Large-scale Particle Image Velocimetry Research Articles

Integrating unmanned aerial systems and LSPIV for rapid, cost-effective stream gauging

Quantifying flow in rivers is fundamental to assessments of water supply, water quality, ecological conditions, hydrological responses to storm events, and geomorphological processes. Image-based surface velocity measurements have shown promise in extending the range of discharge conditions that can be measured in the field. The use of Unmanned Aerial Systems (UAS) in image-based measurements of surface velocities has the potential to expand applications of this method. Thus far, few investigations have assessed this potential by evaluating the accuracy and repeatability of discharge measurements using surface velocities obtained from UAS. This study uses large-scale particle image velocimetry (LSPIV) derived from videos captured by cameras on a UAS and a fixed tripod to obtain discharge measurements at ten different stream locations in Illinois, USA. Discharge values are compared to reference values measured by an acoustic Doppler current profiler, a propeller meter, and established stream gauges. The results demonstrate the effects of UAS flight height, camera steadiness and leveling accuracy, video sampling frequency, and LSPIV interrogation area size on surface velocities, and show that the mean difference between fixed and UAS cameras is less than 10%. Differences between LSPIV-derived and reference discharge values are generally less than 20%, not systematically low or high, and not related to site parameters like channel width or depth, indicating that results are relatively insensitive to camera setup and image processing parameters typically required of LSPIV. The results also show that standard velocity indices (between 0.85 and 0.9) recommended for converting surface velocities to depth-averaged velocities yield reasonable discharge estimates, but are best calibrated at specific sites. The study recommends a basic methodology for LSPIV discharge measurements using UAS that is rapid, cost-efficient, and does not require major preparatory work at a measurement location, pre- and post-processing of imagery, or extensive background in image analysis and PIV.

Application of an Automated Discharge Imaging System and LSPIV during Typhoon Events in Taiwan

An automated discharge imaging system (ADIS), which is a non-intrusive and safe approach, was developed for measuring river flows during flash flood events. ADIS consists of dual cameras to capture complete surface images in the near and far fields. Surface velocities are accurately measured using the Large Scale Particle Image Velocimetry (LSPIV) technique. The stream discharges are then obtained from the depth-averaged velocity (based upon an empirical velocity-index relationship) and cross-section area. The ADIS was deployed at the Yu-Feng gauging station in Shimen Reservoir upper catchment, northern Taiwan. For a rigorous validation, surface velocity measurements were conducted using ADIS/LSPIV and other instruments. In terms of the averaged surface velocity, all of the measured results were in good agreement with small differences, i.e., 0.004 to 0.39 m/s and 0.023 to 0.345 m/s when compared to those from acoustic Doppler current profiler (ADCP) and surface velocity radar (SVR), respectively. The ADIS/LSPIV was further applied to measure surface velocities and discharges during typhoon events (i.e., Chan-Hom, Soudelor, Goni, and Dujuan) in 2015. The measured water level and surface velocity both showed rapid increases due to flash floods. The estimated discharges from ADIS/LSPIV and ADCP were compared, presenting good consistency with correlation coefficient R = 0.996 and normalized root mean square error NRMSE = 7.96%. The results of sensitivity analysis indicate that the components till (τ) and roll (θ) of the camera are most sensitive parameters to affect the surface velocity using ADIS/LSPIV. Overall, the ADIS based upon LSPIV technique effectively measures surface velocities for reliable estimations of river discharges during typhoon events.

