Large scale PIV applied to flow interaction downstream a semi-open barrier

Abstract

Large scale Particle Image Velocimetry (PIV) can be of great use to quantify the surface flow velocities of highly turbulent environmental flows. Preliminary results show that the turbulent flow scales of a jet downstream a semi-open barrier can be quantified using unseeded PIV. The technique, executed with a digital camera, can produce sufficiently accurate estimates of surface velocity and length scales of turbulent structures up to at least 50 m downstream the barrier.

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4th International Symposium of Shallow Flows, Eindhoven University of Technology, NL, 26-28 June 2017
Large scale PIV applied to flow interaction downstream a semi-open barrier
M.C. Verbeek1, R.J. Labeur1, and W.S.J. Uijttewaal1
1Department of Civil Engineering, Delft University of Technology
Email: m.c.verbeek@tudelft.nl
Keywords: Large scale PIV, flow scale interaction, vortex shedding, Horizontal Axis Tidal Turbines
Abstract
Large scale Particle Image Velocimetry (PIV) can be of great use to quantify the surface flow velocities of highly
turbulent environmental flows. Preliminary results show that the turbulent flow scales of a jet downstream a semi-open
barrier can be quantified using unseeded PIV. The technique, executed with a digital camera, can produce sufficiently
accurate estimates of surface velocity and length scales of turbulent structures up to at least 50 m downstream the barrier.
1. Introduction
Electric power generation from tidal currents can play a substantial role to reach carbon reduction targets in Europe. In
areas within relatively low tidal elevation differences Horizontal Axis Tidal Turbines (HATTs) can be deployed in gates or
semi-open barriers. Vital to the development of these renewable power stations is the insight in the hydro-environmental
impacts (Adcock et al., 2015; Laws and Epps, 2016), not at least because they are often deployed in high-value nature
areas along the coast.
M
Roompot 8
Roompot 9
North Sea Eastern Scheldt
R.N
R.1
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.
.
R.2
.
.
.
Roompot 7
Figure 1: (left) Top view of field location and (right) un-calibrated image obtained from location M.
In 2015, five HATTs (1.25MW in total) were installed in one of the gates of the Eastern Scheldt storm surge barrier.
The HATT wakes may influence the jet downstream the gate.
Turbulent jets have large centre velocities and show lateral velocity gradients that induce the entrainment of low-momentum
from the ambient flow. This induces turbulent fluctuations that are organised in periodic vortex structures. This period
vortex shedding occurs also downstream the neighbouring barrier pillars of the Eastern Scheldt barrier. When HATTs are
deployed in one of the gates, this pattern is alternated. A HATT array produces high frequent bound and free vortices that
induced additional mixing in the jet. The vortices interact with the low period structures that are shed from the barrier and
produce a complex flow pattern where several physical flow scales are present.
We discuss the application of large scale PIV to a highly turbulent environmental flow downstream a semi-open barrier
without the use of tracers. The first results show that PIV is a promising method to identify both velocities and length
scales of the turbulent structures present.

4th International Symposium of Shallow Flows, Eindhoven University of Technology, NL, 26-28 June 2017
2. Method
At October 25 (2016), the flow downstream the 8th Roompot gate of the Eastern Scheldt barrier was monitored using
a high resolution digital camera at 24Hz (Fig. 1). During the full day the five HATT devices in this gate were in operation.
The uncalibrated camera images are distorted by the lens convexity, internal camera parameters and the view angle. The
lens distortion is corrected using the Matlab Computer Vision System Toolbox. Successively, the routine as reported by
De Vries et al.(2011) is followed to calibrate the images. The image coordinates are transformed to earth coordinates
using a projection matrix which contains both the internal and external camera parameters. The external parameters,
namely the tilt, roll and azimuth of the device, are solved for with a least square fit of more than 15 ground control points
in the image view (the close locations are visible in Fig. 1 as R1-RN). The mean re-projection error amounted 1.9 px for
an 8.3·106px image.
The velocity field is calculated using the PIV-lab software (Thielicke et al.,2014). A cross-correlation optimization
routine matches the pattern in a predefined frame using a Gaussian for the peak fitting. The frame sizes of analysis
matched the size of the propagating flow structures in the images (up to 0.5 m). To validate the obtained velocity vectors
the 16Hz Nortek ADCP data of Tocardo International BV is used. The ADCPs measure horizontal velocities at three
lateral positions at a depth of -3 m NAP 0 m to 25 m up and downstream of the analysed barrier gate.
Figure 2: (left) Calibrated image where P8, P9 and HATT 1-5 denote the upstream location of the pillars and HATTs
respectively, (right) the 5-min averaged flow velocity field at end of the tidal flood phase obtained with PIV (10:05-
10:10UTC+1, flow is to the right).
3. Results
The obtained surface velocity vectors match within a 0.5 ms−1range to the ADCP-measurements, which can mostly
be attributed to the 5 m depth difference between the PIV and ADCP-measurements.
The calculated velocity vectors amounts to approximately 1.5 ms−1in jets of the neighbouring gates and and approxi-
mately 1 ms−1in the wake of the pillars (see Fig. 2). In a time sequence of the vectors the shedding process of the large
vortex structures from the barrier pillars (Fig. 3) can be observed. The length scales can be separated from the smaller
fluctuations downstream the HATTs (at the centre of Fig. 3). Future work will focus on the characterization of these differ-
ent vortex structures resulting from both the barrier and the HATTs and identify their length scales and turbulent transports.
4. Conclusion
PIV was applied to a unseeded highly turbulent environmental jet downstream a semi-open barrier with Horizontal
Axis Tidal Turbines (HATTs). The first results show that PIV is a promising method to identify both velocities and length
scales. The turbulent structures resulting from pillar vortex shedding and the HATTs are captured by the PIV-algorithm.

4th International Symposium of Shallow Flows, Eindhoven University of Technology, NL, 26-28 June 2017
Figure 3: a-c) Time sequence of 3-second averaged flow velocity. A large vortex (indicated with the yellow arrow) is shed
from the upper pillar. Smaller structures are present in the flow downstream the HATTs (in the middle).
Acknowledgements
This research is funded by the Netherlands Organisation for Scientific Research (NWO) and partners within The New
Delta project.
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