The present study experimentally investigates the onset of ventilation of surface-piercing hydrofoils. Under steady-state conditions, the depth-based Froude number Fr and the angle of attack α define regions in which distinct flow regimes are either locally or globally stable. To map the boundary between these stability regions, the parameter space (α,Fr) was systematically surveyed by increasing α until the onset of ventilation while maintaining a constant Fr. Two simplified model hydrofoils were examined: a semi-ogive with a blunt trailing edge and a modified NACA 0010-34. Tests were conducted in a towing tank under quasi-steady-state conditions for aspect ratios of 1.0 and 1.5, and for Fr ranging from 0.5 to 2.5. Ventilation occurred spontaneously for all test conditions as α increased. Three distinct trigger mechanisms were identified: nose, tail and base ventilation. Nose ventilation is prevalent at Fr<1.0 and Fr<1.25 for aspect ratios of 1.0 and 1.5, respectively, and is associated with an increase in the inception angle of attack. Tail ventilation becomes prevalent at higher Fr, and the inception angle of attack exhibits a negative trend. Base ventilation was only observed for the semi-ogive profile, but it did not lead to the development of a stable ventilated cavity. Notably, the measurements indicate that the boundary between bistable and globally stable regions is not uniform and extends to significantly higher α than previously estimated. A revised stability map is proposed to reconcile previously published and current data, demonstrating how two alternative paths to a steady-state condition can lead to different flow regimes.

Predicting unsteady loads on plate-like objects during unsteady motion is important in many applications, such as ship manoeuvring, flight and biological propulsion. The drag force on a starting plate that moves normal to its surface can be severely underestimated during the acceleration phase when conventional methods are used to incorporate the effects of acceleration. These methods often introduce an inviscid added mass force that has its origin in potential flow. However, the flow field around a starting plate quickly diverges from potential flow after the start of the motion due to the continuous creation of vorticity at the plate surface. Following the concept of drag by Burgers (1921 Proc. K. Ned. Akad. Wet. 23, 774–782), we propose a model to predict the creation of vorticity on the plate surface and its advection into the vortex loop at the plate edges, based on Stokes’ first problem. This model shows that the acceleration drag force is a history force, in contrast to the inviscid added mass force that is proportional to the instantaneous acceleration of the plate. We perform experiments on starting plates over a large range of accelerations, velocities, fluid viscosities and plate geometries for which the model gives accurate predictions for the drag force during acceleration and during the relaxation phase immediately after the acceleration ceases. This model is extended to also predict the drag forces on accelerating plates during a starting motion with a non-constant acceleration.

In tribonucleation, a liquid-to-gas phase transition induced by a local pressure drop (cavitation) is highly undesirable, as it causes surface erosion and noise. A paradigmatic flow characteristic of tribonucleation problems is the flow between two coaxial disks. The flow is produced by the rapid upward movement of the top disk, which is initially at rest and in contact with the bottom disk. An analytical model, the so-called negative squeeze film, is typically used to predict the flow in the gap between the disks in this class of problems. Such a model considers an azimuthally uniform inflow in the gap between the disks. In this study, we experimentally show that if a negligibly small misalignment between the axes of the two disks is introduced, the inflow is not azimuthally uniform as expected from the negative squeeze film, but an entry jet appears in the flow between the disks. This entry jet is associated with the formation of two counter-rotating vortices. From reconstructing the pressure field from PIV velocity data in the vortex regions, we find that the local pressure is lower than the vapor pressure. This indicates that the gaseous phase in the cores of the vortices, which is observed from shadowgraphy visualizations in our study, should be attributed to cavitation. The negative-squeeze-film model, however, largely fails to predict the minimum pressure. Therefore, the onset of cavitation is not correctly captured by the analytical model.

Measurement of three-dimensional velocity field of an opaque material (foamed gypsum slurry) flowing under a roller is explored using a PIV surface-tracking technique employoing two types of software. The foamed slurry was deposited on a moving belt pulling it under a rotating roller. The case of the water-to-stucco ratio (WSR) of 75 was studied at 0.19 wt%, and 1.86 wt% of foam added. The cases of roller co-rotation with the belt, no rotation, and counter-rotation were explored. The effect of the added foam on widening of the slurry layer on a roller was also studied. Particle Image Velocimetry (PIV) was used to measure the surface velocity flow field in both top and side views. A significant rejection flow of slurry before the roller was observed in some cases, with its severity varying with the roller’s rotating direction, its angular speed, as well as the foam content. One of the main aims of the present work is in the comparison of two PIV software: PIVlab (Matlab source, Germany) and PIVware (provided by TUDelft).

