The use of hot surface films for measuring skin friction is examined. It is shown that all existing theories, which neglect heat conduction within the substrate, are inadequate in predicting the variation of heat loss from the film with skin friction. A more physically realistic theory is presented which accounts for the conductive and convective heat transfer into the flow and also heat conduction within the substrate. This leads to a more general relationship between skin friction and heat loss from the film. Experiments conducted in flat plate laminar and turbulent boundary layer flow show that this relationship is more accurate than previous forms. The time and temperature dependence of the heat loss from the hot film are also explored theoretically and experi- mentally. The effect of surface misalignment of the film is shown to alter significantly the convective heat transfer. This effect is more pronounced in laminar flow than in turbulent flow. Using a glue-on hot film probe, calibrations relating the heat loss to skin friction were found to be different in laminar and turbulent flow. An experimental operating procedure for the use of hot surface films for measuring skin friction is proposed which significantly increases the accuracy of the technique.

In einem hydronumerischen Modell zur Prognose des Strömungs- und Wasserstandsfeldes flacher Gewässer wird eine auf der Turbulenztheorie basierende Tiefenkorrektur für den Bodenreibungsbeiwertwert eingeführt und auf der Basis von Berechnungen für die DarßZingster Boddenkette überprüft. Für einen ausgewählten Meßpunkt der Darß-Zingster Boddenkette wurde der Reibungsbeiwert sowohl am Boden als auch unter einer Eisschicht experimentell bestimmt. Die Auswirkungen einer Tiefenkorrektur des Reibungsbeiwertes wurden in einem vertikal integrierten hydronumerischen Modell untersucht. Dabei wurden die experimentell bestimmten Reibungsbeiwerte, sowohl für die Reibung am Boden, als auch für die Reibung unter einer Eisschicht im Modell benutzt. Das verwendete Modell zeigt mit den gemessenen Reibungsbeiwerten nach Einführung der Tiefenkorrektur eine bessere Übereinstimmung zwischen Modelldaten und amtlichen Pegelregistrierungen. / For simulation of the flow and water level field in shallow water is introduced a depth correction of the bottom drag coefficient based on the theory of turbulence in a hydronumerical model and tested on calculations for the "Darß-Zingster Boddenkette". The drag coefficient was determined experimentally on the bottom and under ice for a selected measurement point in the "Darß-Zingster Boddenkette". The result of the depth correction was tested with a depth integrated hydronumerical model. The experimentally deterrnined drag coefficient on the bottom and under ice was introduced in the model. The used model shows with the measured drag coefficient after using the depth correction better agreement between the numerically simulated and measured water level.

The present understanding of the structure and dynamics of turbulent boundary layers on aerodynamically smooth walls has been clarified over the last decade or so. However, the dynamics of turbulent boundary layers over rough surfaces is much less well known. Nevertheless, there are many industrial and environmental flow applications that require understanding of the mean velocity and turbulence in the immediate vicinity of the roughness elements.</p> <p>This thesis reports the effects of surface roughness on the flow characteristics in a turbulent boundary layer. Both experimental and numerical investigations are used in the present study. For the experimental study, comprehensive data sets are obtained for two-dimensional zero pressure-gradient turbulent boundary layers on a smooth surface and ten different rough surfaces created from sand paper, perforated sheet, and woven wire mesh. The physical size and geometry of the roughness elements and freestream velocity were chosen to encompass both transitionally rough and fully rough flow regimes. Three different probes, namely, Pitot probe, single hot-wire, and cross hot-film, were used to measure the velocity fields in the turbulent boundary layer. A Pitot probe was used to measure the streamwise mean velocity, while the single hot-wire and cross hot-film probes were used to measure the fluctuating velocity components across the boundary layer. The flow Reynolds number based on momentum thickness, , ranged from 3730 to 13,550. The data reported include mean velocity, streamwise and wall-normal turbulence intensities, Reynolds shear stress, triple correlations, as well as skewness and flatness factors. Different scaling parameters were used to interpret and assess both the smooth- and rough-wall data at different Reynolds numbers, for approximately the same freestream velocity. The appropriateness of the logarithmic law and power law proposed by George and Castillo (1997) to describe the mean velocity in the overlap region was also investigated. The present results were interpreted within the context of the Townsends wall similarity hypothesis. </p> <p>Based on the mean velocity data, a novel correlation that relates the skin friction to the ratio of the displacement and boundary layer thicknesses, which is valid for both smooth- and rough-wall flows, was proposed. In addition, it was also found that the application of a mixed outer scale caused the velocity profile in the outer region