This is the third volume of the concurring series of Reports in Applied Mechanics, which is based on the outcome of the advanced project course TMPM10 in Applied Mechanics at Link¨oping University during the autumn of 2024. The course lay-up is based on several industrial or in-house research related projects within the field of Solid Mechanics concerning fatigue, topology optimisation, structural dimensioning, contacts etc, and Fluid Mechanics concerning fluid dynamics, flow, aerodynamics, heat transfer etc. The students tackle industry or forefront research relevant projects in close collaboration with industry from near and neighbouring regions or the university and work in project groups to solve the given tasks within the time limit of the course. Close collaboration with the industry/university is necessary to define planning, update and feedback for further evaluation at the industry/university.

This year there were a total of nine projects performed during the course of 2024, seven within Solid Mechanics and two in Fluid Mechanics. Some projects were performed in tight collaboration with industry partners, and had a close application to real industrial problems. The other were related to in-house research projects, pushing the research front forward. New knowledge had to be learned and skills were obtained. This has been a good opportunity for the students to show-off all their gained knowledge and apply it in the best possible way to make innovative solutions in the respective projects. Something they all managed to do with success!

Daniel Leidermark & Magnus Andersson

This is the first volume of the concurring series of Reports in Applied Mechanics, which is based on the outcome of the advanced project course TMPM07 in Applied Mechanics at Link¨oping University during the autumn of 2022. The course lay-up is based on several industrial related projects within the field of Solid Mechanics, concerning fatigue, topology optimisation, structural dimensioning, contacts etc, and Fluid Mechanics, concerning fluid dynamics, flow, aerodynamics, heat transfer etc. The students tackle industry relevant projects in close collaboration with industry from near and neighbouring regions and work in project groups to solve the given tasks within the time limit of the course. Close collaboration with the industry is necessary to define planning, update and feedback for further evaluation at the industry.

Three projects were performed during the course of 2022, two within Solid Mechanics and one in Fluid Mechanics. The projects were all performed in tight collaboration with industry partners, and had a close application to real industrial problems. A good opportunity for the students to show-off all their gained knowledge and apply in the best possible way to make innovative solutions in the respective projects. Something they all managed to do with success!

This is the second volume of the concurring series of Reports in Applied Mechanics, which is based on the outcome of the advanced project course TMPM10 in Applied Mechanics at Linköping University during the autumn of 2023. The course lay-up is based on several industrial or in-house research related projects within the field of Solid Mechanics concerning fatigue, topology optimisation, structural dimensioning, contacts etc, and Fluid Mechanics concerning fluid dynamics, flow, aerodynamics, heat transfer etc. The students tackle industry or forefront research relevant projects in close collaboration with industry from near and neighbouring regions or the university and work in project groups to solve the given tasks within the time limit of the course. Close collaboration with the industry is necessary to define planning, update and feedback for further evaluation at the industry. This year there were a total of six projects performed during the course of 2023, four within Solid Mechanics and two in Fluid Mechanics. Some projects were performed in tight collaboration with industry partners, and had a close application to real industrial problems. The other were related to in-house research projects, pushing the research front forward. This has been a good opportunity for the students to show-off all their gained knowledge and apply it in the best possible way to make innovative solutions in the respective projects. Something they all managed to do with success!

At this very moment, there are literally millions of people who suffer from various types of cardiovascular diseases (CVDs), many of whom will experience reduced quality of life or premature lift expectancy. The detailed underlying pathogenic processes behind many of these disorders are not well understood, but were abnormal dynamics of the blood flow (hemodynamics) are believed to play an important role, especially atypical flow-mediated frictional forces on the intraluminal wall (i.e. the wall shear stress, WSS). Under normal physiological conditions, the flow is relatively stable and regular (smooth and laminar), which helps to maintain critical vascular functions. When these flows encounter various unfavorable anatomical obstructions, the flow can become highly unstable and irregular (turbulent), giving rise to abnormal fluctuating hemodynamic forces, which increase the bloodstream pressure losses, can damage the cells within the blood, as well as impair essential structural and functional regulatory mechanisms. Over a prolonged time, these disturbed flow conditions may promote severe pathological responses and are therefore essential to foresee as early as possible.

