Halldór Pálsson

Aggregated dynamic simulation model of district heating networks

Abstract The dynamic properties of district heating (DH) networks include water flow and propagation of heat from production plants to consumers. Mathematical models of such networks can be applied, either for general understanding of DH systems, or in combination with production planning and optimization. One type of mathematical model involves a full physical modeling of the network, taking into account individual pipes, dimensions, material properties etc. Such full models tend to be computationally intensive when applied in network simulations, which can be a problem when considering large DH systems. In the current paper, a method is presented in which a fully described model of a DH network is replaced by a simplified one, with the purpose of reducing simulation time. This simplified model is generated by gradually reducing the topological complexity of the original network. The method is validated by applying it on a real case study, in which a network with over 1000 pipes is reduced to less than 10 pipes. The results show that such relatively simple networks can maintain most of the dynamic characteristics of the original networks.

Risk of failure during gait for direct skeletal attachment of a femoral prosthesis: A finite element study

Direct skeletal attachments for transfemoral amputees have been the subject of clinical trials since the early nineties. This method of attachment allows the amputee an unrestricted range of motion around the hip joint, better sitting comfort, improved sensory feedback through osseoperception, improved limb control and reduced soft tissue problems. However, the length of the rehabilitation period is perceived as a shortcoming by the amputees and the clinicians. The aim of the present study is to estimate the risk of failure during gait, for a patient with direct skeletal attachment of a femoral prosthesis, using finite element analysis (FEA). Material properties and loads were derived from subject-specific data and implant stability assumed secured by bone ingrowth into a porous implant surface. A simplified FEA was used to optimize the implant geometry with respect to load bearing capacity. The resulting geometry was then implemented in a subject-specific FE study. The results indicate that the risk of failure for the implant system is approximately three times greater than what can be expected for an intact femur. The main conclusion, based on the risk of failure factors calculated, is that it is likely that a porous-coated implant could be beneficial for osseointegrated fixation. It is also suggested that the proposed methodology can be used in future studies exploring the mechanical stability of osseointegrated fixation in the view of improving direct skeletal attachments for lower limb amputees.

Continuous-Time Linear Parameter-Varying Identification of a Cross Flow Heat Exchanger: A Local Approach

In this paper, the problem of deriving a dynamical model of a cross flow heat exchanger is considered. In order to take into account the dependency of the systems dynamics on the hot and the cold mass flow rates in an explicit way, an input-output linear parameter-varying (LPV) model is used. A local approach composed of three steps is carried out to identify this LPV model. A parameter estimation scheme is introduced in which cost functions are minimized by using specific nonlinear programming methods. In this study, a finite volume physical model simulator is exploited to simulate and to generate the data. Simulations are performed to demonstrate the benefits of the suggested approach.

Influence of Parameter Variations on the Braking Torque of a Magnetorheological Prosthetic Knee

Microprocessor-controlled prosthetic knees, which rely on magnetorheological (MR) technology, have the potential to increase the comfort and quality of life of amputees. The focus of this study is on a prosthetic knee which is currently on the market and manufactured by the company Ossur Inc. The knee uses magnetic fields to vary the viscosity of the MR fluid, and thereby its flexion resistance. The torque transmissibility of the knee greatly depends on the magnetic field intensity in the MR fluid. The objective of this study is to investigate the strength of the magnetic field and the braking torque in the knee, for a few selected design parameters, and to determine which changes can be made to the existing design in order to maximize the torque output. The magnetic field in the fluid is evaluated by finite element analysis and the torque is calculated by using a Bingham visco-plastic model. A parametric study is carried out for several design parameters where the effect of variation in each parameter on the braking torque is observed. The results of this study give a valuable insight into which parameters should be prioritized for future changes of the knee, with regard to strength and comfortability.

Interactive graph-cut segmentation for fast creation of finite element models from clinical ct data for hip fracture prediction

Abstract In this study, we propose interactive graph cut image segmentation for fast creation of femur finite element (FE) models from clinical computed tomography scans for hip fracture prediction. Using a sample of N = 48 bone scans representing normal, osteopenic and osteoporotic subjects, the proximal femur was segmented using manual (gold standard) and graph cut segmentation. Segmentations were subsequently used to generate FE models to calculate overall stiffness and peak force in a sideways fall simulations. Results show that, comparable FE results can be obtained with the graph cut method, with a reduction from 20 to 2–5 min interaction time. Average differences between segmentation methods of 0.22 mm were not significantly correlated with differences in FE derived stiffness (R2 = 0.08, p = 0.05) and weakly correlated to differences in FE derived peak force (R2 = 0.16, p = 0.01). We further found that changes in automatically assigned boundary conditions as a consequence of small segmentation differences were significantly correlated with FE derived results. The proposed interactive graph cut segmentation software MITK-GEM is freely available online at https://simtk.org/home/mitk-gem.

Probabilistic production simulation including combined heat and power plants

Expansion planning of combined heat and power systems requires simulation tools for estimating key indicators for decision making. For power systems, so-called probability production simulation techniques have been used extensively. These techniques have been extended to include combined heat and power (CHP) systems with a single heat area and back-pressure type CHP units. In this paper, existing simulation methods for CHP are extended to include extraction power plants. Furthermore, the case of multiple heat areas is addressed and three different simulation strategies are presented for such systems. The results show that an assumption of perfectly correlated heat demands in all heat areas gives results that are very similar to a general case, whereas working with a system with all heat areas aggregated into one gives rather poor results. It is demonstrated that it is possible to use the traditional concepts and methods for power-only analysis on a CHP system. Additional concepts are presented, depending on the heat criterion applied. It is concluded that probabilistic production simulation including CHP units can be performed with reasonable effort and accuracy.

