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The flow distribution and central processing unit (CPU) temperatures inside a rack of thirty 1 U (single rack unit) Sun Fire V20z servers retrofitted with direct-to-chip liquid cooling and two cool...

The paper compares two serrated plate-fin Heat Exchanger (HE) corrugation modelling methods using Computational Fluid Dynamics (CFD). The first method follows closely recent literature studies and models a finite length single channel of a corrugation layer inside the HE core. The second method utilises the conjugate heat transfer methodology and models a section of the HE core with both cold and hot fluid streams separated by a solid conducting wall (HE corrugation). The results of latter model are then extrapolated for the full dimensions of a HE core layer to obtain flow and heat transfer characteristics. The conjugate heat transfer analysis methodology presented is novel and eliminates the need for analytical/empirical modelling currently widely used within industry. Furthermore, it provides more detailed information about the flow and heat transfer inside the HE core enabling potential for more efficient HE designs. Predictions at the corrugation level were carried out at 88 ⩽ Re corrug ⩽ 2957 with mesh independence studies completed for all the computational domains. The results obtained in the HE corrugation predictions were then implemented to the multi-scale HE unit model where the flow inside the HE core was modelled using two porous media simplifications whilst the heat transfer was simplified using the effectiveness source term. The HE unit predictions were validated against industrial experimental data with good agreement found between the numerical and experimental results. All the simulations were completed using the open-source CFD package OpenFOAM.

Electrohydrodynamic (EHD) air blowers are uniquely positioned to overcome the limitations of miniaturized mechanical fans in small-scale and consumer electronic devices. A novel cooling system design using optimized EHD blowers integrated with a plate-fin heat sink is presented and proposed for thin consumer electronics such as laptop applications. A three-dimensional (3D) numerical model is developed and validated to solve the coupled equations of EHD flow and conjugate heat transfer and predict the cooling performance of the integrated EHD system. For a range of heat sink heights from 6 to 12 mm, a parametric study is performed to investigate the influence of geometric parameters and operating conditions on the thermal performance of the EHD systems based on heat sink thermal resistance and the highest operating temperature. Numerical results demonstrate that the proposed EHD cooling system is able to provide effective cooling performance and maintain the temperature within the safe and typical operating range. Under a range of thermal design power (TDP) up to 30 W, trends of predicted operating temperatures show that the developed EHD cooling systems have great potential to compete with mechanical blowers in low-profile laptops with higher TDP, lower device height and reduced installation volume compared to a selected list of current standard laptops available commercially.

An experimental and numerical investigation of water-cooled serpentine rectangular minichannel heat sinks (MCHS) has been performed to assess their suitability for the thermal management of gallium nitride (GaN) high-electron-mobility transistors (HEMTs) devices. A Finite Element-based conjugate heat transfer model is developed, validated experimentally and used to determine the optimal minichannel width and number of minichannels for a case with a uniform heat flux of 100 W/cm2. The optimisation process uses a 30 point Optimal Latin Hypercubes Design of Experiments, generated from a permutation genetic algorithm, and accurate metamodels built using a Moving Least Square approach. A Pareto front is then constructed to enable the compromises available between designs with a low pressure drop and those with low thermal resistance to be explored and an appropriate minichannel width and number of minichannels to be chosen. These parameters are then used within conjugate heat transfer models of a serpentine MCHS with silicon, silicon carbide, diamond and graphene heat spreaders placed above a GaN HEMT heating source of area 4.8 × 0.8 mm2, generating 1823 W/cm2. A nanocrystalline diamond (NCD) layer with thickness of 2 µm is mounted on the top surface of the GaN HEMT to function as a heat spreader to mitigate the hot spots. The effect of volumetric flow rate and heat spreader thickness on the chip temperature has been investigated numerically and each of these has been shown to be influential. For example, at a volumetric flow rate of 0.10 l/min, the maximum chip temperature can be reduced from 124.7 °C to 96.7 °C by employing a 25 µm thick graphene heat spreader attached to the serpentine MCHS together with a NCD layer compared with a serpentine MCHS without these heat spreaders.

Purpose This aim of this work is to investigate different modelling approaches for air-cooled data centres. The study employs three computational methods, which are based on finite element, finite volume and lattice Boltzmann methods and which are respectively implemented via commercial Multiphysics software, open-source computational fluid dynamics code and graphical processing unit-based code developed by the authors. The results focus on comparison of the three methods, all of which include models for turbulence, when applied to two rows of datacom racks with cool air supplied via an underfloor plenum. Design/methodology/approach This paper studies thermal airflows in a data centre by applying different numerical simulation techniques that are able to analyse the thermal airflow distribution for a simplified layout of datacom racks in the presence of a computer room air conditioner. Findings Good quantitative agreement between the three methods is seen in terms of the inlet temperatures to the datacom equipment. The computational methods are contrasted in terms of application to thermal management of data centres. Originality/value The work demonstrates how the different simulation techniques applied to thermal management of airflow in a data centre can provide valuable design and operational understanding. Basing the analysis on three very different computational approaches is new and would offer an informed understanding of their potential for a class of problems.

