Boundary Layer Measurements Research Articles

A Spatiotemporal Estimation Framework for Real-World LIDAR Wind Speed Measurements

Despite significant advances in the remote sensing of fluid flows, light detection and ranging (LIDAR) measurement equipment still presents the problems of having only radial (line-of-sight) wind speed measurements (Cyclops' dilemma). Substantial expanses of unmeasured flow still remain and range weighting errors have a considerable influence on LIDAR measurements. Clearly, more information needs to be extracted from LIDAR data. With this motivation in mind, this brief shows that it is possible to estimate the wind velocity, wind direction, and absolute pressure over the entire spatial region of interest. A key challenge is that most established estimation techniques cater for systems that are finite-dimensional and described by ordinary differential equations (ODEs). By contrast, many fluid flows are governed by the Navier-Stokes equations, which are partial differential-algebraic equations (PDAEs). We show how a basis function decomposition method in conjunction with a pressure Poisson equation (PPE) formulation yields a spatially continuous, strangeness-free, reduced-order dynamic model for which a modified DAE form of the unscented Kalman filter (UKF) algorithm is used to estimate unmeasured velocities and pressure using sparse measurements from wind turbine-mounted LIDAR instruments. The approach is validated for both synthetic data generated from large eddy simulations of the atmospheric boundary layer and real-world LIDAR measurement data. Results show that a reconstruction of the flow field is achievable, thus presenting a validated estimation framework for potential applications including wind gust prediction systems and the preview control of wind turbines.

Trailing-edge flow manipulation using streamwise finlets

The use of streamwise finlets as a passive flow and aerodynamic noise-control technique is considered in this paper. A comprehensive experimental investigation is undertaken using a long flat plate, and results are presented for the boundary layer and surface pressure measurements for a variety of surface treatments. The pressure–velocity coherence results are also presented to gain a better understanding of the effects of the finlets on the boundary layer structures. The results show that the flow behaviour downstream of the finlets is strongly dependent on the finlet spacing. The use of finlets with coarse spacing leads to a reduction in pressure spectrum at mid- to high frequencies and an increase in spanwise length scale in the trailing-edge region due to flow channelling effects. For the finely distributed finlets, the flow is observed to behave similarly to that of a permeable backward-facing step, with significant suppression of the high-frequency pressure fluctuations but an elevation at low frequencies. Furthermore, the convection velocity is observed to reduce downstream of all finlet treatments. The trailing-edge surface pressure spectrum results have shown that, in order to obtain maximum unsteady pressure reduction, the finlet spacing should be of the order of the thickness of the inner layer of the boundary layer. A thorough study is provided for understanding of the underlying physics of both categories of finlets and their implications for controlling the flow and noise generation mechanism near the trailing edge.

Development and Evaluation of a Long‐Term Data Record of Planetary Boundary Layer Profiles From Aircraft Meteorological Reports

AbstractHigh‐quality measurements of planetary boundary layer (PBL) profiles are important for advancing our understanding of land‐atmosphere coupling and PBL processes and improving the predictive capability of numerical models. Now with more than 10 years of observations from Aircraft Meteorological Data Reports (AMDAR), a decade‐long (from 2007 to 2016) data record of hourly PBL profiles is developed over 54 U.S. airports. Comparisons between the derived hourly profiles and nearby radiosonde data, which are typically only available twice a day (at 00 and 12 UTC), show good agreement between the two data sets. The root‐mean‐square errors (RMSEs) between the AMDAR and radiosonde data are found to be dependent on the distance between the two data sets, especially at lower levels. The RMSEs of temperature generally decrease as the altitude increases, while the RMSEs of specific humidity and wind are positively correlated with the measurement magnitude. At perfect collocations (i.e., the separation distance is zero), the seasonal RMSEs at measurement levels with pressure larger than 850 hPa are 1.16–1.52 K, 0.64–1.25 g/kg, and 2.00–2.26 m/s for temperature, humidity, and wind components, respectively. Compared to the RMSEs, the mean biases of AMDAR profiles are much smaller and are less dependent on the separation distance. The RMSEs show little dependence on flight phases (ascent or descent) in the lower troposphere. Overall, our results indicate that the aircraft measurements are accurate enough to be an alternative to the radiosonde data and are better suited for investigating the diurnal variation of PBL due to their higher temporal resolution.

