1,721,027 research outputs found
Snowflakes shape characterization via bayesian inference: exploring the challenges
Full text linkThis work explores the challenges underlying the inference of characteristic parameters describing the aerodynamics of blowing snowflakes, with application to in-flight snow accretion and engine ingestion. Due to the shortage of experimental observations, the classical Bayesian problem is formulated with respect to synthetic data generated from statistics of falling snow measurements. The goal is to expose issues possibly hindering the aerodynamic shape inference process, in order to anticipate barriers and envisage solutions to apply when a comprehensive experimental data set will be available. This paper provides guidelines for implementing novel experiments, including specifications concerning the desirable accuracy and precision of measurement systems
VirtuaSchlieren: A hybrid GPU/CPU-based schlieren simulator for ideal and non-ideal compressible-fluid flows
Full text linkA schlieren post-processing tool for CFD simulations of ideal and non-ideal compressible-fluid flows is presented. The software VirtuaSchlieren simulates light propagation across a non-homogeneous medium to predict the schlieren images from an actual measurement apparatus. Trajectories of a large number of light rays-in the order of tens of millions-are reconstructed by numerical integration, from the light source to the screen plane. To this purpose, kd-tree search algorithms are implemented to retrieve the position of each ray within the CFD computational grid at each time step. The local value of the refraction index was retrieved from the interpolated value of the density at the center of each cell. The simple Gladstone-Dale and the Lorentz-Lorenz models are implemented to compute the value of the refractive index. Two search algorithms are evaluated, namely, the approximate nearest neighbor (ANN) and a simplified kd-tree search technique. The latter is implemented on both CPU and GPU architectures. The hybrid GPU/CPU implementation was successfully tested against a reference experimental schlieren image of the supersonic flow around a conical body. Numerical simulations of the supersonic expansion of non-ideal flows are also presented
Blockage and Three-Dimensional Effects in Wind-Tunnel Testing of Ice Accretion over Wings
Full text linkWind-tunnel testing of ice accretion over aircraft wings is simulated numerically to investigate the effects of tunnel blockage and of the wall–wing interference at the wall–model juncture. The open-source OpenFOAM software is used to compute the flowfield and the trajectories of water droplets. The PoliMIce ice-prediction software is applied to solve the multiphase flow around the body surface and to compute the thickness of the iced layer. Blockage effects are investigated for diverse operating conditions. Similar to dry testing, wind-tunnel blockage is found to produce a variation of the apparent angle of attack of the airfoil with respect to the freestream. However, different from dry testing, the blockage correction is found to be a function of time, due to the continuous modification of the airfoil shape in time resulting from ice accretion. The wall–wing interference results in the occurrence of a shadow region where water droplets do not impinge. In the rime-ice regime, the shadow region is therefore free of ice. In the glaze-ice regime, instead, the liquid film over the wing moves toward the endwall and ice is formed in the shadow region
SU2-COOL: Open-source framework for non-ideal compressible fluid dynamics
Full text linkWe present a fully open-source framework for the numerical simulation of Non-Ideal Compressible Fluid Dynamics (NICFD). The open-source Computational Fluid Dynamics suite SU2 is coupled to the open-source thermophysical library CoolProp, which includes state-of-the-art thermodynamic models of numerous pure fluids and mixtures relevant to applications. Accurate thermodynamic models are needed due to non-ideal operating conditions in which the fluid thermodynamics cannot be described by the simple ideal-gas law (Pv = RT). The coupling interface implements new C++ classes, which allow the automatic exchange of information between SU2 and CoolProp, and it is made directly available as an additional module integrated into the open-source SU2 suite. To assess the performance of the NICFD simulation framework, we present three test cases: a nozzle flow exhibiting non-ideal thermodynamics effects, a nozzle flow with non-monotone Mach number variation, a representative non-ideal gasdynamics effect, and a non-classical rarefaction oblique shock over a wedge. Results are verified against available experiment data and solutions obtained with different implementations of non-ideal thermodynamics in SU2. Performance of the new framework is assessed on user-friendliness, scalability, solution accuracy, and computational efficiency
Non-ideal oblique shock waves
Full text linkFrom the analysis of the isentropic limit of weak compression shock waves, oblique shock waves in which the post-shock Mach number is larger than the pre-shock Mach number, named non-ideal oblique shocks, are admissible in substances characterized by moderate molecular complexity and in the close proximity to the liquid-vapour saturation curve. Non-ideal oblique shocks of finite amplitude are systematically analysed, clarifying the roles of the pre-shock thermodynamic state and Mach number. The necessary conditions for the occurrence of non-ideal oblique shocks of finite amplitude are singled out. In the parameter space of pre-shock thermodynamic states and Mach number, a new domain is defined which embeds the pre-shock states for which the Mach number increase can possibly take place. The present findings are confirmed by state-of-the-art thermodynamic models applied to selected commercially available fluids, including siloxanes and hydrocarbons currently used as working fluids in renewable energy systems
Local Solution to the Unsteady Stefan Problem for In-Flight Ice Accretion Modeling
Full text linkA new model for in-flight ice accretion is presented for both rime and glaze conditions. The model is based on the local, exact solution of the unsteady Stefan problem for the temperature profiles within the ice layer in glaze conditions. The new model moves from Myers’s formulation, and it includes an unsteady description of the heat diffusion problem within the ice layer. Moreover, the local value of the air temperature outside the boundary layer is used to compute convective heat fluxes, in place of the constant freestream temperature value considered in Myers’s model. A source term is introduced to take into account mass transfer at the boundary separating rime and glaze regions. The model was implemented in the ice accretion software PoliMIce to perform numerical simulations of inflight ice accretion over two-dimensional airfoils in both rime and glaze ice regimes. The open-source computational fluid dynamics software OpenFOAM was used to compute the aerodynamic flowfield and to reconstruct water droplet trajectories. Numerical results suggest that the modifications introduced with respect to the original Myers model improve significantly the accuracy of the predicted ice shapes for the considered test cases. The introduction of the local value of air temperature was found to be essential for the formation of the well-known two-horn ice shape, due to the occurrence of a local glaze to rime transition. The diverse contributions to the heat fluxes are discussed for both the proposed and the Myers models
Non-ideal compressible-fluid effects in oblique shock waves
Full text linkThe non-monotone dependence of the speed of sound along adiabatic transformations is demonstrated to result in the admissibility of non-ideal increase of the flow Mach number across oblique shock waves, for pre-shock states in close proximity of the liquid-vapour saturation curve. This non-ideal behaviour is primarily associated with a less-than-unity value of the fundamental derivative of gasdynamics and, therefore, non-ideal shock waves are expected to be observed in flows of fluids with moderate molecular complexity. The simple yet qualitatively sound van der Waals model is used to confirm the present findings and to provide exemplary non-ideal shock waves
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