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Article
Publication date: 6 April 2010

R. Elshereef, J. Vlachopoulos and A. Elkamel

The main purpose of this paper is to present and compare two different models for bubble growth and foam formation and to conduct a thorough assessment in terms of their numerical…

Abstract

Purpose

The main purpose of this paper is to present and compare two different models for bubble growth and foam formation and to conduct a thorough assessment in terms of their numerical implementation and prediction accuracy.

Design/methodology/approach

The two models are assessed and validated against experimental measurements. The first model is known as a single bubble growth model and treats the foaming process as a single bubble growing in a large pool with enough gas available for growth, while the second model (cell model) takes into account the finiteness of gas supply availability as well as the effects of surrounding bubbles. The models are based on the application of the conservation of continuity and momentum principles and on constitutive equations to represent the viscosity of the melt. The models are numerically implemented using a finite difference scheme and their predictions are compared against experimental measurements.

Findings

The results demonstrate that the single bubble model predicts an infinite bubble growth with time due to the assumption of unlimited supply of the blowing agent. Meanwhile the cell model gives an equilibrium bubble size because it accounts for gas depletion. From this work, it was concluded that the cell model is the best model that adequately describes experimental data.

Practical implications

The problem of bubble growth and foam formation is of great importance in the process industry as it plays a key role in diverse technological fields such as the production of foamed plastics.

Originality/value

The findings here are important for the appropriate modeling of bubble growth and foam formation and for scheduling and optimizing the process. A simple model will suffice for the early stage of the process while a cell model is more appropriate for the entire duration of the process.

Details

Engineering Computations, vol. 27 no. 3
Type: Research Article
ISSN: 0264-4401

Keywords

Content available
Article
Publication date: 28 June 2018

Shun Chen, Shiyuan Zheng and Hilde Meersman

The occurrence and unpredictability of speculative bubbles on financial markets, and their accompanying crashes, have confounded economists and economic historians worldwide. The…

1222

Abstract

Purpose

The occurrence and unpredictability of speculative bubbles on financial markets, and their accompanying crashes, have confounded economists and economic historians worldwide. The purpose of this paper is to diagnose and detect the bursting of shipping bubbles ex ante, and to qualify the patterns of shipping price dynamics and the bubble mechanics, so that appropriate counter measures can be taken in advance to reduce side effects arising from bubbles.

Design/methodology/approach

Log periodic power law (LPPL) model, developed in the past decade, is used to detect large market falls or “crashes” through modeling of the shipping price dynamics on a selection of three historical shipping bubbles over the period of 1985 to 2016. The method is based on a nonlinear least squares estimation that yields predictions of the most probable time of the regime switching.

Findings

It could be concluded that predictions by the LPPL model are quite dependent on the time at which they are conducted. Interestingly, the LPPL model could have predicted the substantial fall in the Baltic Dry Index during the recent global downturn, but not all crashes in the past. It is also found that the key ingredient that sets off an unsustainable growth process for shipping prices is the positive feedback. When the positive feedback starts, the burst of bubbles in shipping would be influenced by both endogenous and exogenous factors, which are crucial for the advanced warning of the market conversion.

Originality/value

The LPPL model has been first applied into the dry bulk shipping market to test a couple of shipping bubbles. The authors not only assess the predictability and robustness of the LPPL model but also expand the understanding of the model and explain patterns of shipping price dynamics and bubble mechanics.

Details

Maritime Business Review, vol. 3 no. 2
Type: Research Article
ISSN: 2397-3757

Keywords

Article
Publication date: 5 September 2023

Nikesh Chowrasia, Subramani S.N., Harish Pothukuchi and B.S.V. Patnaik

Subcooled flow boiling phenomenon is characterized by coolant phase change in the vicinity of the heated wall. Although coolant phase change from liquid to vapour phase…

Abstract

Purpose

Subcooled flow boiling phenomenon is characterized by coolant phase change in the vicinity of the heated wall. Although coolant phase change from liquid to vapour phase significantly enhances the heat transfer coefficient due to latent heat of vaporization, eventually the formed vapor bubbles may coalesce and deteriorate the heat transfer from the heated wall to the liquid phase. Due to the poor heat transfer characteristics of the vapour phase, the heat transfer rate drastically reduces when it reaches a specific value of wall heat flux. Such a threshold value is identified as critical heat flux (CHF), and the phenomenon is known as departure from nucleate boiling (DNB). An accurate prediction of CHF and its location is critical to the safe operation of nuclear reactors. Therefore, the present study aims at the prediction of DNB type CHF in a hexagonal sub-assembly.

