This work explores the dynamic response of a turbulent boundary layer to large-scale reactive opposition control, at a friction Reynolds number of Reτ≈2240. A surface-mounted hot-film is employed as the input sensor, capturing large-scale fluctuations in the wall-shear stress, and actuation is performed with a single on/off wall-normal blowing jet positioned 2.4δ downstream of the input sensor, operating with an exit velocity of vj=0.4U∞. Our study builds upon the work of Abbassi et al. [Int. J. Heat Fluid Flow 67, 30 (2017)0142727X10.1016/j.ijheatfluidflow.2017.05.003] and includes a control-calibration experiment and a performance assessment using PIV- and PTV-based flow field analyses. With the control-off calibration-experiment conducted a priori, a transfer kernel is identified so that the velocity fluctuations that are to-be-controlled can be estimated. The controller targets large-scale high-speed zones in an "opposing"mode and low-speed zones in a "reinforcing"mode. A desynchronized mode was tested for reference and consisted of a statistically similar control mode, but without synchronization to the incoming velocity fluctuations. An energy-attenuation of about 40 % is observed for the opposing control mode in the frequency band corresponding to the passage of large-scale motions. This proves the effectiveness of the control in targeting large-scale motions: an energy-intensification of roughly 45% occurs for the reinforcing control mode instead, while no change in energy, within the wall-normal range targeted, appears with the desynchronized control mode. Moreover, direct measures of the skin-friction drag are inferred from PTV data. Results indicate that the opposing control logic yields the lowest wall-shear stress (3% lower than the desynchronized control, and 10% lower than the uncontrolled flow). Finally, a FIK-decomposition of the skin-friction coefficient revealed that the off-the-wall turbulence follows a consistent trend with the PTV-based wall-shear stress measurements, although biased by an increased shear in the wake of the boundary layer given the formation of a plume due to the jet-in-crossflow actuation.

Wall-pressure fluctuations are a practically robust input for real-time control systems aimed at modifying wall-bounded turbulence. The scaling behaviour of the wall-pressure-velocity coupling requires investigation to properly design a controller with such input data so that it can actuate upon the desired turbulent structures. A comprehensive database from direct numerical simulations (DNS) of turbulent channel flow is used for this purpose, spanning a Reynolds-number range. Spectral analysis reveals that the streamwise velocity is most strongly coupled to the linear term of the wall pressure, at a Reynolds-number invariant distance-from-the-wall scaling of (and for the wall-normal velocity). When extending the analysis to both homogeneous directions in and, the peak coherence is centred at and for and, and and, respectively. A stronger coherence is retrieved when the quadratic term of the wall pressure is concerned, but there is only little evidence for a wall-attached-eddy type of scaling. An experimental dataset comprising simultaneous measurements of wall pressure and velocity complements the DNS-based findings at one value of k, with ample evidence that the DNS-inferred correlations can be replicated with experimental pressure data subject to significant levels of (acoustic) facility noise. It is furthermore shown that velocity-state estimations can be achieved with good accuracy by including both the linear and quadratic terms of the wall pressure. An accuracy of up to 72 % in the binary state of the streamwise velocity fluctuations in the logarithmic region is achieved; this corresponds to a correlation coefficient of 0.6. This thus demonstrates that wall-pressure sensing for velocity-state estimation - e.g. for use in real-time control of wall-bounded turbulence - has merit in terms of its realization at a range of Reynolds numbers.

Acoustic spectra of rotor noise yield frequency distributions of energy within pressure time series. However, they are unable to reveal phase relations between different frequency components while these play a role in the fundamental understanding of low-frequency intensity modulation of higher-frequency rotor noise. A methodology to quantify interfrequency modulation is applied to a comprehensive acoustic dataset of a fixed-pitch benchmark rotor, operating at a low Reynolds number and at advance ratios ranging from J = 0 to 0.61. Our findings strengthen earlier observations in case of a hovering rotor, in which the modulation of the high-frequency noise is strongest around an elevation angle of θ = −20° (below the rotor plane). For the nonzero advance ratios, modulation becomes dominant in the sector −45° ≲ θ ≲ 0° and is most pronounced at the highest advance ratio tested (J = 0.61). Intensity modulation of high-frequency noise is primarily the consequence of a far-field observer experiencing a cyclic sweep through the noise directivity pattern of relatively directive trailing-edge noise. This noise component becomes more intense with increasing J and is associated with broadband features of the partially separated flow over the rotor blades.