to collapse onto the same curve, irrespective of Reynolds numbers and roughness conditions. The present results showed that there is a common region within the overlap region of the mean velocity profile where both the log law and power law are indistinguishable, irrespective of the surface conditions. For the power law formulation, functional relationships between the roughness shift, and the power law coefficient and exponent were developed for the transitionally rough flows. The present results also suggested that the effect of surface roughness on the turbulence field depends to some degree on the specific characteristics of the roughness elements and also the component of the Reynolds stress tensor being considered. </p> <p>In the case of the numerical study, a new wall function formulation based on a power law was proposed for smooth and fully rough wall turbulent pipe flow. The new formulation correctly predicted the friction factors for smooth and fully rough wall turbulent pipe flow. The existing two-layer model realistically predicted the velocity shift on a log-law plot for the fully rough turbulent boundary layer. The two-layer model results also showed the effect of roughness is to enhance the level of turbulence kinetic energy and Reynolds shear stress compared to that on a smooth wall. This enhanced level extends into the outer region of the flow, which appears to be consistent with present and recent experimental results for the boundary layer.

The present understanding of the structure and dynamics of turbulent boundary layers on aerodynamically smooth walls has been clarified over the last decade or so. However, the dynamics of turbulent boundary layers over rough surfaces is much less well known. Nevertheless, there are many industrial and environmental flow applications that require understanding of the mean velocity and turbulence in the immediate vicinity of the roughness elements.</p> <p>This thesis reports the effects of surface roughness on the flow characteristics in a turbulent boundary layer. Both experimental and numerical investigations are used in the present study. For the experimental study, comprehensive data sets are obtained for two-dimensional zero pressure-gradient turbulent boundary layers on a smooth surface and ten different rough surfaces created from sand paper, perforated sheet, and woven wire mesh. The physical size and geometry of the roughness elements and freestream velocity were chosen to encompass both transitionally rough and fully rough flow regimes. Three different probes, namely, Pitot probe, single hot-wire, and cross hot-film, were used to measure the velocity fields in the turbulent boundary layer. A Pitot probe was used to measure the streamwise mean velocity, while the single hot-wire and cross hot-film probes were used to measure the fluctuating velocity components across the boundary layer. The flow Reynolds number based on momentum thickness, , ranged from 3730 to 13,550. The data reported include mean velocity, streamwise and wall-normal turbulence intensities, Reynolds shear stress, triple correlations, as well as skewness and flatness factors. Different scaling parameters were used to interpret and assess both the smooth- and rough-wall data at different Reynolds numbers, for approximately the same freestream velocity. The appropriateness of the logarithmic law and power law proposed by George and Castillo (1997) to describe the mean velocity in the overlap region was also investigated. The present results were interpreted within the context of the Townsends wall similarity hypothesis. </p> <p>Based on the mean velocity data, a novel correlation that relates the skin friction to the ratio of the displacement and boundary layer thicknesses, which is valid for both smooth- and rough-wall flows, was proposed. In addition, it was also found that the application of a mixed outer scale caused the velocity profile in the outer region to collapse onto the same curve, irrespective of Reynolds numbers and roughness conditions. The present results showed that there is a common region within the overlap region of the mean velocity profile where both the log law and power law are indistinguishable, irrespective of the surface conditions. For the power law formulation, functional relationships between the roughness shift, and the power law coefficient and exponent were developed for the transitionally rough flows. The present results also suggested that the effect of surface roughness on the turbulence field depends to some degree on the specific characteristics of the roughness elements and also the component of the Reynolds stress tensor being considered. </p> <p>In the case of the numerical study, a new wall function formulation based on a power law was proposed for smooth and fully rough wall turbulent pipe flow. The new formulation correctly predicted the friction factors for smooth and fully rough wall turbulent pipe flow. The existing two-layer model realistically predicted the velocity shift on a log-law plot for the fully rough turbulent boundary layer. The two-layer model results also showed the effect of roughness is to enhance the level of turbulence kinetic energy and Reynolds shear stress compared to that on a smooth wall. This enhanced level extends into the outer region of the flow, which appears to be consistent with present and recent experimental results for the boundary layer.

