Systematic Validation Study of an Unsteady Cavitating Flow over a Hydrofoil Using Conditional Averaging: LES and PIV

We present results of Large-eddy simulations (LES) modeling of steady sheet and unsteady cloud cavitation on a two-dimensional hydrofoil which are validated against Particle image velocimetry (PIV) data. The study is performed for the angle of attack of 9∘ and high Reynolds numbers ReC of the order of 106 providing a strong adverse pressure gradient along the surface. We employ the Schnerr–Sauer and Kunz cavitation models together with the adaptive mesh refinement in critical flow regions where intensive phase transitions occur. Comparison of the LES and visualization results confirms that the flow dynamics is adequately reproduced in the calculations. To correctly match averaged velocity distributions, we propose a new methodology based on conditional averaging of instantaneous velocity fields measured by PIV which only provides information on the liquid phase. This approach leads to an excellent overall agreement between the conditionally averaged fields of the mean velocity and turbulence intensity obtained experimentally and numerically. The benefits of second-order discretization schemes are highlighted as opposed to the lower-order TVD scheme.

Ежекторний детонаційний двигун на екологічно чистих компонентах палива

The subject of research in the article is engines operating on the detonation principle of converting the energy of the working fluid. In recent years, there has been an exponential growth in the number of scientific papers devoted to detonation engines, and the most promising direction is the study of detonation engines with an ejector nozzle (EN). The work aims to obtain the results of studies of the defining characteristics of a detonation engine with an ejector nozzle. The main tasks are the scientific analysis of the working process of the pulse detonation engine with EN; modeling of working processes occurring in the flow path of the engine; numerical implementation of a mathematical model and a computational experiment. Methods, for the numerical implementation of the model of a detonation engine with an ejector, a finite-difference TVD scheme of the second order of accuracy was used. According to the results of the work performed, we observe two regions on the pressure curves, within which the pressure remains unchanged for a certain time interval (pressure plateau). An increase in the length of the ejector leads to an increase in the duration of the stage of the outflow of detonation products from the flow path of the engine, an increase in the added mass of atmospheric air, and contributes to a significant increase in the specific impulse of thrust. The value of the thrust impulse was obtained by integrating the excess pressure on the traction wall over time. Conclusions. The scientific novelty is as follows. The change in pressure overtime on the traction wall of the detonation chamber when using cylindrical EHs of different lengths was investigated by the method of numerical simulation. The value of the thrust coefficient of the ejector nozzle for the starting conditions is obtained. The studies carried out in this work are aimed at analyzing the operating mode of a promising propulsion system and are aimed at modeling the gas-dynamic processes of a pulsed detonation engine with an ejector to obtain the data necessary for preliminary design, consideration of alternative design options, and an operational assessment of the possible characteristics of an engine with an ejector. The main advantages of the engine are the ultra-high-speed of energy release in the detonation process, which leads to an increase in the efficiency of the thermodynamic cycle, simplification and cost reduction of the design, and a significant gain in in-flight performance.

Analysis of the Effectiveness of Variants of the Exhaust Compartment of a Steam Turbine

The results of a numerical study of the gas-dynamic and power characteristics of the exhaust compartment of low-pressure cylinder, including the last stage with a blade of 1100 mm length and the exhaust path, in terms of their interaction are presented. Numerical models of exhaust compartment variants, including steam extraction in front of last stage diaphragm, two steam suctions from the interring gap with a dehumidification chamber, above-band leakages, steam injection from the dehumidification chamber into the diffuser channel have been developed. The flow in each computational subdomain is described by the complete system of non-stationary Reynolds – Favre equations averaged by Navier – Stokes. Turbulent effects are described with the SST Menter model in the stage, and with the modified Spalart – Allmaras turbulence model in the path. The integration of system of equations was carried out using an author’s software package. The calculated subdomains were approximated by hexagonal grids. The solver used an implicit difference TVD-scheme of finite volumes of the 2nd order of accuracy based on the solution of the Riemann problem. A variant of the algorithm based on splitting the computational process for multiprocessor platforms was used in the calculation. The model of stage used an averaging the mass, momentum and energy fluxes in the circumferential direction in the interstage gap. One channel of the diaphragm with a pre-connected fragment, as well as one channel of the working ring and the flow in the branch pipe were calculated. Parameters were exchanged between the stage and the branch pipe on the basis of mass flow averaging. The calculations were based on a table model of wet steam in the approximation of equilibrium condensation. The variants of improvement of the flow part of the exhaust compartment were considered. A study of the operation of the compartment options for the nominal mode of the K-220-44-2M turbine of the Loviisa nuclear power plant was performed. The effectiveness of lowering the cap of the collecting chamber, controlling the flow using sheet ribs above the diffuser shell, expanding the collecting chamber in the plane of the horizontal connector, and ensuring the release of excess steam from the moisture removal chamber through the tangential slots in the lower half of the convex shell of the diffuser were analyzed.

