Turbulent Boundary Layer Measurements Research Articles

A micro-hot-wire anemometry probe with elongated stubs for turbulent boundary layer measurements

A micro-hot-wire anemometry probe with elongated stubs for turbulent boundary layer measurements

Stochastic modelling of the instantaneous velocity profile in rough-wall turbulent boundary layers

The statistical properties of uniform momentum zones (UMZs) are extracted from laboratory and field measurements in rough wall turbulent boundary layers to formulate a set of stochastic models for the simulation of instantaneous velocity profiles. A spatiotemporally resolved velocity dataset, covering a field of view of $8 \times 9\,{\rm m}^2$ , was obtained in the atmospheric surface layer using super-large-scale particle image velocimetry (SLPIV), as part of the Grand-scale Atmospheric Imaging Apparatus (GAIA). Wind tunnel data from a previous study are included for comparison (Heisel et al., J. Fluid Mech., vol. 887, 2020, R1). The probability density function of UMZ attributes such as their thickness, modal velocity and averaged vertical velocity are built at varying elevations and modelled using log-normal and Gaussian distributions. Inverse transform sampling of the distributions is used to generate synthetic step-like velocity profiles that are spatially and temporally uncorrelated. Results show that in the wide range of wall-normal distances and $Re_\tau$ up to $\sim O(10^6)$ investigated here, shear velocity scaling is manifested in the velocity jump across shear interfaces between adjacent UMZs, and attached eddy behaviour is observed in the linear proportionality between UMZ thickness and their wall normal location. These very same characteristics are recovered in the generated instantaneous profiles, using both fully stochastic and data-driven hybrid stochastic (DHS) models, which address, in different ways, the coupling between modal velocities and UMZ thickness. Our method provides a stochastic approach for generating an ensemble of instantaneous velocity profiles, consistent with the structural organisation of UMZs, where the ensemble reproduces the logarithmic mean velocity profile and recovers significant portions of the Reynolds stresses and, thus, of the streamwise and vertical velocity variability.

Boundary layer measurements of the flow along a streamwise-oriented circular cylinder using particle tracking velocimetry

Particle tracking velocimetry (PTV) measurements of an axisymmetric turbulent boundary layer (ATBL) around a long circular cylinder in axial flow were carried out in a large-scale water tunnel at different velocities and at different measurement positions along the cylinder. A relatively thick ATBL developed with ratios of δ/a between 4.6 and 8.1 and a/δν between 313 and 1027, where δ is the boundary layer thickness, δν the viscous length scale and a the cylinder radius. At ratios of δ/a above unity and a/δν above 250, transverse curvature effects are known to influence the boundary layer in the outer region. This was observed in the measured mean velocity profiles as the profiles fall below the planar logarithmic law of the wall. Shape factors close to unity with a rather full velocity profile were determined that explain the relatively high skin friction coefficient known to be present for such ATBL flows. Additionally, measurements of the turbulence intensity and Reynolds stress profiles are presented. The non-intrusive PTV measurement approach allowed to gather near-wall velocity data down to a wall-distance below 10 wall-units. The present study provides valuable information regarding ATBL flows along streamwise-oriented cylinders in the flow regime indicated above by the ranges of δ/a and a/δν. In particular, it fills a gap in the literature as far as experimentally obtained higher-order statistical quantities are concerned, such as root-mean-square velocity fluctuations and Reynolds stresses.

Perturbative model for the second-order velocity structure function tensor in turbulent shear flows

A model for the second order velocity structure function tensor, based on Kolmogorov's theory and valid for the inertial range, is proposed, incorporating the effect of anisotropy as a linear perturbation to the standard isotropic form. Structure functions measured from direct numerical simulations of channel flow and from experimental measurements in turbulent boundary layers are compared with predicted behavior and reasonable agreement is found.

Extension of the KDO turbulence/transition model to account for roughness

Wall roughness significantly influences both laminar-turbulent transition process and fully developed turbulence. A wall roughness extension for the KDO turbulence/transition model is developed. The roughness effect is introduced via the modification of the k and νt boundary conditions. The wall is considered to be lifted to a higher position. The difference between the original position and the higher position, named as equivalent roughness height, is linked to the actual roughness height. The ratio between the two heights is determined by reasoning. With such a roughness extension, the predictions of the KDO RANS model agree well with the measurements of turbulent boundary layer with a sand grain surface, while the KDO transition model yields accurate cross-flow transition predictions of flow past a 6:1 spheroid.

