The heat sealability of laminated films with linear low density polyethylene (LLDPE) and low density polyethylene (LDPE) as the sealant materials was investigated. A laboratory heat sealer was used to study the response of laminated films to temperature, time, and pressure. Platen temperature was confirmed as primary factor in controlling heat-seal strength. Dwell time must be sufficiently long to bring the interfacial temperature to a desired level. When the desired heat-seal strength has been achieved, further increase of dwell time did not improved heat-seal strength. Platen pressure had little effect above the level required to flatten the materials for good contact. Bar sealing process window for each sample were developed. The optimum combination of platen temperature and dwell time for each laminated film can be obtained in the respec-tive process windows. Strength of heat-seal and its failure modes are closely related. Plateau initiation temperature closely corresponds to the final melting point of sealant materials. Relatively higher platen temperature was required to seal laminated films with lower thermal conductance. Required dwell time corresponds closely to the heat flow rate of bar sealing process. Laminated films made from extrusion lamination process provided lower level of achievable heat seal strength when compared with the laminated films made from dry-bond lamination process.
The influence of transient flows on vehicle stability was investigated by large eddy simulation. To consider the dynamic response of a vehicle to real-life transient aerodynamics, a dimensionless parameter that quantifies the amount of aerodynamic damping for vehicle subjects to pitching oscillation is proposed. Two vehicle models with different stability characteristics were created to verify the parameter. For idealized notchback models, underbody has the highest contribution to the total aerodynamic damping, which was up to 69%. However, the difference between the aerodynamic damping of models with distinct A-and C-pillar configurations mainly depends on the trunk-deck contribution. Comparison between dynamically obtained phase-averaged pitching moment with quasisteady values shows totally different aerodynamic behaviors.
Conventionally, the pitching instability of road vehicles has been controlled mechanically through the application of suspension systems. The present study demonstrates how unsteady aerodynamics can be exploited for such control by properly configuring vehicle body shapes. To discern the effect of unsteady aerodynamics on road vehicle stability, large eddy simulation has been conducted to simulate the flow past simplified vehicle models. Forced-sinusoidal-pitching oscillation was imposed on the models during the simulation to probe their dynamic responses. Numerical results were compared with wind tunnel measurements for validation, and good agreement is attained. Unsteady flow structures above the rear section of the vehicles were found to significantly affect their pitching stability. Depending on the vehicle body shape configurations, the induced aerodynamic force tended to either enhance or restrain the vehicles' pitching instability.
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