Bulletin of the American Physical Society
77th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 24–26, 2024; Salt Lake City, Utah
Session X20: Cavitation and Aerated Flows |
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Chair: Tadd Truscott, King Abdullah Univ of Sci & Tech (KAUST) Room: 250 D |
Tuesday, November 26, 2024 8:00AM - 8:13AM |
X20.00001: Beyond spheres: water entry of small disks and cones Nathan B Speirs, Caleb Becker, Cody Roundy, Daniel Webb, Muneo Okiishi, JESSE L BELDEN, Aren M Hellum When a solid body enters the water with sufficiently high speed, a cavity forms that is ventilated by air from the atmosphere. The shapes and collapse mechanism of these cavities fall into four different regimes depending on the Bond and Weber numbers. The vertical water entry of spheres impacting normal to a flat free surface is perhaps the most canonical solid body entry problem and has been used extensively to study cavity dynamics. As such, the cavity regimes have not been completely characterized for other canonical impactor geometries such as disks and cones. In this study we experimentally investigate the water entry of disks and cones and show that they produce the same cavity regimes as spheres and discuss how they differ. |
Tuesday, November 26, 2024 8:13AM - 8:26AM |
X20.00002: Water entry with disrupted splash closure JESSE L BELDEN, Nathan B Speirs, Aren M Hellum When a body impacts the water surface it typically opens a gas-filled cavity. At sufficiently high Weber number, the cavity seals when the above-surface splash domes over on itself. This behavior is characteristic of the surface seal cavity closure regime. Mansoor et al. [1] showed that by inhibiting splash closure, one can induce a deep seal closure normally seen at lower impact speeds. However, as speed is increased it is unclear if inhibiting the splash closure will continue to result in deep seal, revert back to surface seal, or result in an entirely different cavity closure mechanism. Here we investigate the mechanisms of cavity closure when the splash sheet is disrupted at moderate to high impact speed (O(10-100 m/s)). Over a large velocity range, we show that the cavity closure mechanism varies and evidently affects the resulting volume and gas composition of the cavity. |
Tuesday, November 26, 2024 8:26AM - 8:39AM |
X20.00003: Holey surfaces repel cavitation bubbles Jefferson B Santos da Silva, Zhao Pan, Nathan B Speirs Cavitation bubbles pose significant challenges in various engineering applications due to the erosion, noise, and vibrations they induce upon collapsing on or near solid surfaces. In this study, we explore the potential of solid surfaces with gas-filled holes to repel cavitation bubbles and mitigate their negative effects. While existing research shows that surfaces with gas-filled holes repel collapsing bubbles, a comprehensive understanding for design is lacking. We investigate a wide range of bubble-to-hole diameter ratios and bubble-hole relative positioning to identify when a solid surface with a single gas-filled hole repels cavitation bubbles. Using high-speed photography, we capture the bubble behavior near these surfaces, which are generated by a pulsed laser. We summarize the results by developing behavioral regime plots for bubble repulsion. Our research on this mechanism may inspire novel surface designs for bubble repulsion to prevent cavitation damage, noise, and vibrations. |
Tuesday, November 26, 2024 8:39AM - 8:52AM |
X20.00004: Enhancing wastewater treatment efficiency through Venturi-assisted confined-tube aeration David MacPhee The most efficient municipal wastewater treatment method is the activated sludge process, which consumes 1-2% of all electricity generated in the USA. The vast majority of this energy is consumed in the aeration process, where oxygen-rich water is required so that microorganisms can grow, multiply, and consume organic matter in the waste stream. Most new facilities use bubble diffusers, where air is forced through small membranes at the bottom of deep basins, creating tiny bubbles which slowly rise to the surface, transferring oxygen in the process. |
Tuesday, November 26, 2024 8:52AM - 9:05AM |
X20.00005: Modeling incipient cavitation during vortex-vortex interaction due to Crow instability Mehedi Hasan Bappy, Aditya Madabhushi, Krishnan Mahesh Cavitation inception can occur at sites of low pressure such as the cores of vortices. In shear flows with separation, cavitation inception tends to occur in streamwise weaker vortical structures compared to the stronger spanwise vortical structures. The prevailing theory suggests that the strong spanwise vortices extend the weaker streamwise vortices, resulting in a significant reduction of pressure within the cores of these weaker vortices, leading to inception. This paper presents a numerical investigation into this phenomenon using Large Eddy Simulation (LES) and subgrid scale (SGS) cavitation inception modeling techniques on a pair of vortices generated by two hydrofoils in a high-velocity water tunnel at two Reynolds numbers of 1.2x106 and 1.7x106. Our simulation captures the intricate interplay between the vortices and reduction of pressure within the secondary vortex cores as a result of its stretching. Additionally, the SGS inception model complements LES with modeling the behavior of dispersed nuclei within the flow reacting to the low pressures and estimates cavitation inception within secondary cores at resolved local pressure ~40 kPa higher than the vapor pressure. The results are in good agreement with the experiments both in terms of inception pressure and its location. |