As the demand for advanced microelectronic devices of small form factor and high power density has increased, the capability of the miniaturized mechanical solutions for effective cooling has become critical. Electrohydrodynamic (EHD) air blowers have great potential as alternative cooling solutions over the traditional mechanical systems to meet the thermal management requirements with flexible design and considerable flow production for effective heat removal. In this paper, a numerical analysis and optimization of wire-to-plane EHD air blowers are performed based on 2-D developed models validated against previous data. For a range of blower heights from 2 to 10 mm, the location and length of the collecting plane located at the blower walls apart from the emitter wire electrode are investigated and optimized based on pumping efficiency using the ranges of fixed operating powers and voltages. Simple relations for each optimization method are presented to determine the optimal blower configuration. Results of flow rate and static pressure obtained by each optimized blower show good agreement with those predicted by EHD scaling laws previously presented.

In this paper the performance of a recently patented additive layer manufactured (ALM) concept inter-layer heat exchanger (HE) is evaluated experimentally and numerically. Two numerical HE models are developed using the conjugate heat transfer (CHT) methodology. The first is an idealised HE core model, consisting of a single period width HE corrugation section (termed superchannel). The second approach uses a fully detailed HE unit model which resolves the flow and heat transfer inside the complete HE unit. A close agreement was found between the HE unit simulations and the experimentally obtained results, such that the fully detailed HE model could be validated. It was also shown that, a full CHT approach is necessary to accurately evaluate complex inter-layer ALM HE core flow and heat transfer behaviour and can serve as an approach for optimising HE designs. The results also reinforce the occurrence of the inter-layer flow mixing inside the HE core of the same flow streams and allows the mass flow to redistribute inside the HE core which is impossible with the current HE generation geometries. The superchannel model results in a slight over-estimation in heat transfer ( Δ T ≈ 4 K on average) making the simplified model acceptable as a conservative estimate. Using validated simulations a parametric study was conducted by changing the solid properties of the full CHT HE model to aluminium to investigate the effects of a significantly more conductive material. This resulted in ≈ 3 % higher heat transfer effectiveness ( ϵ ) of the HE unit. All the simulations were carried out using CFD package OpenFOAM.

Electrohydrodynamic (EHD) airflows provide a means of generating air motion by a gas discharge across two electrodes without the need for moving parts. A two-dimensional numerical model for a wire-to-grid EHD air blower is developed, validated against previous data, and used to investigate the influence of key design parameters on blower efficiency, including the emitter wire diameter, the blower height, the locations of the collecting surfaces and grid wires, and the collecting grid density. The optimal locations of both collectors from the corona wire are determined based on blower efficiency at a fixed operating power, resulting in improvements in the average outlet velocity between 9% and 15%, depending on blower thickness. The presence of the grid has a strong influence on the electric field distribution and increases the blower performance, with higher flow, lower operating voltage and reduced blower size. An investigation into the effect of grid density reveals that using coarse collecting grids is generally beneficial, leading to higher efficiency with lower pressure losses.

This paper presents a combined experimental and numerical investigation of single-phase water flow and heat transfer in serpentine rectangular microchannels embedded in a heated copper block. The performance of four different microchannel heat sink (MCHS) configurations are investigated experimentally, the first having an array of straight rectangular microchannels (SRMs), while the other have single (SPSMs), double (DPSMs) and triple path multi-serpentine rectangular microchannels (TPSMs). Three-dimensional conjugate heat transfer models are developed for both laminar and turbulent single-phase water flows in each of these MCHSs and the governing flow and energy equations solved numerically using finite elements. The numerical predictions of pressure drop ( Δ P ) and average Nusselt number ( Nu avg ) are in good agreement with experimental data, and indicated that the single path serpentine microchannel (SPSM) leads to a 35% enhancement of the Nu avg at a volumetric flow rate of 0.5 l / min and a 19% reduction in total thermal resistance ( R th ) compared to the conventional SRM heat sink. However, this enhancement is at the expense of a large (up to ten-fold) increase in Δ P compared to the SRM heat sink, so that a suitable compromise must be struck between heat transfer and pressure drop in practical MCHS designs.

A combination of laboratory experiments and a system model are used to carry out the first investigation into the potential for cold air to bypass IT equipment within data centres (DCs) employing aisle containment, and the effect of this bypass on DC electricity consumption. The laboratory experiments involved applying a differential pressure across commercially available server racks and aisle containment systems and measuring the resulting air flow. The potential to minimise bypass by sealing leakage paths and redesigning racks was investigated and quantified experimentally. A new system model is developed using a combination of manufacturer data, empirical relationships and experimental results to predict the impact of bypass on the power consumption of the various components of a DC’s cooling infrastructure. The results show that, at typical cold aisle pressures, as much as 20% of the supplied air may bypass servers by finding alternate paths through the server rack itself. This increases the required flow rate from air conditioning units (ACUs). The system model predicts that: (i) practical measures undertaken to reduce this bypass could reduce total power consumption by up to 8.8% and (ii) excessive pressure differentials across the containment system could also increase power consumption, by up to 16%.

