Aerodynamic Drag - Theoretical Aspects of Far-field/near-field Balance

Theoretical Aspects of Far-field/near-field Balance

The drag force calculation can be performed using the integral of force balance in the freestream direction as

\int_{S=S_{\infty}+S_D+S_A}\left[\rho\,u\,\vec{q}+\left(p-p_{\infty}\right) \vec{i} - \vec{\tau}_{x}\right].\vec{n}dS\,=\,0

which surrounds the body represents the union of two unconnected surfaces,

S = \underbrace{S_{A}}_{Aircraft\,Surf.}\;+\;\underbrace{S_{D}+S_{\infty}}_{Far\,Surf.}

where is the airplane surface, is the outlet surface and represents both the lateral and inlet surfaces. In general, the far-field control volume is located in the boundaries of the domain and its choice is user-defined. In Subsection \ref{sGF}, further considerations concerning to the correct selection of the far-field boundary are given, allowing for desired flow characteristics.

Equation (\ref{ta1}) can be decomposed into two surface integrals, yielding

\int_{S_{A}}\left[\rho\,u\,\vec{q}+\left(p-p_{\infty}\right) \vec{i} -\vec{\tau}_{x}\right]\,.\,\vec{n}\,dS =-\int_{S_{D}+S_{\infty}}\left[\rho\,u\,\vec{q}+\left(p-p_{\infty}\right) \vec{i} -\vec{\tau}_{x}\right]\,.\,\vec{n}\,dS

The right-hand side integral in Eq.\ (\ref{ta2}) represents the reaction forces of the airplane. The left-hand side integral in Eq.\ (\ref{ta2}) represents the total force exerted by the fluid. Mathematically, these two integrals are equivalent. However, the numerical integration of these terms will hardly lead to the same result, because the solution is approximated. In the terminology of Computational Fluid Dynamics (CFD), when the integration is performed using the left-hand side integral in Eq.\ (\ref{ta2}), the near-field method is employed. On the other hand, when the integration of the right-hand side in Eq. (\ref{ta2}) is computed, the far-field method is considered.

The drag force balance is assured mathematically by Eq.\ (\ref{ta2}), that is, the resultant drag force evaluated using the near-field approach must be equal to the drag force extracted by the far-field approach. The correct drag breakdown considered in this work is

\underbrace{D_f + D_{pr}}_{near-field} = \underbrace{D_i + D_w + D_v}_{far-field}

Read more about this topic:  Aerodynamic Drag

Famous quotes containing the words theoretical, aspects and/or balance:

    “The desire to serve the common good must without fail be a requisite of the soul, a necessity for personal happiness; if it issues not from there, but from theoretical or other considerations, it is not at all the same thing.”
    —Anton Pavlovich Chekhov (1860–1904)

    “I suppose an entire cabinet of shells would be an expression of the whole human mind; a Flora of the whole globe would be so likewise, or a history of beasts; or a painting of all the aspects of the clouds. Everything is significant.”
    —Ralph Waldo Emerson (1803–1882)

    “Forget dating. Forget striking a balance between work and family. Most single parents, whether they are divorced, widowed, or single by choice, report that discipline is by far the toughest issue.”
    —Jean Callahan (20th century)