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.

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.

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 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.

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.

Turbulent convection systems are known to give rise to prominent large-scale circulation. At the same time, the 'background' (or 'small-scale') turbulence is also highly relevant and e.g. carries the majority of the heat transport in the bulk of the flow. Here, we investigate how the small-scale turbulence is interlinked with the large-scale flow organization of Rayleigh-Bénard convection. Our results are based on a numerical simulation at Rayleigh number in a large aspect ratio cell to ensure a distinct scale separation. We extract local magnitudes and wavenumbers of small-scale turbulence and find significant correlation of large-scale variations in these quantities with the large-scale signal. Most notably, we find stronger temperature fluctuations and increased small-scale transport (by about 10Â % of the global Nusselt number) in plume impacting regions and opposite trends in the plume emitting counterparts. This concerns wall distances up to (thermal boundary layer thickness). Local wavenumbers are generally found to be higher on the plume emitting side compared to the impacting one. A second independent approach by means of conditional averages confirmed these findings and yields additional insight into the large-scale variation of small-scale properties. Our results have implications for the modelling of small-scale turbulence.

Opposition-control of the energetic cycle of near wall streaks in wall-bounded turbulence, using numerical approaches, has shown promise for drag reduction. For practical implementation, real-time opposition control is only realizable if there is a degree of coherence between the turbulent velocities passing a sensor and the target point within the flow; for practicality, a sensor (and actuator) should be wall-based to avoid parasitic drag. As such, we here inspect the feasibility of real-time control of the near wall cycle, by considering the coherence between a measurable wall-quantity, being the wall-shear stress fluctuations, and the streamwise and wall-normal velocity fluctuations in a turbulent boundary layer. Synchronized spatial and temporal velocity data from two direct numerical simulations and a fine large eddy simulation at Re_τ≈590 and 2000 are employed. This study shows that the spectral energy of the streamwise velocity fluctuations that is stochastically incoherent with wall signals is independent of Reynolds number in the near wall region (up to the viscous-scaled wall-normal height z⁺≈20). Consequently, the streamwise energy-fraction that is stochastically wall-coherent grows with Reynolds number due to the increasing range of energetic large scales. This thus implies that a wall-based control system has the ability to manipulate a larger portion of the total turbulence energy at off-wall locations, at higher Reynolds numbers, while the efficacy of predicting/targeting the small scales of the near wall cycle remains indifferent with varying Reynolds number. Coherence values of 0.55 and 0.4 were found between the streamwise and wall-normal velocity fluctuations at the near wall peak in the energy spectrogram, respectively, and the streamwise fluctuating friction velocity. These coherence values, which are considerably lower than 1 (maximum possible coherence) suggest that a closed-loop drag reduction scheme targeting near wall cycle streaks alone (based on sensed friction velocity fluctuations) will be of limited success in practice.

A method for calculating the effective Gol'dberg number for diverging waveforms is presented, which leverages known features of a high-speed jet and its associated sound field. The approach employs a ray tube situated along the Mach wave angle where the sound field is not only most intense, but advances from undergoing cylindrical decay to spherical decay. Unlike other efforts, a "piecewise-spreading regime" model is employed, which yields, separately, effective Gol'dberg numbers for the cylindrically and spherically spreading regions in the far field. The new approach is applied to a plethora of experimental databases, encompassing both laboratory-and full-scale jet noise studies. The findings demonstrate how cumulative nonlinear distortion is expected to form in the acoustic near field of laboratoryscale round jets where pressure amplitudes decay cylindrically; waveformdistortion is not expected in the acoustic far field where waveform amplitudes diverge spherically. On the other hand, where full-scale jet studies are concerned, effective Gol'dberg number calculations demonstrate how cumulative waveform distortion is significant in both the cylindrical-and spherical-spreading regimes. The laboratory-scale studies also reveal a pronounced sensitivity to humidity conditions, relative to the full-scale counterpart.

The mean wall shear stress, τ¯ _w, is a fundamental variable for characterizing turbulent boundary layers. Ideally, τ¯ _w is measured by a direct means and the use of floating elements has long been proposed. However, previous such devices have proven to be problematic due to low signal-to-noise ratios. In this paper, we present new direct measurements of τ¯ _w where high signal-to-noise ratios are achieved using a new design of a large-scale floating element with a surface area of 3 m (streamwise) × 1 m (spanwise). These dimensions ensure a strong measurement signal, while any error associated with an integral measurement of τ¯ _w is negligible in Melbourne’s large-scale turbulent boundary layer facility. Wall-drag induced by both smooth- and rough-wall zero-pressure-gradient flows are considered. Results for the smooth-wall friction coefficient, C_f≡ τ¯ _w/ q_∞, follow a Coles–Fernholz relation Cf=[1/κln(Reθ)+C]-2 to within 3 % (κ= 0.38 and C= 3.7) for a momentum thickness-based Reynolds number, Re_θ> 15 , 000. The agreement improves for higher Reynolds numbers to <1 % deviation for Re_θ> 38 , 000. This smooth-wall benchmark verification of the experimental apparatus is critical before attempting any rough-wall studies. For a rough-wall configuration with P36 grit sandpaper, measurements were performed for 10 , 500 < Re_θ< 88 , 500 , for which the wall-drag indicates the anticipated trend from the transitionally to the fully rough regime.

Wavelet analysis is employed to examine amplitude and frequency modulations in broadband signals. Of particular interest are the streamwise velocity fluctuations encountered in wall-bounded turbulent flows. Recent studies have shown that an important feature of the near-wall dynamics is the modulation of small scales by large-scale motions. Small- and large-scale components of the velocity time series are constructed by employing a spectral separation scale. Wavelet analysis of the small-scale component decomposes the energy in joint time–frequency space. The concept is to construct a low-dimensional representation of the small-scale time-varying spectrum via two new time series: the instantaneous amplitude of the small-scale energy and the instantaneous frequency. Having the latter in a time-continuous representation allows a more thorough analysis of frequency modulation. By correlating the large-scale velocity with the concurrent small-scale amplitude and frequency realizations, both amplitude and frequency modulations are studied. In addition, conditional averages of the small-scale amplitude and frequency realizations depict unique features of the scale interaction. For both modulation phenomena, the much studied time shifts, associated with peak correlations between the large-scale velocity and small-scale amplitude and frequency traces, are addressed. We confirm that the small-scale amplitude signal leads the large-scale fluctuation close to the wall. It is revealed that the time shift in frequency modulation is smaller than that in amplitude modulation. The current findings are described in the context of a conceptual mechanism of the near-wall modulation phenomena.