The importance of accounting for the temperature dependence of air properties in numerical simulations of air flows over pinned heat sinks is demonstrated by comparisons with recently published experiments. Numerical simulations, based on a conjugate heat transfer analysis, using the RANS-based modified k-ω turbulence model, with temperature-dependent air properties, are shown to be in significantly better agreement with experimental measurements of pressure drop, heat transfer coefficient, and heat sink base temperature, than those which employ constant air properties.

The evaporation characteristics of sessile droplets on heated hydrophobic and hydrophilic surfaces are investigated. Results are reported for the evaporation of water droplet volumes covering a range of shapes dominated by surface tension or gravity and over a range of temperatures between 40 and 60 °C. The weight evolution and total time of evaporation is measured using a novel self-contained heating stage on a high resolution analytical balance, which has advantages over visualization measurement techniques as it allows free choice of the initial droplet size and surface and the ability to record the droplet evaporation right through to the final stages of droplet life. Evaporation is modeled through a combination of a constant contact area and a constant contact angle model with the switch from the former to the latter occurring when the contact angle falls below its predetermined receding value. Theoretical results compare well with the experimental results for the hydrophobic substrate. However, a significant deviation is observed for the hydrophilic substrate due to the combined effects of the droplet surface cooling due to evaporation and buoyancy effects that are not included in the model. The proposed method of using the stick–slip model offers a convenient means of modeling droplet evaporation by mimicking the drying modes based on initial measurements of the static and receding contact angles.

The benefits of using pin heat sinks (PHSs) with single, rectangular slotted or notched pin perforations, are explored computationally, using a conjugate heat transfer model. Results show that the heat transfer increases monotonically while the pressure drop decreases monotonically as the size of the rectangular perforation increases. Performance comparisons with PHSs with multiple circular perforations show favourable heat transfer and pressure drop characteristics. However, the reduced manufacturing complexity of rectangular notched pins in particular provide strong motivation for their use in practical applications. Detailed parameterisation and optimisation studies into the benefits of single rectangular notch perforations demonstrate the scope for improving heat transfer and reducing mechanical fan power consumption yet further by careful design of pin density and pin perforations in PHSs.

The benefits of using pin fin heat sinks with multiple perforations are investigated using complementary experimental and Computational Fluid Dynamics (CFD) methods. An experimental heat sink with multiple perforations is designed and fabricated and parameter studies of the effect of perforated pin fin design on heat transfer and pressure drops across the heat sinks undertaken. Experimental data is found to agree well with predictions from a CFD model for the conjugate heat transfer into the cooling air stream. The validated CFD model is used to carry out a parametric study of the influence of the number and positioning of circular perforations, which shows that the Nusselt number increases monotonically with the number of pin perforations, while the pressure drop and fan power required to overcome the pressure drop all reduce monotonically. Pins with five perforations are shown to have a 11% larger Nu than for corresponding solid pin cases. These benefits arise due to not only the increased surface area but also heat transfer enhancement near perforations through the formation of localised air jets. In contrast, the locations of the pin perforations are much less influential. When examined in the context of CPU cooling, a conjugate heat transfer analysis shows that improved heat transfer with pin perforations translates into significantly reduced processor case temperatures with the additional benefit of a reduction in the weight of the heat sink's pins. To achieve these benefits care must be taken to ensure that pin perforations are aligned with the dominant flow direction and manufactured with a good quality surface finish.

A Concept Heat Exchanger (HE) design manufactured using the Additive Layer Manufacturing (ALM) technique Selective Laser Melting (SLM) is proposed and numerically evaluated. It is composed of a HE corrugation which introduces inter-layer flow conduits between the parallel HE layers of the same fluid. These pathways are provided by hollow elliptical tubes which serve several functions: to disturb the flow to promote heat transfer, to provide additional heat transfer area and to minimise flow maldistribution inside the HE core. The corrugation is incorporated into a counter-flow prototype HE unit model meaning to exploit the installation volume and design freedom made possible via ALM. Three Computational Fluid Dynamics (CFD) models are utilised to evaluate the performance of the proposed HE unit. Firstly, a traditional two step HE design methodology is utilised which works by initially evaluating a fully symmetric channel of the proposed HE corrugation (termed single channel). Then the results this model are incorporated into a simplified HE unit model. The second approach evaluates the HE unit performance based on a fully detailed CFD analysis that fully resolves flow and heat transfer inside the HE core. The third modelling approach involves splitting the inter-layer HE unit model into parts, which results in HE header models and allows simplification of the HE core into a single corrugation period width HE core model (termed superchannel). The results of these models are then compared to a conventional pin–fin HE unit model, formed by blocking the elliptical inter-layer conduits. It was found that in all the HE unit models the pressure drop is similar whilst the heat transfer was enhanced by between 7% and 13% in terms of the overall Δ T by the inter-layer channels (increasing with the Reynolds number). All simulations were completed using a CFD package OpenFOAM.

