Advanced Flow Diagnostics
Participants: F. Scarano, B.W. van Oudheusden, Roeland de Kat, Daniele Ragni
Activities:
Advanced image interrogation procedures
New methods (digital algorithms) aiming at enhancing the image interrogation performances are currently investigated and developed at TU Delft Aerospace Engineering (Aerodynamic group). The aim is to extend the velocity and vorticity dynamic range from both the lower and the upper end.
The Wi ndow D isplacement (and Deformation) I terative M ultigrid approach (WIDIM) is based on a correlation matching method that makes use of a first-order (bi-linear) predictor for the selection of the regions to be matched. The interrogation windows are subjected to a relative transformation that maximizes the particle image pattern correspondence as shown in figure 1.
Figure 1 Interrogation (a) and search window in the basic (b) and WIDIM (b') method.
The performance assessment through synthetic images of reference displacement distribution confirms the expected enlargement of the dynamic range of about two orders of magnitude (figure 2). The method is applied over a broad variety of flow situations demonstrating its validity and robustness.
Figure 2: Assessment of measurement errors for the different interrogation methods.
Recently developed PIV image interrogation schemes allow using interrogation windows of different size depending on local the need for spatial resolution. The research focuses on solution or measurement adaptive methods with an iterative structure.
The latest developed method is AR-WIDIM which combines the advantage of multigrid window deformation and adaptive resolution. The method has been applied successfully on aircraft trailing vortices flow in a towing tank. The research is now focusing on non-isotropic measurement methods that change not only the size of the interrogation windows but also the shape and orientation.
Tomographic PIV
Conventional PIV techniques allow velocity measurements to be performed in planes cutting through the flow domain of interest. For this reason, they only provide part of the information of the flow field. By combining flow field observations from different directions simultaneously and using tomographic reconstruction techniques, the instantaneous three-dimensional velocity field can be measured. Recent efforts deal with the development of this advanced and novel technique and its application to unsteady flow phenomena, such as wakes and boundary layers.
Figure 3: Application of a tomographic PIV technique to the three-dimensional visuzlization of the instantaneous structure of a cylinder wake.
PIV-based Non-Intrusive Determination of Aerodynamic Loads
The research aims at the development and application of a novel non-intrusive experimental method to determine the aerodynamic loads on objects from flow velocity data. The load determination is achieved by combining the flow field information with basic integral momentum principles according to Newton’s third law (action = - reaction). The flow velocity is measured by a technique called Particle Image Velocimetry (PIV), which relies on following the motion of particles transported by the flow.
The PIV-based loads-determination technique has been validated for flows that are constant in time, or for time-average values of the loads. Moreover, the recent developments of the PIV technique, including high speed illumination-imaging systems, provide nowadays the perspective to perform time-resolved flow measurement. This would enable the determination of instantaneous loads in unsteady flows. More in particular, the present project will address the development of procedures, in terms of hardware solutions, experimental arrangements and data post-processing, in order to make it a technique that can be applied relatively routinely in fluid-dynamic research practice. The primary area of application is that of low-speed aerodynamics and industrial fluid dynamics, wind engineering in particular. The extension of the method to the high-speed flow regime(compressible flow) will also receive attention, in view of its relevance for industrial aeronautical practice.
Related Publications:
1. B.W. van Oudheusden, F. Scarano, E.W.F. Casimiri: Non-intrusive load characterization of an airfoil using PIV. Exp. in Fluids 40 (2006) 988-992.
2. B.W. van Oudheusden, F. Scarano, E.W.M. Roosenboom, E.W.F. Casimiri, L.J. Souverein: Evaluation of integral forces and pressure fields from planar velocimetry data for incompressible and compressible flows, Exp. in Fluids 42 (2007) 153-162.
3. B.W. van Oudheusden, E.W.F. Casimiri, F. Scarano: Aerodynamic load characterization of a low speed aerofoil using particle image velocimetry, Aeronautical Journal 112, Number 1130 (2008), 197-205.
4. B.W. van Oudheusden: Principles and application of velocimetry-based planar pressure imaging in compressible flows with shocks, Exp in Fluids, 45 (2008), 657-674.
5. L.J. Souverein, B.W. van Oudheusden, F. Scarano: Particle Image Velocimetry based loads determination in supersonic flows, 45th AIAA Aerospace Science Meeting and Exhibit, 8-11 Jan 2007, Reno, USA, AIAA-2007-0050, 10 pp.
6. B.W. van Oudheusden, L.J. Souverein: Evaluation of the pressure field from PIV in a shock wave boundary layer interaction, 7th International Symposium on Particle Image Velocimetry, Rome, Italy, 11-14 Sept. 2007.
7. R. de Kat, B.W. van Oudheusden, F. Scarano: Instantaneous planar pressure field determination around a square-section cylinder based on time-resolved stereo-PIV, 14th Int. Symposium on Applications of Laser Techniques to Fluid Mechanics, Lisbon, Portugal, 7-10 July, 2008.
8. A. Ashok , D. Ragni, B.W. van Oudheusden, F. Scarano: Transonic airfoil aerodynamic characterization by means of PIV, 14th Int. Symposium on Applications of Laser Techniques to Fluid Mechanics, Lisbon, Portugal, 7-10 July, 2008.