Near surface experimental exploration of tornado vortices

Large scale and high-resolution surface pressure and Particle Image Velocimetry measurements were carried out in a new testing chamber, the WindEEE Dome, to investigate the structure of stationary tornado-like vortices and their surface layer over a broad range of swirl ratios and at the highest radial Reynolds numbers ever achieved in laboratory simulations of tornadoes. The large size of vortices simulated in this facility allows for the laboratory resolution needed for near surface exploration.It is shown that at low swirls, tornado vortices are unstable and highly characterized by wandering. A qualitative investigation of the vortex wandering and a preliminary estimation of the effect of vortex wandering on time-averaged pressures are performed. Also, surface pressure data are used to examine the Reynolds number dependency of the flow. A clear demarcation of the surface layer behavior inside versus outside of the vortex core is observed. Normalizing axial profiles of core radii and tangential velocities in the region outside of the vortex core with the core radius and height of the maximum tangential velocity suggests self-similarity of the flow with future implications for empirical modeling. Axial profiles of the radial velocity are used to determine the vortex structure at various swirl ratios.

Exploiting LSPIV to assess debris-flow velocities in the field

Abstract. The assessment of flow velocity has a central role in quantitative analysis of debris flows, both for the characterization of the phenomenology of these processes and for the assessment of related hazards. Large-scale particle image velocimetry (LSPIV) can contribute to the assessment of surface velocity of debris flows, provided that the specific features of these processes (e.g. fast stage variations and particles up to boulder size on the flow surface) are taken into account. Three debris-flow events, each of them consisting of several surges featuring different sediment concentrations, flow stages, and velocities, have been analysed at the inlet of a sediment trap in a stream in the eastern Italian Alps (Gadria Creek). Free software has been employed for preliminary treatment (orthorectification and format conversion) of video-recorded images as well as for LSPIV application. Results show that LSPIV velocities are consistent with manual measurements of the orthorectified imagery and with front velocity measured from the hydrographs in a channel recorded approximately 70 m upstream of the sediment trap. Horizontal turbulence, computed as the standard deviation of the flow directions at a given cross section for a given surge, proved to be correlated with surface velocity and with visually estimated sediment concentration. The study demonstrates the effectiveness of LSPIV in the assessment of surface velocity of debris flows and permit the most crucial aspects to be identified in order to improve the accuracy of debris-flow velocity measurements.

MEASUREMENT OF INUNDATING FLOW FROM A BROKEN ENBANKMENT BY USING VIDEO IMAGES SHOOT FROM A MEDIA HELICOPTER

The heavy rain disaster in the Kinugawa River basin that occurred along with the passage of the Typhoon 18 caused the embankment destruction in the middle reach of the river on September 10, 2015. Due to the overflow, the houses in the vicinity of the embankment collapsed, causing a flood inundation spreading over a wide area. Because the embankment breakwater occurred during the daytime, the state of the inundating flow was recorded from various angles by media helicopters or drones. In this study, we developed a method to extract quantitative flow information from a helicopter video image in which the shooting position and angles are changed one after another, because it was taken in emergency. In the analysis, the images were orthorectified after stabilizing the images, from which surface velocity distributions were measured by image-based technique such as the large-scale particle image velocimetry (LSPIV) or the space-time image velocimetry (STIV). As a result, the time change of water entering from the broken embankment and the total inundated water volume during the disaster were estimated. In addition, two-dimensional surface velocity distributions were analysed to show the spreading of inundated flow.

Applying adaptive LS-PIV with dynamically adjusting detection region approach on the surface velocity measurement of river flow

An image based surface velocity measurement of river flow that applies the adaptive large-scale particle Image velocimetry (LS-PIV) with dynamically adjusting detection region (DADR) approach is presented in this research. The image based measurement is a safe and efficient method. However, the flow conditions, weather or environment changes caused this type of measurement was hard to adapt each situation. In this paper, the DADR method was proposed to determine the better detection region whenever the flow condition changed. The proposed adaptable interrogation area (IA) size determining algorithm was applied to automatically correct the IA size and overcome the lower signal to noise rate (SNR) and higher errors produced by the conventional measurement with the fixed IA under varied environments. Based on the experimental results, our proposed approaches are accurate, robust and adaptable enough to overcome multiple flow and weather conditions presented in this paper.

Streamflow Observations From Cameras: Large‐Scale Particle Image Velocimetry or Particle Tracking Velocimetry?