Design/methodology/approach

Computational fluid dynamics (CFD) simulations are performed to predict DNB in a hexagonal sub-assembly. The methodology uses an Eulerian–Eulerian multiphase flow (EEMF) model in conjunction with multiple size group (MuSiG) model. The breakup and coalescence of vapour bubbles are accounted using a population balance approach.

Findings

Bubble departure diameter parameters in EEMF framework are recalibrated to simulate the near atmospheric pressure conditions. The predictions from the modified correlation for bubble departure diameter are found to be in good agreement against the experimental data. The simulations are further extended to investigate the influence of blockage (b) on DNB type CHF at low operating pressure conditions. Larger size vapour bubbles are observed to move away from the corner sub-channel region due to the presence of blockage. Corner sub-channels were found to be more prone to experience DNB type CHF compared to the interior and edge sub-channels.

Practical implications

An accurate prediction of CHF and its location is critical to the safe operation of nuclear reactors. Moreover, a wide spectrum of heat transfer equipment of engineering interest will be benefited by an accurate prediction of wall characteristics using breakup and coalescence-based models as described in the present study.

Originality/value

Simulations are performed to predict DNB type CHF. The EEMF and wall heat flux partition model framework coupled with the MuSiG model is novel, and a detailed variation of the coolant velocity, temperature and vapour volume fraction in a hexagonal sub-assembly was obtained. The present CFD model framework was observed to predict the onset of vapour volume fraction and DNB type CHF. Simulations are further extended to predict CHF in a hexagonal sub-assembly under the influence of blockage. For all the values of blockage, the vapour volume fraction is found to be higher in the corner region, and thus the corner sub-channel experiences CHF. Although DNB type CHF is observed in corner sub-channel, it is noticed that the presence of blockage in the interior sub-channel promotes the coolant mixing and results in higher values of CHF in the corner sub-channel.

Details

International Journal of Numerical Methods for Heat & Fluid Flow, vol. 33 no. 12
Type: Research Article
ISSN: 0961-5539

Keywords

Article
Publication date: 6 June 2016

Charalambos Pitros and Yusuf Arayici

The purpose of this paper is to provide a decision support model for the early diagnosis of housing bubbles in the UK during the maturity process of the phenomenon.

1086

Abstract

Purpose

The purpose of this paper is to provide a decision support model for the early diagnosis of housing bubbles in the UK during the maturity process of the phenomenon.

Design/methodology/approach

The development process of the model is divided into four stages. These stages are driven by the normal distribution theorem coupled with the case study approach. The application of normal distribution theory is allowed through the usage of several parametric tools. The case studies tested in this research include the last two UK housing bubbles, 1986 to 1989 and 2001/2002 to 2007. The central hypothesis of the model is that during housing bubbles, all speculative activities of market participants follow an approximate synchronisation, and therefore, an irrational, synchronous and periodic increase on a wide range of relevant variables must occur to anticipate the bubble component. An empirical application of the model is conducted on UK housing market data over the period of 1983-2011.

Findings

The new approach successfully identifies the well-known UK historical bubble episodes over the period of 1983-2011. The study further determines that for uncovering housing bubbles in the UK, house price changes have the same weight with the debt–burden ratio when their velocity is positive. Finally, the application of this model has led us to conclude that the model’s outputs fluctuate approximately in line with phases of the UK real estate cycle.

Originality/value

This paper proposes a new measure for studying the presence of housing bubbles. This measure is not simply an ex post detection technique but dating algorithms that use data only up to the point of analysis for an on-going bubble assessment, giving an early warning diagnostic that can assist market participants and regulators in market monitoring.

Details

International Journal of Housing Markets and Analysis, vol. 9 no. 2
Type: Research Article
ISSN: 1753-8270

Keywords

Article
Publication date: 28 October 2013

Xiang Wang, Guangya Zhu and Ke Li

The present study aims to resolve the adjustment problem of cavitation bubble number density in simulations of the cavitating flows within the diesel injection nozzle holes using…

Abstract

Purpose

The present study aims to resolve the adjustment problem of cavitation bubble number density in simulations of the cavitating flows within the diesel injection nozzle holes using a two-fluid cavitation model.

