Session Q28: General Fluid Dynamics II

Effect of a single gap on the dynamics of flexible plates

Wind tunnel experiments and direct numerical simulations were conducted to investigate the impact of a single square perforation in the reconfigurations and unsteady dynamics of flexible rectangular plates at a porosity of 0.028. The analysis considered various Reynolds, Re, and Cauchy, Ca, numbers and the relative location of the perforation along the vertical axis of the plates. High-frequency force balance was used to obtain the unsteady aerodynamics force. Particle tracking velocimetry (PTV) and particle image velocimetry (PIV) were used to characterize the flexible plate deformation and flow field around the plates. Results show the modulation of the single perforation on the unsteady dynamics of the plate. Despite that the perforation imposed minor changes in mean reconfiguration, it significantly reduced tip fluctuation of the plates with perforations near the base. PTV shows two regimes where tip fluctuation sharply increases as a function of Ca and perforation location. Spectral analysis shows that the increase of tip motions is linked to the synchronization of the flow-induced frequency, fv, with the bending and twisting mode frequency depending on Ca. A plate softening effect may be produced for specific perforation locations. We propose a simple formulation for estimating fv considering the effective porosity and Vogel exponent, which is particularly useful for quantifying critical lock-in velocity for flexible plates.

    

Vehicle hydroplaning speed predicted via computational fluid dynamics

Pneumatic hydroplaning has been identified as one of the major causes of wet weather traffic accidents. Therefore, knowledge of potential hydroplaning speeds is crucial in designing roadways to reduce crash risks. It has been shown that the tire inflation pressure is the predominant factor affecting the hydroplaning speed while factors such as the water film thickness also contribute heavily to this phenomenon. A numerical model has been formulated for the simulation of a smooth tire sliding over a flooded pavement using the finite volume method. The current study adopts a generalized methodology that does not need a starting tire load and an inflation pressure, which essentially simplifies the complexity of the model. Alternative turbulence models have been used in the simulation process and model performances have been compared. The model verification has been done with respect to the well-known National Aeronautics and Space Administration (NASA) equation which was developed by previous researchers using experimental findings involving trucks at NASA’s Langley research facility. Predictions of the developed model were also compared with those of other existing empirical and numerical models for a range of water film thicknesses. In addition, the model behavior under changing loads has also been investigated. From the numerical analysis performed it was concluded that the accurate representation of hydrodynamics underneath the tire is crucial for numerically predicting the correct lift force on the tire. Specifically, it was revealed that modeling of turbulence in the flow is essential to obtain a better agreement with experimental results. Additionally, it was shown that changing the tire load has only a minor effect on the hydroplaning speed.

    

Trajectory prediction of a freely rising cylinder

In the present study, we present a model that can predict the trajectory of a freely rising cylinder. For this purpose, the motion of the cylinder is recorded using a high-speed camera in the range of 0.15 < ρ^* < 1.0 and 275 < Ga < 13000, where ρ^* is the ratio of the cylinder density to the fluid density and Ga is the Galileo number. The rising motion of the cylinder is modeled based on the two-dimensional wave equation as the two-dimensional cylinder rises in the form of a sine wave. For the modeling, dimensional analysis is conducted by using physical variables, and the two main parameters, 1-ρ^* and Ga, are obtained. A newly defined Galileo number, Ga^*, is defined by the relationship between 1-ρ^* and Ga, which can be expressed in the form of a power function. In addition, empirical formulae are derived based on the Ga^*, to predict the amplitude, wavelength, and period of the wave. The proposed model is shown to accurately predict the motion of the freely rising cylinder, and the time at which the rising cylinder arrives at a specific position. Some more details about the trajectory prediction will be discussed in the presentation.   

A meta-fluid with multistable equation of state and internal energy

Investigating and tailoring the thermodynamic properties of different fluids is crucial to many fields. For example, the efficiency, operation range, and environmental safety of applications in energy and refrigeration cycles are highly affected by the properties of the respective available fluids. Here, we suggest combining gas, liquid and multistable elastic capsules to create an artificial fluid with a multitude of stable states. We study, theoretically and experimentally, the suspension's internal energy, equilibrium pressure-density relations, and their stability for both adiabatic and isothermal processes. We show that the elastic multistability of the capsules endows the fluid with multistable thermodynamic properties, including the ability of capturing and storing energy at standard atmospheric conditions, not found in naturally available fluids.   