Studies on Insertion Effects of Hot-Wire Probe upon the Velocity Measurement of Boundary Layer with Separation and Reattachment

Studies on Insertion Effects of Hot-Wire Probe upon the Velocity Measurement of Boundary Layer with Separation and Reattachment

Wind load assumptions for permeable cladding elements considering the installation context

Prestigious buildings are mainly characterized by the design and functionality of their cladding concept. The combination of closed and porous elements has become popular in recent years because it allows a concerted control of translucency and reflection by daylight and enables an illumination during night-time. In many cases, permeable wire meshes made of stainless steel are used for this aim and a wide variety of installation concepts exist. In order to determine realistic wind loads, the flow resistance of the mesh and its specific installation context has to be taken into account. In the present paper, fundamental investigations based on experiments in the boundary layer wind tunnel and measurements on-site are presented. These disclose the principal relations between mesh resistance and the integration thereof within the building skin, with respect to the characteristic load values. Based on these studies, load recommendations are formulated for buildings with single layer porous structures (like parking garages) as well as, for buildings with permeable cladding elements which have an offset to a closed inner façade.

Roughness effects of diatomaceous slime fouling on turbulent boundary layer hydrodynamics

Biofilm fouling significantly impacts ship performance. Here, the impact of biofilm on boundary layer structure at a ship-relevant, low Reynolds number was investigated. Boundary layer measurements were performed over slime-fouled plates using high resolution particle image velocimetry (PIV). The velocity profile over the biofilm showed a downward shift in the log-law region (ΔU+), resulting in an effective roughness height (ks) of 8.8 mm, significantly larger than the physical thickness of the biofilm (1.7 ± 0.5 mm) and generating more than three times as much frictional drag as the smooth-wall. The skin-friction coefficient, Cf, of the biofilm was 9.0 × 10−3 compared with 2.9 × 10−3 for the smooth wall. The biofilm also enhances turbulent kinetic energy (tke) and Reynolds shear stress, which are more heterogeneous in the streamwise direction than smooth-wall flows. This suggests that biofilms increase drag due to high levels of momentum transport, likely resulting from protruding streamers and surface compliance.

Development of a novel anemometry technique for velocity and temperature measurement

Hot-wire anemometry is a widely used measurement technique for obtaining local velocities or temperatures in a flow-field. Operated under constant temperature or constant current the small diameter wires can resolve high-frequency velocity or temperature fluctuations with a high degree of spatial and temporal resolution. Therefore, more or less sophisticated hardware, e.g. using a Wheatstone bridge for constant temperature anemometry (CTA), is used with relatively high acquisition costs. In this paper a new technique is used, where the wire temperature or current is digitally controlled via a FPGA unit. The electric circuit is simpler than the usual set-ups and the hardware used is very robust, allowing to operate the control unit under harsh environments. This allows also to reduce the cable lengths, and therefore, eventually to increase the frequency response of the hot wire. The application of this technique for measurements of aerodynamic and thermal boundary layers under varying Reynolds numbers for laminar and turbulent boundary layers is demonstrated. The results are compared to measurements with standard CTA system and show that similar results are obtained with this new technique.

Progresses in turbulent Rayleigh-Bénard convection over rough surfaces

Turbulent convection over a rough surface is a common scenario that one often encounters in nature and in many industrial processes. It is of great interest and especially useful to reveal the properties of this type of flow. In this paper, we present an overview of recent progresses in turbulent Rayleigh-Benard convection over rough surfaces, including how surface roughness enhances/reduces the global heat transfer through the system, whether roughness could be used as a route to the ultimate regime of thermal convection. We also point out the main remaining issues and challenges and giving an outlook on the future development. Rough surface to affect the turbulent transport need to further research, based on exquisite measurement of boundary layers. Rough surface causes plumes generating and separating, and at the same time evolution of one-roll state and multi-roll state in the large scale circulation, is seldom studied. Structure of rough surface lead to symmetry influence on flow structure, which requires further probe.

