Bubble growth processes are highly complex processes, which are not only dependent on the foaming process parameters (temperature, pressure and blowing agent concentration) but also on the type and ...structure of the polymer used. Since the elongational viscosity at the bubble wall during bubble growth also depends on these influencing factors, the so-called transient elongational viscosity plays a key role in describing the gas bubble growth behavior in polymer melts. The model-based description of the transient elongational viscosity function is difficult due to its dependence on time, Hencky strain and strain rate. Therefore, representative viscosities or shear viscosity models are usually used in the literature to predict the bubble growth behavior. In this work, the transient equibiaxial elongational viscosity function at the bubble wall during bubble growth is described holistically for the first time. This is achieved by extending the so-called molecular stress function (MSF) model by superposition principles (temperature, pressure and blowing agent concentration) and by using the elongational deformation behavior (Hencky strain and strain rate) at the bubble wall during the initial, and thus viscosity-driven, bubble growth process. Therefore, transient uniaxial elongational viscosity measurements are performed and the non-linear MSF model parameters of the two investigated polymers PS (linear polymer chains) and PLA (long-chain branched polymer chains) are determined. By applying the superposition principles and by changing the strain mode parameter to the equibiaxial case in the MSF model, the transient equibiaxial viscosity master curve is obtained and used to describe the bubble growth process. The results show that the extended MSF model can fully predict the transient equibiaxial elongational viscosity function at the bubble wall during bubble growth processes. The bubble growth behavior over time can then be realistically described using the defined transient equibiaxial elongational viscosity function at the bubble wall. This is not possible, for example, with a representative viscosity and therefore clearly demonstrates the influence and importance of knowing the transient deformation behavior that prevails at the bubble wall during bubble growth processes.
Abstract
Based on the traditional falling ball method, the U-tube design is more suitable for measuring transparent or translucent liquids with a large viscosity coefficient. An electromagnetic relay ...is placed at the upper end of one side of the tube to ensure that the ball falls vertically through the axis of the tube. The other side is sealed with water to prevent the pollution of the liquid to be measured and improve the measurement accuracy. At the same time, it solves the problem of oil leakage in the existing device for a long time, which is convenient for device maintenance and cleaning.
The wind speed response to mesoscale SST variability is investigated over the Agulhas Return Current region of the Southern Ocean using the Weather Research and Forecasting (WRF) Model and the U.S. ...Navy Coupled Ocean-Atmosphere Mesoscale Prediction System (COAMPS) atmospheric model. The SST-induced wind response is assessed from eight simulations with different subgrid-scale vertical mixing parameterizations, validated using Quick Scatterometer (QuikSCAT) winds and satellite-based sea surface temperature (SST) observations on 0.25 degree grids. The satellite data produce a coupling coefficient of s sub(U) = 0.42 m s super(-1) degree C super(-1) for wind to mesoscale SST perturbations. The eight model configurations produce coupling coefficients varying from 0.31 to 0.56 m s super(-1) degree C super(-1). Most closely matching QuikSCAT are a WRF simulation with the Grenier-Bretherton-McCaa (GBM) boundary layer mixing scheme (s sub(U) = 0.40 m s super(-1) degree C super(-1)), and a COAMPS simulation with a form of Mellor-Yamada parameterization (s sub(U) = 0.38 m s super(-1) degree C super(-1)). Model rankings based on coupling coefficients for wind stress, or for curl and divergence of vector winds and wind stress, are similar to that based on s sub(U). In all simulations, the atmospheric potential temperature response to local SST variations decreases gradually with height throughout the boundary layer (0-1.5 km). In contrast, the wind speed response to local SST perturbations decreases rapidly with height to near zero at 150-300 m. The simulated wind speed coupling coefficient is found to correlate well with the height-averaged turbulent eddy viscosity coefficient. The details of the vertical structure of the eddy viscosity depend on both the absolute magnitude of local SST perturbations, and the orientation of the surface wind to the SST gradient.
Purpose
The flow phenomenon of particle image velocimetry has revealed the transition process of the complex multi-scale vortex between the boundary layer and mainstream region. Nonetheless, present ...computational fluid dynamics methods inadequately distinguish the discernable flows in detail. A multi-physical field coupling model, which was applied in rotor-stator fluid machinery (Umavathi, 2015; Syawitri et al., 2020), was put forward to ensure the identification of multi-scale vortexes and the improvement of performance prediction in torque converter.
Design/methodology/approach
A newly-developed multi-physical field simulation framework that coupled the scale-resolving simulation method with a dynamic modified viscosity coefficient was proposed to comparatively investigate the influence of energy exchange on thermal and flow characteristics and the description of the flow field in detail.
