Boundary Layer Profile Measurements Research Articles

Experimental characterization of the transonic test section flow in a Ludwieg tube

A new Ludwieg tube for transonic testing has been put into operation. For this, an existing shock tube has been modified to a Ludwieg tube. The ideal operating principle is described by theory and corresponding theoretical values are compared to experimental ones. Pressure histories as well as test section flow visualization demonstrate a better performance concerning flow quality and testing time compared to the former shock tube. Static as well as total pressure measurements have been performed within the test section. Pitot rake measurements show the homogeneity of the test section flow field. Additional boundary layer profile measurements give evidence of the temporal growth of the test section wall boundary layer. These results are compared with theoretical ones deduced from a theoretical approach for a boundary layer behind an expansion wave. Both, boundary layer and Pitot rake measurements allow to discuss the influence of a turbulent boundary layer on these measurements. Cookie cutters at the entrance to the test section act as boundary layer bleed which partially avoid the entering of the tube wall boundary layer into the test section. Their influence on the test section flow is discussed as well.

Measurements of Boundary Layer Profiles with In Situ Sensors and Doppler Lidar

AbstractA new in situ measurement system and lidar processing algorithms were developed for improved measurements of boundary layer profiles. The first comparisons of simultaneous Doppler lidar–derived profiles of the key turbulence statistics of the two orthogonal horizontal velocity components (longitudinal and transverse) are presented. The instrument requirements for accurate observations of stably stratified turbulence were determined. A region of stably stratified low turbulence with constant gradients of temperature and velocity was observed above the nocturnal boundary layer using high-rate sensors. The important turbulence parameters were estimated, and turbulence spectra were consistent with new theoretical descriptions of stratified turbulence. The impact of removing the larger-scale velocity features in Doppler lidar estimates of turbulent velocity variance and length scales was investigated. The Doppler lidar–derived estimates of energy dissipation rate ε were found to be insensitive to spatial filtering of the large-scale atmospheric processes. The in situ and lidar-derived profiles were compared for the stable boundary layer in a suburban environment.

Measurements of Boundary Layer Profiles in an Urban Environment

AbstractBoundary layer profiles of mean temperature, velocity, and small-scale turbulence from in situ sensors, Doppler lidar, sodar, and rawinsondes are intercompared for an urban environment. A new Doppler lidar algorithm is presented to produce high-resolution profiles of small-scale velocity statistics. The lidar-derived profiles are robust and accurate even for challenging conditions such as stable boundary layers with a low-level jet, low turbulence, and low wind speed. Similar results are expected for other locations and convective conditions.

Effects of Gurney Flaps on a NACA0012 Airfoil

Experimental measurements of surface pressure distributions and wake profiles were obtained for a NACA0012 airfoil to determine the lift, drag, and pitching-moment coefficients for various configurations. The addition of a Gurney flap increased the maximum lift coefficient from 1.37 to 1.74, however there was a drag increment at low-to-moderate lift coefficient. In addition, the boundary layer profile measurements were taken using a rake of total pressure probes at the 90% chord location on the suction side. The effective Gurney flap height is about 2% of chord length, which provides the highest lift-to-drag ratio among the investigated configurations when compared with the clean NACA0012 airfoil. In this case, the device remains within the boundary layer.

Morphological variation in coral aggregations: branch spacing and mass flux to coral tissues