Fully resolved measurements of turbulent boundary layer flows up to

Fully resolved measurements of turbulent boundary layers are reported for the Reynolds number range $Re_{\unicode[STIX]{x1D70F}}=6000{-}20\,000$. Despite several decades of research in wall-bounded turbulence there is still controversy over the behaviour of streamwise turbulence intensities near the wall, especially at high Reynolds numbers. Much of it stems from the uncertainty in measurement due to finite spatial resolution. Conventional hot-wire anemometry is limited for high Reynolds number measurements due to limited spatial resolution issues that cause attenuation in the streamwise turbulence intensity profile near the wall. To address this issue we use the nano-scale thermal anemometry probe (NSTAP), developed at Princeton University to conduct velocity measurements in the high Reynolds number boundary layer facility at the University of Melbourne. The NSTAP has a sensing length almost one order of magnitude smaller than conventional hot-wires. This enables us to acquire fully resolved velocity measurements of turbulent boundary layers up to $Re_{\unicode[STIX]{x1D70F}}=20\,000$. Results show that in the near-wall region, the viscous-scaled streamwise turbulence intensity grows with $Re_{\unicode[STIX]{x1D70F}}$ in the Reynolds number range of the experiments. A second outer peak in the streamwise turbulence intensity is also shown to emerge at the highest Reynolds numbers. Moreover, the energy spectra in the near-wall region show excellent inner scaling over the small to moderate wavelength range, followed by a large-scale influence that increases with Reynolds number. Outer scaling in the outer region is found to collapse the energy spectra over high wavelengths across various Reynolds numbers.

Developments in acoustics for studying wave-driven boundary layer flow and sediment dynamics over rippled sand-beds

The processes of sediment entrainment, transport, and deposition over bedforms are highly dynamic and temporally and spatially variable. Obtaining measurements to understand these processes has led to ongoing developments in instrumentation for studying near-bed sediment dynamics, with the outputs applied to the development and assessment of sediment transport modelling. In the present study results are reported from three acoustic systems deployed to make observations of bedforms, bedload, suspended concentration and horizontal and vertical velocity components. To evaluate the instruments a series of near-bed boundary layer measurements were collected in a large scale flume facility over a rippled bed of medium sand under regular waves. The observations were conducted as part of Joint Research Activities within the EU funded Hydralab project. The suite of acoustic instruments consisted of a Bedform And Suspended Sediment Imager, BASSI, a three dimensional acoustic ripple profiler, 3D-ARP, and three Acoustic Concentration and Velocity Profilers, ACVP's. Here results are reported from the deployment of the instruments, to illustrate the ongoing developments in acoustics and the expanding capability of the application of acoustics to the study of near-bed flow and sediment dynamics.

Trailing-edge flow and noise control using porous treatments

This paper is concerned with the application of porous treatments as a means of flow and aerodynamic noise reduction. An extensive experimental investigation is undertaken to study the effects of flow interaction with porous media, in particular in the context of the manipulation of flow over blunt trailing edges and attenuation of vortex shedding. Comprehensive boundary layer and wake measurements have been carried out for a long flat plate with solid and porous blunt trailing edges. Unsteady velocity and surface pressure measurements have also been performed to gain an in-depth understanding of the changes to the energy–frequency content and coherence of the boundary layer and wake structures as a result of the flow interaction with a porous treatment. Results have shown that permeable treatments can effectively delay the vortex shedding and stabilize the flow over the blunt edge via mechanisms involving flow penetration into the porous medium and discharge into the near-wake region. It has also been shown that the porous treatment can effectively destroy the spanwise coherence of the boundary layer structures and suppress the velocity and pressure coherence, particularly at the vortex shedding frequency. The flow–porous scrubbing and its effects on the near-wall and large coherent structures have also been studied. The emergence of a quasi-periodic recirculating flow field inside highly permeable surface treatments has also been investigated. Finally, the paper has identified several important mechanisms concerning the application of porous treatments for aerodynamic and aeroacoustic purposes, which can help more effective and tailored designs for specific applications.