Supplying coolant through internal coolant channels is a common method of transporting large thermal loads away from the tool in twist-drill machining to increase tool life, aid chip evacuation and avoid catastrophic tool failure. In this work a finite element-based numerical model of the machining process is loosely coupled with a finite volumebased numerical method for predicting the distribution of coolant inside the borehole. These methods are employed to study the effect of channel position on cutting geometry lubrication and uses response surface models to show that all designs do not fully flood the borehole and that not all areas of the tool geometry are lubricated with coolant. Visual analysis of results show that coolant, for all designs, primarily \ud lubricates the area between the cutting edge and the coolant hole exit, however depending on application requirements coolant channel positioning can be used to modify coolant supply to the axial rake, for chip evacuation or to the cutting edge for heat removal.

Data centers are complex systems that require sophisticated operational management approaches to provide the availability of digital services against the backdrop of cost and energy efficiency. To achieve this, data center telemetry data is required since, as is commonly said it is not possible to manage what cannot be measured. This paper details how it is possible to construct the key data center infrastructure management (DCIM) elements of monitoring and measuring by a combination of available opensource software tools that permit both scalability and an environment where analytics can be employed on the data center operation, which can offer relevant insight into energy efficient operational practices.

A novel lattice Boltzmann method (LBM) based 3D computational fluid dynamics (CFD) technique has been implemented on the graphics processing unit (GPU) for the purpose of simulating the indoor environment in real-time. We study the time evolution of the turbulent airflow and temperature inside a test chamber and in a simple model of a four-bed hospital room. The predicted results from LBM are compared with traditional CFD based large eddy simulations (LES). Reasonable agreement between LBM results and LES method is observed with significantly faster computational times.

A methodology using Computational Fluid Dynamics (CFD) was developed to predict the flow and heat transfer performance of a single two dimensional sinusoidal channel of a Heat Exchanger (HE) at a Reynolds number (Re) range of 5 ≤ Re ≤ 500. The impact of different modelling assumptions was thoroughly evaluated which has not has been done in detail before. Two computational domains were used: a single period sinusoidal channel for fully periodic flow predictions and finite length channel consisting of 6 sinusoidal channel periods. Mesh and time independence was achieved for both domains whilst results with periodic domain were compared to numerical results in the literature. Laminar, k-ε and k-ω SST predictions were assessed throughout the Reynolds range with unsteady flow onset detected at Re ≈ 200 using laminar and k-ω SST models. The impact of different accuracy numerical discretisation schemes is assessed throughout the Re range and it was found that second order accuracy schemes should be used to fully capture the unsteady flow. Comparison between open-source CFD package OpenFOAM and Ansys was Fluent was performed and agreement was ‘ found.

Aisle containment is increasingly common in data centres, and is widely believed to improve efficiency and effectiveness of cooling. Investigations into the impacts of aisle containment on the behavior and power consumption of cooling infrastructure and servers have been limited. Nor has the impact of supply air conditions on these factors been extensively investigated. This work uses measurements of bypass in a test data centre and observations on server behavior in a wind tunnel, in conjunction with a system model, to investigate the efficiency with which computations can be undertaken in an aisle contained data centre, and how this is impacted by supply air conditions.

Datacenter energy consumption constitutes a large portion of global energy consumption. Particularly, a large amount of this energy is consumed by the datacenter cooling system. Subsequently, many innovative cooling technologies have been developed to reduce energy consumption and increase cooling performance. In this work, an experimental setup was designed and constructed which comprises a direct liquid-cooled server, rack-level cooling and compressor-free external cooling system. This study tracks the heat generated from IT processes to the environment. In addition, the power usage effectiveness (PUE) and the air handling unit (AHU) performance are investigated. The objectives were studied under different datacenter operation scenarios, and AHU configurations.

Real-time fluid simulation is an active field of research in computer graphics, but they usually focus on visual impact rather than physical accuracy. However, by combining a lattice Boltzmann model with the parallel computing power of a graphics processing unit, both real-time compute capability and satisfactory physical accuracy are now achievable. The implementation of an optimized 3D real-time thermal and turbulent fluid flow solver with a performance of half a billion lattice node updates per second is described in detail. The effects of the hardware error checking code and the competition between appropriate boundary conditions and performance capabilities are discussed.

The natural convection cooling system of a fully immersed server in a dielectric liquid is analysed numerically, where the servers are in sealed capsules and submerged in in a dielectric fluid. A reduced order flow model is developed under a saturated porous media flow assumption using a Darcy flow regime and the Stokes equations solved numerically using successive over relaxation and time marching techniques. The simplified model is shown to agree well with predictions from full Navier-Stokes flow analyses and then used to study the role of spatial parameters on the convective heat transfer, in particular the effect of the locations and separations of two heat sources representing two central processing units (CPUs). The flow and heat transfer characteristics are analysed for a range of modified Rayleigh numbers between 0.5 and 300 and a correlation for Nusselt number is obtained which shows that the thermal behaviour is most strongly influenced by the modified Rayleigh number and that the vertical separation of the CPUs is more influential than the vertical position of the lower CPU.

