Convection Velocity Measurements Research Articles

A low-effort and inexpensive methodology to determine beam separation distance of multi-foci FLDI

A new method is presented to measure the separation distance between probing volumes of closely spaced multi-foci Focused Laser Differential Interferometers (FLDI). The accuracy and precision of this distance measurement directly translate into the quality of convection velocity measurements performed by means of arrays of FLDI. The suggested method is based on the detection of a propagating weak blast wave, generated with a simple and inexpensive apparatus using an automotive spark plug. Demonstration is conducted using an FLDI with two foci (D-FLDI). The generated blast wave is probed at multiple distances from its source to verify its weakening into an acoustic pulse, which offers ideal conditions to the proposed methodology. D-FLDI separation distance measurement using the new approach is compared to measurements using beam profiler images and to the alternative currently established in the literature, based on the FLDI response to a moving weak lens. Tests are made on varying internal configurations of the D-FLDI, while the distance between the two systems is kept constant. Results show the present method to have improved accuracy and robustness in comparison with the moving lens approach, while requiring significantly less effort. Measured separation distances obtained from blast wave detections in a single location are within 0.5 % of the reference value measured through the beam profiler. This procedure is therefore a practical and reliable alternative to the measurement using beam profiler imaging, with similar quality. Its advantages concern associated costs, flexibility when measuring in constrained spaces such as in proximity to walls, and applicability to systems in which beam imaging is not an option, such as multi-point line FLDI.

Coherent Structure Eduction and Convection Velocity in Compressible Blunt-Body Wakes

An iterative eduction procedure has been developed to study coherent structures in blunt-base cylinder wakes at 0 and 10 deg angles of attack to an M = 2.46 freestream flow. The eduction procedure uses an initial wavelet pattern to find candidate coherent structures of a particular scale and then uses the average intensity pattern of the educed structures to create an updated pattern. This iterative procedure continues until the average structure converges, indicating that a self-consistent intensity pattern has been described. Using this eduction procedure, the average appearance of the structures is described. Moreover, a convection velocity measurement is made by directly tracking individual coherent structures, unlike conventional imaging-based convection velocity measurements, which are not conditioned on coherent structures. The evolutionary trends of the structures are also quantified. The coherent structure averages show many of the same trends found in more conventional autocorrelation analysis. For example, the structures are inclined toward the freestream flow direction, and the angle of this inclination is affected by the recompression and reattachment processes. Convection velocity measurements show that the convection velocity is dependent on the transverse position of the structure in the shear layer, particularly for small structures. Finally, the coherent structures evolve in such a way that they become poorly representative of the average coherent structure pattern within 5-15 μs after being first identified.

Large-scale structure visualization and convection velocity in supersonic blunt-base cylinder wakes

Time-correlated imaging has been used to study supersonic blunt-base cylinder wakes at 0° and 10° angle-of-attack to an M=2.46 flow. Most of the temporal evolution of the large-scale structures consists of swelling, shrinking, or rollover, with interactions between structures seeming to play a small role. While structures occasionally merge, rotational pairing events are not observed in the visualizations. The position of the recompression shock in the windward part of the angle-of-attack wake appears to fluctuate on rapid time scales, with disturbances in the shock position tending to follow the convection of large-scale structures in the adjoining shear layer. A consistent difference in the structure convection velocity seems to exist between the high-speed and low-speed sides of the shear layers in this study. The stream selection rule is followed for both the axisymmetric and angle-of-attack wakes before reattachment, but the recompression and reattachment processes greatly influence the structure convection velocity in the axisymmetric wake. Convection velocity measurements in the end-view visualizations show that the large-scale structures have a significant circumferential velocity component in the angle-of-attack wake during and after reattachment, while the large-scale structures in the axisymmetric wake do not, as expected.