Ducted rotors are configurations known to outperform their unducted reference baselines when aerodynamic performance is concerned. Aside from aerodynamic benefits in hover, a duct also affects acoustic emissions. One of the most contended design parameters of a duct-rotor assembly is the radial distance between the blade tip and the duct wall, referred to as the “tip gap”. The present study explains how the aerodynamic performance of a ducted-rotor system is affected by the tip-gap distance, taking into account the performance of the rotor and those of the duct's inlet lip and diffuser sections. Separate thrust measurements of the rotor and duct establish that the latter can generate up to half of the total thrust of the assembly. Static wall-pressure measurements along the inner wall of the duct reveal a low pressure suction zone over the duct's inlet lip area. This allows the assembly to generate more thrust than the rotor alone, even though the duct's diffuser section generates a drag component (negative thrust). From the velocity fields it is further shown that the performance-deterioration with an increasing tip gap distance is associated with a contraction of the rotor slipstream in the duct diffuser.

Helmholtz resonators flush-mounted in a wall beneath turbulent boundary layer flow are studied by focusing on their flow-induced excitation and effect on the grazing turbulent flow. A particular focus lies on single resonators tuned to the most intense spatio-temporal fluctuations in the near-wall vertical velocity and wall-pressure, residing at a spatial scale of (Formula presented.) (or temporal scale of (Formula presented.)). Resonators are examined in a boundary layer flow at (Formula presented.). Two neck-orifice diameters of (Formula presented.) and 102 are considered, and for each value of (Formula presented.) three different resonance frequencies are studied (corresponding to a period of (Formula presented.), as well as one lower, and one higher, period). The response of the TBL flow is analysed by employing velocity data from hot-wire anemometry and particle image velocimetry measurements. Passive resonance only affects streamwise velocity fluctuations in the region (Formula presented.), while vertical velocity fluctuations due to resonance reach up to (Formula presented.). A narrow-band increase in streamwise turbulence kinetic energy at the resonance scale co-exists with a more than 20% attenuation of lower-frequency energy. Current findings on single resonator cases will aid in the development of passive surfaces with distributed resonators for boundary-layer flow control.

We investigate the impact of a single miniature Helmholtz resonator on wall-bounded turbulence using time-resolved planar particle image velocimetry. A particular aim is to explain the mechanism by which a resonator alters the turbulent velocity fluctuations of different scales. A grazing flow configuration is studied in which the resonator is embedded in the wall beneath a turbulent boundary layer at a friction Reynolds number of Reτ≈2300; the resonator is designed so that its resonance frequency matches the peak frequency of the wall-pressure spectrum. It is found that the resonator amplifies velocity fluctuations near its resonance frequency, while it attenuates the energy of subresonance scales. Underlying mechanisms responsible for these changes in energy are discussed in view of the resonator's local impedance condition. It is posited that large-scale velocity fluctuations in the wall-normal velocity, at temporal frequencies below resonance, are subject to a phase-opposed wall-normal velocity perturbation when the TBL flow convects over the resonator's orifice. This yields a decrease of large-scale energy in u′u′¯,-u′v′¯, and v′v′¯. In addition, modifications of the wall-shear stress field downstream of the resonator are addressed. Insights from this research will contribute to the development of surface designs for passive skin-friction control using arrays of miniature resonators.

Wall-pressure spectra and coherence between wall-pressure and streamwise velocity in a turbulent pipe flow are presented. An experimental investigation was conducted in the CICLoPE long-pipe facility at friction Reynolds numbers in the range of 4700≲Re_τ≲46000. Wall-pressure energy spectra reveal an alignment of the inner-spectral peak location in terms of λ_x⁺≈250, as well as an increase in overall energy content with increasing Re_τ. Linear coherence spectra between wall-pressure and streamwise velocity in the logarithmic region follow a Reynolds-number-independent wall-scaling. Identification of such a universal scaling contributes to compelling evidence that wall-pressure sensing, as an input for real-time flow control, is a feasible approach for implementation in practical engineering systems.

