The understanding of secondary flow behavior has become an important aspect in the design of modern gas turbines. Secondary flow gives rise to aerodynamic losses, distorts the thermal field and affects the flow conditions at the exit of a passage negatively. Therefore, reducing secondary flow is a major concern for efficiency improvement. Many passive control-methods have been suggested by turbine designers and researchers, and one very promising modification is blade leading edge contouring near the endwall. At the Division of Heat and Power Technology KTH, Stockholm, a detailed experimental investigation of three filleted nozzle guide vanes in an annular sector cascade has been performed, providing excellent experimental data for numerical validation of complex turbine flows. Based on the above, a numerical study and aerodynamic investigation for a leading edge filleted vane and baseline vane has been performed. The potential effect of the leading edge fillet on flow structure and secondary losses has been evaluated based on a number of flow parameters, and computational predictions have been compared to experimental results. The numerical investigation has shown some differences in the flow behavior between the filleted and baseline case. All results indicate that the fillet affects the flow structure in regions close to the hub endwall. It shifts the position of vortices and loss core. However, the overall effect on reducing secondary losses downstream of the passage is insignificant. Additionally, the numerical results show good qualitative agreement with experimental results.

In this thesis, secondary flow in a two stage, low aspect ratio turbine is investigated using CFD. A parameter study is carried out to investigate how the turbine performance is affected by the choice of aspect ratio. This is done in two steps, first by changing the blade height and then the blade size. The study shows that increasing the aspect ratio will lead to a significant increase of efficiency, but the effect diminishes for large aspect ratios, at which the efficiency moves towards an asymptotic value. Furthermore it is shown that increasing the aspect ratio to a certain value by changing the blade height results in a higher efficiency compared to changing the blade size, which is due to the difference in hub-to-tip ratio. An attempt to quantify the secondary losses is also made by looking at the radial kinetic energy at the outlet of a blade row. It turns out though, that the radial kinetic energy does not follow the same trend as the total pressure loss coefficient, which implies that it can not be used to quantify the secondary losses. Lastly, an effort to improve the method used for generating blade profiles is made, and the updated method is used to redesign rotor 2 to reduce losses.

Engine designers continually push the combustor exit temperature higher to produce more power from gas turbine engines. These high turbine inlet temperatures, coupled with high turbulence levels and flow field non-uniformities, make turbine vane and endwall cooling a very critical issue in engine design. To appropriately cool these surfaces, knowledge of the passage flow field and endwall temperature distribution at representative engine conditions is necessary. A combustor test section was used to simulate realistic turbine inlet profiles of turbulence, normalized temperature, normalized total pressure, and normalized streamwise velocity to study the flow field in a turbine vane passage and the adiabatic temperature distribution on the endwall. The combustor liner film-cooling and exit slot flows were varied independently to determine their relative effect on endwall cooling in the downstream turbine vane. Flow field measurements revealed the presence of a previously unmeasured third vortex in the vane passage. The tertiary vortex was located above the passage vortex and had rotation opposite to the passage vortex. Increasing the amount of slot flow reduced the size and strength of the nearwall vortices, while increasing the size and strength of the tertiary vortex. Adiabatic endwall temperature measurements revealed higher temperatures surrounding the base of the vane. The endwall measurements also showed that the exit slot flow was effective at cooling only a region of the endwall near the vane leading edge on the suction side. Increasing slot flow was found to have a larger thermal benefit to the endwall relative to increasing combustor liner film-cooling. / Master of Science