The scleractinian (reef-building) coral Agaricia tenuifolia (Dana) is one of the most common constituents of the barrier reef of Belize, Central America. This species grows almost exclusively in aggregations of clonemates and conspecifics, in which rows of thin, upright blades line up behind one another facing the dominant direction of flow. We quantified patterns in colony morphology, light levels and mainstream flow over a range of physical habitats (fore reef, patch reef and lagoon locations) near Carrie Bow Cay and in the Pelican Cays. Water flow and light levels both decreased with depth on the fore reef. Light levels in the lagoon environment (1 m depth) were comparable to those at the same depth on the fore reef, but flow speeds were markedly lower. Aggregation size, branch spacing, height and width all varied with location. Mean branch spacing increased with depth on the fore reef by approximately 50%, but total branch height increased by only 20–25%, indicating that the shape of colonies did not remain constant. Colonies in the 1 m lagoon habitat (high light, low flow) were very similar to those at 1 m on the fore reef (high light, high flow). These results thus suggested that colony morphology was insensitive to the flow regime, despite previous studies that have linked flow-dependent mass flux to both coral respiration and symbiont (zooxanthellae) photosynthesis. Because of this discrepancy, we examined the effect of one aggregation parameter, branch spacing, to test the null hypothesis that mass flux to a coral's tissues is unaffected by colony morphology. We used two non-dimensional parameters, the Reynolds number ( Re) and the Sherwood number ( Sh), to examine the interaction between flow, morphology and mass transport. Using physical scaling arguments, we measured water loss rates from scale models in air as proxies for gas flux from corals in water. We created two types of solitary models, horizontal (unifacial) and upright (bifacial) plates and two types of aggregations, widely-spaced (5 cm between rows) and tightly-spaced (2.5 cm spacing), to examine how morphology affects mass flux to a branch's surface under conditions of uniform flow. Measurements at two Re (4 000 and 21 000) and two turbulence levels in uniform flow showed that mass flux is significantly higher in solitary models compared to aggregations. Mass flux from branches within aggregations was highest at branch tips and decreased closer to the bottom. Measurements of boundary layer profiles overlying aggregations indicated higher boundary layer diffusivities to the surface of the tightly-spaced aggregation, per unit of substrate area. However, the increased amount of tissue surface area in these aggregations led to a lower flux per unit of coral tissue. Our results suggest that the coral A. tenuifolia displays different aggregation structures in response to light but not water flow, at least in shallow, high light environments. Nonetheless, our laboratory experiments show that branch spacing within an aggregation has significant effects on the flux of gases to the surface of corals. Because photosynthesis depends upon both mass flux and light, this apparent contradiction between field patterns and laboratory results suggests that A. tenuifolia and its symbionts may adapt physiologically rather than morphologically to variation in the local flow regime. The optimal branch spacing in any given environment is thus unlikely to result from a single selective pressure but rather from a suite of environmental parameters acting in concert, including light, water flow, sedimentation rate, hydromechanical stresses and competition for space.

Landsat surface temperatures and radio soundings to obtain regional surface fluxes

Thermal infrared radiances recorded by the thematic mapper of the Landsat 5 satellite, in combination with wind velocity and temperature profiles recorded by radiosondes, were used to derive surface fluxes of sensible heat during the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE). FIFE was conducted in 1987 and 1989 in a strongly dissected tallgrass prairie area of the Flint Hills in northeastern Kansas. The scale of the surface temperature measurements was made compatible with the effective fetch of the boundary layer profile measurements by aggregation. The methods of calculation were based on known Monin‐Obukhov flux‐profile relationships and on bulk similarity formulations for the atmospheric boundary layer. The surface temperatures were used both with and without atmospheric correction. The latent heat fluxes (evaporation) were derived from those of sensible heat by means of the energy budget. The calculated fluxes were generally in good agreement with the means of the fluxes measured in the field by all (22 in 1987 and 14 in 1989) ground stations in the 15×15 km experimental area.

Effects of a change of terrain height and roughness on a wind profile

We have measured profiles of an onshore wind 200 m upwind and 200 m downwind from an abruptly rising shore using a remote-sensing Doppler lidar anemometer. Data were taken at heights between 4.7 and 66.5 m above sea level. Results show that the onshore velocity vector slopes upward 16.6 to 9.6°, the amount depending on the height of measurement, due to the combined effects of a 1.7-m high bluff shoreline and the frictionally decelerated flow over land. The profile 200 m inland has the expected deceleration at lower levels because of increased surface roughness and implies a velocity vector at 66.5 m height with an upward slope of approximately 18° (2.6 m s-1 upward component, 8.4 m s-1 vector magnitude), an acceleration to 0.3 m s-1 greater than the upwind value, or a combination of both effects. All three options are consistent with mass continuity. The experiment exhibits the usefulness and limitations of a backscatter Doppler lidar for boundary-layer profile measurements in a horizontally inhomogeneous environment.