AbstractImage‐based methodologies, such as large scale particle image velocimetry (LSPIV) and particle tracking velocimetry (PTV), have increased our ability to noninvasively conduct streamflow measurements by affording spatially distributed observations at high temporal resolution. However, progress in optical methodologies has not been paralleled by the implementation of image‐based approaches in environmental monitoring practice. We attribute this fact to the sensitivity of LSPIV, by far the most frequently adopted algorithm, to visibility conditions and to the occurrence of visible surface features. In this work, we test both LSPIV and PTV on a data set of 12 videos captured in a natural stream wherein artificial floaters are homogeneously and continuously deployed. Further, we apply both algorithms to a video of a high flow event on the Tiber River, Rome, Italy. In our application, we propose a modified PTV approach that only takes into account realistic trajectories. Based on our findings, LSPIV largely underestimates surface velocities with respect to PTV in both favorable (12 videos in a natural stream) and adverse (high flow event in the Tiber River) conditions. On the other hand, PTV is in closer agreement than LSPIV with benchmark velocities in both experimental settings. In addition, the accuracy of PTV estimations can be directly related to the transit of physical objects in the field of view, thus providing tangible data for uncertainty evaluation.

Measuring velocity field and heat transfer during natural fire spread over large inclinable bench

This study examines the spread of a natural fire over an inclinable bench for several vegetative fuel-load and slope conditions and on an intermediate scale (18 m2). A pioneering combination of heat flux measurements and a large-scale particle image velocimetry system is presented. Three different fuel loads and two slopes (0° and 30°) are investigated. The results provide new insights into the fluid mechanics of fire spread, elucidating the air entrained flow motion generated by the fire as a function of the fuel load and slope in particular. For horizontal fires, increases in the vertical velocities and the size of the mixing layer, which is formed by the elevation of hot gases in the atmosphere, are observed with increasing fuel load. The air ahead of the fire is aspired far from the fire, suggesting convective cooling of the unburned vegetation. For 30°-upslope fires, the measurements show a convective heat transfer generated by hot gases advancing towards the unburned vegetation, which is supported by the elongation of the fire head. The experimental technique presented in this study is, therefore, effective for extracting valuable data for fire modeling.

Rectification of Image Velocity Results (RIVeR): A simple and user-friendly toolbox for large scale water surface Particle Image Velocimetry (PIV) and Particle Tracking Velocimetry (PTV)

LSPIV (Large Scale Particle Image Velocimetry) and LSPTV (Large Scale Particle Tracking Velocimetry) are used as relatively low-cost and non-intrusive techniques for water-surface velocity analysis and flow discharge measurements in rivers or large-scale hydraulic models. This paper describes a methodology based on state-of-the-art tools (for example, that apply classical PIV/PTV analysis) resulting in large-scale surface-flow characterization according to the first operational version of the RIVeR (Rectification of Image Velocity Results). RIVeR is developed in Matlab and is designed to be user-friendly. RIVeR processes large-scale water-surface characterization such as velocity fields or individual trajectories of floating tracers. This work describes the wide range of application of the techniques for comparing measured surface flows in hydraulic physical models to flow discharge estimates for a wide range of flow events in rivers (for example, low and high flows).

RAPTOR‐UAV: Real‐time particle tracking in rivers using an unmanned aerial vehicle

