For more information on the project please contact the coordinator: Dr. Ni Yan, n.yan@tudelft.nl
This project is made possible by
FP7
the EU
Project description
WP1 - Management and Coordination
The overall objective of the management WP is the smooth delivery of the project, and in particular:
- To co‐ordinate and supervise activities to be carried out
- To carry out the overall administrative and financial management of the project
- To manage the contract with the European Commission and the Consortium Agreement
- To manage contacts with the European Commission
- To monitor quality and timing of project deliverables
- To establish effective internal and external communication procedures
Lead Partner: Technische Universiteit Delft
WP2 - 3D Tomographic PIV
Tomographic PIV has appeared very recently in the scenario of 3D measurement techniques (Elsinga et al., 2006).
Since then, it has been applied to several aerodynamic problems. The present state-of-the-art for tomographic PIV reveals major bottlenecks both on the side of hardware requirements as well as in regards to computational cost.
The aims of this work-package are the development and the optimization of three-dimensional particle image velocimetry (3D-PIV) for applications in external aerodynamics and internal flows in engines. The main tasks of this work package are:
1) identify minimum hardware requirements for tomographic-PIV measurements
2) define optimization rules for illumination and imaging especially in view of industrial applications.
3) Study how the technique can be made robust and versatile to be operated in industrial environment.
The major part of the activities is directed towards the development of improved experimental procedures in terms of illumination, 3D-reconstruction algorithms and techniques for 3D-motion estimation.
During the first part of the project, tomographic experiments have been listed and illumination and the recording configurations have been adapted for industrial applications. In particular, a fundamental point is the improvement of the calibration of the camera for the reconstruction procedure.
Different reconstruction algorithms have been developed and compared during these two first years. An adaptive correlation has been implemented in a different way to reduce the computational cost and to obtain a better interactive tool for the motion estimation. Finally, a database of tomographic experiments and one numerically simulated case is being prepared and analyzed for comparing and optimizing the different approaches.
Since then, it has been applied to several aerodynamic problems. The present state-of-the-art for tomographic PIV reveals major bottlenecks both on the side of hardware requirements as well as in regards to computational cost.
The aims of this work-package are the development and the optimization of three-dimensional particle image velocimetry (3D-PIV) for applications in external aerodynamics and internal flows in engines. The main tasks of this work package are:
1) identify minimum hardware requirements for tomographic-PIV measurements
2) define optimization rules for illumination and imaging especially in view of industrial applications.
3) Study how the technique can be made robust and versatile to be operated in industrial environment.
The major part of the activities is directed towards the development of improved experimental procedures in terms of illumination, 3D-reconstruction algorithms and techniques for 3D-motion estimation.
During the first part of the project, tomographic experiments have been listed and illumination and the recording configurations have been adapted for industrial applications. In particular, a fundamental point is the improvement of the calibration of the camera for the reconstruction procedure.
Different reconstruction algorithms have been developed and compared during these two first years. An adaptive correlation has been implemented in a different way to reduce the computational cost and to obtain a better interactive tool for the motion estimation. Finally, a database of tomographic experiments and one numerically simulated case is being prepared and analyzed for comparing and optimizing the different approaches.
WP3 - High-speed PIV and long-range micro-PIV
Highly spatially and temporally resolved measurements of the flow over periodic hills (measurements performed at TU Munich)
The flow over periodic hills is a common test case for the validation of numerical flow simulations (see ERCOFTAC test case Nr. 81). The numerical prediction is quite difficult, since flow separation and reattachment are not fixed in space and time due to the absence of sharp edges. Furthermore, the separated and fully three-dimensional flow from the previous hill impinges on the next hill and results in very complex flow features including turbulent splashing, Taylor-Görtler vortices and a very thin shear layer with developing Kelvin-Helmholtz instabilities (Rapp & Manhart, 2011). The complex dynamic of the flow can easily be seen in the instantaneous images in Fig. 1.