Validation of a Numerical Quasi-One-Dimensional Model for Wave Rotor Turbines With Curved Channels

A wave rotor is a shock-driven pressure exchange device that, while relatively rarely studied or indeed, employed, offers significant potential efficiency gains in a variety of applications including refrigeration and gas turbine topping cycles. This paper introduces a quasi-one-dimensional (Q1D) wave action model implemented in matlab for the computation of the unsteady flow field and performance characteristics of wave rotors of straight or cambered channel profiles. The purpose here is to introduce and validate a rapid but reliable method of modeling the performance of a power-generating wave rotor where little such insight exists in open literature. The model numerically solves the laminar one-dimensional (1D) Navier–Stokes equations using a two-step Richtmyer time variation diminishing (TVD) scheme with minmod flux limiter. Additional source terms account for viscous losses, wall heat transfer, flow leakage between rotor and stator endplates as well as torque generation through momentum change. Model validation was conducted in two steps. First of all, unsteady and steady predictive capabilities were tested on three-port pressure divider rotors from open literature. The results show that both steady port flow conditions as well as the wave action within the rotor can be predicted with good agreement. Further validation was done on an in-house developed and experimentally tested four-port, three-cycle, throughflow microwave rotor turbine featuring symmetrically cambered passage walls aimed at delivering approximately 500 W of shaft power. The numerical results depict trends for pressure ratio, shaft power, and outlet temperature reasonably well. However, the results also highlight the need to accurately measure leakage gaps when the machine is running in thermal equilibrium.

Numerical simulation of trans-critical flows in open channels using a FVM scheme

The present study makes efforts to simulate the behavior of fully developed stationary shocks, caused by the incidence of supercritical flow with a cross-barrier in an open channel. The numerical solution of nonlinear governing shallow flow equations has been implemented by the application of a second-order Roe TVD scheme. The obtained results from numerical experiment are compared with some measured in a laboratory setup. It can be deduced by comparison of the flow depths in numerical and measured experiments in three different cases of cross-barrier width of 6, 12 and 16 cm that the numerical scheme of Roe is a robust and capable method for simulation of complicated stationary shocks in shallow water flow.

A New TVD Scheme for Gradient Smoothing Method Using Unstructured Grids

An improved [Formula: see text]-factor algorithm for implementing total variation diminishing (TVD) scheme has been proposed for the gradient smoothing method (GSM) using unstructured meshes. Different from the methods using structured meshes, for the methods using unstructured meshes, generally the upwind point cannot be clearly defined. In the present algorithm, the value of upwind point has been successfully approximated for unstructured meshes by using the GSM with different gradient smoothing schemes, including node GSM (nGSM) midpoint GSM (mGSM) and centroid GSM (cGSM). The present method has been used to solve hyperbolic partial differential equation discontinuous problems, where three classical flux limiters (Superbee, Van leer and Minmod) were used. Numerical results indicate that the proposed algorithm based on mGSM and cGSM schemes can avoid the numerical oscillation and reduce the numerical diffusion effectively. Generally the scheme based on cGSM leads to the best performance among the three proposed schemes in terms of accuracy and monotonicity.

Computational study of flow around a NACA 0012 by using Roe FVM scheme and davis-yee TVD scheme

<p>Currently CFD had been considered as an important tool for solving engineering problems. The application of CFD had been used intensively in aircraft industries in design a new aircraft or in the effort of improvement on the exiting aircraft. In term of CFD computer code, the CFD code differs with any others may due to the difference in the numerical scheme have been used. Therefore, the present work presents the comparison result between two developed computer codes with ANSYS-FLUENT software to the case of transonic steady flow past through airfoil NACA 0012. The first computer code used a finite difference method with numerical scheme according to Davis-Yee TVD scheme. Meanwhile, the second computer code used a Roe’s cell centre finite volume scheme. The flow analysis is carried out at two Mach number, M (0.65 &amp; 0.8). Each Mach number applied to two different angles of attacks (0° &amp; 5°). The flow domain discretized by use of C-topology with 193x63 grid points. The comparison in term of the pressure coefficient, along the airfoil surface are presented. From the result, indicated that developed computer code is able to capture the presence of shock wave in the flow field.</p>