Conditional Sampling Technique Research Articles

Conditional sampling and state space reconstruction

This work looks into the existing links between conditional sampling techniques for coherent structure identification and methods for state space reconstruction which come up in the field of nonlinear time series analysis. It is shown that the sampling condition used by the VITA method corresponds to a round hyper-cylinder, having as its axis the main diagonal of an embedding space reconstructed with the delay method. Such a geometrical interpretation allows us to better understand certain typical characteristics of the method and to see bursting events as excursions of the system leaving the high-dimension basic turbulence.

Vortex interaction in the formation region of a grooved circular cylinder

Based on the conditional sampling technique, the present study in the formation region of the flow over a partially grooved circular cylinder of smooth half and groove half of θ g=+90° over a Reynolds number range of 7.7×10 3–1.3×10 5 establishes the presence of the Gerrard–Bloor secondary vortices in the subcritical, critical, supercritical and transcritical regimes of a smooth cylinder. Quantitatively, there is evidence of mutual interaction of the primary vortex sheet and the secondary vortices. Due to the presence of the secondary vortices, the development of the primary vortex sheet is affected, depending on the energy of the small-scale vortices. The effect of the primary vortex sheet is to enhance the development of the secondary vortices under its strain field. The degree of mutual interaction depends on the energy of the secondary vortices. The present findings on the small-scale streamwise wake vortices on the groove half also indicate similar phenomena.

Laser Doppler anemometry and conditional sampling

Abstract This paper reviews a series of conditional sampling techniques developed at Oxford that allow pseudo-instantaneous velocity field pictures to be synthesised from a succession of single-point measurements made with a two-component laser Doppler anemometer (LDA). The first attempt to apply this technique was by Marwood in his study of roof-edge vortices. This pressure conditional velocity method involved extracting appropriate velocity values after examination of pre-recorded simultaneous pressure and velocity time series. In order to obtain the required data from a reasonably short experiment, Marwood accepted velocities within a small range of pressure trigger levels and calculated the flow field mean values of the samples obtained. Subsequently, bending-moment-conditional velocity sampling has been applied to the detection of identifiable coherent flow structures over a forest canopy. In this application, a significant improvement in trigger detection is under investigation. Instead of setting a trigger based upon a single instantaneous bending moment, the bending moment time series is being examined using the relatively new technique of wavelet analysis. In this case, a wavelet function is chosen to detect the short-term fluctuations in bending moment which might be expected to follow the impact of a discrete canopy-penetrating gust. Within a bending moment event the wavelet analysis will give a precise marker on the velocity time series, and once again, repeated experiments with velocity components measured by the LDA at a sequence of locations provide the necessary information to build a pseudo-instantaneous picture of the gust velocity field above and within the forest canopy. This paper reports that preliminary experiments using this method have been unsuccessful because of the inability of the wavelet analysis to specifically detect coherent gusts in a bending moment time series. However, when an inverted analysis is carried out (using velocity-conditional bending-moment wavelet analysis) clear bending moment events can be recognised to coincide with gust fluctuations of similar wavelength as the tree natural frequency. This suggests that the method is viable, but not set up appropriately. More interestingly it leads to the question of whether coherent gusts produced by trees in a forest can lead to resonance damage to the trees themselves.

Intermittent Flow and Thermal Structures of Accelerating Transitional Boundary Layers: Part 1—Mean Quantities

A conditional sampling technique was employed to separate the turbulent and nonturbulent parts of accelerated boundary layers undergoing laminar-turbulent transition on a uniformly heated flat plate. Tests were conducted with zero pressure gradient and two levels of streamwise acceleration parameter: K = 0.07 × 10−6 and 0.16 × 10−6. The conditionally sampled distribution of the skin friction coefficients revealed that the values for Cf in the nonturbulent and turbulent portions deviated significantly from the respective laminar and turbulent correlations. These deviations increased as acceleration increased. Reconstructing the local average Cf values using the laminar and fully turbulent correlations consistently overestimated the unconditioned Cf values. Using the conditionally sampled data for reconstructing Cf values provided better results, but does not necessarily result in the same unconditioned Cf values. The mean velocity profiles from the turbulent portions had the appearance of a low-Reynolds-number turbulent boundary layer with a large wake region. In the late transition region, as acceleration increased, the wake region in the turbulent portion was suppressed relative to the unconditioned result. The integral parameters, δ*, θ, and shape factor, H, were conditionally sampled and analyzed.