This experiment investigated the effects of a non-axisymmetric endwall contour and upstream purge flow on the secondary flow of an inlet guide vane. Three cases were tested in a transonic wind tunnel with an exit Mach number of 0.93-a flat endwall with no upstream purge flow, the same flat endwall with upstream purge flow, and a 3D contoured endwall with upstream purge flow. All cases had a backward-facing step upstream of the vanes. Five-hole probe measurements were taken 0.2, 0.4, and 0.6 Cx downstream of the vane row trailing edge, and were used to calculate loss coefficient, secondary velocity, and secondary kinetic energy. Additionally, surface static pressure measurements were taken to determine the vane loading at 4% spanwise position. Surface oil flow visualizations were performed to analyze the flow qualitatively. No statistically significant differences were found between the three cases in mass averaged downstream measurements. The contoured endwall redistributed losses, rather than making an improvement distinguishable beyond experimental uncertainty. Flow visualization found that the passage vortex penetrated further in the spanwise direction into the passage for the contoured endwall (compared to the flat endwall), and stayed closer to the endwall with a blowing ratio of 1.5 with a flat endwall (compared to no blowing with flat endwall). This was corroborated by the five hole probe results. / Master of Science / This experiment investigated effects of a specially designed endwall (the wall of a jet engine where the vanes end) and adding extra flow upstream through a slot on the inefficiencies of a jet engine vane (a stationary part of the engine that looks like a wing). Three cases were tested in a high-speed wind tunnel at almost the speed of sound-a flat endwall with no extra flow upstream, the same flat endwall with extra flow upstream, and the specially designed endwall with extra flow upstream. All cases had a backward-facing step (a step in the direction as if you are walking downstairs) upstream of the vanes. Measurements of flow direction and pressure were taken at three locations close to the vanes, and were used to calculate parameters relating to efficiency. Additionally, measurements were taken to verify that the vanes functioned correctly. Different colored paints (that do not stick) were used to see how the flow changed between each case. Measurements showed there were no major differences in overall efficiency between the three cases. The specially designed endwall made some areas more efficient, and others less efficient, rather than making the overall vane more efficient. The colored paints showed that a region of spinning flow went further away from the wall with the specially designed endwall. The paints also found that the same region of spinning flow stayed closer to wall when extra flow was added upstream. This was corroborated by the five hole probe results. The results from the paints agreed with the measurements of flow direction and pressure. In conclusion, neither the specially designed endwall or the extra flow made much difference in the overall efficiency (instead, they made some parts more efficient and other parts less efficient).

A set of large-scale laboratory experiments were conducted to study channel meander migration. Factors affecting the migration of banklines, including the ratio of curvature to channel width, bend angle, and the Froude number were tested in the experiments. The effect of each factor on the evolution of channel plan form was evaluated and quantified. The channel bankline displacement was modeled by a hyperbolic function with the inclusion of an initial migration rate and a maximum migration distance. It is found that both the initial migration rate and maximum migration distance exhibit a Gaussian distribution along a channel bend. Correlations between the distributions and the controlling parameters were then studied. Two sets of equations were developed for predicting the initial migration rate and the maximum migration distance. With the initial migration rate and maximum migration distance being developed as a function of geometric and flow parameters, a hyperbolic-function model can be applied to estimate the bankline migration distance. The prediction of channel centerline migration was also developed in this study. The channel centerline was represented with a combination of several circular curves and straight lines. Each curve with the radius of curvature and bend angle was used to describe the channel bend geometry. HEC-RAS was applied to estimate the flow hydraulic properties along the channel by adjusting the channel bed slope. The intersections of two consecutive centerlines were found to be the inflection points of the centerline migration rate. Phase lag to the bend entrance was measured and correlated with the bend length and water depth. The migration rate between two successive inflection points demonstrated a growth and decay cycle. A sine function was used to model the centerline migration rate with regression analysis of the measurement data. The method was applied to four sites of four natural rivers in Texas. The results showed that the prediction equation provides agreeable results to the centerline migration of natural rivers.

