A comprehensive set of experiments were performed to document the separated flow over a three-dimensional (3-D) bump with the purpose of generating a benchmark experimental database useful in validating computational fluid dynamics flow simulations and improving model development. The emphasis of this manuscript is on the 3-D topographical and topological features of the separated flow that forms downstream of the bump and its sensitivity to upstream flow conditions. The bump model geometry was designed to provide well-defined and repeatable smooth-body flow separation conditions that were suitable for both experiments and simulations. The bump had a Gaussian streamwise profile with a constant maximum height equal to 8.5 % of its width over the central 60 % of its span. The remaining 40 % were outboard spanwise portions that gradually taper to zero using an error function profile to minimize tunnel sidewall boundary layer interaction effects. The model was immersed in a canonical turbulent boundary layer that was developed on a suspended flat plate in the Notre Dame Mach 0.6 closed-circuit wind tunnel. To document the effect of the incoming boundary layer thickness on the flow separation, the bump model could be located at two streamwise positions. The measurements of the flow separation region included fluorescent surface flow visualization, wall shear stress using oil-film interferometry, mean and dynamic surface pressure, hot-wire anemometry and planar and stereoscopic particle image velocimetry. It is shown that the surface flow separation topology is characterized by the `owl-face pattern of the first kind'. This flow topology consists of four singular points - two saddle points at the bump centrespan and two foci located at a spanwise-symmetric position. It is shown that the spanwise separation of the twin foci increases with Reynolds number indicating a corresponding increase in the spanwise extent of the flow separation. The two surface foci represent the footprint of vortices that lift off the ramp surface and form an arch vortex time-mean off-surface flow topology aft of the bump.

Experiments were performed that (i) document the effect of the steady spanwise buffer layer blowing on the mean characteristics of the turbulent boundary layer for a range of momentum thickness Reynolds numbers from 4760 to 10 386, and (ii) document the effect of the buffer layer blowing on the unsteady characteristics and coherent vorticity in a boundary layer designed to provide sufficiently high spatial resolution. The spanwise buffer layer blowing of the order of $u_{\tau }$ is produced by a surface array of pulsating direct current (pulsed-DC) plasma actuators. This was found to substantially reduce the wall shear stress that was directly measured with a floating element coupled with a force sensor. The direct wall shear measurements agreed with values derived using the Clauser method to within $\pm 0.85$ %. The degree to which the buffer layer blowing affected $\tau _w$ was found to primarily depend on the inner variable spanwise spacing between the pulsed-DC actuator electrodes, i.e. ‘blowing sites’. Utilizing pairs of $[u,v]$ and $[u,w]$ hot-wire sensors, the latter experiments correlated significant reductions in the $\omega _y$ and $\omega _x$ vorticity components that resulted from the buffer layer blowing and translated into lower Reynolds stresses and turbulence production. The time scale to which these observed changes in the boundary layer characteristics would return to the baseline condition was subsequently documented. This revealed a recovery length of $x^+ \approx 86\,000$ that translated to a streamwise fetch of $x \approx 66\delta$ . Finally, a comparison with the recent work by Cheng et al. (2021, J. Fluid Mech. vol. 918, A24) and Wei & Zhou (2024 in TSFP13 , June 25–28, 2024) that followed our experimental approach to achieve comparable wall shear stress (drag) reductions has led to a new scaling based on the baseline boundary layer $\textit{Re}_{\tau }$ and buffer layer blowing velocity.

A novel photogrammetry procedure is proposed for projecting 3D surface oil flow visualizations from 2D images over an arbitrary computer-aided design (CAD) model of a 3D surface. The technique yields a 3D view of the oil flow pattern on the model surface from different view angles and allows rendering it along with off-surface particle image velocimetry (PIV) flow field measurements. The technique is based on a recently developed simplified photogrammetry procedure proposed by Gluzman (Exp Fluids 63:118, 2022), which is modified to account for the full, large flow field over the 3D surface. The procedure requires only two reference markers on the given surface geometry CAD model, a small checkerboard placed on a flat surface near the 3D geometry, and a single camera that should capture at least three images after the experiment from any view angle in order to calibrate the projection. The proposed procedure allows removing the elaborate steps typically required in photogrammetry while retaining the same measurement accuracy. In particular, we demonstrate the application of the technique to study turbulent smooth-body flow separation over a Gaussian bump geometry in Gray (Part I-Exp Investig, 2023). Our photogrammetric procedure allows us to obtain a 3D view from different angles of the surface oil flow pattern warped over the CAD model geometry and coupled with stereo PIV plane flow field cuts, which provides important insights into the character of the flow separation.

