Engine In-cylinder Flow Research Articles

Dynamic Mode Decomposition for the Comparison of Engine In-Cylinder Flow Fields from Particle Image Velocimetry (PIV) and Reynolds-Averaged Navier–Stokes (RANS) Simulations

Validation of Reynolds-averaged Navier–Stokes (RANS) simulation results against experimental data such as flow measurements from particle image velocimetry (PIV) remains a challenge for the development of thermal propulsion systems. This is partly due to cycle-to-cycle variations (CCVs) in the air motion and partly due to uncertainties in the PIV measurement technique, complicating the question of what constitutes a fair validation target for the RANS model. Indeed, an inappropriate validation target can misguide subsequent adjustments of a RANS model. In this work, the ensemble-averaged PIV field is first investigated for its suitability as a validation target for RANS simulations. The relevance index and the velocity histogram distance are used as quantitative metrics to assess the similarity of the ensemble-averaged field to the full dataset of individual PIV cycles. While a high similarity is seen between the average PIV flow field and the individual cycles on the tumble plane, the similarity is lower and more variable on the cross-tumble plane, where there are significant CCVs. Standard (space-only, phase-dependent) proper orthogonal decomposition (POD) is employed as an alternative method of data processing with the aim of providing a fairer comparison to RANS simulations. The cycle-dependence of the standard POD modes is shown to be an aspect that results in many validation targets and an excessively broad validation range, limiting its utility in this context. Dynamic mode decomposition (DMD) and sparsity-promoting dynamic mode decomposition (SPDMD) are then proposed as alternative solutions, capable of extracting flow structures at specific frequencies. The background 0 Hz SPDMD modes exhibit an ability to produce more realistic flow fields with velocity magnitudes that are significantly closer to the individual cycles.

A Study on In-Cylinder Flow of Small Engine Using Steady-State Flow Benches

In-cylinder air flow structures are known to strongly impact on the performance and combustion of internal combustion engines (ICE). Therefore, the aim of this paper is to experimentally study an IC engine in-cylinder flow under steady-state conditions. Steady-state flow bench methods can be used to characterize the in-cylinder flow engine. The experiments were carried out on the small cylinder head with pressure difference across the inlet valves of 100 to 300 mmH2O. The experimental results are presented in terms of the measured air flow rate, flow coefficient and discharge coefficient. Moreover, the CFD used to illustrate the evolution of tumble motion in-cylinder engine. The results show that air volume flow rate and flow coefficient increased as the valve lift increases but the coefficient of discharge decrease if valve lift increase. The higher the pressure drop the higher the mass flow rate value. However, the higher the pressure drop the lower the value of flow coefficient and discharge coefficient.

非定常Adjoint 法によるCFD 逆解析技術の開発とエンジン筒内流れへの適用

非定常Adjoint 法によるCFD 逆解析技術の開発とエンジン筒内流れへの適用

Experimental study on an IC engine in-cylinder flow using different steady-state flow benches

In-cylinder air flow structures are known to strongly impact on the performance and combustion of internal combustion engines (ICE). Therefore the aim of this paper is to experimentally study an IC engine in-cylinder flow under steady-state conditions. Different methods can be used to characterize the in-cylinder flow which are optical engines and laser diagnostics, computational fluid dynamic and steady-state flow bench. Here we are concentrating on two different types of flow benches. The first (Ricardo) uses the impulse torque meter method while the other (FEV) uses the paddle wheel technique. The experiments were carried out on the same cylinder head and the same pressure difference across the inlet valves of 600mmH2O in order to compare the results. The experimental results are presented in terms of the measured air flow rate, flow coefficient, discharge coefficient and non-dimensional rig tumble. Moreover, number of modifications were conducted on the FEV flow bench in order to apply particle image velocimetry measurements on the vertical tumble plane, which passing through the middle of the cylinder at different valve lifts. The results show that a reasonably good level of agreement can be achieved between both methods, providing the methods of calculations of the various parameters are consistent.

Development of CFD Inverse Analysis Technology Using the Transient Adjoint Method and Its Application to Engine In-Cylinder Flow

Development of CFD Inverse Analysis Technology Using the Transient Adjoint Method and Its Application to Engine In-Cylinder Flow

