175 research outputs found
Critical point anomalies include expansion shock waves
No full textFrom first-principle fluid dynamics, complemented by a rigorous state equation accounting for critical anomalies, we discovered that expansion shock waves may occur in the vicinity of the liquid-vapor critical point in the two-phase region. Due to universality of near-critical thermodynamics, the result is valid for any common pure fluid in which molecular interactions are only short-range, namely, for so-called 3-dimensional Ising-like systems, and under the assumption of thermodynamic equilibrium.In addition to rarefaction shock waves, diverse non-classical effects are admissible, including composite compressive shock-fan-shock waves, due to the change of sign of the fundamental derivative of gasdynamics.Aerodynamics, Wind Energy, Flight Performance and PropulsionAerospace Engineerin
UNSTRUCTURED PERIODIC GRID GENERATION AROUND 2D TURBINE CASCADES
No full textA novel grid generation algorithm for periodic unstructured grid around two-dimensional highly-staggered turbine cascades, including multiply connected blade geometries, is presented. The idea is to generate the mesh in a transformed space in which the periodic boundaries are coincident and internal to the computational domain so that no special treatment along these curves is required. The mesh in the transformed u-v space is computed by means of front-advancing/Delaunay technique and the resulting grid is transformed back into the x-y space of physical variables after introducing a suitable cut, which translates to periodic boundaries in the x-y plane. The cut is not arbitrary and it is automatically managed by the algorithm. The proposed transformation is conformal and therefore no elements are distorted in the process. In this way, the prescribed element size and mesh quality are easily attained and the uniqueness of the periodic nodes is guaranteed by the fact that they are indeed coincident in the space u-v. Numerical simulations of a VKI LS-89 turbine blade are presented to support the present approach
Siloxanes: A new class of candidate Bethe-Zel’dovich-Thompson fluids
No full textThis paper presents a new class of Bethe-Zel’dovich-Thompson fluids, which are expected to exhibit nonclassical gasdynamic behavior in the single-phase vapor region. These are the linear and cyclic siloxanes, light silicon oils currently employed as working fluids in organic Rankine cycle turbines. State-of-the-art multiparameter equations of state are used to describe the thermodynamic properties of siloxanes and to compute the value of the fundamental derivative of gasdynamics ?, whose negative sign is the herald of nonclassical gasdynamics. Siloxane fluids starting from D6 and cyclic siloxanes of greater complexity, and MD3M and linear siloxanes of greater complexity are predicted to exhibit a thermodynamic region in which ? is negative and hence nonclassical wavefields are admissible. As an exemplary case, a nonclassical rarefaction shock wave propagating in fluid D6 is studied to demonstrate the possibility of using siloxane fluids in nonclassical gasdynamic applications and to experimentally verify the existence of nonclassical wavefields in the vapor phase. The sensitivity of the present results to the considered thermodynamic model of the fluid is also briefly discussed.Process and EnergyMechanical, Maritime and Materials Engineerin
Molecular interpretation of nonclassical gas dynamics of dense vapors under the van der Waals model
No full textThe van der Waals polytropic gas model is used to investigate the role of attractive and repulsive intermolecular forces and the influence of molecular complexity on the possible nonclassical gas dynamic behavior of vapors near the liquid-vapor saturation curve. The decrease of the sound speed upon isothermal compression is due to the well-known action of the van der Waals attractive forces and this effect is shown here to be comparatively larger for more complex molecules with a large number of active vibrational modes; for these fluids isentropic flows are in fact almost isothermal. Contributions to the speed of sound resulting from intermolecular forces and the role of molecular complexity are analyzed in details for both isothermal and isentropic transformations. Results of the exact solution to the problem of a finite pressure perturbation traveling in a still fluid are presented in three exemplary cases: ideal gas, dense gas and nonclassical gas behavior. A classification scheme of fluids based on the possibility of exhibiting different gas dynamic behaviors is also proposed.Process and EnergyMechanical, Maritime and Materials Engineerin
The influence of molecular complexity on expanding flows of ideal and dense gases
No full textThis paper presents an investigation about the effect of the complexity of a fluid molecule on the fluid dynamic quantities sound speed, velocity, and Mach number in isentropic expansions. Ideal-gas and dense-gas expansions are analyzed, using the polytropic ideal gas and Van der Waals thermodynamic models to compute the properties of the fluid. In these equations, the number of active degrees of freedom of the molecule is made explicit and it is taken as a measure of molecular complexity. The obtained results are subsequently verified using highly accurate multiparameter equations of state. For isentropic expansions, the Mach number does not depend on the molecular weight of the fluid but only on its molecular complexity and pressure ratio. Remarkably enough, the Mach number can either increase or decrease with molecular complexity, depending on the considered pressure ratio. The exit speed of sound and flow velocity, however, are dependent on both molecular complexity and weight, as well as on the inlet total temperature. The exit flow velocity is found to be a monotonically increasing function of molecular complexity for all expansion ratios, whereas the speed of sound monotonically increases with molecular complexity only at high pressure ratios. The speed of sound is not monotone for pressure ratios around 3, which leads to the Mach number being nonmonotone at pressure ratios around 10. It should be noted that the sound speed and flow velocity depend much more strongly on molecular weight than on molecular complexity, which in realistic expansions often obscures the influence of the latter. Quantitative differences are observed between ideal and dense-gas expansions, which are dependent on the reduced inlet conditions. The present study concludes with the numerical simulation of two-dimensional expansions in a turbine nozzle to document the occurrence of real-gas effects and their dependence on molecular complexity in realistic applications.Process and EnergyMechanical, Maritime and Materials Engineerin
Dynamics of cylindrical converging shock waves interacting with aerodynamic obstacle arrays