By combining the experimental capabilities of the partners of the consortium time-resolved planar and volumetric measurements were performed at two Reynolds numbers (8,000 and 33,000). Of special interest is the thin shear layer at the hill top. Using the advanced single-pixel ensemble-correlation method and particle tracking the steep gradient can be fully resolved as can be seen in Fig. 2. The much finer resolution compared to standard evaluation techniques was evaluated systematically (Kähler et al., 2012a,2012b) and an obvious improvement is observed looking at the small symbols. The vector spacing in each direction is 180 µm which corresponds to one Kolmogorov length scale Δxy ~ η for the lower Reynolds number and Δxy ~ 2.6η for Re = 33,000. The Reynolds stresses are of paramount importance for the validation of the turbulence models. A method to determine the Reynolds stresses from the correlation peak rather than using successive vector fields was recently developed by Scharnowski et. al (2012). The method has the advantage that it does not suffer from either underestimation due to velocity smoothing by outlier filters or overestimation due to strong outliers and thus provides reliably the Reynolds stresses with a grid spacing typical for DNS and finer than for LES within a field of view of 0.5 m length.
As it turned out that the region of the evolving shear layer from the hill top is highly populated with three-dimensional vortices with various orientations, only time-resolved tomographic PIV measurements are suitable to characterize these structures (Elsinga, 2006; Scarano, 2013). The high complexity of the flow, within a volume of 80x80x20 mm3, located downstream of the hill-crest can be already seen in Fig. 3. As expected from the broad frequency spectra obtained by the 2D measurements, a manifold of differently sized vortical structures can be observed, including very small and randomly appearing ones as well as large structures that seem to be inclined with the mean flow at a defined angle.
These results display that the combined usage of planar high resolution PIV, time-resolved PIV and tomographic PIV is required to obtain an unambiguous description of the flow phenomena, as each technique has its specific strengths and weaknesses. This result is of paramount importance for industrial flow investigations or for the acquisition of data sets for the validation of numerical flow simulations, were the requirement regarding the accuracy are extremely high. In the future more detailed analysis of the whole data set will provide a much deeper inside to the flow and allows for a better validation of numerical methods which will greatly increase the impact and visibility of the AFDAR project to the scientific community.
The flow over periodic hills is a common test case for the validation of numerical flow simulations (see ERCOFTAC test case Nr. 81). The numerical prediction is quite difficult, since flow separation and reattachment are not fixed in space and time due to the absence of sharp edges. Furthermore, the separated and fully three-dimensional flow from the previous hill impinges on the next hill and results in very complex flow features including turbulent splashing, Taylor-Görtler vortices and a very thin shear layer with developing Kelvin-Helmholtz instabilities (Rapp & Manhart, 2011). The complex dynamic of the flow can easily be seen in the instantaneous images in Fig. 1.
By combining the experimental capabilities of the partners of the consortium time-resolved planar and volumetric measurements were performed at two Reynolds numbers (8,000 and 33,000). Of special interest is the thin shear layer at the hill top. Using the advanced single-pixel ensemble-correlation method and particle tracking the steep gradient can be fully resolved as can be seen in Fig. 2. The much finer resolution compared to standard evaluation techniques was evaluated systematically (Kähler et al., 2012a,2012b) and an obvious improvement is observed looking at the small symbols. The vector spacing in each direction is 180 µm which corresponds to one Kolmogorov length scale Δxy ~ η for the lower Reynolds number and Δxy ~ 2.6η for Re = 33,000. The Reynolds stresses are of paramount importance for the validation of the turbulence models. A method to determine the Reynolds stresses from the correlation peak rather than using successive vector fields was recently developed by Scharnowski et. al (2012). The method has the advantage that it does not suffer from either underestimation due to velocity smoothing by outlier filters or overestimation due to strong outliers and thus provides reliably the Reynolds stresses with a grid spacing typical for DNS and finer than for LES within a field of view of 0.5 m length.
