Marcus Megerle

Measurement of digital particle image velocimetry precision using electro-optically created particle-image displacements

The displacement (velocity) precision achieved with digital particle image velocimetry (PIV) was measured. The purpose of this work was to determine the precision and sensitivity of digital PIV using real rather than theoretical images at 1 and 2 mm spatial resolution. The displacement measurement precision was determined by measuring the RMS noise from 60 identical displacement distributions. This work is unique in that it uses electro-optical image shifting to create a repeatable image displacement distribution of random particle fields. The displacement variance between images is caused by the shot-to-shot variation in: (1) the particle-image fields, (2) the camera noise and (3) the variance in the correlation peak detection. In addition to magnification variations, the particle-number density, imaging-lens f-stop and image-plane position errors were varied to determine the best configuration. The results indicate that both the ensemble-mean and the RMS fluctuations of the image displacements are affected by these parameters and comparisons with results found in the literature are presented. The extents of these variations are quantified. This variance does not, of course, include errors due to random gradients and out-of-plane pairing losses, which exist in real turbulent flows.

Particle-image velocimetry measurement errors when imaging through a transparent engine cylinder

When making particle-image velocimetry measurements through the quartz cylinder of a reciprocating engine, the particle images are aberrated. This work quantifies the practical field-of-view and the errors in the velocity measurements caused by those aberrations. Electro-optical image shifting was used to create a repeatable particle-image displacement distribution for 60 images. Ensemble averaging of these images is used to quantify the rms errors due to the shot-to-shot variation in (1) the particle-image fields, (2) the camera noise, (3) the variance in the correlation-peak detection and (4) the particle-image aberrations. These results demonstrate that the field-of-view is restricted to the centre 66 mm of the 86 mm inside-diameter cylinder due to decreased accuracy, decreased image-to-image precision and decreased displacement-peak detectability of the image-displacement correlation. The correlation-peak detectability was degraded by both particle-image aberrations and decreased transmission of the scattered light.

The influence of swirl ratio on turbulent flow structure in a motored HSDI diesel engine - A combined experimental and numerical study

Simultaneous two-component measurements of gas velocity and multi-dimensional numerical simulation are employed to characterize the evolution of the in-cylinder turbulent flow structure in a re-entrant bowl-in-piston engine under motored operation. The evolution of the mean flow field, turbulence energy, turbulent length scales, and the various terms contributing to the production of the turbulence energy are correlated and compared, with the objectives of clarifying the physical mechanisms and flow structures that dominate the turbulence production and of identifying the source of discrepancies between the measured and simulated turbulence fields. Additionally, the applicability of the linear turbulent stress modeling hypothesis employed in the k-e model is assessed using the experimental mean flow gradients, turbulence energy, and length scales.

An experimental assessment of turbulence production, reynolds stress and length scale (Dissipation) modeling in a swirl-supported di diesel engine

Simultaneous measurements of the radial and the tangential components of velocity are obtained in a high-speed, direct-injection diesel engine typical of automotive applications. Results are presented for engine operationwith fuel injection, but without combustion, for three different swirl ratios and four injection pressures. With the mean and fluctuating velocities, the r-θ plane shear stress and the mean flow gradients are obtained. Longitudinal and transverse length scales are also estimated via Taylors hypothesis. The flow is shown to be sufficiently homogeneous and stationary to obtain meaningful length scale estimates. Concurrently, the flow and injection processes are simulated with KIVA-3V employing a RNG k-e turbulence model. The measured turbulent kinetic energy k, r-θ plane mean strain rates ( , , and ), deviatoric turbulent stresses ( , - 2/3k, and -2/3k), and the r-θ plane turbulence production terms are compared directly to the simulated results. The model predicts the qualitative trends in k well, but under-predicts the magnitude of the late-cycle turbulence at the higher swirl ratios. The mean strain rates, turbulent stresses, and turbulence production terms generally agree qualitatively. Both the experimental and the simulated results indicate that redistribution of the mean flow angular momentum by the fuel injection event is an important source of late-cycle turbulence. This redistribution enhances r-θ plane turbulence production at low swirl ratios through formation of unstable mean flow distributions with negative radial gradients in mean flow angular momentum. Additionally, r-z plane vortical flow structures are formed by a competition between the inward displacement of high angular momentum fluid by the fuel jets and the centrifugal forces acting to force the high momentum fluid back to the bowl periphery. Model results indicate that these flow structures can also be important sources of turbulence at higher swirl ratios. The measured length scales, mean strain rates, and turbulent kinetic energy are used to assess directly the validity of the isotropic eddy viscosity hypothesis. This modeling hypothesis is found to provide good estimates of and -2/3k, despite the high levels of flow swirl. However, the data indicate that the modeled is underestimated when unstable mean flow distributions exist. Further, under some conditions, no model based solely on local flow properties is likely to predict the measured . Poor agreement is found between the measured and modeled -2/3k. The under-prediction of k at high swirl, coupled with the generally good agreement in , , and P r θ , suggest that the under-prediction of k is predominantly due to an over-estimation of e after the injection event. This suggestion is further supported by a comparison of the temporal evolution of the measured and modeled length scales.