Fluids at supercritical pressures exhibit large variations in density near the pseudo-critical line, such that buoyancy plays a crucial role in their fluid dynamics. Here, we experimentally investigate heat transfer and turbulence in horizontal hydrodynamically developed channel flows of carbon dioxide at 88.5 bar and 32.6∘C, heated at either the top or bottom surface to induce a strong vertical density gradient. In order to visualise the flow and evaluate its heat transfer, shadowgraphy is used concurrently with surface temperature measurements. With moderate heating, the flow is found to strongly stratify for both heating configurations, with bulk Richardson numbers Ri reaching up to 100. When the carbon dioxide is heated from the bottom upwards, the resulting unstably stratified flow is found to be dominated by the increasingly prevalent secondary motion of thermal plumes, enhancing vertical mixing and progressively improving heat transfer compared with a neutrally buoyant setting. Conversely, stable stratification, induced by heating from the top, suppresses the vertical motion, leading to deteriorated heat transfer that becomes invariant to the Reynolds number. The optical results provide novel insights into the complex dynamics of the directionally dependent heat transfer in the near-pseudo-critical region. These insights contribute to the reliable design of heat exchangers with highly property-variant fluids, which are critical for the decarbonisation of power and industrial heat. However, the results also highlight the need for further progress in the development of experimental techniques to generate reliable reference data for a broader range of non-ideal supercritical conditions.

An experimental setup was designed and built to explore the spreading of a slurry layer during deposition on a moving belt before a rotating roller. The case of the water-to-stucco ratio of 75 was studied. The roller could be at rest (no rotation) or in co-rotation and counter-rotation compared to the directional motion of the spreading plastic belt (parchment paper). The widening of the slurry layer was measured and compared with the predictions of the theory developed previously by the present group, and the predicted maximum width reasonably agreed with the experimental observations. Particle image velocimetry was used to measure the velocity field at the surface of the slurry layer in top and side views. A flow pattern was observed before the slurry bypassed under the roller, in the domain where variations in entrainment led to different surface profiles. Depending on the roller's rotating direction and speed, the slurry was either more effectively drawn under the roller, forming a mild ridge, or less effectively entrained, resulting in a pronounced ridge.

Interferometric particle imaging (IPI) is used to measure both the size distribution and concentration of microbubbles (with a diameter less than 100 micron) in water. Using a new method for calibration makes it possible to obtain quantitative results for the concentration of microbubbles. The results are validated using imaging with a long-range microscope shadowgraph (LMS). Estimates of the size distribution and concentration from both IPI and LMS agree within uncertainty limits. The relative uncertainty in the IPI concentration estimation is about 10% and is mostly due to the finite number of detected bubbles. It is shown that the performance of the bubble-image detection algorithm needs to be quantified to obtain a reliable estimate of the concentration obtained with IPI.

Wave-structure interactions of flexible membrane-type materials are an emerging research field, driven by their potential in renewable energy and breakwater concepts. This study proposes stereoscopic digital image correlation (DIC) as a scalable method for spatiotemporal measurements of fluid-structure interactions in wave tanks. The scalability is presented by two setups with domain dimensions ranging from O(10−1 m) to O(101 m). The calibrations of 5 adjacent and synchronized stereoscopic camera pairs are projected on a common frame of reference to cover the large domain. The presented methodology includes suggestions on the calibration method, and a practical speckle application technique is proposed. The benefits of the method are highlighted by the preliminary indication of a dynamic scaling law for wave-structure interactions. This work can serve as a foundation for further development and application of stereoscopic DIC for such structures. It is expected that this large domain method will contribute to further physical understanding of the fluid-structure interactions of large floating structures in waves.

This paper presents a microfluidic approach that dynamically controls the hydrodynamic flow and the streamlines to enable complex multi-particle manipulations within a single device. The approach combines the design of a flow-through microfluidic Hele-Shaw flow cell together with an optimization procedure to find a priori optimal particle pathlines, and an effective proportional-integral-derivative (PID) feedback controller to provide real-time control over the particle manipulations. In the device, particles are manipulated with hydrodynamic forces, by using a uniform flow through the flow cell and three inlets perpendicular to the flow cell. The streamlines within the device are manipulated by injecting or extracting fluid through the three inlets. The Hele-Shaw geometry allows a fast and accurate prediction of the particle trajectory, meaning only a simple PID controller is required to correct for particle deviations. The robustness of this approach is demonstrated by implementing multiple functions within the device, including particle trapping, particle sorting, particle separation, and assembly. The real-time control procedure affords accurate particle manipulation, with a maximum error on the order of the diameter of the particle.