Vertical structures of physical and chemical properties of urban boundary layer and formation mechanisms of atmospheric pollution

Air quality in China has been continuously improved since the implementation of “Atmospheric Pollution Prevention and Control Action Plan” in 2013. However, the mass concentrations of fine particles particularly in Beijing, Tianjin and Hebei are still much higher than the National Ambient Air Quality Standard (35 μg m−3 as an annual average), and severely polluted events also occur occasionally in autumn and winter seasons. In recent years, Chinese research scientists have made significant progresses in vertical measurements of air pollution, formation mechanisms and source apportionment of atmospheric aerosol, and also the interactions between aerosol and atmospheric boundary layer, which greatly support the scientific prevention and control of atmospheric pollution. Despite this, many questions on atmospheric pollution have not been well addressed yet. This paper first summarizes the recent research progresses in vertical measurements of air pollution in China, in particular, the measurements from the research platform of the Beijing 325 m meteorological tower. In addition to the routine long-term measurements of boundary layer (turbulence, CO2 and H2O flux) and meteorological parameters (temperature, relative humidity, wind speed and wind direction) at 7 and 15 heights, respectively, this platform can have simultaneous real-time measurements of aerosol particle composition (organics, sulfate, nitrate, ammonium, chloride, and black carbon), gaseous species (CO, SO2, and O3), particle number size distributions, and optical properties (extinction and absorption coefficients) at ground level and 260 m, and also the continuously vertical measurements from ground to 260 m using the tower container equipped with specific instruments. Then, the vertical differences of aerosol composition in different seasons in Beijing, and their interactions with boundary layer are discussed. Also, the response of aerosol chemistry to regional source emission control are elucidated based on vertical measurements during the Asia-Pacific Economic Conference (APEC) summit in 2014 and the 70th anniversary parade in 2015. This paper also reviews the latest research progress on formation mechanisms of secondary inorganic aerosol (nitrate and sulfate) and secondary organic aerosol, and the life cycles of severe haze events in China. In particular, the heterogeneous formation of sulfate during severe haze episodes strongly depends on pH values of aerosol liquid water content. While techniques for direct measurements of aerosol particle pH are essential yet challenging, laboratory experiments are more needed now to prove the hypothesis of sulfate formation mechanisms. Compared with inorganic species, the formation and evolutionary mechanisms of secondary organic aerosol (SOA) are far more complex in polluted environments, and their roles in haze formation need to be better characterized. Finally, this paper lists some future recommendations for air pollution measurements (e.g., from long-term to vertical measurements in terms of significant emission changes in the near future), formation mechanism of secondary aerosol, and molecular composition and physical and chemical properties (phase, volatility, oxidation state, etc.) of organic aerosol.

Skin friction estimation in a strong decelerating flow

The paper presents the analysis of the turbulent boundary layer developed on a flat plate subjected to an Adverse Pressure Gradient (APG) and approaching separation. The aim of the study is to examine the effects of pressure gradient on a non-equilibrium boundary layer while indicating local areas of the equilibrium flow. The emphasis is on the analysis of mean flow velocity and the estimation of skin friction. It is known that accurate measurements of skin friction were considered as a difficult and demanding task despite of various measuring techniques available. A great challenge is especially the measurement of a strong decelerated turbulent boundary layer because of low shear stress and possible large measuring errors. To date, the oil film or oil drop interferometry technique, because of its high accuracy, has become a basis of turbulent-boundary-layer research. In our research, this technique has been used as a reference method for comparing with the traditional Clauser chart method, which generally is considered as not suitably for non-canonical flows. In the paper, however, a correction of the method is proposed, which allows one to increase its range of applicability. This corrected Clauser chart method (CCCM) involves only one iteration while other proposed in the literature methods employ a twofold iterative procedure. The comparison of the methods for the non-canonical turbulent boundary layer, i.e. adverse pressure gradient with a strong flow history effect has been presented. It has been shown that CCCM can be successfully used for small and medium pressure gradients, where the Clauser-Rotta pressure gradient parameter β does not exceed level close to 11.

Experimental Validation of a Boundary Layer Convective Heat Flux Measurement Technique

If a steady thermal boundary layer is sufficiently thick, wall heat fluxes and associated convective heat transfer coefficients can be directly calculated from measured temperature distributions taken within it using a traversing thermocouple probe. The boundary layer can be laminar, turbulent, or transitional and on a surface of arbitrary surface temperature distribution and geometry. Herein, this technique is presented and validated in a steady, turbulent, two-dimensional boundary layer on a flat, uniform-heat-flux wall. Care is taken to properly account for radiation from the wall and conduction within the thermocouple wire. In the same setting, heat flux measurements are made for verification purposes using an energy balance on a segment of the test wall carefully designed to minimize and include radiation and conduction effects. Heat flux values measured by the boundary layer measurement technique and by the energy balance measurement agree to within 4.4% and the difference between the two lie completely within their respective measurement uncertainties of 5.74% and 0.6%.