AbstractRiver system measurement and mapping using UAVs is both lean and agile, with the added advantage of increased safety for the surveying crew. A common parameter of fluvial geomorphological studies is the flow velocity, which is a major driver of sediment behavior. Advances in fluid mechanics now include metrics describing the presence and interaction of coherent structures within a flow field and along its boundaries. These metrics have proven to be useful in studying the complex turbulent flows but require time‐resolved flow field data, which is normally unavailable in geomorphological studies. Contactless UAV‐based velocity measurement provides a new source of velocity field data for measurements of extreme hydrological events at a safe distance, and could allow for measurements of inaccessible areas. Recent works have successfully applied large‐scale particle image velocimetry (LSPIV) using UAVs in rivers, focusing predominantly on surficial flow estimation by tracking intensity differences between georeferenced images. The objective of this work is to introduce a methodology for UAV based real‐time particle tracking in rivers (RAPTOR) in a case study along a short test reach of the Brigach River in the German Black Forest. This methodology allows for large‐scale particle tracking velocimetry (LSPTV) using a combination of floating, infrared light‐emitting particles and a programmable embedded color vision sensor in order to simultaneously detect and track the positions of objects. The main advantage of this approach is its ability to rapidly collect and process the position data, which can be done in real time. The disadvantages are that the method requires the use of specialized light‐emitting particles, which in some cases cannot be retrieved from the investigation area, and that the method returns velocity data in unscaled units of px/s. This work introduces the RAPTOR system with its hardware, data processing workflow, and provides an example of unscaled velocity field estimation using the proposed method. First experiences with the method show that the tracking rate of 50 Hz allows for position estimation with sub‐pixel accuracy, even considering UAV self‐motion. A comparison of the unscaled tracks after Savitzky–Golay filtering shows that although the time‐averaged velocities remain virtually the same, the filter reduces the standard deviation by more than 40% and the maxima by 20%. Copyright © 2017 John Wiley & Sons, Ltd.

PIV experimental study of the large-scale dynamic airflow structures in an aircraft cabin: Swing and oscillation

The dynamic behaviors of airflows, which are common in aircraft cabins with mixing ventilations, significantly influence the thermal sensation of passengers and contaminant transmissions. In this paper, to analyze the large-scale dynamic characteristics in aircraft cabins, a series of long-term large-scale particle image velocimetry (PIV) measurements were performed in an aircraft cabin mockup to provide necessary basic data. Two typical airflow fields, jet zones characterized as large-scale circulations and collision zones featured as interactions of two lateral jets, were selected as the research objects of the dynamic behaviors in the cabin. In collision zones, the temporal characteristics of the swing motions were presented via proper orthogonal decomposition (POD) method, and the swing periods were 25.2 s–29.8 s and 13.0 s–19.8 s under isothermal and cooling conditions, respectively. By analyzing the occurrence probability of swings, the swing-induced contaminant transmission regularity was presented. In jet zones, the dynamic behavior of large-scale circulation was revealed by identifying the circulation center of each instantaneous flow field. However, the center oscillated in a small range near the breath zone of the passenger seating in the middle of one side, and the plumes under cooling conditions further reduced the range, which explained the lock-up phenomenon founded in contaminant transmission studies in the cabin. Finally, several vortex identification methods with raw and reconstructed instantaneous airflow fields were compared, and their practicalities were discussed.

Apply Image Technology to Flow Discharge Estimation

In this paper, an image based approach of the water flow information measurement is proposed. Applying the image based measurement is a safely and efficiently non-contact method. The image based flow discharge estimation method is proposed in this paper. The multi virtual water level probes (MVWLP) method is proposed to estimate water level. This approach mainly applies the color space adjustable technique to reduce noises and uses the adaptive edge detection to extract the water line. Then, the velocity area method (VAM) is applied to estimate flow discharge in this study. The depth-averaged velocity of sampling verticals of the designed transection can be obtained by the image-based surface velocity estimation method denoted large-scale particle image velocimetry (LS-PIV). We convince that the proposed methods are accurate, robust and adaptable enough to overcome multiple conditions.

Object Motion Detection and Data Processing in Large-Scale Particle Image Velocimetry

Large-Scale Particle Image Velocimetry (LSPIV) is a non-intrusive imaging measurement method of the river surface flow velocity. However, in natural environments with the impact of the shadow and s...