Hydrodynamics of accelerated spheres pulled along the air-water interface

We report the experimental study of floating spheres accelerated horizontally along an air-water interface. At low speeds, the spheres float on the surface, however, as speed is increased, the behavior of the spheres becomes more erratic as the spheres oscillate above and below the surface, forming horizontal cavities underwater and large skipping above. To a certain extent, the underwater horizontal cavities are attached to the sphere surface just above the equator instead of entirely wrapped around it and exhibit narrower shapes and lengths.  High-speed imaging is used to capture the generation of the horizontal air cavities and the effect of the pulling angle on the transition from the floating to the skipping mode. The hollow spheres are in the subcritical Reynolds number range of Re ≈ 2 × 10⁴ to 2 × 10⁵ with density ratios between 0.5 g/cm³ and 0.65 g/cm³. We show when stable skipping modes occur and unravel the dynamics of the erratic hydrodynamic behavior.   

Complex Fluids Latent Space Exploration Towards Accelerated Predictive Modeling

A subtle change in complex fluids microstructures leads to a totally emergent mesoscopic response and macroscopic functionality. In most cases, the predictive structure-property relationships are missing due to the curse of dimensionality and the lack of understanding of the mechanisms that bridge the governing physics interacting across a broad range of spatiotemporal scales. In this study, we introduce a custom data intelligent technique that integrates the non-negative tensor factorization and hierarchical clustering models, dubbed as NTFh. The NTFh model decomposes a physical dataset into lower rank representations with the intention to expose "explainable" latent features. As a proof of concept, we applied NTFh to Darcy friction dataset, where the underlying physics is well established as ground truth. Our findings proved that NTFh can (i) extract the latent features as a combination of physical attributes that simplify the understanding of the dynamics, and (ii) reveal various mechanisms and transitions hidden in the data without prior knowledge. We also show that NTFh can be used to extract latent interdependencies and construct explicit structure-property relationships that bridge physics interacting across a broad range of spatiotemporal scales.   

The impact of ice cover on the secondary flow structures at the bend apex

River ice plays a significant role in river flow characteristics in winter. However, the impacts of the frozen surface on river hydrodynamics are still unknown. In this work, we investigate the impact of the ice cover on the secondary flow patterns by (1) conducting field surveys, and (2) performing large-eddy simulation (LES). Measurements are carried out using Acoustic Doppler Current Profiler (ADCP), Sontek M9, in a river bend of Red River in Fargo. A minimum of six ice holes are opened at each designated cross-section at the bend apex during the Winter seasons of 2021 and 2022. The classical rotation-based Rozovskii method is employed to generate secondary flow patterns over the cross-sections. Measurement and LES results show that the existence of ice cover alters the secondary flow patterns drastically in comparison to the free surface condition. Here we show the existence of multi-cell structures in the secondary patterns. We hypothesize that the presence of the ice cover is responsible for such an emergence of secondary flows.   

On the enhanced attractive load capacity of resonant flexural squeeze-film levitators

Typical rigid-body squeeze-film levitation systems produce repulsive forces of up to several kilograms, and under a limited range of operating conditions, attractive forces of less than a gram. In a 2021 experimental study, researchers demonstrated attractive levitation of several hundred grams using an oscillator that displayed pronounced elastic standing-wave deformations. In this presentation, we employ the method of matched asymptotic expansions to model the viscoacoustic fluid flow in such flexural squeeze-film systems. Our analysis reveals that, while the weak attractive forces produced by rigid-body systems occur as a result of a sufficient drop in mean pressure near the outer periphery of the film, the much stronger attraction generated by resonant flexural systems owes instead to local extrema of mean underpressure near the nodes of the standing wave. Both the attractive load capacity and the range of operating conditions under which attractive forces can be generated are found to correlate strongly with the associated wavenumber. The results of our study offer fundamental insights that may guide the development of future non-contact levitation devices and their implementation in relevant applications such as mobile soft robots and surface-mount technologies.