Chevron-shaped protrusions have been proposed in the literature for turbulent skin friction reduction. However, there is no consensus on the performance of this passive flow control technique; both an increase and a decrease in drag have been observed in previous studies. There is also no experimental evidence to support the working mechanism behind the drag reduction effect that has been postulated in the literature. In this study, direct force measurements were used to replicate experiments from the literature and, in addition, were used to test new array configurations to characterise the effect of individual design parameters on drag performance. A total of 23 different protrusion configurations were investigated in a turbulent boundary layer flow. In addition to the integral force measurements, particle image velocimetry was used to measure wall-parallel velocity fields in order to extract the statistical sizing and energy of the near-wall cycle turbulence. All configurations increased the drag between 2% and 10% for a friction Reynolds number of 1700. The drag reduction reported in the literature could not be replicated; however, these findings agreed with an experimental and numerical study that reported drag increase. The trend observed in the low-speed streak spacing from the PIV experiments was consistent with that observed in the balance data. Nevertheless, no evidence was found to support the working mechanism proposed in the literature. These results cast doubt on the proposed drag reduction potential of chevron-shaped protrusions. In the authors’ view, the results of this study strengthen previous conclusions regarding their minor increase in drag. Future studies to further approach a consensus are proposed.

We investigate the underlying physics behind the change in amplitude modulation coefficient in noncanonical wall-bounded flows in the framework of the inner-outer interaction model (IOIM) [Baars, Phys. Rev. Fluids 1, 054406 (2016)2469-990X10.1103/PhysRevFluids.1.054406]. The IOIM captures the amplitude modulation effect, and here we focus on extending the model to noncanonical flows. An analytical relationship between the amplitude modulation coefficient and IOIM parameters is derived, which is shown to capture the increasing trend of the amplitude modulation coefficient with an increasing Reynolds number in a smooth-wall dataset. This relationship is then applied to classify and interpret the noncanonical turbulent boundary layer results reported in previous works. We further present the case study of a turbulent boundary layer after a rough-to-smooth change. Both single-probe and two-probe hotwire measurements are performed to acquire streamwise velocity time series in the recovering flow on the downstream smooth wall. An increased coherence between the large-scale motions and the small-scale envelope in the near-wall region is attributed to the stronger footprints of the overenergetic large-scale motions in the outer layer, whereas the near-wall cycle and its amplitude sensitivity to the superposed structures are similar to that of a canonical smooth-wall flow. These results indicate that the rough-wall structures above the internal layer interact with the near-wall cycle in a similar manner as the increasingly energetic structures in a high-Reynolds number smooth-wall boundary layer.

Equation 3 for the induced velocity factor a in the section III.B should be: (Formula presented) Instead of: (Formula presented) The first sentence of Chapter V should read that an angle of attack of 8° positions the propeller in a strong adverse pressure gradient, not a strong advance ratio.

The unsteady flow behaviour of two side-by-side rotors in ground proximity is experimentally investigated. The rotors induce a velocity distribution interacting with the ground causing the radial expansion of the rotor wakes. In between the rotors, an interaction of the two wakes takes place, resulting in an upward flow similar to a fountain. Two types of flow topologies are examined and correspond to two different stand-off heights between the rotors and the ground: the first one where the height of the fountain remains below the rotor disks, and a second one where it emerges above, being re-ingested. The fountain unsteadiness is shown to increase when re-ingestion takes place, determining a location switch from one rotor disk to the other, multiple times during acquisition. Consequently, variable inflow conditions are imposed on each of the two rotors. The fountain dynamics is observed at a frequency that is about two orders of magnitude lower than the blade passing frequency. The dominant characteristic time scale is linked to the flow recirculation path, relating this to system parameters of thrust and ground stand-off height. The flow field is analysed using proper orthogonal decomposition, in which coupled modes are identified. Results from the modal analysis are used to formulate a simple dynamic flow model of the re-ingestion switching cycle.

