Subcritical transition to turbulence in quasi-two-dimensional shear flows

The transition to turbulence in ducts, pipes or other conduits is a crucial phenomenon. It determines the energy consumption and heat or mass exchange in countless processes: whether cooling circuits of heat exchangers, pipelines or chemical reactors to cite but a few. The transition occurs at relatively low flow rates as a response to perturbations exceeding a critical amplitude (such transitions are subcritical) through an intrinsically three-dimensional (3D) mechanism. However, fluid motion can be restricted to two dimensions, if it is stratified, subject to rapid rotation or intense magnetic fields, for example in rotating machines or in the liquid metal cooling circuits of nuclear fusion reactors. Subcritical turbulence has yet to be observed in 2D or quasi-2D flows, let alone a transition to it. Here we use stability analysis and direct numerical simulations on the example of a duct flow driven by the motion of its lateral walls to provide the first evidence of turbulence in subcritical quasi-2D shear flows. We further show that the scenario leading to turbulence mostly relies on the nonlinear dynamics of so-called Tollmien-Schlichting waves, rather than on perturbations experiencing fast, transient growth. Although the transition is subcritical, it cannot take place at such low flow rates as in 3D flows, because these waves are severly damped. This alternative scenario opens a new route to turbulence that calls for exploration. This new landscape may reset current strategies to promote or to hinder quasi-2D turbulence in practical applications, including in fusion reactors.Comment: Combined main paper (7 pages, 5 figures) and supplementary information (16 pages, 6 figures, 5 tables), submitted for consideration to Nature Physic

A framework for integrated environmental health impact assessment of systemic risks

Traditional methods of risk assessment have provided good service in support of policy, mainly in relation to standard setting and regulation of hazardous chemicals or practices. In recent years, however, it has become apparent that many of the risks facing society are systemic in nature – complex risks, set within wider social, economic and environmental contexts. Reflecting this, policy-making too has become more wide-ranging in scope, more collaborative and more precautionary in approach. In order to inform such policies, more integrated methods of assessment are needed. Based on work undertaken in two large EU-funded projects (INTARESE and HEIMTSA), this paper reviews the range of approaches to assessment now in used, proposes a framework for integrated environmental health impact assessment (both as a basis for bringing together and choosing between different methods of assessment, and extending these to more complex problems), and discusses some of the challenges involved in conducting integrated assessments to support policy

Transition to turbulence in quasi-two-dimensional MHD flow driven by lateral walls

This manuscript has been accepted for publication in Physical Review Fluids, see https://journals.aps.org/prfluids/accepted/d5074S28J6b11905012b7cb06505e8f2149dd5f20. This work investigates the mechanisms that underlie transitions to turbulence in a three-dimensional domain in which the variation of flow quantities in the out-of-plane direction is much weaker than any in-plane variation. This is achieved using a model for the quasi-two-dimensional magnetohydrodynamic flow in a duct with moving lateral walls and an orthogonal magnetic field. In this environment, conventional subcritical routes to turbulence, which are highly three-dimensional, are prohibited. To elucidate the remaining mechanisms involved in quasi-two-dimensional turbulent transitions, the magnetic field strength and degree of antisymmetry in the base flow are varied, the latter via the relative motion of the lateral duct walls. Introduction of any amount of antisymmetry to the base flow drives the critical Reynolds number infinite, as the TS instabilities take on opposite signs of rotation, and destructively interfere. However, an increasing magnetic field strength limits interaction between the instabilities, permitting finite critical Reynolds numbers. The transient growth only mildly depends on the base flow, with negligible differences for friction parameters $H \gtrsim 30$. Direct numerical simulations, initiated with random noise, indicate that for $H \leq 1$, supercritical exponential growth leads to saturation, but not turbulence. For higher $3 \leq H \leq 10$, a turbulent transition occurs, and is maintained at $H=10$. For $H \geq 30$, the turbulent transition still occurs, but is short lived, as the turbulent state quickly collapses. In addition, for $H \geq 3$, an inertial subrange is identified, with the perturbation energy exhibiting a $-5/3$ power law dependence on wave number.Comment: 44 pages, 18 figures, accepted for publication in Physical Review Fluids, see https://journals.aps.org/prfluids/accepted/d5074S28J6b11905012b7cb06505e8f2149dd5f2

