Hot-wire Arrays Research Articles

고속(KTX) 및 기존 철도차량의 열차풍 현상 비교 분석

A series of field tests were performed to develop aerodynamic characteristic evaluation method and countermeasure technology in conventional and high-speed railway. The strength of rolling stock-induced wind which affect the people and substructure in platform and nearby track were investigated. The slipstream of passing trains was measured by hot-wire array system. The speed of trains was 110 - 125㎞/h for conventional ones and 300㎞/h for high-speed ones. The streamlined shape trains cause about 50% smaller-scale slipstream compared to the non-streamlined ones.

The surface hot wire as a means of measuring mean and fluctuating wall shear stress

The paper describes some applications of a wall shear stress sensor technique which is based on hot-wire anemometry. The “surface hot wire” is a flush-mounted thermal resistive wire with a tiny slot underneath. The arrangement of this sensor guarantees an improved signal-to-noise ratio compared to a common surface hot film. The setup and the application of single sensors and of surface hot-wire arrays are shown. Some results are presented that were acquired in several experiments in the field of laminar-turbulent transition.

The spatial resolution of hot-wire arrays for the measurement of small-scale turbulence

The measurement of small-scale turbulence quantities with hot-wire arrays has proven difficult due to the difficulty of measuring spatial derivatives of velocity and temperature fluctuations reliably. This paper reviews recent developments in accounting for spatial resolution effects on these arrays. In particular, the effects of wire length, separations between wires and effective wire inclinations are considered for an X-probe, parallel-wire arrays and vorticity arrays. Expressions for correcting spectra and variances of velocity (or temperature) derivatives and vorticity components are presented. Sufficient evidence is now available to validate these corrections, at least when local isotropy is a reasonable approximation. Data from direct numerical simulations (DNS) have played an important role in the context of this validation.

Observation of traveling waves in the three-dimensional boundary layer along a yawed cylinder

Experimental investigations are made of crossflow instability of the three-dimensional boundary layer along a yawed circular cylinder. To explore the effect of environmental disturbances on transition, comparative experiments are performed in high- and low-turbulence wind tunnels. Independent of turbulence level, unsteady disturbances with definite frequencies instead of stationary vortices are observed in hot-wire surveys of the unstable flow filed. Detailed measurements with the aid of hot-wire arrays have confirmed that the unsteady disturbances are identical with the so-called crossflow traveling waves predicted by linear stability theory as the most unstable disturbances, although some discrepancies still remain between experimental results and theoretical predictions.

Organized structures in a compressible, turbulent boundary layer

Experimental results are presented that show the existence of organized structures in a compressible, turbulent boundary layer. Results were obtained using arrays of hot wires and wall pressure transducers in a Mach-3 zero-pressure-gradient boundary layer. The VITA method of conditional sampling was used to deduce average pressure events at the wall and mass flux events throughout the boundary layer; these results show qualitative similarity to those found in incompressible flows. By conditioning upon the middle hot wire from a three-wire probe, evidence is found suggesting that structures exist of a height comparable with the boundary-layer thickness. Furthermore, two-point conditional sampling was used to show that an average pressure event could be extracted by conditioning upon mass-flux events. From this procedure we found that the structures maintain their shape as they travel downstream and also that their spanwise extent is very limited.The inferred angle from correlations between two hot wires, and between a hot wire and a wall-pressure transducer, indicate that the average structure is inclined at approximately 45° for a large part of the boundary layer. This result agrees well with structures observed in schlieren photographs of supersonic boundary layers. Measurements of the instantaneous angle show a wide distribution of structure angles, and the general behaviour of the large-scale structures is consistent with the hairpin loop model of wall turbulence.

