Flow, Turbulence and Combustion最新文献

The objectives of this study were (i) to create a unique test case of an unsteady flow around a cone in a Ludwieg tube for the purpose of validating computational codes; (ii) to test a new method for determining non-stationary aerodynamic loads using a strain-gauge balance; and (iii) to test a new method for determining unsteady pressure using a Pitot-Prandtl-type probe with a cavity between the sensor and the flow. The results of testing of a 10° cone in the TsAGI short-duration UT-1M Ludwieg-type tube at Mach number M = 6 are presented. During the runs, the following parameters were measured simultaneously: non-stationary aerodynamic loads using an internal six-component strain-gauge balance, dynamic behavior of the cone and balance using three-axis accelerometers, unsteady pressure on the cone surface, total and static pressure using a Pitot-Prandtl-type probe, cone surface temperature using a temperature sensitive paint, flow parameter fluctuations on the cone surface using a constant voltage film thermoanemometer. In some tests, the flow was visualized using the Schlieren method. Before the tests, numerical calculations of the flow around the cone in the facility were performed with the EWT-TsAGI software package to select the location of the oversized cone and the Pitot-Prandtl-type probe in the test section. The conducted studies confirmed the applicability of the new methods for measuring non-stationary forces and unsteady pressure in short-duration wind tunnels.

It is well-known that kinetic energy produced artificially by an inadequate numerical discretization of nonlinear transport terms may lead to a blow-up of the numerical solution in simulations of fluid dynamical problems such as incompressible turbulent flows. However, the community seems to be divided whether this problem should be resolved by the use of discretely energy-preserving or dissipative discretization schemes. The rationale for discretely energy-preserving schemes is often based on the expectation of exact conservation of kinetic energy in the inviscid limit, which mathematically relies on the assumption of sufficient regularity of the solution. There is the (contradictory) phenomenological observation in turbulence that flows dissipate energy in the limit of vanishing molecular viscosity, an “anomalous” phenomenon termed dissipation anomaly or the zeroth law of turbulence. As already conjectured by Onsager, the Euler equations may dissipate kinetic energy through the formation of singularities of the velocity field. With the proof of Onsager’s conjecture in recent years, a consequence for designing numerical methods for turbulent flows is that the smoothness assumption behind conservation of energy in the inviscid limit becomes indeed critical for turbulent flows. The velocity field rather has to be expected to show singular behavior towards the inviscid limit, supporting the dissipation of kinetic energy. Our main argument is that designing numerical methods against the background of this physical behavior is a strong rationale for the construction of dissipative (or dissipation-aware) numerical schemes for convective terms. From that perspective, numerical dissipation does not appear artificial, but as an important ingredient to overcome problems introduced by energy-conserving numerical methods such as the inability to represent anomalous dissipation as well as the accumulation of energy in small scales, which is known as thermalization. This work discusses stabilized (H^1), (L^2), and (H(mathrm{div}))-conforming finite element methods for incompressible flows with a focus on the energy-stability of the numerical method and its dissipation mechanisms to predict inertial dissipation. Finally, we discuss the achievable convergence rate for the kinetic energy in under-resolved turbulent flow simulations.

The consideration of the inherently non-local characteristics of turbulence is an open challenge and subject to many investigations. Recent approaches rely on the utilization of spatially configured Neural Networks such as e.g. Convolutional Neural Networks to account for non-local effects (Comput. Methods Appl. Mech. Eng. 384:113927, 2021). Nevertheless, approaches featuring Neural Networks are not easily available for Gene Expression Programming. An alternative option, to consider non-local effects, is the use of partial differential equations (PDE) like an additional convection-diffusion equation as is done for example in several transition models such as the (gamma)- model by Menter et al. (Flow Turbul. Combust. 583–619, 2015). Consequently, instead of only modeling a local correction factor directly using GEP, we equip the input quantities with an additional optional convection-diffusion equation of which we model the production term, diffusion constants and boundary type. The methodology is applied on a set of low pressure turbine testcases in order to find transition models. Resulting expressions are further analysed in terms of underlying mechnims and logical foundations.

This work examines the momentum boundary layer evolution on the piston top of the Darmstadt optically accessible Internal Combustion Engine (ICE). For this purpose, a 3D-CFD wall-resolved Large Eddy Simulation (LES) under motored conditions was deployed. The piston wall is resolved down to 25 (upmu)m, corresponding to ({y^ + } < 1). For statistical purposes and to compare with experimental data, 33 consecutive engine cycles are simulated. A large-scale tumble motion characterizes the flow field. This flow impinges on the piston on the exhaust side, it moves along the flat piston wall and detaches on the intake side. The near-wall velocities of the simulations align well with the experiment. Analysis revealed regions of Favorable Pressure Gradient (FPG) on the exhaust side and Adverse Pressure Gradient (APG) on the intake side, separated by a sharp pressure inversion zone. The near-wall flow accelerates and then decelerates until detachment. Analysis of the non-dimensional ({u^ + } - {y^ + }) profiles reveals the absence of a logarithmic region in the boundary layer. This scaling procedure is sensitive to thermo-physical properties like density and viscosity that vary across the boundary layer, which complicates comparisons with canonical studies. The shape factor of the boundary layer suggests a fully turbulent state despite the low momentum thickness-based Reynolds number. The boundary layer height increases from the exhaust towards the intake side, especially in the presence of strong pressure gradients. Pressure gradients acting perpendicular to the boundary layer are observed. The comparison of ensemble-averaged and single-cycle instantaneous data shows high levels of cyclic fluctuations.

