Boundary Layer Measurements Research Articles

大型風洞による高レイノルズ数乱流境界層実験の試み

High Reynolds number turbulent boundary layer measurement is performed in the RTRI's Large-scale low-noise wind tunnel in Maibara Japan. The momentum thickness Reynolds number is up to approximately 7×104 . The mean velocity profile shows a clear log-law region with Kármán constant 0.38. The turbulent intensity profile also shows the log-law scaling predicted by attached eddy model. These data will be used as the fundamental data base made in Japan which is also applicable to industrial application including railway technology.

Wind Tunnel Testing of Airfoils for Wind Turbine Applications

Accurate wind tunnel measurements of the lift and drag of airfoil sections are critical for the design and performance evaluation of wind turbine blades. As blades continue to increase in size, the demand for highly accurate wind tunnel results at progressively larger Reynolds numbers has also increased. Performing these wind tunnel measurements requires precise experimental control, and three challenges for these measurements are model surface quality, pressure tap effects, and model deflections under aerodynamic loading. These challenges were systematically studied in the Virginia Tech Stability Wind Tunnel using a DU96-W-180 airfoil geometry at a chord Reynolds number (Rec) of 3.0 × 106. Naphthalene sublimation showed turbulent wedges caused by surface imperfections; removing these imperfections increased the lift curve slope by 3%. Pressure tap diameter effects were investigated by placing taps of varying size at the same chord location on the airfoil. These measurements showed a steady pressure bias correlated to tap diameter when making measurements in turbulent boundary layers, and naphthalene visualizations showed a turbulent wedge created by pressure taps at the leading edge. Finally, laser distance sensors were used to measure model deflections/rotations under aerodynamic loading, improving upon the traditional angle of attack measurement. Addressing these challenges has improved the accuracy of lift measurements in the Stability Wind Tunnel and emphasized the need for precise experimental controls when performing these types of wind tunnel measurements.

Temporally optimized spanwise vorticity sensor measurements in turbulent boundary layers

Multi-element hot-wire anemometry was used to measure spanwise vorticity fluctuations in turbulent boundary layers. Smooth wall boundary layer profiles, with very good spatial and temporal resolution, were acquired over a Karman number range of 2000–12,700 at the Melbourne Wind Tunnel at the University of Melbourne and the University of New Hampshire’s Flow Physics Facility. A custom hot-wire probe was necessary to simultaneously obtain velocity and spanwise vorticity measurements centered at a fixed point in space. A custom calibration/processing scheme was developed to utilize single-wall-parallel wires to optimize the accuracy of the measured wall-normal velocity fluctuations derived from the sensor’s $$\times$$ -array.

A simple two-dimensional parameterisation for Flux Footprint Prediction (FFP)

Abstract. Flux footprint models are often used for interpretation of flux tower measurements, to estimate position and size of surface source areas, and the relative contribution of passive scalar sources to measured fluxes. Accurate knowledge of footprints is of crucial importance for any upscaling exercises from single site flux measurements to local or regional scale. Hence, footprint models are ultimately also of considerable importance for improved greenhouse gas budgeting. With increasing numbers of flux towers within large monitoring networks such as FluxNet, ICOS (Integrated Carbon Observation System), NEON (National Ecological Observatory Network), or AmeriFlux, and with increasing temporal range of observations from such towers (of the order of decades) and availability of airborne flux measurements, there has been an increasing demand for reliable footprint estimation. Even though several sophisticated footprint models have been developed in recent years, most are still not suitable for application to long time series, due to their high computational demands. Existing fast footprint models, on the other hand, are based on surface layer theory and hence are of restricted validity for real-case applications. To remedy such shortcomings, we present the two-dimensional parameterisation for Flux Footprint Prediction (FFP), based on a novel scaling approach for the crosswind distribution of the flux footprint and on an improved version of the footprint parameterisation of Kljun et al. (2004b). Compared to the latter, FFP now provides not only the extent but also the width and shape of footprint estimates, and explicit consideration of the effects of the surface roughness length. The footprint parameterisation has been developed and evaluated using simulations of the backward Lagrangian stochastic particle dispersion model LPDM-B (Kljun et al., 2002). Like LPDM-B, the parameterisation is valid for a broad range of boundary layer conditions and measurement heights over the entire planetary boundary layer. Thus, it can provide footprint estimates for a wide range of real-case applications. The new footprint parameterisation requires input that can be easily determined from, for example, flux tower measurements or airborne flux data. FFP can be applied to data of long-term monitoring programmes as well as be used for quick footprint estimates in the field, or for designing new sites.

