Topic
Freestream
About: Freestream is a research topic. Over the lifetime, 3428 publications have been published within this topic receiving 56147 citations.
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TL;DR: In this paper, the authors investigated the relationship between the fluid-thermal parameters of jet and film cooling effectiveness using a row of inclined holes and concluded that the flow predictions are greatly affected by the selection of the turbulence model.
Abstract: Introduction G AS turbines require proper cooling mechanisms to protect the airfoils from thermal stresses generated by exposure to hot combustion gases. The problem becomes aggravated by the growing trend to use higher turbine inlet temperatures to generate more power. Thus, film cooling is used as a cooling mechanism, and it works in the form of row of holes located in the spanwise direction, through which cold jets are issued into the hot crossflow. The penetration of cold jets into the main flow creates a complex flowfield. Systematic investigation of such flowfield started in late 1950s. Figure 1 shows the schematic of a single round jet injected in the crossflow at an angle α = 35 deg. The figure also describes the boundary conditions applied at different faces. Even though use of symmetry boundary condition at the hole centerline would reduce the computational time by half, its use is avoided as it prevents the possibility of capturing the unsteady asymmetric vortical flow patterns. This geometry is well accepted by the gas-turbine community and has been extensively studied1 for cooling performance for a wide range of blowing ratios, M = ρ j Vj/ρfsVfs, where ρ and V are density and normal velocity, respectively, for jet j and freestream fs. Goldstein2 correlated film cooling effectiveness η = (Tfs − T )/ (Tfs − Tj ) with the parameter x/Mb, where x is the downstream distance; M is the blowing ratio; b is the slot width; and Tfs, T , and Tj are the temperatures of crossflow, blade, and jet, respectively. Sinha et al.1 carried out experimental work to study the relationship between the fluid-thermal parameters of jet and film cooling effectiveness using a row of inclined holes. The mixing of a jet in a cross stream is a fully three-dimensional phenomenon.3 Amer et al.4 pointed out that the flow predictions are greatly affected by the selection of the turbulence model. Roy5 documented the cooling performance of 12 different arrangements of holes with a combination of blowing ratio M , distance between the holes L , and jet angle α using a upwind-biased finite volume code and standard k–ω turbulence closure model. Garg and Rigby6 resolved the plenum and hole pipes for a three-row showerhead film cooling arrangement with Wilcox’s k–ω turbulence model. Heidmann et al.7 used Reynolds-averaged Navier–Stokes (RANS) to compute the heat transfer for a realistic turbine vane with 12 rows of film cooling holes with shaped holes and plena resolved. Though these studies provide good details of the flow, the anisotropic dynamic nature of the spanwise vortices that affect the film cooling process are more complex than that can be captured by the mixing models used in aforementioned papers. Acharya8 compared the re-
16 citations
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26 Feb 2015TL;DR: The aerodynamic performance of inverted wings on racing-car configurations is most critical when cornering; however, current wind tunnel techniques are generally limited to the straight-line condition as discussed by the authors.
Abstract: The aerodynamic performance of inverted wings on racing-car configurations is most critical when cornering; however, current wind tunnel techniques are generally limited to the straight-line condition. The true cornering condition introduces complexity because of the curvature of the freestream flow. This results in an increase in the tangential velocity with increasing distance from the instantaneous center of rotation and causes the front wing to be placed at a yaw angle. Numerical simulations were used to consider an 80% scale front wing when steady-state cornering with radii ranging from 60m to 7.5m, and yaw angles ranging from 1.25° to 10°. The changes to the pressure distribution near the endplates caused the wake structure to become highly asymmetric. Both the primary longitudinal vortices and the secondary longitudinal vortices differed in strength, and the vortex core positions shifted in the vertical direction and the spanwise direction. The change in the position became more substantial further downstream as the structures tended toward the freestream direction. The effects on the wing surface pressure distribution resulted in the introduction of yawing and rolling moments, as well as a side force and an increase in drag. The results demonstrate the importance of evaluating the cornering condition if that is where a good performance is most sought after.
