Ebrahim Ghahramani - Academia.edu (original) (raw)

Papers by Ebrahim Ghahramani

Journal of Fluid Mechanics, Jul 13, 2021

Cavitating flows include vapour structures with a wide range of different length scales, from mic... more Cavitating flows include vapour structures with a wide range of different length scales, from micro-bubbles to large cavities. The correct estimation of small-scale cavities can be as important as that of large-scale structures, because cavitation inception as well as the resulting noise, erosion and strong vibrations occur at small time and length scales. In this study, a multi-scale cavitating flow around a sharp-edged bluff body is investigated. For numerical analysis, while popular homogeneous mixture models are practical options for large-scale applications, they are normally limited in the representation of small-scale cavities. Therefore, a hybrid cavitation model is developed by coupling a mixture model with a Lagrangian bubble model. The Lagrangian model is based on a four-way coupling approach, which includes new submodels, to consider various small-scale phenomena in cavitation dynamics. Additionally, the coupling of the mixture and the Lagrangian models is based on an improved algorithm that is compatible with the flow physics. The numerical analysis provides a detailed description of the multi-scale dynamics of cavities as well as the interactions between vapour structures of various scales and the continuous flow. The results, among others, show that small-scale cavities not only are important at the inception and collapse steps, but also influence the development of large-scale structures. Furthermore, a comparison of the results with those from experiment shows considerable improvements in both predicting the large cavities and capturing the small-scale structures using the hybrid model. More accurate results (compared with the traditional mixture model) can be achieved even with a lower mesh resolution.

Tip vortex cavitating flow is known as challenging to study. The objective of this paper is to in... more Tip vortex cavitating flow is known as challenging to study. The objective of this paper is to investigate the effective parameters in numerical modelling of tip vortex cavitation (TVC) inception through the comparison of three different models. The models are (1) a commonly used homogenous mixture model, in which inception is based on pressure drop criterion; (2) a Lagrangian bubble model, in which cavitation is initiated from free nuclei in the liquid; and (3) a hybrid Eulerian-Lagrangian model, in which the cavities are initiated based on the pressure drop criterion, but the growth of initially small cavities are modelled using the more accurate Lagrangian equations. The simulations are conducted on the tip vortex flow around an elliptical foil. The results show that the commonly applied pressure drop assumption is not a sufficient criterion for cavitation inception. Also, it is seen that the water quality and nuclei transport towards the vortex core influence the cavity pattern at inception.

In this study a hybrid Eulerian mixture-Lagrangian bubble model is developed for numerical simula... more In this study a hybrid Eulerian mixture-Lagrangian bubble model is developed for numerical simulation of cavitating flows. In this model, the large scale cavities are represented in the Eulerian framework using the homogeneous mixture model, while the small sub-grid structures are tracked as Lagrangian bubbles. Also, at each time step small cavity structures in the Eulerian framework are transformed to the Lagrangian framework to be treated as sub-grid bubbles and vice versa. Using this model, it is possible to represent various cavity structures of different length scales with reasonable computational cost.

Sustainability, Aug 30, 2022

This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY

Sustainability

Many industrial processes make use of sodium because sodium is the fifth most abundant metal and ... more Many industrial processes make use of sodium because sodium is the fifth most abundant metal and the seventh most abundant element on Earth. Consequently, there are many sodium-containing industrial wastes that could potentially be used for carbon capture, paving the way towards a circular and biobased economy. For example, a common industrial chemical is NaOH, which is found in black liquor, a by-product of the paper and pulp industry. Nonetheless, the literature available on CO2 absorption capacity of aqueous NaOH is scarce for making a fair comparison with sodium-containing waste. Therefore, to fill this gap and set the foundation for future research on carbon capture, the CO2 absorption capacity of NaOH solutions in a concentration range of 1–8 w/w% was evaluated, a wider range compared with currently available data. The data set presented here enables evaluating the performance of sodium-based wastes, which are complex mixtures and might contain other compounds that enhance or ...