Enhancing the accuracy of measurement techniques in high Reynolds number turbulent boundary layers for more representative comparison to their canonical representations

Existing differences between experimental, computational and theoretical representations of a particular flow do not allow one-to-one comparisons, prevent us from identifying the absolute contributions of the various sources of uncertainty in each approach, and highlight the importance of developing suitable corrections for experimental techniques. In this study we utilize the latest Pitot tube correction schemes to develop a technique which improves on the outcome of hot-wire measurements of mean velocity profiles in ZPG turbulent boundary layers over the range 11500<Reθ<21500. Measurements by Bailey et al. (2013), carried out with probes of diameters ranging from 0.2 to 1.89 mm, supplemented by other data with larger diameters up to 12.82 mm, are used first to develop a somewhat improved Pitot tube correction which is based on viscous, shear and near-wall schemes (which contribute with around 85% of the effect), together with a turbulence scheme which accounts for 15% of the whole correction. The correction proposed here leads to similar agreement with available high-quality datasets in the same Reynolds number range as the one proposed by Bailey et al. (2013), but this is the first time that the contribution of the turbulence scheme is quantified. In addition, four available algorithms to correct wall position in hot-wire measurements are tested, using as benchmark the corrected Pitot tube profiles with artificially simulated probe shifts and blockage effects. We find that the κB-Musker correction developed in this study produces the lowest deviations with respect to the introduced shifts. Unlike other schemes, which are based on a prescribed near-wall region profile description, the κB-Musker is focused on minimizing the deviation with respect to the κ̃B̃ relation, characteristic of wall-bounded turbulent flows. This general approach is able to locate the wall position in probe measurements of the wall-layer profiles with around one half the error of the other available methods. The difficulties encountered during the development of adequate corrections for high-Re boundary layer measurements highlight the existing gap between the conditions that can be reproduced and measured in the laboratory and the so-called canonical flows.

Documentation of the role of large-scale structures in the bursting process in turbulent boundary layers

The scaling of the bursting frequency with Reynolds number is investigated experimentally by means of hot-wire measurements in turbulent boundary layers over the range 1580 &amp;lt; Reθ &amp;lt; 23 700. Bursting events are detected by means of the u-level technique, and the effect of filtering and the choice of the threshold are also assessed. We find that the inner scaling produces excellent collapse of the high-pass filtered data in the buffer and log regions, regardless of the cutoff frequency and the threshold. This is due to the fact that the high-pass filter attenuates the low frequency component of the streamwise velocity signal, which is associated with large-scale structures in the flow. On the other hand, the bursting frequency scales with outer variables when the time-series is low-pass filtered with very low cutoff frequencies, highlighting the connection between bursting phenomena and the outer flow.

Characteristics of the Springtime Alpine Valley Atmospheric Boundary Layer Using Self-Organizing Maps

AbstractVertical profiles of wind velocity and air temperature from a sound detection and ranging (sodar) radio acoustic sounding system (RASS)-derived dataset within an alpine valley of the New Zealand Southern Alps were analyzed. The data covered the month of September 2013, and self-organizing maps (SOM; a data-clustering approach that is based on an unsupervised machine-learning algorithm) are used to detect topological relationships between profiles. The results of the SOM were shown to reflect the physical processes within the valley boundary layer by preserving valley boundary layer dynamics and its response to wind shear. By examining the temporal evolution of ridgetop wind speed and direction and SOM node transitions, the sensitivity of the valley boundary layer to ridgetop weather conditions was highlighted. The approach of using a composite variable (wind speed and potential temperature) with SOM was successful in revealing the coupling of dynamics and atmospheric stability. The results reveal the capabilities of SOM in analyzing large datasets of atmospheric boundary layer measurements and elucidating the connectivity of ridgetop wind speeds and valley boundary layers.

