Chapter 9
Characterisation of Flow Properties
at the Surface
9.1 The Action of a Fluid at the Wall
The main forces acting locally on a surface experiencing a fluid flow are due to the
pressure and the wall friction, the latter being responsible for heat transfer between
the surface and the fluid. The direction of the shear stresses is of other practical
importance. The pressure and wall shear stress or skin friction is of obvious importance in determining vehicle lift and drag in the absence of direct force measurement.
However, the aerodynamic forces are very rarely determined from the integration of
the local contribution as it would require extremely dense measurements of pressure
and friction over the whole surface. The distribution of pressure on a vehicle is necessary to determine the local loads on the structure, and also provides an indication
of the organisation of the flow locally, for instance a pressure plateau revealing a
separated region. This is less true in three-dimensional flows where separation is
a more complex process that cannot be inferred from the sole inspection of wall
pressure distributions.
The knowledge of the skin friction is useful not only to determine the drag, but
also to validate the theoretical models. This quantity is indeed difficult to predict with
precision, since its calculation depends on both the precision of the numerical scheme
(the friction being proportional to the derivatives of the velocity) and the robustness
of the physical model. In addition, in two-dimensional planar or axisymmetric flows,
the change in the sign of the shear stress at the wall is the best indicator of separation.
The ability of computer codes to predict the location of this point is a good
indicator of their accuracy. In three-dimensional flows, the definition of separation
is more subtle, as the transverse velocity component is not equal to zero and drives
the line of separation in its direction, except in some particular situations.
Heat transfers (or flux) are of vital importance for flying at high Mach numbers
(greater than 3), where some parts of the structure are subjected to considerable
heating in hypersonics (re-entry body in particular). Also, high temperature flows
© Springer Nature Switzerland AG 2020
B. Chanetz et al., Experimental Aerodynamics,
Springer Tracts in Mechanical Engineering,
https://doi.org/10.1007/978-3-030-35562-3_9
199
Characterisation of Flow Properties
at the Surface
9.1 The Action of a Fluid at the Wall
The main forces acting locally on a surface experiencing a fluid flow are due to the
pressure and the wall friction, the latter being responsible for heat transfer between
the surface and the fluid. The direction of the shear stresses is of other practical
importance. The pressure and wall shear stress or skin friction is of obvious importance in determining vehicle lift and drag in the absence of direct force measurement.
However, the aerodynamic forces are very rarely determined from the integration of
the local contribution as it would require extremely dense measurements of pressure
and friction over the whole surface. The distribution of pressure on a vehicle is necessary to determine the local loads on the structure, and also provides an indication
of the organisation of the flow locally, for instance a pressure plateau revealing a
separated region. This is less true in three-dimensional flows where separation is
a more complex process that cannot be inferred from the sole inspection of wall
pressure distributions.
The knowledge of the skin friction is useful not only to determine the drag, but
also to validate the theoretical models. This quantity is indeed difficult to predict with
precision, since its calculation depends on both the precision of the numerical scheme
(the friction being proportional to the derivatives of the velocity) and the robustness
of the physical model. In addition, in two-dimensional planar or axisymmetric flows,
the change in the sign of the shear stress at the wall is the best indicator of separation.
The ability of computer codes to predict the location of this point is a good
indicator of their accuracy. In three-dimensional flows, the definition of separation
is more subtle, as the transverse velocity component is not equal to zero and drives
the line of separation in its direction, except in some particular situations.
Heat transfers (or flux) are of vital importance for flying at high Mach numbers
(greater than 3), where some parts of the structure are subjected to considerable
heating in hypersonics (re-entry body in particular). Also, high temperature flows
© Springer Nature Switzerland AG 2020
B. Chanetz et al., Experimental Aerodynamics,
Springer Tracts in Mechanical Engineering,
https://doi.org/10.1007/978-3-030-35562-3_9
199