Wear

The objective of this study is to investigate the collapsing behavior of cavitation, which leads ... more The objective of this study is to investigate the collapsing behavior of cavitation, which leads to the erosion of material. An experimental examination was conducted in a channel with a semi-circular cylinder obstacle, which serves as a "vortex cavity" generator. Cavitation was achieved by employing a range of pressure differences over the test section and a high-speed camera was used to observe the cavitation behavior. The flow field behind the semi-circular cylinder was investigated as a characteristic example of bluff bodies that exhibit a distinct, separated vortex flow in their wake. The cases with the bluff body were also compared to the ones without the bluff body. Erosion tests were performed using paint (stencil ink). The intensity of cavitation is characterized by the cavitation number (σ); the lower the cavitation number, the higher the cavitation intensity. The erosion (removal of paint) after 40 min of operation revealed distinct and repeatable results. For a high cavitation number, a large number of von Karman-vortex-like cavities are shed downstream of the obstacle. This results in a higher number of collapse events and, ultimately, more erosion. On the other hand, at lower cavitation numbers, the erosion took place at the cavity's closure line. It was seen that with the increase in cavitation intensity, the erosion area increases. Moreover, the bluff body obstacle promotes and localizes cavitation-induced erosion on the sample plate compared to the cases without the bluff body. This ultimately means that in the cases with the bluff body, less power is required in the system to cause erosion. The erosion patterns caused by the bluff body cavitation are more repeatable compared to the cases without the bluff body due to the localized cavitation load. The erosion pattern from the paint test is also compared with a material loss test (30 h of operation). A very good qualitative agreement is found between the two tests, with the paint test requiring approximately two orders of magnitude less running time of the facility. We demonstrate that paint tests, combined with this geometry, provide an efficient and economical way to investigate erosion patterns compared to expensive material loss tests.

Proceedings of the 11th International Symposium on Cavitation (CAV2021), 2021

Most incompressible models to simulate cavitating flows relies on a rudimentary mixture assumptio... more Most incompressible models to simulate cavitating flows relies on a rudimentary mixture assumption of vapour and liquid, that does not account for the dynamics of small-scale bubbles in cloudy cavitation. This PhD project aims to develop a Eulerian sub-grid mixture model for incompressible LES, that implements mixture properties for vapour clouds, extracted from DNS data, coupled with Lagrangian bubble models for very sparse clouds. The new model will yield a more realistic condensation process, derived from vapour cloud dynamics, with seamless transition to micro-bubble dynamics. In the current study, the governing equations are improved to avoid non-realistic flow variations during the Eulerian-Lagrangian transition.

In this study a hybrid Eulerian mixture - Lagrangian bubble model is developed for numerical simu... more In this study a hybrid Eulerian mixture - Lagrangian bubble model is developed for numerical simulation of cavitating flows. In this model, the large scale cavities are represented in the Eulerian framework using the homogeneous mixture model, while the small sub-grid structures are tracked as Lagrangian bubbles. Also, at each time step small cavity structures in the Eulerian framework are transformed to the Lagrangian framework to be treated as sub-grid bubbles and vice versa. Using this model, it is possible to represent various cavity structures of different length scales with reasonable computational cost.

Journal of Fluid Mechanics, 2021

Computers & Fluids, 2018

Cavitating multi-phase flows include an extensive range of cavity structures with different lengt... more Cavitating multi-phase flows include an extensive range of cavity structures with different length scales, from micro bubbles to large sheet cavities that may fully cover the surface of a device. To avoid high computational expenses, incompressible transport equation models are considered a practical option for simulation of large scale cavitating flows, normally with limited representation of the small scale vapour structures. To improve the resolution of all scales of cavity structures in these models at a moderate additional computational cost, a possible approach is to develop a hybrid Eulerian mixture-Lagrangian bubble solver in which the larger cavities are considered in the Eulerian framework and the small (subgrid) structures are tracked as Lagrangian bubbles. A critical step in developing such hybrid models is the correct transition of the cavity structures from the Eulerian mixture to a Lagrangian discrete bubble framework. In this paper, such a multi-scale model for numerical simulation of cavitating flows is described and some encountered numerical issues for Eulerian-Lagrangian transition are presented. To address these issues, a new improved formulation is developed, and simulation results are presented that show the issues are overcome in the new model.

International Journal of Multiphase Flow, 2018

In this paper, the performance of three different numerical approaches in cavitation modelling ar... more In this paper, the performance of three different numerical approaches in cavitation modelling are compared by studying two benchmark test cases to understand the capabilities and limitations of each method. Two of the methods are the well established compressible thermodynamic equilibrium mixture model and the incompressible transport equation finite mass transfer mixture model, which are compared with a third method, a recently developed Lagrangian discrete bubble model. In the Lagrangian model, the continuum flow field is treated similar to the finite mass transfer approach, however the cavities are represented by individual bubbles. Further, for the Lagrangian model, different ways to consider how the fluid pressure influences bubble dynamics are studied, including a novel way by considering the local pressure effect in the Rayleigh-Plesset equation. The first case studied is the Rayleigh collapse of a single bubble, which helps to understand each model behaviour in capturing the cavity interface and the surrounding pressure variations. The special differences between the Lagrangian and finite mass transfer models in this case clarify some possible origin for some limitations of the latter method. The second investigated case is the collapse of a cluster of bubbles, where the collapse of each bubble is affected by the dynamics of surrounding bubbles. This case confirms the importance of considering local pressure in the improved form of the Rayleigh-Plesset equation and illustrates the influence of the liquid compressibility for cavity modelling and appropriate capturing of the collapse pressure.