Clinical measurements of blood flow characteristics are often performed non-invasively by modalities such as ultrasound and magnetic resonance imaging (MRI). High-fidelity MRI techniques may be used to attain a general view of the overall large-scale flow features in the heart and larger vessels but cannot be used for estimating small-scale flow variations nor capture the WSS characteristics. Since the era of modern computers, fluid motion can now also be predicted by computational fluid dynamics (CFD)simulations, which can provide discrete mathematical approximations of the flow field with much higher details (resolution) and accuracy compared to other modalities. CFD simulations rely on the same fundamental principles as weather forecasts, the physical laws of fluid motion, and thus can not only be used to assess the current flow state but also to predict (foresee) important outcome scenarios in e.g. intervention planning. To enable blood flow simulations within certain cardiovascular segments, these CFD models are usually reconstructed from MRI-based anatomical and flow image-data. Today, patient-specific computational hemodynamics are essentially only performed within the research field, where much emphasis is dedicated towards understanding normal/abnormal blood flow physiology, developing better individual-based diagnostics/treatments, and evaluating the results reliability/generality in order to approach clinical applicability.

In this thesis, advanced CFD methods were adopted to simulate realistic patient-specific turbulent hemodynamics in constricted arteries reconstructed from MRI data. The main focus was to investigate novel, comprehensive ways to characterize these abnormal flow conditions, in the pursuit of better clinical decision-making tools; from more in-depth analyzes of various turbulence-related tensor characteristics to descriptors that evaluate the hemodynamics more globally in the domain. Results from the studies in this thesis suggest that these turbulence descriptors can be useful to: i) target cardiovascular sites prone to specific turbulence characteristics, both in the bulk flow and on the intraluminal wall, ii) provide a more extensive view of the general flow severity within malformed vascular regions, and iii) evaluated and potentially improve cardiovascular modeling strategies and MRI-measured turbulence data.

The benefit of these descriptors is that they all, in principle, can be measured by different MRI procedures, making them more accessible from a clinical perspective. Although the significance of these suggested flow-mediated phenotypes has not yet been evaluated clinically, this work opens many doors of opportunities for making more thorough and longitudinal patient-specific studies, including large cohorts of patients with various CVDs susceptible to turbulent-like conditions, as well as performing more in-depth CFD-MRI validation analyzes.

Just nu finns det bokstavligen miljontals människor som lider av olika typer av hjärt- och kärlsjukdomar, av vilka många kommer att uppleva nedsatt livskvalitet samt förkortad livslängd. De underliggande patogena orsakerna bakom dessa åkommor är fortfarande inte väl förstådda, men där onormal blodflödesdynamik (hemodynamik) tros spela en viktig roll, särskilt oregelbundna friktionskrafter på kärlväggens insida (väggskjuvspänningen). Under normala fysiologiska förhållanden är blodflödet relativt stabilt och regelbundet (laminärt), vilket hjälper till att bibehålla kritiska kärlfunktioner. När dessa flöden stöter på olika ogynnsamma anatomiska hinder kan flödet bli mycket instabilt och oregelbundet (turbulent) och ge upphov till onormala fluktuerande flödeskrafter vilket resulterar i förhöjda tryckförluster i blodomloppet, försämring av väsentliga strukturella och funktionella regleringsmekanismer i kärlen, samt stundvis skador på diverse blodkroppar och ge upphov till blodproppar. Över en längre tidsperiod kan dessa abnormala flödesförhållanden främja allvarliga patologiska förändringar och är därför viktiga att kartlägga så tidigt som möjligt.