Findings
Regardless of whether quantitative or qualitative, its description ability on turbulence statistics, pressure-streamline, vortex structure and eddy viscosity ratio were visually experimentally and numerically analyzed. The results revealed that the modification of transmission medium viscous can identify flows more exactly between the viscous sublayer and outer boundary layer. Compared with RANS and large eddy simulation, a stress-blended eddy simulation model with a dynamic modified viscosity coefficient, which was further used to achieve blending on the stress level, can effectively solve the calculating problem of the transition region between the near-wall boundary layer and mainstream region.
Research limitations/implications
This indeed provides an excellent description of the transient flow field and vortex structure in different physical flow states. Furthermore, the experimental data has proven that the maximum error of the external performance prediction was less than 4%.
Originality/value
An improved model was applied to simulate and analyze the flow mechanism through the evolution of vortex structures in a working chamber, to deepen the designer with a fundamental understanding on how to reduce flow losses and flow non-uniformity in manufacturing.
The viscosity of lysosome is reported to be a key indicator of lysosomal functionality. However, the existing mechanical methods of viscosity measurement can hardly be applied at the cellular or ...subcellular level. Herein, a BODIPY-based two-photon fluorescent probe was presented for monitoring lysosomal viscosity with high spatial and temporal resolution. By installing two morpholine moieties to the fluorophore as target and rotational groups, the TICT effect between the two morpholine rings and the main fluorophore scaffold endowed the probe with excellent viscosity sensitivity. Moreover, Lyso-B succeeded in showing the impact of dexamethasone on lysosomal viscosity in real time.
Increased vegetable consumption could decrease the risk of cardiovascular disease (CVD), obesity and certain cancers. Smoothies are one of the ways to increase vegetable consumption. The aim of this ...study was to develop a vegetable-containing smoothie with improved nutritional value and consumer acceptability. A survey was conducted on university students (from UK and KSA) to assess their opinions, followed by tasting sessions with several potential smoothies. Following this, the most acceptable smoothie recipe was selected for further analysis. Some key nutritional parameters; ascorbate, antioxidants, phenolics, sugars and fibre of the novel smoothie were measured and compared with commercial smoothies available in the UK market. Also, the retention and stability, during 1, 3 and 5 days of storage at 4°C of these nutrients were analysed. The survey revealed that there was good awareness of the 5 A DAY requirement in the UK, but only fair awareness in the KSA student sample of increasing fruit and vegetable (F&V) intake. Price was a potential barrier for students. Tasting trials indicated that the UK student’s acceptance was limited by firstly taste then texture and colour. The novel smoothie based on beetroot and red pepper appeared to be most acceptable. The nutritional value of this smoothie (ascorbate 30 mg/100mL, antioxidant capacity 1031μmoles Trolox Equivalents/100mL, phenolics 57mg Gallic Acid Equivalents/100mL and fibre 6g/100mL) appears to be higher than current commercial smoothies. It was also slightly lower in total sugars with 9g/100mL. This smoothie retained the nutritional benefits of the constituent F&Vs during processing. Also, the nutritional value of the designed smoothie was stable during storage at 4oC over 5 days, apart from ascorbate which declined dramatically. Potential commercial barriers included the high viscosity. Preliminary experiments were carried out to investigate the potential to reduce viscosity using enzyme cocktails. The smoothie containing 47% vegetables and 53% fruit could be one of the easiest ways of increasing F&V consumption, particularly amongst students, to maintain a healthy diet.
Understanding the outcomes of binary droplet collisions is important to many areas of physics and technology. For a given system, the collision outcomes can be mapped, in the parameter space of ...impact parameter (offset between the droplets centres) and Weber number, with clear regimes boundaries. As Weber number increases, the collision outcomes are bouncing, coalescence, stretching separation at high offset and reflexive separation. This work, in collisions of miscible droplets, experimentally and theoretically investigates the role of the viscosity, viscosity difference, size and size ratio as they have impacts on the collisions’ outcomes and hence the position of the regime boundaries in the regime map. Increasing the viscosity, decreasing the size and decreasing the size ratio (d_s/d_l), qualitatively, have the same impact on the collisions outcomes, as both enhance the coalescence regime by suppressing the other regimes. Collisions of droplets with non-identical viscosities show the following behaviours. The bouncing boundary shows an intermediate position compared to the identical viscosity cases, however, at high viscosity difference, a new regime was observed, namely, partial bouncing which is bouncing with a very thin ligament between the droplets. Likewise, in equal-size collisions, and in unequal size collisions where the large droplet has the higher viscosity, the boundary of reflexive separation regime shows an intermediate position. However, in unequal-size collisions, if the larger droplet has the lower viscosity the boundary remains similar to the identical viscosity case of the low viscosity droplet. Finally, stretching separation always shows a boundary similar to that of the identical viscosity case of the low viscosity droplet. New models for the boundaries of the bouncing and the reflexive separation regimes were developed to consider the effects of the viscosity, the impact parameter and the size ratio.