FluidsDraskic, M.Westerweel, J.Pecnik, R. display sharp, non-linear variations of thermodynamic properties when they are heated at a supercritical pressure. As such, near-pseudo-critical heat transfer is often characterized by large variations in density, leading to sharp near-wall accelerations or strong stratifications when buoyancy becomes dominant. We study the modulation of heat transfer and turbulence by non-negligible buoyancy in such property-variant flows, for the development of near-pseudo-critical heat exchangers for supercritical energy conversion systems. In particular, a liquid-like, horizontal base flow of carbon dioxide at 88.5 bar and 32.6 ^∘C is considered, which is subjected to a vertical heat flux of up to 12.0 kW/m² at Reynolds numbers of up to Re_Dh≤10.000. Here, optical- and surface temperature measurements are used concurrently to evaluate the flow. Integratced visualizations of the flow field show the onset of strong stratifications with limited heating rates in the near-pseudo-critical region. During unstable stratification, the channel flow is dominated by the upward motion of thermal plumes. When the stratification is stable, any vertical motion and turbulence present in an equivalent neutrally buoyant flow is suppressed. As a result, wall heat is removed more effectively in the unstably stratified configuration than in a forced convective flow, whereas the opposite is true for a stably stratified flow. The difference in the perceived heat transfer between the considered configurations increases as buoyancy becomes more dominant.

In an experiment on a turbulent jet, we detect interfacial turbulent layers in a frame that moves, on average, along with the turbulent-nonturbulent interface. This significantly prolongs the observation time of scalar and velocity structures and enables the measurement of two types of Lagrangian coherent structures. One structure, the finite-time Lyapunov field (FTLE), quantifies advective transport barriers of fluid parcels while the other structure highlights barriers of diffusive momentum transport. These two complementary structures depend on large-and small-scale motion and are therefore associated with the growth of the turbulent region through engulfment or nibbling, respectively. We detect the turbulent-nonturbulent interface from cluster analysis, where we divide the measured scalar field into four clusters. Not only the turbulent-nonturbulent interface can be found this way, but also the next, internal, turbulent-turbulent interface. Conditional averages show that these interfaces are correlated with barriers of advective and diffusive transport when the Lagrangian integration time is smaller than the integral timescale. Diffusive structures decorrelate faster since they have a smaller timescale. Conditional averages of these structures at internal turbulent-turbulent interfaces show the same pattern with a more pronounced jump at the interface indicative of a shear layer. This is quite an unexpected outcome, as the internal interface is now defined not by the presence or absence of vorticity, but by conditional vorticity corresponding to two uniform concentration zones. The long-time diffusive momentum flux along Lagrangian paths represents the growth of the turbulent flow into the irrotational domain, a direct demonstration of nibbling. The diffusive flux parallel to the turbulent-nonturbulent interface appears to be concentrated in a diffusive superlayer whose width is comparable with the Taylor microscale, which is relatively invariant in time.

This study investigates the turbulent/non-turbulent interface (TNTI) in self-similar turbulent axisymmetric jet flows, focusing on a novel approach named ’move with the flow’ where the image acquisition is space-based rather than time-based. Experiments were conducted at three different Reynolds numbers (9000, 12000, and 31000), utilizing particle image velocimetry (PIV) and laser-induced fluorescence (LIF) techniques. The core of this research was developing a traverse system specifically designed to follow the evolving flow structures at the TNTI synchronously. We succeeded in tracking large-scale events in TNTI from creation to dissolution.

Correction to: Scientific Reportshttps://doi.org/10.1038/s41598-024-68971-x, published online 13 August 2024 In the original version of this Article a previous rendition of Figure 2B, Figure 4 and Figure 5D was published. The original Figure 2, 4 and 5 and accompanying legends appear below. (Figure presented.) (Figure presented.) (Figure presented.) (A) Representative bubble dynamics for different channel geometries. (B) Universal motion of bubbles within channels with different size, shape and length. The dashed line represents the developed theory, Eq. (2). The marker colors represent the hydraulic diameters (d_h), the shapes represent the cross-section and the facecolor represent the lengths (L). The graphical marker symbols and colors established here are followed throughout this article. The black arrow represents the region of deviation(s) from the expected dynamics. The threshold laser energy absorbed for bubble formation estimated from experiments (E_th,exp) against theory (E_th,theory) presented in Eq. (5). (A,B) Representative dynamic bubble size curves illustrating the emergence of instabilities. The zones of the instabilities are highlighted using a shaded rectangular area. The arrows represent if the instabilities occur before or after X_max. (A) Illustrates the experimental data for different d_h with similar oscillation time. The instabilities emerge with increasing d_h. (B) Illustrates the data for d_h = 200 µm with increasing laser energies. The instabilities disappear with increasing E_abs. (C) Flow stability diagram with the transition line at W_o = 734. The markers represent the experiments and the lines represent the analytical estimate. The numbers correspond to the channel hydraulic diameters (in µm) with the dashed and solid lines representing the channel lengths L = 25 and 50 mm, respectively. (D) The dimensionless convective timescale against the L/d_h aspect ratio. The partition line is a linear relation between the x and y axes with 45 × 10⁻⁶ as the slope and the origin as the intercept. The original Article has been corrected.