16 citations
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TL;DR: In this article, the authors investigated the relationship between wind turbine geometry and the wake structure of a vertical-axis wind turbine and the tip-speed ratio, or the tangential speed of the turbine blade relative to incoming wind speed.
Abstract: The wake structure of a vertical-axis wind turbine (VAWT) is strongly dependent on the tip-speed ratio, $$\lambda$$
, or the tangential speed of the turbine blade relative to the incoming wind speed. The geometry of a turbine can influence $$\lambda$$
, but the precise relationship among VAWT geometric parameters and VAWT wake characteristics remains unknown. To investigate this relationship, we present the results of an experiment to characterize the wakes of three VAWTs that are geometrically similar except for the ratio of the turbine diameter (D), to blade chord (c), which was chosen to be $$D/c =$$
3, 6, and 9. For a fixed freestream Reynolds number based on the blade chord of $$Re_c = 1.6\times 10^3$$
, both two-component particle image velocimetry (PIV) and single-component hot-wire anemometer measurements are taken at the horizontal mid-plane in the wake of each turbine. PIV measurements are ensemble averaged in time and phase averaged with each rotation of the turbine. Hot-wire measurement points are selected to coincide with the edge of the shear layer of each turbine wake, as deduced from the PIV data, which allows for an analysis of the frequency content of the wake due to vortex shedding by the turbine.
16 citations
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TL;DR: In this article, a mathematical model and simulation of airborne particle collisions with a 38 m, 1.5 MW horizontal axis wind turbine blade are presented using a two-dimensional inviscid flowfield solver coupled with a particle position code.
Abstract: A mathematical model and simulation of airborne particle collisions with a 38 m, 1.5 MW horizontal axis wind turbine blade are presented. Two types of particles were analyzed, namely insects and sand grains. Computations were performed using a two-dimensional inviscid flowfield solver coupled with a particle position code. Three locations along the blade were considered and characterized by airfoils of the DU series. The insect simulations estimated the residual debris thickness on the blade, while sand simulations computed the surface erosion rate. Results show that the impact locations along the blade depend on angle of attack, freestream velocity, airfoil shape, and particle mass. Particles were found to collide primarily at the leading edge. The volume of insect debris per unit span was maximum at r/R = 0.75. The erosion rate due to sand was maximum on the low pressure side of the blade. An erosion rate approximately ten times higher was observed at r/R = 0.75, as compared with the section at r/R = 0....
16 citations
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TL;DR: In this article, a high-fidelity simulation of the shock/transitional boundary layer interaction caused by a 15-degrees axisymmetrical compression ramp is performed at a freestream Mach number of 5 and a transitional Reynolds number.
Abstract: A high-fidelity simulation of the shock/transitional boundary layer interaction caused by a 15-degrees axisymmetrical compression ramp is performed at a freestream Mach number of 5 and a transitional Reynolds number. The inlet of the computational domain is perturbed with a white noise in order to excite convective instabilities. Coherent structures are extracted using Spectral Proper Orthogonal Decomposition (SPOD), which gives a mathematically optimal decomposition of spatio-temporally correlated structures within the flow. The mean flow is used to perform a resolvent analysis in order to study non-normal linear amplification mechanisms. The comparison between the resolvent analysis and the SPOD results provides insight on both the linear and non-linear mechanisms at play in the flow. To carry out the analysis, the flow is separated into three main regions of interest: the attached boundary layer, the mixing layer and the reattachment region. The observed transition process is dependent on the linear amplification of oblique modes in the boundary layer over a broad range of frequencies. These modes interact nonlinearly to create elongated streamwise structures which are then amplified by a linear mechanism in the rest of the domain until they break down in the reattachment region. The early nonlinear interaction is found to be essential for the transition process.
16 citations