Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session Q09: Cavitation and Multiphase Flows |
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Chair: Aren Hellum, Naval Undersea Warfare Center Room: Georgia World Congress Center B214 |
Tuesday, November 20, 2018 12:50PM - 1:03PM |
Q09.00001: Predicting Cavitation Erosion Propensity and Severity in Fuel Injection Systems Gina Maureen Magnotti, Michele Battistoni, Kaushik Saha, Sibendu Som The objective of this work is to identify computational metrics that can characterize the erosive potential of cavitation within injector orifices. While a commonly employed cavitation erosion metric, namely the maximum local pressure, was found to describe single impact events, no additional information could be determined regarding the material erosion process. To improve representation of the incubation period, a new metric was derived based on the cumulative energy absorbed by the solid material from repeated hydrodynamic impacts. The stored energy metric was then implemented into CONVERGE. Large eddy simulations for turbulent cavitating flow through channel geometries were performed, where the multiphase and multi-component flow was represented using a homogeneous mixture modeling approach. Through comparison against available experimental data, the stored energy metric was found to accurately predict the influence of flow conditions on the incubation period before material erosion. Additionally, detailed analysis of cavitation cloud collapse events highlighted the strong correlation between cloud collapse mechanisms and their erosive potential. |
Tuesday, November 20, 2018 1:03PM - 1:16PM |
Q09.00002: Measurements of cavitation inception in a turbulent shear layer Omri Ram, Karuna Agarwal, Joseph Katz The mechanisms that govern the location and frequency of cavitation inception events in turbulent shear flows have not been well characterized. Water tunnel experiments in the shear layer generated by a 10 mm (h) backward facing step at speeds of 5.3, 10.5 and 16 m/s examine the mean flow, Reynolds stresses, and the onset of cavitation. High-resolution 2D PIV confirm that the separating boundary layer is turbulent with Re_{τ}=807, 1504 and 2345, respectively. The profiles of mean velocity as well as the normal and shear Reynolds stresses collapse when scaled by the freestream velocity and reattachment length. The latter decreases with increasing Reynolds number, varying between 5.3h to 6.2h. Highspeed silhouette imaging in perpendicular views captures early phases of cavitation events, corresponding to cavitation indices in the 0.45 to 0.55 range, primary at the higher two speeds. Inception occurs in the quasi-streamwise braids developing between the spanwise vortices shortly after they roll up in the near field of the shear layer. Hence, the intermittently forming cavities appear as inclined elongated strings. The transition from a microscopic nucleus to a fully formed 5-7 mm strings occur in less than 250 μs, and they break up and collapse in about 1ms. |
Tuesday, November 20, 2018 1:16PM - 1:29PM |
Q09.00003: Lift
production using differential cavity ventilation on a symmetric hydrofoil Aren Hellum, David Yamartino Cavitation near lifting surfaces is typically viewed as a problem to be mitigated, because local cavitation on highly loaded surfaces both reduces the lift and invites damage near the point of cavity collapse. However, this loss of lift indicates that it may be possible to produce controllable forces by artificially inducing a cavity on one side of a symmetric hydrofoil by using ventilation gas. Experiments have been performed in order to test this idea, and the results are in agreement with the working theory. The produced forces are robust, equivalent to 4° angle-of-attack using a NACA0009 cross-section. Time-dependent and hysteretic features of the system have also been identified. |
Tuesday, November 20, 2018 1:29PM - 1:42PM |
Q09.00004: Experimental investigation of turbulence within unsteady cavitation Ilyass Khlifa, Olivier Coutier-Delgosha An experimental study based on fast X-ray imaging was performed to investigate the turbulence within unsteady cavitating flows. Cloud cavitation in a small scale Venturi type section, characterized by periodical large-scale oscillations of the sheet cavity, was investigated. The flow was seeded with radio-opaque tracers, and acquisition was performed at the Advanced Photon Source facility of the Argonne National Laboratory. Simultaneous measurements of the liquid and the vapour velocities were obtained by PIV-like image cross-correlations applied to particle and bubble images, while the distribution of the vapour volume fraction was derived from local X-ray absorption. These data were used to determine the distributions of the turbulent kinetic energy k and the turbulent shear stress τ, for several conditions of cavitating flows. The results were then confronted to Reboud’s modification of two-equation eddy viscosity models, which is commonly used to improve the turbulence modelling by reducing the mixture turbulent viscosity in the low void ratio areas of the flow. A nice agreement between the current results and this empirical modification was obtained, explaining a posteriori why Reboud’s correction leads to major improvements in the simulation of cloud cavitation. |
