Experimental Fluid-Structure Interaction of a Flexible Plate Under Gust Excitation

Abstract

The reduction of atmospheric pollutants emission is one of the biggest challenges of modern aeronautical industry. This is testified by the commitment of international institutions such as the European Commission, with its \textit{Flightpath 2050} document issued in 2011, and by research organization such as NASA, with its \textit{Fixed Wing} project. This attention towards the reduction of fuel burn and consequently of emissions constitutes a drive towards the employment of lightweight aerospace structures, leading in this way to the rise of a variety of so-called aeroelastic problems. Despite the many progresses made in the development of numerical tools for the prediction of aeroelastic phenomena, some aspects still constitute a challenge in the computational field and as a consequence wind-tunnel testing remains an important tool for the study of aeroelastic problems.

Modern aeroelastic models employed in wind-tunnel tests are usually heavily instrumented in their inside in order to host common measurement devices such as pressure sensors, accelerometers and strain gauges. These devices provide accurate experimental data for the characterization of both flow and structure, however they are not ideal both in terms of structural intrusiveness and in terms of the complexity of the model’s instrumentation phase. At this purpose, optical metrology offers a valid alternative for both flow and structural measurements. In particular, quantitative flow visualization techniques such as Particle Image Velocimetry (PIV) allow for a thorough characterization of flow fields which for certain aspects results superior to more traditional point-wise measurements. A literature survey on the combination of PIV with optical structural measurements for Fluid-Structure Interaction (FSI) experiments reveals how volumetric flow measurements are almost never carried out. Since a large variety of aeroelastic phenomena involves the presence of 3D unsteady flows and large scale structures, a gap in the simultaneous assessment of volumetric flow fields and of structural displacements emerges from the literature review. Besides this, it is observed how at least two systems are always used to obtain quantitative measurements of both flow and structure, leading to complex setups with challenges related to both the synchronization and the optical isolation of the two systems. In the field of large-scale aerodynamic testing, the introduction of Helium Filled Soap Bubbles (HFSB) as flow tracers for PIV measurements and the development of efficient particle tracking algorithms such as Shake-the-Box (STB) have enabled a substantial extension of the achievable measurement domain at an affordable computational cost. The development of Robotic Volumetric PIV has allowed to exploit even further the potential of HFSB and STB, revealing a capability of volumetric measurements in domains of several cubic meters. Furthermore, being based on particle tracking, Robotic Volumetric PIV appears as a good candidate for the tracking of markers on a structure and thus for the realization of aeroelastic measurements where a 3D flow and a large structure are characterized with a single measurement system. The aim of this work is then to assess the feasibility of Robotic Volumetric PIV for aeroelastic investigations. This is achieved studying the dynamic response of a flexible aluminium plate subjected to gust excitation. The experiment is carried out in the Open Jet Facility at Delft University of Technology, which is equipped with a gust generator. Phase-averaged structural displacement of the entire plate together with the volumetric near flow field is measured, with a total measurement volume of approximately 150 litres. Small circular markers are applied to the surface of the plate in order to carry out the structural measurement, which is validated by means of a Scanning Vibrometer. The assessment of the FSI phenomenon is conducted at a wind-tunnel speed of 12 m/s and at a reduced frequency of 0.045. In this way, the capability of Robotic Volumetric PIV to deliver unprecedented quantitative volumetric flow visualization coupled to the measurement of structural displacement over large scales is demonstrated. The challenges faced to achieve such objective include the possibility to distinguish between flow particles and structural markers in the acquired images, the validity of the instantaneous structural displacements measured by the PIV system, the feasibility of the phase-average approach and the consistency of the combined structural and flow information. A visualization of the FSI phenomenon is finally presented, together with a quantitative analysis of its dynamics.The reduction of atmospheric pollutants emission is one of the biggest challenges of modern aeronautical industry. This is testified by the commitment of international institutions such as the European Commission, with its \textit{Flightpath 2050} document issued in 2011, and by research organization such as NASA, with its \textit{Fixed Wing} project. This attention towards the reduction of fuel burn and consequently of emissions constitutes a drive towards the employment of lightweight aerospace structures, leading in this way to the rise of a variety of so-called aeroelastic problems. Despite the many progresses made in the development of numerical tools for the prediction of aeroelastic phenomena, some aspects still constitute a challenge in the computational field and as a consequence wind-tunnel testing remains an important tool for the study of aeroelastic problems. Modern aeroelastic models employed in wind-tunnel tests are usually heavily instrumented in their inside in order to host common measurement devices such as pressure sensors, accelerometers and strain gauges. These devices provide accurate experimental data for the characterization of both flow and structure, however they are not ideal both in terms of structural intrusiveness and in terms of the complexity of the model’s instrumentation phase. At this purpose, optical metrology offers a valid alternative for both flow and structural measurements. In particular, quantitative flow visualization techniques such as Particle Image Velocimetry (PIV) allow for a thorough characterization of flow fields which for certain aspects results superior to more traditional point-wise measurements. A literature survey on the combination of PIV with optical structural measurements for Fluid-Structure Interaction (FSI) experiments reveals how volumetric flow measurements are almost never carried out. Since a large variety of aeroelastic phenomena involves the presence of 3D unsteady flows and large scale structures, a gap in the simultaneous assessment of volumetric flow fields and of structural displacements emerges from the literature review. Besides this, it is observed how at least two systems are always used to obtain quantitative measurements of both flow and structure, leading to complex setups with challenges related to both the synchronization and the optical isolation of the two systems. In the field of large-scale aerodynamic testing, the introduction of Helium Filled Soap Bubbles (HFSB) as flow tracers for PIV measurements and the development of efficient particle tracking algorithms such as Shake-the-Box (STB) have enabled a substantial extension of the achievable measurement domain at an affordable computational cost. The development of Robotic Volumetric PIV has allowed to exploit even further the potential of HFSB and STB, revealing a capability of volumetric measurements in domains of several cubic meters. Furthermore, being based on particle tracking, Robotic Volumetric PIV appears as a good candidate for the tracking of markers on a structure and thus for the realization of aeroelastic measurements where a 3D flow and a large structure are characterized with a single measurement system. The aim of this work is then to assess the feasibility of Robotic Volumetric PIV for aeroelastic investigations. This is achieved studying the dynamic response of a flexible aluminium plate subjected to gust excitation. The experiment is carried out in the Open Jet Facility at Delft University of Technology, which is equipped with a gust generator. Phase-averaged structural displacement of the entire plate together with the volumetric near flow field is measured, with a total measurement volume of approximately 150 litres. Small circular markers are applied to the surface of the plate in order to carry out the structural measurement, which is validated by means of a Scanning Vibrometer. The assessment of the FSI phenomenon is conducted at a wind-tunnel speed of 12 m/s and at a reduced frequency of 0.045. In this way, the capability of Robotic Volumetric PIV to deliver unprecedented quantitative volumetric flow visualization coupled to the measurement of structural displacement over large scales is demonstrated. The challenges faced to achieve such objective include the possibility to distinguish between flow particles and structural markers in the acquired images, the validity of the instantaneous structural displacements measured by the PIV system, the feasibility of the phase-average approach and the consistency of the combined structural and flow information. A visualization of the FSI phenomenon is finally presented, together with a quantitative analysis of its dynamics.