Tuesday, November 26, 2024 9:05AM - 9:18AM |
X20.00006: Compressible large eddy simulations of sheet-to-cloud cavity transition on a NACA0015 hydrofoil Seungnam Kim, Krishnan Mahesh We investigate the different cavitation regimes around a NACA0015 hydrofoil with changing cavitation number, under the same conditions as experiment using high-fidelity compressible large eddy simulations with a water-vapor mixture model. Fundamental shedding mechanisms, such as re-entrant jet flows and propagating shock waves are investigated to understand the formation of cloud cavitation from stable sheet cavities. Cavity lengths, moment, lift and drag forces, as well as spanwise mean cavity volume are compared to available experimental data with promising agreement. We numerically quantify and visualize sheet-to-cloud transitions in each regime demarcated by different cavitation numbers and investigate how such complex phenomena affect the overall hydrofoil loads. |
Tuesday, November 26, 2024 9:18AM - 9:31AM |
X20.00007: Ventilated two-dimensional supercavitation behind a moving vertical flat plate in a still fluid: Experiment and theory Jeonghyeon Nam, Yeunwoo Cho This study examines ventilated two-dimensional supercavity formation both in a deep-water condition and in a free-surface bounded environment where a vertical flat plate (h by b, h<<b) is in motion and the fluid is at rest. In the experiment, the vertical flat plate (with height h) horizontally moves with a constant speed (U) while two-dimensional compressed air is injected both in the upward and downward vertical directions with a volume flow rate (Q) behind the flat plate. Then, for a fixed geometry of the plate, depending on the Froude numbers (Fr=U/(gh)1/2) and the air-entrainment coefficients (Cv=Q/Uhb), two-dimensional supercavites of various sizes are generated behind the moving plate both in deep water and near the free surface. The experimentally observed supercavity shapes are compared with those from a theoretical re-entrant jet model and they agree with each other very well. . |
Tuesday, November 26, 2024 9:31AM - 9:44AM |
X20.00008: Unsteady Load Analysis of Partial Cavitation of a Hydrofoil at High Reynolds Numbers Reza Nouri, Daniel Foti Cavitation is the primary cause of erosion on lifting surfaces and ship propellers. In real-world conditions, the Reynolds number on a marine hydrofoil typically reaches millions, and surface pressure fluctuates frequently and with high magnitude, making the hydrofoil prone to structural damage. This study investigates the unsteady loads of partial cavitation using large eddy simulation over a modified NACA-16 hydrofoil at various angles of attack, cavitation numbers, and high Reynolds numbers from 1.4 to 50 million. The Schnerr-Sauer cavitation model and a two-phase flow approach based on the volume of fluid method are employed. The computational domain matches the dimensions of the Large Cavitation Channel with a test section of 3.05 by 3.05 by 13.1 meters. To validate the method, the pressure distribution and global forces are compared with previous experimental data. Additionally, a small tripping fence is added near the leading edge of the hydrofoil to improve the uniformity of cavitation generation. The cavitation aggressiveness is assessed, and regions on the hydrofoil with a high risk of erosion are estimated. While the fence increases the spanwise uniformity of the cavity, it can increase the time-averaged drag coefficient. Statistical analysis shows strong agreement between the frequency content of forces and experimental data. Overall, the method performs very well across the range of conditions investigated. |
Tuesday, November 26, 2024 9:44AM - 9:57AM |
X20.00009: Experimental Characterization of the Unsteady Flow Structure and Cavitation Inception in the Tip Region of a Ducted Marine Propeller Ayush Saraswat, Kirtivardhan Singh, Chintan Panigrahi, Joseph Katz In ducted marine propellers, the tip flow vortices include a primary tip leakage vortex (TLV) and multiple counter- and co-rotating secondary vortices. Near design conditions, flow visualizations show that cavitation inception occurs within co-rotating secondary vortices as they are entrained into the TLV, and presumably stretched by it, circumferentially downstream of the blade trailing edge. Due to the inherent small timescales, intermittency, and limited spatial extent of this phenomenon, its signature is smeared, but still visible, in the ensemble-averaged flow field. Hence, high-speed (30 kHz) stereo-PIV is employed to measure the time-resolved evolution of the flow in the tip region of a two-bladed acrylic propeller in a refractive index-matched facility. The tip diameter and tip gap are 300.6 and 2.1 mm respectively, and Re=1.0-1.2 x 106. Data acquired in meridional planes capture the evolution of the tip leakage flow, TLV roll-up, formation and entrainment of secondary vortices, vortex-wake interaction, entrainment of the endwall boundary layer, interactions with the main passage flow, and eventual vortex breakup. The TLV is a 3D structure with its core surrounded by perpendicularly aligned vorticity originating from the blade wake and from the endwall. These results serve as a guide for the ongoing 3D tomographic PTV, which will be used for calculating the pressure field in the region of cavitation inception. Sample results of the latter will be presented. |