Method to minimize polymer degradation in drag-reduced non-Newtonian turbulent boundary layers

Polymer solutions are often used to produce drag-reduced fluid flows, in which the drag reduction is achieved due to the solutions’ non-Newtonian shear-thinning and viscoelastic properties. However, experiments using polymer solutions are typically challenging due to the tendency of the polymer to degrade when subjected to intense shearing. The degradation reduces the amount of drag reduction as the experiment progresses, which limits the experiment duration and the accuracy of the results. Here we introduce a method to avoid the degradation of the polymer solution by driving the flow with a paddlewheel instead of a conventional pump. The solution is shown to undergo very little degradation during the paddlewheel’s operation. The method is then applied to perform novel measurements of a drag-reduced turbulent boundary layer at two different Reynolds numbers, both with and without a suspended particle phase. The effects of carrier fluid rheology and Reynolds number on the particle concentration and velocity profiles are explored, as well as the effect on total drag of the flow. For a given fluid type and Reynolds number, the drag is found to be nearly constant with the global particle volume fraction, suggesting that the particles have a limited ability to modulate the drag. Remarkably, the particle velocity fluctuations are greater in the non-Newtonian cases, possibly due to enhanced collisions in the near-wall region.

Research on velocity measurements of the hypersonic turbulent boundary layer based on the nano-tracer-based planar laser scattering technique

As one of the most typical flow structures in hypersonic flow, the hypersonic turbulent boundary layer (TBL) is of great significance in the optimization of the aerodynamic configuration design of hypersonic vehicles. The nano-tracer-based planar laser scattering technique was utilized to obtain velocity results of the fully developed hypersonic (Mach 6.0) TBL. The scale-invariant feature transformation algorithm was used to obtain velocity information. Based on the results of mean velocity and fluctuating velocity distribution, the feasibility of this method has been verified.

Volumetric measurements of a self-similar adverse pressure gradient turbulent boundary layer using single-camera light-field particle image velocimetry

As a novel volumetric particle image velocimetry technique, single-camera light-field PIV (LF-PIV) is able to acquire three-dimensional flow fields through a single camera. Compared with other multi-camera 3D PIV techniques, LF-PIV has distinct advantages, including concise hardware setup and low optical access requirements. Its capability has proven effective in many experimental investigations. In this study, the use of LF-PIV in measuring a self-similar adverse pressure-gradient turbulent boundary layer (APG-TBL) is demonstrated. Experiments are performed in a large water tunnel at the Laboratory for Turbulence Research in Aerospace and Combustion (LTRAC), Monash University. Sets of 250 light-field PIV image pairs are captured covering both the inner and outer regions of the boundary layer. Instantaneous 3D velocity fields are reconstructed using a GPU accelerated density ray tracing multiplicative reconstruction technique (DRT-MART) and three-dimensional cross-correlation methods. The LF-PIV results are compared with two-dimensional PIV (2D-PIV) measurements of the same flow. Comparable accuracy to 2D-PIV is achieved for first- and second-order velocity statistics above approximately $$ y/\delta_{1} = 1 $$ .

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.

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.

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.

An experimental investigation of an unsteady adverse pressure gradient turbulent boundary layer: embedded shear layer scaling

An experimental investigation of an unsteady adverse pressure gradient turbulent boundary layer is described. It is demonstrated that the local flow physics is largely dominated by an inflectional instability which gives rise to an embedded shear layer contained within the boundary layer. Experimental measurements are presented which are fully consistent with the presence of clockwise spanwise-oriented coherent vorticity within the embedded shear layer. Using embedded shear layer scaling parameters in the form of the shear layer vorticity thickness and the velocity defect at the upper inflection point, both the mean and the phase-averaged boundary layer velocity profiles exhibit similarity in both space and time over a large wall-normal extent. In a similar manner, the profiles of the streamwise-component turbulence intensity and Reynolds stress also exhibit similarity when scaled with the embedded shear layer parameters. The embedded shear layer scaling of previously published adverse pressure gradient turbulent boundary layer measurements confirms its generic applicability in a wide range of flow-field geometries and extending to high Reynolds numbers.

Image pre-processing method for near-wall PIV measurements over moving curved interfaces

PIV measurements near a moving interface are always difficult. This paper presents a PIV image pre-processing method that returns high spatial resolution velocity profiles near the interface. Instead of re-shaping or re-orientating the interrogation windows, interface tracking and an image transformation are used to stretch the particle image strips near a curved interface into rectangles. Then the adaptive structured interrogation windows can be arranged at specified distances from the interface. Synthetic particles are also added into the solid region to minimize interfacial effects and to restrict particles on both sides of the interface. Since a high spatial resolution is only required in high velocity gradient region, adaptive meshing and stretching of the image strips in the normal direction is used to improve the cross-correlation signal-to-noise ratio (SN) by reducing the velocity difference and the particle image distortion within the interrogation window. A two dimensional Gaussian fit is used to compensate for the effects of stretching particle images. The working hypothesis is that fluid motion near the interface is ‘quasi-tangential flow’, which is reasonable in most fluid-structure interaction scenarios. The method was validated against the window deformation iterative multi-grid scheme (WIDIM) using synthetic image pairs with different velocity profiles. The method was tested for boundary layer measurements of a supersonic turbulent boundary layer on a flat plate, near a rotating blade and near a flexible flapping flag. This image pre-processing method provides higher spatial resolution than conventional WIDIM and good robustness for measuring velocity profiles near moving interfaces.