Relying on thermal air management in a data centre is becoming less effective as heat densities from the Information Technology (IT) equipment continue to rise. Direct liquid cooling is more efficient at transferring the waste heat, but requires liquid loops passing as close as possible to the heat source. A new Computational Fluid Dynamics (CFD) strategy is developed for data centre scenarios where a liquid loop heat exchanger is attached at the rear of server racks (back doors), which can avoid the need to separate the cold and hot air streams in traditional hot/cold aisle arrangements. The effectiveness of additional fans in the back door heat exchangers is investigated using the three-dimensional CFD model of a simplified three-aisle, six-rack data centre configuration.

Estrogen receptors (ERs) regulate gene transcription by interacting with regulatory elements. Most information regarding how ER activates genes has come from studies using a small set of target genes or simple consensus sequences such as estrogen response element, activator protein 1, and Sp1 elements. However, these elements cannot explain the differences in gene regulation patterns and clinical effects observed with estradiol (E(2)) and selective estrogen receptor modulators. To obtain a greater understanding of how E(2) and selective estrogen receptor modulators differentially regulate genes, it is necessary to investigate their action on a more comprehensive set of native regulatory elements derived from ER target genes. Here we used chromatin immunoprecipitation-cloning and sequencing to isolate 173 regulatory elements associated with ERalpha. Most elements were found in the introns (38%) and regions greater than 10 kb upstream of the transcription initiation site (38%); 24% of the elements were found in the proximal promoter region (

Mixing in microchannels is an important process since certain microfluidic applications require the rapid mixing of species. This paper investigates the mixing in the microchannels via the possibility of utilising the spatial zeta potential variations with the opposite sign to that of the homogeneous surface to create the circulation zones within the bulk flow. The effects of the number of heterogeneous patches, the strength of applied external voltage and zeta potential are analyzed to determine the mixing efficiency. In addition, the replacement of patches with heterogeneous surface blocks shows the improvement of mixing efficiency. The fluid flow is simulated via the single relaxation time lattice Boltzmann method (LBM) with a forcing term from the electric field. The concentration of species within the channel is governed by an advection‐diffusion equation with Peclet number of the order 1000.

The contact line problem, where a liquid/fluid interface moves relative to a solid boundary (either slipping or spreading), is an important feature of many engineering and naturally occurring free-surface flows. This paper discusses the current state-of-the-art in applying wetting line models to computational fluid dynamics simulations and contrasts and compares it with a new wetting model (based on one physical parameter, notably the solid boundary surface affinity) for the finite-density multiphase lattice Boltzmann method (LBM). Results of two-dimensional filament (natural and forced) spreading flows over different types of surfaces are presented to illustrate the capability and drawbacks of the new dynamic wetting model for the finite-density multiphase LBM.

An adaptive finite element algorithm is described for the stable solution of three-dimensional free-surface-flow problems based primarily on the use of node movement. The algorithm also includes a discrete remeshing procedure which enhances its accuracy and robustness. The spatial discretisation allows an isoparametric piecewise-quadratic approximation of the domain geometry for accurate resolution of the curved free surface. The technique is illustrated through an implementation for surface-tension-dominated viscous flows modelled in terms of the Stokes equations with suitable boundary conditions on the deforming free surface. Two three-dimensional test problems are used to demonstrate the performance of the method: a liquid bridge problem and the formation of a fluid droplet.

A direct comparison between experimental and numerical results for the passage of an array of 3D flat-top, square shaped surface features through an EHL point contact is presented. Results for pure rolling conditions show that the features’ deformation in the high-pressure region is governed by their ability to entrap lubricant both underneath and in the grooves during their passage through the inlet zone. Film perturbations associated with each defect occur as locally enhanced regions of lubricant and film thickness micro-constrictions. Under sliding conditions the features sustain further deformations as they traverse the high-pressure conjunction and meet the highly viscous lubricant entrapped in the grooves, which moves at a different velocity. Lubricant is also seen to accumulate just in front or behind the features depending on the slide-to-roll ratio. Overall, the results highlight the importance of understanding the effects of the defects structure and the lubricant rheology on the film thickness to unravel the effects of real roughness patterns.

An arbitrary Lagrangian–Eulerian (ALE) finite element method is described for the solution of three-dimensional free-surface flow problems. The focus of this work is on extending the algorithm to include a dynamic contact line model allowing the fluid free surface, in the steady case, to form a prespecified static contact angle with a solid boundary and, in the transient case, to move along the solid boundary. This widens the applicability of the algorithm to important industrial applications such as forced spreading of fluids and gravity-driven flow on inclined surfaces. Copyright © 2004 John Wiley & Sons, Ltd.