Turbulent impinging jet (TIJ) flows are a canonical type of flow that is present in nature and in a wide range of industrial applications, making their study indispensable. Among them, multiple co-flowing and angled jets offer possibilities for various practical applications. However, fundamental information on these particular jet configurations is scarce, and there is also a lack of data for validating numerical simulations of these jet flows. Therefore, this paper presents an experimental analysis of isothermal plane turbulent impinging co-flowing and angled jets at moderate Reynolds numbers (Re_jet ≈ 8,700 and 10,000) and height-to-width ratio (γ = 40.5) utilizing 2D particle image velocimetry (PIV). It also validates the results of several RANS turbulence models that are commonly used for simulating single straight TIJs: standard k-ε (SKE) model, realizable k-ε model (RKE), renormalization group (RNG) k-ε model, baseline (BSL) k-ω model, shear-stress transport (SST) k-ω model, and a Reynolds-stress model (RSM). The analysis and validation focus on detailed velocity measurements while also providing insights into turbulence parameters. Results reveal strong similarities between the two analyzed TIJs and single straight TIJs at the developed free-jet (or combined jet region for the co-flowing jets configuration) and impingement regions. The validation study demonstrates that relatively inexpensive RANS simulations in combination with typical k-ε turbulence models are capable of resolving the mean velocity field of the two investigated TIJ configurations with good accuracy, which is especially the case for the RNG k-ε turbulence model that yields a very good match with the PIV data throughout.

Lean, premixed, swirl-stabilized flames are widely used in modern Dry Low Emissions gas turbine combustors; however, the turbulent combustion process under those conditions is known to be extremely sensitive and prone to instabilities. Numerical simulations can be a valuable tool in predicting the effects of alternative fuels; however, the sensitivity of the results to different models ought to be outlined. In this work, we present the results of Large Eddy Simulations performed on the CECOST burner with both Finite Rate Chemistry and Flamelet Progress Variable combustion models, non-adiabatic boundary conditions, and radiation modeling. The results highlight a surprising sensitivity of the simulation results in terms of mean fields, flame macrostructure, and flame dynamics. We discuss the model effects on the coupling mechanisms between turbulence and combustion, e.g., thermal expansion, and we conclude that, in particularly sensitive cases, they are capable of locally altering the flowfield to the extent it influences key flow structures on which flame stabilization relies. Additionally, the interaction between the smallest resolved scales of turbulence and the flame front is also affected, resulting in distinct flame dynamics.

The combustion of conventional Jet A, alongside two alternative jet fuels, C1 and C5, is simulated with Large Eddy Simulations (LES) in a generic single-cup spray combustor during idle and cruise conditions. The spray is modeled using Lagrangian particle tracking and the combustion chemistry of each fuel is modeled by skeletal reaction mechanisms. The volatility and atomizability of each fuel directly affect the spray penetration depth, with Jet A having the longest spray and C5 the shortest. All fuels have qualitatively similar flames at idle conditions, but the Jet A flame is relatively lifted at cruise conditions. C1 and C5 have similar flames despite different spray lengths, likely due to the rapid breakup of C1. The fuels produce different emission profiles, which is connected to their respective H/C ratios, equivalence ratios, and aromatics contents. NO_x emissions are particularly affected by the mixture fraction in the flame, resulting in high NO_x emissions for the compact C1 and C5 flames. Thermoacoustic oscillations are observed in all simulations but are strongest for C1 and C5, which we hypothesize is a result of their high volatility.

Parameter measurement of gas–liquid two-phase flow with a high gas void fraction has received great attention in the research field of multiphase flow. As a flowmeter on the sonar principle, the line array-mounted piezoelectric sensors can be applied for gas void fraction measurement of gas–liquid two-phase flow. Firstly, the formula for calculating the density and sound velocity of the medium is analyzed, including: the gas equation of air, the AGA8 and AGA10 standards of natural gas, and the IAPWS-IF97 model of water, combined with the Wood formula and the one-dimensional acoustic attenuation formula of the pipeline, it is deduced that in the gas void fraction > 50% high zone where the gas well is located, the mixed sound velocity increases with the increase of the gas void fraction, showing a monotony increasing correspondence. Secondly, the MVDR beamforming algorithm is applied to solve the “acoustic ridge” in the (k - omega) domain based on the one-dimensional acoustic model of the pipeline to realize the mixed sound velocity measurement of two-phase flow. Finally, the air and water standard device was used to replicate on-site gas well high gas void fraction experiments. The relative error was 2.62% and repeatability was 1.50% for vertically mounted large array spacing. The accuracy meets the requirements for gas well measurements, verifying the feasibility of using the acoustic method to measure gas void fraction. Therefore, it is of great engineering guiding significance to realize the measurement of gas void fraction in the on-site gas well.

Experiments and numerical modeling on vehicle aerodynamics were conducted in a Reynolds (Re) number one order of magnitude lower than that of typical full-scale application. Drag coefficient, velocity profile measurements and flow visualization were the focus with the proposition of comparing scale effects of a 1:10 sedan passenger vehicle model with numerical data from full-scale (1:1) based on the Reynolds Averaged Navier–Stokes (RANS) approach. After the validation of the numerical approach at 1:10 scale, additional investigation of sharp and rounded fillets presented on the car’s geometry showed to be relevant to the calculation of the separating shear layers and drag prediction, although the general wake structures are qualitatively similar. Effects of the reduced scale are translated to low Reynolds number where viscous effects starts to play a role. Detailed flow features such as recirculating regions and reversing flow acts on the model’s surface while the near wake velocity field is well captured and evaluated both experimentally and numerically. The results permitted to characterize flow details based on Re number flow, to show the effects of sharp corners on the model and to scrutinize the influence of scale effects on vehicle’s aerodynamics.