The results of an experimental investigation of smooth-body adverse pressure gradient (APG) turbulent boundary layer flow separation and reattachment over a two-dimensional ramp are presented. These results are part of a larger archival smooth-body flow separation data set acquired in partnership with NASA Langley Research Center and archived on the NASA Turbulence Modeling Resource website. The experimental geometry provides initial canonical turbulent boundary layer growth under nominally zero pressure gradient conditions prior to encountering a smooth, two-dimensional, backward facing ramp geometry onto which a streamwise APG that is fully adjustable is imposed. Detailed surface and off-surface flow field measurements are used to fully characterize the smooth-body APG turbulent boundary layer separation and reattachment at multiple spanwise locations over the ramp geometry. Unsteady aspects of the flow separation are characterized. It is shown that the first and second spatial derivatives of the streamwise static surface pressure profile are sufficient to determine key detachment and reattachment locations. The imposed streamwise APG gives rise to inflectional mean velocity profiles and the associated formation of an embedded shear layer, which is shown to play a dominant role in the subsequent flow development. Similarity scaling is developed for both the mean velocity and turbulent stresses that is found to provide self-similar collapse of profiles for different regions of the ramp flow. Despite the highly non-equilibrium flow environment, a new similarity scaling proved capable of providing self-similar turbulent stress profiles over the full streamwise extent of flow separation and downstream reattachment.

The dynamic response of a zero-pressure gradient turbulent boundary layer (TBL) to an active flow control actuator was experimentally investigated. The experimental TBL had a sufficiently low momentum thickness Reynolds number, such that there were no energetic organized large-scale structures in the outer region, as evidenced from measurements of the premultiplied wavenumber–frequency spectra. The periodically pulsed plasma actuator, placed inside the outer region of TBL, introduced a synthetic large-scale structure, and the boundary-layer response to this synthetic structure downstream of the actuator at select wall-normal and streamwise locations was investigated. The turbulence amplitude modulating effect of the synthetic large-scale structure, within the inner and log-linear regions of the boundary layer, was isolated and analyzed using a phase-locked analysis. The dynamic interaction of the synthetic large-scale structure and smaller-scale turbulent motions was quantified using a modulation coefficient, and a strong positive correlation within the inner and log regions of the boundary layer was measured. The streamwise development of the synthetic large-scale structure and its modulating effect on the near-wall turbulence across several streamwise locations are described. Profiles of the phase speed at these streamwise locations were extracted and were found to be constant within the log region, further confirming a strong coupling between the near-wall fluctuations in turbulence intensity and the synthetic large-scale motions introduced in the outer region. Overall, the turbulence amplitude modulation effect induced by the synthetic large-scale structure was found to be dynamically similar to the large-scale modulation measured in canonical TBLs at higher Reynolds numbers.

This study focuses on the details of the geometry and dynamics of sidewall vortices observed in supersonic wind tunnels with a rectangular cross section of the nozzle and the test section. The formation of sidewall vortices limits the accuracy of the data measured during wind tunnels' testing due to a reduced area of uniform core flow results. Most of the test data presented in this work are generated using Mie scattering visualization for M = 4 flow, with CO2 seeded up to 7% mole fraction. The Mie scattering results are complemented by data from fast pressure sensor and schlieren visualization. It is shown that the formation of vortices is caused by a transverse pressure gradient realized in the supersonic nozzle due to the gas under-expansion. The vortex external mixing layer is strongly perturbed in time but remains globally geometrically similar with streamwise distance. The vortex-generated dominant flow disturbances are in the frequency range of f = 10–50 kHz, doubling the magnitude of baseline power spectral density. The authors' viewpoint is that sidewall vortex generation is a more generic phenomenon than was thought previously.

