Effect of Leading Edge Geometry on Boundary Layer Transition-An Experimental Approach (original) (raw)
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Effect of a Sinusoidal Leading Edge Pattern on Flat Plate Boundary Layer Flow
2014
Aerofoils with a sinusoidal leading edge pattern have been the subject of the study during the recent decade due to its considerable effect on the aerodynamics performance. The leading edge variation results in the formation of streamwise counterrotating vortices which affect the boundary layer development on aerofoils. To understand the influence of the leading edge variation to the boundary layer flow development, the current study was carried out using a flat plate model. A single hot-wire anemometer was used to measure the streamwise velocity at Reynolds number (based on the wavelength of the leading edge patterns) of 3080 corresponding to free-stream velocity of 3 m/s. The results showed that downstream of the troughs of the leading edge pattern, counterrotating pairs of vortices form and develop along the streamwise direction. Inflection points on the velocity profiles at upwash region imply the appearance of the vortices with mushroom like structure which diffuse further down...
Experiments in Fluids, 2012
This work is concerned with the design of a leading edge for a flat-plate model used to study laminar and transitional boundary layers. For this study, the flow over the complete boundary-layer model, including leading edge, flat section, and trailing-edge flap, is modeled. The effect of important geometrical features of the leading edge on the resulting pressure distribution, starting from the well-known symmetric modified super ellipse, is investigated. A minimal pressure gradient on the measurement side of the plate is achieved using an asymmetrical configuration of modified super ellipses, with a thickness ratio of 7/24. An aerodynamic shape optimization is performed to obtain a novel leading edge shape that greatly reduces the length of the non-zero pressure gradient region and the adverse pressure gradient region compared to geometries defined by ellipses. Wind tunnel testing is used to validate the numerical solutions.
Experimental Study of Boundary Layer Transition on a Heated Flat Plate
1991
A detailed investigation to document momentum and thermal development of boundary layers undergoing natural transition on a heated flat plate was performed. Experimental results of both overall and conditionally sampled characteristics of laminar, transitional and low Reynolds number turbulent[boundary layers are presented. Measurements were done in a low-speed, closed-loop wind tunne1_with a freestream velocity of I00 ft/s and zero pressure gradient over a range of freestream turb_lence intensities from 0.4 to 6 percent. The distributions of skin friction, heat transfer rate and Reynolds shear stress were a11 consistent with previously published data. Reynolds analogy factors for momentum thickness Reynolds number, Re8 < 2300 were found to be well predicted by laminar and turbulent correlations which accounted for an unheated starting length and uniform heat flux. A small dependence of turbulent results on the freestream turbulence intensity was observed. 19. Security Classif. (of the report) 20, Security Classif. (of this page) 21. No. of pages Unclassified Unclassified 16 NASA FORM 1626 OCT 88
Length Scale of Free Stream Turbulence and Its Impact on Bypass Transition in a Boundary Layer
Journal of Applied Fluid Mechanics
An experimental investigation was carried out to study the turbulent flow over a flat plate in a subsonic wind tunnel. The enhanced level of turbulence was generated by five wicker grids with square meshes, and different parameters (diameter of the grid rod d = 0.3 to 3 mm and the grid mesh size M = 1 to 30 mm). The velocity of the flow was measured by means of a 1D hot-wire probe, suitable for measurements in a boundary layer. The main aim of the investigation was to explore the influence of the free stream turbulence length scale on the onset of laminar-turbulent bypass transition in a boundary layer on a flat plate. For this purpose, several transition correlations were presented, including intensity and length scales of turbulence, both at the leading edge of a plate and at the onset of transition. The paper ends with an attempt to create a correlation, which takes into account a simultaneous impact of turbulence intensity and turbulence scale on the boundary layer transition. To assess the isotropy of turbulence, the skewness factor of the flow velocity distribution was determined. Also several longitudinal scales of turbulence were determined and compared (integral scale, dissipation scale, Taylor microscale and Kolmogorov scale) for different grids and different velocities of the mean flow U = 4, 6, 10, 15, 20 m/s.