Tuesday, November 26, 2024 9:57AM - 10:10AM |
X20.00010: Effect of Advance Ratio on the Flow Structure and Cavitation Inception in the Tip Region of a Ducted Marine Propeller Kirtivardhan Singh, Ayush Saraswat, Chintan Panigrahi, Joseph Katz The flow in the tip region of a ducted marine propeller, with blades made of acrylic, is studied in the JHU refractive index-matched turbomachine facility. The propeller has a tip diameter and gap of 300.6 and 2.1 mm respectively, and Re=1.0-1.2 x 106. Characterization of incipient cavitation is performed using high-speed flow visualization at design and off-design advance ratios (J=Vz/nD), with Vz being the spatially averaged inlet axial velocity, n being the rotor rps, and D, the rotor diameter. At design J=0.85, cavitation inception occurs in secondary vortices aligned perpendicularly to the primary tip leakage vortex (TLV), circumferentially downstream of the blade trailing edge (TE). Conversely, below design flowrate, J=0.68, inception occurs along the primary TLV, in the mid-chord region, circumferentially upstream of the TE. Ensemble-averaged velocity distributions in a series of meridional planes, obtained using stereo-PIV, reveal that with decreasing J there is: (i) an upstream shift in blade loading, (ii) earlier rollup and breakdown of the TLV, as well as entrainment of the opposite sign vorticity away from the endwall. Furthermore, while at J=0.85, the turbulent kinetic energy peaks in the region of secondary and counter-rotating vortices, predominantly owing to interaction of the backward tip leakage flow with the main passage flow, at J=0.68, the peak is in the vicinity of the TLV center. Dominant turbulence production mechanisms for normal Reynolds stresses will also be discussed. |
Tuesday, November 26, 2024 10:10AM - 10:23AM |
X20.00011: Characterizing cavitating flow over a hydrofoil using X-ray densitometry Elijah D Andrews, Harish Ganesh, Steven Louis Ceccio Hydrodynamic cavitation is a significant source of erosion on the surface of propulsors and rudders. Understanding the effect of cavity properties on cavitation intensity is key to the prediction and mitigation of erosive cavitation. Quantification of surface flow fields during cavitation is an important step in understanding the relationship between the erosivity of a cavity and its topology. In this study, we use measurements of hydrodynamic loads, in combination with high-speed optical and X-ray imaging, to characterize cavity topology. Time-synchronized surface pressure measurements are correlated with x-ray measurements to identify pressure variations caused by flow mechanisms near the surface. The findings are compared with pressure and erosion measurements from full-scale experiments performed at the Large Cavitation Channel in Memphis, Tennessee. |
Tuesday, November 26, 2024 10:23AM - 10:36AM |
X20.00012: Simulation of cavitation over a wedge using various cavitation and turbulence models Dhruv Girish Apte, Mahdi Lavari, Diego Vaca-Revelo, Aswin Gnanaskandan, Olivier COUTIER-DELGOSHA Cavitation occurs when the local liquid pressure drops sharply below vapor pressure, forming vapor bubbles that collapse as they exit the low-pressure region and result in noise, vibration, and erosion damage thus, being performance-degrading to hydraulic machinery. Conversely, cavitation has promising applications in geothermal energy and non-invasive surgery. The process is characterized by the cavitation number, the ratio of the difference between local pressure and vapor pressure to the dynamic pressure. Numerical modeling of cavitating flows requires coupling a cavitation model with a turbulence model to capture the cavitation-turbulence interplay accurately. This study investigates cavitation over a wedge across various cavitation numbers using different cavitation and turbulence models, emphasizing cavity shapes and localized void fraction comparisons in OpenFOAM. The study employs the Saito, Merkle, and Schnerr-Sauer cavitation models, along with k-omega Shear Stress Transport (SST) with & without the Reboud viscosity correction, Spalart-Allmaras and the Delayed Detached Eddy Simulation (DDES) models. On comparison with experimental data, results indicate that while all cavitation models predict similar cavity lengths, they differ in cavity widths at the inception point, at the wedge's top. The turbulence models show similar cavity shapes regardless of the eddy viscosity correction, highlighting challenges in simulating cavitation and turbulence model development. |
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