Kliniska mätningar av blodflödesdynamik utförs ofta icke-invasivt av modaliteter som ultraljud och magnetisk resonanstomografi (MRI). Avancerade MRI-tekniker kan användas för att återskapa en allmän bild av de storskaliga flödesstrukturerna i hjärtat och de större kärlen men är inte lämpad för att uppskatta småskaliga flödesvariationer samt väggskjuvspänningens karaktär i detalj. Sedan introduktionen av moderna datorer så kan numera flödesmönster även estimeras av strömningsimuleringar (beräkningsströmningsdynamik), en metod som på engelska kallas ”computational fluid dynamics” eller CFD, vilket ger en diskret matematisk approximation av flödesfältet med mycket högre spatiell och temporal detaljnivå (upplösning) och noggrannhet jämfört med andra modaliteter. CFD simuleringar vilar på samma grundläggande principer som väderprognoser, de fysiska lagarna som beskriver hur ett strömningsfält beter sig, och kan således inte bara användas för att bedöma det aktuella flödestillståndet utan också för att försöka förutsäga utfallsscenarier vid exempelvis olika kirurgiska interventioner. För att möjliggöra blodflödesimuleringar inom vissa kardiovaskulära segment så rekonstrueras vanligtvis CFD-modeller från MRI-baserade anatomisk- och flödsbilddata. Idag är patientspecifika blodflödesberäkningar i huvudsak en forskningsdiciplin, där mycket vikt läggs vid att förstå normal/onormal blodflödesfysiologi, utveckla bättre individbaserad diagnostik/behandlingar och utvärdera resultatets tillförlitlighet/generalitet för att närma sig klinisk tillämpbarhet.

I denna avhandling användes avancerade CFD simuleringar för att beräkna realistiska turbulenta flödesförhållanden i patientspecifika förträngda bloodkärlsmodeller återskapade från MRI mätningar. Huvudfokus var att undersöka nya, omfattande sätt att karakterisera dessa onormala blodflöden i strävan efter bättre kliniska beslutsverktyg, från mer fördjupade analyser av olika turbulensrelaterade tensoregenskaper till deskriptorer som utvärderar blodflödesdynamiken mer globalt i domänen. Resultat från studierna i denna avhandling antyder att dessa turbulensrelaterade deskriptorer kan vara användbara för att: i) karlägga kardiovaskulära regioner exponerad av olika turbulent karakteristik, både i friströmen samt på kärlväggen, ii) ge en mer omfattande bild av flödes abnormalitet inom missbildade kärlregioner, och iii) utvärdera och potentiellt förbättra kardiovaskulära modelleringsstrategier samt MRI mätningar av turbulens.

Fördelen med dessa flödesdeskriptorer är att de alla, principiellt, kan mätas med olika MRI-tekniker, vilket gör dem mer tillgängliga ur ett kliniskt perspektiv. Ä ven om värdet av dessa föreslagna analysmetoder ännu inte har utvärderats kliniskt, öppnar detta arbete många dörrar för möjligheter att göra mer grundliga och longitudinella patientspecifika studier, inklusive stora kohorter av patienter med olika kardiovaskulär sjukdomar som förorsakar liknande turbulenta flödesförhållanden, samt utför mer fördjupade CFD-MRI valideringsanalyser.

Turbulence and flow eccentricity can be measured by magnetic resonance imaging (MRI) and may play an important role in the pathogenesis of numerous cardiovascular diseases. In the present study, we propose quantitative techniques to assess turbulent kinetic energy (TKE) and flow eccentricity that could assist in the evaluation and treatment of stenotic severities. These hemodynamic parameters were studied in a pre-treated aortic coarctation (CoA) and after several virtual interventions using computational fluid dynamics (CFD), to demonstrate the effect of different dilatation options on the flow field. Patient-specific geometry and flow conditions were derived from MRI data. The unsteady pulsatile flow was resolved by large eddy simulation (LES) including non-Newtonian blood rheology. Results showed an inverse asymptotic relationship between the total amount of TKE and degree of dilatation of the stenosis, where turbulent flow proximal the constriction limits the possible improvement by treating the CoA alone. Spatiotemporal maps of TKE and flow eccentricity could be linked to the characteristics of the jet, where improved flow conditions were favored by an eccentric dilatation of the CoA. By including these flow markers into a combined MRI-CFD intervention framework, CoA therapy has not only the possibility to produce predictions via simulation, but can also be validated pre- and immediate post treatment, as well as during follow-up studies.

