This paper presents the application, and implications of line arrays of light emitting diodes (LEDs) as an illumination source for planar particle image velocimetry (PIV) measurements in large-scale hydraulic laboratories. The use of class 4 lasers, commonly applied as illumination source in these PIV measurements, requires strict safety precautions (i.e. to prevent safety hazards for people working in their vicinity), specifically when applied at hydraulic experimental setups that are not located in a laser lab. To examine the applicability of an alternative light source, differences between LED- and laser-based illumination for PIV are analyzed. A theoretical analysis, in which a so-called motion blur parameter ((Formula presented) ) is introduced, shows that for moderate flow velocities in large-scale setups, image blur can be avoided, even for relatively long (millisecond) pulse widths. The light sheet thickness, width and intensity of a pulsed laser and a line array of both continuous and pulsed LEDs are measured and compared. Based on these properties a safety assessment is made, from which it is concluded that the application of arrays of LEDs for PIV measurements applied in liquid flows requires significantly less safety precautions than in case a class 4 PIV laser is used. Planar (2D) PIV measurements have been performed with both a pulsed and a continuous LED as light source for two testcases in large hydraulic scale-models. Time-averaged velocity field results from the LED-based PIV measurements show good resemblance to both PIV measurements obtained with a class 4 laser as well as to pitot tube measurements. It is shown that the time-averaged PIV vector fields are influenced by motion blur, resulting in a distinct bias towards smaller velocities when increasing motion blur. The two testcases show that linear LED arrays can serve as a suitable alternative illumination source for planar PIV measurements in large-scale hydraulic laboratories in case motion blur remains limited. Specifically, LED line arrays are considered useful for the time-average quantification of predominantly 2D, low to moderate flows in a relatively large domain.

Surge vessels are important for pipeline system resilience, especially for large drinking water mains. Optimal modeling of the heat transfer in these vessels in, often low-dimensional, hydraulic models can enable an optimal design and reduce material usage. In this work, Large Eddy Simulation (LES) of heat transfer for air expansion in surge vessels is performed and a 0D numerical model is devised. The flow as found in the LES is elaborately described. Four cases (one small scale and three full scale) are analyzed. The LES simulations and the 0D model agree very well for the volume average air temperature, hinting at relatively conventional convective heat transfer at the vessel walls, despite downflow and stratification in the bulk. A remarkable observation regarding the behavior of the polytropic number with time is made. Furthermore, the temperature is compared to experimental measurements for two different vessels. The comparison with the experimental data for the first vessel shows a large difference in temperature at sensor locations, whereas the comparison with the experimental data for the second vessel agrees up to 10% of the temperature drop, where both numerical models (the LES and the 0D model) predict a lower temperature than the physical measurement indicates. The differences are discussed. Future work will focus on modeling condensation.

Modelling of the initial pressure pulse amplitudes due to water hammer is well known, but including the modelling of the damping due to the wall friction and dissipation in the fluid in subsequent pulses is less straightforward. Some analytical solutions and detailed measurements exist for impulsively accelerating flow, while measurements and highly detailed simulations exist for impulsively decelerating turbulent flow in straight pipes. The authors are not aware of any detailed measurements of instantaneous velocity fields in a straight pipe for typical water hammer events. Measurements of this kind will enable validation of detailed flow simulations that might become attainable on short term and comparison with detailed analytical models. The authors recently presented a new facility for detailed flow field measurements of water hammer in a pipe (2). In this paper we present the quantification of the steady state flow and turbulence in the mentioned facility using planar Particle Image Velocimetry (PIV). We apply a calibration based on optical ray tracing analysis to the PIV data and present measurements up to a shear Reynolds number of Reτ = 827 and a spatial resolution in wall normal direction of at least Δy⁺ = 7.1, as found via a PIV analysis using non-square interrogation areas of 8 × 32 pixel. The presented PIV method and calibration lead to a match with literature data for steady flow and turbulence of up to 1.8% in turbulent normal stress (u^′+) peak height. The difference can likely be explained by the slightly lower value of Re_t than that for the reference case. This will enable the detailed study of the dissipative near-wall behavior and also of turbulent vortical interactions further from the wall in the logarithmic region to aid the understanding of the dissipation in impulsive water hammer flows in pipes.

We present a facility for measurements of transient flow behavior due to rapid valve closure at initial Reynolds numbers up to Re τ = 1237. The facility allows the deployment of non-intrusive laser based diagnostic techniques such as Particle Image Velocimetry, Stereo Particle Image Velocimetry and Laser Doppler Anemometry. Steady state planar PIV and pressure drop measurements have been performed as well as planar PIV and absolute pressure measurements of the flow following rapid valve closure. The steady state measurements indicate that a clear logarithmic layer is present for the measured conditions at Re τ > 344 with a von-Kármán constant of k = 0.40. Resulting velocity and vorticity fields for an example transient case show the rich flow physics, including generation and advection of vorticity.