Many of the flows in nature are turbulent. To modify turbulent flows, nature serves itself with different types of coatings. Sharks have riblets-like structures on their skin, fishes have slime with polymers and the surface of the lotus flower has superhydrophobic properties. However many times these naturally occurring coatings also serve other purposes. Due to millions of years of adaption, there are anyway many reasons to be inspired by these. The present work is an investigation of nature inspired coatings with the aim of passive flow manipulations. The goal of the investigation has not been to achieve drag reduction, but to achieve a better understanding of the effect of these coatings on turbulent flows. Simulations have been performed in a channel flow configuration, where the boundary condition on one wall has been modified. A macroscopic description has been used to simulate superhydrophobic and porous-like surfaces and a microscopic description has been used to simulate suspended fibers, both rigid and flexible, attached to the channel wall. For the macroscopic description, a pseudo-spectral method was used and for the microscopic description a lattice-Boltzmann method was used. The superhydrophobic modification was implemented using a general slip tensor formulation. In agreement with earlier results, drag reduction was achieved with slip in the streamwise direction and slip in the spanwise direction resulted in drag increase. Non-zero off-diagonal terms in the slip tensor resulted in a slight drag increase, but with rather similar flow behaviour. Transpiration, imitating a porous media, gave rise to drag increase and severely modified the turbulent structures, forming two-dimensional structures elongated in the spanwise direction. For the short fibers, neither rigid nor flexible fibers modified the velocity field to a large extent. The fibers gave rise to recirculation regions and these were seen to be stronger below high-speed streaks. Flexible fibers showed similarities to porous media through a coupling of wallnormal velocity and pressure fluctuations, and this was not seen for the rigid fibers. The fiber deflections were seen to correlate well with the pressure fluctuations. / Många naturligt förekommande flöden är turbulenta. Naturen har också gett upphov till flera typer av ytskikt som kan påverka dessa. Hajars skinn har räfflor, fiskar har slem som innehåller polymerer och lotusblommans yta har superhydrofobiska egenskaper, men ofta har dessa naturliga ytskikt också andra egenskaper. På grund av miljoner år av anpassning så finns det ändå många skäl att studera dessa. Detta arbete är en studie av naturinspirerade ytskikt, där målet har varit passiva flödesmanipulationer. Målet har inte varit att åstadkomma en ytfriktionsminskning, utan att få en bättre förståelse om hur dessa ytskikt påverkar turbulenta flöden. Simuleringar har utförts i en kanalliknande geometri, där en kanalväggs randvillkor har modifierats. En makroskopisk beskrivning har använts för att simulera superhydrofobiska och porösa ytor och en mikroskopisk beskriving har använts för att simulera fibrer, både stela och böjbara, fastsatta på en kanalvägg. För flödet med det makroskopiskt beskrivna randvillkoret har en pseudospektral metod använts och för flödet med det mikroskopiskt beskrivna randvillkoret har en lattice-Boltzmannmetod använts. Den superhydrofobiska ytan implementerades genom en generell tensorformulering. Ett randvillkor med nollskild hastighet i kanalens riktning gav upphov till en ytfriktionsminskning och ett randvillkor med nollskild hastighet vinkelrät mot kanalens riktning gav upphov till en ökad ytfriktion, i överensstämmelse med tidigare resultat. Nollskilda icke-diagonala tensorelement gav upphov till en smärre ökning av ytfriktionen, utan att nämnvärt förändra flödet. De porösa ytorna gav upphov till en ytfriktionsökning och hade stor inverkan på de turbulenta strukturerna. Dessa ytor bildade tvådimensionella struturer vinkelrät mot kanalens riktning. Varken de stela eller de böjbara fibrerna gav upphov till stora ändringar i hastighetsfältet. Däremot uppstor cirkulationszoner och dessa var starkare under stråkstrukturer med hög hastighet. De böjbara fibrerna uppvisade likheter med porösa material genom en interaktion mellan det vertikala hastighetsfältet och de turbulenta tryckfluktuationerna. Denna interaktion uppstod inte för de stela fibrerna. Fibrernas böjning korrelerade också i stor utsträckning till tryckfluktuationerna.