Search for Large-Scale Coherent Structures in the Nearly Self-Preserving Region of a Turbulent Axisymmetric Jet

In an attempt to explore the existence of large-scale coherent structures in the nearly self-preserving region of an axisymmetric free air jet, a 2.54 cm air jet at a Reynolds number ReD = 6.8 × 104 has been investigated for x/D≥40 via both long and short time-averaged space-time correlation measurements. Conventional space-time correlation data with probe separations in the stream wise direction by as much as 25 diameters suggest the passage of large-scale organized structures. The radial extent of these structures is about one local jet diameter, and the azimuthal extent is about a quadrant of the cross-section. Recurring quasi-periodic patterns are observed in the time series of short time-averaged correlations between longitudinal velocity fluctuations obtained with two arrays of hot-wires separated in the streamwise direction. These orderly patterns provide direct evidence for the existence of these structures. Quantitative details of these structures are now being investigated.

Large structure in a turbulent boundary layer

Using an array of hot wires and wall shear stress probes results are presented which support the hypothesis that an organized structure exists in the boundary layer. From the correlation between wall shear stress and velocity it is shown that this structure is at an oblique angle to the free stream. The inferred angle from this correlation is found to agree remarkably with recent flow visualization photographs. Evidence is found that the large scale motion in the organized structure produces a slowly varying component in the wall shear stress and, importantly, a high frequency large amplitude fluctuation occurring near the maximum in the slowly varying wall shear. It is suggested that this latter fluctuation is associated with the bursting phenomenon. A model which accounts for these results is described. It leads to a description of the bursting process in terms of a rotational instability and while too approximate to give more than good order of magnitude estimates, it gives values of the wall streak spacing which are not inconsistent with previous measurements.

Survey and new measurements of turbulent structure near the wall

A survey of recent measurements of turbulent structure near the wall which are applicable to the phenomenon of drag reduction by polymer additives is presented. Knowledge of the vorticity in the wall region provides a framework for understanding turbulent structure and the effects of polymer additives. New measurements using a small, calibrated streamwise vorticity probe are described. Near the wall where the turbulent production and dissipation are a maximum the streamwise vorticity is a maximum and is highly intermittent. New measurements of the Reynolds stress have also been made with a very small, computer calibrated hot-wire array. Intermittent small scale structures producing rapid oscillations of the hot-wire signals are detected near the wall which are too small to be resolved by the array. Their spatial scale is much less than the Kolmogoroff length. It is suggested that they are associated with streamwise vorticity.

Treatment of the Annulus Wall Boundary Layer Using a Secondary Flow Hypothesis

Hitherto, theories of annulus wall boundary layer development in axial compressors have assumed an axially-symmetric flow in which the blade action has been replaced by a force field. A more rigorous treatment of the momentum equations in the annulus boundary layer by Mellor and Wood demonstrated the presence of certain terms, after the equations had been averaged in the pitch-wise direction, which arise from the truly three-dimensional character of the flow. These terms, which may be described as the gradients of apparent stresses, were not regarded by them (apart from a discussion of tip clearance) as having importance for the problem. In the present work a second equation of the annulus wall boundary layer is obtained by consideration of the work of these apparent stresses. By integration of the system of equations over a single blade row, two equations are obtained relating various integral quantities at inlet to and exit from the row. Each equation contains terms which depend upon apparent stresses connected with the relative velocity field at the exit plane. An experiment is described in which the six turbulent stresses in the stationary frame downstream of a single rotor, determined by means of a multiple hot wire array, are used to evaluate each term of the aforementioned equations. The integral quantities thus determined are shown to be reasonably consistent with the predictions from the two equations, in particular, for the case of the hub boundary layer. Theoretical solutions of the two integral equations require a secondary flow hypothesis so that the departure from collateral flow at blade row exit is determined by the solution.

Spatial resolution of the vorticity meter and other hot-wire arrays

Analyses of the spatial resolution of the Kovasznay vorticity meter and of arrays for measuring velocity derivatives in isotropic turbulence are presented. Pao's three-dimensional spectrum is used, allowing calculations of the measured values of the one-dimensional vorticity spectrum and mean-square velocity derivatives to be compared with their predicted values. It is found that for accurate measurement the array size should be of the order of the Kolmogorov microscale of the turbulence field.