This paper explores integrating artificial intelligence (AI) segmentation models, particularly the Segment Anything Model (SAM), into fluid mechanics experiments. SAM’s architecture, comprising an image encoder, prompt encoder, and mask decoder, is investigated for its application in detecting and segmenting objects and flow structures. Additionally, we explore the integration of natural language prompts, such as BERT, to enhance SAM’s performance in segmenting specific objects. Through case studies, we found that SAM is robust in object detection in fluid experiments. However, segmentations related to flow properties, such as scalar turbulence and bubbly flows, require fine-tuning. To facilitate the application, we have established a repository (https://github.com/AliRKhojasteh/Flow_segmentation) where models and usage examples can be accessed.

Cavitation occurs when the local pressure, induced by high local velocities, drops below the vapor pressure, leading to the formation of vapor bubbles. The subsequent collapse of these bubbles can cause noise, erosion, and vibrations. Recent studies show that cavitation is sensitive to water quality, i.e., the nuclei populations, the chemical composition of water, and the presence of particles. Motivated by investigating the effects of water quality on cavitation, experiments are performed in a dedicated experimental facility. This consists in two co-axial disks that are initially at rest and mutually in contact, in a tank filled with water. The fast diverging movement of the top disk with respect to the bottom one produces a jet flow inside the gap between the disks, which leads to the formation of two counter-rotating vortices. The local pressure drop induced by high flow velocities leads to a phase change. To characterize the phenomenon, two optical techniques are applied, i.e., shadowgraphy and particle image velocimetry (PIV). In performing PIV reconstruction, the sum of correlation enhances the spatial resolution of the velocity vector fields. The pressure field in the region where the vortices occur is obtained from velocity data. The water quality effects on cavitation are investigated by adding salt and using water with an abundance of nuclei inside the tank.

Oscillatory flow in confined spaces is central to understanding physiological flows and rational design of synthetic periodic-actuation based micromachines. Using theory and experiments on oscillating flows generated through a laser-induced cavitation bubble, we associate the dynamic bubble size (fluid velocity) and bubble lifetime to the laser energy supplied—a control parameter in experiments. Employing different channel cross-section shapes, sizes and lengths, we demonstrate the characteristic scales for velocity, time and energy to depend solely on the channel geometry. Contrary to the generally assumed absence of instability in low Reynolds number flows (<1000), we report a momentary flow distortion that originates due to the boundary layer separation near channel walls during flow deceleration. The emergence of distorted laminar states is characterized using two stages. First the conditions for the onset of instabilities is analyzed using the Reynolds number and Womersley number for oscillating flows. Second the growth and the ability of an instability to prevail is analyzed using the convective time scale of the flow. Our findings inform rational design of microsystems leveraging pulsatile flows via cavitation-powered microactuation.

Measurements were conducted in the fully developed turbulent flow in a pipe with internal diameter D at a Reynolds number of Re _D= 1.6 × 10 ⁵ . The pipe walls were equipped with regularly spaced square ribs of relative height h/ D= 0.154 , while the pitch-to-roughness height was varied between p/ h= 1.67 and p/ h= 6.67 . The measurements include mean velocity components, Reynolds shear and normal stresses and pressure losses. It is investigated whether the effects of the large roughness on the (time and axially averaged) velocity profile can be described by the classical rough-wall formulation by allowing the value of the von Kármán constant to deviate from its standard value of 0.41.

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.

We report results on the instantaneous drag force on plates that are accelerated in a direction normal to the plate surface, which show that this force scales with the square root of the acceleration. This is associated with the generation and advection of vorticity at the plate surface. A new scaling law is presented for the drag force on accelerating plates, based on the history force for unsteady flow. This scaling avoids previous inconsistencies in using added mass forces in the description of forces on accelerating plates.