Next generation aircraft design focuses on highly integrated airframe/engine architectures that exploit advantages in system level efficiency and performance. One such design concept incorporates boundary layer ingestion which locates the turbofan engine inlet near enough to the lifting surface of the aircraft skin that the boundary layer is ingested and reenergized. This process reduces overall aircraft drag and associated required thrust, resulting in fuel savings and decreased emissions; however, boundary layer ingestion also creates unique challenges for the turbofan engines operating in less than optimal inlet flow conditions. The engine inlet flow profiles predicted from boundary layer ingesting aircraft architectures contain complex distortions that affect the engine operability, durability, efficiency, and performance. One component of these complex distortion profiles is off-axial secondary flow, commonly referred to as swirl. As a means to investigate the interactions of swirl distortion with turbofan engines, an experiment was designed to measure distorted flow profiles in an operating turbofan research engine. Three-dimensional flow properties were measured at discrete planes immediately upstream and immediately downstream of the fan rotor, isolating the component for analysis. Constant speed tests were conducted under clean and distorted test conditions. For clean tests, a straight cylindrical inlet duct was attached to the fan case; for distorted tests, a StreamVane swirl distortion generator was inserted into the inlet duct. The StreamVane was designed to induce a swirl distortion matching results of computation fluid dynamics models of a conceptual blended wing body aircraft at a plane upstream of the fan. The swirl distortion was then free to develop naturally within the inlet duct before being ingested by the engine. Results from the investigation revealed that the generated swirl profile developed, mixed, and dissipated in the inlet duct upstream of the fan. Measurements immediately upstream of the fan rotor leading edge revealed 50% reduction in measured flow angle magnitudes along with evidence of fanwise vortex convection when compared to the StreamVane design profile. The upstream measurements also indicated large amounts of secondary flow entered the fan rotor. Measurements immediately downstream of the fan rotor trailing edge demonstrated that the fan processed the distortion and further reduced the intensity of the swirl; however, non-uniform secondary flow persisted at this plane. The downstream measurements confirmed that off-design conditions entered the fan exit guide vanes, likely contributing to cascading performance deficiencies in downstream components and reducing the performance of the propulsor system. / Master of Science

In urban areas, discharging wastewater into rivers is a common way to dispose of contaminants, and it is usually the most economical. Accurate information about how effluents are distributed in the receiving water body is desirable when designing industrial plants. Flow structures will be influenced by an effluent’s dilution processes during the mixing. Meanwhile, the cross-stream motions resulting from the streamline curvature can redistribute both the velocity and the shear stress, which favors the mixing behavior compared to a straight channel. However, the interactions between jet mixing behavior and the bend flow requires further investigation. In the present study, jets with different densities were discharged horizontally into a laboratory flume with a 135-degree open channel bend, and both the main and secondary flow behaviors in the bend were observed after the introduction of effluents. The acquired three- dimensional velocity data were used to validate numerical models of the effluent-bend flow. Numerical turbulence models such as the standard k-ε eddy viscosity model, non-linear k-ε model (Shih quadratic k-ε), and the k-ω SST (shear stress transport) model were employed to evaluate their accuracy. OpenFOAM was selected in the analysis for proposing better numerical models since it gives high-quality results to individualized complex fluid flows, and as an open source CFD software it can be beneficial to further develop and maintain. The first part of this study presents the implementation of the physical modelling of the proposed problems. Detailed descriptions of the experimental process were elaborated. Specifically, the three velocity components at four cross-sectional planes in the bend section were measured with and without saltwater jets by using the stereo Particle Image Velocimetry (PIV) ii technique in the laboratory flume. The experimental results show that the more pronounced effects with the jets were found at the beginning and exit of the bend. Although the jets had little effect on the maximum streamwise velocity, it was found that the occurrence of the negatively buoyant jets would affect the patterns and properties of the secondary flow in the bend. The second part of this study investigated the mechanisms underlying the two cells system, particularly when interacting with a discharged effluent jet. Detailed experimental data were used in interpreting the large center-region cell as well as small structures in the 135-degree open channel bend. A term-by-term analysis of the downstream vorticity equation was executed to investigate the various mechanisms underlying these cross-stream flow motions considering the influence of the negatively buoyant jets. The results indicated the generation and the dissipation of the streamwise vorticity with the effective terms of the vorticity equation. The third part of this study evaluated the performance of three different turbulence models with the experimental measurements. It can be concluded that fully 3D numerical models are capable of simulating the primary flow pattern in a strongly curved channel with the presence of a negatively buoyant jet. The comparison also shows that, although the outer bank cell was not predicted, the k-omega SST model can satisfactorily predict some of the smaller flow features in bend flow, such as the inner bank circulation cell and the overall form of the vorticity distributions. The results enable more reliable predictions for the characteristics and development of jets in a bend.