Bubble screens are used at sea locks to mitigate salt intrusion into inland water systems. In this paper the effectiveness of a bubble screen in delaying the mixing of salt and freshwater via lock exchange was studied. Laboratory-scale experiments investigating the flow field and mixing caused by a bubble screen are presented. The tests include both the homogeneous situation of freshwater on both sides of the screen and the inhomogeneous situation where there is an initial density difference across the screen, which leads to a density current after the lock gate is removed or opened. Optical measurement techniques were applied, giving spatially detailed flow velocities and densities. The parameters varied between tests are the airflow discharge and the bubble size. The results show that the bubble size in the screen had a significant effect with a screen with bubbles of 1-2 mm being more effective at generating a surface flow in the homogeneous case but less effective at keeping the fresh and salt sides separated in the inhomogeneous case, when compared with a screen of 4-6 mm bubbles. The point of maximum effectiveness for separating salt and fresh sides was also shown to be dependent on bubble size.

In this paper are presented PIV measurements of turbulent pipe flow at bulk Reynolds numbers Re _D between 3.4 × 10 ⁵ and 6.9 × 10 ⁵ . So-called single-pixel correlation is applied that yields a superior spatial resolution that is slightly larger than the equivalent size of a pixel in the flow. The location and shape of the averaged correlation peak give the mean velocity and normal and Reynolds stresses. A novel aspect of the single-pixel correlation approach is the extension to determine the 2-point spatial correlation of the velocity fluctuations and the spectrum of the longitudinal velocity fluctuations. Detailed results are presented for Re _D = 4.98 × 10 ⁵ , corresponding to a shear Reynolds number Re _τ = 10.3 × 10 ³ , with a spatial resolution in wall units of Δy⁺ = 19.

Spatio-temporal wave patterns due to wave field–structure interaction can be very complex to measure and analyze when using (intrusive) point probes. Free-surface field measurements can offer much needed insight in this domain. Nevertheless, to the best of the authors’ knowledge, these methods are rarely used in experimental offshore engineering and research. In these fields, typical domain sizes are at least in the order of several m², whereas (optical) free-surface field measurements are often not performed in domains with dimensions larger than roughly 0.5 × 0.5 m². In the current work, the optical free-surface measurement technique named Free-Surface Synthetic Schlieren (FS-SS) is applied to measure the interaction between an incident wave field and a surface piercing cylinder, or monopile, in a domain of several m² for the first time. The FS-SS method is validated for wave fields with wavelengths λ ≪ L to λ ≫ L, where L is the domain size in the direction of wave propagation. It is found that the incorporation of an additional water level measurement improves the agreement between intrusive wave height meters and the FS-SS measurement for large wavelengths (λ / L> 0.5) as compared to assuming a zero-mean free-surface. Wave field–monopile interaction is measured for two values of D/ λ: D/ λ= 0.1 and D/ λ= 0.2 , where D is the monopile diameter. For the case D/ λ= 0.2 the interaction wave field is analyzed by subtracting the measured wave field in the absence of a structure, from the measured interaction wave fields. The measured difference wave field reveals many interaction phenomena such as locations of amplification, both near the monopile and further away, a wake that has certain similarities with a Kelvin wake, and a circular small wavelength diffraction pattern. Additionally, the embedding of the measurements in a wave breaking regime map is presented. In this map, the applicability for certain wave conditions can be clearly visualized. It is concluded that the FS-SS method, including the proposed improvement using additional sensor data, is a useful addition to the toolbox of hydraulic engineers and researchers, and that especially the measured locations of wave amplification in the far field will not be easily detected using (arrays of) point probes.

Abstract: Ultrasound imaging velocimetry (UIV) refers to the technique wherein ultrasound images are analysed with 2D cross-correlation techniques developed originally in the framework of particle image velocimetry. Applying UIV to opaque, particle-laden multiphase flows have long been considered to be an attractive prospect. In this study, we demonstrate how fundamental differences in acoustical imaging, as compared to optical imaging, manifest themselves in the 2D cross-correlation analysis. A chief point of departure from conventional particle image velocimetry is the strong variation in the intensity profile of the acoustic wavefield, primarily caused by the attenuation of ultrasonic waves in particle-laden flows. Attenuation necessitates using a larger ensemble of correlation planes to obtain satisfactory time-averaged velocity profiles. For a given combination of imaging and flow conditions, attenuation sets upper limits on volume fraction, penetration depth, as well as temporal resolutions that may be accessed confidently. This behaviour is demonstrated in two experimental datasets and is also supported by a modified cross-correlation theory. The modification is brought about by incorporating a lumped model of ultrasonic backscattering in suspensions into existing spatial cross-correlation analysis. The two experimental datasets correspond to two distinct particle-laden pipe flows: (1) a neutrally buoyant non-Brownian suspension in a laboratory-scale flow facility, wherein particle sizes are comparable to the ultrasonic wavelength and (2) a non-Newtonian slurry in an industrial-scale flow facility, wherein particle sizes are much smaller than the ultrasonic wavelength. We illustrate how and to what extent correlation averaging can counteract the adversity caused by attenuation. The work herein offers a template for one to evaluate the performance of UIV in particle-laden flows. Graphical abstract: [Figure not available: see fulltext.].