Measurements and modeling of reynolds stress and turbulence production in a swirl-supported, direct-injection diesel engine

Measured and numerically predicted components of the mean rate-of-strain tensor 〈Sij〉 and the Reynolds stress 〈u′ru′θ〉 are examined and compared to elucidate the source and scrutinize the modeling of late-cycle turbulence production in swirl-supported, direct-injection diesel engines. The experiments are performed with combustion in the engine inhibited, to eliminate the complicating influence of heat release on turbulence generation and to reduce the problem to one more closely approximating constant-density turbulence. Both the measurements and the calculations indicate that the primary influence of the mean flow swirl on turbulence production is confined to two separate periods: (1) shortly after the end of injection and (2) in the late-cycle period, when large positive levels of 〈u′r,u′θ〉 are observed. Formation of the positive Reynolds stress coincides with the development of a negative radial gradient in mean angular momentum, indicating an unstable mean flow field. At this time, the measured velocity fluctuations show a large increase, approximately doubling in magnitude compared to fluctuations measured without fuel injection. Predicted velocity fluctuations, obtained via k-e turbulence modeling, show a similar late-cycle increase, although the magnitude of the increase is not quantitatively captured. To evaluate its applicability during the period in which the unstable, negative radial gradient in angular momentum is present, the isotropic eddy viscosity hypothesis is examined. The Reynolds stress estimated from the measured 〈Srθ〉 using the eddy viscosity hypothesis is found to mimic the measured stress with reasonable accuracy, and the measured and calculated r-θ plane turbulence production terms are shown to have excellent qualitative and quantitative agreement. The quantitative agreement, however, appears largely providential, as the measured and predicted values of 〈Srθ〉 differ by a factor of 2. This discrepancy is compensated for by the underpredicted turbulent kinetic energy and seemingly high values of the dissipation.

The Influence of Swirl and Injection Pressure on Post-Combustion Turbulence in a HSDI Diesel Engine

The effect of variable flow swirl and injection pressure on the generation of post-combustion turbulence is investigated in a HSDI diesel engine operating at a simulated idle condition. The engine is operated in three modes: motored, injected (fuel injected into a N2 environment), and fired. The radial and tangential components of flow velocity are measured simultaneously for each operating condition. Radial and tangential gradients in the mean velocities are also measured, which, with the variance of the individual velocity components and the Reynolds stress, allow turbulence production due to r-9 plane fluid motion to be evaluated. Through comparison of the temporal evolution of turbulent kinetic energy k for the three operating conditions, the influence of the fuel injection event and the subsequent combustion is assessed, as is the influence of variable flow swirl and injection pressure. The turbulence production is investigated in detail for injected engine operation, where the complicating effects of combustion-induced turbulence and large density variations are eliminated. The measured production and the evolution of the numerically predicted mean flow structures are evaluated for consistency with a simple picture wherein the fuel injection process redistributes the angular momentum of the swirling flow, creating an unstable flow that significantly enhances the flow turbulence. The simple picture is consistent with the majority of the trends in the data, and provides a mental model that is useful for understanding the effects of varying flow swirl, combustion chamber geometry, and fuel injection parameters.