Turbulent Structure in Channel Flow through an Expanded Region.

Effects of impulsive change in pressure gradient in a two-dimensional turbulent channel flow have been experimentally investigated. The rapid change of pressure gradient was accomplished by passing an expanded region. Reynolds shear stress and the statistical properties, i.e., integral length scale, skewness/flatness factors and turbulent diffusions, were investigated in the relaxing region behind the expanded region. An organized structure was examined using conditional sampling techniques. Mean frequency of the bursting was obtained using the VITA Technique. The contributions to uv in the four quadrants on the u-v plane were examined by means of the quadrant analysis method. Reynolds shear stress decreases in the near-wall region, and increases in the outer region. The frequency of the bursting decreases with the pressure gradient. The contribution of the ejection to Reynolds shear stress decreases in the near-wall region, and increases in the outer region due to pressure gradient.

Three-dimensional structure of space-time correlation on coherent vortices generated behind dune crest

Large-scale coherent vortices such as separated vortices and kolk-boils are often observed and generated behind crests of sand dunes in fluvial rivers. The kolk-boil vortex greatly affects transport of suspended sediment which develops up to the free surface. In the present study, both visualization by dye-injection technique and turbulence measurements with a laser Doppler anemometer were conducted in order that the elemental structure of vortices could be clarified. 3-D space-time structures of the coherent vortices are discussed by evaluating conventional correlation coefficients and also by using conditional sampling techniques. Instantaneous velocity-velocity combinations at different points were simultaneously measured by means of two sets of laser Doppler anemometers and an automatic traversing system. Refined physical models of coherent vortices are proposed in the present study.

A simple experimental equation for the bursting cycle

This Brief Communication presents a simple second-order differential equation extracted from experimental data, which can mimic the velocity fluctuations that are typical of bursting. The starting time series concerns the longitudinal component of turbulent velocity measured near the wall in a hydraulically smooth pipe flow. By means of standard conditional sampling techniques, we found the typical behavior of velocity fluctuations during the bursting events, to which we then applied the trajectory method in order to extract the equation of motion. The resulting equation, containing quadratic and cubic nonlinearities, follows the original time series very well, and may represent a useful starting point for the construction of more complex models of this phenomenon.

Measurements of a supersonic turbulent boundary layer by focusing schlieren deflectometry

Some novel, non-intrusive, high-frequency, localized optical measurements of turbulence in compressible flows are described. The technique is based upon focusing schlieren optics coupled with high-speed quantitative measurement of light intensity fluctuations in the schlieren image. Measurements of density gradient fluctuations confined to a thin slice of the flowfield are thus obtained. The new instrument was used to investigate the structure of a two-dimensional, adiabatic, wind tunnel wall boundary layer at a Mach number of 3. The measurements were compared to data obtained using hot-wire anemometry and good agreement was found between the two. Distributions of broadband convection velocity of large-scale structures through the boundary later were also measured. In marked contrast to earlier results, it is shown here that the convection velocity is essentially identical to the local mean velocity. Further, results obtained using the VITA conditional sampling technique shed new light on the turbulent boundary layer structure. Overall, the data presented herein serve to validate the new measurement technique.

Dynamics of interaction modes in excited annular jets

Abstract Evolution of coherent structures and their interaction dynamics are educed in the near field of an acoustically excited basic annular jet using conditional sampling technique based on a multiple triggering criterion to detect the two dominating modes of structure pattern. Acoustic excitation is applied with an aim to better organize the phase alignment of initial rolling and pairing process in the outer shear layer. Negligible modification of the time-averaged flow field results from the excitation. The educed coherent vorticities show that the two modes of evolution are due to the corresponding two modes of shedding pattern of the wake structures from the centerbody, namely the mode one wake and the mode zero wake. In both modes, the shear-layer mode jet vortex rings in the outer layer are perturbed by the shedding of wake structures in the inner region and interaction involving primary merging of three successive jet vortex rings or their partial circumferential sections is found. This results in the formation of wake-induced structures of the corresponding mode pattern, which possesses concentration of coherent vorticity and fluid circulation over a large spatial extent at 1 x / D

Effect of Trailing Edge Suction on Coherent Structures in Near Wake

Experimental results, obtained from hot-wire measurements using a conditional sampling technique, demonstrate feasibility of controlling large-scale spanwise vortices (coherent structures) in the near wake region behind a rectangular base by means of suction through a slit at just one of the trailing edges. The suction thus employed, is found to influence the near wake topology with strong asymmetry and disturb the net vorticity balance. Moreover, a significant reduction in the base drag is achieved as a consequence of the trailing edge suction. The mechanism of the drag reduction is understood to lie in a marked change in the wake dynamics including attenuation in the size and strength of the coherent structures.

