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- Resonant gravity-wave drag enhancement in linear stratified flow over mountainsPublication . Teixeira, M. A. C.; Miranda, P. M. A.; Argaín, José Luís Almaguer; Valente, M. A.High-drag states produced in stratified flow over a 2D ridge and an axisymmetric mountain are investigated using a linear, hydrostatic, analytical model. A wind profile is assumed where the background velocity is constant up to a height z(1) and then decreases linearly, and the internal gravity-wave solutions are calculated exactly. In flow over a 2D ridge, the normalized surface drag is given by a closed-form analytical expression, while in flow over an axisymmetric mountain it is given by an expression involving a simple 1D integral. The drag is found to depend on two dimensionless parameters: a dimensionless height formed with z(1), and the Richardson number, Ri, in the shear layer. The drag oscillates as z(1) increases, with a period of half the hydrostatic vertical wavelength of the gravity waves. The amplitude of this modulation increases as Ri decreases. This behaviour is due to wave reflection at z(1). Drag maxima correspond to constructive interference of the upward- and downward-propagating waves in the region z < z(1), while drag minima correspond to destructive interference. The reflection coefficient at the interface z = z(1) increases as Ri decreases. The critical level, z(c), plays no role in the drag amplification. A preliminary numerical treatment of nonlinear effects is presented, where z(c) appears to become more relevant, and flow over a 2D ridge qualitatively changes its character. But these effects, and their connection with linear theory, still need to be better understood.
- A local grid refinement technique based upon Richardson extrapolationPublication . Coelho, P. J.; Argaín, José Luís AlmaguerA grid-embedding technique for the solution of two-dimensional incompressible flows governed by the Navier-Stokes equations is presented. A single coarse grid covers the whole domain, and local grid refinement B carried out in the regions of high gradients without changing the basic grid structure. A finite volume method with collocated primitive variables is employed, ensuring conservation at the interfaces of embedded grids, as well as global conservation. The method is applied to the simulation of a turbulent flow past a backward facing step, the flow over a square obstacle, and the flow in a sudden pipe expansion, and the predictions are compared with data published in the literature. They show that neither the convergence rate nor the stability of the method are affected by the presence of embedded grids. The grid-embedding technique yields significant savings in computing time to achieve the same accuracy obtained wing conventional grids. (C) 1997 by Elsevier Science Inc.
- Modelação numérica de escoamentos atmosféricos : efeitos orográficos e de camada limitePublication . Argaín, José Luís AlmaguerNeste trabalho desenvolveu-se um modelo numérico bidimensional para estudar escoamentos na vizinhança de obstáculos.
- Estimation of surface-layer scaling parameters in the unstable boundary layer: Implications for orographic flow speed-upPublication . Argaín, José Luís Almaguer; Teixeira, Miguel A. C.; Miranda, Pedro M. A.A method is proposed for estimating the surface-layer depth and the friction velocity as a function of stability (here quantified by the Obukhov length, L) over the complete range of unstable flow regimes. This method extends that developed previously for stable conditions by Argain et al. (Boundary-Layer Meteorol 130:15-28, 2009), but uses a qualitatively different approach. The method is specifically used to calculate the fractional speed-up in flow over a ridge, although it is suitable for more general boundary-layer applications. The behaviour of and as a function of L is indirectly assessed via calculation of using the linear model of Hunt et al. (Q J R Meteorol Soc 29:16-26, 1988) and its comparison with the field measurements reported in Coppin et al. (Boundary-Layer Meteorol 69:173-199, 1994) and with numerical simulations carried out using a non-linear numerical model, FLEX. The behaviour of estimated from the linear model is clearly improved when is calculated using the method proposed here, confirming the importance of accounting for the dependences of and on L to better represent processes in the unstable boundary layer.
- The importance of friction in mountain wave drag amplification by scorer parameter resonancePublication . Teixeira, M. A. C.; Argaín, José Luís Almaguer; Miranda, P. M. A.A mechanism for amplification of mountain waves, and their associated drag, by parametric resonance is investigated using linear theory and numerical simulations. This mechanism, which is active when the Scorer parameter oscillates with height, was recently classified by previous authors as intrinsically nonlinear. Here it is shown that, if friction is included in the simplest possible form as a Rayleigh damping, and the solution to the TaylorGoldstein equation is expanded in a power series of the amplitude of the Scorer parameter oscillation, linear theory can replicate the resonant amplification produced by numerical simulations with some accuracy. The drag is significantly altered by resonance in the vicinity of n/l0 = 2, where l0 is the unperturbed value of the Scorer parameter and n is the wave number of its oscillation. Depending on the phase of this oscillation, the drag may be substantially amplified or attenuated relative to its non-resonant value, displaying either single maxima or minima, or double extrema near n/l0 = 2. Both non-hydrostatic effects and friction tend to reduce the magnitude of the drag extrema. However, in exactly inviscid conditions, the single drag maximum and minimum are suppressed. As in the atmosphere friction is often small but non-zero outside the boundary layer, modelling of the drag amplification mechanism addressed here should be quite sensitive to the type of turbulence closure employed in numerical models, or to computational dissipation in nominally inviscid simulations. Copyright (c) 2012 Royal Meteorological Society
- Drag produced by trapped lee waves and propagating mountain waves in a two-layer atmospherePublication . Teixeira, M. A. C.; Argaín, José Luís Almaguer; Miranda, P. M. A.The surface drag force produced by trapped lee waves and upward propagating waves in non-hydrostatic stratified flow over a mountain ridge is explicitly calculated using linear theory for a two-layer atmosphere with piecewise-constant static stability and wind speed profiles. The behaviour of the drag normalized by its hydrostatic single-layer reference value is investigated as a function of the ratio of the Scorer parameters in the two layers l2/l1 and of the corresponding dimensionless interface height l1H, for selected values of the dimensionless ridge width l1a and ratio of wind speeds in the two layers. When l2/l1 1, the propagating wave drag approaches 1 in approximately hydrostatic conditions, and the trapped lee wave drag vanishes. As l2/l1 decreases, the propagating wave drag progressively displays an oscillatory behaviour with l1H, with maxima of increasing magnitude due to constructive interference of reflected waves in the lower layer. The trapped lee wave drag shows localized maxima associated with each resonant trapped lee wave mode, occurring for small l2/l1 and slightly higher values of l1H than the propagating wave drag maxima. As l1a decreases, i.e. the flow becomes more non-hydrostatic, the propagating wave drag decreases and the regions of non-zero trapped lee wave drag extend to higher l2/l1. These results are confirmed by numerical simulations for l2/l1 = 0.2. In parameter ranges of meteorological relevance, the trapped lee wave drag may have a magnitude comparable to that of propagating wave drag, and be larger than the reference single-layer drag. This may have implications for drag parametrization in global climate and weather-prediction models. Copyright (c) 2012 Royal Meteorological Society