Stereoscopic multi-planar PIV measurements of in-cylinder tumbling flow

The non-reacting flow field within the combustion chamber of a motored direct-injection spark-ignition engine with tumble intake port is measured. The three-dimensionality of the flow necessitates the measurement of all three velocity components via stereoscopic particle-image velocimetry in multiple planes. Phase-locked stereoscopic PIV is applied at 15 crank angles during the intake and compression strokes, showing the temporal evolution of the flow field. The flow fields are obtained within a set of 14 axial planes, covering nearly the complete cylinder volume. The stereoscopic PIV setup applied to engine in-cylinder flow and the arising problems and solutions are discussed in detail. The three-dimensional flow field is reconstructed and analyzed using vortex criteria. The tumble vortex is the dominant flow structure, and this vortex varies significantly regarding shape, strength, and position throughout the two strokes. The tumble vortex center moves clockwise through the combustion chamber. At first, the tumble has a c-shape which turns into an almost straight tube at the end of the compression. Small-scale structures are analyzed by the distribution of the turbulent kinetic energy. It is evident that the symmetry plane only represents the 3D flow field after 100 CAD. For earlier crank angles, both kinetic energy (KE) and turbulent kinetic energy (TKE) in the combustion chamber are well below the KE and TKE in the symmetry plane. This should be taken into account when the injection and breakup of the three-dimensional fuel jet are studied. The mean kinetic energy is conserved until late compression by the tumble motion. This conservation ensures through the excited air motion an enhancement of the initial air-fuel mixture which is of interest for direct-injection gasoline engines.

Large Eddy Simulation of Diesel Engine In-cylinder Flow

The numerical simulation of the in-cylinder flow of a realistic Diesel engine is presented. Over the past three decades most of the CFD research for internal combustion engines (ICE) has been carried out using the Reynolds Averaged Navier Stokes (RANS) approach. Despite the achievements obtained in engine design, numerical investigations with RANS models can only give insight into the mean behaviour of the in-cylinder flow. By contrast, Large–Eddy Simulation (LES) allows a better description of the in-cylinder flow motion and gives access to the cycle-to-cycle variations. Multi-cycle simulations of a motored case using LES and a hybrid LES/RANS approach are compared against experimental data and results obtained with a RANS methodology. The cycle-to-cycle variations in global in-cylinder characteristics such as pressure, swirl number and tumble number are reported and an analysis of the in-cylinder flow for each stroke of the engine cycle is presented. The interaction between spray and turbulence is also analysed with closed-chamber simulations. The effects of the hybrid RANS/LES approach on mixing of the vapour phase of the fuel is described and compared to the results obtained with a standard RANS approach. This work gives an insight into the numerical simulation of Diesel engine and illustrates some of the advantages of the LES methodology over RANS. The trade-off between computational cost and details in the flow description using a hybrid RANS/LES approach is discussed.

Some subtleties concerning fluid flow and turbulence modeling in 4.-valve engines

In this paper some results concerning the structure and evolution of fluid flow pattern during induction and compression in 4- valve engines with tilted valves were presented. Results were obtained by dint of multidimensional modeling of non-reactive flows in arbitrary geometry with moving boundaries. During induction fluid flow pattern was characterized with organized tumble motion followed by small but clearly legible deterioration in the vicinity of the bottom dead center. During compression the fluid flow pattern is entirely three-dimensional and fully controlled by vortex motion located in the central part of the chamber. In order to annihilate negative effects of tumble deterioration and to enhance swirling motion one of the intake valves was deactivated. Some positive and negative effects of such attempt were elucidated. The effect of turbulence model alteration in the case of excessive macro flows was tackled as well. Namely, some results obtained with eddy-viscosity model i. e. standard k-e model were compared with results obtained with k-ξ-f model of turbulence in domain of 4-valve engine incylinder flow. Some interesting results emerged rendering impetus for further quest in the near future.

A methodology for analysis of diesel engine in-cylinder flow and combustion

A methodology is developed for the analysis of diesel engine in-cylinder processes and combustion. Beginning from CAD data of the engine geometry, the methodology involves use of a commercial code AVL FIRE for simulation of suction stroke, and an open-source code KIVA-3V for simulation of the closed-valve part of the diesel cycle. For this, an algorithm is first developed to map a generalised three-dimensional Computational Fluid Dynamics (CFD) solution from an unstructured mesh in AVL FIRE to a structured mesh in KIVA-3V to provide initial conditions for the closed-valve simulations. For simulation of diesel combustion process, an integrated KIVA-3V code is developed by incorporating two well-validated models into the standard code: the Shell hydrocarbon auto-ignition model for simulation of diesel auto-ignition under conditions of high temperature and pressure, and the Characteristic-time model for simulation of subsequent diesel combustion. The integrated code is validated and calibrated against experimental pressure measurements in a naturally aspirated direct injection diesel engine. These tools are then used for exploring the potential of a constant-speed, turbocharged diesel engine towards emission reduction. The case study involves combustion simulations for exploring multiple injection strategy for the engine concerned.

A nonlinear eddy-viscosity model based on an elliptic relaxation approach

The present paper proposes a nonlinear eddy-viscosity model, which combines an elliptic relaxation concept with the nonlinear stress–strain modelling. The basis model used here is the variant of model, namely ζ–f model. This recently proposed model essentially improves the numerical robustness over the original model; it is even used as the default turbulence model instead of the standard industrial k–ε model in the in-house commercial computational fluid dynamics (CFD) code, which was applied for all computational work presented here. This paper aims to extend the capability of the ζ–f model by employing the nonlinear stress–strain relationship including both quadratic and cubic nonlinear terms, and with that, to eliminate deficiencies arising from the linear Boussinesq assumption. The ζ–f model is applied in conjunction with universal wall treatment, which combines the integration up to the wall with wall functions. Computational results are compared with available experimental data for four different applications: a backward-facing step, a 180° turn-around duct, a swirling combustor and a three-dimensional (3D) in-cylinder engine flow. The results obtained clearly show where the main benefits from a more general eddy-viscosity model could be expected.

