Flat-plate transitional 2D boundary layers flows with or without pressure gradient. No temperature variations.
Free-stream velocity: Uo = 1.2, 3.7, 5.0, 5.4, 5.9, 8.4, 9.4 and 19.8 m/s
Upstream turbulence intensity: Tuo = 0.9%, 3.0%, 6.0%, 6.6%
The wind tunnel used in the present experiments is of the closed circuit type and is described in detail in ref.2. Its major elements include a centrifugal fan blowing through a large plenum fitted with turbulence reducing honeycomb gauzes, a small 2-D contraction, leading to a highly versatile working section, and a wide-angle diffuser. The working section is of 2m length, O.71m width and 0.26m height with full-length perspex sidewalls for good optical access, see figure 1. The bottom wall can be inclined sufficiently to produce a zero pressure gradient on the test surface, which is hung from the ceiling of the working section. High performance flow cleaning filters are employed in the return ducting, together with a water-cooled heat exchanger. As a result of the careful design, the working section free-stream turbulence intensity is O.2+/-O.O5 over the operating velocity range of 0 to 25m/s, and the air temperature can be controlled to +/-0.1 C degrees during the course of a day's operation.
Measurements are made on a flat perspex test surface, which is hung from the roof of the working section. The test plate has numerous static pressure tappings and surface plugs, which are interchangeable and flush to within 25Ám (when the tunnel is operated at a nominal 20oC). The (1700mm length, 20mm thick, 710mm wide) test surface is extremely flat and employs a small leading edge radius of 0.75mm with a 5 degree chamfer on the other side to the (lower) test-surface. The test plate leading edge is mounted 4Omm from the working section upper wall, and the circulation about it is controlled by a combination of a trailing edge flap and adjusting the pressure drop across the working section exit plane (by means of gauzes). Moreover, the test plate is inclined at 0.5 degree to the main flow vector which, together with the circulation control measures, ensures attached, steady leading edge flow with the stagnation streamline located on the test surface.
The turbulence generating grids used in these experiments are also detailed in ref. 2, together with the turbulence intensity characteristics. These can be located at the entrance plane of the working section, at the downstream end of the 2-D contraction. They are made of square or round bars and woven wire mesh, and are positioned 610mm upstream of the test plate leading edge. The grids have been designed according to the methods of ref. 3, and can generate test plate free-stream turbulence intensities from 0.5% to 7% and integral length scales from 5mm to 30mm. With the grids in location, it was found that the generated turbulence extremely homogeneous and isotropic, with streamwise-to-normal fluctuating velocity ratio of about 1.005.
A DISA 55M series anemometer system was used to make velocity measurements in the wind tunnel. Custom designed single and X-wire probes (total wire length l.5mm, sensing length 0.5mm, wire diameter 2.5Ám) were used to measure both the free-stream turbulence properties and the boundary layer velocity profiles. Gold-plated wire probes have been used to minimise prong interference, with the small sensing length chosen to minimise spatial averaging from both the turbulent eddies and from the mean velocity shear. The primary purpose for using the single wire probes is to get sufficiently close to the test surface in order that accurate measurements of the boundary layer integral parameters could be made. In practice, the probe prongs were always placed on the test surface with the traverse proceeding away from this surface, ending at about three times the boundary layer thickness. The minimum wire height was measured to be 29(m but because the wall proximity error was found to be significant (for y+ < 3), a certain number of measured points were deleted from the profiles before analysing the data. The single wire probes also provide an initial estimate of the streamwise component of fluctuating velocities, which may be compared with the X-wire results.
The X-wires (both U-V and U-W components) were used to measure normal and shear stress distributions through the boundary layers. They provide measurements of all three components of normal stresses, together with two components of shear stress ( and ) A number of techniques have been examined to measure surface shear stress. In the case of a turbulent boundary layer, the law of the wall has been used with the Karman and offset constants of 0.41 and 5.20 respectively.
A Preston tube has also been used (making use of Patel's calibration), and comprises a square-cut circular tube of 2.49mm outside diameter, located on the surface of a test surface plug.
The momentum balance approach was also used and it was found that all three techniques yielded skin friction coefficients, which agreed to within 2% of each other. Attempts were made at calibrating both a heated film gauge (DISA R45) and a small (50(m height) razor blade device in both turbulent and laminar boundary layers. The aim was to produce a 'universal' calibration which could be used in both laminar and turbulent flows, and by implication in the transitional zone also. Unfortunately, these attempts failed for various reasons. As a result, it was decided to use the momentum balance technique in both the laminar and transitional zones. This was compared with the wall-slope approach in laminar boundary layers with negligible free-stream turbulence and found to agree to better than 1%.
The single wire measurements have been made at typically 15 streamwise locations, and up to 46 points within each boundary layer. Sample times of approximately 30 seconds per measurement have been used, with a bandwidth of 3kHz on-line, and 10kHz recorded Recognising the value of the results and the probable requirement for further analysis, all of the raw data have been simultaneously recorded on an instrumentation FM tape recorder (SE 7000M). The results, may be accessed readily, knowing the unique file number. The file name is constructed from the file number, preceded and succeeded by the characters as outlined in the table notes. The preceding characters denote the probe type: 1W being the single wire, XV being the U-V cross-wire probe, and XW being the U-W cross-wire probe. The code name identifies each of the seven flow conditions examined.
|None ...||Wind tunnel residual turbulence.|
|SMR ...||Square mesh, round wires.|
|PR ...||Parrallel array,round rods.|
|PS ...||Parrallel array,square rods.|
It should first be noted that corrections have been applied to the data to account for the influence of velocity shear on the UW probe measurements. Even so, the actual magnitude of the results shown will be slightly in error, due to the influence of spatial averaging over the sensor length, ref. 3. It is possible that the single wire measurements of u' are more accurate than those using the cross-probes. The difference becoming greater in proportion to the velocity shear.
In the present study we employ a flat test plate with a small leading edge radius. This approach can readily lead to flow separation at the leading edge, which would render any further measurements pointless. To overcome this problem, in the wind tunnel in use it is possible to incline both the test plate and the opposite tunnel wall in such a way as to ensure zero pressure gradient over the bulk of the test plate length. In addition, use is made of the test plate trailing edge flap, coupled with the variable pressure drop working section exit plane gauzes, which effectively control the circulation about the plate and thus overcome the tendency to separate near the leading edge.
Previous and Reference Numerical Solutions
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