The influence of the modulus–density relationship and the material mapping method on the simulated mechanical response of the proximal femur in side-ways fall loading configuration

Contributing to slow advance of finite element (FE) simulations for hip fracture risk prediction, into clinical practice, could be a lack of consensus in the biomechanics community on how to map properties to the models. Thus, the aim of the present study was first, to systematically quantify the influence of the modulus-density relationship (E-ρ) and the material mapping method (MMM) on the predicted mechanical response of the proximal femur in a side-ways fall (SWF) loading configuration and second, to perform a model-to-model comparison of the predicted mechanical response within the femoral neck for all the specimens tested in the present study, using three different modelling techniques that have yielded good validation outcome in terms of surface strain prediction and whole bone response according to the literature. We found the outcome to be highly dependent on both the E-ρ relationship and the MMM. In addition, we found that the three modelling techniques that have resulted in good validation outcome in the literature yielded different principal strain prediction both on the surface as well as internally in the femoral neck region of the specimens modelled in the present study. We conclude that there exists a need to carry out a more comprehensive validation study for the SWF loading mode to identify which combination of MMMs and E-ρ relationship leads to the best match for whole bone and local mechanical response. The MMMs tested in the present study have been made publicly available at https://simtk.org/home/mitk-gem.

Morphology based anisotropic finite element models of the proximal femur validated with experimental data

Finite element analysis (FEA) of bones scanned with Quantitative Computed Tomography (QCT) can improve early detection of osteoporosis. The accuracy of these models partially depends on the assigned material properties, but anisotropy of the trabecular bone cannot be fully captured due to insufficient resolution of QCT. The inclusion of anisotropy measured from high resolution peripheral QCT (HR-pQCT) could potentially improve QCT-based FEA of the femur, although no improvements have yet been demonstrated in previous experimental studies. This study analyzed the effects of adding anisotropy to clinical resolution femur models by constructing six sets of FE models (two isotropic and four anisotropic) for each specimen from a set of sixteen femurs that were experimentally tested in sideways fall loading with a strain gauge on the superior femoral neck. Two different modulus-density relationships were tested, both with and without anisotropy derived from mean intercept length analysis of HR-pQCT scans. Comparing iso- and anisotropic models to the experimental data resulted in nearly identical correlation and highly similar linear regressions for both whole bone stiffness and strain gauge measurements. Anisotropic models contained consistently greater principal compressive strains, approximately 14% in magnitude, in certain internal elements located in the femoral neck, greater trochanter, and femoral head. In summary, anisotropy had minimal impact on macroscopic measurements, but did alter internal strain behavior. This suggests that organ level QCT-based FE models measuring femoral stiffness have little to gain from the addition of anisotropy, but studies considering failure of internal structures should consider including anisotropy to their models.

Material mapping strategy to improve the predicted response of the proximal femur to a sideways fall impact

Sideways falls are largely responsible for the highly prevalent osteoporotic hip fractures in todays society. These injuries are dynamic events, therefore dynamic FE models validated with dynamic ex vivo experiments provide a more realistic simulation than simple quasi-static analysis. Drop tower experiments using cadaveric specimens were used to identify the material mapping strategy that provided the most realistic mechanical response under impact loading. The present study tested the addition of compression-tension asymmetry, tensile bone damage, and cortical-specific strain rate dependency to the material mapping strategy of fifteen dynamic FE models of the proximal femur, and found improved correlations and reduced error for whole bone stiffness (R2 = 0.54, RSME = 0.87kN/mm) and absolute maximum force (R2 = 0.56, RSME =0.57kN), and a high correlation in impulse response (R2 = 0.82, RSME =12.38kg/s). Simulations using fully bonded nodes between the rigid bottom plate and PMMA cap supporting the femoral head had higher correlations and less error than simulations using a frictionless sliding at this contact surface. Strain rates over 100/s were observed in certain elements in the femoral neck and trochanter, indicating that additional research is required to better quantify the strain rate dependencies of both trabecular and cortical bone at these strain rates. These results represent the current benchmark in dynamic FE modeling of the proximal femur in sideways falls. Future work should also investigate improvements in experimental validation techniques by developing better displacement measurements and by enhancing the biofidelity of the impact loading wherever possible.

Inducing Swirling Flow in Heat Exchanger Pipes for Reduced Fouling Rate

Fouling buildup in circular heat exchanger pipes is a common problem in the industry. Various methods have been proposed to increase the shear stress of the fluid near the pipe wall. This is known to reduce fouling rate, as well as increasing the heat transfer rate of heat exchangers. In this article, the effect of fouling-rate reduction is investigated by increasing friction in a circular pipe. This is done by inducing swirling flow at the pipe entrance, which in turn increases velocities close to the wall and consequently the shear stress. Results are obtained from a three-dimensional finite-volume computational fluid dynamics (CFD) code, where the pipe is modeled along with a swirling device at the entrance. Flow conditions are set to the laminar regime, where the effects of swirling flow are much more influential than in turbulent flow. It is concluded that considerable increase in friction can be obtained, but at the cost of increase in pressure drop.