Mean turbulence statistics in boundary layers over high-porosity foams

This paper reports turbulent boundary layer measurements made over open-cell reticulated foams with varying pore size and thickness, but constant porosity ($\unicode[STIX]{x1D716}\approx 0.97$). The foams were flush-mounted into a cutout on a flat plate. A laser Doppler velocimeter (LDV) was used to measure mean streamwise velocity and turbulence intensity immediately upstream of the porous section, and at multiple measurement stations along the porous substrate. The friction Reynolds number upstream of the porous section was $Re_{\unicode[STIX]{x1D70F}}\approx 1690$. For all but the thickest foam tested, the internal boundary layer was fully developed by ${<}10\unicode[STIX]{x1D6FF}$ downstream from the porous transition, where $\unicode[STIX]{x1D6FF}$ is the boundary layer thickness. Fully developed mean velocity profiles showed the presence of a substantial slip velocity at the porous interface (${>}30\,\%$ of the free-stream velocity) and a mean velocity deficit relative to the canonical smooth-wall profile further from the wall. While the magnitude of the mean velocity deficit increased with average pore size, the slip velocity remained approximately constant. Fits to the mean velocity profile suggest that the logarithmic region is shifted relative to a smooth wall, and that this shift increases with pore size until it becomes comparable to substrate thickness $h$. For all foams, the turbulence intensity was found to be elevated further into the boundary layer to $y/\unicode[STIX]{x1D6FF}\approx 0.2$. An outer peak in intensity was also evident for the largest pore sizes. Velocity spectra indicate that this outer peak is associated with large-scale structures resembling Kelvin–Helmholtz vortices that have streamwise length scale $2\unicode[STIX]{x1D6FF}{-}4\unicode[STIX]{x1D6FF}$. Skewness profiles suggest that these large-scale structures may have an amplitude-modulating effect on the interfacial turbulence.

Correction to: Design and implementation of a hot-wire probe for simultaneous velocity and vorticity vector measurements in boundary layers

It has been discovered that the original manuscript presented data that had a sign error associated with the application of Taylor’s frozen turbulence hypothesis.

Towards fully-resolved PIV measurements in high Reynolds number turbulent boundary layers with DSLR cameras

In this study, we describe a multi-camera large field-of-view (FOV) planar-PIV experiment to capture the wide range of scales that coexist in high Reynolds number turbulent boundary layers. The proposed measurements are designed to capture spatial flow features over a greater range than current common practices, and at significantly lower cost. With this goal in mind, specialist PIV cameras are substituted with modern consumer full-frame digital cameras, which are typically available at a fraction of the cost, with higher resolution sensors. These cameras are configured to capture single-frame double-exposed images (DE-PIV), but at a much higher spatial resolution than what is available from specialist PIV cameras that capture double-frame single-exposure images (SE-PIV). This work discusses a set of simulations and experiments to quantitatively assess the quality of the PIV velocity fields from these two approaches for large field-of-view measurements. Our findings confirm that despite the known loss-of-accuracy associated with DE-PIV, the use of high-resolution cost-effective consumer cameras provides an economically feasible PIV solution with the necessary performance and accuracy for high spatial range measurements in wall-bounded turbulent flows.