Rotor noise comprises harmonic features, related to the blade passing frequency, as well as broadband noise. Even though acoustic spectra yield frequency-distributions of acoustic energy within pressure time series, they do not reveal phase-relations between different frequency components. The latter are of critical importance for the development of prediction- and auralization-algorithms, because these phase-relations can result in low- frequency intensity modulation of higher-frequency rotor noise. Baars et al. (AIAA Paper 2021-0713) outlined a methodology to quantify inter-frequency modulation, which in the current work is applied to a comprehensive acoustic dataset of a laboratory-scale rotor at advance ratios ranging from J = 0 to 0.61. PIV measurements of the blade-induced flow disturbances complement the acoustic data to elucidate how the vortical flow structures of one blade impact the inflow of the consecutive blade. The findings strengthen earlier observations for the case of a hovering rotor (J = 0), in which the modulation of the high-frequency noise is strongest at angles of θ ≈ −20° (below the rotor plane). For the non-zero advance ratios, the modulation becomes dominant in the sector −45° ≲. θ ≲ 0°, and is maximum in strength for the highest advance ratio tested (J = 0.61). It is hypothesized that the intensity-modulation of high-frequency noise relates to the appearance of different separated-flow features over the suction side of the low Reynolds-number rotor-blades. As recently detailed in the articles by Grande et al. (AIAA J. 60:2 & AIAA J. 60:9, 2022), with increasing J, the separation goes from a fully laminar separation, to one that reattaches and forms a laminar separation bubble, to one that fully separates in a turbulent state. With an increase of modulation strength with J we conjecture that trailing-edge/shedding noise, associated with the broadband features of the separated flow, causes the modulation due to a far-field observer experiencing a periodic sweep through the noise directivity patterns. Even though the high-frequency noise is more intense in the hover scenario, the degree of modulation is less since the high-frequency noise field is dominated by turbulence- ingestion noise that has a more omnidirectional nature.

Rotating discs, flush-mounted within the wall beneath a turbulent boundary layer, affect the large-scale flow dynamics. An organized array of rotating discs is a surrogate for the transverse-oscillating wall concept that is known to reduce turbulent friction drag, since the convecting flow encounters a travelling wave, particularly in the spanwise centre of the discs. This work experimentally assesses the flow manipulation of this wall-based actuation method using planar and stereoscopic particle image velocimetry (PIV). Experiments were conducted in a developing turbulent boundary layer, at Re_τ ≈ 910, and with an optimized viscous-scaled sizing and layout of the discs following the direct numerical simulation (DNS) study of Ricco & Hahn (2013). Planar PIV in the streamwise-wall-normal plane over the spanwise centre of the discs revealed a reduction of the in-plane Reynolds stresses, suggesting a suppression of the near-wall turbulence auto-generation process. Wall-parallel planes of velocity data at a height of 70 viscous units above the wall revealed two distinct types of streamwise-oriented regions, comprising low- and high-momentum pathways. These spanwise alternating regions were also captured using the stereo-PIV measurements downstream of the disc-array. It was observed that the mean boundary layer flow is pulled closer to the wall in the disc center, resulting in a higher mean velocity and a less intense streamwise Reynolds stress for a given wall-normal height. With this effect being maximum in the disc center, while being absent between the discs, this type of flow manipulation could be optimized in terms of turbulence suppression (and potentially in terms of friction drag reduction at high Reynolds numbers), by considering larger discs.

A large-scale spanwise and wall-normal array of sonic anemometers in the atmospheric surface layer is used to acquire all three components of instantaneous fluctuating velocity as well as temperature in a range of stability conditions. These data permit investigation of the three-dimensional statistical structure of turbulence structures. Based on a similar dataset, Krug et al. (Boundary-Layer Meteorol., vol. 172, 2019, pp. 199-214) reported a self-similar range of wall-attached turbulence structures under both unstable and near-neutral stability conditions. They considered only a wall-normal array and thus assessed statistical structure in the wall-normal direction, in relation to the streamwise wavelength. The present work extends the view of a self-similar range of turbulence structures, by including the statistical structure in the spanwise direction. Moreover, by analysing the phase shift between synchronized measurements in the spectral domain, it is inferred how a scale-dependent inclination angle in the streamwise/wall-normal plane varies with stability. Results suggest that the self-similar wall-attached structures have similar aspect ratios between streamwise/wall-normal scales and streamwise/spanwise scales such that for both near-neutral and unstable conditions. Under the most unstable conditions, coherent structures with are inclined at angles as high as relative to the solid boundary, while larger scales exhibit inclination angles of approximately. For near-neutral stability conditions, the angle tends towards for all scales. It is noted that in the near-neutral condition, the structure inclination angle and the aspect ratio - and thus the statistical modelling of coherent structures in the atmospheric surface layer - are highly sensitive to the value of the stability parameter.

