CFD versus PIV
Computational Fluid Dynamics (CFD) is playing an increasingly important role in today’s maritime industry. Report explains MARIN’s efforts to improve quality levels and outlines the next steps towards full 3D flow understanding.
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Figure 1: Tuft observation (above) and calculation (below); indication of flow separation
owadays, more complex flows are being calculated and these await comparison and validation with
measured data. Guaranteeing and increasing the quality of calculated and measured results becomes an even bigger challenge. For years qualitative data has been obtained from flow visualisation tests using tufts (Figure 1), paint-smearing and dye injection, followed by quantitative flow data from Pitot tube measurements. For general com- parison with CFD such techniques suffice. However, these methods have limited applicability and partly obstruct the flow. A flexible system that copes with complex geometries but maintains and eventually increases the quality level, is needed. Therefore, MARIN has taken the next step in the use of Particle Image Velocimetry (PIV).
Christian Veldhuis
c.veldhuis@
marin.nl
PIV PIV is a method to determine the ve- locities in a fluid in an optical, non-invasive way. The flow measurement with PIV is based on measurements of the displacement of a particle in a target plane between two successive light pulses with a time delay. The flow is seeded with micrometre-sized particles and the target plane is illuminated with a laser light sheet. Two digital cameras record the particle positions. Special image- processing software analyses the movements of particles in subsections of the PIV image using correlation techniques. By using two
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Refit “Queen Elizabeth 2”
cameras in a stereoscopic arrangement the instantaneous three-velocity components are derived in the measuring plane.
PIV and CFD comparison For the flow around a twin-screw, open-shaft vessel PIV measurements and steady viscous flow calculations (RANS using a SST k-omega turbulence model) have been performed. For the measurements the model was equipped with a full shaft line arrangement with I and V-brackets. Measurements have been carried out at several locations along the shaft line at port and starboard (Figure 2). In the calculation no shaft rotation was mod- elled and shaft brackets were disregarded because when nicely aligned, no significant effect is expected. Here we focus on the wake field at port side. Figure 3 shows the comparison of the axial velocity field with and without, shaft rotation in the measurement. The distinct “wake peak” between shaft and hull caused by the shaft shadow is clearly visible. The thickness of the boundary layer is nicely captured and the agreement between PIV and CFD is good. PIV facilitates a higher spatial resolution compared to Pitot tube measurements, so more flow details can be captured. The influence of the inward rotating shaft is clearly visible and the effect of the V-bracket (yellow-orange contour level) is
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Start Trials & Monitoring activities
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