Gravity currents are flows that are driven by a density difference and include pyroclastic flows, landslides and turbidity currents. Gravity currents can be a geohazard and have significant economic ...impact to connected industries. This thesis focuses on two important questions relating to gravity current dynamics: How do pulses or surges affect the flow dynamics? And, how does a viscosity contrast affect the mixing process between the current and its ambient? Real-world gravity current flows rarely exist as a single discrete event, but are instead made up of multiple surges. These are studied by the sequential release of two lock gates. The first release creates a gravity current, while the second creates a pulse that eventually propagates to the head of the first current. A shallow-water model is used to analyse the flow structure in terms of two parameters: the densimetric Froude number at the head of the current, Fr, and a dimensionless time between releases, t_re. The pulse speed exhibits negative acceleration for a region of (Fr, t_re)-space. Critically for sediment-laden gravity currents, pulsed flows may change from erosional to depositional further affecting their dynamics. Experimental modelling using glycerol/water mixtures reveals that the pulse can cause a rapid dilution of the current and transition to fully turbulent behaviour. In a lock-exchange configuration, the effect of a viscosity contrast between the ambient and the current is studied using fully resolved direct numerical simulation of the Navier-Stokes equations. Viscosity acts to both dissipate energy in the bulk of the current and locally inhibit mixing at the interface. Energy lost to viscous heating is dominant when the viscosity contrast is large, i.e. ten times the ambient. However, when the viscosity contrast is small but non-zero, the reduced mixing of the current enables a more efficient transfer between kinetic and potential energy and the total energy lost to mixing and viscous dissipation is reduced when compared to an equal viscosity case.
The production of anatomically complex tissues and organs with high biological function requires bioinks to have contradictory material properties. Properties that enable bioinks to be mechanically ...self-sufficient and accurate in terms of geometric fidelity may not be inherently compatible for cell viability and vice versa. Such is the practical dilemma of bioprinting, leading to the development of bioinks with balanced mechanical and biological properties that do not excel in either respect. In this thesis, the development of a customised, modular, extrusion-based 3D bioprinter and two novel supportive bath strategies is described. This custom bioprinter is able to extrude low-concentration, low-viscosity bioinks deep into the developed support baths and suspend the extruded bioink in 3D space. Printing structures in this manner reduces the demand for mechanically strong bioinks during the fabrication process as the structure's weight is supported by the bath in all dimensions. These supportive strategies enable the production of larger and geometrically more complex anatomical structures whilst using a low-concentration, low-viscosity alginate hydrogel bioink. Therefore the material's mechanical needs for bioprinting are addressed in such a way that encourages the use of bioinks with qualities that can be biologically more favourable. The support baths detailed in this thesis includes a quiescently gelled gelatine-based approach and a fluidised-agar fluid gel approach. The gelatine baths are prepared in a very simple, reliable, and repeatable two-step manner, and printed structures embedded within the gel are removed gently and easily by utilising gelatine's physiologically relevant melting temperature to liquefy the support. Blood vessel-like structures and noses were fabricated in this manner. Agar fluid gel support baths are also simple to produce and only require a gelled puck of agar be blended prior to its application as a supportive material. Agar fluid gel baths have been used successfully to support the fabrication of geometrically challenging structures such as bucky balls and Eiffel towers as well as replicate anatomical models such as ears, noses, brains, and hearts, which are easily separated from their supports by washing away the residual fluid gel.
Temporal vertical eddy viscosity coefficient (VEVC) in an Ekman layer model is estimated using an adjoint method. Twin experiments are carried out to investigate the influences of several factors on ...inversion results, and the conclusions of twin experiments are 1) the adjoint method is a capable method to estimate different kinds of temporal distributions of VEVCs; 2) the gradient descent algorithm is better than CONMIN and L-BFGS for the present problem, although the posterior two algorithms perform better on convergence efficiency; 3) inversion results are sensitive to initial guesses; 4) the model is applicable to different wind conditions; 5) the inversion result with thick boundary layer depth (BLD) is slightly better than thin BLD; 6) inversion results are more sensitive to observations in upper layers than those in lower layers; 7) inversion results are still acceptable when data noise exists, indicating the method can sustain noise to a certain degree; 8) a regularization method is proved to be useful to improve the results for present problem; and 9) the present method can tolerate the existence of balance errors due to the imperfection of governing equations. The methodology is further validated in practical experiments where Ekman currents are derived from Bermuda Testbed Mooring data and assimilated. Modeled Ekman currents coincide well with observed ones, especially for upper layers. The results demonstrate that the assumptions of depth dependence and time dependence are equally important for VEVCs. The feasibility of the typical Ekman model, the imperfection of Ekman balance equations, and the deficiencies of the present method are discussed. This method provides a potential way to realize the time variations of VEVCs in ocean models.
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