Losses associated with turbulent flows dissipate a significant amount of generated energy. Such losses originate from the drag force, which is often described as the sum of the pressure drag and the friction drag. This thesis sets out to explore the hypothesis that passive wall motions driven by fluid mechanical forces are able to reduce the friction drag in fully developed turbulent boundary layers. Firstly, the streamwise and spanwise opposition controls proposed by Choi et al. (1994, Journal of Fluid Mechanics) are revisited to identify beneficial wall motions. Near-wall streamwise or spanwise velocity fluctuations are measured along a detection plane parallel to the wall (sensing). For streamwise control, the wall velocities are set to be equivalent to the measured streamwise velocity fluctuations, whereas for spanwise control, the wall velocities are set to have the same magnitude but opposite direction as the measured spanwise velocity fluctuations (actuation). Direct numerical simulations of canonical turbulent channel flows are carried out at low (Reτ ≈ 180) and intermediate (Reτ ≈ 1000) Reynolds numbers to quantify the effect of the distance between the wall and the detection plane. The investigation reveals the primary differences between the mechanisms underlying the two active in-plane controls. The modified flow features and turbulence statistics show that the streamwise control amplifies the most energetic streamwise velocity fluctuations and damps the near-wall vorticity fluctuations. In comparison, the spanwise control induces near-wall vorticity in order to counteract the quasi-streamwise vortices of the near-wall cycle and suppress turbulence production. Although, the working principles of the active controls are fundamentally different, both achieve drag reduction by mitigating momentum transfer between the velocity components. Secondly, two theoretical passive compliant wall models are proposed, the aim being to sustain beneficial wall motions identified by active flow control simulations. In the proposed models, streamwise or spanwise in-plane wall motions are governed by an array of independent one-degree-of-freedom damped harmonic oscillators. Unidirectional wall motions are driven by local streamwise or spanwise wall shear stresses. A weak coupling scheme is implemented to investigate the interaction between the compliant surface models and the turbulent flow in the channel by means of direct numerical simulations. A linear analytical solution of the coupled differential equation system is derived for laminar pulsatile channel flows allowing verification and validation of the numerical model. The obtained analytical solution is utilised to map the parameter space of the passive controls and estimate the effect of the wall motions. It is shown that depending on the control parameters, the proposed compliant walls decrease or increase the vorticity fluctuations at the wall similarly to the active controls. This is confirmed by direct numerical simulations. On the one hand, when the control parameters are chosen appropriately, the passive streamwise control damps the near-wall vorticity fluctuations and sustains the same drag reduction mechanism as the active streamwise control. This leads to modest, 3.7% and 2.3% drag reductions at low and intermediate Reynolds numbers. On the other hand, the spanwise passive control is not capable of increasing the near-wall vorticity fluctuations as dictated by the active spanwise control. For this reason, passive spanwise wall motions can increase the friction drag by more than 50%. The results emphasise the necessity of anisotropy for a practical compliant wall design. The present work demonstrates for the first time that passive wall motions can decrease friction drag in fully turbulent wall-bounded flows. The thesis sheds light on the working principle of an active streamwise control, and proposes a passive streamwise control exploiting the same drag reduction mechanism. An analytical model is developed to give a ready prediction of the statistical behaviour of passive in-plane wall motions. Whereas streamwise passive wall motions are found beneficial when the control parameters are chosen appropriately, solely spanwise passive wall motions lead to a drag penalty.

