equations of mass, momentum and energy) are solved, fringe cells which are solved by interpolation from the field cells from the other respective mesh, and dead cells which are irrelevant for the solution.

In some situations, the geometry is restricted by certain building requirements. This is, for example, the case if a transition piece for a circular to a square form needs to be designed for maximum flow uniformity. The figure shows a close-up of the mesh and the velocity contours of the geometry of an air duct.

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