Humberto Medina

Reynolds Number Effects on Fully-Developed Pulsed Jets Impinging on Flat Surfaces

A systematic study of the effect of the Reynolds number on the fluid dynamics and turbulence statistics of pulsed jets impinging on a flat surface is presented. It has been suggested that the influence of the Reynolds number may be somewhat different for a jet subjected to pulsation when compared to an equivalent steady jet. A comparative study of both steady and pulsating jets is presented for a Reynolds number range from Re=4,730 to Re=10,000. All the other factors that affect the flowfield are kept constant, which are H/d=3, St=0.25, and d=30.5  mm. It was found that for the range of the Reynolds numbers tested, pulsation results in a shortening of the jet core, the centerline axial velocity component declines more rapidly, and higher values of the radial velocity component for r/d>0.75 are observed. As the Reynolds number increases, the jet spreads more rapidly, the turbulent kinetic energy and nondimensional turbulent fluctuations decrease, and the flowfield near the impinging surface changes drastic...

Study of the fluid mechanics of transitional steady and pulsed impinging jets using a high-speed PIV system

H. Medina∗ Centre of Excellence for Integrated Aircraft Technology, School of Mechanical and Aeronautical Engineering, The Queen’s University of Belfast, Belfast, BT9 5AH, United Kingdom E. Benard † Department of Aerospace Engineering, University of Glasgow, Glasgow, G12 8QQ, United Kingdom J.C. Huang‡ Centre of Excellence for Integrated Aircraft Technology, School of Mechanical and Aeronautical Engineering, The Queen’s University of Belfast, Belfast, BT9 5AH, United Kingdom S. Raghunathan§ Centre of Excellence for Integrated Aircraft Technology, School of Mechanical and Aeronautical Engineering, The Queen’s University of Belfast, Belfast, BT9 5AH, United Kingdom

Investigation of Utilizing Single Surface Depressions in Optimization of NLF Aerofoil Design

An environment has been created for the optimisation of aerofoil profiles with inclusion of small surface features. For TS wave dominated flows, the paper examines the consequences of the addition of a depression on the aerodynamic optimisation of an NLF aerofoil, and describes the geometry definition fidelity and optimisation algorithm employed in the development process. The variables that define the depression for this optimisation investigation have been fixed, however a preliminary study is presented demonstrating the sensitivity of the flow to the depression characteristics. Solutions to the optimisation problem are then presented using both gradient-based and genetic algorithm techniques, and for accurate representation of the inclusion of small surface perturbations it is concluded that a global optimisation method is required for this type of aerofoil optimisation task due to the nature of the response surface generated. When dealing with surface features, changes in the transition onset are likely to be of a non-linear nature so it is highly critical to have an optimisation algorithm that is robust, suggesting that for this framework, gradient-based methods alone are not suited.

Effects of Pulsation on the Flow Field of Turbulent Impinging Jets

The current study examines the effect of pulsation on the flow field of turbulent impinging jets. Previous work identified significant differences in the transitional and turbulent jet responses to pulsation, and this study considers the effect of the Strouhal number on the structure of turbulent impinging jets. This is of particular interest due to the widespread use of jets (for instance, in heat and mass transfer applications), and there is an interest in the utilisation of pulsating jets as alternative to steady jets to increase heat and mass transfer rates. Preliminary findings suggest that in order to achieve improvements in heat and mass transfer rates, a turbulent jet is preferable to an equivalent transitional jet as their behaviour is easier to predict, and for a number of frequencies, they exhibit favourable flow conditions when compared to the equivalent steady jet case. The similariry in the observed turbulence characteristics between the steady and pulsating cases also suggests that a steady jet model could be a viable baseline for computational modelling of a turbulent pulsating jet.

The Effect of Swirl on the Flow Uniformity in Automotive Exhaust Catalysts

In aftertreatment system design, flow uniformity is of paramount importance as it affects aftertreatment device conversion efficiency and durability. The major trend of downsizing engines using turbochargers means the effect of the turbine residual swirl on the flow needs to be considered. In this paper, this effect has been investigated experimentally and numerically. A swirling flow rig with a moving-block swirl generator was used to generate swirling flow in a sudden expansion diffuser with a wash-coated diesel oxidation catalyst (DOC) downstream. Hot-wire anemometry (HWA) was used to measure the axial and tangential velocities of the swirling flow upstream of the diffuser expansion and the axial velocity downstream the monolith. With no swirl, the flow in the catalyst monolith is highly non-uniform with maximum velocities near the diffuser axis. At high swirl levels, the flow is also highly nonuniform with the highest velocities near the diffuser wall. An intermediate swirl level exists where the flow is most uniform. To gain further insight into the mechanisms controlling flow redistribution, numerical simulations have been performed using the commercial CFD code STARCCM+. With no swirl, the central jet transverses the diffuser, and a drastic flow redistribution takes place near the monolith face due to its high resistance. Immediately downstream of the sudden expansion, the flow separates from the diffuser wall forming a separation zone around the central jet. Increasing swirl reduces the size of this separation zone, and eventually leads to the formation of the central recirculation zone characteristic of high swirl flows. At intermediate swirl levels, the size of the wall separation zone is reduced considerably, while the axial adverse pressure gradient is insufficient to cause a central recirculation. Such a flow regime occurs at relatively low swirl levels (S ~ 0.23). This may have positive implications for aftertreatment system design with low residual swirl levels from the turbine, which might be tuned by adjusting the distance between the turbine and the catalyst or employing guide vanes. The findings can be directly transferred to other aftertreatment systems with a catalyst or particulate filter. Moreover, swirling flows with an obstruction or a high resistance device downstream (e.g. a heat exchanger or filter) are present in many other applications such as cooling flows, combustion and turbomachinery. Therefore the results are relevant to a much wider research and industrial community.