Granular convection driven by shearing inertial forces

Convection velocity measurements in vertically vibrated granular materials are presented. The convection velocity close to the walls grows quadratically with the difference between the maximum and critical, or excess, amplitude (proposed as a dynamic parameter to describe related problems) and it is shown numerically that the average bed-bottom relative velocity during the distancing between them, grows linearly with the squared as well. This is interpreted as the signature of an inertial shearing force or momentum transfer proportional to the bed-container relative velocity, acting mainly during the bed-plate distancing part of each cycle which leads to the formation of the convective flux.

A procedure for turbulent structure convection velocity measurements using time-correlated images

This paper describes the development of a technique used to determine the convection velocity of large-scale turbulent structures captured in time-correlated images. The crux of the procedure centers on a cross-correlation routine that is used to determine the convection distance of eddies during the time separation between the image acquisitions. The convection velocity is then estimated as the convection distance divided by the time separation of the image-pair. This cross-correlation routine is capable of analyzing very large data sets in a completely automated manner, thereby improving the accuracy and objectivity of the results over manual or partially automated procedures. Guidelines for optimizing the experimental and computational components of this technique are also presented.

Large structure convection velocity measurements in compressible transverse injection flowfields

Temporally correlated Rayleigh/Mie scattering images were used to examine the convection characteristics of the large-scale vortex structures developing in flowfields created by sonic transverse injection through circular and elliptical nozzles into a Mach 1.98 crossflow. Both compressibility and injector geometry were found to have significant influences on the convection characteristics of the large eddies. High compressibility injection cases produce eddies having dramatically larger near-field convection velocities than those in low compressibility cases. Farther downstream, the large-scale vortices tend to travel at velocities nearer the freestream veocity. Injector geometry primarily affects the near-field behavior.

Effects of Convection and Decay of Turbulence on the Wall Pressure Wavenumber-Frequency Spectrum

Cross-spectral and cross-correlation data from experiments and numerical simulations have shown the turbulent wall pressure convection velocity to vary with the streamwise sensor spatial separation. This variation is due to the spatial decay rates of turbulent structures in the inner and outer regions of the boundary layer. Its effect is shown to have a significant impact on the distribution of energy in the wavenumber-frequency spectrum Φ (k1, ω). The standard Corcos model is known to over predict the wavenumber-frequency spectrum at low wavenumbers. This is shown to result from its constant convection velocity assumption. The spectral levels at sub convective and lower wavenumbers are shown to be directly influenced by the spatial variation in convection velocity. Convection velocity measurements from past investigations that cover the range 285 ≤ Rθ ≤ 29,000 are compared, and an outer variable scaling is shown to effectively collapse the data.

Convection Velocity Measurements in the Crevices of Furniture

Previous to this study, research has shown that small air veloci ties can strongly influence the ignition propensity of a cigarette on upholstered furniture simulations. However, data were not available to determine if such velocities existed in practice. The two objectives of the current effort were to de termine (1) the typical steady velocities near the cushion of a warm chair due strictly to free convection and (2) the typical unsteady velocities near the crev ice of a warm chair due to a combination of free convection and forced convec tion as a result of a person vacating the chair. Typical steady velocities were 40 mm/s. Typical velocities in the unsteady conditions were 15 mm/s. For the unsteady conditions large velocities remained long after the chair cooled to room temperature due to large vortices in the flow with long time constants.

Convection velocity measurements in a cylinder wake

The convection velocity of vortices in the wake of a circular cylinder has been obtained by two different approaches. The first, implemented in a wind tunnel using an array of X-wires, consists in determining the velocity at the location of maximum spanwise vorticity. Four variants of the second method, which estimates the transit time of vortices tagged by heat or dye, were used in wind and water tunnels over a relatively large Reynolds number range. Results from the two methods are in good agreement with each other. Along the most probable vortex trajectory, there is only a small streamwise increase in the convection velocity for laminar conditions and a more substantial variation when the wake is turbulent. The convection velocity is generally greater than the local mean velocity and does not depend significantly on the Reynolds number.

Convection velocity measurements in hypersonic sphere wakes.

Convection velocity measurements in hypersonic sphere wakes.