Design/methodology/approach

The basic rule that determines the variations of cavitation bubble number density has been checked through the scaling analysis of a two-fluid model under the assumption of hydrodynamic similarity of the cavitating flows. Moreover, a phenomenological model for the number density of cavitation bubbles that takes the hydrodynamic effect into account has been developed through the combined analysis of cavitation bubble dynamics and internal flow characteristics of diesel injection nozzle holes. This new model has also been validated by the discharge coefficient measures in a wide range of injection conditions.

Findings

The values of cavitation bubble number density must rationally match changes both in liquid quality effect and in hydrodynamic effect corresponding to different cavitating flows. The validation results show that the two-fluid cavitation model together with this new cavitation bubble number density model predicts well both the cavitation content inside the diesel nozzle hole and the relationship between discharge coefficient and cavitation number, and the new cavitation bubble number density model has the potential to further expand the application range of the two-fluid cavitation model.

Originality/value

This study provides insight into hydrodynamic effect corresponding to cavitating flows inside diesel nozzle holes and presents an idea to model the cavitation bubble number density phenomenologically. The model idea and the developed model are useful to researchers and engineers in the area of nozzle internal flow and cavitating flow.

Details

International Journal of Numerical Methods for Heat & Fluid Flow, vol. 23 no. 8
Type: Research Article
ISSN: 0961-5539

Keywords

Article
Publication date: 3 May 2013

Xiaoliang Liu, Guenther Filler and Martin Odening

The authors' paper aims to deal with the question whether speculative bubbles are present in agricultural commodity prices.

3274

Abstract

Purpose

The authors' paper aims to deal with the question whether speculative bubbles are present in agricultural commodity prices.

Design/methodology/approach

The authors apply a regime switching regression model to test the hypothesis that agricultural prices contain periodically collapsing bubbles. Using daily futures prices for six agricultural commodities, the authors calculate net convenience yields from which price fundamentals are derived.

Findings

The authors discover pronounced deviations between observed prices and their fundamental values. However, they do not find evidence for the presence of periodically and partially collapsing speculative bubbles for five of six commodities. Except for soybeans, the signs and the significance of the estimated coefficients are not entirely in line with the predictions of the theoretical model.

Originality/value

The authors' study adds to the heated discussion on the impact of speculative behavior on agricultural commodity prices. So far, most contributions in the literature either use theoretical arguments for the (non‐) existence of bubbles or apply indirect tests which are plagued by low statistical reliability. In contrast, the authors apply a direct test. They find that the outcome of empirical bubble tests depends on the considered bubble type and on the testing procedure. In view of these ambiguities, definite statements on the presence of speculative bubbles as well as demands for limitations of speculative positions in commodity futures markets should be carefully reconsidered.

Details

Agricultural Finance Review, vol. 73 no. 1
Type: Research Article
ISSN: 0002-1466

Keywords

Article
Publication date: 20 November 2019

Daniel Hagemann and Monika Wohlmann

The global financial and economic crisis resulting from the US housing crisis has shown that house prices can have far-reaching consequences for the real economy. For…

Abstract

Purpose

The global financial and economic crisis resulting from the US housing crisis has shown that house prices can have far-reaching consequences for the real economy. For macroprudential supervision, it is, therefore, necessary to identify house price bubbles at an early stage to counteract speculative price developments and to ensure financial market stability. This paper aims to develop an early warning system to signal speculative price bubbles.

Design/methodology/approach

The results of explosivity tests are used to identify periods of excessive price increases in 18 industrialized countries. The early warning system is then based on a logit and an ordered logit regression, in which monetary, macroeconomic, regulatory, demographic and private factors are used as explanatory variables.

Findings

The empirical results show that monetary developments have the highest explanatory power for the existence of house price bubbles. Further, the study reveals currently emerging house price bubbles in Norway, Sweden and Switzerland.

Practical implications

The results implicate a new global housing boom, particularly in those countries that did not experience a major price correction during the global financial crisis.

Originality/value

The ordered logit model is an advanced approach that offers the advantage of being able to differentiate between different phases of a house price bubble, thereby allowing a multi-level assessment of the risk of speculative excesses in the housing market.