Enhancing Noise Attenuation in Exhaust Mufflers on Response to Baffle Configuration

Using baffles in exhaust mufflers is known to improve transmission loss and reduce the noise emitted to the environment. This paper postulates that baffle cut ratio affects the muffler performance in the same effect as a shell-and-tube heat exchanger, consequently the baffle cut ratio should affect the muffler performance. This study presents a parametric study on the effect of baffle configuration on predicted transmission loss and pressure drop. The effect of baffle cut ratio, baffle spacing, number of baffle holes, and hole pattern distribution on transmission loss was investigated. Results showed that reducing the baffle cut ratio increased the transmission loss at intermediate frequencies by up to 45% while decreasing the spacing between muffler plates improved the muffler transmission loss by 40%. The assessment of the baffle effect on flow using a thermal baffle approach model indicated a sudden drop in fluid temperature in axial flow direction by 15% as the baffle cut ratio changed from 75% to 25. To the best of authors knowledge, the effect of baffle cut ratio configuration on acoustic response and back pressure has not been previously reported or investigated.

Effect of grid-filter width definition on implicitly filtered large eddy simulations using OpenFOAM

Turbulent flow over a 3D backward-facing step at a step-height Reynolds number, Reh, of 5100 is simulated in OpenFOAM using Large Eddy Simulation (LES) to investigate how the definition of the grid-filter width, Δ, influences the computed solutions. As part of this study, two different Δ definitions are tested: Δ=ΔxΔyΔz3 and Δ = max(Δx, Δy, Δz). The LES results show good agreement when compared with DNS and experimental data. The simulation results are used to further evaluate the effect of grid non-uniformity on the computation of the subgrid-scale eddy viscosity, νSGS.

Evaluation of transition-sensitive eddy-viscosity turbulence models for separated flow in OpenFOAM

A recently published transition-sensitive turbulence model, k−kL−ω−υ2 [1], is implemented in the open-source CFD package OpenFOAM, and its performance is evaluated in comparison with k−kL−ω [2] and υ2− f [3] models. On T3A and T3B flat plate cases, the k−kL−ω−υ2 model gives accurate transitional predictions. On a flapped NACA 23012 aerofoil, it is found to give only a small improvement over the k−kL−ω model (under 5% reduction in error for lift coefficient) compared with experimental results obtained at the Coventry University wind tunnel, showing limited effects of the extra transport equation which was added to sensitise the model to rotation and curvature effects. Assessment of fluctuating kinetic energy and the new wall-normal turbulent velocity scale shows overprediction near the wall compared to the υ2− f model which indicates a delayed prediction of separation.

A Behaviour Awareness Mechanism to Support Collaborative Learning

Awareness has been identified as a key element that affects the quality of collaboration. Several studies indicate that awareness mechanisms to support collaborative learning activities should include factors and stimuli from the students’ context and social interactions. This contributes to enhance the collaboration process and the learning experience of the students. This paper proposes a behaviour awareness mechanism to support collaborative learning in undergraduate learning scenarios. This mechanism has been designed to provide personal and social awareness to students about both, their own and their peers learning behaviour. Moreover, this mechanism encourages reflection and promotes social interactions among students in order to improve the effectiveness of collaborative learning. The article also describes and evaluates a prototype of the proposed mechanism and its implementation in a collaborative mobile learning application, using a case study. The preliminary results show that this proposal helps promote collaborative learning in undergraduate learning contexts.

Effect of three-dimensional surface perturbations on boundary layer transitional characteristics

The influence of a three-dimensional surface perturbation on the transitional boundary layer has been investigated experimentally. The perturbation height and longitudinal location relative to the leading edge have been varied to represent the effects of moving the location of a rivetted joint on a typical aerodynamic surface. A single hot wire probe was used survey the flow at a freestream turbulence intensity of 0.08% (Re=1.4x10 6 ) to 0.12% (Re=1.7x10 6 ). The experimental results indicate that while, for most configurations tested, the presence of a surface perturbation reduces the transitional Reynolds number, there are potential benefits to be realised with careful positioning of the rivet joint, promoting an increase in the transition onset location of up to 5%. This has a series of implications with respect to manufacturing tolerance analysis, and the manner in which tolerances are assigned to joints.