Estimating bed shear stress from remotely measured surface turbulent dissipation fields in open channel flows

AbstractSynoptic information on bed shear stress is necessary in predicting the transport of sediments and environmental contaminants in rivers and open channels. Existing methods of estimating bed shear stress typically involve measuring vertical profiles of streamwise velocity or Reynolds stress and taking advantage of the logarithmic or the constant stress region, respectively, to determine friction velocity and subsequently, bed shear stress. While effective, these methods yield local measurements of bed shear stress only. Direct measurements of bed shear stress can also be obtained through measurements with a drag plate. However, this method yields average shear stress information over the area of the plate and is impractical for large‐scale implementation in the field.Here we present a method capable of providing continuous synoptic measurements of bed shear stress over a large field‐of‐view. A series of Large‐Scale Particle Image Velocimetry (LSPIV) and Acoustic Doppler Velocimetry (ADV) measurements were made in a variety of flows generated in a wide‐open channel facility. Turbulent dissipation is calculated on the free surface from the 2‐D LSPIV results and is correlated with near‐surface ADV measurements of turbulent dissipation in the water column. The ADV results are consistent with the Nezu (1977) established relationship for the vertical variation of turbulent dissipation in the water column. Knowledge of the correlation between free‐surface and near‐surface dissipation values coupled with Nezu's (1977) relationship allow a robust and accurate estimate of friction velocity to be made and subsequently, shear stress at the bed can be estimated.

Unseeded large scale PIV measurements corrected for the capillary-gravity wave dynamics

Large scale particle image velocimetry (LSPIV) has become a reliable and accurate technique to measure water surface velocity field in open channels and rivers. The LSPIV technique is based on a camera view framing the water surface, complemented with image-processing methods to compute water surface displacements between consecutive frames. Using LSPIV, high flow velocities, such as flood conditions, are accurately measured, whereas determinations of low flow regimes is more challenging, especially in the absence of floating material carried by the stream. In fact, in unseeded conditions, typical surface feature dynamics must be taken into account. Indeed, besides surface structures advected by the current, capillary-gravity waves propagate on the surface, with their own speed and direction. In this study, the discrimination between these phenomena is discussed, providing a new method to distinguish and to correct unseeded LSPIV measurements with capillary-gravity wave dynamics. Current velocity obtained with such a correction at low flow speed in an artificial channel and in a natural river is in fair agreement with reference observations indicating the need of accounting for the speed and direction of capillary-gravity waves on the surface to obtain reliable measurements of the current vector field, and, as a further result, of the flow discharge over a river cross-section.

HFSB-seeding for large-scale tomographic PIV in wind tunnels

A new system for large-scale tomographic particle image velocimetry in low-speed wind tunnels is presented. The system relies upon the use of sub-millimetre helium-filled soap bubbles as flow tracers, which scatter light with intensity several orders of magnitude higher than micron-sized droplets. With respect to a single bubble generator, the system increases the rate of bubbles emission by means of transient accumulation and rapid release. The governing parameters of the system are identified and discussed, namely the bubbles production rate, the accumulation and release times, the size of the bubble injector and its location with respect to the wind tunnel contraction. The relations between the above parameters, the resulting spatial concentration of tracers and measurement of dynamic spatial range are obtained and discussed. Large-scale experiments are carried out in a large low-speed wind tunnel with 2.85 × 2.85 m2 test section, where a vertical axis wind turbine of 1 m diameter is operated. Time-resolved tomographic PIV measurements are taken over a measurement volume of 40 × 20 × 15 cm3, allowing the quantitative analysis of the tip-vortex structure and dynamical evolution.

Validation of large-scale particle image velocimetry to acquire free-surface flow fields in vegetated rivers

The reliability of large-scale particle image velocimetry (LSPIV) methodology to measure a 2D surface velocity field in a vegetated lowland stream is evaluated. To this end, measurements of the free-surface flow field obtained with LSPIV are compared with measurements with an electromagnetic current meter (ECM) close to the surface at four different locations. The measurements were performed monthly, allowing the evaluation of the LSPIV measurements in relation to different vegetated conditions. The difference observed between the mean velocities measured with ECM and LSPIV remains low in winter, whereas an increase is observed in summer. Inappropriate particle seeding density and unsteadiness of the flow are the main sources of LSPIV reliability reduction. Nonetheless, the seasonal average frequency of reliable LSPIV measurements is 97%, 95% and 78% in winter, spring and summer, respectively. The results illustrate that LSPIV is an inexpensive methodology, which provides high-resolution and reliable data to study the flow-field distribution in vegetated rivers, provided some considerations are taken into account to deal with the added complexity of the vegetation presence and the field conditions.

