Apart from providing some new experimental data the paper reviews previous investigations concerning fluctuating lift acting on a stationary circular cylinder in cross-flow. In particular, effects of ...Reynolds number in the nominal case of an infinitely long and nonconfined cylinder in a smooth oncoming flow are discussed. The Reynolds number range covered is from about Re=47 to 2×10
5, i.e., from the onset of vortex shedding up to the end of the subcritical regime. At the beginning of the subcritical regime (Re≃0.3×10
3) a spanwise correlation length of about 30 cylinder diameters is indicated, the correlation function being based on near-cylinder velocity fluctuations in outer parts of the separated shear layer. In between Reynolds numbers 1.6×10
3 and 20×10
3, an approximate 10-fold increase in the sectional r.m.s. lift coefficient is indicated. This range contains a fundamental change-over from one flow state to another, starting off at Re≃5×10
3 and seemingly fully developed at Re≃8×10
3.
The paper is concerned with some aspects of the fluctuating lift acting on a stationary circular cylinder in cross flow, in particular effects of Reynolds number in the nominal case of a ...large-aspect-ratio cylinder at small to vanishing blockage and free-stream turbulence, respectively. The Reynolds number range covered is from about Re=47–2·2×105, i.e., from the onset of vortex shedding up to the point where a subsequent increase in Re gives a rapid fall in the mean drag coefficient, the all-familiar drag crisis. A review of 2-D numerical simulations suggests that the r.m.s. lift coefficient (CL′) within the laminar shedding regime can be approximated asε/30+ε2/90, whereε =(Re−47)/47. For all Reynolds numbers above the inception of three-dimensional flow instabilities, i.e., for Re>(160–190), the near-wake flow is supposed to be partially correlated along the span. The lift fluctuations on a finite (spanwise) length of the cylinder are then not only dependent on the sectional lift variations but also on the spanwise correlation of the lift-related flow. At around Re=230, which is the approximate onset Re for mode B instability, the one-sided spanwise correlation length (Λ) is about twice the wavelength of the most unstable mode A instability,Λ /d≈7. Up to Re=260–300 the spanwise correlation increases dramatically, the indicated peak value being Λ/d≈30. From 3-D numerical simulations, the correspondingCL′ is approximately 0·5, which coincidentally is about the same value as found experimentally just before the rapid fall when entering the critical regime. Dramatic variations of both sectionalCL′ and Λ/d occur within the range Re≃0·3×103–2·2×105. For instance, at around Re=1·6×103a local minimum of about CL′=0·045 is indicated, at Re≃16×103the corresponding CL′-value is ten times higher. At Re=5·1×103there is a peak in spanwise correlation, Λ/d≈15.
Context. Strong electron cooling on the neutral gas in cometary comae has been predicted for a long time, but actual measurements of low electron temperature are scarce. Aims. Our aim is to ...demonstrate the existence of cold electrons in the inner coma of comet 67P/Churyumov-Gerasimenko and show filamentation of this plasma. Methods. In situ measurements of plasma density, electron temperature and spacecraft potential were carried out by the Rosetta Langmuir probe instrument, LAP. We also performed analytical modelling of the expanding two-temperature electron gas. Results. LAP data acquired within a few hundred km from the nucleus are dominated by a warm component with electron temperature typically 5–10 eV at all heliocentric distances covered (1.25 to 3.83 AU). A cold component, with temperature no higher than about 0.1 eV, appears in the data as short (few to few tens of seconds) pulses of high probe current, indicating local enhancement of plasma density as well as a decrease in electron temperature. These pulses first appeared around 3 AU and were seen for longer periods close to perihelion. The general pattern of pulse appearance follows that of neutral gas and plasma density. We have not identified any periods with only cold electrons present. The electron flux to Rosetta was always dominated by higher energies, driving the spacecraft potential to order − 10 V. Conclusions. The warm (5–10 eV) electron population observed throughout the mission is interpreted as electrons retaining the energy they obtained when released in the ionisation process. The sometimes observed cold populations with electron temperatures below 0.1 eV verify collisional cooling in the coma. The cold electrons were only observed together with the warm population. The general appearance of the cold population appears to be consistent with a Haser-like model, implicitly supporting also the coupling of ions to the neutral gas. The expanding cold plasma is unstable, forming filaments that we observe as pulses.
Growth and development in middle- and low-income countries often come at an environmental cost, but is that trade-off always necessary? This study uses a computable general equilibrium model to ...estimate the macroeconomic and environmental impact of the world's most significant plurilateral trade agreement, the African Continental Free Trade Agreement (AfCFTA). We build a novel dataset using the Global Trade Analysis Project data, which allows us to estimate the effect on emissions of carbon dioxide (CO2), non-CO2, and other pollutants. In terms of macroeconomic impact, African nations benefit from gross domestic product growth by 1.2% and employment by 2.1%, with less developed economies, such as Togo and Benin, obtaining the largest macroeconomic gains from trade liberalization. On aggregate, we estimate that the agreement will lead to a marginal, 0.3% increase in CO2 emissions, a 19.6% increase in non-CO2 greenhouse gas emissions, and a 21.5% decline in air pollutants. We find considerable heterogeneity across countries. For Nigeria, the rest of Central Africa, and South-Central Africa, the AfCFTA is expected to reduce emissions, while in Ethiopia, Cameroon, and Burkina Faso, estimations show an increase. Transit countries connected to large ports, such as Togo and Benin, are most negatively impacted. We conclude that while the AfCFTA implementation is expected to lead to notable improvements in air quality by reducing air pollutants, the resulting increase in climate-related emissions may require member countries to make concerted efforts to deal with the adverse effects.
Results of numerical simulations are presented for flow past a stationary circular cylinder at low Reynolds numbers (Re=50–200). The simulations were carried out using a finite-volume code employing ...a fractional step method with second-order accuracy in both space and time. A sensitivity study on numerical parameters concerning the domain size, grid independence and time step resolution was carried out in detail for Re=100. Global time-averaged results on force coefficients, non-dimensional velocities and pressures, including their corresponding r.m.s. values, as well as various quantities related to the separation and vortex shedding characteristics are presented. A non-monotonous streamwise velocity recovery in the intermediate wake is observed for Re>50, a phenomenon that has been grossly overlooked in the past. There are two plateaus along the wake centerline, in particular for Re=200. The first, which is the most distinct, ranges from about x=9 to x=16 at a wake deficit velocity of 0.38, x being counted in diameters behind the cylinder axis; the second one appears from x=25 to x=28 at a wake deficit velocity of 0.54. This phenomenon seems to be related to an associated change-over in the orientation of the von Kármán vortices and the merging trends, especially for Re=200 beyond x=25, as observed from instantaneous vorticity fields. Three-dimensional simulations using spanwise lengths of 10 and 12 (diameters) were carried out at Re=200. After a long initial phase with regular three-dimensional mode A flow features increasing very slowly in amplitude, the flow went into a state with distinct pulsating forces acting on the cylinder, the pulsations being seemingly randomly localized across the cylinder span. In this second, much more chaotic, flow state, the time-averaged results were in agreement with previous experiments and with parts of previous numerical studies.
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