Tuesday, November 20, 2018 1:42PM - 1:55PM |
Q09.00005: Probable sites of cavitation inception and associated pressure field in a turbulent shear layer Karuna Agarwal, Omri Ram, Joseph Katz Experiments performed downstream of a backward facing step, where the mean flow and turbulence statistics are well characterized, examine the early phases of cavitation events, and correlate them with the corresponding pressure field. Cavities detected using high speed silhouette imaging at cavitation indices of 0.45-0.55 and speeds of 10.5 and 16 m/s are used to compute probability maps that depict their location and frequency. These events are more likely to occur between 45% - 75% of the reattachment length, well upstream of the peaks in turbulence level. Occurring primarily in the braids between the spanwise vortices dominating the near field of the shear layer, the correlation between probability of cavitation and the turbulence scales in the shear layer is not evident. Hence, the dynamics of these quasi streamwise vortices is measured using 3D time-resolved tomographic PIV, enhanced by particle tracking. The spatial resolution of these measurements is sufficient for characterizing the ~1mm diameter cores of the secondary vortices. The corresponding evolution of the instantaneous pressure distributions is derived by integrating the material acceleration. Conditional spatial sampling is used for correlating the pressure field with the cavitation events. |
Tuesday, November 20, 2018 1:55PM - 2:08PM |
Q09.00006: Numerical Study on Supercavitation Turbulence Han Liu, Lian Shen Supercavitation uses a bubble of gas inside a liquid large enough to encompass an object travelling through the liquid so that the skin friction on the object can be greatly reduced and high speed can be obtained. In this study, direct numerical simulation is used to investigate the turbulence in supercavitation. The study builds on an in-house simulation code that uses the coupled level set and volume of fluid method to accurately capture the interface between the water and gas phases. A ventilated disk cavitator is used for the bubble generation, and it is modeled by a sharp interface immersed boundary method. Based on the simulation data, we have discovered two structures near the closure of the cavity, the shear layer (SL) and the jet layer (JL), which have distinct flow patterns. Through the analyses of turbulent kinetic energy (TKE) and its budget, different roles played by SL and JL in the TKE transportation and generation processes are revealed. Using a kinematic criterion, conditional statistics are obtained in these two layers. Based on conditional temporal spectra, we have identified the dominant frequencies in SL and JL, with the highest one belonging to the transverse motion of JL and determined by the frequency of the vortex shedding at the closure of cavity. |
Tuesday, November 20, 2018 2:08PM - 2:21PM |
Q09.00007: Cavitation in shear layer of a backward facing step Anubhav Bhatt, Harish Ganesh, Steven Ceccio The backward facing step is used as the baseline geometry in experimental studies on separation and reattachment in turbulent flows. Separated shear flow behind a backward facing wedge can experience cavitation when the local pressure is close to vapor pressure in the shear layer. Current study focuses on quantifying the developing and fully developed cavitation in the shear layer and separation region formed downstream a backward facing step. As the cavitation number is lowered from the point of inception, the shear flow begins to fill with vapor forming into a fully developed/filled cavitation and then, with lower pressure, a super-cavity. The underlying multiphase flow features are examined using time-resolved X-Ray densitometry and high speed videography. The void fraction measurements are used to quantify vapor production, convection and shedding. These measurements reveal the presence of propagating condensation shock-waves associated with partial cavity shedding. In addition, 2-D planar PIV is performed to characterize the turbulent boundary layer upstream of the liquid separation at the step edge. |
Tuesday, November 20, 2018 2:21PM - 2:34PM |
Q09.00008: Furthering the Understanding of Multi-modal Shedding on a Cavitating NACA0015 Hydrofoil using Cluster-Based Reduced-Order Modeling (CROM) and Dynamic Mode Decomposition (DMD) Harish Ganesh, Daniel Knister, Shivam Barwey, Malik Hassanaly, Venkatramanan Raman, Eric Johnsen, Steven Ceccio Cavitation dynamics on a NACA0015 hydrofoil is known to be multi-modal with abrupt changes in shedding frequency corresponding to a change in cavitation number. By using time-resolved X-ray densitometry based void-fraction field measurements, the underlying physical mechanisms responsible for the observed behavior have been found to be the occurrence of a three-step shedding process which involved either partial or full-collapse of growing cavity due to shed cloud-collapse. In addition, occurrence of propagating bubbly shock waves as a dominant mechanism of shedding also coincided with the observance of multi-modal behavior. It was also observed that the shedding process under certain conditions involved abrupt oscillations from three-step shedding to a single-step shedding. Even though the flow structures responsible for this transition have been identified, the driving force behind the transition between modes is not clear. In this study, we use CROM and DMD on instantaneous void-fraction fields to understand the transition dynamics and relate them to physical processes. In addition, comparison between the two methods for a cavitating flow is also presented. |
