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.

In this thesis, the resistance of a single degree of freedom liner is investigated experimentally. Semi-empirical resistance modelling fails at higher Mach numbers (M₀>0.3) and sound pressure levels (SPL>130dB), and experimental work is missing in these conditions. An experiment combining in-situ impedance measurements and phase-locked particle image velocimetry is selected. This combined approach allows to evaluate the resistance, in-orifice velocity, friction velocity, vorticity and turbulence intensity.

The high grazing velocity and low acoustically induced velocity form a wide range of velocity scales that the PIV measurement must resolve. This sets a requirement for the dynamic velocity range that typical PIV with advanced interrogation techniques is not expected to achieve. A multi-frame approach is selected, which combines measurements with different pulse separation times. A second benefit of such a multi-dt approach is that information from different pulse separation times allows to decompose the PIV measurement noise from the turbulent fluctuations. Limits on the pulse separation time are seen in the freestream velocity, acoustic frequency, correlation strength and spatial filtering. The resulting timing ranges are evaluated before the experiment, and experimentally confirmed to be appropriate.

The decomposition between PIV measurement noise and turbulence is shown to be not applicable to the current data, as the measurement noise is correlated with the pulse separation time. This is caused by turbulence intensity and laser sheet reflections near the liner's face sheet. A DVR of 600 is achieved, and an improvement with respect to a grid refinement approach is seen. The difference between multi-dt and grid refinement vanishes towards the wall, and the DVR itself is also seen to decrease, due to the increase in measurement noise towards the wall. It is shown that the DVR must exceed the expected range of velocity scales by approximately 25% to accurately resolve phase-dependent quantities.

Results for the friction velocity found from a momentum-integral method and from a log-law fit, modified for surface roughness, transpiration and inclusion of the outer layer, agree well with direct drag measurements. The boundary layer is seen to develop in a periodic manner over the orifices, and the influence of the aero-acoustic interaction at the liner on the local friction velocity can not be neglected. The in-orifice velocity fluctuation RMS is consistent between the in-situ impedance and PIV measurements. A lumped-element model suggested in literature is shown to accurately predict the in-orifice velocity scale, but it requires an accurate value for the orifice quality factor. The quality factor is associated to the reduction in effective open orifice area, known as a vena contracta. It is thereby inversely proportional to the friction velocity, and the in-orifice velocity scale. The current results on the quality factor agree qualitatively and quantitatively to statements and suggestions from literature. The resistance is confirmed to be a function of the friction velocity and the in-orifice velocity \gls{rms}, with both velocity scales associated to vena contracta. A velocity scale associated to the intensity of vortex shedding from the orifices is introduced by integrating the acoustically induced vorticity near the wall. The onset of non-linear resistance is confirmed at a ratio between the in-orifice and friction velocity scales of 2, and is clearly linked to the intensity of vortex shedding.