A pure hydrodynamic instability in shear flows and its application to astrophysical accretion disks

We provide the possible resolution for the century old problem of hydrodynamic shear flows, which are apparently stable in linear analysis but shown to be turbulent in astrophysically observed data and experiments. This mismatch is noticed in a variety of systems, from laboratory to astrophysical flows. There are so many uncountable attempts made so far to resolve this mismatch, beginning with the early work of Kelvin, Rayleigh, and Reynolds towards the end of the nineteenth century. Here we show that the presence of stochastic noise, whose inevitable presence should not be neglected in the stability analysis of shear flows, leads to pure hydrodynamic linear instability therein. This explains the origin of turbulence, which has been observed/interpreted in astrophysical accretion disks, laboratory experiments and direct numerical simulations. This is, to the best of our knowledge, the first solution to the long standing problem of hydrodynamic instability of Rayleigh stable flows.Comment: 14 pages including 10 figures; accepted for publication in Ap

Origin of nonlinearity and plausible turbulence by hydromagnetic transient growth in accretion disks: Faster growth rate than magnetorotational instability

We investigate the evolution of hydromagnetic perturbations in a small section of accretion disks. It is known that molecular viscosity is negligible in accretion disks. Hence, it has been argued that a mechanism, known as Magnetorotational Instability (MRI), is responsible for transporting matter in the presence of weak magnetic field. However, there are some shortcomings, which question effectiveness of MRI. Now the question arises, whether other hydromagnetic effects, e.g. transient growth (TG), can play important role to bring nonlinearity in the system, even at weak magnetic fields. Otherwise, whether MRI or TG, which is primarily responsible to reveal nonlinearity to make the flow turbulent? Our results prove explicitly that the flows with high Reynolds number (Re), which is the case of realistic astrophysical accretion disks, exhibit nonlinearity by TG of perturbation modes faster than that by modes producing MRI. For a fixed wave vector, MRI dominates over transient effects, only at low Re, lower than its value expected to be in astrophysical accretion disks, and low magnetic fields. This seriously questions (overall) persuasiveness of MRI in astrophysical accretion disks.Comment: 9 pages including 7 figures; version published in Physical Review

Capital controls and capital flow volatility during global shocks: Indian experience

This paper examines the issue of capital controls in India and their effectiveness in stabilizing the capital flow volatility in the economy. It documents the evolution of the capital controls regime in India since its economic liberalization in 1991 and focuses on India’s experience with capital controls in the period leading up to the Global Financial Crisis (GFC) of 2008 until the taper tantrum aftermath in 2013. We construct a capital controls index based on data from the IMF’s Annual Report on Exchange Arrangements and Exchange Restrictions (AREAER). The index shows careful ease of capital controls by Indian policymakers in the financial sectors, i.e., the capital market, money market and direct investment, over the 2001–2008 period. The index further shows that the process of decontrol in the capital market stagnated during 2009–2014 due to the GFC to insulate the Indian economy from global shocks. This paper further explores the impact of capital controls in managing capital flow volatility in the context of the GFC. Using the tobit estimation approach, we show that capital controls effectively reduce capital flow volatility in the pre-GFC period followed by a limited impact post-GFC. This complements the capital account liberalization process during pre-GFC period. Our findings support India’s prudent approach to capital account management as financial markets evolve to manage risk efficiently in a large economy

“THE ECONOMIC CRISIS AND ITS AFTERMATH IN THE NORDIC AND BALTIC COUNTRIES DO AS WE SAY AND NOT AS WE DO”

The book The Economic Crisis and its Aftermath in the Nordic and Baltic Countries Do As We Say and Not As We Do written by Professor Dr. Hilmar Þór Hilmarsson is unique in many respects. It provides a broad view about the Nordic-Baltic countries, their response to the crisis of 2008/09 and the post-crisis consequences. Both regional and international aspects have been analyzed, namely regional institutions such as the European Union (EU) and global institutions such as the International Monetary Fund (IMF) in the operational sphere where policy choices of these nations get influenced. The author points out that Barry Eichengreen (Professor of Economics and Political Science at UC Berkeley) suggested that the title of this book should include: ‘Do as we say and not as we do’, reflecting author’s view mainly to a comparison between how Nordic countries may want the Baltic States to respond to crisis as compared to how they would respond themselves in comparable situations. In order to support this statement, the author mentions some instances, for example, comparison of how Finland and Sweden responded to the banking crisis they experienced in the early 1990s with how the Baltic States responded to the 2008/09 global economic and financial crisis, arguably under Nordic (mainly Swedish) influence. All those countries are then influenced by regional and global institutions. Thus, the author framed the title of the book as “The Economic Crisis and its Aftermath in the Nordic and Baltic Countries: Do as we say and not as we do”