Investigation of the Effects of Ship Induced Waves on the Littoral Zone with Field Measurements and CFD Modeling

Waves induced by ship movement might be harmful for the habitat in the littoral zone of rivers due to the temporally increasing bed shear stress, the high-energy breaking waves and the consequently related detachment of benthic animals. In order to understand the complex hydrodynamic phenomena resulting from littoral waves, we present the testing of a novel methodology that incorporates field observations and numerical tools. The study is performed at a section of the Danube River in Hungary and analyzes the influence of different ship types. The field methods consist of parallel acoustic measurements (using Acoustic Doppler Velocimetry (ADV)) conducted at the riverbed and Large Scale Particle Image Velocimetry (LSPIV) of the water surface. ADV measurements provided near-bed flow velocities based on which the wave induced currents and local bed shear stress could be estimated. The LSPIV was able to quantify the dynamics of the breaking waves along the bank. Furthermore, computational fluid dynamics (CFD) modeling was successfully applied to simulate the propagation and the breaking of littoral waves. The used techniques complement each other well and their joint application provides an adequate tool to support the improvement of riverine habitats.

A novel permanent gauge-cam station for surface-flow observations on the Tiber River

Abstract. Flow monitoring of riverine environments is crucial for hydrology and hydraulic engineering practice. Besides few experimental implementations, flow gauging relies on local water level and surface-flow velocity measurements through ultrasonic meters and radars. In this paper, we describe a novel permanent gauge-cam station for large-scale and continuous observation of surface flows, based on remote acquisition and calibration of video data. Located on the Tiber River, in the center of Rome, Italy, the station captures 1 min videos every 10 min over an area oriented along the river cross section of up to 20.6 × 15.5 m2. In a feasibility study, we demonstrate that accurate surface-flow velocity estimations can be obtained by analyzing experimental images via particle tracking velocimetry (PTV). In medium illumination conditions (70–75 lux), PTV leads to velocity estimations in close agreement with radar records and is less affected by uneven lighting than large-scale particle image velocimetry. Future efforts will be devoted to the development of a comprehensive test bed infrastructure for investigating the potential of multiple optics-based approaches for surface hydrology.

Photogrammetric discharge monitoring of small tropical mountain rivers: A case study at Rivière des Pluies, Réunion Island

AbstractReliable discharge measurements are indispensable for an effective management of natural water resources and floods. Limitations of classical current meter profiling and stage‐discharge ratings have stimulated the development of more accurate and efficient gauging techniques such as nonintrusive photogrammetric techniques. Despite many successful applications of large‐scale particle image velocimetry (LSPIV) for short‐term measurements during flood events, there are still very few studies that address its use for long‐term monitoring of small mountain rivers. To fill this gap, this study targets the development and testing of largely autonomous photogrammetric discharge measurement system with a special focus on the application to small mountain river with high discharge variability in the tropics. It proposes several enhancements concerning camera calibration, more efficient processing in image geometry, the automatic detection of the water level as well as the statistical calibration and estimation of the discharge from multiple profiles. A case study which comprises the analysis of several thousand videos spanning over 2.5 year is carried out to test the robustness and accuracy of different processing steps. Comparisons against classical current meter profiling show a mean absolute percentage error of 9.0% after the statistical calibration of the system. The study suggests that LSPIV can already be considered as a valuable tool for the monitoring of torrential flows, whereas further research is still needed to fully integrate nighttime observation and stereophotogrammetric capabilities.