Experiments were performed to document changes in and recovery of the turbulence characteristics of a boundary layer whose viscous drag has been reduced by 62%. The drag reduction involves generating a steady spanwise velocity component on the order of uτ, within the sublayer using an array of pulsed-DC plasma actuators. Emphasis is placed on changes in coherent 3-D vortical motions associated with near-wall turbulence production that result from the drag reduction and the temporal response of these structures during and following drag reduction. This involves measurements of all three velocity components in a 3-D region within the boundary layer using a pair of dual [u, v] and [u, w] hot-wire sensors during and following actuation shutoff as the boundary layer recovers to its baseline state. Under reduced drag, these documented a decrease in the turbulence intensity levels of all three velocity components through most of the boundary layer. Conditional sampling is used to construct the coherent 3-D vortical structures associated with “burst-sweep” events. These revealed a reduction of the wall normal vorticity component, ωy, associated with the mean flow distortion caused by quasi-steady streamwise vorticity associated with the wall streak structure[Kline et al., 1967]. The significance of the ωy component comes from Schoppa and Hussain [2002] who proposed it to be a critical parameter in an autonomous mechanism for self-sustained wall turbulence generation. The results support this mechanism in both the observed suppression of the near-wall turbulence producing events, and the resulting decrease in the viscous drag. After quantifying the drag-reduced boundary layer in the drag-reduced space, the temporal response of terminating the actuation was studied to determine how physical properties such as the wall-layer streamwise velocity fluctuations, frequency of “burst-sweep” events, spatially-averaged ωy levels, and the y+ center return from the drag-reduced state to the baseline levels.

This study is focused on the characterization and modeling of aviation fuel cavitation physics in radial flow in a thin layer between two disks—a geometry highly relevant to aviation fuel pump systems. This involves a lower circular disk with a centrally located fuel injection port and a matching disk resting on top of the lower disk. In the described experiments, we have quantified and compared cavitating disk behavior for distilled water and JP-5 fuel at various inlet supply pressures via high-speed imaging and radial pressure measurements. High-speed imaging data were used to quantify the radial collapse location of cavitation voids. An enhanced gradient shadowgraphy method was employed to obtain the spatial–temporal evolution of propagating bubbly shock waves. This technique revealed unsteady shock waves propagating in a spiral motion in JP-5 fuel, while a standing bubbly shock was observed in water. In our modeling efforts, the Rayleigh–Plesset equation was adapted to a spatial form in order to predict the radial location of cavitation bubble collapse. Further work incorporated the spatial Rayleigh–Plesset equation into the barotropic model that has been used previously for cavitating nozzle flows and generalized it to radial flow geometry in order to reproduce radial pressure profiles obtained from the experiment. The model predictions of the radial location of bubble collapse and the radial pressure profiles are shown to be in excellent agreement with the experiments. This approach will be valuable for predicting aviation fuel cavitation in complex fuel system geometries.

The dynamic response of a zero pressure gradient turbulent boundary layer (TBL) to an active flow control actuator was experimentally studied using continous laser particle imaging velocimetry (PIV). In previous experiments using a single hot-wire, it was shown that the synthetic large-scale structure (LSS) introduced by the plasma-based actuator, located in the outer region of TBL, had a strong modulating and reorganizing effect on the near-wall turbulence. In the study reported here, an actuator, optimized for the experimental TBL, was placed at the upper boundary of the log-linear region of the TBL to produce a spanwise uniform, periodic synthetic LSS in order to study the response of the TBL to these large-scale perturbations. Planar PIV over a narrow streamwise region was used to measure the time-resolved, two-dimensional velocity downstream of the actuator at a series of streamwise point locations. Using PIV, the modulating effect of the synthetic LSS on the near-wall turbulence is described in more detail. The results are discussed and compared with previous hot-wire measurements and numerical simulations of the actuated TBL.

This paper reports the results of experiments in which flush-mounted pulsed-direct-current plasma actuator arrays were used to achieve skin-friction drag reduction on a NACA 63012A airfoil at a zero attack angle over the Mach number range of [Formula: see text] corresponding to a chord Reynolds number range of [Formula: see text]. The objective of the experiments was to document both the level of drag reduction and the net power savings achievable over this Mach number range. The arrays were designed to produce a spanwise array of opposed wall jets confined to the turbulent boundary-layer buffer region. These served to intervene in the autonomous near-wall mechanism that produces buffer-layer streamwise vorticity and is largely responsible for high levels of friction drag. It was demonstrated that the actuator array produces unprecedented levels of friction drag reduction with significant net power savings. Perhaps most interesting was the observation that the actuator authority has to be reduced for increased Mach numbers in order to keep the wall jets confined to the turbulent boundary-layer buffer region. This gave rise to a net power savings that scaled as [Formula: see text]. Unlike many previous experiments with plasma actuators, these results show that this form of flow control would not be authority limited in actual flight applications.

