13 research outputs found
Vertical Coherence of Turbulence in the Atmospheric Surface Layer: Connecting the Hypotheses of Townsend and Davenport
Statistical descriptions of coherent flow motions in the atmospheric boundary
layer have many applications in the wind engineering community. For instance,
the dynamical characteristics of large-scale motions in wall-turbulence play an
important role in predicting the dynamical loads on buildings, or the
fluctuations in the power distribution across wind farms. Davenport (Quarterly
Journal of the Royal Meteorological Society, 1961, Vol. 372, 194-211) performed
a seminal study on the subject and proposed a hypothesis that is still widely
used to date. Here, we demonstrate how the empirical formulation of Davenport
is consistent with the analysis of Baars et al. (Journal of Fluid Mechanics,
2017, Vol. 823, R2) in the spirit of Townsend's attached-eddy hypothesis in
wall turbulence. We further study stratification effects based on two-point
measurements of atmospheric boundary-layer flow over the Utah salt flats. No
self-similar scaling is observed in stable conditions, putting the application
of Davenport's framework, as well as the attached eddy hypothesis, in question
for this case. Data obtained under unstable conditions exhibit clear
self-similar scaling and our analysis reveals a strong sensitivity of the
statistical aspect ratio of coherent structures (defined as the ratio of
streamwise and wall-normal extent) to the magnitude of the stability parameter
Wall-pressure--velocity coupling in high-Reynolds number wall-bounded turbulence
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 the controller can
actuate upon the desired turbulent structures. A comprehensive database from
direct numerical simulations of turbulent channel flow is used for this
purpose, spanning a Reynolds-number range . A spectral
analysis reveals that the streamwise velocity is most strongly coupled to the
linear term of the wall-pressure, at a 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 weak evidence for a wall-attached-eddy type of scaling. Experimental
data are explored in the second part of this work: wall-pressure data are
denoised and subsequently used for predicting the binary-state of the
streamwise velocity fluctuations in the logarithmic region. A binary estimation
accuracy of up to 72% can be achieved by including both the linear and
quadratic terms of the wall-pressure. This study demonstrates that a controller
for wall-bounded turbulence (solely relying on wall-pressure data) has merit in
terms of a sufficient state estimation capability, even in the presence of
significant facility noise
Inner-scaled Helmholtz resonators with grazing turbulent boundary layer flow
Response details are presented of small-scale Helmholtz resonators excited by
grazing turbulent boundary layer flow. A particular focus lies on scaling of
the resonance, in relation to the spatio-temporal characteristics of the
near-wall velocity and wall-pressure fluctuations. Resonators are tuned to
different portions of the inner-spectral peak of the boundary-layer
wall-pressure spectrum, at a spatial scale of (or
temporal scale of ). Following this approach, small-scale
resonators can be designed with neck-orifice diameters of minimum intrusiveness
to the grazing flow. Here we inspect the TBL response by analysing velocity
data obtained with hot-wire anemometry and particle image velocimetry
measurements. This strategy follows the earlier work by Panton and Miller (J.
Acoust. Soc. Am. 526, 800, 1975) in which only the change in resonance
frequency, due to the grazing flow turbulence, was examined. Single resonators
are examined in a boundary layer flow at . Two
neck-orifice diameters of and 102 are considered, and for each
value of three different resonance frequencies are studied (targeting the
spatial scale of , as well as sub- and
super-wavelengths). Passive resonance only affects the streamwise velocity
fluctuations in the region , while the vertical velocity
fluctuations are seen in a layer up to . A narrow-band
increase in streamwise turbulence kinetic energy at the resonance scale
co-exists with a more than 20% attenuation of lower-frequency (larger scale)
energy. Current findings inspire further developments of passive surfaces that
utilize the concept of changing the local wall-impedance for boundary-layer
flow control, using miniature resonators as a meta-unit
Time-Frequency Analysis of Rocket Nozzle Wall Pressures During Start-up Transients
Surveys of the fluctuating wall pressure were conducted on a sub-scale, thrust- optimized parabolic nozzle in order to develop a physical intuition for its Fourier-azimuthal mode behavior during fixed and transient start-up conditions. These unsteady signatures are driven by shock wave turbulent boundary layer interactions which depend on the nozzle pressure ratio and nozzle geometry. The focus however, is on the degree of similarity between the spectral footprints of these modes obtained from transient start-ups as opposed to a sequence of fixed nozzle pressure ratio conditions. For the latter, statistically converged spectra are computed using conventional Fourier analyses techniques, whereas the former are investigated by way of time-frequency analysis. The findings suggest that at low nozzle pressure ratios -- where the flow resides in a Free Shock Separation state -- strong spectral similarities occur between fixed and transient conditions. Conversely, at higher nozzle pressure ratios -- where the flow resides in Restricted Shock Separation -- stark differences are observed between the fixed and transient conditions and depends greatly on the ramping rate of the transient period. And so, it appears that an understanding of the dynamics during transient start-up conditions cannot be furnished by a way of fixed flow analysis