Boundary-layer profile measurements in a combustion driven MHD generator

The velocity, temperature, and electron number density profiles were measured in the electrode wall boundary layer of a combustion driven MHD generator. Both subsonic and supersonic conditions were run. The experimental results are compared with predictions of a two-dimensional turbulent boundary-layer computation. For the subsonic condition, high levels of freestream turbulence were measured, about 10-12 percent. The measured velocity profile was fatter than that predicted, although the temperature and electron number density profiles were in agreement. This difference is tentatively ascribed to the high freestream turbulence levels. There was no measurable MHD effect for the subsonic case. For the supersonic condition, the measured velocity, temperature, and electron number density profiles fell under the predicted profiles. The discrepancy may be due to three-dimensional recirculation effects. There was a small amount of MHD interaction, the degree of which was in agreement with predictions. Electron number density nonequilibrium was not identified, but the degree of nonequilibrium predicted was small. Under the appropriate supersonic conditions, primarily at freestream temperatures below 2400K, ionization nonequilibrium is predicted to occur.

Experiment on Turbulent Boundary Layers on a Concave Wall

SummaryThe present experiment describes the behaviour of a turbulent boundary layer on a concave wall. At the onset of curvature there appears a fairly coherent wavelike transverse profile of mean velocity. This disturbance might be interpreted as a kind of large scale Taylor-Görtler type instability superimposed on a conventional turbulent boundary layer; further downstream the coherence degenerates as the turbulence level increases. Boundary-layer profile measurements were made at positions of maxima and minima of transverse profiles of (U-component) mean velocity. The boundary layer at the minima positions is found to be twice as thick as that at the maxima positions. Also, turbulent intensities inside the boundary layer are substantially increased as a result of the concave curvature of the surface.

Effects of upstream wall temperatures on hypersonic tunnel wall boundary-layer profile measurements.

Effects of upstream wall temperatures on hypersonic tunnel wall boundary-layer profile measurements.

Effects of mass transfer on hypersonic turbulent boundary-layer properties.

Turbulent boundary-layer profile measurements on a sharp 7.25° half angle porous cone have been made at Mach 8. The data obtained have been analyzed in the form of integral parameters. These results were correlated as a function of the blowing parameter and compared to currently used thickness prediction methods employed in the aerospace industry. Revised correlations based on the available data have been generated. The sensitivity of the velocity profile exponent to distance from the end of transition and mass addition has been established. Finally, the applicability of the Crocco theory for flows with mass addition has been examined.

Measurement of fluid velocity and refractive index gradient using lasers

Two instruments which make use of the properties of gas laser beams are described. In the velocity meter the laser provides intense illumination at two lines separated by a known distance. Dust particles crossing these lines scatter light, and the time interval between the two pulses of scattered light gives the particle velocity. The meter is simple and accurate and the method avoids the need to insert probes into the flow. In the deflection meter, the laser beam after passing through the object is detected by opposed photocells whose output is thus a measure of the beam deflection. The device is extremely sensilive, and possible applications include deteclion or measurement of temperature or concentration gradients, boundary layer profile measurements and turbulance detection.

Some Observations on dr. Coleman's Comments

The method referred to in Dr. Coleman's notes was developed with the collaboration of my colleague Mr. J. B. Edwards of Handley Page Research Department. The purpose was to obtain a rational estimate of suction quantities and suction distribution, linked up with measurements of boundary layer profiles and suction quantities on wind tunnel models, and also to assess the effect of a certain degree of roughness of the order to be expected on actual wings. Existing theoretical methods ignore roughness which, however, is a most important parameter not only in wind tunnel tests, but also in flight at higher values of the unit Reynolds number; surface roughness obviously limits the intensity of suction which can be applied at a spanwise suction strip.It was intuitively assumed that the removal of fluid by suction was equivalent to cutting off the lower portion of the boundary layer profile at the upstream edge of the suction strip and that a rapid re-adjustment of the boundary layer profile within a short distance took place.