Correction NoticeThe sentence, "The reduced separation extent in the latter is attributed to the larger momentum thickness which makes the boundary layer more resistant to the shear layer instability that begins growing in the APG region, ultimately leading to TBL separation.", in the first paragraph of subsubsection 2 (Centerline pressure distribution: sensitivity to bump location) of subsection B (Surface static pressure) of section III (Results and Discussion) should be removed.

View Video Presentation: https://doi.org/10.2514/6.2022-1209.vid This study was motivated by the need for a benchmark data set for the characterization of smooth body separation over a wall-mounted, Gaussian surface. Skin friction values were computed using a modified oil film interferometry technique that implements a flexible calibration board to correct for perspective and geometric distortions. Experiments were conducted at the University of Notre Dame to show how this method can be used to acquire skin friction values for models with surface curvature. Two test cases conducted at Re_L = 2×10^6 and 4 × 106 were investigated. Image data was processed to determine the centerspan streamwise development of the wall shear stress. The results were compared to a spanwise-periodic direct numerical computation at Re_L = 2 × 10^6 to show the method's effectiveness.

In this paper the skewness of streamwise velocity fluctuations in a canonical zero pressure gradient turbulent boundary layer is spectrally decomposed via the bispectrum. In particular, it is shown that the real part of the bispectrum allows the individual triad interactions contributing to the skewness to be characterized. These measurements are presented for a range of wall-normal locations associated with positive, zero and negative skewness. The individual contributions are also summed to obtain partial and cumulative sums of the skewness as a function of frequency. Complementary conditional sampling measurements demonstrate that the main contribution to boundary layer skewness is intimately associated with ejection-sweep and sweep-ejection events and the degree of asymmetry of their characteristic velocity signatures.

Experiments were performed to document changes in the turbulence characteristics of a boundary layer whose viscous drag has been reduced by 62%. The drag reduction involves generating a steady spanwise velocity component on the order of u-tau, within the sublayer using an array of pulsed-DC plasma actuators. Emphasis is placed on changes in coherent 3-D vortical motions associated with near-wall turbulence production that result from the drag reduction. This involves measurements of all three velocity components in a 3-D region within the boundary layer using a pair of dual [u,v] and [u,w] hot-wire sensors. Under reduced drag, these documented a decrease in the turbulence intensity levels of all three velocity components through most of the boundary layer. Conditional sampling is used to construct the coherent 3-D vortical structures associated with “burst-sweep” events. These revealed a reduction of the wall normal vorticity component, ωy, associated with the mean flow distortion caused by quasi-steady streamwise vorticity associated with the wall streak structure[Kline et al., 1967]. The significance of the ωy component comes from Schoppa and Hussain [2002] who proposed it to be a critical parameter in an autonomous mechanism for self-sustained wall turbulence generation. The results support this mechanism in both the observed suppression of the near-wall turbulence producing events, and the resulting decrease in the viscous drag.

An accurate and straightforward skin-friction measurement procedure over a three-dimensional (3D) bump geometry is proposed. The procedure employs oil-film interferometry (OFI) measurements coupled with photogrammetry techniques. Photogrammetry is vital for accurate quantitative skin friction measurements over curved surfaces and at different oil flow imaging settings. However, it requires information on surface geometry obtained from multiple reference points on the surface and placing the camera in the exact location and orientation before and after the wind-tunnel run, which complicates the application of OFI. A new simplified method reported here is based on camera calibration algorithms and a flexible small checkerboard. The proposed procedure removes the elaborate steps typically performed in photogrammetry while retaining the same measurement accuracy. In particular, it allows obtaining OFI data without a priori knowledge of model geometry and only taking images after experiments at arbitrary undocumented camera locations. The simplified method was first validated with the Clauser method over a flat surface in a canonical turbulent boundary layer. Then the method was applied to obtain the skin-friction distribution for turbulent flow over 3D Gaussian bump geometry, which exhibits smooth body flow separation. The obtained high-quality benchmark data over the bump geometry are aimed to serve as validation data for numerical simulations for the challenging task of predicting flow separation.