Experimental study of low-frequency electrostatic fluctuations in a magnetized toroidal plasma

An experimental study of low-frequency electrostatic fluctuations is presented for a plasma produced by a steady-state discharge in a magnetized toroidal plasma device without a rotational transform. A significant intermittency of the fluctuations is observed. Thus the evolution and propagation of large coherent vortical structures is demonstrated by a conditional sampling technique. The flutelike nature of the structures is explicitly demonstrated. The analysis includes measurements of fluctuations in plasma density, electric potential, and electron temperature. The relevance of the observations to anomalous transport in the device is pointed out. The performance of the conditional sampling technique is compared to a simple correlation analysis by a Monte Carlo simulation.

Experimental Study on Organized Structure in a Turbulent Channel Flow through a Contracted Region.

Effects of impulsive change in pressure gradient in a two-dimensional turbulent channel flow have been experimentally investigated. The rapid change of pressure gradient was produced by passage through a contracted region. An organized structure was examined using conditional sampling techniques in the relaxing region behind the contracted region. Mean frequency of the bursting was obtained using the VITA Technique. The contributions to uv in the four quadrants on the u-v plane were examined by means of the quadrant analysis method. The frequency of the bursting increases with the pressure gradient. The contribution of the sweep to Reynolds shear stress increases near the wall region due to pressure gradient. On the other hand, the contribution of the ejection increases in the outer region. These results are consistent with the finding that the Reynolds shear stress increases by the impulsive disturbance due to the pressure gradient.

Image Measurement of the Wave Height Distributions around a Ship Hull in Regular Wave

Present paper describes an image measurement system for the wave height distribution around ship hulls in regular wave. The wave profile on a laser light sheet is visualized by the difference of light scattering between air and water. Detecting the boundary of bright and dark area on the visualized image with an image processing technique, the instantaneous wave profile on the sheet illumination is obtained. The wave height distribution in calm water can be obtained by reconstructing the distribution from the measured time-series data on a computer.The conditional sampling technique enables to obtain the wave height distribution in regular wave. The wave pattern around a ship in regular wave changes periodically, and its period is based on the encountering interval of wave and ship hull. The measured time-series wave profiles are sorted using their positions on the model fixed coordinate system and phases of encountering. It is the first try to obtain the time series wave pattern based on an experiment.

Space-time correlation structures of horizontal coherent vortices in compound open-channel flows by using particle-tracking velocimetry

In compound open-channel flows, it is very important to clarify an interaction mechanism between the mainchannel and flood-plains induced by coherent vortices, so-called the “intermittent upward secondary currents” and the “large-scale horizontal vortices”, as shown in Figure 1. In the present study, time-averaged structures of secondary currents were revealed experimentally by making use of a two-component fiber-optic Laser Doppler Anemometer (LDA). Next, a new quantitative visualization technique called the “Particle-Tracking Velocimetry (PTV)” was used to investigate evolutionary patterns of horizontal large-scale vortices, which are the ultimate interest in a shallow water flood-plain. By this technique, instantaneous velocity components in arbitrary horizontal plane were obtained at all grid points of laser-light-sheet (LLS) from continuous four images of one particle, and space-time correlation structures of coherent vortices between the main-channel and flood-plain were investigated by using a conditional sampling technique.

CONDITIONAL SAMPLING AND SCALE ANALYSIS OF THE MARINE ATMOSPHERIC MIXED LAYER -- SOFIA EXPERIMENT

During the SOFIA experiment, performed in the Azores region in June 1992, airborne missions were conducted in the atmospheric boundary layer with two aircraft instrumented for turbulence measurements. We show how the conditional sampling technique, applied to the velocity, temperature and moisture fluctuations, is able to describe the various parcels which constitute the turbulent field. Each parcel, so identified, is characterized by its fractional area and by its contribution to the transfers of sensible heat and latent heat. On the other hand, a scale analysis is conducted by filtering the turbulent signals in five non-overlapping frequency bands, defined according to the characteristic turbulent scales. The contribution of each band to the turbulent energy and to the transfers is thus presented. The importance of the lowest frequencies, which are generally removed from the signals by high-pass filtering before computing turbulent fluxes, is shown. In the final section, the conditional sampling technique is applied to the signals filtered in the various bands. Despite a slight deformation of the eddies due to the filtering technique, the contribution of each parcel can be estimated at the various scales analysed.