Rivers are considered as one of the most important surface water resources on the earth. During the time, most of the rivers on the earth experienced evolution and changes. River bends and confluences are one of the common cases in most rivers. There is a significant impact of the flow on the cross-sectional profile of river bends and confluences. Secondary currents are one of the important features that characterize flow in river bends and confluences. In such currents, fluid particles follow a helical path instead of moving nearly parallel to the axis of the channel. The local imbalance between the vertically varying centrifugal force and the cross-stream pressure gradient results in generating the secondary flow and raising a typical motion of the helical flow. Several studies, including experimental or mathematical, have been conducted to examine flow characteristics in curved open channels, river meanders, or confluences. In this research, the influence of secondary currents is studied on the elevation of water surface and the hydraulic structures in channel bends and confluences by employing a 3D OpenFOAM numerical model. The research implements a 3D OpenFOAM numerical model to simulate the horizontal distribution of the flow. In addition, the progress in unraveling and understanding the bend and confluent dynamics is discussed. The finite volume method in OpenFOAM software is used to simulate and examine the behavior of the secondary current. Thereafter, a comparison between the experimental data and a numerical model is conducted. Two sets of experimental data are used as the dataset for these two experiments are complete and validated; the data provided by Rozovskii (1961) for a sharply curved channel, and the dataset provided by Shumate (1998) for a confluent channel. Two solvers in OpenFOAM software were selected to solve the problem regarding the experiment: InterFoam and PisoFoam. InterFoam is a transient solver for incompressible flow that is used with open channel flow with Free Surface Model. PisoFoam is a transient solver for incompressible flow that is used with closed channel flow and Rigid-Lid Model. Various turbulence models (i.e., Standard k-ε, Realizable k-ε) are applied in the numerical model to assess the accuracy of turbulence models in predicting the behavior of the flow. The accuracies of various turbulence models are examined and discussed.

A three-component laser Doppler anemometer was used to measure the fully developed, turbulent flow in a closed, symmetric, smooth-wall compound channel. Measurements were made across one quadrant of the cross-section since the flow was assumed symmetric. Measurements were made for a single channel Reynolds number. All mean velocity components were calculated and are reported. The mean velocity field results are in good agreement with results reported for similar geometries. The vector plots and the axial vorticity distribution reveal the existence of secondary flow cells in both the main channel and the flood plain. The maximum values of the secondary velocities are at the comer region, on the interface between the main channel and the flood plain. In this region the mean velocity gradients are large, indicating that this might be an area of high turbulence production. The distributions of all Reynolds stresses across the cross-section are reported. The Reynolds stress distributions show peak values near the interface corner region and small values near the center-line and on the axes of symn1etry of the channel. The turbulence kinetic energy distribution verifies the existence of high turbulence energy fluid in the comer region. / Master of Science

An experimental investigation was performed to understand the development and suppression of the secondary flow structures within a compact, serpentine jet engine inlet duct. By employing a variety of flow diagnostic techniques, the formation of a pair of counter-rotating vortices was revealed. A modular fluidic actuator system that would apply several different methods of flow control was then designed and manufactured to improve duct performance. At the two bends of the inlet, conformal flow control devices were installed to deliver varying degrees of boundary layer suction, suction and steady fluid injection, and suction and oscillatory injection. Testing showed that suction alone could delay flow separation and improve the pressure recovery of the duct by as much as 70%. However, this technique was not able to rid the duct completely of the nonuniformities that exist at the engine face plane. Suction with steady blowing, however, increased pressure recovery by 37% and reduced distortion by 41% at the engine face. Suction with pulsed injection had the least degree of success in suppressing the secondary flow structures, with improvements in pressure recovery of only 16.5% and a detrimental impact on distortion. The potential for gains in the aerodynamic efficiency of serpentine inlets by active flow control was demonstrated in this study.