In einem hydronumerischen Modell zur Prognose des Strömungs- und Wasserstandsfeldes flacher Gewässer wird eine auf der Turbulenztheorie basierende Tiefenkorrektur für den Bodenreibungsbeiwertwert eingeführt und auf der Basis von Berechnungen für die DarßZingster Boddenkette überprüft. Für einen ausgewählten Meßpunkt der Darß-Zingster Boddenkette wurde der Reibungsbeiwert sowohl am Boden als auch unter einer Eisschicht experimentell bestimmt. Die Auswirkungen einer Tiefenkorrektur des Reibungsbeiwertes wurden in einem vertikal integrierten hydronumerischen Modell untersucht. Dabei wurden die experimentell bestimmten Reibungsbeiwerte, sowohl für die Reibung am Boden, als auch für die Reibung unter einer Eisschicht im Modell benutzt. Das verwendete Modell zeigt mit den gemessenen Reibungsbeiwerten nach Einführung der Tiefenkorrektur eine bessere Übereinstimmung zwischen Modelldaten und amtlichen Pegelregistrierungen. / For simulation of the flow and water level field in shallow water is introduced a depth correction of the bottom drag coefficient based on the theory of turbulence in a hydronumerical model and tested on calculations for the 'Darß-Zingster Boddenkette'. The drag coefficient was determined experimentally on the bottom and under ice for a selected measurement point in the 'Darß-Zingster Boddenkette'. The result of the depth correction was tested with a depth integrated hydronumerical model. The experimentally deterrnined drag coefficient on the bottom and under ice was introduced in the model. The used model shows with the measured drag coefficient after using the depth correction better agreement between the numerically simulated and measured water level.

There is a wide variety of tails, fins, scales, riblets and surface coatings, which are used by motile animals in nature. Since organisms currently living on earth have gone through millions of years of evolution, one can expect that their design is optimal for their tasks, including locomotion. However, the exterior of living animals has range of different functions, from camouflage to heat insulation; therefore it is a very challenging task to isolate mechanisms, which are beneficial to reduce the motion resistance of the body. There are two general categories of mechanisms existing in locomotion and flow control. The first is active flow control, when an organism is actively moving some parts or the whole body (exerts energy) in order to modify the surrounding flow field (for example, flapping bird wings). The second is passive flow control, in which an organism has an appendage or a coating, which is not actively controlled (no energy is spent), but is interacting with surrounding flow in a beneficial way. Our aim is to find novel mechanisms for passive flow control. We start by looking at a simple model of an appendage (splitter plate) behind a bluff body (circular cylinder). If a recirculation region forms behind the body, already in this simple system there is a symmetry breaking effect for sufficiently short plates, which passively generates turn and drift of the body. We have found that this effect is caused by the pressure forces in the recirculation region, which pushes the plate away from the vertical in a manner similar to how a straight inverted pendulum falls under the influence of gravity. In order to investigate this symmetry breaking, we developed an extension of the immersed boundary projection method, in which the rigid body dynamics and fluid dynamics are coupled implicitly. The method is capable of solving for particle motion in a fluid for very small density ratios. We also explain our findings by a simple yet quantitative reduced-order model and soap-film experiments. To extend our work, we investigate flow around bodies, which are coated by a porous and elastic material. We have analysed various theoretical approaches to modeling a coating in a continuous manner. We aim to solve the governing equations numerically. We have selected multi-scale expansion approach, of which we present some initial results. / Många djur använder sig av fjäll, päls, hår eller fjädrar för att öka sin förmåga att förflytta sig i luft eller vatten. Evolutionen har främjat ojämna, sträva eller gropiga ytor, vilka har en tendens att minska det totala motståndet som uppstår när en kropp rör sig i vatten eller luft, jämfört med en helt slät och jämn yta.Det finns två kategorier av metoder för manipulering av strömning (så kallad flödeskontroll). Den första är en aktiv