Salt water intrusion through the New Sea Lock of IJmuiden, Netherlands requires mitigation to ensure availability of enough fresh water further inland. For this purpose, a salt screen has been proposed for selective withdrawal of salt water from the Noordzeekanaal in the vicinity of the lock complex. Formulas to assess the withdrawal rate of selective withdrawal are based on idealized layouts and conditions. In the case of IJmuiden, the flow surrounding a salt screen has a strong nonuniform character, such that these formulas are not applicable to predict the correct withdrawal rate and the effectiveness of selective withdrawal accurately. In this case physical scale modeling or computational fluid dynamics (CFD) modeling can be applied. This article discusses the limitations of the formulas for a three-dimensional (3D) flow application near the locks of IJmuiden and presents the use of CFD and physical scale model research to assess the flow patterns around the salt screen and the effectiveness of selective withdrawal. The CFD model was validated against the physical scale model and represented the complex flow fields around the salt screen to within acceptable deviations for both steady and transient states. This gives confidence in applying these more advanced modeling tools for the design and positioning of salt screens in confined complex 3D flow areas.

In this paper we present PIV measurements of turbulent pipe flow at Reynolds numbers between 3.4×10⁵ and 6.9×10⁵. We apply a so-called 'single-pixel correlation' that yields a superior spatial resolution (Westerweel et al., 2004). We use the location and shape of the averaged correlation peak to obtain the mean velocity and normal and Reynolds stresses (Scharnowski et al., 2012). A novel aspect of the single-pixel correlation approach is the extension to determine the spatial correlation of the velocity fluctuations. In this paper we present the results for Re = 4.98×10⁵, corresponding to a shear Reynolds number Re_τ = 10.3×10³, with a spatial resolution of ∆y⁺ = 18.

In this paper the development of a high-power pulsed LED line light and its use to apply particle image velocimetry (PIV) during wave impact measurements are described. An electrical circuit that generates high-current pulses is designed and built, which is used to overdrive a number of commercially available LEDs. The limit for this overdrive-capacity is determined as function of pulse duration for various commercial available LEDs. Two systems of cylindrical convex lenses are designed to act as a collimator and reduce divergence of the LED bundle and the resulting light sheet properties (maximum light intensity and sheet thickness) are investigated. An array of LEDs of 60 cm length (referred to as the LED line light) is designed and manufactured. For the two lens systems, the LED line light provides proper light sheet conditions to illuminate measurement regions in the order of either 0.3 × 0.3 m ², or 1 × 1 m ², at a sufficiently constant light sheet thickness of 5 mm. The application of the LED line light is demonstrated by quantifying the instantaneous flow field of a wave impacting on a blunt object in a wave flume. PIV measurements are conducted at an acquisition rate of 25 frame pairs per second, quantifying maximum flow velocities in the order of 1.0 m s ^-1 at a LED pulse width of 200 µs. The system, consisting of the LED line light, a CMOS camera and open source PIV processing software provides the possibility to perform 2D planar PIV measurements for a fraction of the costs of a commercially available laser based PIV system.

In order to analyse the flow characteristics of free-surface vortexes and to validate the Burgers vortex model by using stereo particle image velocimetry, experiments are conducted in a 600 mm diameter vortex tank. Measured axial velocities indicate that 10–25% of the flow is transported through the vortex core. The velocity profiles show that the axial flow is concentrated in a domain bounded by two times the core radius. Despite Burgers’ assumption of radially independent axial velocity profiles, the model quantifies the tangential velocity profile within a relative uncertainty of circa 10%. The measurements show that it seems valid to use Burgers’ model to obtain an estimate for the core radius by taking the average axial velocity over a radial domain of approximately 2.2 times the core radius. The Burgers model quantifies the air core depth with an uncertainty of 20% relative to the measurements. When compared with the magnitude of vorticity diffusion by molecular viscosity, the experiments show that there is no significant diffusion by radial turbulence.