x→−∞
(
S→0
)
:V
(
0,t
)
=K𝑒
(
)
(
9
)
X→+∞
(
S→∞
)
:V
(
S,t
)
→0
(
10
)
Each time step is forced to satisfy by the
projection method: 𝑉
=max (𝑉
,𝐾−𝑒
).
The discrete equations can be constructed as a
tridiagonal linear system A𝑉
=B𝑉
+b, which
is solved iteratively using the projected successive
over-relaxation method (PSOR) to ensure that the
solution satisfies the advance exercise constraint
(Marengo et al., 2021).
This study constructs a variational inequality
model for American option pricing based on the
Black-Scholes equation, and reveals the essential
features of the free boundary through mathematical
analysis. In the continuous-time financial framework,
the American option exercise strategy is transformed
into a coupled problem of parabolic partial
differential equations with complementary
conditions, the logarithmic price transformation is
introduced to eliminate the singularity of the
equations, and the uniqueness of the existence of the
weak solution is proved by using Sobolev space
theory. The local Lipschitz continuity of the critical
price function S*(t) and its asymptotic behaviour of
convergence to the strike price are rigorously derived
for the non-smooth property of the free boundary. A
finite difference method in Crank-Nicolson format is
proposed, which is combined with a projection
relaxation algorithm to deal with the early strike
constraints, to establish the convergence theory of the
numerical solution in terms of operator splitting. The
model is further extended to fractional order
derivatives to portray market memory effects (Chen
et al., 2024), and sparse tensor product spaces are
constructed to solve the high-dimensional problem
dimensionality catastrophe. This study provides a
rigorous mathematical framework for American
derivatives pricing and deepens the theoretical
knowledge of the free boundary dynamics
mechanism.
2.2 Turbulence Modelling and
Aerodynamic Efficiency
Optimization
The mathematical description of fluid motion is the
intersection of classical mechanics and engineering
science. the Navier-Stokes system of equations, as the
controlling equations of viscous fluid dynamics, has
nonlinear characteristics originating from the
coupling of inertial and viscous terms, which
profoundly reflects the energy transfer and
dissipation mechanisms of fluid motion. The system
of equations is derived from the laws of conservation
of mass and momentum:
ρ
∂u
∂t
+u∇u=−∇p+μ∇
𝑢+𝑓
(
∇𝑢=0
)(
11
)
he coupling relationship between the velocity
field u and the pressure field p dominates complex
phenomena such as flow separation and vortex
evolution. In the field of aeronautical engineering, the
solution of the equations is directly related to the
optimisation of the aerodynamic performance of the
aircraft, whose core objective is to reduce wind
resistance by suppressing turbulence dissipation, and
thus to improve fuel efficiency (Chau & Zingg,
2021).
Theoretical studies have shown that the
distribution of pressure gradient ∇p on the airfoil
surface is a key regulating parameter for aerodynamic
efficiency (Deng et al., 2022). The region of inverse
pressure gradient is prone to trigger boundary layer
separation, leading to a surge in differential pressure
drag. Adjusting the airfoil curvature by constructing
a shape function can optimise the mathematical
properties of the pressure distribution function p(x)
and shift the separation point back. For example,
increasing the radius of curvature of the leading edge
delays the flow instability, while controlling the
trailing edge curvature attenuates the intensity of
trailing vortex shedding. This type of optimisation is
essentially a problem of solving a generalised
extremum problem under the constraints of the N-S
equations, which is mathematically expressed as:
𝑚𝑖𝑛
𝐶
(
𝛤
)(
12
)
Such that
𝑁
(
𝑢,𝑃;𝛤
)
=0
(
13
)
where Γ is the airfoil geometry parameter, CD is
the drag coefficient, and N is the N-S equation
operator.
Current theoretical challenges focus on the
construction of closed models for high Reynolds
number turbulence (Zhang et al., 2023). The classical
RANS method simplifies the pulsation correlation
term by introducing the turbulent viscosity μt, but its
prediction of anisotropic turbulence suffers from
systematic bias. The data assimilation technique
embeds the flow field observation data into the PDE
constrained optimisation framework, which provides
a new idea to improve the generality of the model.
The theoretical progress of the N-S equations