Large-Scale Particle Image Velocimetry – a Reliable Tool for Modeling Free-Surface Flows

Investigators

M. Muste, A. Kruger, and A. Bradley

Background

Large-scale particle image velocimetry (LSPIV) is an extension of conventional PIV for velocity measurements in large-scale flows (Fujita et al., 1998).  While the image- and data-processing algorithms are similar to those used in conventional PIV, adjustments are required for illumination, seeding, and pre-processing of the recorded images.   The adjustments are mainly connected with the large imaged areas and the oblique angle used for imaging the flows.

Case study

              LSPIV experiments were conducted in conjunction with a 1:16 scale model shown in Figure 1.  Flow features in the model were extensively documented by Acoustic Doppler Velocimeter measurements (Nakato et al., 1999) and numerical simulations (Huang et al., 2000).

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Figure 1.  Layout of the model: a) model photo; b) model bathymetry.

LSPIV Setup

A digital camera connected to a video-cassette recorder was used to record images.  Two camera positions, as shown in Figure 2, were used for measurements (the size of the imaged areas was approximately 3.4 ´ 14.3 m and 3.4 ´ 19.2 for the upstream and downstream reach, respectively.  The seeding material consisted of biodegradable and harmless for the environment foam peanuts (95% cornstarch with synthetic additives).  Transformation of the images to remove perspective distortion as well as the subsequent image processing were conducted on a PC using in-house developed software (Fujita et al., 1998).

Figure 2. The experimental arrangement.

Results

LSPIV free-surface measurements overlaid on the model image are shown in Figure 3.

Figure 3.  Free-surface velocity over the model area

Validation of the accuracy of LSPIV measurements was made by comparing them with ADV measurements and results of numerical simulations.  This comparison is shown in Figure 4.

The difference between LSPIV and ADV measurements and numerical simulation results are reflecting the backwater effect present in the vicinity of the tailgate (not accounted for in the simulations).

 Figure 4.  Comparison of LSPIV, ADV, and numerical simulation velocities at 0.6 of the flow depth for a cross-section (prototype dimensions). 

LSPIV provides instantaneous whole-field vector fields covering large flow areas while most of the existing laboratory measurement instruments are local, i.e., measurements are made in point or along a line (profilers).  Besides velocities, LSPIV can provide spatial and temporal flow features of the flow (flow recirculation, flow-structure interaction).  LSPIV used in conjunction with bathymetry information and assumed velocity distribution over the depth can estimate flow discharge in the modeled flow.  The seeding used for tracing the flow has minimum interaction with the underlying flow, which makes the technique practically non-intrusive.  The technique is fully digital facilitating handling, storing and visualization of the measurements.  The technique is user-friendly because the raw information is flow images, easily to be interpreted by the investigator. 

Future Plans

Current efforts are directed to implementation of the technique for field conditions and additions of hardware components to make LSPIV a real-time measurement tool.

References

Fujita, I., Muste, M. and Kruger, A. (1998). “Large-Scale Particle Image Velocimetry for Flow Analysis in Hydraulic Applications,” J. Hydr. Res., 36(3), pp. 397-414.

Huang, J., Patel, V.C., and Lai, Y.G. (2000).  “A 3D PISO-Based Navier-Stokes Equation Solver for River Flows: Part II. Application to a Reach of the Chattahoochee River,” Proceedings of the 4^th Hydroinformatics International Conference, Iowa City, IA.

Muste, M., Xiong, Z., Bradley, A., and Kruger, A. (2000).  “Large-Scale Particle Image Velocimetry – a Reliable Tool for Physical Modeling,” Proceedings of ASCE 2000 Joint Conference on Water Resources Engineering and Water Resources Planning & Management, Minneapolis, MN.

Nakato, T., De Jong, D., and Odgaard, J. (1999).  “Hydraulic Model Study of Dekalb County Raw Water Intake on the Chattahoochee River, Located Near the Holcomb Bridge, Norcross, Georgia,” IIHR Limited Distribution Report No. 281, IIHR, Iowa City, IA.