An inflow element is a component of an aerodynamic model. The inflow model is associated with a wing or a rotor and inherits from this wing or rotor a list of lifting lines. At each airstation of the lifting lines, lift and circulation are computed using the two-dimensional unsteady aerodynamic model. In turn, this circulation is convected in the flow and creates an unsteady inflow field, which is computed using on the model defined in the present section. The interaction between the structural dynamics, two-dimensional unsteady aerodynamic, and inflow models is depicted in a schematic manner in fig. 1.
Two types of inflow model can be defined, depending on the value of the parameter InflowType.
If InflowType = DYNAMIC_INFLOW, the number of inflow modes, 0 < N ≤ 48, must be defined and determines the number harmonics for the states used for the solution over the inflow disk. The choice of the number and location of the airstations of the associated lifting lines will greatly affects the accuracy and efficiency of the solution: the number of airstations must be increased as the number of inflow states increases. Table 1 indicates the number of states corresponding to a given number of inflow modes. The computational cost of the inflow model grows with the cube of the number of states.
|of modes||of states||of modes||of states||of modes||of states||of modes||of states|
The dynamic inflow model is defined in an inflow frame shown in fig. 2. At first, frame FR = [O, BR = (r1, r2, r3)] is defined. If the inflow model is associated with a wing, point O is the wing root point and frame FR the wing root frame. If the inflow model is associated with a rotor, point O is the hub point and frame FR the rotor frame. In both cases, frame FR is a non-rotating frame.
Next, an intermediate axis system is defined as follows: it has its origin at point O, and basis E = (e1, e2, e3) is obtained by a 180 degree rotation of basis R,
e1 = r1, e2 = - r2, e3 = - r3,
as depicted in fig. 2. Let V∞ denote the far field flow velocity. The non-rotating inflow frame, FI = [O, I = (i1, i2, i3)], is then defined as follows: it has its origin at point O, and basis I is obtained as follows
i3 = e3, i2 = (i3 x V∞)/||i3 x V∞||, i1 = i2 x i3.
If the far field flow velocity vanishes, i.e. in the hover case, the inflow frame is defined as follows
i3 = e3, i2 = e1, i1 = i2 x i3.
Figure 2 also indicates the disk effective angle of attack, αe, and the wake skew angle χ = π/2 - αe.