Wall Interference Assessment of Isolated Fixed-Pitch Propeller Tests

Abstract

The need to reduce greenhouse gas emissions
is emergent throughout every industrial sector worldwide. For aviation, this
has opened a market gap for small scale electric aircraft configurations
(e-V/STOL). These configurations are very diverse and they are classified based
on how lift is obtained. However, their flight envelope consists of the same
operational regimes which in turn, are very diverse compared with the ones of a
traditional aircraft. This challenges the way wind tunnel testing was developed
and urges the need for new testing techniques and wall correction methods. It
is known for over a century that the flow inside a wind tunnel is not exactly
the same as a free-air flow. This has to do with the existence of the wind
tunnel test section boundaries which affect the flow over the model. Thus, the
measured wind tunnel data deviate from the desired free-air performance measurements,
except if the model is extremely small relative to the test section size. This
is almost never satisfied due to physical scaling and structural
considerations. Wall or boundary interference, is the quantification of those
effects with the aim of correcting the wind tunnel performance measurements to
their actual free-flight values. This is achieved by wall correction methods.
These methods were developed in absence of high computing power and their
purpose is to perform real-time adjustments to wind tunnel data. Consequently,
they depend on simplified modeling assumptions, such as the linearized potential
flow. As a result, these corrections might prove ineffective in generating
reliable outcomes when dealing with non-linear boundary interference fields
characterized by significant variations. For a given model, it is generally
known that there is a limit at which wind tunnel test data are able to
represent free-air conditions. This limit is called wind tunnel flow breakdown
and it occurs when the model wake impinges on the boundaries of the wind tunnel
test section, causing flow phenomena which would never occur in a free-flight
situation. This phenomenon succeeds the non-linearities of boundary
interference described above and is the worst case scenario for wind tunnel
tests. The wall corrections completely fail to produce any valid outcome and
the wind tunnel data are considered useless. This happens at low-speed/high-thrust
conditions which are typical for a V/STOL aircraft during transition from hover
to forward flight. Thus, powered wind tunnel testing at such conditions poses
challenges in terms of data representation to an interference-free, free-air
situation. Such conditions may also be encountered when the aircraft is
operating in-ground vicinity and thus, the interference field for those cases
is of interest. In the latter case, it is expected that the magnitude of the
corrections and the wind tunnel flow breakdown limits would be significantly
lower. This Thesis aims to investigate wall interference effects between the
rotor and the wind tunnel test section boundaries at low-speed/high-thrust
conditions, establish the flow breakdown limits for a given propeller and test
section and to gain insight into the effect that the bottom wall of the test
section exhibits to the boundary interference field. This is realized by
conducting wind tunnel tests on two different fixed-pitch propellers with
varying incidence angles in both a closed-wall and a 3/4 open-jet test section
of the NLR Aeroacoustic Wind Tunnel. Static wall pressure measurements are
acquired and the flow-field in the vicinity of the propeller is quantified
using large-scale, Tomographic Particle Tracking Velocimetry (PTV) with the use
of helium filled soap bubbles (HFSB) as tracers. Finally, the interference-free
performance data for these propellers are obtained from the second test
campaign performed in the industrial Low-Speed-Wind Tunnel (LST) of the DNW. The
global flow topology of the wall-bounded tests in the AWT-closed test section
is presented by means of wall pressure measurements and PTV. It was found that
a distinct rise of the pressure coefficient leading to a maximum peak,
corresponds to wake impingement. This firstly occurs on the advancing side of
the rotor due to the larger vortex which is deflected further downward than the
one on retreating side. A flow breakdown criterion which makes use of the
static pressure distribution on the bottom wall of the test section is
introduced. It aligns well with existing benchmarks available in literature for
closedwall test sections. This criterion can be applied to any test section
consisting of a solid lower wall, as long as the distribution of static
pressure is monitored. On the other hand, the comparison between the LST and
AWT-closed test section data do not show any signs of flow breakdown. This
could prompt additional questions regarding the necessity of an even larger
test section for obtaining the interference free propeller performance data. Various
wall correction methods are utilized in order to evaluate their applicability
for powered wind tunnel testing and to assess the impact of wall interference.
It is shown that the developed Two-VariableMethod (TVM) does not provide
satisfying results with respect to lift interference, when compared with the
Heyson method. Nevertheless, blockage predictions follow the expected trends
even for lifting cases, at least for the higher advance ratios where no flow
impingement is present. For a simple non-lifting case, this method is deemed
more reliable since its underlying assumptions are satisfied to a greater extend
and its blockage predictions are also in good agreement with the Glauert’s
method. For the purely lifting angle of attack, the comparison between the two
test campaigns is not satisfactory both for in-ground and out-of-ground effect.
This discrepancy arises from the fact that the predictions of all wall
correction methods applied do not abide by the trend that is discernible based
on the deviations of the bounded (AWT) to the unbounded (LST) data. That is
mainly attributed to the load measurement system and to the possibly stalling
conditions at the higher wind tunnel speeds. For the 0◦ incidence angle, there
was a very good agreement between the AWT-closed test section and the LST data
for both flight conditions (OGE, IGE). This was validated by both applied
correction procedures (Glauert’s method for off-center propellers &
Two-Variable-Method). Even in that case, the AWT-3/4 open-jet results were not
in good agreement implying effects which may not only be attributed to boundary
interference.