Design and implementation of a hot-wire probe for simultaneous velocity and vorticity vector measurements in boundary layers

A multi-sensor hot-wire probe for simultaneously measuring all three components of velocity and vorticity in boundary layers has been designed, fabricated and implemented in experiments up to large Reynolds numbers. The probe consists of eight hot-wires, compactly arranged in two pairs of orthogonal ×-wire arrays. The ×-wire sub-arrays are symmetrically configured such that the full velocity and vorticity vectors are resolved about a single central location. During its design phase, the capacity of this sensor to accurately measure each component of velocity and vorticity was first evaluated via a synthetic experiment in a set of well-resolved DNS fields. The synthetic experiments clarified probe geometry effects, allowed assessment of various processing schemes, and predicted the effects of finite wire length and wire separation on turbulence statistics. The probe was subsequently fabricated and employed in large Reynolds number experiments in the Flow Physics Facility wind tunnel at the University of New Hampshire. Comparisons of statistics from the actual probe with those from the simulated sensor exhibit very good agreement in trend, but with some differences in magnitude. These comparisons also reveal that the use of gradient information in processing the probe data can significantly improve the accuracy of the spanwise velocity measurement near the wall. To the authors’ knowledge, the present are the largest Reynolds number laboratory-based measurements of all three vorticity components in boundary layers.

The heat/mass transfer analogy for a backward facing step

Heat and mass transfer from a surface to a stream of fluid are governed by Fourier’s law and Fick’s law respectively,which are mathematical manifestations of the process of diffusion. In the realm of transport processes, the mathematical equations describing the two phenomena can become analogous under certain assumptions and boundary conditions.From an engineering perspective, it is difficult to measure heat transfer coefficients in separated flows because of high spatial thermal gradients and the intrusive nature of the various techniques. The analogous mass transfer measurement using the naphthalene sublimation, on the other hand, overcomes these challenges and presents significant advantages of speed, economy, better resolution and accuracy over its heat transfer counter-part. However, quantitatively, the diffusion rates of heat in air and naphthalene in air are different. So, the physical and mathematical similarity between the two processes can be utilized effectively only when the analogy factor (F=Nu/Sh) is determined.This study investigates the heat/mass transfer analogy in a turbulent separated flow behind a backward facing step. The heat (Nu) and mass (Sh) transfer measurements were made using the thermal boundary layer measurement technique and the naphthalene sublimation measurement respectively under near identical flow conditions. Analogous boundary conditions of uniform temperature and constant concentration were imposed on the active surfaces, which is the recirculation-reattachment region behind the backward facing step.The Nu and Sh values thus obtained were used to calculate the analogy factor, F, which was found to be 0.692.

Reynolds number and roughness effects on turbulent stresses in sandpaper roughness boundary layers

Multicomponent turbulence measurements in rough-wall boundary layers are presented and compared to smooth-wall data over a large friction Reynolds number range (δ+). The rough-wall experiments used the same continuous sandpaper sheet as in the study of Squire et al. [J. Fluid Mech. 795, 210 (2016)JFLSA70022-112010.1017/jfm.2016.196]. To the authors' knowledge, the present measurements are unique in that they cover nearly an order of magnitude in Reynolds number (δ+≃2800-17400), while spanning the transitionally to fully rough regimes (equivalent sand-grain-roughness range, ks+≃37-98), and in doing so also maintain very good spatial resolution. Distinct from previous studies, the inner-normalized wall-normal velocity variances, w2, exhibit clear dependencies on both ks+ and δ+ well into the wake region of the boundary layer, and only for fully rough flows does the outer portion of the profile agree with that in a comparable δ+ smooth-wall flow. Consistent with the mean dynamical constraints, the inner-normalized Reynolds shear stress profiles in the rough-wall flows are qualitatively similar to their smooth-wall counterparts. Quantitatively, however, at matched Reynolds numbers the peaks in the rough-wall Reynolds shear stress profiles are uniformly located at greater inner-normalized wall-normal positions. The Reynolds stress correlation coefficient, Ruw, is also greater in rough-wall flows at a matched Reynolds number. As in smooth-wall flows, Ruw decreases with Reynolds number, but at different rates depending on the roughness condition. Despite the clear variations in the Ruw profiles with roughness, inertial layer u, w cospectra evidence invariance with ks+ when normalized with the distance from the wall. Comparison of the normalized contributions to the Reynolds stress from the second quadrant (Q2) and fourth quadrant (Q4) exhibit noticeable differences between the smooth- and rough-wall flows. The overall time fraction spent in each quadrant is, however, shown to be nearly fixed for all of the flow conditions investigated. The data indicate that at fixed δ+ both Q2 and Q4 events exhibit a sensitivity to ks+. The present results are discussed relative to the combined influences of roughness and Reynolds number on the scaling behaviors of boundary layers.