The Numerical Analysis and Experimental Research of an Aspirated Counter-Rotating Compressor

Domain scaling method was used in the unsteady numerical simulation of a dual-stage counter-rotating compressor. Numerical simulation results show that there exist much high entropy low energy fluids in the tip region on the near stall condition. In order to remove the high entropy flows near the shroud casing and improve the performance of the counter-rotating compressor, the boundary layer removal was conducted on the shroud casing over front rotor (R1) and rear rotor (R2) blade tip region. By shroud casing boundary layer suction, the flow field in the tip area improves greatly and the efficiency of the compressor increases by 0.58%. Based on the numerical results, the boundary layer suction system and measurement platform were established afterwards. The experiment was carried out with 0.9% relative suction rate in R1 and R2 tip region shroud casing respectively at 70% corrected rotating speed. Experimental results also show that shroud casing boundary layer suction can improve the compressor's performance effectively on the small flow rate conditions and has a positive role to enlarge the stable operating range.

Generating an artificially thickened boundary layer to simulate the neutral atmospheric boundary layer

We aim to generate an artificially thickened boundary layer in a wind tunnel with properties similar to the neutral atmospheric boundary layer. We implement a variant of Counihan's technique which uses a combination of a castellated barrier, elliptical vortex generators, and surface roughness to create a thick boundary layer in a relatively short wind tunnel. We demonstrate an improved spanwise uniformity when compared to Counihan's original design by using a tighter vortex generator spacing with a smaller wedge angle which keeps the frontal area approximately constant while keeping the turbulence intensity and power spectral density unchanged. With this arrangement we were able to generate a 1:1000 scale atmospheric boundary layer at Reθ∼106 displaying logarithmic mean velocity behavior, a constant stress region, and turbulence intensities consistent with atmospheric boundary layer measurements and high Reynolds number laboratory rough-wall boundary layers. In addition, the power spectral density of the longitudinal velocity fluctuations agrees well with von Kármán's model spectrum, and the turbulence integral length scale agrees well with data from atmospheric boundary layer measurements.

Cross-correlation Doppler global velocimetry (CC-DGV)

A flow velocimetry method, cross-correlation Doppler global velocimetry (CC-DGV), is presented as a robust, simplified, and high dynamic range implementation of the Doppler global/planar Doppler velocimetry technique. A sweep of several gigahertz of the vapor absorption spectrum is used for each velocity sample, with signals acquired from both Doppler-shifted scattered light within the flow and a non-Doppler shifted reference beam. Cross-correlation of these signals yields the Doppler shift between them, averaged over the duration of the scan. With presently available equipment, velocities from 0ms−1 to over 3000ms−1 can notionally be measured simultaneously, making the technique ideal for high speed flows. The processing routine is shown to be robust against large changes in the vapor pressure of the iodine cell, benefiting performance of the system in facilities where ambient conditions cannot be easily regulated. Validation of the system was performed with measurements of a model wind turbine blade boundary layer made in a 1.83m by 1.83m subsonic wind tunnel for which laser Doppler velocimetry (LDV) measurements were acquired alongside the CC-DGV results. CC-DGV uncertainties of ±1.30ms−1, ±0.64ms−1, and ±1.11ms−1 were determined for the orthogonal stream-wise, transverse-horizontal, and transverse-vertical velocity components, and root-mean-square deviations of 2.77ms−1 and 1.34ms−1 from the LDV validation results were observed for Reynolds numbers of 1.5 million and 2 million, respectively. Volumetric mean velocity measurements are also presented for a supersonic jet, with velocity uncertainties of ±4.48ms−1, ±16.93ms−1, and ±0.50ms−1 for the orthogonal components, and self-validation done by collapsing the data with a physical scaling.