It has been established that the dynamics of large-scale structures (LSS) in the outer region of turbulent boundary layers (TBL) and the near-wall small-scale turbulence are correlated. In the study reported here, a plasma-based active flow control device was placed within the TBL to introduce periodic motions into the outer region. The boundary layer Reynolds number was low enough, Reτ = 700 that no naturally occurring energetic coherent large-scale structure was present. Via actuation, a periodic synthetic large-scale structure was introduced into the TBL, and the TBL’s response to this structure in the near-wall region was studied using both hot-wire anemometry and planar PIV. In previous experiments, it was shown that this large-scale structure had a strong modulating effect on the near-wall turbulence downstream of the actuator. In this study an optimized actuator design is tested, and changes to the modulating effect are discussed. In addition, planar PIV is implemented to measure the spatially and temporally resolved two-dimensional velocity field downstream of the actuator. The PIV measurements are shown to be consistent with the hot-wire measurements and capable of capturing the synthetic LSS dynamics and their modulating effect on the near wall turbulence. Ongoing PIV work and the benefits of supplementing the original hot-wire measurements with PIV are discussed.

Bubble breakup and associated gaseous cavitation inception triggered by a single bubble injected into the flow of aviation fuel flowing in a converging–diverging nozzle are characterized by employing advanced computer vision (CV) algorithms. The obtained statistics from CV blob detection analysis provide unprecedented quantitative data from non-intrusive imaging techniques and reveal valuable insights regarding the kinematics of the bubble spatial–temporal evolution, breakup, and gaseous cavitation inception mechanisms in aviation fuels. A distinct constant velocity of the bubble before its breakup and constant group velocity of the resulting void cloud after the breakup are observed despite the continuous streamwise variation in fluid velocity in the nozzle. It is found that though the initial bubble size plays an essential role in the resulting void fraction variation after the breakup, it does not play a role in the bubble breakup location nor the resulting bubble terminal velocity before and void group velocity after the breakup. We define a unique dimensionless number, σb, that is able to distinguish between the breakup dynamic parameters for different fuels and flow regimes. Linear dependence is observed between the breakup location and σb. The obtained results shed some light on the kinematic evolution of a group of nonspherical cavities and will prove useful in modeling complex fuel cavitation mechanisms.

View Video Presentation: https://doi.org/10.2514/6.2022-3342.vid The experimental progress in characterizing the flow separation downstream of the Boeing bump model is presented. Two-component particle image velocimetry was implemented to provide mean velocity and turbulence data in the region downstream of the bump apex. The experimental setup, including the incoming parameters of the upstream boundary layer and skin friction development, is documented comprehensibly. The measurement calibration and processing of the image data is described in detail. Initial observations of the vector fields include the development of the mean separation and reattachment points, as well as the characteristics of the turbulent wake. Embedded shear layer scaling is implemented to highlight the inviscid instability located in the inflection point of the mean velocity profile. Comparisons to a blindly conducted delayed detached eddy simulation is made to show its promising future, and current limitations. This work is meant to highlight the obvious features of the flow, and to announce the current state of the experimental campaign.

A smooth body, adverse pressure gradient (APG), turbulent boundary layer (TBL) separation is experimentally studied. The geometry features canonical TBL development prior to encountering a smooth, two-dimensional convex ramp geometry of finite span onto which a streamwise APG that is fully adjustable is imposed. Both large- and small-scale separations are studied, and all data are archived on the NASA Turbulence Modeling Resource website. This paper describes the large-scale separation case with focus on the surface topography and topology of both separation and reattachment. Despite the spanwise uniform approach TBL and ramp geometry, the separation is highly three-dimensional but the reattachment is spanwise uniform. The surface flow topology is characterized by the ‘owl-face pattern of the fourth kind’ – found to be highly repeatable over multiple experiments. This ubiquitous topology has been reported for a variety of flows including inclined bodies of revolution. It is demonstrated that the APG and the secondary flow associated with the sidewall–ramp juncture is responsible for the formation of the surface separation patterns.

This paper develops a spatial input-output approach to investigate the dynamics of a turbulent boundary layer subject to a localized single frequency excitation. This method uses one-way spatial integration to reformulate the problem in terms of spatial evolution equations. The technique is used to examine the effect of localized periodic actuation at a given temporal frequency, based on an experimental set-up in which an active large-scale is introduced into the outer layer of a turbulent boundary layer. First, the large-scale structures associated with the phase-locked modal velocity field obtained from spatial input-output analysis are shown to closely match those computed based on hot-wire measurements. The approach is then used to further investigate the response of the boundary layer to the synthetically generated large-scale. A quadrant trajectory analysis indicates that the spatial input-output response produces shear stress distributions consistent with those in canonical wall-bounded turbulent flows in terms of both the order and types of events observed. The expected correspondence between the dominance of different quadrant behavior and actuation frequency is also observed. These results highlight the promise of a spatial input-output framework for analyzing the formation and streamwise evolution of structures in actuated wall-bounded turbulent flows.