Turbulent and wall impinging blood flow causes abnormal shear forces onto the lumen and may play an important role in the pathogenesis of numerous cardiovascular diseases. In the present study, wall shear stress (WSS) and related flow parameters were studied in a pre-treated aortic coarctation (CoA) as well as after several virtual interventions using computational fluid dynamics (CFD). Patient-specific geometry and flow conditions were derived from magnetic resonance imaging (MRI) data. Finite element analysis was performed to acquire six different dilated CoAs. The unsteady pulsatile flow was resolved by large eddy simulation (LES) including non-Newtonian blood rheology. Pre-intervention, the presence of jet flow wall impingement caused an elevated WSS zone, with a distal region of low and oscillatory WSS. After intervention, cases with a more favorable centralized jet showed reduced high WSS values at the opposed wall. Despite significant turbulence reduction post-treatment, enhanced regions of low and oscillatory WSS were observed for all cases. This numerical method has demonstrated the morphological impact on WSS distribution in an CoA. With the predictability and validation capabilities of a combined CFD/MRI approach, a step towards patient-specific intervention planning is taken.  

Turbulent blood flow is often associated with some sort of cardiovascular disease, e.g. sharp bends and/or sudden constrictions/expansions of the vessel wall. The energy losses associated with the turbulent flow may increase the heart workload in order to maintain cardiac output (CO). In the present study, the amount of turbulent kinetic energy (TKE) developed in the vicinity of an aortic coarctation was estimated pre-intervention and in a variety of post-intervention configurations, using scale-resolved image-based computational fluid dynamics (CFD). TKE can be measured using magnet resonance imaging (MRI) and have also been validated with CFD simulations [1], i.e. a parameter that not only can be quantified using simulations but can also be measured by MRI.

Patient-specific geometry and inlet flow conditions were obtained using contrast-enhanced MR angiography and 2D cine phase-contrast MRI, respectively. The intervention procedure was mimicked using an inflation simulation, where six different geometries were obtained. A scale-resolving turbulence model, large eddy simulation (LES), was utilized to resolve the largest turbulent scales and also to capture the laminar-to-turbulent transition. All cases were simulated using baseline CO and with a 20% CO increase to simulate a possible flow adaption after intervention.

For this patient, results shows a non-linear decay of the total amount of TKE integrated over the cardiac phase as the stenotic cross-sectional area is increased by the intervention.  Figure 1 shows the original segmented geometry and two dilated coarctation with corresponding volume rendering of the TKE at peak systole. Due to turbulent transition at a kink upstream the stenosis further dilation of the coarctation tends to restrict the TKE to a plateau, and continued vessel expansion may therefore only induce unnecessary stresses onto the arterial wall. 

This patient-specific non-invasive framework has shown the geometrical impact on the TKE estimates. New insight in turbulence development indicates that the studied coarctation can only be improved to a certain extent, where focus should be on the upstream region, if further TKE reduction is motivated. The possibility of including MRI in a combined framework could have great potential for future intervention planning and follow-up studies.  

[1] J. Lantz, T. Ebbers, J. Engvall and M. Karlsson, Numerical and Experimental Assessment of Turbulent Kinetic Energy in an Aortic Coarctation, Journal of Biomechnics, 2013. 46(11): p. 1851-1858.

In the presented work, two studies using ComputationalFluid Dynamics (CFD) have been conducted on a generictruck-like model with and without a trailer unit at a speed of 80km/h. The purpose is to evaluate drag reduction possibilitiesusing externally fitted devices. A first study deals with a flapplaced at the back of a rigid truck and inclined at seven differentangles with two lengths. Results show that it is possible todecrease drag by 4%. In a second study, the flap has been fittedon the tractor and trailer units of a truck-trailer combination.Four settings were surveyed for this investigation, one of whichproved to decrease drag by up to 15%. A last configurationwhere the gap between the units has been closed has also beenevaluated. This configuration offers a 15% decrease in drag.Adding a flap to the closed gap configuration decreases drag by18%. New means of reducing aerodynamic drag of heavy-duty(HD) vehicles will be important in the foreseeable future inorder to improve the fuel economy. The possibilities of reducingdrag are prevalent using conceptual design.