Space- and time-resolved interferometric measurements of the thermal boundary layer at a periodically magnetized gadolinium plate

By means of a Mach-Zehnder interferometer we examine the transient dynamics of heat transfer from two periodically magnetized gadolinium (Gd) plates into a heat transfer fluid (n-decane). We demonstrate that the propagation of the thermal fronts emanating from the Gd plates after magnetization or demagnetization obeys a t-dependence. A finite time required for magnetization and demagnetization causes a spatially delayed propagation of the thermal fronts. The diffusive heat flux, derived from the temperature profiles, experiences a drop down by about 80% after first 3 s while the percentage of thermal energy transferred into n-decane experiences a maximum there. Although limited to heat transfer into a stagnant fluid, the present works provides reasons for lower bounds of geometry and operation frequency of a simplified parallel-plate structure in the diffusive limit. In this way, the results are instructive for an efficient design of a parallel-plate magnetic regenerator with forced convection.

Crossflow Instability on a Yawed Cone at Mach 6

Boundary-layer measurements were performed in a Mach 6, low-disturbance wind tunnel on a 7∘ cone at 5.6∘ angle of incidence such that the model was primarily subject to crossflow instability. Constant-temperature hot-wire anemometry was used to measure the streamwise mass flux at a series of planes normal to the cone axis. A dominant stationary wave is observed to achieve nonlinear saturation at approximately 23% with maximum rms fluctuations of approximately 8%. Additionally, traveling crossflow waves were observed in a broad frequency band centered at f = 40kHz and secondary instabilities were observed in a broad band centered at f = 100kHz. Measurements show excellent agreement with previously published computational results and are qualitatively similar to studies performed in subsonic flows. Transition to turbulence was not observed to occur under these conditions.

Experimental study on flow visualization of hypersonic flat plate boundary layer

The classical problem of flat plate boundary layer which involves turbulence and transition is still hot, and a mass of work should be done to reach a high accuracy measurement of this flow, especially under the condition of high velocity. In the present paper, the application of the nano-based planar laser scattering (NPLS) method in a hypersonic short-duration facility is explored, and then the high accuracy measurement of a flat plate boundary layer is studied. The Mach number of the main flow is 7.3, the total pressure is 4.8 MPa, and the total temperature is 680 K. Through analysis and tests, the synchronization control of the NPLS system with the test facility is realized, and with the quantitative control, the tracer particle is uniformly seeded. Based on this, the transient boundary layer flow in the short-duration tunnel is visualized with high resolution, and the transition from laminar to turbulent flows is captured. The development characteristic of the flow is studied finally.

Feather roughness reduces flow separation during low Reynolds number glides of swifts.

Swifts are aerodynamically sophisticated birds with a small arm and large hand wing that provides them with exquisite control over their glide performance. However, their hand wings have a seemingly unsophisticated surface roughness that is poised to disturb flow. This roughness of about 2% chord length is formed by the valleys and ridges of overlapping primary feathers with thick protruding rachides, which make the wing stiffer. An earlier flow study of laminar-turbulent boundary layer transition over prepared swift wings suggested that swifts can attain laminar flow at a low angle of attack. In contrast, aerodynamic design theory suggests that airfoils must be extremely smooth to attain such laminar flow. In hummingbirds, which have similarly rough wings, flow measurements on a 3D printed model suggest that the flow separates at the leading edge and becomes turbulent well above the rachis bumps in a detached shear layer. The aerodynamic function of wing roughness in small birds is, therefore, not fully understood. Here, we performed particle image velocimetry and force measurements to compare smooth versus rough 3D-printed models of the swift hand wing. The high-resolution boundary layer measurements show that the flow over rough wings is indeed laminar at a low angle of attack and a low Reynolds number, but becomes turbulent at higher values. In contrast, the boundary layer over the smooth wing forms open laminar separation bubbles that extend beyond the trailing edge. The boundary layer dynamics of the smooth surface varies non-linearly as a function of angle of attack and Reynolds number, whereas the rough surface boasts more consistent turbulent boundary layer dynamics. Comparison of the corresponding drag values, lift values and glide ratios suggests, however, that glide performance is equivalent. The increased structural performance, boundary layer robustness and equivalent aerodynamic performance of rough wings might have provided small (proto) birds with an evolutionary window to high glide performance.

