Geometric Sensitivity

CFD simulation involves the creation of a discrete mathematical model of an object, assembly, region in space, and so on. In order to obtain accurate results from the CFD analysis, analysts must ensure that they are using an appropriate representation of the geometry that they are trying to simulate. When simulating flow involving complex geometric features, it is necessary to evaluate the degree to which the discretized representation of the geometry matches the actual component of interest. For example, it is common practice to remove small features such as small bolt holes and fasteners when simulating complex assemblies. These features may or may not have a significant impact on the results of the simulation. Analysts need to determine the appropriate level of detail required to achieve an accurate representation of the flow field. This often times requires a geometry sensitivity study to determine.

Another aspect of proper geometry modeling is how well the mesh matches the geometry. For curved geometries, it is necessary to have sufficient element density to resolve the curvature smoothly and provide an appropriate reproduction of the shapes. In addition to the mesh density, it is also necessary to ensure that the underlying CAD model provided appropriate resolution of the geometry. When constructing meshes from discrete CAD models you must pay particular attention to this to ensure that the discrete model is an appropriate representation of the true geometry.

The third geometric modeling aspect is the extent of the domain used for analysis. When modeling external flows, it is necessary to model a sufficiently large domain that minimizes the impact of the numerical constraints applied to the model. For example, when trying to simulate an airfoil in an infinite medium it is necessary to size the domain large enough so that the surrounding region is not constraining the flow around the airfoil in an unrealistic manner. Similarly, for internal flows, the boundaries of the domain should be located far away from the location of any phenomena of interest in the domain to minimize any interference in the results due to the presence of the boundaries.

A final aspect of the geometric modeling applies to the problems that have a rotating component within the domain. Examples of such flows are the flows in pumps and turbomachines where fluid interacts with a rotating part, adding swirl to the flow. The common approach while modeling such cases is splitting the domain into a rotating part which immediately surrounds the rotating component (rotor), and the static part that makes up the rest of the domain. While modeling such cases, it is very important to understand the sensitivity of the location of the interface which will separate the static and the rotating part. An interface located either too close or too far from the rotor reduces the accuracy of the solution. For an accurate solution it is advisable to test the sensitivity of the solution to the location of this interface. Unfortunately, there are no set guidelines to determine this, as the optimum location is a function of the dimensions of the domain and the rotor, the relative velocity between the fluid and the rotor, and so on.

The Backward Facing Step Case

A look at the schematic of the backward facing step case indicates that the outlet boundary of the domain has been modeled at a distance of 50 times the step height downstream from the step location. It is desired that the extent of the recirculation zone and the subsequent reattachment of the flow downstream of the step are not affected by the proximity to the boundary.

The extent to which the boundaries should be distanced from the region of interest should be evaluated on a case by case basis. It can be argued that using a distance of 50 times the step height is a very conservative estimate for the backward facing step (BFS) problem, which may be considered a relatively simple problem. Using such a conservative value is not always feasible especially as geometries and models become complex. The modeling aim should be to minimize the interference of boundaries on the results. Unless there is a constraint on the model that prevents shifting the boundaries sufficiently far from the region of interest, or you are interested in studying the effect of boundaries on the model, this practice should be followed.