Correlation of the Wall Skin-Friction and Streamwise Velocity Fluctuations in a Turbulent Boundary Layer(II)

Conditional sampling techniques are utilized to investigate the relation between the wall skin-friction and stream wise velocity fluctuations in a turbulent boundary layer. Conditionally averaged results using a peak detection and the VITA (variable-interval time-averaging) technique show that a high skin friction is associated with high frequency components of the wall skin-friction fluctuations. The conditionally averaged wall skin-friction fluctuations obtained by using the VITA technique have a positively-skewed characteristics compared with the conditionally averaged stream wise velocity fluctuations. It is confirmed that there exists a phase shift between the wall skin-friction and stream wise velocity fluctuations, which was also found from the long-time averaged space-time correlations. The amount of phase shift between the wall skin-friction and stream wise velocity fluctuations is the same as that from the long-time averaged space-time correlations and does not change despite the variation of the detection threshold.

Investigation of Flow Field around a Darrieus Rotor by Flow Visualization and Image Analysis.

Flow around a Darrieus wind turbine operating at low tip-speed ratios is studied by flow visualization using the dye injection technique and also by measuring the velocity field using a particle imaging and conditional sampling technique. The instantaneous flow patterns observed upon the visualization pictures correspond well to the measured velocity distributions, vorticity contours and velocity magnitude maps. It is found that two pairs of mushroom-type vortices are shed from the blade during a rotation of the rotor, which is due to the dynamic stall phenomenon. These stall vortices are produced by the flow separation over the suction side of the blade and the formation of roll-up vortices from the pressure side. The stall vortices at small incidence angles advance to the returning blade side, while those at large incidence angles grow along the moving blade and interact with flow near the blade. The effect of tip-speed ratios on the growth of stall vortices is also examined.

Simple battery powered device for flux measurements by conditional sampling

The design, construction and field operation of a simple electronic system for the measurement of trace gas fluxes using the conditional sampling technique is described. The system was tested by measuring CO2 fluxes from peatland. Different sampling modes can be used depending on the chemical being measured. The simplicity of the equipment should facilitate measurements at remote but globally significant sites.

The spatial structure of the downward momentum flux on sea derived from a SAR image

The spatial structure of the atmospheric surface layer over the sea has been investigated through a radar image taken by an airborne Synthetic Aperture Radar (SAR) at C-band (5.3 GHz) and wind stress data recorded on board of the C.N.R. research platform located in the northern Adriatic Sea. The SAR image shows quasi-ordered structures of radar backscatter hundreds of meters long, while the wind stress time series are characterized by the well known intermittent bursts of tenth of seconds. A conditional sampling technique has been applied both to the wind stress time series and the SAR image, to get the mean structure of the downward momentum flux and the one and two dimensional shape of the structures of the image. It has been shown that a part of the backscatter structures represents the imprint of the downward wind stress. They appear like microfronts, 50 m wide in the up-downwind direction, elongated in the crosswind direction. The crosswind size is in average four times larger than the up-downwind one. The downward wind stress structures cover the 13% of the total area.

Design, construction and operation of flux measurement systems using the conditional sampling technique

The design, construction and field operation of computer-based systems for the measurement of trace gas fluxes using the conditional sampling technique are described. A simple system which sampled air into sampling bags was used to measure CH 4 and N 2O fluxes from peatland and agricultural land, respectively. The system was subsequently automated by making real-time measurements of the gas mixing ratios in the sampling lines thus providing continuous measurements for periods of up to several days. Minor modifications enabled measurement of total hydrocarbons, CO 2 and non-methane hydrocarbon fluxes from a forest ecosystem. Intercomparison of conditional sampling with other techniques (eddy covariance, gradient and aircraft boundary layer budget) was encouraging with good agreement between flux measurements of CH 4, CO 2, N 2O and sensible heat. The system has been developed in a sufficiently simple and robust manner to enable extended field measurements. A number of theoretical problems remain including the absolute accuracy of the gas analysis procedures and real-time coordinate rotation procedures to deal with non-uniform terrain.