LDV measurement of boundary layer on rotating blade surface in wind tunnel

Wind turbines generate electricity due to extracting energy from the wind. The rotor aerodynamics strongly depends on the flow around blade. The surface flow on the rotating blade affects the sectional performance. The wind turbine surface flow has span-wise component due to span-wise change of airfoil section, chord length, twisted angle of blade and centrifugal force on the flow. These span-wise flow changes the boundary layer on the rotating blade and the sectional performance. Hence, the thorough understanding of blade surface flow is important to improve the rotor performance. For the purpose of clarification of the flow behaviour around the rotor blade, the velocity in the boundary layer on rotating blade surface of an experimental HAWT was measured in a wind tunnel. The velocity measurement on the blade surface was carried out by a laser Doppler velocimeter (LDV). As the results of the measurement, characteristics of surface flow are clarified. In optimum tip speed operation, the surface flow on leading edge and r/R=0.3 have large span-wise velocity which reaches 20% of sectional inflow velocity. The surface flow inboard have three dimensional flow patterns. On the other hand, the flow outboard is almost two dimensional in cross sectional plane.

Unsteady boundary layer measurement on an oscillating (pitching) supercritical airfoil in compressible flow using multiple hot-film sensors

Aerodynamic characteristics of an airfoil in a dynamic motion are highly affected by the behavior of the viscous boundary layer. Boundary layer condition, and the related phenomena, depends on such different parameters as the oscillation type, flight Mach number, angle of attack, etc. In compressible and transonic regimes, the shock-boundary layer interaction causes complex phenomena such as λ-shock and separation bubble on the airfoil. When oscillation and unsteady effects are also added, a complicated flow is resulted which could not be easily investigated using theoretical and computational methods. Therefore, in this paper a series of static and dynamic (pitch) tests at pre stall angle of attacks were performed in a high speed wind tunnel to study the steady and unsteady behavior of the compressible boundary layer. Measurements involved pressure coefficient distribution and qualitative analysis of output voltage of hot films. Some sinusoidal pitching motions were performed on the SC (2)-0410 airfoil at Mach numbers of 0.4, 0.5, and 0.6 with the maximum Reynolds number of 8.8 million per meter. The effects of compressibility, reduced frequency, mean angle and oscillation amplitude and free stream Mach number were investigated. Results show asymmetry between transition and relaminarization during pitching motion, and delay in transition with respect to steady conditions. Also, the location of the shock, shock induced separation bubble, and their motions due to the change of instantaneous angle of attack were detected and interpreted using the outputs of hot-film sensors.

Boundary layer laminarization by convex curvature and acceleration

Particle image velocimetry boundary layer measurements are presented for flows over convex surfaces, and subject to flow acceleration. These mechanisms are present in gas reactor core designs, which is of concern because it is known that they can cause boundary layer laminarization which can have a negative effect on surface heat transfer. A wind tunnel was constructed where flow acceleration and curvature effects were applied separately, and simultaneously to investigate the separate and combinative influence of these mechanisms. Applied acceleration for the different test cases varied from K=1.6×10−6 to 1×10−5. Curvature values of δ/R=0.015 to δ/R=0.05 were tested. The applied acceleration spans across the threshold value when laminarization is expected to occur for flat plate geometries.Measurements were taken using particle image velocimetry. Mean flow parameters including mean velocity profiles, boundary layer thickness, and shape factor are presented as a function of streamwise position during the boundary layer response as it is subjected to the laminarization causing mechanisms. Velocity profiles are normalized by both outer and inner variables.It is observed that as the flow is subjected to these mechanisms, the boundary layer mean flow parameters diverge from their turbulent values and approach laminar characteristics. The linear viscous sublayer increases in thickness, the overall boundary layer thickness is reduced, and the shape factor increases. The response in the boundary layer mean flow parameters is more pronounced and occurs more quickly when subject to both mechanisms simultaneously.