Tuesday, November 20, 2018 2:34PM - 2:47PM |
Q09.00009: Surface Pressure Measurements to Characterize Bubbly Shock Properties on a Cavitating NACA0015 Hydrofoil Juliana Wu, Anubhav Bhatt, Harish Ganesh, Steven Ceccio Cavitation partial cavity dynamics on a NACA0015 hydrofoil is shown to be multimodal. For given fixed inlet conditions, the cavity growth and shedding processes can lock into differing, multistep processes, resulting in varying shedding frequencies (Strouhal numbers). In the present study, time-resolved X-ray densitometry based void-fraction flow field measurements, high-speed cinematography, and surface pressure measurements are performed to examine such cavitation dynamics. X-ray visualization revealed the presence of both reentrant liquid flow and bubbly shock propagation, both of which can contribute to cavity pinch-off and shedding. The shedding dynamics on the examined hydrofoil is weakly multi-modal, compared to that observed on a hydrofoil with a smaller planform. Also, significantly faster bubbly shock propagation speeds were observed on the present model compared to the smaller model. By measuring surface pressure on the hydrofoil, it is shown that occurrence of cavitation at higher mean pressures requires stronger pressure rises across the shocks thereby increasing the shock speed. |
Tuesday, November 20, 2018 2:47PM - 3:00PM |
Q09.00010: A Comparison of Experimental-based Data Driven Models for Predicting Spontaneous Cavitation Mode Switching on a NACA 0015 Hydrofoil Shivam Barwey, Malik Hassanaly, Venkatramanan Raman, Harish Ganesh, Daniel Knister, Eric Johnsen, Steven Louis Ceccio Hydrodynamic cavitation on a NACA 0015 hydrofoil can experience multimodal dynamics for the same operating conditions, in which the transition from one mode to another occurs spontaneously. Predicting these transitions is crucial in naval applications to prevent cavitation erosion and performance loss. As experimental data in the form of time-resolved X-ray densitometry depicting this phenomenon is available, the present study focuses on the forecasting application and comparison of two data driven reduced-order modeling techniques: dynamic mode decomposition (DMD) and cluster based reduced order modelling (CROM). In DMD, the system is represented in a reduced sense by selecting characteristic non-orthogonal modes which are obtained from an assumed linear mapping of the snapshot set, where the modes contain system temporal behavior. Unlike modal techniques such as DMD, CROM provides a probabilistic prediction tool entirely free of underlying PDE manipulation. Dynamics are captured in a forward Markov-based model in terms of a discretized phase space, which can be used to determine the probability of future transition via a PDF transport approximation. A metric is developed to compare predictive capabilities of these two ROMs to forecast the sporadic cavitation mode transition. |
Tuesday, November 20, 2018 3:00PM - 3:13PM |
Q09.00011: Cavitation Dynamics in Wakes Behind Bluff Bodies Lisa Deijlen, Anubhav Bhatt, Harish Ganesh, Juliana Wu, Steven Ceccio The shedding frequency of cavitating wakes behind bluff bodies depends strongly on the extent of the cavitation in the near wake. As the extent of cavitation increases from inception to an attached super-cavity, the wake shedding frequency attains a peak. The physical mechanism responsible for these observed changes in shedding dynamics with the extent of the wake cavitation is yet to be fully understood. In the current study, we employ time resolved X-ray densitometry and high-speed videography to study the cavitation dynamics in the wake of a triangular, nominally two-dimensional wedge in a re-circulating water tunnel to understand the underlying mechanisms responsible for cavity formation and shedding. It is proposed that alteration of the near wake dynamics by cavitation and the effect of compressibility in bubbly wakes that can explain the observed change in shedding dynamics. This hypothesis is tested via injection of non-condensable gas into the near wake of the bluff body. |
Tuesday, November 20, 2018 3:13PM - 3:26PM |
Q09.00012: LES of propeller cavitation Mrugank P Bhatt, Krishnan Mahesh Cavitation over a five bladed marine propeller (P4381) is studied using large eddy simulations (LES) at Re = 894,000. The simulations use the homogeneous mixture model, where the mixture of water and vapor is treated as a single compressible fluid. The numerical method developed by Gnanaskandan and Mahesh (IJMF. vol. 70, April 2016) is extended to solve the governing equations in the rotating frame of reference using absolute velocity. Flow over the propeller is simulated at design advance ratio (J = 0.89) under wetted (σ =∞) and thrust break down conditions (σ = 0.6). Propeller loads obtained from the simulations are compared to the experiments and flow field obtained using LES is discussed. |
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