In this article the aerodynamic optimisation of the windshield of the Ducati Multistrada motorbike is presented. Two cases have been considered: the bike equipped with its original windshield and with the windshield Variotouring by MRA.
RBF Morph has been used to deform the CFD model of the motorbike, considering three deforming actions: modification of the driver height, of the driver posture, defined by the hunching angle, and of the windshield deflector angle. A CFD mesh has been prepared according to best practices used for external aerodynamic analysis. Solid models from CAD have been processed to obtain a valid surface mesh. The virtual wind tunnel has then been added to the motorbike model and only half model has been considered exploiting symmetry. Considering that the study is focused on the performance on the windshield, only the relevant components of the vehicle has been represented in the model. The surface mesh has been used to generate the volume mesh. The model size is about a few millions of cells.
The set-up stage for changing the driver angle (or height) starts with the definition of an encapsulation box. Encapsulation domains of various shape (box, sphere, cylinder) can be used to limit the action of the morpher. For complex shapes the encapsulation domain can be defined combining an arbitrary number of such shapes (only the effective envelope will be used to locate source points).The number of points located on the surface is defined imposing a proper point spacing. The effect of encapsulation is to give a near zero solution on the boundary (in fact zero value is imposed only on the source points, the zero values in other points on the encapsulation surface depends on spacing). Furthermore the geometrical information of the encapsulation is used to apply the morphing only to the mesh nodes that fall inside the domain. Moving encapsulations are also available (not used in this example). They work in a similar way of the domains but prescribe a given deformation field inside the encapsulation and on the boundary points accordingly. This means that the mesh deformation is applied only to the nodes contained inside the domain encapsulations and outside of the moving ones.
To complete the set-up, two sets of source points on surfaces are defined: the first one is composed by all the mesh nodes that belong to the helmet, the second one is composed by all the nodes on the bike and on the windshield. As can be observed in the figure, only the nodes that fall inside the domain are selected (i.e. the encap domain works also as a selection encapsulation to limit the action of the “on surface” selection, custom selection encapsulation can also introduced for each surfaces set). For the first set a rigid movement is imposed (a rotation about driver ankles or a displacement along driver neck), for the second set a zero rigid movement is imposed to preserve the original shape of the bike components. The remaining nodes that fall inside the domain (i.e. the fluid and the body of the driver) remain free to deform under the action of the morpher.
Before accept the solution a preview of modifiers can be examined, as represented in the above figure, where the preview of driver rotation is performed on motorbike surfaces and on two cutting planes. The preview on the surfaces can be completed trying some preliminar morph (the undo button helps for interactive tweaking). In this case the worst combinations of the parameters (i.e. maximum driver rotation for maximum and minimum driver height) are tested to check mesh validity and quality.
After the completion of set-up stage the solution can be used to explore several combinations. In this case the use of the multi morph command is used to explore 15 combinations changing driver angle in the range [0 deg -- 15 deg] and the deflector angle in the range [-10 deg -- 10 deg].