PIV measurements of the bottom boundary layer under nonlinear surface waves

Sediment in the nearshore is largely mobilized in the wave bottom boundary layer (wbbl) hereby emphasizing the importance of this relatively thin layer to nearshore morphology. This paper presents a laboratory experiment where hydrodynamic properties of the wbbl were quantified by measuring flow velocities using Particle Image Velocimetry. The bottom of the wave flume was rigid with a single bar profile. The measurements consisted of the velocities of the wbbl with a resolution in the order of 10 points in time and space for skewed and asymmetric waves. In the wbbl there was significant transformation of velocity skewness and asymmetry. Negative wbbl streaming was generated by asymmetric waves. Boundary layer development and generation of vortex tubes were observed. The velocity measurements included the turbulent components resulting in quantification of turbulent stresses that were of similar magnitude as the viscous stresses.

Pressure Gradient Effects on Smooth- and Rough-Surface Turbulent Boundary Layers—Part II: Adverse Pressure Gradient

An experimental investigation has been conducted to identify the effects of pressure gradient and surface roughness on turbulent boundary layers. In Part II, smooth- and rough-surface turbulent boundary layers with and without adverse pressure gradient (APG) are presented at a fixed Reynolds number (based on the length of flat plate) of 900,000. Flat-plate boundary layer measurements have been conducted using a single-sensor, hot-wire probe. For smooth surfaces, compared to the zero pressure gradient (ZPG) boundary layer, the APG boundary layer has a higher mean velocity defect throughout the boundary layer and lower friction coefficient. APG decreases the streamwise normal Reynolds stress for y less than 0.4 times the boundary layer thickness and increases it slightly in the outer region. For rough surfaces, APG reduces the roughness effects of increasing the mean velocity defect and normal Reynolds stress for y less than 23 and 28 times the average roughness height, respectively. Consistently, for the same roughness, APG decreases the integrated streamwise turbulent kinetic energy. APG also decreases the roughness effect on the friction coefficient, roughness Reynolds number, and roughness shift. Compared to the ZPG boundary layers, the roughness effects on integral boundary layer parameters—boundary layer thickness and momentum thickness—are weaker under APG. Thus, contrary to the favorable pressure gradient (FPG) in part I, APG reduces the roughness effects on turbulent boundary layers.

Estimating Sensible and Latent Heat Fluxes Using the Integral Method from in situ Aircraft Measurements

Abstract In September 2009, several Aerosonde unmanned aerial vehicles (UAVs) were flown from McMurdo Station to Terra Nova Bay, Antarctica, with the purpose of collecting three-dimensional measurements of the atmospheric boundary layer (ABL) overlying a polynya. Temperature, pressure, wind speed, and relative humidity measurements collected by the UAVs were used to calculate sensible and latent heat fluxes (SHF and LHF, respectively) during three flights. Fluxes were calculated over the depth of the ABL using the integral method, in which only measurements of the mean atmospheric state (no transfer coefficients) were used. The initial flux estimates assumed that the observations were Lagrangian. Subsequent fluxes were estimated using a robust and innovative methodology that included modifications to incorporate adiabatic and non-Lagrangian processes as well as the heat content below flight level. The SHF ranged from 12 to 485 W m−2, while the LHF ranged from 56 to 152 W m−2. The importance of properly measuring the variables used to calculate the adiabatic and non-Lagrangian processes is discussed. Uncertainty in the flux estimates is assessed both by varying the calculation methodology and by accounting for observational errors. The SHF proved to be most sensitive to the temperature measurements, while the LHF was most sensitive to relative humidity. All of the flux estimates are sensitive to the depth of the boundary layer over which the values are calculated. This manuscript highlights these sensitivities for future field campaigns to demonstrate the measurements most important for accurate flux estimates.