That's a wrap!
Stephen Ferguson, CD-adapco

Dealing with the consequences of imperfect CAD has long been the biggest bottleneck in CFD, and was, until recently, the single largest obstacle in implementing flow-simulation in the design process.

While CD-adapco can justifiably claim to lead the market in CAD embedded CFD, there still exists a group of problems for which the CAD geometries are too big or too complex to be handled from within the CAD environment. These problems typically involve large assemblies of complex CAD parts, many of which contain more detail than is necessary for a CFD calculation.

Prior to the arrival of CD-adapco’s automatic surface wrapping technology, the only viable alternative was manual repair; a process that is both time-consuming and expensive. For a typical case, producing a meshing quality triangulated surface from complex or dirty CAD could take days or even weeks, consuming up to 50% of the engineering budget of a CFD project, while tying up a highly qualified CFD engineer in monotonous manual labour.

A typical example is an underhood analysis (the application for which the surface wrapper was originally developed). A typical underhood geometry can consist of several hundred individual components. In the early stages of the design – the time at which CFD analysis is most critical – many of these components will touch or intersect. Before the advent of surface wrapping, extracting a single surface in this type of situation was extremely time consuming or sometimes even impossible.

CD-adapco’s key motivation in developing surface wrapping technology was to eliminate manual repair from the CFD process altogether, guaranteeing a faster route from CAD to flow-simulation, and freeing CFD engineers to concentrate on engineering analysis.

The surface wrapper works by ‘shrink-wrapping’ a high-quality triangulated surface mesh onto the geometry, closing holes in the geometry and joining disconnected and overlapping surfaces. The surface wrapper quickly calculates the wetted surface of the geometry, automatically discarding surfaces that are outside the flow-domain, instantly eliminating unnecessary detail.

Although the emphasis of the surface wrapper is on fully automatic repair, the tool is also fully customizable, allowing users to specify the level of resolution on a surface-by-surface basis if necessary, or using volume regions to specify larger areas of refinement. All size specifications are relative to a global base size, so that once set up, the wrapped surface can be fine tuned by just altering a single parameter.

The wrapper can also be used to quickly and automatically extract flow volumes from complex solid parts. Users have the option of wrapping from the outside of the geometry, or the inside. For complex geometries seed points can be defined within a geometry in order to prescribe which internal volume the wrapper will extract.

Importantly the surface wrapper respects the fidelity of the original surface. Unlike other surface-wrappers, CD-adapco’s wrapper automatically respects the sharp edges and corners of the original model, as well as any other “feature curves” that the user chooses to prescribe.

Before wrapping, the user has the option of defining seed points inside and outside of the geometry, which allow the surface wrapper to automatically identify and display any leakage paths between the internal and external geometries. Having identified the size and the location of any holes, the user is able specify a closure tolerance which the wrapper uses to close any leakage paths and generate a single continuous wetted surface.

The idea of surface wrapping is not new (CD-adapco first implemented a surface wrapper in pro-STAR in 2004), however the surface wrapper implemented in STAR-CD V4 and STAR-CCM+ V2 is based on entirely new technology that is unique in one significant respect: CD-adapco’s new surface wrapper guarantees a closed manifold surface at every wrap.

The surface wrapper is both speed and memory efficient, producing a high quality wrapped surface of 10 million triangles from a 5 million triangle input mesh typically takes less than 30 minutes and requires less than 3Gb of memory. The surface preparation time for an underhood analysis has been reduced from several weeks (using manual repair) to under a hour using CD-adapco’s surface wrapping technology.

The path from “imperfect” CAD geometry to CFD mesher has never been easier or more automatic.

Major automotive companies such as Daimler Chrysler are already benefiting from huge time saving in the design of vehicle underbonnet, external aerodynamics and passenger compartments. Daimler Chrysler’s Walter Bauer described CD-adapco’s surface wrapper as:

“The most automatic approach that exists. It has saved us weeks of clean-up when carrying out our air flow studies and provides us with significant time and cost savings.”