In-cylinder engine flow measurement using stereoscopic molecular tagging velocimetry (SMTV)

The stereoscopic molecular tagging velocimetry (SMTV) technique is used to obtain the multiple point measurement of an instantaneous three-component velocity field inside the cylinder of an internal combustion (IC) engine assembly. A novel image processing technique is implemented to obtain the velocity data. The technique has the advantage that it eliminates the geometric details required to obtain the three components of the velocity field. The procedure involves two major steps: (i) calibration process and (ii) data acquisition and reduction. Cycle-to-cycle variations of the three-component velocity field and out-of-plane vorticity are presented inside an engine cylinder. Preliminary results show that cycle-to-cycle variations are more prominent in the velocity component perpendicular to the tumble plane, as opposed to the in-plane components. Such new insights will help better understand the details of these flows and further improve CFD models for IC engines.

Use of Proper Orthogonal Decomposition for reconstructing the 3D in-cylinder mean-flow field from PIV data

Based on PIV measurements obtained in planes along the main flow direction, a snapshot POD based on the full velocity field is performed. The dominant POD coefficients associated to the mean flow, are interpolated in the whole space domain according to boundary flow properties. From the knowledge of these POD coefficients, the entire mean-flow field is reconstructed and a 3D mean-flow representation is obtained. An application is presented using PIV measurements performed in in-cylinder engine flow. To cite this article: Ph. Druault, C. Chaillou, C. R. Mecanique 335 (2007).

Proper orthogonal decomposition of in-cylinder engine flow into mean component, coherent structures and random Gaussian fluctuations

A snapshot proper orthogonal decomposition (POD) is performed from 2D time-resolved PIV measurements obtained in the tumble plane of a Spark Ignition engine flow. Based on this filtering approach, the in-cylinder flow field is decomposed into a mean part, a coherent part and a turbulent incoherent part. The analysis of the one-point statistical moments of orders 3 and 4 (skewness and flatness coefficients) as well as the analysis of the probability density function of the velocity field show that the POD extracts an incoherent random velocity field having Gaussian distribution properties. These small amplitude background fluctuations are also homogeneous. We then demonstrate the ability of the POD procedure, based on an energy criterion, in separating the coherent part and random Gaussian fluctuations. Based on this POD flow partitioning, providing a new triple decomposition of the instantaneous turbulent flow, the flow engine cycle-to-cycle variations associated with each velocity field contribution are then accessed. Thus, one of the first full field quantitative analyses of the cyclic variabilities associated with each part of the in-cylinder flow field is provided from the analysis of the corresponding POD temporal coefficients.

Use of proper orthogonal decomposition for time interpolation from PIV data

Based on time-resolved PIV (TRPIV) data, a new proper orthogonal decomposition (POD) application is developed to recover the time information between two consecutive PIV time measurements. Indeed, by performing a POD over the full PIV velocity field snapshots, POD coefficients are time-interpolated providing a continuous space–time description of the turbulent flow field. An application of this interpolation method is proposed from TRPIV velocity fields obtained in the tumble plane of in-cylinder engine flow. Available flow dynamical representations then allow some cycle-to-cycle variation analyses based on the instantaneous flow pictures as well as on the statistical data. It has been shown that the cyclic variabilities increase during the compression process and at the end of the compression stage, they decrease to reach those obtained during the intake process.

Swirl characterisation in steady and unsteady flows with PIV measurements

A detailed aerodynamic analysis of the swirl characteristics of an in-cylinder flow generated by a cylinder head engine is presented and that can be easily extended to the case of real in-cylinder engine flow. The article contributes to the reflection related to the pertinence to keep the characterisation of the swirl generation from the cylinder head in steady rigs.

Concentration fields in a confined two-gas mixture and engine in-cylinder flow: laser tomography measurement by Mie scattering

Gaseous mixture concentration characterisation by Mie scatter laser tomography has been widely used, except when experimental conditions turn out to be difficult especially in confined flow. This paper describes a complete formulation of the image processing required for a confined flow including effects from wall reflection, quality of the seeding, thermodynamic flow characteristics and instantaneous laser profile disparity. Two applications are described; the first one on a free and confined jet (turbulent, isothermal, isobaric flow) which shows the potential of the technique and the processing needed to reconstitute the mean concentration field, thus demonstrating the reliability of the processing. Second, an in-cylinder engine concentration measurement is put forward. Normal running and a stratified concept are given as examples. The results show the excellent potential of the technique to help engine developers. Thus, this paper presents the problems connected with measurement of concentration fields by laser tomography and Mie scattering from a theoretical and experimental approach.

5. Large eddy simulation of engine in-cylinder flow

5. Large eddy simulation of engine in-cylinder flow