An experimental investigation is conducted to study the aerodynamic behavior of a two-rotor system in ground proximity. The counter-rotating rotors are placed side-by-side in the hovering condition. The time-averaged and unsteady flow behavior is studied when the rotor-to-rotor lateral distance and the distance between the rotors and the ground are varied. The experiments are performed using three-dimensional large-scale volumetric velocimetry with helium-filled soap bubbles as tracers, tracked by the particle motion analysis technique “Shake-The-Box.” The mean velocity field reveals the wake deflection due to the ground plane and the formation of toroidal-shape regions of separated flow below each rotor. The interaction of the wall jets formed by slipstream deflection results in a separation line with the flow emerging from the wall in a fountain-like pattern. Regimes of flow re-ingestion occur when the rotors are sufficiently far apart. The flowfield exhibits the tendency toward asymmetric states, during which the fountain flow column and the domain of re-ingestion shift closer to one of the rotors. A generic classification of flow regimes is proposed in relation to the behavior of two rotors in ground effect.

Since wall-pressure fluctuations would form a practically-robust input to a real-time active controller of wall-bounded turbulence, it is of high practical interest to study the scaling behavior of the wall-pressure-velocity coupling. This work investigates the coupling of the wall-pressure fluctuations with the streamwise and wall-normal velocity fluctuations. Both the gain (or coherence) and phase spectra of the wall-pressure-velocity transfer kernel are assessed using a comprehensive database, available from direct numerical simulations of turbulent channel flow. With data spanning a decade in friction Reynolds number Re_τ ∼ 550-5200, a 1D analysis (in terms of the streamwise wavelength, λ_x) reveals that the streamwise velocity and wall-pressure are most strongly coupled at a self-similar wall-scaling of λ_x/y ≈ 14. For the wall-normal velocity component, the strongest coupling appears at approximately half this ratio (λ_x/y ≈ 8.5). An analysis of the kernel's phase demonstrates that both the coherent fluctuations of streamwise and wall-normal velocity obey a forward-leaning inclination angle of α ≈ 30^◦. When extending the analysis to 2D (as a function of λ_x and λ_z), the peak-coherence for p_w and u still resides close to λ_x/y ≈ 14 and is reasonably symmetric around λ_x/λ_z = 2.3. The 2D coherence for pw and v peaks around λ_x/λ_z = 1.0. Both the 2D coherence for pw and u, and p_w and v, adhere to a wall-scaling with y. Scaling behaviours identified in this work will aid the efficacy of real-time controllers, by for instance the implementation of data-derived FIR filters to only control velocity structures that are captured through wall-pressure measurements.

Abstract: This work combines the latest advancements in time marching of 3D vector fields from tomographic particle image velocimetry, with an adapted version of Lighthill’s formulation, for the prediction of far-field jet noise. Three-dimensional velocity vector fields of the jet flow are first reconstructed from a tomographic volume of 4× 3× 9.5 Dj3, with D_j = 5 cm being the jet-exit diameter. (The jet-exit Mach number M_j ranges from 0.10 to 0.20.) The obtained vector fields are then used as input to a recently developed procedure for the time marching of the vorticity field, which relies upon the vortex-in-cell methodology. This yields time series of each three-dimensional velocity field, from which the far-field pressure is computed via Lilley’s acoustic analogy (through evaluation of the Lighthill’s stress tensor). It is shown that the estimate of the far-field noise spectrum compares well with the spectrum measured directly from a far-field microphone in the anechoic A-tunnel facility of TU Delft, in the Strouhal number range from approximately 1 to 12. Graphical Abstract: [Figure not available: see fulltext.]

A quantitative assessment of the acoustic source field produced by a laboratory-scale heated jet with a gas dynamic Mach number of 1.55 and an acoustic Mach number of 2.41 is performed using arrays of microphones that are traversed across the axial and radial plane of the jet's acoustic field. The nozzle contour comprises a method of characteristics shape so that shock-related noise is minimal and the dominant sound production mechanism is from Mach waves. The spatial topography of the overall sound pressure level is shown to be dominated by a distinct lobe residing on the principal acoustic emission path, which is expected from flows of this kind with supersonic convective acoustic Mach numbers. The sound field is then analysed on a per-frequency basis in order to identify the location, strength, convection velocity and propagation angle of the various axially distributed noise sources. The analysis reveals a collection of unique data-informed polar patterns of the sound intensity for each frequency. It is shown how these polar patterns can be propagated to any point in the far field with extreme accuracy using the inverse square law. Doing so allows one to gauge the kinds of errors that are encountered using a nozzle-centred source to calculate sound pressure spectrum levels and acoustic power. It is proposed that the measurement strategy described here be used for situations where measurements are being used to compare different facilities, for extrapolating measurements to different geometric scales, for model validation or for developing noise control strategies.