No full textCylindrical converging shock waves interacting with an array of aerodynamic obstacles are investigated numerically for diverse shock strengths and for different obstacle configurations in air in standard conditions. The considered number of obstacles N is 4, 6, 8, 16, and 24. Obstacles are lenticular airfoils with thickness-to-chord ratios t/c of 0.07, 0.14, and 0.21. The distances of the airfoil leading edge from the shock focus point rLE/rref LE are 1, 2, and 2.5, where rref LE = 7 is the dimensionless reference distance from the origin. Considered impinging shock Mach numbers Ms are 2.2, 2.7, and 3.2 at the reference distance from the origin. The reference experimental configuration (N = 8,t/c = 0.14,rLE = 7,Ms = 2.7) was proposed by Kjellander et al. ["Thermal radiation from a converging shock implosion," Phys. Fluids 22, 046102 (2010)]. Numerical results compare fairly well to available one-dimensional models for shock propagation and to available experimental results in the reference configuration. Local reflection types are in good agreement with the classical criteria for planar shock waves. The main shock reshaping patterns are identified and their dependence on the shock strength and obstacle configuration is exposed. In particular, different shock patterns are observed after the leading edge reflection, which results in polygonal shock wave with N, 2N, 3N, and 4N sides. The largest temperature peak at the origin is obtained for the 8-and the 16-obstacle configurations and for the smallest thickness to length ratio, 0.07, located at distance from the origin of 2rref LE. In terms of compression efficiency at the origin, the 16-obstacle configuration is found to perform slightly better than the reference 8-obstacle configuration-with an efficiency increase of about 2%-3%, which is well within the model accuracy-thus confirming the goodness of the obstacle arrangement proposed by Kjellander and collaborators
The flexible asymmetric shock tube (FAST): A Ludwieg tube facility for wave propagation measurements in high-temperature vapours of organic fluids
No full textThis paper describes the commissioning of the flexible asymmetric shock tube (FAST), a novel Ludwieg tube-type facility designed and built at Delft University of Technology, together with the results of preliminary experiments. The FAST is conceived to measure the velocity of waves propagating in dense vapours of organic fluids, in the so-called non-ideal compressible fluid dynamics (NICFD) regime, and can operate at pressures and temperatures as high as 21 bar and 400 ?C, respectively. The set-up is equipped with a special fast-opening valve, separating the high-pressure charge tube from the low-pressure plenum. When the valve is opened, a wave propagates into the charge tube. The wave speed is measured using a time-of-flight technique employing four pressure transducers placed at known distances from each other. The first tests led to the following results: (1) the leakage rate of 5×10?4mbarl s?1 for subatmospheric and 5×10?2mbarl s?1 for a superatmospheric pressure is compatible with the purpose of the conceived experiments, (2) the process start-up time of the valve has been found to be between 2.1 and 9.0 ms, (3) preliminary rarefaction wave experiments in the dense vapour of siloxane D6 (dodecamethylcyclohexasiloxane, an organic fluid) were successfully accomplished up to temperatures of 300?C, and (4) a method for the estimation of the speed of sound from wave propagation experiments is proposed. Results are found to be within 2.1 % of accurate model predictions for various gases. The method is then applied to estimate the speed of sound of D6 in the NICFD regime.Aerodynamics, Wind Energy & PropulsionAerospace Engineerin
Helicopter Shipboard Operation: Effect of Atmospheric Boundary Layer on Turbulent Ship Airwake and Rotor Aerodynamic Loads
No full textThis paper studies the effect of two different boundary layer models on the airwake of the Simple Frigate Shape 1 and further on the unsteady aerodynamic loads of a scaled helicopter model operating inside the ship airwake. The unsteady airwake of the isolated SFS1 is computed in a time-accurate approach using the open-source SU2 solver, implementing two types of boundary conditions: a Uniform Flow (UF) and a steady Atmospheric Boundary Layer (ABL), where the reduction of the velocity due to the surface roughness is also considered. The simulations are performed in two wind conditions, including headwind and 30◦ from the port-side (R30). The airwake data are implemented into a multibody simulation of a scaled helicopter model, developed using the open-source multibody software MBDyn, based on the one-way coupling approach. Hover tests are performed at different positions with respect to the deck and the unsteady aerodynamic loads are compared in frequency domain. In addition to the increase of unsteadiness in the red wind compared to the headwind simulation, the results indicate that in both wind conditions, the unsteadiness is reduced with the presence of the steady ABL. Since the unsteady loads are expected to directly affect the pilot workload, the results highlight the importance of modelling a realistic boundary layer considering both steady and turbulent profiles
On the Fundamental Derivative of Gas Dynamics in the Vapor-Liquid Critical Region of Single-Component Typical Fluids
No full textThis document describes the results of an investigation on the variation of the so-called fundamental derivative of gas dynamics, Γ, in the vapor–liquid critical region of well-measured substances, namely methane, carbon dioxide and water, for which accurate, scaled fundamental equations are available. The results demonstrate that for a pure fluid in the single-phase thermodynamic regime, Γ diverges to +∞ independent of the direction of approach of the vapor–liquid critical point. Furthermore, in the two-phase thermodynamic regime, Γ diverges to −∞ independent of the direction of approach of the vapor–liquid critical point. These two qualitative results, as well as the value of the exponent giving the power-law dependence of Γ along the critical isochore as a function of |T − TC|/T (T is the temperature and “C” indicates its critical point value), namely ≈−0.89, are similar for all pure, non-ionized fluids belonging to the class of 3-dimensional Ising-like systems, i.e., systems governed by short-range forces
Admissibility region for rarefaction shock waves in dense gases
No full textIn 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 ? 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 ? <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 ? <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.Process and EnergyMechanical, Maritime and Materials Engineerin
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