A comprehensive analysis is presented of the internal flow structures that can arise in the coating bead of a twin roll coater, operating in forward mode, as inlet flux is varied seamlessly from a starved to a flooded condition. This is accomplished via the application of lubrication theory combined with the acquisition of accurate finite element solutions of the full two free surface flow problem. The former is used to map out associated control-space diagrams in terms of the non-dimensional quantities, roll speed ratio, inlet flux and Capillary number, and, further, to predict the distribution of eddies and liquid transfer paths as a function of the main operating parameters. The dynamic contact angle is predicted as a function of the operating conditions for both the computations and analysis. Detailed results are presented which: (i) provide a complete picture of attendant eddy structures; (ii) quantify the strengths of primary, secondary and asymmetric liquid transfer jets; (iii) reveal the presence of a weak secondary asymmetric transfer jet in the intermediate to flooded inlet flow regime; (iv) describe stagnation and free surface locations as a function of inlet flux and Capillary number; (v) demonstrate the unexpected coexistence of the primary and asymmetric jets in the low speed ratio meniscus regime.

The use of boundary-conforming finite element methods is considered for the solution of surface-tension-dominated free-surface flow problems in three dimensions. This class of method is based upon the use of a moving mesh whose velocity is driven by the motion of the free surface, which is in turn determined via a kinematic boundary condition for the normal velocity. The significance of the method used to compute the normal direction at the finite element node points for a C0 piecewise-polynomial free surface is investigated. In particular, it is demonstrated that the concept of mass-consistent normals on an isoparametric quadratic tetrahedral mesh is flawed. In this case an alternative, purely geometric, normal is shown to lead to a far more robust numerical algorithm.

Recent experimental observations by Félix-Quiñonez, Ehret and Summers ( Trans. ASME, J. Tribology, 2003, 125, 252–259) for a single, flat-top transverse ridge passing through an elastohydrodynamic lubrication (EHL) point contact under various slide-roll ratios have revealed some unexpected features. In particular, under sliding conditions the ridge was observed to sustain additional deformations in the high-pressure region of the contact and an entrapped amount of lubricant was seen to accompany the ridge during its passage through the conjunction and, contrary to current knowledge, appeared not to move at the entraining velocity. This paper analyses the experimental observations by means of a direct comparison with numerical simulations. The numerical results show that the main contributing factor towards the experimentally observed accumulation of lubricant is a reduced lubricant viscosity via shear thinning effects. For the operating conditions considered in this study, internal heating of the lubricant is found to be negligible. In addition, it is shown that the amount of lubricant accumulation is dependent on the geometry of the ridge. Compared to a defect with a Gaussian shape, it is further shown that a greater accumulation of lubricant occurs with the flat-top ridge due to the generation of larger pressure gradients.

An image analysis technique is employed to measure lubricant film thickness fluctuations introduced by a single flat-top transverse ridge passing under pure rolling conditions through an elastohydrodynamically lubricated contact. The experimental observations are compared with the film thickness profiles predicted by the discrete Fourier transform method for real surface roughness given by Morales-Espejel et al. in 2000. The results show that the theoretical approach adequately predicts the overall film thickness, thus giving credence to its potential practical application. On a local scale, however, important differences are seen regarding the deformed shape of the feature.

Spray cooling is one of the most promising methods of cooling high heat flux electronics. Depending on the type of the nozzle, spray cooling can be categorized as single-phase or two-phase. In the latter, which is known to be more effective, a secondary gas is used to further pressurize the liquid and form smaller droplets at higher velocities. The gas is also assumed to assist the spreading phase by imposing normal and tangential forces on the droplet free surface which adds to the complicated hydrodynamics of the droplet impact. Moreover, the order of magnitude of droplet size in spray cooling is 10−6 m, thereby introducing a low Weber and Reynolds numbers’ impact regime which heretofore has not been well understood. A 3D lattice Boltzmann method was implemented to simulate the impact of a single micro-droplet on a dry surface both in ambient air and under a stagnation gas flow. Two cases were closely compared and correlations were proposed for the instantaneous spreading diameter. Contrary to recent findings at higher impact Weber and Reynolds numbers, it was found that a stagnation flow only significantly affects the spreading phase for Ca* ≥ 0.35 but has little influence on the receding physics.

An industrially-important variant of reverse roll coating is studied in which the metering gap sits beneath a large, liquid reservoir, the influence of which is investigated via complimentary experimental, analytical (lubrication) and computational (finite element) methods, and for which gravitational effects are shown to be influential. Experimental measurements for both the flow rate and wetting line position are given over a wide range of roll speed ratio and capillary number and it is shown that, provided the wetting line is sufficiently far from the nip, the flow rate depends linearly on the reservoir level. A key feature of the mathematical models is that, unlike previous reverse roll coating studies, the variation of dynamic contact angle with metering roll speed has been accounted for. The lubrication model also uses boundary conditions which incorporate free surface, surface tension and wetting line effects and the predictions from both models are found to be in generally good agreement with experiment. Finally, streamlines obtained from Finite Element solutions of the flow in the reservoir and wetting line regions are found to compare well with corresponding experimental flow visualisations. The flow in the reservoir is recirculating in nature, the size and number of recirculations depending on the reservoir geometry.

Metrics commonly used to assess the energy efficiency of data centres are analysed through performing and critiquing a case study calculation of energy efficiency. Specifically, the metric Power Usage Effectiveness (PUE), which has become a de facto standard within the data centre industry, will be assessed. This is achieved by using open source specifications for a data centre in Prineville, Oregon, USA provided by the Open Compute Project launched by the social networking company Facebook. The usefulness of the PUE metric to the IT industry is critically assessed and it is found that whilst it is important for encouraging lower energy consumption in data centres, it does not represent an unambiguous measure of energy efficiency.