Life cycle inventory of biodiesel and petroleum diesel for use in an urban bus. Final report

This report presents the findings from a study of the life cycle inventories for petroleum diesel and biodiesel. It presents information on raw materials extracted from the environment, energy resources consumed, and air, water, and solid waste emissions generated. Biodiesel is a renewable diesel fuel substitute. It can be made from a variety of natural oils and fats. Biodiesel is made by chemically combining any natural oil or fat with an alcohol such as methanol or ethanol. Methanol has been the most commonly used alcohol in the commercial production of biodiesel. In Europe, biodiesel is widely available in both its neat form (100% biodiesel, also known as B1OO) and in blends with petroleum diesel. European biodiesel is made predominantly from rapeseed oil (a cousin of canola oil). In the United States, initial interest in producing and using biodiesel has focused on the use of soybean oil as the primary feedstock mainly because the United States is the largest producer of soybean oil in the world. 170 figs., 148 tabs

A viscous vortex model for predicting the drag reduction of riblet surfaces

This paper introduces a viscous vortex model for predicting the optimal drag reduction of riblet surfaces, eliminating the need for expensive direct numerical simulations (DNSs) or experiments. The footprint of a typical quasi-streamwise vortex, in terms of the spanwise and wall-normal velocities, is extracted from smooth-wall DNS flow fields in close proximity to the surface. The extracted velocities are then averaged and used as boundary conditions in a Stokes-flow problem, wherein riblets with various cross-sectional shapes are embedded. Here, the same smooth-wall-based boundary conditions can be used for riblets, as we observe from the DNSs that the quasi-streamwise vortices remain unmodified apart from an offset. In particular, the position of these vortices remain unpinned above small riblets. The present approach is compared with the protrusion-height model of Luchini et al. (J. Fluid Mech., vol. 228, 1991, pp. 87–109), which is also based on a Stokes calculation, but represents the vortex with only a uniform spanwise velocity boundary condition. The key novelty of the present model is the introduction of a wall-normal velocity component into the boundary condition, thus inducing transpiration at the riblet crests, which becomes relevant as the riblet size increases. Consequently, the present model allows for the drag-reduction prediction of riblets up to the optimal size. The present approach does not rely on the scale separation formally required by homogenisation techniques, which are only applicable for vanishingly small riblets

An Overview of Biodiesel and Petroleum Diesel Life Cycles

This report presents the findings from a study of the life cycle inventories for petroleum diesel and biodiesel. It presents information on raw materials extracted from the environment, energy resources consumed, and air, water, and solid waste emissions generated

The role of nonlinear interactions in the onset of drag increase in flow over riblets

Abstract Characterizing the mechanisms that contribute to the onset of drag increase over micro-grooves (riblets) as the spacing increases is critical to design strategies for riblet-based drag reduction. This study decomposes the roughness function to investigate different mechanisms associated with the breakdown of drag reduction as riblet spacing is increased. We obtain the roughness function through direct numerical simulations (DNS) in a minimal channel and restricted nonlinear (RNL) models. Both the traditional RNL decomposition and an augmented RNL (ARNL) model that includes additional nonlinear interactions are employed as computationally tractable, reduced order representations of the flow field. RNL and ARNL results are compared to those of DNS in minimal channels to investigate the role of the different scale-dependent nonlinear interactions contributing to the roughness function. A comparison of the co-spectra arising from the minimal channel DNS with that from RNL and ARNL simulations indicates that general trends are captured by both reduced order models. However, the additional nonlinearity introduced in the ARNL model produces closer correspondence in the observed structural features of the DNS results. In particular, the ARNL better captures the signatures of the dispersive flow and the texture-coherent fluctuations. There is also a noticeable improvement observed in the profiles of the added stress contributions obtained with the ARNL model versus the RNL model.</jats:p

Environmental Life Cycle Implications of Fuel Oxygenate Production from California Biomass

Historically, more than 90% of the excess agricultural residue produced in California (approximately 10 million dry metric tons per year) has been disposed through open-field burning. Concerns about air quality have prompted federal, state, and local air quality agencies to tighten regulations related to this burning and to look at disposal alternatives. One use of this biomass is as an oxygenated fuel. This report focuses on quantifying and comparing the comprehensive environmental flows over the life cycles of two disposal scenarios: (1) burning the biomass, plus producing and using MTBE; and (2) converting and using ETBE