Experimental study on the density characteristics of a supersonic turbulent boundary layer

An experimental study on the density characteristics of a zero-pressure-gradient flat plate turbulent boundary layer at Ma=3.0 is performed by the density field measurement method based on Nano-tracer planar laser scattering (NPLS) technology. The mean and the fluctuating characteristics of the density field of the boundary layer are analyzed. And the spectrum analyses of density fluctuations are performed by utilizing Taylor's hypothesis to convert spatial measurements into pseudo-temporal measurements. The mean density profile increases away from the wall, which accords well with the density profile deduced from the mean velocity distribution by using the adiabatic Crocco-Busemann relation. The root mean square (RMS) of the density fluctuations increases in the logarithmic region with a peak value of 0.2ρ∞, and its probability density distribution follows a normal distribution. However, the RMS of density fluctuations decreases in the outer region of the boundary layer. According to the spectrum analysis, the density fluctuations are characterized in a wide range of frequencies throughout the boundary layer, with the maximum frequency on the order of 1 MHz. The low frequency fluctuations are predominant near the wall and in the outer region of the turbulent boundary layer. However, the proportion of high-frequency fluctuations is nearly equal to that of low-frequency fluctuations in the logarithmic region. The combined NPLS and PIV technique provide a simultaneous density and velocity measurements of the present turbulent boundary layer. The high frequency fluctuations in the supersonic turbulent boundary layer may be induced by the density fluctuations, which are caused by the convection of the turbulent structures with nonuniform density distributions. And the contribution of the velocity fluctuations only to the low frequency fluctuations is observed. There are good similarities between the density fluctuations and the mass flux fluctuations for both the probability density distribution and the spectrum characteristics. On the contrary, a large difference between the fluctuations of velocity and density is identified. Therefore, the strong density fluctuations inside supersonic turbulent boundary layers, as well as its difference between the velocity fluctuations, should be one of the most important differences between compressible and incompressible turbulent boundary layers.

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.

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 < Reθ < 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.

Comparison of PIV and Hot-Wire statistics of turbulent boundary layer

The paper shows a cross checking of turbulent boundary layer measurements using large field of view PIV and hot-wire anemometry techniques. The time-resolved PIV method was used for the experiments. The measuring plane was oriented perpendicularly to the wall and parallel to the mean flow. Hot wire measurement has been performed using the special probe with perpendicular hot wire. The HW point measurements were performed in the same place as PIV experiments. The hot-wire probe has the wire length of l+ < 20 in considered range of Reynolds numbers. Various evaluation methods were applied on PIV data. The profiles of statistical characteristics of streamwise velocity components were evaluated from the data. Mean values, standard deviations as well as skewness and kurtosis coefficients were compared for a few values of Reθ. Reynolds number ranges from 1000 to 5500. The result shows that with the increasing Reynolds number the attenuation of fluctuations maximum in PIV measurements occurs with respect to Hot-Wire measurements, however representation of velocity fluctuations using the PIV method is satisfactory. The influence of wall-normal fluctuation component on Hot-Wire near wall peak was also investigated.

Obtaining accurate mean velocity measurements in high Reynolds number turbulent boundary layers using Pitot tubes

AbstractThis article reports on one component of a larger study on measurement of the zero-pressure-gradient turbulent flat plate boundary layer, in which a detailed investigation was conducted of the suite of corrections required for mean velocity measurements performed using Pitot tubes. In particular, the corrections for velocity shear across the tube and for blockage effects which occur when the tube is in close proximity to the wall were investigated using measurements from Pitot tubes of five different diameters, in two different facilities, and at five different Reynolds numbers ranging from ${\mathit{Re}}_{\theta } = 11\hspace{0.167em} 100$ to 67 000. Only small differences were found amongst commonly used corrections for velocity shear, but improvements were found for existing near-wall proximity corrections. Corrections for the nonlinear averaging of the velocity fluctuations were also investigated, and the results compared to hot-wire data taken as part of the same measurement campaign. The streamwise turbulence-intensity correction was found to be of comparable magnitude to that of the shear correction, and found to bring the hot-wire and Pitot results into closer agreement when applied to the data, along with the other corrections discussed and refined here.

Acoustic Doppler Velocimeter in Measurements of Turbulent Boundary Layer Flows

Acoustic Doppler Velocimeter (ADV)’s applications in measuring velocity profiles of turbulent boundary layers (TBLs) are investigated at a flow Reynolds number of 36000 in a lab-made open channel. The 3D instantaneous velocity signals at different heights of TBLs are obtained, which present very strong abnormal noises in the near wall regions because of the aliasing of Doppler signal spectra. The minimum/maximum threshold filter is adopted for the signal to remove the noises reflected by walls. The mean streamwise velocities and turbulence intensities profiles are then calculated correctly. The measuring results consist with the law of turbulent boundary layer flow. The ADV has a good performance in measurements of TBLs with a simple implementation of filter to the sampled velocity signals. The present research is guidable for applications of ADV in practice.