BACKGROUND Left ventricular noncompaction (LVNC) cardiomyopathy is a heterogeneous condition that is gaining recognition as a distinct clinical entity. It is characterized by numerous, excessively prominent ventricular trabeculations with deep trabecular recesses and the formation of 2 distinct layers of myocardium. It is often unrecognized clinically for decades and poses distinct life-threatening complications. Accurate diagnosis is key to minimizing risks associated with LVNC. PURPOSE This article will introduce clinicians to the defining criteria and diagnostic process for recognition of LVNC. Left ventricular noncompaction is an important clinical entity not easily recognized at first glance but has important clinical ramifications for treatment requiring an accurate diagnosis. CONCLUSIONS Left ventricular noncompaction is present at birth but can remain clinically silent for decades. It is found both in isolation and as a part of other congenital cardiac, extracardiac, neuromuscular, and genetic syndromes. Left ventricular noncompaction is associated with the major complications of heart failure, tachyarrhythmias, and systemic thromboembolism. CLINICAL IMPLICATIONS Transthoracic echocardiography will likely remain the initial screening modality, but modality limitations, questionable findings, and clinical presentations suspicious for noncompaction should be followed up promptly with cardiac magnetic resonance imaging for definitive diagnosis and treatment.

Three practically relevant, Stokes flows in closed, rectangular cavities are considered. The first involves a solid-walled cavity where flow is driven by the motion of either one or both of its horizontal bounding walls; the other two have an upper free surface and are driven either by the motion of vertical side walls or by a horizontally-moving lower wall. Each problem is formulated as a biharmonic boundary value problem (bvp) for the streamfunction. The relative merits of two different coefficient determination methods for the corresponding analytical solutions are assessed and, in addition, each solution is compared with its numerical counterpart obtained using a finite element formulation of the governing equations. It is shown that, provided the number N of terms in each solution is sufficiently large, they are in extremely good agreement and, similarly, they compare well with work from other published theoretical and experimental studies. Streamlines are presented, over a wide range of operating parameters, for the geometries containing an upper free surface. For the flow generated by two moving vertical side walls two flow transformation mechanisms are identified. For cavities with small and decreasing width to depth (aspect) ratios, there is a sequence of critical aspect ratios at which flow bifurcations arise with a centre becoming a saddle point and vice versa, whereas for large aspect ratios increasing the ratio further leads to eddy growth from the lower wall, resulting in a regular sequence of separatrices along the cavity width. In the case of flow generated by a horizontally-moving lower wall the streamlines are simpler and exhibit the regular array of separatrices reported previously for flow in a solid-walled cavity with a single moving wall.

The flow structure in the reservoir of a ceramic tape casting head is examined using a linear finite element formulation of the governing fluid mechanical equations, written in terms of the streamfunction and vorticity. Streamline plots are presented for representative reservoir shapes and depth-to-width (aspect) ratios. Analysis predicts that the flow is characterised by an ever-present primary recirculation, adjacent to the moving web, and that the size and number of secondary recirculations above the primary one depend upon both the aspect ratio of the reservoir and the angle of inclination of the side walls. In particular, it is shown that the number of recirculations spanning the entire width of the reservoir increases at critical values of the aspect ratio, when eddies emanating from the upper corners merge together. The analysis also shows that for a given reservoir depth, it is possible to postpone the development of these secondary recirculations by ‘opening out’ the casting head, i.e. making the sides further away from the vertical, which may be a useful control mechanism for the design of such systems.

In the design of a data centre (DC), engineers are increasingly using specialized commercial computational fluid dynamics (CFD) software packages to predict thermal loads for various cooling configurations. This paper is the first to demonstrate how a set of open source software tools, namely OpenFOAM, Octave and paraview can be used to simulate and visualize the airflow in a real live 55kW private DC, with a particular focus on energy savings and reduction of hotspots.

Liquid immersion of datacom electronics can be configured as a cooling mechanism when components are in direct contact with a high dielectric strength liquid. This paper analyses the heat transfer capabilities of datacom electronics when they are enclosed in a cassette with a dielectric liquid and sandwiched against a cold plate positioned in parallel with the printed circuit board. A proxy server motherboard with controllable heat cells and temperature sensors is used to conduct a series of heat transfer experiments. The results of which demonstrate that the thermal conductance processes via the naturally convecting dielectric liquid improves as the power demand of the electronics increases.

Two industrially important free surface flows arising in polymer processing and thin film coating applications are modelled as lid-driven cavity problems to which a creeping flow analysis is applied. Each is formulated as a biharmonic boundary-value problem and solved both analytically and numerically. The analytical solutions take the form of a truncated biharmonic series of eigenfunctions for the streamfunction, while numerical results are obtained using a linear, finite-element formulation of the governing equations written in terms of both the streamfunction and vorticity. A key feature of the latter is that problems associated with singularities are alleviated by expanding the solution there in a series of separated eigenfunctions. Both sets of results are found to be in extremely good agreement and reveal distinctive flow transformations that occur as the operating parameters are varied. They also compare well with other published work and experimental observation.