Details

Journal of European Real Estate Research , vol. 12 no. 3
Type: Research Article
ISSN: 1753-9269

Keywords

Book part
Publication date: 12 December 2007

Gary J. Rangel and Subramaniam S. Pillay

We tested for evidence of stock price bubbles in the Malaysian stock market from 1978 to 2004. Four different tests were used namely excess volatility tests, unit…

Abstract

We tested for evidence of stock price bubbles in the Malaysian stock market from 1978 to 2004. Four different tests were used namely excess volatility tests, unit root/co-integration tests, duration dependence tests, and the intrinsic bubbles model. All four tests indicate that during the sample period, there was evidence of stock price bubbles. All tests results conform to the theoretical literature on asset price bubbles except for the results on the intrinsic bubbles model, which concludes that Malaysian investors under react to information on dividends. We find this result hardly surprising as anecdotal evidence does indicate that Malaysian investors place more importance on capital gains as compared to dividends. Although we do not go into a debate on whether authorities should be prick the bubble to stem its negative effects, we argue that transparent information dissemination will ensure that the stock market becomes more efficient in pricing stocks.

Details

Asia-Pacific Financial Markets: Integration, Innovation and Challenges
Type: Book
ISBN: 978-0-7623-1471-3

Article
Publication date: 22 September 2020

Qianqian Mao, Yanjun Ren and Jens-Peter Loy

The purpose of this paper is to detect the existence of price bubbles and examine the possible contributing factors that associate with price bubble occurrences in China…

Abstract

Purpose

The purpose of this paper is to detect the existence of price bubbles and examine the possible contributing factors that associate with price bubble occurrences in China agricultural commodity markets.

Design/methodology/approach

Using recently developed rolling window right-side augmented Dickey–Fuller test, we first detect the dates of price bubbles in China's two important agricultural commodity markets, namely corn and soybeans. Then, we use a penalized maximum likelihood estimation of a multinomial logistic model to estimate the contributing factors of price bubbles in both markets, respectively.

Findings

Results from the bubble detection indicate that price bubbles account for 5.48% (3.91%) of the studied periods for corn (soybeans). More importantly, we find that market liquidity and speculation have opposite effects on the occurrences of bubbles in the corn and soybeans market. World stocks-to-use and exchange rates affect the occurrences of bubbles in a different way for each commodity, as well. Price bubbles are more likely associated with strong economic activity, high interest rates and low inflation levels.

Originality/value

This is the first study considering commodity-specific features into the formation of price bubbles. Through accurately identifying the bubble dates and fixing the estimation bias of rare events models, this study enables us to obtain robust results for each commodity. The results imply that China's corn and soybeans market respond differently to the speculative activity and external shocks from international markets. Therefore, future policy regulations on commodity markets should focus on more commodity-specific factors when aiming at avoiding bubble occurrences.

Details

China Agricultural Economic Review, vol. 13 no. 1
Type: Research Article
ISSN: 1756-137X

Keywords

Article
Publication date: 3 August 2010

Jung‐Suk Yu and M. Kabir Hassan

The purpose of this paper is to examine the existence of rational speculative bubbles in the Middle East and North African (MENA) stock markets.

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Abstract

Purpose

The purpose of this paper is to examine the existence of rational speculative bubbles in the Middle East and North African (MENA) stock markets.

Design/methodology/approach

To complement shortcomings of the traditional bubble tests, such as unit root tests and cointegration tests, mainly relying on expectations of future steams of dividends, the authors employ fractional integration tests and duration dependence tests.

Findings

Despite recent extreme fluctuations of MENA stock markets, fractional integration tests built on autoregressive fractionally integrated moving average models do not support the possibility of bubbles in the MENA stock markets. Similarly, duration dependence tests based on nonparametric Nelson‐Aalen hazard functions not only reject the existence of bubbles but also support equality of hazard functions between domestic and the US‐based investors without regard to the rapid financial liberalization and integration in the MENA stock markets.

Originality/value

The reliable results of bubble tests of the MENA stock markets provide domestic and international investors as well as policy makers with invaluable benchmark to better understand the irregular and highly fluctuating stock market behaviors of the MENA stock markets compared to other developed and emerging stock markets. For domestic and international investors, the formal analysis of MENA stock markets behavior including rational speculative bubbles will help them in their portfolio decisions and hedging purposes. Similarly, the empirical results of bubble tests in the paper will be also helpful to policymakers in MENA countries to take actions to improve the functioning of these dynamic markets.

Details

Studies in Economics and Finance, vol. 27 no. 3
Type: Research Article
ISSN: 1086-7376

Keywords

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