309 research outputs found
Numerical simulations of ice accretion on wind turbine blades: are performance losses due to ice shape or surface roughness?
Ice accretion on wind turbine blades causes both a change in the shape of its sections and an increase in surface roughness. These lead to degraded aerodynamic performances and lower power output. Here, a high-fidelity multi-step method is presented and applied to simulate a 3 h rime icing event on the National Renewable Energy Laboratory 5 MW wind turbine blade. Five sections belonging to the outer half of the blade were considered. Independent time steps were applied to each blade section to obtain detailed ice shapes. The roughness effect on airfoil performance was included in computational fluid dynamics simulations using an equivalent sand-grain approach. The aerodynamic coefficients of the iced sections were computed considering two different roughness heights and extensions along the blade surface. The power curve before and after the icing event was computed according to the Design Load Case 1.1 of the International Electrotechnical Commission. In the icing event under analysis, the decrease in power output strongly depended on wind speed and, in fact, tip speed ratio. Regarding the different roughness heights and extensions along the blade, power losses were qualitatively similar but significantly different in magnitude despite the well-developed ice shapes. It was found that extended roughness regions in the chordwise direction of the blade can become as detrimental as the ice shape itself
Shock Tube Flows Past Partially Opened Diaphragms
Unsteady compressible flows resulting from the incomplete burst of the shock tube
diaphragm are investigated both experimentally and numerically for different initial
pressure ratios and opening diameters. The intensity of the shock wave is found
to be lower than that corresponding to a complete opening. A heuristic relation is
proposed to compute the shock strength as a function of the relative area of the
open portion of the diaphragm. Strong pressure oscillations past the shock front are
also observed. These multi-dimensional disturbances are generated when the initially
normal shock wave diffracts from the diaphragm edges and reflects on the shock
tube walls, resulting in a complex unsteady flow field behind the leading shock wave.
The limiting local frequency of the pressure oscillations is found to be very close to the
ratio of acoustic wave speed in the perturbed region to the shock tube diameter. The
power associated with these pressure oscillations decreases with increasing distance
from the diaphragm since the diffracted and reflected shocks partially coalesce into
a single normal shock front. A simple analytical model is devised to explain the
reduction of the local frequency of the disturbances as the distance from the leading
shock increases
Admissibility Region for Rarefaction Shock Waves in Dense Gases
In the vapour phase and close to the liquid–vapour saturation curve, fluids made of complex molecules are expected to exhibit a thermodynamic region in which the fundamental derivative of gasdynamic G is negative. In this region, non-classical
gasdynamic phenomena such as rarefaction shock waves are physically admissible, namely they obey the second law of thermodynamics and fulfil the speed-orienting condition for mechanical stability. Previous studies have demonstrated that the
thermodynamic states for which rarefaction shock waves are admissible are however not limited to the G <0 region. In this paper, the conditions for admissibility of rarefaction shocks are investigated. This results in the definition of a new thermodynamic region – the rarefaction shocks region – which embeds the G <0 region. The rarefaction shocks region is bounded by the saturation curve and by the locus of the states connecting double-sonic rarefaction shocks, i.e. shock waves in
which both the pre-shock and post-shock states are sonic. Only one double-sonic shock is shown to be admissible along a given isentrope, therefore the double-sonic states can be connected by a single curve in the volume–pressure plane. This curve is named
the double sonic locus. The influence of molecular complexity on the shape and size of the rarefaction shocks region is also illustrated by using the van der Waals model; these results are confirmed by very accurate multi-parameter thermodynamic models applied to siloxane fluids and are therefore of practical importance in experiments aimed at proving the existence of rarefaction shock waves in the single-phase vapour region as well as in future industrial applications operating in the non-classical regime
- …