Aerofoil wake-induced transition characteristics on a flat-plate boundary layer
Journal of Fluid Mechanics, 2021
This paper presents an experimental investigation of the characteristics of laminarturbulent transition occurring on a flat-plate boundary layer due to the interaction with a non-impinging aerofoil wake. Previous studies have tended to focus on transition induced by free-stream turbulence or by the wake of a circular cylinder, both of which exhibit different forcing characteristics to the present experimental arrangement. A tripped NACA 0014 aerofoil was used to generate a fully turbulent wake, upstream of and at various heights above a laminar, flat-plate boundary layer, in the UK National Low-turbulence Wind Tunnel at City, University of London. Hot-wire measurements conducted in the pre-transitional region reveal the wall-normal and spanwise structure of the disturbances induced within the boundary layer and the rate of growth of disturbance energy. Disturbance profiles generally (but not uniquely) follow the non-modal distribution obtained from transient growth theory, but energy growth rates are mainly exponential rather than algebraic. Energy spectra demonstrate the existence of mixed transitional features (both natural and bypass) in the boundary layer. Two-point spatial correlations reveal the presence of a streaky structure, but with spanwise scale much larger than the boundary layer thickness, in contrast to the trends seen in free-stream turbulence-induced bypass transition and cylinder wake-induced transition. The gap between aerofoil and flat plate affects both the evolution of non-modal disturbance profile and the appearance of the streaky structure; the spacing of the streaks was also found to scale with the vertical gap between aerofoil and flat plate. Overall, the combination of observed characteristics is quite different from the forced transition mechanisms previously reported in the literature.
Impact of a 3D Plate on the Structure of a Turbulent Boundary Layer
Journal of engineering physics and thermophysics, 2021
The authors have performed 3D numerical investigations into the velocity fi eld behind a 3D thin plate by the LES method. The plate was located at a height of 0.002 m in a turbulent boundary layer formed in a water channel. The chord length of the plate and its span-wise size were equal to 0.01 m and 0.024 m respectively. The Reynolds number calculated from the half-width of the channel and the velocity on its axis was equal to 7500. It has been shown that under the impact of the wake of the plate, the longitudinal velocity normalized to the stagnation velocity grew in the logarithmic region at a distance to x/δ = 3.8, and pulsations of all the velocity components decreased in the buffer region to the distance x/δ = 0.8. As the lower shear layer of the wake approached the surface, longitudinal pulsations reached their minima at the wall at x/δ = 1.8, whereas vertical and transverse pulsations became higher than those in an unperturbed boundary layer. The calculated characteristics of the velocity fi eld satisfactorily correlated with the relevant characteristics obtained in experimental investigation with similar initial and boundary conditions. An analysis of the velocity fi eld has revealed the mechanism of impact of the wake on the structural change of wall fl ow.The involvement of the wall medium in the lower shear layer generated the outflow of the medium from the channel wall, and the involvement in the upper shear layer formed the inflow of a high-speed medium to the buffer region. The medium′s outflow from the wall led to a reduction in the velocity gradient at the surface. Shear stresses decreased at a distance of four thicknesses of the boundary layer, and their local reduction amounted to as much as ~30%. Trailing vortices descending from the plate′s side edges created a nonuniform transverse-velocity distribution. This non-uniformity caused the vorticity to form in the buffer region. The arising system of small-size vortices blocked the feed of the high-speed medium to the wall, retarding the growth in the shear stresses on the surface in the interval 1.4 ≤ x/δ ≤ 2.6. The vortex system degenerated upon the stabilization of the transverse velocity and shear fl ow on the surface was restored.
Advances in Unsteady Boundary Layer Transition Research, Part I: Theory and Modeling
The International Journal of Rotating Machinery
This two-part article presents recent advances in boundary layer research that deal with the unsteady boundary layer transition modeling and its validation. A new unsteady boundary layer transition model was developed based on a universal unsteady intermittency function. It accounts for the effects of periodic unsteady wake flow on the boundary layer transition. To establish the transition model, an inductive approach was implemented; the approach was based on the results of comprehensive experimental and theoretical studies of unsteady wake flow and unsteady boundary layer flow. The experiments were performed on a curved plate at a zero streamwise pressure gradient under a periodic unsteady wake flow, where the frequency of the periodic unsteady flow was varied. To validate the model, systematic experimental investigations were performed on the suction and pressure surfaces of turbine blades integrated into a high-subsonic cascade test facility, which was designed for unsteady boundary layer investigations. The analysis of the experiment's results and comparison with the model's prediction confirm the validity of the model and its ability to predict accurately the unsteady boundary layer transition.