Density Field Reconstruction of an Overexpanded Supersonic Jet using Tomographic Background-Oriented Schlieren
A Tomographic Background-Oriented Schlieren (TBOS) technique is developed to
aid in the visualization of compressible flows. An experimental setup was
devised around a sub-scale rocket nozzle, in which four cameras were set up in
a circular configuration with 30{\deg} angular spacing in azimuth. Measurements
were taken of the overexpanded supersonic jet plume at various nozzle pressure
ratios (NPR), corresponding to different flow regimes during the start-up and
shut-down of rocket nozzles. Measurements were also performed for different
camera parameters using different exposure times and f-stops in order to study
the effect of measurement accuracy. Density gradients and subsequently
two-dimensional line-of-sight integrated density fields for each of the camera
projections are recovered from the index of refraction field by solving a
Poisson equation. The results of this stage are then used to reconstruct
two-dimensional slices of the (time-averaged) density field using a tomographic
reconstruction algorithm employing the filtered back-projection and the
simultaneous algebraic reconstruction technique. By stacking these
two-dimensional slices, the (quasi-) three-dimensional density field is
obtained. The accuracy of the implemented method with a relatively low number
of sparse cameras is briefly assessed and basic flow features are extracted
such as the shock spacing in the overexpanded jet plume
Quantifying inner-outer interactions in non-canonical wall-bounded flows
We investigate the underlying physics behind the change in amplitude
modulation coefficient in non-canonical wall-bounded flows in the framework of
the inner-outer interaction model (IOIM) (Baars et al., Phys. Rev. Fluids 1
(5), 054406). The IOIM captures the amplitude modulation effect, and here we
focus on extending the model to non-canonical 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 non-canonical
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 over-energetic 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
Opposition flow control for reducing skin-friction drag of a turbulent boundary layer
This work explores the dynamic response of a turbulent boundary layer to
large-scale reactive opposition control, at a friction Reynolds number of
. A 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 downstream
of the input sensor, operating with an exit velocity of . Our control efforts follow the work by Abbassi et al. [Int. J.
Heat Fluid Fl. 67, 2017], but includes a control-calibration experiment and a
performance assessment using PIV- and PTV-based flow field analyses. The
controller targets large-scale high-speed zones when operating in ``opposing"
mode and low-speed zones in the ``reinforcing" mode. 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, since an
energy-intensification of roughly 45% occurs for the reinforcing control mode
instead. Skin friction coefficients are inferred from PTV data to yield a
direct measurement of the wall-shear stress. Results indicate that the opposing
control logic can lower the wall-shear stress by about 3% with respect to a
desynchronised control strategy, and by roughly 10% with respect to the
uncontrolled flow. A FIK-decomposition of the skin-friction coefficient was
performed, revealing 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 Unsteadiness and Side Loads in Overexpanded Rocket Nozzles
Surveys of both the static and dynamic wall pressure signatures on the interior surface of a sub-scale, cold-flow and thrust optimized parabolic nozzle are conducted during fixed nozzle pressure ratios corresponding to FSS and RSS states. The motive is to develop a better understanding for the sources of off-axis loads during the transient start-up of overexpanded rocket nozzles. During FSS state, pressure spectra reveal frequency content resembling SWTBLI. Presumably, when the internal flow is in RSS state, separation bubbles are trapped by shocks and expansion waves; interactions between the separated flow regions and the waves produce asymmetric pressure distributions. An analysis of the azimuthal modes reveals how the breathing mode encompasses most of the resolved energy and that the side load inducing mode is coherent with the response moment measured by strain gauges mounted upstream of the nozzle on a flexible tube. Finally, the unsteady pressure is locally more energetic during RSS, albeit direct measurements of the response moments indicate higher side load activity when in FSS state. It is postulated that these discrepancies are attributed to cancellation effects between annular separation bubbles