metod, där organismer aktivt rör hela eller delar av kroppen (förbrukar energi) för att manipulera omgivande strömningsfält. Den andra metoden är passiv, där organismer har utväxter eller ytbeläggningar som de inte är aktivt har kontroll över (ingen energi förbrukas), men som samverkar med omgivande strömningsfält på ett fördelaktigt sätt. Vårt mål är att hitta nya mekanismer för passiv flödeskontroll.Vi börjar med att studera en enkel modell för hur en utväxt samverkar med en strömmande fluid genom att fästa en platta på en cirkulär cylinder. Om en vak (så-kallad återcirkulationsregion) bildas bakom kroppen, bryts symmetrin i strömningsfältet då plattan är tillräckligt kort. Som en konsekvens av detta roterar kroppen och driver i sidled. Vi visar att detta fenomen orsakas av tryckkrafter i återcirkulationsregionen, som förskjuter plattan från dess vertikala läge. Vi argumenterar att denna mekanism är samma mekanism som får en inverterad pendel att falla under inverkan av gravitation. För att analysera symmetribrytningen, utvecklade vi en numerisk metod (immersed boundary projection method), som implicit kopplar stelkropps- och strömningsdynamik. Med hjälp av denna metod kan vi simulera partiklar i fluider med väldigt låga densitetsskillnader. Våra resultat förklaras även med hjälp av en enkel modell av låg ordning och med hjälp av såphinneexperiment.Som nästa steg i vårt arbete, ämnar vi att studera strömningen kring kroppar som är belagda av tät, porös och elastisk beläggning. Vi har analyserat möjliga tillvägagångssätt för att modellera beläggningar med kontinuumteori. Vi har valt en metod baserad på en flerskalig expansionsmetod, från vilken vi presenterar våra preliminära resultat. / <p>QC 20150119</p>

Rising fuel cost has motivated increased fuel efficiency for freight trains. At cruising speed,the largest contributing factor to the fuel consumption is aerodynamic drag. As a result of stagnationand flow separation on and around lead and trailing cars, the first and last railcars experiencegreater drag than intermediate cars. Accordingly, this work focused on reducing drag on lead locomotivesby designing and optimizing an add-on nose fairing that is feasible for industrial operation.The fairing shape design was performed via computational fluid dynamic (CFD) software.The simulations consisted of two in-line freight locomotives, a stretch of rails on a raised subgrade,a computational domain, and a unique fairing geometry that was attached to the lead locomotive ineach non-baseline case. Relative motion was simulated by fixing the train and translating the rails,subgrade, and ground at a constant velocity. An equivalent uniform inlet velocity was applied atzero degree yaw to simulate relative motion between the air and the train.Five fairing families-Fairing Families A-E (FFA-FFE)-are presented in this thesis.Multidimensional regressions are created for each family to approximate drag as a function ofthe design variables. Thus, railroad companies may choose an alternative fairing if the recommendedfairing does not meet their needs and still have a performance estimate. The regression forFFE is used as a surrogate model in a surrogate based optimization. Results from a wind tunneltest and from CFD are reported on an FFE geometry to validate the CFD model. The wind tunneltest predicts a nominal drag reduction of 16%, and the CFD model predicts a reduction of 17%.A qualitative analysis is performed on the simulations containing the baseline locomotive, the optimalfairings from FFA-FFC, and the hybrid child and parent geometries from FFA & FFC. Theanalysis reveals that optimal performance is achieved for a narrow geometry from FFC becausesuction behind the fairing is greatly reduced. Similarly, the analysis reveals that concave geometriesboost the flow over the top leading edge of the locomotive, thus eliminating a vortex upstreamof the windshields. As a result, concave geometries yield greater reductions in drag.The design variable definitions for each family were strategically selected to improve manufacturability,operational safety, and aerodynamic performance relative to the previous families.As a result, the optimal geometry from FFE is believed to most completely satisfy the constraintsof the design problem and should be given the most consideration for application in the railroadindustry. The CFD solution for this particular geometry suggests a nominal drag reduction of 17%on the lead locomotive in an industrial freight train.