1,720,969 research outputs found
On the three-dimensional structure of turbulent spots
No full textA detailed measurement of turbulent spots propagating in a laminar boundary layer over a flat plate was made at a zero pressure gradient and three favourable pressure gradients. Data were recorded across the span of turbulent spots at a number of streamwise locations along the plate using a hot-wire probe and surface-mounted hot films. This work aims at extending the existing studies that are largely based upon measurements made in the plane of symmetry of the spots and away from the wall and investigating the three-dimensional structure of turbulent spots and its dependence on streamwise pressure gradients. The results from the present experiment reveal some interesting aspects of the overall structure of the spots and the role that regions with negative velocity perturbations play in spanwise growth of turbulent spots. The presence of a spanwise overhang is also confirmed which is believed to be responsible for the smaller spreading angle of spots measured at the wall. This finding is expected to have important implications to the development of improved transition models which are used to predict heat transfer and skin friction for turbine blades
On the Three-Dimensional Structure of Turbulent Spots
No full textA detailed measurement of turbulent spots propagating in a laminar boundary layer over a flat plate was made at a zero pressure gradient and three favorable pressure gradients. Data were recorded across the span of turbulent spots at a number of streamwise locations along the plate using a hot-wire probe and surface-mounted hot films. In this work we aim at extending the existing studies that are largely based upon measurements made in the plane of symmetry of the spots and away from the wall and investigating the three-dimensional structure of turbulent spots and its dependence on streamwise pressure gradients. The results from the present experiment reveal some interesting aspects of the overall structure of the spots and the role that regions with negative velocity perturbations play in the spanwise growth of turbulent spots. The presence of a spanwise overhang is also confirmed that is believed to be responsible for the smaller spreading angle of spots measured at the wall. This finding is expected to have important implications to the development of improved transition models that are used to predict heat transfer and skin friction for turbine blades
Visualisation of turbulent wedges under the influence of favourable pressure gradients using both shear-sensitive and temperature sensitive liquid crystals
No full textEmmons discovered in 1951 that boundary layer transition occurred when some arrow shaped disturbances, or turbulent spots, formed randomly and grew as they propagated downstream until they merged and covered the entire flow field. Using Emmons’ spot hypothesis, and with a knowledge of the spot’s propagation rate and spreading angle, one could model the boundary layer intermittency distribution more accurately, hence the transition process
An investigation of airfoil tonal noise at different Reynolds numbers and angles of attack
No full textThis paper presents the results of an experimental study into the aeroacoustics of airfoil instability noise and a linear stability analysis of the Tollmien–Schlichting (T–S) waves which develop on the pressure surface. The far field noise produced by a NACA0012 airfoil was measured in a low-noise and low-turbulence open jet wind tunnel. Noise measurements were made at the three angles of attack: 0°, 1.4° and 4.2°, and covered a range of Reynolds numbers between 1.0 × 105 and 6.0 × 105. An improved understanding of the causal link between the radiated tonal noise and the hydrodynamic instabilities which develop on the pressure surface of an airfoil for a wide range of Reynolds numbers and angles of attack is reported in this paper. The tonal noise produced by the boundary layer which develop on the suction surface was found to be negligible. It was also observed that the total T–S wave amplification on the pressure surface was generally in good agreement with the measured noise spectra. Based on the experimental and predicted data presented in this paper, it was found that effective tone noise generation requires the incoming T–S waves to be amplified by a separated boundary layer first. As the angle of attack increases, the change in pressure gradient on the pressure surface causes the instability noise to change from a broadband hump of moderate intensity level to become more tonal of higher noise intensit
On the momentum and thermal structures of turbulent spots in a favourable pressure gradient
No full textThis paper represents the results from an experimental investigation of the flow physics behind the difference in the transition zone length indicated by the momentum boundary layer and thermal boundary layer parameters observed on the suction surfaces of gas turbine blades. The experiments were carried out on turbulent spots created artificially in an otherwise laminar boundary layer developing over a heated flat plate in a zero pressure gradient and a favourable pressure gradient. A specially designed miniature triple wire probe was used to measure the streamwise velocity U, transverse velocity component V and temperature T simultaneously during the passage of the spots. In this paper, the general characteristics of the ensemble-averaged velocity and temperature perturbations, rms fluctuations and the second moment turbulent quantities are discussed and the influence of favourable pressure gradient on these parameters is examined. When a favourable pressure gradient is present, unlike in the velocity boundary layer where significant velocity fluctuations (or Reynolds shear stress) occur both on the plane of symmetry and the spanwise periphery, high temperature fluctuations (or turbulent heat fluxes) are confined in the plane of symmetry. The difference in the levels of velocity/temperature fluctuations at these two locations gives an indication of the effectiveness of momentum/heat transfer across the span of the spots. The results of this study show that the heat transfer within a spot is inhibited more than that of the momentum transfer at the presence of a favourable pressure gradient. This phenomenon is expected to slow down the spanwise growth of turbulent spots in the transitional thermal boundary layer, leading to a longer transitional zone length indicated by the heat transfer parameters as reported in the literature
Design and characterisation of a quiet, low turbulence open jet blow down wind tunnel in ISVR
No full textThis technical report presents the design of an open-jet, blow-down wind tunnel that was newly commissioned in the anechoic chamber at the ISVR, University of Southampton, UK. This wind tunnel is intended for the measurement of airfoil trailing edge self-noise but can be extended to other aeroacoustic applications. With the primary objectives of achieving acoustically quiet and low turbulence air jet up to 120m/s through a 150mm x 450mm nozzle, several novel noise and flow control techniques were implemented in the design. Both the acoustical and aerodynamic performances of the open jet wind tunnel were calibrated in detail after its fabrication. It is found that the background noise of the facility is adequately low for a wide range of exit jet velocity. The potential core of the free jet is characterised by a low turbulence level of about 0.1%. A benchmark test by submerging a NACA0012 airfoil with a tripped boundary layer at zero angle of attack into the potential core of the free jet was carried out. It was confirmed that the radiatingaerofoil trailing edge self-noise has levels significantly above the rig noise over a wide range of frequencies. The low noise and low turbulence characteristics of this open jet wind tunnel are comparable to the best facilities in the world, and isbelieved to be the first of its kind in the UK
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