As it turned out that the region of the evolving shear layer from the hill top is highly populated with three-dimensional vortices with various orientations, only time-resolved tomographic PIV measurements are suitable to characterize these structures (Elsinga, 2006; Scarano, 2013). The high complexity of the flow, within a volume of 80x80x20 mm3, located downstream of the hill-crest can be already seen in Fig. 3. As expected from the broad frequency spectra obtained by the 2D measurements, a manifold of differently sized vortical structures can be observed, including very small and randomly appearing ones as well as large structures that seem to be inclined with the mean flow at a defined angle.
These results display that the combined usage of planar high resolution PIV, time-resolved PIV and tomographic PIV is required to obtain an unambiguous description of the flow phenomena, as each technique has its specific strengths and weaknesses. This result is of paramount importance for industrial flow investigations or for the acquisition of data sets for the validation of numerical flow simulations, were the requirement regarding the accuracy are extremely high. In the future more detailed analysis of the whole data set will provide a much deeper inside to the flow and allows for a better validation of numerical methods which will greatly increase the impact and visibility of the AFDAR project to the scientific community.
- Elsinga GE, Scarano F, Wieneke B, van Oudheusden BW (2006) Tomographic particle image velocimetry. Exp Fluids 41: 933-947
- Kähler CJ, Scharnowski S, Cierpka C (2012a) On the resolution limit of digital particle image velocimetry. Exp Fluids 52:1629–1639
- Kähler CJ, Scharnowski S, Cierpka C (2012b) On the uncertainty of digital PIV and PTV near walls. Exp Fluids 52:1641–1656
- Rapp C, Manhart M (2011) Flow over periodic hills: an experimental study. Exp Fluids 51, 247–269.
- Scarano F (2013) Tomographic PIV: principles and practice. Meas Sci Technol 24, 012001
- Scharnowski S, Hain R, Kähler CJ (2012) Reynolds stress estimation up to single-pixel resolution using PIV-measurements. Exp Fluids 52, 985–1002.
WP4 - Combined Diagnostics for combustion, aeroacoustics and aero-elasticity
Coupling PIV with complementary diagnostic methods is the only approach enabling to extend measurement capabilities to problems that involve both the fluid flow process and other thermo-chemical, acoustic or structural interactions. This requires a skilful combination of PIV with other diagnostics, which success strongly depends upon the nature of such combinations (i.e. optical/electronic interference, ensuring simultaneous and compatible outputs). The interpretation of inhomogeneous experimental data also requires developing proper post-processing and visualizing techniques, which are almost non-existent to date.
In WP4, combination of PIV with different diagnostics in various configurations at laboratory-scale is demonstrated to bring new knowledge in terms of technical difficulties and potential interests for future uses in industrial configurations for aeronautic. Experiments in combustion facilities will verify the feasibility to integrate and simultaneously operate PIV and spectroscopic imaging methods such as laser induced incandescence (LII) or planar laser induced fluorescence (PLIF) for correlated flow diagnostics in combustors (T4.1). The combination of TR-PIV and microphones is intended to provide the basis for aero-acoustic analysis of aerodynamic systems (T4.2). Finally the field of flow induced vibrations and aero-elasticity will be covered by setting up a combined measurement technique based on TR- PIV and IPCT for non-linear aero-elastic interactions in subsonic flows (T4.3).
More...
In WP4, combination of PIV with different diagnostics in various configurations at laboratory-scale is demonstrated to bring new knowledge in terms of technical difficulties and potential interests for future uses in industrial configurations for aeronautic. Experiments in combustion facilities will verify the feasibility to integrate and simultaneously operate PIV and spectroscopic imaging methods such as laser induced incandescence (LII) or planar laser induced fluorescence (PLIF) for correlated flow diagnostics in combustors (T4.1). The combination of TR-PIV and microphones is intended to provide the basis for aero-acoustic analysis of aerodynamic systems (T4.2). Finally the field of flow induced vibrations and aero-elasticity will be covered by setting up a combined measurement technique based on TR- PIV and IPCT for non-linear aero-elastic interactions in subsonic flows (T4.3).
More...