Instantaneous Velocity Fluctuations Research Articles

Simultaneous measurement of velocity and temperature fluctuations in the boundary layer of a supersonic flow

The instantaneous velocity and temperature behavior of a Mach 3 turbulent boundary layer was studied using two closely spaced constant temperature hot wires. By operating the wires at different temperatures, it was possible to obtain the instantaneous velocity and temperature fluctuations and to evaluate the strong Reynolds analogy. Our analysis revealed that the strong Reynolds analogy is a good approximation for describing the instantaneous velocity and temperature behavior for the inner 70% of the boundary layer.

底吹き気泡による水浴内流動のPTV計測

A mixing flow in a cylindrical vessel agitated by a bubbling jet is measured quantitatively applying particle tracking velocimetry by the binary cross correlation method. Analysing hundreds of video frames by the method, instantaneous velocity distributions, particles paths and velocity fluctuations are measured in a relatively short time. Although a two-dimensional measurement are used in the present experiments, results such as particles paths and instantaneous velocity distributions could be discussed considering threedimensional factors.

Interpretation of Measured Tracer Concentration Fluctuations Using a Sinusoidal Meandering Plume Model

Abstract Simultaneous instantaneous concentration and wind velocity fluctuations were measured 100 to 752 m downwind of a point source release of SF6 tracer during two field studies conducted amid rolling wheat fields and at a flat desert site in eastern Washington. Data from stable, neutron, and unstable conditions are interpreted using a meandering plume model where the meander is defined to be sinusoidal and the instantaneous plume profile is Gaussian. A sensitivity analysis of the model shows that the characteristic concentration time scale is a direct function of the meander time scale and the receptor position relative to the meander centerline. For narrow instantaneous plumes relative to the meander amplitude, the predicted mean crosswind profiles of concentration, intermittency factor, concentration fluctuation intensity, and peak-to-mean ratios exhibit bimodal distributions. Conditional (nonzero) concentration fluctuation intensifies calculated from the model are scattered about 1.0;, the scatter...

B102 二値化相関法PTVによる円筒容器内気泡混合流れの解析

A mixing flow in a cylindrical vessel agitated by a bubbling jet is analyzed quantitatively by using the binary cross correlation method, which is a particle tracking velocimetry (PTV). With this method quantitative results such as instantaneous velocity distributions, velocity fluctuations and particle paths can be obtained easily in a short time. Instantaneous motions of dispersed air bubbles in a water vessel are tracked by establishing correspondence of particle pairs between two sequentially frozen pictures.As a result, a pair of circulation flow pattern was found in the upper region of the vessel and the mixing flow was regarded as three dimensional turbulent flow with large and impulsive fluctuations superposed frequently on the general velocity variation at low speed, though the image processing technique was of a two dimensional type.

An anemometry technique for turbulence measurements at low velocities

A method using symmetrically bent V-shaped hot-wires has been developed for the accurate measurement of low-speed turbulence. Directional characteristics of V-shaped hot-wires at low velocities are investigated, and a generalized expression is derived for the effective cooling velocity. The measurement with V-shaped hot-wires in a pseudo turbulent field, which is artificially produced by shaking the hot-wires with an accurately known motion in a steady flow, has confirmed that the expression for the effective cooling velocity is also valid for instantaneous velocity fluctuations. The accuracy of a practical technique comprising two V-shaped hot-wires in an X arrangement is investigated by an error analysis in simulated Gaussian velocity fields using a digital computer.

Taylor's hypothesis and the probability density functions of temporal velocity and temperature derivatives in a turbulent flow

Equations for the instantaneous velocity and temperature fluctuations in a turbulent flow are used to assess the effect of a fluctuating convection velocity on Taylor's hypothesis when certain simplifying assumptions are made. The probability density function of the velocity or temperature derivative is calculated, with an assumed Gaussian probability density function of the spatial derivative, for two cases of the fluctuating convection velocity. In the first case, the convection velocity is the instantaneous longitudinal velocity, assumed to be Gaussian. In the second, the magnitude of the convection velocity is equal to that of the total velocity vector whose components are Gaussian. The calculated probability density function shows a significant departure, in both cases, from the Gaussian distribution for relatively large amplitudes of the derivative, at only moderate values of the turbulence intensity level. The fluctuating convection velocity affects normalized moments of measured velocity and temperature derivatives in the atmospheric surface layer. The effect increases with increasing order of the moment and is more significant for odd-order moments than even-order moments.

Measurements and observations of turbulent recirculating jet flows

An extensive study has been made of the initial mixing regions of free and confined coaxial air jets with recirculation. Measurements of the axial and radial mean velocity profiles show that the time-averaged characteristics of the two flowfields are substantially different, and that significant mean radial velocities are present in both test cases. Root mean square (rms) and probability density distributions of the instantaneous velocity fluctuations define extensive regions of large scale motion associated with recirculation.

Flapping of a plane jet

Correlations of the instantaneous velocity fluctuations on opposite sides of a subsonic plane jet have been measured. These indicate that there is a measurable flapping of the jet. The frequency of the flapping is estimated by taking the correlation with one signal delayed in time.