On improvement of PIV image interrogation near stationary interfaces

Abstract

In this paper the problem posed by interfaces when present in PIV measurements is addressed. Different image pre-processing, processing and post-processing methodologies with the intention to minimize the interface effects are discussed and assessed using Monte Carlo simulations. Image treatment prior to the correlation process is shown to be incapable of fully removing the effects of the intensity pedestal across the object edge. The inherent assumption of periodicity in the signal causes the FFT-based correlation technique to perform the worst when the correlation window contains a signal truncation. Instead, an extended version of the masking technique introduced by Ronneberger et al. (Proceedings of the 9th international symposium on applications of laser techniques to fluid mechanics, Lisbon, 1998) is able to minimize the interface-correlation, resolving only the particle displacement peak. Once the displacement vector is obtained, the geometric center of the interrogation area is not the correct placement. Instead, the centre of mass position allows an unbiased representation of the wall flow (Usera et al. in Proceedings of the 12th international symposium on applications of laser techniques to fluid mechanics, Lisbon, 2004). The aforementioned concepts have been implemented in an adaptive interrogation methodology (Theunissen et al. in Meas Sci Technol 18:275-287, 2007) where additionally non-isotropic resolution and re-orientation of the correlation windows is applied near the interface, maximizing the wall-normal spatial resolution. The increase in resolution and robustness are demonstrated by application to a set of experimental images of a flat-plate, subsonic, turbulent boundary layer and a hypersonic flow over a double compression ramp.

High-resolution PIV measurements of a transitional shock wave–boundary layer interaction

“…The resulting velocity fields are post-processed with a normalized median filter to remove spurious vectors (Westerweel and Scarano 2005), and vector relocation (Theunissen et al 2008) is performed on vectors for which the interrogation window is overlapping the wall mask.…”

“…The log region can be clearly distinguished in the PIV data, and also the start of the viscous sublayer is resolved. Figure 14 also highlights the need for vector relocation (Theunissen et al 2008) when performing accurate near-wall measurements. Without relocation, the near-wall velocity vectors are placed too close to the wall, and the resulting velocity is biased towards a higher value.…”

On image pre-processing for PIV of single- and two-phase flows over reflecting objects

“…5. The problem of elimination of particles with a displacement smaller than one particle diameter by the image subtraction algorithm (Theunissen et al 2008) is not applicable here, since the magnification combined with the size of the applied interrogation area is not detailed enough to resolve the boundary layer flow around the spacer.…”

“…Laser intensity can vary between images or image pairs due to differences in the two lasers (in case of a double-pulsed YAG laser), objects or bubbles can introduce strong reflections of light, and/or reflection from channel walls in confined flows introduce glow. Several approaches are available in literature to tackle these problems (Seol and Socolofsky 2008;Lindken and Merzkirch 2002;Honkanen and Nobach 2005;Westerweel 1993;Shavit et al 2007;Theunissen et al 2008). In the presence of static objects or in two-phase flow, fluorescent particles and a color filter are often used to avoid interference of the object edges in the correlation map.…”

“…Since this approach also reduces the relative intensity of bright objects (bubble or object reflections) compared to the particle intensities, the relative contribution of these objects in the correlation function is also reduced (Shavit et al 2007). Combining image normalization with background subtraction solves the problems of temporal and spatial variation in the intensity distribution, as discussed by Theunissen et al (2008) recently.…”