The method of variable-coefficient harmonic balance (VCHB) presented in Summers (1995) is applied to the Duffing and the periodically forced van der Pol oscillator equations. A rationale is given of how Poincare-Andronov-Hopf (PAH) bifurcations in the amplitude evolution equations of VCHB relate to the various local bifurcations arising in the solution of these two oscillator equations. In order to demonstrate the crucial role played by PAH bifurcations in the amplitude evolution equations, the theory is applied in its simplest form, that is VCHB with a one-harmonic solution expansion, to the Duffing oscillator equation with a single well and softening-type nonlinearity. A single PAH bifurcation in the amplitude evolution equations is evaluated and the frequency of this bifurcation is then used to construct the curve of symmetry breaking bifurcations of the periodic solutions of the Duffing equation in (ω, F ) control space. In general, the periodic solution is represented by a truncated Fourier series with several harmonics and, therefore, by virtue of the size of the problem, numerical methods are employed to perform the algebra and the analysis. This procedure yields many various eigenvalues to track and, hence, an increased number of PAH bifurcations. The features of nonlinear resonances, period-doubling bifurcations, symmetry breaking bifurcations, subharmonic and superharmonic entrainments and NaimarkSacker bifurcations are tracked throughout control parameter space by tracing the critical eigenvalues associated with the PAH bifurcations. The local bifurcations of the two oscillator equations are classified by evaluating the imaginary part of the critical eigenvalues. In the case of the forced van der Pol equation with order one nonlinearity, the 1:1 and 3:1 superharmonic entrainment boundaries are evaluated, and the symmetry breaking and period-doubling bifurcation boundaries are derived for the first time by a semi-analytic approach.

Three mathematical models are developed for meniscus roll coating in which there is steady flow of a Newtonian fluid in the narrow gap, or nip, between two contrarotating rolls in the absence of body forces.The zero flux model predicts a constant pressure gradient within the central core and two eddies, each with an inner structure, in qualitative agreement with observation. The small flux model takes account of a small inlet flux and employs the lubrication approximation to represent fluid velocity as a combination of Couette and Poiseuille flows. Results show that the meniscus coating regime is characterized by small flow rates (λ [Lt ] 1) and a sub-ambient pressure field generated by capillary action at the upstream meniscus. Such flows are found to exist for small modified capillary number, Ca(R/H0)1/2 [lsim ] 0.15, where Ca and R/H0 represent capillary number and the radius to semi-gap ratio, respectively.A third model incorporates the full effects of curved menisci and nonlinear free surface boundary conditions. The presence of a dynamic contact line, adjacent to the web on the upper roll, requires the imposition of an apparent contact angle and slip length. Numerical solutions for the velocity and pressure fields over the entire domain are obtained using the finite element method. Results are in accord with experimental observations that the flow domain consists of two large eddies and fluid transfer jets or ‘snakes’. Furthermore, the numerical results show that the sub-structure of each large eddy consists of a separatrix with one saddle point, two sub-eddies with centres, and an outer recirculation.Finally finite element solutions in tandem with lubrication analysis establish the existence of three critical flow rates corresponding to a transformation of the pressure field, the emergence of a ‘secondary snake’ (another fluid transfer jet) and the disappearance of a primary snake.

This paper explores the application of the method of variable-coefficient harmonic balance to nonautonomous nonlinear equations of the form XsF(X, t:λ), and in particular, a one-degree-of-freedom nonlinear oscillator equation describing escape from a cubic potential well. Each component of the solution, X(t), is expressed as a truncated Fourier series of superharmonics, subharmonics and ultrasubharmonics. Use is then made of symbolic manipulation in order to arrange the oscillator equation as a Fourier series and its coefficient are evaluated in the traditional way. The time-dependent coefficients permit the construction of a set of amplitude evolution equations with corresponding stability criteria. The technique enables detection of local bifurcations, such as saddle-node folds, period doubling flips, and parts of the Feigenbaum cascade. This representation of the periodic solution leads to local bifurcations being associated with a term in the Fourier series and, in particular, the onset of a period doubled solution can be detected by a series of superharmonics only. Its validity is such that control space bifurcation diagrams can be obtained with reasonable accuracy and large reductions in computational expense.

A simplified analytical solution has been obtained for the radial and tangential displacements on the surface of a thin, hemispherical layer of porous-elastic articular cartilage firmly bonded to a rigid foundation. A static pressure distributed according to a paraboloid of revolution is applied simulating cartilage compression by a porous indenter. The solution method is in the form of an asymptotic series and uses Laplace transforms. The analytical predictions are in qualitative agreement with the behaviour of biphasic articular cartilage reported in the literature. A direct comparison with numerical simulations using commercially available Finite Element Modelling (FEM) software was also carried out for conditions relevant to natural hip joints and the results show a good quantitative agreement overall.